Process of wastewater treat. / Proces preciscavanja otpadnih voda by AG-Metal Zenica

Wastewater Treatment

According to the Code of Federal Regulations (CFR) 40 CFR Part 403, regulations were established in the late 1970s and early 1980s to help publicly owned treatment works (POTW) control industrial discharges to sewers. These regulations were designed to prevent pass-through and interference at the treatment plants and interference in the collection and transmission systems. Pass-through occurs when pollutants literally pass through a POTW without being properly treated, and cause the POTW to have an effluent violation or increase the magnitude or duration of a violation. Interference occurs when a pollutant discharge causes a POTW to violate its permit by inhibiting or disrupting treatment processes, treatment operations, or processes related to sludge use or disposal.

18.1 WASTEWATER OPERATORS Like waterworks operators, wastewater operators are highly trained and artful practitioners and technicians of their trade. Both operators are also required by the states to be licensed or certified to operate a wastewater treatment plant. When learning wastewater operator skills, there are a number of excellent texts available to aid in the training process. Many of these texts are listed in Table 18.1.

We begin certain sections (which discuss unit processes) with frequent reference to Figure 18.1. It is important to begin these sections in this manner because wastewater treatment is a series of individual steps (unit processes) that treat the wastestream as it makes its way through the entire process. It logically follows that a pictorial presentation along with pertinent written information enhances the learning process. It should also be pointed out that even though the model shown in Figure 18.1 does not include all unit processes currently used in wastewater treatment, we do not ignore the other major processes: trickling filters, rotating biological contactors (RBCs), and oxidation ponds.

18.2 WASTEWATER TERMINOLOGY AND DEFINITIONS Wastewater treatment technology, like many other technical fields, has its own unique terms with their own meaning. Though some of the terms are unique, many are common to other professions. Remember that the science of wastewater treatment is a combination of engineering, biology, mathematics, hydrology, chemistry, physics, and other disciplines. Many of the terms used in engineering, biology, mathematics, hydrology, chemistry, physics, and others are also used in wastewater treatment. Those terms not listed or defined in the following section will be defined as they appear in the text.

18.1.1 THE WASTEWATER TREATMENT PROCESS: THE MODEL

18.2.1 TERMINOLOGY

Figure 18.1 shows a basic schematic of an example wastewater treatment process providing primary and secondary treatment using the activated sludge process. This is the model, prototype, and paradigm used in this book. Though it is true that in secondary treatment (which provides biochemical oxygen demand [BOD] removal beyond what is achievable by simple sedimentation), there are actually three commonly used approaches (trickling filter, activated sludge, and oxidation ponds). For instructive and illustrative purposes, we focus on the activated sludge process throughout this handbook. The purpose of Figure 18.1 is to allow the reader to follow the treatment process step-by-step as it is presented (and as it is actually configured in the real world) and to assist understanding of how all the various unit processes sequentially follow and tie into each other.

Activated sludge the solids formed when microorganisms are used to treat wastewater using the activated sludge treatment process. It includes organisms, accumulated food materials, and waste products from the aerobic decomposition process. Advanced waste treatment treatment technology used to produce an extremely high quality discharge. Aerobic conditions in which free, elemental oxygen is present. Also used to describe organisms, biological activity, or treatment processes that require free oxygen. Anaerobic conditions in which no oxygen (free or combined) is available. Also used to describe organisms, biological activity or treatment processes that function in the absence of oxygen.

ÂŠ 2003 by CRC Press LLC

AND

DEFINITIONS

528

Handbook of Water and Wastewater Treatment Plant Operations

TABLE 18.1 Recommended Reference and Study Material 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

Kerri, K.D. et al., Advanced Waste Treatment, A Field Study Program, 2nd ed., California State University, Sacramento, 1995. U.S. Environmental Protection Agency, Aerobic Biological Wastewater Treatment Facilities, EPA 430/9–77–006, Washington, D.C., 1977. U.S. Environmental Protection Agency, Anaerobic Sludge Digestion, EPA-430/9–76–001, Washington, D.C., 1977. American Society for Testing Materials, Section 11: Water and environmental technology, in Annual Book of ASTM Standards, Philadelphia, PA. Guidelines Establishing Test Procedures for the Analysis of Pollutants, Federal Register (40 CFR 136), April 4, 1995, Vol. 60, No. 64, p. 17160. HACH Chemical Company, Handbook of Water Analysis, 2nd ed., Loveland, CO, 1992. Kerri, K.D. et al., Industrial Waste Treatment: A Field Study Program, Vols. 1 and 2, California State University, Sacramento, CA, 1996. U.S. Environmental Protection Agency, Environmental Monitoring Systems Laboratory-Cincinnati, Methods for Chemical Analysis of Water and Wastes, EPA-6000/4–79–020, revised March 1983 and 1979 (where applicable). Water Pollution Control Federation (now called Water Environment Federation), O & M of Trickling Filters, RBC and Related Processes, Manual of Practice OM-10, Alexandria, VA, 1988. Kerri, K.D. et al., Operation of Wastewater Treatment Plants: A Field Study Program, Vols. 1 and 2, 4th ed., California State University, Sacramento, 1993. American Public Health Association, American Water Works Association-Water Environment Federation, Standard Methods for the Examination of Water and Wastewater, 18th ed., Washington, D.C., 1992. Kerri, K.D. et al., Treatment of Metal Wastestreams, 2nd ed., California State University, Sacramento, 1993. Price, J.K., Basic Math Concepts: For Water and Wastewater Plant Operators, Technomic Publ., Lancaster, PA, 1991. Haller, E., Simplified Wastewater Treatment Plant Operations, Technomic Publ., Lancaster, PA, 1999. Qaism, S.R., Wastewater Treatment Plants: Planning, Design, and Operation, Technomic Publ., Lancaster, PA, 1994.

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

Primary treatment

Secondary treatment

Influent Collection system

Air

Screening and comminution

Grit chamber

Screenings

Grit

Primary settling

Aeration

Chlorine Secondary settling

Effluent

Chlorine contact tank

Activated sludge

Thickener

Anaerobic digester

Sludge dewatering Sludge disposal

FIGURE 18.1 Schematic of an example wastewater treatment process providing primary and secondary treatment using activated sludge process. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Anoxic conditions in which no free, elemental oxygen is present. The only source of oxygen is combined oxygen, such as that found in nitrate compounds. Also used to describe biological activity of treatment processes that function only in the presence of combined oxygen. Average monthly discharge limitation the highest allowable discharge over a calendar month. Average weekly discharge limitation t h e h i g h e s t allowable discharge over a calendar week. Biochemical oxygen demand (BOD) the amount of organic matter that can be biologically oxidized © 2003 by CRC Press LLC

under controlled conditions (5 days @ 20∞C in the dark). Biosolids (from 1977) solid organic matter recovered from a sewage treatment process and used especially as fertilizer (or soil amendment); usually used in plural (from Merriam-Webster’s Collegiate Dictionary, 10th ed., 1998). Note: In this text, biosolids is used in many places (activated sludge being the exception) to replace the standard term sludge. The author views the term sludge as an ugly, inappropriate four-letter word to describe biosolids. Biosolids

Wastewater Treatment

is a product that can be reused; it has some value. Because biosolids has value, it certainly should not be classified as a waste product, and when biosolids for beneficial reuse is addressed, it is made clear that it is not. Buffer a substance or solution which resists changes in pH. Carbonaceous biochemical oxygen demand (CBOD5) the amount of biochemical oxygen demand that can be attributed to carbonaceous material. Chemical oxygen demand (COD) the amount of chemically oxidizable materials present in the wastewater. Clarifier a device designed to permit solids to settle or rise and be separated from the flow. Also known as a settling tank or sedimentation basin. Coliform a type of bacteria used to indicate possible human or animal contamination of water. Combined sewer a collection system that carries both wastewater and storm water flows. Comminution a process that shreds solids into smaller, less harmful particles. Composite sample a combination of individual samples taken in proportion to flow. Daily discharge the discharge of a pollutant measured during a calendar day or any 24-h period that reasonably represents a calendar day for the purposes of sampling. Limitations expressed as weight is total mass (weight) discharged over the day. Limitations expressed in other units are average measurements of the day. Daily maximum discharge the highest allowable values for a daily discharge. Detention time the theoretical time water remains in a tank at a given flow rate. Dewatering the removal or separation of a portion of water present in a sludge or slurry. Discharge monitoring report (DMR) the monthly report required by the treatment plant’s National Pollutant Discharge Elimination System (NPDES) discharge permit. Dissolved oxygen (DO) free or elemental oxygen that is dissolved in water. Effluent the flow leaving a tank, channel, or treatment process. Effluent limitation any restriction imposed by the regulatory agency on quantities, discharge rates, or concentrations of pollutants that are discharged from point sources into state waters. Facultative organisms that can survive and function in the presence or absence of free, elemental oxygen. © 2003 by CRC Press LLC

529

Fecal coliform a type of bacteria found in the bodily discharges of warm-blooded animals. Used as an indicator organism. Floc solids which join together to form larger particles which will settle better. Flume a flow rate measurement device. Food-to-microorganism ratio (F:M) an activated sludge process control calculation based upon the amount of food (BOD or COD) available per pound of mixed liquor volatile suspended solids. Grab sample an individual sample collected at a randomly selected time. Grit heavy inorganic solids such as sand, gravel, egg shells, or metal filings. Industrial wastewater wastes associated with industrial manufacturing processes. Infiltration/inflow extraneous flows in sewers; simply, inflow is water discharged into sewer pipes or service connections from such sources as foundation drains, roof leaders, cellar and yard area drains, cooling water from air conditioners, and other clean-water discharges from commercial and industrial establishments. Defined by Metcalf & Eddy as follows:1 • Infiltration water entering the collection system through cracks, joints, or breaks. • Steady inflow water discharged from cellar and foundation drains, cooling water discharges, and drains from springs and swampy areas. This type of inflow is steady and is identified and measured along with infiltration. • Direct flow those types of inflow that have a direct stormwater runoff connection to the sanitary sewer and cause an almost immediate increase in wastewater flows. Possible sources are roof leaders, yard and areaway drains, manhole covers, cross connections from storm drains and catch basins, and combined sewers. • Total inflow the sum of the direct inflow at any point in the system plus any flow discharged from the system upstream through overflows, pumping station bypasses, and the like. • Delayed inflow stormwater that may require several days or more to drain through the sewer system. This category can include the discharge of sump pumps from cellar drainage as well as the slowed entry of surface water through manholes in ponded areas. Influent the wastewater entering a tank, channel, or treatment process.

530

Handbook of Water and Wastewater Treatment Plant Operations

Inorganic mineral materials such as salt, ferric chloride, iron, sand, gravel, etc. License a certificate issued by the state board of waterworks or wastewater works operators authorizing the holder to perform the duties of a wastewater treatment plant operator. Mean cell residence time (MCRT) the average length of time a mixed liquor suspended solids particle remains in the activated sludge process. May also be known as sludge retention time. Mixed liquor the combination of return activated sludge and wastewater in the aeration tank. Mixed liquor suspended solids (MLSS) the suspended solids concentration of the mixed liquor. Mixed liquor volatile suspended solids (MLVSS) the concentration of organic matter in the mixed liquor suspended solids. Milligrams/Liter (mg/L) a measure of concentration. It is equivalent to parts per million. National Pollutant Discharge Elimination System permit permit that authorizes the discharge of treated wastes and specifies the condition, which must be met for discharge. Nitrogenous oxygen demand (NOD) a measure of the amount of oxygen required to biologically oxidize nitrogen compounds under specified conditions of time and temperature. Nutrients substances required to support living organisms. Usually refers to nitrogen, phosphorus, iron, and other trace metals. Organic materials that consist of carbon, hydrogen, oxygen, sulfur, and nitrogen. Many organics are biologically degradable. All organic compounds can be converted to carbon dioxide and water when subjected to high temperatures. Pathogenic disease causing. A pathogenic organism is capable of causing illness. Point source any discernible, defined, and discrete conveyance from which pollutants are or may be discharged. Part per million (ppm) an alternative (but numerically equivalent) unit used in chemistry is milligrams per liter. As an analogy, think of this unit as being equivalent to a full shot glass in a swimming pool. Return activated sludge solids (RASS) the concentration of suspended solids in the sludge flow being returned from the settling tank to the head of the aeration tank. Sanitary wastewater wastes discharged from residences and from commercial, institutional, and similar facilities that include both sewage and industrial wastes.

ÂŠ 2003 by CRC Press LLC

Scum the mixture of floatable solids and water that is removed from the surface of the settling tank. Septic a wastewater that has no dissolved oxygen present. Generally characterized by black color and rotten egg (hydrogen sulfide) odors. Settleability a process control test used to evaluate the settling characteristics of the activated sludge. Readings taken at 30 to 60 min are used to calculate the settled sludge volume and the sludge volume index. Settled sludge volume (SSV) the volume in percent occupied by an activated sludge sample after 30 to 60 minutes of settling. Normally written as SSV with a subscript to indicate the time of the reading used for calculation (SSV60) or (SSV30). Sewage wastewater containing human wastes. Sludge the mixture of settleable solids and water that is removed from the bottom of the settling tank. Sludge retention time (SRT) see mean cell residence time. Sludge volume index (SVI) a process control calculation that is used to evaluate the settling quality of the activated sludge. Requires the SSV30 and mixed liquor suspended solids test results to calculate. Storm sewer a collection system designed to carry only storm water runoff. Storm water runoff resulting from rainfall and snowmelt. Supernatant the amber-colored liquid above the sludge that is in a digester. Wastewater the water supply of the community after it has been soiled by use. Waste activated sludge solids (WASS) the concentration of suspended solids in the sludge, which is being removed from the activated sludge process. Weir a device used to measure wastewater flow. Zoogleal slime the biological slime which forms on fixed film treatment devices. It contains a wide variety of organisms essential to the treatment process.

18.3 MEASURING PLANT PERFORMANCE To evaluate how well a plant or treatment unit process is operating, performance efficiency or percent removal is used. The results can be compared with those listed in the plantâ&#x20AC;&#x2122;s operation and maintenance manual (O & M) to determine if the facility is performing as expected. In this chapter sample calculations often used to measure plant performance and efficiency are presented.

Wastewater Treatment

531

18.3.1 PLANT PERFORMANCE

AND

%VM Reduction =

EFFICIENCY

Note: The calculation used for determining the performance (percent removal) for a digester is different from that used for performance (percent removal) for other processes. Care must be taken to select the right formula The following equation is used to determine plant performance and efficiency: % Removal =

(18.1)

[Influent Concentration

- Effluent Concentration] ¥ 100

[%VM

]

- %VM out ¥ 100

(%VM in ¥

]

(18.2)

%VM out )

EXAMPLE 18.3 Problem: Using the digester data provided below, determine the percent volatile matter reduction for the digester. Data:

Influent Concentration Raw sludge volatile matter = 74% Digested sludge volatile matter = 54%

EXAMPLE 18.1 Problem: The influent BOD is 247 mg/L and the plant effluent BOD is 17 mg/L. What is the percent removal?

%VM Reduction =

[0.74 - 0.54] ¥ 100

[0.74 - (0.74 ¥ 0.54)]

= 59%

Solution: % Removal =

[247

mg L - 17 mg L ] ¥ 100 247 mg L

= 93%

18.3.2 UNIT PROCESS PERFORMANCE AND EFFICIENCY Equation 18.1 is used again to determine unit process efficiency. The concentration entering the unit and the concentration leaving the unit (i.e., primary, secondary, etc.) are used to determine the unit performance. EXAMPLE 18.2 Problem: The primary influent BOD is 235 mg/L and the primary effluent BOD is 169 mg/L. What is the percent removal?

% Removal =

[235

mg L - 169 mg L ] ¥ 100 235 mg L

18.4 HYDRAULIC DETENTION TIME The term detention time (DT) or hydraulic detention time (HDT) refers to the average length of time (theoretical time) a drop of water, wastewater, or suspended particles remains in a tank or channel. It is calculated by dividing the water or wastewater in the tank by the flow rate through the tank. The units of flow rate used in the calculation are dependent on whether the detention time is to be calculated in seconds, minutes, hours or days. Detention time is used in conjunction with various treatment processes, including sedimentation and coagulation and flocculation. Generally, in practice, detention time is associated with the amount of time required for a tank to empty. The range of detention time varies with the process. For example, in a tank used for sedimentation, detention time is commonly measured in minutes. The calculation methods used to determine detention time are illustrated in the following sections.

= 28%

18.4.1 DETENTION TIME 18.3.3 PERCENT VOLATILE MATTER REDUCTION IN SLUDGE The calculation used to determine percent volatile matter (%VM) reduction is more complicated because of the changes occurring during sludge digestion:

DAYS

Use Equation 18.3 to calculate the detention time in days:

HDT (d ) =

( )

Tank Volume ft 3 ¥ 7.48 gal ft 3 Q (gal d )

(18.3)

532

Handbook of Water and Wastewater Treatment Plant Operations

Note: The tank volume and the flow rate must be in the same dimensions before calculating the hydraulic detention time.

EXAMPLE 18.4 Problem: An anaerobic digester has a volume of 2,400,000 gal. What is the detention time in days when the influent flow rate is 0.07 MGD?

Solution: DT (d ) =

Wastewater treatment is designed to use the natural purification processes (self-purification processes of streams and rivers) to the maximum level possible. It is also designed to complete these processes in a controlled environment rather than over many miles of a stream or river. Moreover, the treatment plant is also designed to remove other contaminants that are not normally subjected to natural processes, as well as treating the solids that are generated through the treatment unit steps. The typical wastewater treatment plant is designed to achieve many different purposes:

2, 400, 000 gal 0.07 MGD ¥ 1, 000, 000 gal MG

= 34 d

18.4.2

DETENTION TIME

HOURS

HDT ( h ) =

( )

Tank Volume ft 3 ¥ 7.48 gal ft 3 ¥ 24 h d (18.4) Q (gal d )

EXAMPLE 18.5 Problem: A settling tank has a volume of 44,000 ft.3 What is the detention time in hours when the flow is 4.15 MGD?

DT ( h ) =

44, 000 ft 3 ¥ 7.48 gal ft 3 ¥ 24 h d 4.15 MGD ¥ 1, 000, 000 gal MG

= 1.9 h

18.4.3

DETENTION TIME

MINUTES

HDT ( min) =

(18.5)

( )

Tank Volume ft 3 ¥ 7.48 gal ft 3 ¥ 1440 min d Q (gal d )

EXAMPLE 18.6 Problem: A grit channel has a volume of 1340 ft.3 What is the detention time in minutes when the flow rate is 4.3 MGD?

Solution: DT ( min ) =

1340 ft 3 ¥ 7.48 gal ft 3 ¥ 1440 min d

= 3.36 min

4, 300, 000 gal d

18.5 WASTEWATER SOURCES AND CHARACTERISTICS

1. 2. 3. 4. 5.

Protect public health. Protect public water supplies. Protect aquatic life. Preserve the best uses of the waters. Protect adjacent lands.

Wastewater treatment is a series of steps. Each of the steps can be accomplished using one or more treatment processes or types of equipment. The major categories of treatment steps are: 1. Preliminary treatment — Removes materials that could damage plant equipment or would occupy treatment capacity without being treated. 2. Primary treatment — Removes settleable and floatable solids (may not be present in all treatment plants). 3. Secondary treatment — Removes BOD and dissolved and colloidal suspended organic matter by biological action. Organics are converted to stable solids, carbon dioxide and more organisms. 4. Advanced waste treatment — Uses physical, chemical, and biological processes to remove additional BOD, solids and nutrients (not present in all treatment plants). 5. Disinfection — Removes microorganisms to eliminate or reduce the possibility of disease when the flow is discharged. 6. Sludge treatment — Stabilizes the solids removed from wastewater during treatment, inactivates pathogenic organisms, and reduces the volume of the sludge by removing water. The various treatment processes described above are discussed in detail later.

Wastewater Treatment

533

18.5.1 WASTEWATER SOURCES The principal sources of domestic wastewater in a community are the residential areas and commercial districts. Other important sources include institutional and recreational facilities and storm water (runoff) and groundwater (infiltration). Each source produces wastewater with specific characteristics. In this section wastewater sources and the specific characteristics of wastewater are described.

18.5.1.1 Generation of Wastewater Wastewater is generated by five major sources: human and animal wastes, household wastes, industrial wastes, storm water runoff, and groundwater infiltration.

1. Human and animal wastes — Contains the solid and liquid discharges of humans and animals and is considered by many to be the most dangerous from a human health viewpoint. The primary health hazard is presented by the millions of bacteria, viruses, and other microorganisms (some of which may be pathogenic) present in the wastestream. 2. Household wastes — Consists of wastes, other than human and animal wastes, discharged from the home. Household wastes usually contain paper, household cleaners, detergents, trash, garbage, and other substances the homeowner discharges into the sewer system. 3. Industrial wastes — Includes industry specific materials that can be discharged from industrial processes into the collection system. Typically contains chemicals, dyes, acids, alkalis, grit, detergents, and highly toxic materials. 4. Storm water runoff — Many collection systems are designed to carry both the wastes of the community and storm water runoff. In this type of system when a storm event occurs, the wastestream can contain large amounts of sand, gravel, and other grit as well as excessive amounts of water. 5. Groundwater infiltration — Groundwater will enter older improperly sealed collection systems through cracks or unsealed pipe joints. Not only can this add large amounts of water to wastewater flows, but also additional grit.

18.5.2 CLASSIFICATION

WASTEWATER

Wastewater can be classified according to the sources of flows: domestic, sanitary, industrial, combined, and storm water. 1. Domestic (sewage) wastewater — Contains mainly human and animal wastes, household © 2003 by CRC Press LLC

wastes, small amounts of groundwater infiltration and small amounts of industrial wastes. Sanitary wastewater — Consists of domestic wastes and significant amounts of industrial wastes. In many cases, the industrial wastes can be treated without special precautions. However, in some cases, the industrial wastes will require special precautions or a pretreatment program to ensure the wastes do not cause compliance problems for the wastewater treatment plant. Industrial wastewater — Consists of industrial wastes only. Often the industry will determine that it is safer and more economical to treat its waste independent of domestic waste. Combined wastewater — Consists of a combination of sanitary wastewater and storm water runoff. All the wastewater and storm water of the community is transported through one system to the treatment plant. Storm water — Contains a separate collection system (no sanitary waste) that carries storm water runoff including street debris, road salt, and grit.

18.5.3 WASTEWATER CHARACTERISTICS Wastewater contains many different substances that can be used to characterize it. The specific substances and amounts or concentrations of each will vary, depending on the source. It is difficult to precisely characterize wastewater. Instead, wastewater characterization is usually based on and applied to an average domestic wastewater. Note: Keep in mind that other sources and types of wastewater can dramatically change the characteristics. Wastewater is characterized in terms of its physical, chemical, and biological characteristics. 18.5.3.1 Physical Characteristics The physical characteristics of wastewater are based on color, odor, temperature, and flow. 1. Color — Fresh wastewater is usually a light brownish-gray color. However, typical wastewater is gray and has a cloudy appearance. The color of the wastewater will change significantly if allowed to go septic (if travel time in the collection system increases). Typical septic wastewater will have a black color. 2. Odor — Odors in domestic wastewater usually are caused by gases produced by the decomposition of organic matter or by other substances

534

Handbook of Water and Wastewater Treatment Plant Operations

added to the wastewater. Fresh domestic wastewater has a musty odor. If the wastewater is allowed to go septic, this odor will significantly change to a rotten egg odor associated with the production of hydrogen sulfide (H2S). 3. Temperature — the temperature of wastewater is commonly higher than that of the water supply because of the addition of warm water from households and industrial plants. However, significant amounts of infiltration or storm water flow can cause major temperature fluctuations. 4. Flow — the actual volume of wastewater is commonly used as a physical characterization of wastewater and is normally expressed in terms of gallons per person per day. Most treatment plants are designed using an expected flow of 100 to 200 gallons per person per day. This figure may have to be revised to reflect the degree of infiltration or storm flow the plant receives. Flow rates will vary throughout the day. This variation, which can be as much as 50 to 200% of the average daily flow is known as the diurnal flow variation. Note: Diurnal means occurring in a day or daily. 18.5.3.2 Chemical Characteristics In describing the chemical characteristics of wastewater, the discussion generally includes topics such as organic matter, the measurement of organic matter, inorganic matter, and gases. For the sake of simplicity, in this handbook we specifically describe chemical characteristics in terms of alkalinity, BOD, chemical oxygen demand (COD), dissolved gases, nitrogen compounds, pH, phosphorus, solids (organic, inorganic, suspended, and dissolved solids), and water. 1. Alkalinity — This is a measure of the wastewater’s capability to neutralize acids. It is measured in terms of bicarbonate, carbonate, and hydroxide alkalinity. Alkalinity is essential to buffer (hold the neutral pH) of the wastewater during the biological treatment processes. 2. Biochemical oxygen demand — This is a measure of the amount of biodegradable matter in the wastewater. Normally measured by a 5-d test conducted at 20∞C. The BOD5 domestic waste is normally in the range of 100 to 300 mg/L. 3. Chemical oxygen demand — This is a measure of the amount of oxidizable matter present in the sample. The COD is normally in the range of 200 to 500 mg/L. The presence of industrial wastes can increase this significantly. © 2003 by CRC Press LLC

4. Dissolved gases — These are gases that are dissolved in wastewater. The specific gases and normal concentrations are based upon the composition of the wastewater. Typical domestic wastewater contains oxygen in relatively low concentrations, carbon dioxide, and hydrogen sulfide (if septic conditions exist). 5. Nitrogen compounds — The type and amount of nitrogen present will vary from the raw wastewater to the treated effluent. Nitrogen follows a cycle of oxidation and reduction. Most of the nitrogen in untreated wastewater will be in the forms of organic nitrogen and ammonia nitrogen. Laboratory tests exist for determination of both of these forms. The sum of these two forms of nitrogen is also measured and is known as total kjeldahl nitrogen (TKN). Wastewater will normally contain between 20 to 85 mg/L of nitrogen. Organic nitrogen will normally be in the range of 8 to 35 mg/L, and ammonia nitrogen will be in the range of 12 to 50 mg/L. 6. pH — This is a method of expressing the acid condition of the wastewater. pH is expressed on a scale of 1 to 14. For proper treatment, wastewater pH should normally be in the range of 6.5 to 9.0 (ideally 6.5 to 8.0). 7. Phosphorus — This element is essential to biological activity and must be present in at least minimum quantities or secondary treatment processes will not perform. Excessive amounts can cause stream damage and excessive algal growth. Phosphorus will normally be in the range of 6 to 20 mg/L. The removal of phosphate compounds from detergents has had a significant impact on the amounts of phosphorus in wastewater. 8. Solids — Most pollutants found in wastewater can be classified as solids. Wastewater treatment is generally designed to remove solids or to convert solids to a form that is more stable or can be removed. Solids can be classified by their chemical composition (organic or inorganic) or by their physical characteristics (settleable, floatable, and colloidal). Concentration of total solids in wastewater is normally in the range of 350 to 1200 mg/L. A. Organic solids — Consists of carbon, hydrogen, oxygen, nitrogen and can be converted to carbon dioxide and water by ignition at 550∞C. Also known as fixed solids or loss on ignition. B. Inorganic solids — Mineral solids that are unaffected by ignition. Also known as fixed solids or ash.

Wastewater Treatment

C. Suspended solids — These solids will not pass through a glass fiber filter pad. Can be further classified as Total suspended solids (TSS), volatile suspended solids, and fixed suspended solids. Can also be separated into three components based on settling characteristics: settleable solids, floatable solids, and colloidal solids. Total suspended solids in wastewater are normally in the range of 100 to 350 mg/L. D. Dissolved solids — These solids will pass through a glass fiber filter pad. Can also be classified as total dissolved solids (TDS), volatile dissolved solids, and fixed dissolved solids. TDS are normally in the range of 250 to 850 mg/L. 9. Water — This is always the major constituent of wastewater. In most cases water makes up 99.5 to 99.9% of the wastewater. Even in the strongest wastewater, the total amount of contamination present is less than 0.5% of the total and in average strength wastes it is usually less than 0.1%. 18.5.3.3 Biological Characteristics and Processes (Note: The biological characteristics of water were discussed in detail earlier in this text.) After undergoing physical aspects of treatment (i.e., screening, grit removal, and sedimentation) in preliminary and primary treatment, wastewater still contains some suspended solids and other solids that are dissolved in the water. In a natural stream, such substances are a source of food for protozoa, fungi, algae, and several varieties of bacteria. In secondary wastewater treatment, these same microscopic organisms (which are one of the main reasons for treating wastewater) are allowed to work as fast as they can to biologically convert the dissolved solids to suspended solids that will physically settle out at the end of secondary treatment. Raw wastewater influent typically contains millions of organisms. The majority of these organisms are nonpathogenic, but several pathogenic organisms may also be present. (These may include the organisms responsible for diseases such as typhoid, tetanus, hepatitis, dysentery, gastroenteritis, and others.) Many of the organisms found in wastewater are microscopic (microorganisms); they include algae, bacteria, protozoa (e.g., amoeba, flagellates, free-swimming ciliates, and stalked ciliates), rotifers, and viruses. Table 18.2 is a summary of typical domestic wastewater characteristics. © 2003 by CRC Press LLC

535

TABLE 18.2 Typical Domestic Wastewater Characteristics Characteristic Color Odor DO pH TSS BOD COD Flow Total nitrogen Total phosphorus Fecal coliform

Typical Characteristic Gray Musty >1.0 mg/L 6.5–9.0 100–350 mg/L 100–300 mg/L 200–500 mg/L 100–200 gal/person/d 20–85 mg/L 6–20 mg/L 500,000–3,000,000 MPN/100 mL

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

18.6 WASTEWATER COLLECTION SYSTEMS Wastewater collection systems collect and convey wastewater to the treatment plant. The complexity of the system depends on the size of the community and the type of system selected. Methods of collection and conveyance of wastewater include gravity systems, force main systems, vacuum systems, and combinations of all three types of systems.

18.6.1 GRAVITY COLLECTION SYSTEM In a gravity collection system, the collection lines are sloped to permit the flow to move through the system with as little pumping as possible. The slope of the lines must keep the wastewater moving at a velocity (speed) of 2 to 4 ft/sec. Otherwise, at lower velocities, solids will settle out and cause clogged lines, overflows, and offensive odors. To keep collection systems lines at a reasonable depth, wastewater must be lifted (pumped) periodically so that it can continue flowing downhill to the treatment plant. Pump stations are installed at selected points within the system for this purpose.

18.6.2 FORCE MAIN COLLECTION SYSTEM In a typical force main collection system, wastewater is collected to central points and pumped under pressure to the treatment plant. The system is normally used for conveying wastewater long distances. The use of the force main system allows the wastewater to flow to the treatment plant at the desired velocity without using sloped lines. It should be noted that the pump station discharge lines in a gravity system are considered to be force mains since the content of the lines is under pressure.

536

Handbook of Water and Wastewater Treatment Plant Operations

Note: Extra care must be taken when performing maintenance on force main systems since the content of the collection system is under pressure.

has a weatherproof motor housing located above the wet well. In this type of station, a submersible centrifugal pump is normally used.

18.6.3 VACUUM SYSTEM

18.6.4.3 Pneumatic Pumping Stations

In a vacuum collection system, wastewaters are collected to central points and then drawn toward the treatment plant under vacuum. The system consists of a large amount of mechanical equipment and requires a large amount of maintenance to perform properly. Generally, the vacuumtype collection systems are not economically feasible.

The pneumatic pumping station consists of a wet well and a control system that controls the inlet and outlet value operations and provides pressurized air to force or push the wastewater through the system. The exact method of operation depends on the system design. When operating, wastewater in the wet well reaches a predetermined level and activates an automatic valve that closes the influent line. The tank (wet well) is then pressurized to a predetermined level. When the pressure reaches the predetermined level, the effluent line valve is opened and the pressure pushes the wastestream out the discharge line.

18.6.4 PUMPING STATIONS Pumping stations provide the motive force (energy) to keep the wastewater moving at the desired velocity. They are used in both the force main and gravity systems. They are designed in several different configurations and may use different sources of energy to move the wastewater (i.e., pumps, air pressure or vacuum). One of the more commonly used types of pumping station designs is the wet well/dry well design. 18.6.4.1 Wet Well–Dry Well Pumping Stations The wet well–dry well pumping station consists of two separate spaces or sections separated by a common wall. Wastewater is collected in one section (known as the wet well section); the pumping equipment (and in many cases, the motors and controllers) is located in a second section known as the dry well. There are many different designs for this type of system, but in most cases the pumps selected for this system are of a centrifugal design. There are a couple of major considerations in selecting centrifugal design:

18.6.4.4 Pumping Station Wet Well Calculations Calculations normally associated with pumping station wet well design (determining design lift or pumping capacity, etc.) are usually left up to design and mechanical engineers. However, on occasion, wastewater operators or interceptor’s technicians may be called upon to make certain basic calculations. Usually these calculations deal with determining either pump capacity without influent (e.g., to check the pumping rate of the station’s constant speed pump) or pump capacity with influent (e.g., to check how many gallons per minute the pump is discharging). In this section we use examples to describe instances on how and where these two calculations are made. EXAMPLE 18.7: DETERMINING PUMP CAPACITY INFLUENT

WITHOUT

1. This design allows for the separation of mechanical equipment (pumps, motors, controllers, wiring, etc.) from the potentially corrosive atmosphere (sulfides) of the wastewater. 2. This type of design is usually safer for workers because they can monitor, maintain, operate, and repair equipment without entering the pumping station wet well.

Problem: A pumping station wet well is 10 ¥ 9 ft. The operator needs to check the pumping rate of the station’s constant speed pump. To do this, the influent valve to the wet well is closed for a 5-min test, and the level in the well dropped 2.2 ft. What is the pumping rate in gallons per minute?

Solution: Note: Most pumping station wet wells are confined spaces. To ensure safe entry into such spaces, compliance with Occupational Safety and Health Administration’s 29 CFR 1910.146 (Confined Space Entry Standard) is required. 18.6.4.2 Wet Well Pumping Stations Another type of pumping station design is the wet well type. This type consists of a single compartment that collects the wastewater flow. The pump is submerged in the wastewater with motor controls located in the space or © 2003 by CRC Press LLC

Using the length and width of the well, we can find the area of the water surface: 10 ft ¥ 9 ft = 90 ft2 The water level dropped 2.2 ft. From this we can find the volume of water removed by the pump during the test: A ¥ D=v 90 ft 2 ¥ 2.2 ft = 198 ft

Wastewater Treatment

537

One cubic foot of water holds 7.48 gal. We can convert this volume in cubic feet to gallons: 198 ft 3 ¥

7.48 gal = 1481 gal 1 ft 3

The test was done for 5 min. From this information, a pumping rate can be calculated: 1481 gal 296.2 = 296.2 gal min 5 min 1 min EXAMPLE 18.8: DETERMINING PUMP CAPACITY INFLUENT

WITH

Problem: A wet well is 8.2 ¥ 9.6 ft. The influent flow to the well, measured upstream, is 365 gal/min. If the wet well rises 2.2 in. in 5 min, how many gallons per minute is the pump discharging?

Solution: Influent = Discharge + Accumulation 365 gal 1 min

= Discharge + Accumulation

We want to calculate the discharge. Influent is known and we have enough information to calculate the accumulation. Volume accumulated = 8.2 ft ¥ 9.6 ft ¥ 2.2 in. ¥ 1 ft 12 in. Accumulation =

108 gal 5 min

7.48 gal 1 ft 3 =

= 108 gal

21.6 gal 1 min

= 21.6 gal min Using Equation 18.7: Influent = Discharge + Accumulation 365 gal min = Discharge + 21.6 Subtracting from both sides: 365 gal min - 21.6 gal min = Discharge + 21.6 gal min - 21.6 gal min 343.4 gal min = Discharge The wet well pump is discharging 343.4 gal each minute.

18.7 PRELIMINARY TREATMENT The initial stage in the wastewater treatment process (following collection and influent pumping) is preliminary treatment. Raw influent entering the treatment plant may contain many kinds of materials (trash). The purpose of preliminary treatment is to protect plant equipment by removing these materials that could cause clogs, jams, or excessive wear to plant machinery. In addition, the removal of various materials at the beginning of the treatment process saves valuable space within the treatment plant. Preliminary treatment may include many different processes. Each is designed to remove a specific type of material — a potential problem for the treatment process. Processes include: wastewater collections (influent pumping, screening, shredding, grit removal, flow measurement, preaeration, chemical addition, and flow equalization). The major processes are shown in Figure 18.1. In this section, we describe and discuss each of these processes and their importance in the treatment process. Note: As mentioned, not all treatment plants will include all of the processes shown in Figure 18.1. Specific processes have been included to facilitate discussion of major potential problems with each process and its operation; this is information that may be important to the wastewater operator.

18.7.1 SCREENING The purpose of screening is to remove large solids, such as rags, cans, rocks, branches, leaves, roots, etc., from the flow before the flow moves on to downstream processes. Note: Typically, a treatment plant will remove anywhere from 0.5 to 12 ft3 of screenings for each million gallons of influent received. A bar screen traps debris as wastewater influent passes through. Typically, a bar screen consists of a series of parallel, evenly spaced bars or a perforated screen placed in a channel (see Figure 18.2). The wastestream passes through the screen and the large solids (screenings) are trapped on the bars for removal. Note: The screenings must be removed frequently enough to prevent accumulation that will block the screen and cause the water level in front of the screen to build up. The bar screen may be coarse (2 to 4-in. openings) or fine (0.75 to 2.0-in. openings). The bar screen may be manually cleaned (bars or screens are placed at an angle of 30∞ for easier solids removal; see Figure 18.2) or mechanically cleaned (bars are placed at 45∞ to 60∞ angle to improve mechanical cleaner operation).

538

Handbook of Water and Wastewater Treatment Plant Operations

mon problem with manually cleaned bar screens is their tendency to clog frequently. This may be caused by excessive debris in the wastewater or the screen being too fine for its current application. The operator should locate the source of the excessive debris and eliminate it. If the screen is the problem, a coarser screen may need to be installed. If the bar screen area is filled with obnoxious odors, flies, and other insects, it may be necessary to dispose of screenings more frequently.

Drain

18.7.1.2 Mechanically Cleaned Screens

Flow in FIGURE 18.2 Bar screen. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

The screening method employed depends on the design of the plant, the amount of solids expected, and whether the screen is for constant or emergency use only. 18.7.1.1 Manually Cleaned Screens Manually cleaned screens are cleaned at least once per shift (or often enough to prevent buildup that may cause reduced flow into the plant) using a long tooth rake. Solids are manually pulled to the drain platform and allowed to drain before storage in a covered container. The area around the screen should be cleaned frequently to prevent a buildup of grease or other materials that can cause odors, slippery conditions, and insect and rodent problems. Because screenings may contain organic matter as well as large amounts of grease they should be stored in a covered container. Screenings can be disposed of by burial in approved landfills or by incineration. Some treatment facilities grind the screenings into small particles; these particles are then returned to the wastewater flow for further processing and removal later in the process. 18.7.1.1.1 Operational Problems Manually cleaned screens require a certain amount of operator attention to maintain optimum operation. Failure to clean the screen frequently can lead to septic wastes entering the primary, surge flows after cleaning, and low flows before cleaning. On occasion, when such operational problems occur, it becomes necessary to increase the frequency of the cleaning cycle. Another operational problem is excessive grit in the bar screen channel. Improper design or construction or insufficient cleaning may cause this problem. The corrective action required is either to correct the design problem or increase cleaning frequency and flush the channel regularly. Another com© 2003 by CRC Press LLC

Mechanically cleaned screens use a mechanized rake assembly to collect the solids and move them (carry them) out of the wastewater flow for discharge to a storage hopper. The screen may be continuously cleaned or cleaned on a time or flow controlled cycle. As with the manually cleaned screen, the area surrounding the mechanically operated screen must be cleaned frequently to prevent buildup of materials, which can cause unsafe conditions. As with all mechanical equipment, operator vigilance is required to ensure proper operation and proper maintenance. Maintenance includes lubricating equipment and maintaining it in accordance with manufacturer’s recommendations or the plant’s O & M manual. Screenings from mechanically operated barscreens are disposed of in the same manner as screenings from manually operated screens. These include landfill disposal, incineration, or the process of grinding into smaller particles for return to the wastewater flow. 18.7.1.2.1 Operational Problems Many of the operational problems associated with mechanically cleaned bar screens are the same as those for manual screens. These include septic wastes entering the primary, surge flows after cleaning, excessive grit in the bar screen channel, and a screen that clogs frequently. Basically the same corrective actions employed for manually operated screens would be applied for these problems in mechanically operated screens. In addition to these problems, mechanically operated screens also have other problems. These include the cleaner failing to operate; and a nonoperating rake, but operating motor. Obviously, these are mechanical problems that could be caused by jammed cleaning mechanism, broken chain, broken cable, or a broken shear pin. Authorized and fully trained maintenance operators should be called in to handle these types of problems. 18.7.1.3 Safety The screening area is the first location where the operator is exposed to the wastewater flow. Any toxic, flammable or explosive gases present in the wastewater can be released at this point. Operators who frequent enclosed bar screen areas should be equipped with personal air monitors. Adequate ventilation must be provided. It is also

Wastewater Treatment

539

important to remember that, due to the grease attached to the screenings this area of the plant can be extremely slippery. Routine cleaning is required to minimize this problem. Note: Never override safety devices on mechanical equipment. Overrides can result in dangerous conditions, injuries, and major mechanical failure. 18.7.1.4 Screenings Removal Computations

(

)

(

)

Screenings Removed ft MG = 3

( )

Screenings ft 3 d

(18.6)

( ) (18.7)

Screenings ft 3 Q (MG)

EXAMPLE 18.9 Problem: A total of 65 gal of screenings are removed from the wastewater flow during a 24-h period. What is the screenings removal reported as cubic feet per day?

Solution: First, convert gallons screenings to cubic feet: 65 gal 7.48 gal ft 3

= 8.7 ft 3 screenings

Next, calculate screenings removed as cubic feet per day:

(

)

Screenings Removed ft 3 d =

8.7 ft 3 1d

= 8.7 ft 3 d

EXAMPLE 18.10 Problem: During 1 week, a total of 310 gal of screenings were removed from the wastewater screens. What is the average screening removal in cubic feet per day?

ÂŠ 2003 by CRC Press LLC

First, gallons screenings must be converted to cubic feet screenings: 310 gal 7.48 gal ft 3

= 41.4 ft 3 screenings

Next, calculate screenings removed as cubic feet per day:

(

)

Screenings Removed ft 3 d =

Operators responsible for screenings disposal are typically required to keep a record of the amount of screenings removed from the wastewater flow. To keep and maintain accurate screeningsâ&#x20AC;&#x2122; records, the volume of screenings withdrawn must be determined. Two methods are commonly used to calculate the volume of screenings withdrawn:

Screenings Removed ft 3 d =

Solution:

41.4 ft 3 7d

= 5.9 ft 3 d

18.7.2 SHREDDING As an alternative to screening, shredding can be used to reduce solids to a size that can enter the plant without causing mechanical problems or clogging. Shredding processes include comminution (comminute means cut up) and barminution devices. 18.7.2.1 Comminution The comminutor is the most common shredding device used in wastewater treatment. In this device all the wastewater flow passes through the grinder assembly. The grinder consists of a screen or slotted basket, a rotating or oscillating cutter, and a stationary cutter. Solids pass through the screen and are chopped or shredded between the two cutters. The comminutor will not remove solids, which are too large to fit through the slots, and it will not remove floating objects. These materials must be removed manually. Maintenance requirements for comminutors include aligning, sharpening and replacing cutters and corrective and preventive maintenance performed in accordance with plant O & M manual. 18.7.2.1.1 Operational Problems Common operational problems associated with comminutors include output containing coarse solids. When this occurs it is usually a sign that the cutters are dull or misaligned. If the system does not operate at all, the unit is either clogged, jammed, a shear pin or coupling is broken or electrical power is shut off. If the unit stalls or jams frequently, this usually indicates cutter misalignment, excessive debris in influent, or dull cutters. Note: Only qualified maintenance operators should perform maintenance of shredding equipment. 18.7.2.2 Barminution In barminution, the barminutor uses a bar screen to collect solids that are shredded and passed through the bar screen

540

Handbook of Water and Wastewater Treatment Plant Operations

for removal at a later process. In operation each device’s cutter alignment and sharpness are critical factors in effective operation. Cutters must be sharpened or replaced and alignment must be checked in accordance with manufacturer’s recommendations. Solids, which are not shredded, must be removed daily, stored in closed containers, and disposed of by burial or incineration. Barminutor operational problems are similar to those listed above for comminutors. Preventive and corrective maintenance as well as lubrication must be performed by qualified personnel and in accordance with the plant’s O & M manual. Because of higher maintenance requirements the barminutor is less frequently used.

18.7.3 GRIT REMOVAL The purpose of grit removal is to remove the heavy inorganic solids that could cause excessive mechanical wear. Grit is heavier than inorganic solids and includes, sand, gravel, clay, egg shells, coffee grounds, metal filings, seeds, and other similar materials. There are several processes or devices used for grit removal. All of the processes are based on the fact that grit is heavier than the organic solids, which should be kept in suspension for treatment in following processes. Grit removal may be accomplished in grit chambers or by the centrifugal separation of sludge. Processes use gravity and velocity, aeration, or centrifugal force to separate the solids from the wastewater. 18.7.3.1 Gravity and Velocity Controlled Grit Removal

18.7.3.1.1 Process Control Calculations Velocity of the flow in a channel can be determined either by the float and stopwatch method or by channel dimensions. BY

FLOAT

AND

STOP-

WATCH

Velocity, feet second =

A float takes 25 sec to travel 34 ft in a grit channel. What is the velocity of the flow in the channel?

Solution: V (ft sec) =

34 ft 25 sec

= 1.4 ft sec

EXAMPLE 18.12: VELOCITY AND CHANNEL DIMENSIONS

FLOW

Note: This calculation can be used for a single channel or tank or multiple channels or tanks with the same dimensions and equal flow. If the flow through each unit of the unit dimensions is unequal, the velocity for each channel or tank must be computed individually. Velocity, fps = Flow, MGD ¥ 1.55 cfs MGD # Chan. in Ser. ¥ Chan Width, ft ¥ Water D, ft Problem: The plant is currently using two grit channels. Each channel is 3 ft wide and has a water depth of 1.2 ft. What is the velocity when the influent flow rate is 3.0 MGD?

Solution:

Gravity and velocity controlled grit removal is normally accomplished in a channel or tank where the speed or the velocity of the wastewater is controlled to about 1 foot per second (ideal), so that grit will settle while organic matter remains suspended. As long as the velocity is controlled in the range of 0.7 to 1.4 ft/sec the grit removal will remain effective. Velocity is controlled by the amount of water flowing through the channel, the depth of the water in the channel, the width of the channel, or the cumulative width of channels in service.

EXAMPLE 18.11: VELOCITY

Problem:

Distance Traveled, feet Time Required, Seconds

V (ft sec) = =

3.0 MGD ¥ 1.55 ft 3 sec MGD 2 Channels ¥ 3 ft ¥ 1.2 ft 4.65 ft 3 sec 7.2 ft 2

= 0.65 ft sec

Note: The channel dimensions must always be in feet. Convert inches to feet by dividing by 12 in./ft. EXAMPLE 18.13: REQUIRED SETTLING TIME Note: This calculation can be used to determine the time required for a particle to travel from the surface of the liquid to the bottom at a given settling velocity. In order to compute the settling time, the settling velocity in feet per second must be provided or determined experimentally in a laboratory.

Wastewater Treatment

Settling Time, seconds =

541

Liquid Depth in Feet Settling, Velocity, fps

Problem: The plant’s grit channel is designed to remove sand and has a settling velocity of 0.085 ft/sec. The channel is currently operating at a depth of 2.2 ft. How many seconds will it take for a sand particle to reach the channel bottom?

Solution: Settling Time (sec) =

2.2 ft 0.085 ft sec

= 25.9 sec

EXAMPLE 18.14: REQUIRED CHANNEL LENGTH Note: This calculation can be used to determine the length of channel required to remove an object with a specified settling velocity. Required Channel Length = Channel Depth, ft ¥ Flow Velocity, fps Settling Velocity, fps Problem: The plant’s grit channel is designed to remove sand and has a settling velocity of 0.070 ft/sec. The channel is currently operating at a depth of 3 ft. The calculated velocity of flow through the channel is 0.80 ft/sec. The channel is 35 ft long. Is the channel long enough to remove the desired sand particle size?

Solution: Required Channel Length (ft ) =

3 ft ¥ 0.80 ft sec 0.070 ft sec

= 34.3 ft Yes, the channel is long enough to ensure all of the sand will be removed.

18.7.3.1.2 Cleaning Gravity type systems may be manually or mechanically cleaned. Manual cleaning normally requires that the channel be taken out of service, drained, and manually cleaned. Mechanical cleaning systems are operated continuously or on a time cycle. Removal should be frequent enough to prevent grit carryover into the rest of the plant. Note: Always ventilate the area thoroughly before and during cleaning activities. © 2003 by CRC Press LLC

18.7.3.1.3 Operational Observations/ Problems/Troubleshooting Gravity and velocity-controlled grit removal normally occurs in a channel or tank where the speed or the velocity of the wastewater is controlled to about 1 ft/sec (ideal), so that grit settles while organic matters remains suspended. As long as the velocity is controlled in the range of 0.7 to 1.4 ft/sec, the grit removal remains effective. Velocity is controlled by the amount of water flowing through the channel, the depth of the water in the channel, by the width of the channel, or the cumulative width of channels in service. During operation, the operator must pay particular attention to grit characteristics for evidence of organic solids in the channel, for evidence of grit carryover into plant, for evidence of mechanical problems, and for grit storage and disposal (housekeeping). Aerated grit removal systems use aeration to keep the lighter organic solids in suspension while allowing the heavier grit articles to settle out. Aerated grit removal may be manually or mechanically cleaned; the majority of the systems are mechanically cleaned. During normal operation, adjusting the aeration rate produces the desired separation. This requires observation of mixing and aeration and sampling of fixed suspended solids. Actual grit removal is controlled by the rate of aeration. If the rate is too high, all of the solids remain in suspension. If the rate is too low, both grit and organics will settle out. The operator observes the same kinds of conditions as those listed for the gravity and velocity-controlled system, but must also pay close attention to the air distribution system to ensure proper operation. The cyclone degritter uses a rapid spinning motion (centrifugal force) to separate the heavy inorganic solids or grit from the light organic solids. This unit process is normally used on primary sludge rather than the entire wastewater flow. This critical control factor for the process is the inlet pressure. If the pressure exceeds the recommendations of the manufacturer, the unit will flood and grit will carry through with the flow. Grit is separated from flow, washed, and discharged directly to a strange container. Grit removal performance is determined by calculating the percent removal for inorganic (fixed) suspended solids. The operator observes the same kinds of conditions listed for the gravity and velocity-controlled and aerated grit removal systems, with the exception of the air distribution system. Typical problems associated with grit removal include mechanical malfunctions and rotten egg odor in the grit chamber (hydrogen sulfide formation), which can lead to metal and concrete corrosion problems. Low recovery rate of grit is another typical problem. Bottom scour, overaeration, or a lack of detention time normally causes this.

542

Handbook of Water and Wastewater Treatment Plant Operations

When these problems occur, the operator must make the required adjustments or repairs to correct the problems.

Solution: First, convert gallon grit removed to cubic feet:

18.7.3.2 Grit Removal Calculations

250 gal

Wastewater systems typically average 1 to 15 ft of grit/MG of flow (sanitary systems average 1 to 4 ft3/MG; combined wastewater systems average from 4 to 15 ft3/MG of flow), with higher ranges during storm events. Generally, grit is disposed of in sanitary landfills. Because of this practice, for planning purposes, operators must keep accurate records of grit removal. Most often, the data is reported as cubic feet of grit removed per million gallons of flow:

(

( )

Grit Volume ft 3

)

Grit Removed ft MG = 3

Q (MG)

( )

(

)

Grit Removed ft 3 MG = =

( )

Grit Volume ft 3 Q (MG ) 33 ft 3 12.2 MGD

= 2.7 ft 3 MGD

EXAMPLE 18.17

( ) 27 (ft yd )

Total Grit ft 3 3

Next, complete the calculation of cubic feet per million gallons:

(18.8)

Over a given period, the average grit removal rate at a plant (at least a seasonal average) can be determined and used for planning purposes. Typically, grit removal is calculated as cubic yards because excavation is normally expressed in terms of cubic yards:

Grit Removal yd 3 =

= 33 ft 3

7.48 gal ft 3

Problem: The monthly average grit removal is 2.5 ft3/MGD. If the monthly average flow is 2,500,000 gal/d, how many cubic yards must be available for grit disposal pit to have a 90d capacity?

Solution: (18.9)

First, calculate the grit generated each day: 2.5 ft 3

EXAMPLE 18.15

1 MG

¥ 2.5 MGD = 6.25 ft 3 d

Problem: A treatment plant removes 10 ft3 of grit in 1 d. How many cubic feet of grit are removed per million gallons if the plant flow was 9 MGD?

The cubic feet grit generated for 90 d would be: 6.25 ft 3 1d

¥ 90 d = 562.5 ft 3

Solution:

(

)

Grit Removed ft 3 MG = =

( )

Q (MG ) 10 ft

9 MGD

= 1.1 ft 3 MGD

EXAMPLE 18.16 Problem: The total daily grit removed for a plant is 250 gal. If the plant flow is 12.2 MGD, how many cubic feet of grit are removed per million gallons of flow?

Convert cubic feet grit to cubic yard grit:

Grit Volume ft 3

562.5 ft 3 27 ft 3 yd 3

= 21 yd 3

18.7.4 PREAERATION In the preaeration process (diffused or mechanical), we aerate wastewater to achieve and maintain an aerobic state (to freshen septic wastes), strip off hydrogen sulfide (to reduce odors and corrosion), agitate solids (to release trapped gases and improve solids separation and settling), and to reduce BOD. All of this can be accomplished by aerating the wastewater for 10 to 30 min. To reduce BOD, preaeration must be conducted from 45 to 60 min.

Wastewater Treatment

543

18.7.4.1 Operational Observations, Problems, and Troubleshooting

18.7.6.1 Operational Observations, Problems, and Troubleshooting

In preaeration grit removal systems, the operator is concerned with maintaining proper operation and must be alert to any possible mechanical problems. In addition, the operator monitors DO levels and the impact of preaeration on influent.

During normal operations, the operator must monitor all mechanical systems involved with flow equalization and must watch for mechanical problems and take the appropriate corrective action. The operator also monitors DO levels, the impact of equalization on influent, and water levels in equalization basins; any necessary adjustments are also made.

18.7.5 CHEMICAL ADDITION Chemical addition is made (either via dry chemical metering or solution feed metering) to the wastestream to improve settling, reduce odors, neutralize acids or bases, reduce corrosion, reduce BOD, improve solids and grease removal, reduce loading on the plant, add or remove nutrients, add organisms, and aid subsequent downstream processes. The particular chemical and amount used depends on the desired result. Chemicals must be added at a point where sufficient mixing will occur to obtain maximum benefit. Chemicals typically used in wastewater treatment include chlorine, peroxide, acids and bases, miner salts (ferric chloride, alum, etc.), and bioadditives and enzymes. 18.7.5.1 Operational Observations, Problems, and Troubleshooting In adding chemicals to the wastestream to remove grit, the operator monitors the process for evidence of mechanical problems and takes proper corrective actions when necessary. The operator also monitors the current chemical feed rate and dosage. The operator ensures that mixing at the point of addition is accomplished in accordance with standard operating procedures and monitors the impact of chemical addition on influent.

18.7.6 EQUALIZATION The purpose of flow equalization (whether by surge, diurnal, or complete methods) is to reduce or remove the wide swings in flow rates normally associated with wastewater treatment plant loading; it minimizes the impact of storm flows. The process can be designed to prevent flows above maximum plant design hydraulic capacity, reduce the magnitude of diurnal flow variations, and eliminate flow variations. Flow equalization is accomplished using mixing or aeration equipment, pumps, and flow measurement. Normal operation depends on the purpose and requirements of the flow equalization system. Equalized flows allow the plant to perform at optimum levels by providing stable hydraulic and organic loading. The downside to flow equalization is the additional costs associated with construction and operation of the flow equalization facilities. ÂŠ 2003 by CRC Press LLC

18.7.7 AERATED SYSTEMS Aerated grit removal systems use aeration to keep the lighter organic solids in suspension while allowing the heavier grit particles to settle out. Aerated grit removal may be manually or mechanically cleaned; the majority of the systems are mechanically cleaned. In normal operation, the aeration rate is adjusted to produce the desired separation, which requires observation of mixing and aeration and sampling of fixed suspended solids. Actual grit removal is controlled by the rate of aeration. If the rate is too high, all of the solids remain in suspension. If the rate is too low, both the grit and the organics will settle out.

18.7.8 CYCLONE DEGRITTER The cyclone degritter uses a rapid spinning motion (centrifugal force) to separate the heavy inorganic solids or grit from the light organic solids. This unit process is normally used on primary sludge rather than the entire wastewater flow. The critical control factor for the process is the inlet pressure. If the pressure exceeds the recommendations of the manufacturer, the unit will flood and grit will carry through with the flow. Grit is separated from the flow and discharged directly to a storage container. Grit removal performance is determined by calculating the percent removal for inorganic (fixed) suspended solids.

18.7.9 PRELIMINARY TREATMENT SAMPLING AND TESTING During normal operation of grit removal systems (with the exception of the screening and shredding processes), the plant operator is responsible for sampling and testing as shown in Table 18.3.

18.7.10 OTHER PRELIMINARY TREATMENT PROCESS CONTROL CALCULATIONS The desired velocity in sewers in approximately 2 ft/sec at peak flow; this velocity normally prevents solids from settling from the lines. When the flow reaches the grit channel, the velocity should decrease to about 1 ft/sec to permit the heavy inorganic solids to settle. In the example

544

Handbook of Water and Wastewater Treatment Plant Operations

TABLE 18.3 Sampling and Testing Grit Removal Systems Process

Location

Test

Frequency

Grit removal (velocity)

Influent Channel Grit Effluent Influent Channel Grit Effluent Influent Influent Effluent Effluent

Suspended solids (fixed) Depth of grit Total solids (fixed) Suspended solids (fixed) Suspended solids (fixed) DO Total solids (fixed) Suspended solids (fixed) Jar test DO DO DO

Variable Variable Variable Variable Variable Variable Variable Variable Variable Variable Variable Variable

Grit removal (aerated)

Chemical addition Preaeration Equalization

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

calculations that follow, we describe how the velocity of the flow in a channel can be determined by the float and stopwatch method and by channel dimensions. EXAMPLE 18.18: VELOCITY AND STOPWATCH

FLOAT

Problem: The plant is currently using two grit channels. Each channel is 3 ft wide and has a water depth of 1.3 ft. What is the velocity when the influent flow rate is 4.0 MGD?

Solution: Velocity, feet second =

Distance Traveled, ft Time required, seconds

Problem: A float takes 30 sec to travel 37 ft in a grit channel. What is the velocity of the flow in the channel?

V (ft sec) = =

4.0 MGD ¥ 1.55 ft 3 sec MGD 2 Channels ¥ 3 ft ¥ 1.3 ft 6.2 ft 3 sec 7.8 ft 2

= 0.79 ft sec

Solution: V (ft sec) =

37 ft 30 sec

= 1.2 ft sec

EXAMPLE 18.19: VELOCITY AND CHANNEL DIMENSIONS

FLOW

Note: This calculation can be used for a single channel or tank or for multiple channels or tanks with the same dimensions and equal flow. If the flow through each of the unit dimensions is unequal, the velocity for each channel or tank must be computed individually. Velocity, fps = Flow, MGD ¥ 1.55 cfs MGD # Chan in Ser ¥ Chan Width, ft ¥ Water Depth, ft © 2003 by CRC Press LLC

Note: Because 0.79 is within the 0.7 to 1.4 level, the operator of this unit would not make any adjustments. Note: The channel dimensions must always be in feet. Convert inches to feet by dividing by 12 in./ft. EXAMPLE 18.20: REQUIRED SETTLING TIME Note: This calculation can be used to determine the time required for a particle to travel from the surface of the liquid to the bottom at a given settling velocity. To compute the settling time, settling velocity in feet per second must be provided or determined by experiment in a laboratory. Settling Time, seconds =

Liquid Depth in ft Settling, Velocity, fps

Wastewater Treatment

545

Problem: The plant’s grit channel is designed to remove sand and has a settling velocity of 0.080 ft/sec. The channel is currently operating at a depth of 2.3 ft. How many seconds will it take for a sand particle to reach the channel bottom?

Solution: Settling Time (sec) =

2.3 ft 0.080 ft sec

= 28.7 sec

EXAMPLE 18.21: REQUIRED CHANNEL LENGTH Note: This calculation can be used to determine the length of channel required to remove an object with a specified settling velocity. Required Channel Length = Channel Depth, ft ¥ Flow Velocity, fps 0.080 fps Problem: The plant’s grit channel is designed to remove sand and has a settling velocity of 0.080 ft/sec. The channel is currently operating at a depth of 3 ft. The calculated velocity of flow through the channel is 0.85 ft/sec. The channel is 36 ft long. Is the channel long enough to remove the desired sand particle size?

Sedimentation may be used throughout the plant to remove settleable and floatable solids. It is used in primary treatment, secondary treatment, and advanced wastewater treatment processes. In this section, we focus on primary treatment or primary clarification, which uses large basins in which primary settling is achieved under relatively quiescent conditions (see Figure 18.1). Within these basins, mechanical scrapers collect the primary settled solids into a hopper where they are pumped to a sludge-processing area. Oil, grease, and other floating materials (scum) are skimmed from the surface. The effluent is discharged over weirs into a collection trough.

18.8.1 PROCESS DESCRIPTION In primary sedimentation, wastewater enters a settling tank or basin. Velocity is reduced to approximately 1 ft/min. Note: Notice that the velocity is based on minutes instead of seconds, as was the case in the grit channels. A grit channel velocity of 1 ft/sec would be 60 ft/min. Solids that are heavier than water settle to the bottom, while solids that are lighter than water float to the top. Settled solids are removed as sludge and floating solids are removed as scum. Wastewater leaves the sedimentation tank over an effluent weir and on to the next step in treatment. Detention time, temperature, tank design, and condition of the equipment control the efficiency of the process. 18.8.1.1 Overview of Primary Treatment

Solution: Required Channel Length (ft ) =

3 ft ¥ 0.85 ft sec 0.080 ft sec

= 31.9 ft Yes, the channel is long enough to ensure all of the sand will be removed.

18.8 PRIMARY TREATMENT (SEDIMENTATION) The purpose of primary treatment (primary sedimentation or primary clarification) is to remove settleable organic and flotable solids. Normally, each primary clarification unit can be expected to remove 90 to 95% settleable solids, 40 to 60% TSS, and 25 to 35% BOD. Note: Performance expectations for settling devices used in other areas of plant operation is normally expressed as overall unit performance rather than settling unit performance. © 2003 by CRC Press LLC

1. Primary treatment reduces the organic loading on downstream treatment processes by removing a large amount of settleable, suspended, and floatable materials. 2. Primary treatment reduces the velocity of the wastewater through a clarifier to approximately 1 to 2 ft/min, so that settling and floatation can take place. Slowing the flow enhances removal of suspended solids in wastewater. 3. Primary settling tanks remove floated grease and scum, remove the settled sludge solids, and collect them for pumped transfer to disposal or further treatment. 4. Clarifiers used may be rectangular or circular. In rectangular clarifiers, wastewater flows from one end to the other, and the settled sludge is moved to a hopper at the one end, either by flights set on parallel chains or by a single bottom scraper set on a traveling bridge. Floating material (mostly grease and oil) is collected by a surface skimmer.

546

Handbook of Water and Wastewater Treatment Plant Operations

5. In circular tanks, the wastewater usually enters at the middle and flows outward. Settled sludge is pushed to a hopper in the middle of the tank bottom, and a surface skimmer removes floating material. 6. Factors affecting primary clarifier performance include: A. Rate of flow through the clarifier B. Wastewater characteristics (strength; temperature; amount and type of industrial waste; and the density, size, and shapes of particles) C. Performance of pretreatment processes D. Nature and amount of any wastes recycled to the primary clarifier 7. Key factors in primary clarifier operation include the following concepts: Retention Time ( h ) =

v (gal) ¥ 24 h d Q (gal d )

(

)

Surface Loading Rate gal d ft 2 = Q (gal d )

( )

Surface Area ft 2

(

)

Solids Loading Rate lb d ft 2 = Solids into Clarifier (lb d )

( )

Surface Area ft 2

Weir Overflow Rate (gal d lineal ft ) = Q (gal d ) Weir Length (lineal ft )

18.8.2 TYPES

SEDIMENTATION TANKS

Sedimentation equipment includes septic tanks, two story tanks, and plain settling tanks or clarifiers. All three devices may be used for primary treatment; plain settling tanks are normally used for secondary or advanced wastewater treatment processes. 18.8.2.1 Septic Tanks Septic tanks are prefabricated tanks that serve as a combined settling and skimming tank and as an unheated–unmixed anaerobic digester. Septic tanks provide long settling times (6 to 8 h or more), but do not separate decomposing solids from the wastewater flow. When the tank becomes full, solids will be discharged with the flow. The process is © 2003 by CRC Press LLC

suitable for small facilities (i.e., schools, motels, homes, etc.), but due to the long detention times and lack of control, it is not suitable for larger applications. 18.8.2.2 Two-Story (Imhoff) Tank The two-story or Imhoff tank is similar to a septic tank in the removal of settleable solids and the anaerobic digestion of solids. The difference is that the two story tank consists of a settling compartment where sedimentation is accomplished, a lower compartment where settled solids and digestion takes place, and gas vents. Solids removed from the wastewater by settling pass from the settling compartment into the digestion compartment through a slot in the bottom of the settling compartment. The design of the slot prevents solids from returning to the settling compartment. Solids decompose anaerobically in the digestion section. Gases produced as a result of the solids decomposition are released through the gas vents running along each side of the settling compartment. 18.8.2.3 Plain Settling Tanks (Clarifiers) The plain settling tank or clarifier optimizes the settling process. Sludge is removed from the tank for processing in other downstream treatment units. Flow enters the tank, is slowed and distributed evenly across the width and depth of the unit, passes through the unit, and leaves over the effluent weir. Detention time within the primary settling tank is from 1 to 3 h (2-h average). Sludge removal is accomplished frequently on either a continuous or intermittent basis. Continuous removal requires additional sludge treatment processes to remove the excess water resulting from the removal of sludge, which contains less than 2 to 3% solids. Intermittent sludge removal requires the sludge be pumped from the tank on a schedule frequent enough to prevent large clumps of solids rising to the surface but infrequent enough to obtain 4 to 8% solids in the sludge withdrawn. Scum must be removed from the surface of the settling tank frequently. This is normally a mechanical process, but may require manual start-up. The system should be operated frequently enough to prevent excessive buildup and scum carryover but not so frequent as to cause hydraulic overloading of the scum removal system. Settling tanks require housekeeping and maintenance. Baffles (devices that prevent floatable solids and scum from leaving the tank), scum troughs, scum collectors, effluent troughs, and effluent weirs require frequent cleaning to prevent heavy biological growths and solids accumulations. Mechanical equipment must be lubricated and maintained as specified in the manufacturer’s recommendations or in accordance with procedures listed in the plant O & M manual.

Wastewater Treatment

Process control sampling and testing is used to evaluate the performance of the settling process. Settleable solids, DO, pH, temperature, TSS and BOD5, as well as sludge solids and volatile matter testing are routinely accomplished.

18.8.3 OPERATOR OBSERVATIONS, PROCESS PROBLEMS, AND TROUBLESHOOTING Before identifying a primary treatment problem and proceeding with appropriate troubleshooting effort, the operator must be cognizant of what constitutes normal operation. (i.e., Is there a problem or is the system operating as per design?) Several important items of normal operation can have a strong impact on performance. In the following section, we discuss the important operational parameters and normal observations. 18.8.3.1 Primary Clarification: Normal Operation In primary clarification, wastewater enters a settling tank or basin. Velocity reduces to approximately 1 ft/min. Note: Notice that the velocity is based on minutes instead of seconds, as was the case in the grit channels. A grit channel velocity of 1 ft/sec would be 60 ft/min. Solids that are heavier than water settle to the bottom, while solids that are lighter than water float to the top. Settled solids are removed as sludge and floating solids are removed as scum. Wastewater leaves the sedimentation tank over an effluent weir and on to the next step in treatment. Detention time, temperature, tank design, and condition of the equipment control the efficiency of the process. 18.8.3.2 Primary Clarification: Operational Parameters (Normal Observations) 1. Flow distribution — Normal flow distribution is indicated by flow to each in-service unit being equal and uniform. There is no indication of short-circuiting. The surface-loading rate is within design specifications. 2. Weir condition — Under this condition, weirs are level, flow over the weir is uniform, and the weir overflow rate is within design specifications. 3. Scum removal — The surface is free of scum accumulations, and the scum removal does not operate continuously. 4. Sludge removal — No large clumps of sludge appear on the surface. The system operates as designed. The pumping rate is controlled to pre© 2003 by CRC Press LLC

547

vent coning or buildup, and the sludge blanket depth is within desired levels. 5. Performance — The unit is removing expected levels of BOD5, TSS, and settleable solids. 6. Unit maintenance — Mechanical equipment is maintained in accordance with planned schedules; equipment is available for service as required. To assist the operator in judging primary treatment operation, several process control tests can be used for process evaluation and control. These tests include the following: 1. 2. 3. 4. 5. 6. 7. 8. 9.

pH (normal range: 6.5 to 9.0) DO (normal range is <1.0 mg/L) Temperature (varies with climate and season) Settleable solids (influent is 5 to 15 mL/L; effluent is 0.3 to 5 mL/L) BOD (influent is 150 to 400 mg/L; effluent is 50 to 150 mg/L) Percent solids (4 to 8%) Percent volatile matter (40% to 70%) Heavy metals (as required) Jar tests (as required)

Note: Testing frequency should be determined on the basis of the process influent and effluent variability and the available resources. All should be performed periodically to provide reference information for evaluation of performance.

18.8.4 PROCESS CONTROL CALCULATIONS As with many other wastewater treatment plant unit processes, process control calculations aid in determining the performance of the sedimentation process. Process control calculations are used in the sedimentation process to determine: 1. 2. 3. 4. 5. 6.

Percent removal Hydraulic detention time Surface loading rate (surface settling rate) Weir overflow rate (weir loading rate) Sludge pumping Percent total solids (% TS)

In the following sections, we take a closer look at a few of these process control calculations and example problems. Note: The calculations presented in the following sections allow you to determine values for each function performed. Keep in mind that an optimally operated primary clarifier should have values in an expected range.

548

Handbook of Water and Wastewater Treatment Plant Operations

18.8.4.1 Percent Removal

EXAMPLE 18.23

The expected range of percent removal for a primary clarifier is:

Problem:

Settleable solids Suspended solids BOD

90–95% 40–60% 25–35%

A circular clarifier has a diameter of 50 ft. If the primary effluent flow is 2,150,000 gal/d, what is the surface overflow rate in gallons per day per square foot?

Solution:

(

18.8.4.2 Detention Time

Surface Overflow Rate gal d ft

The primary purpose of primary settling is to remove settleable solids. This accomplished by slowing the flow down to approximately 1 ft/min. The flow at this velocity will stay in the primary tank from 1.5 to 2.5 h. The length of time the water stays in the tank is called the hydraulic detention time.

(gal d ) ) = QArea (ft 2 ) =

2, 150, 000 0.785 ¥ 50 ft ¥ 50 ft

= 1096 gal d ft

18.8.4.4 Weir Overflow Rate (Weir Loading Rate) 18.8.4.3 Surface Loading Rate (Surface Settling Rate and Surface Overflow Rate) Surface loading rate is the number of gallons of wastewater passing over 1 ft2 of tank/d. This can be used to compare actual conditions with design. Plant designs generally use a surface loading rate of 300 to 1200 gal/d/ ft2. Other terms used synonymously with surface loading rate are surface overflow rate and surface settling rate. The equation for calculating the surface loading rate is as follows:

(

)

Surface Loading Rate gal d ft 2 = Q (gal d )

(

( )

Settling Tank Area ft 2 EXAMPLE 18.22 Problem:

The settling tank is 120 ft in diameter and the flow to the unit is 4.5 MGD. What is the surface loading rate in gallons per day per square foot?

Q (gal d ) Weir Length (ft )

(

Surface Loading Rate gal d ft = =

Q (gal d )

)

( )

(18.11)

Problem: The circular settling tank is 90 ft in diameter and has a weir along its circumference. The effluent flow rate is 2.55 MGD. What is the weir overflow rate in gallons per day per foot?

Solution:

(

Weir Overflow Rate gal d ft 2 =

)

Weir Overflow Rate gal d ft 2 =

EXAMPLE 18.24 (18.10)

Solution:

Weir overflow rate (weir loading rate) is the amount of water leaving the settling tank per linear foot of weir. The result of this calculation can be compared with design. Normally weir overflow rates of 10,000 to 20,000 gal/d/ft are used in the design of a settling tank:

)

2.55 MGD ¥ 1, 000, 000 gal MG 3.14 ¥ 90 ft

= 9023 gal d ft

Settling Tank Area ft 2

18.8.4.5 Sludge Pumping

4.5 MGD ¥ 1, 000, 000 gal MGD

Determination of sludge pumping (the quantity of solids and volatile solids removed from the sedimentation tank) provides accurate information needed for process control of the sedimentation process:

0.785 ¥ 120 ft ¥ 120 ft

Wastewater Treatment

549

Solids Pumped (lb d ) = Pump Rate ¥ Pump Time ¥ 8.34 lb gal ¥ % Solids Volume of Solids (lb d ) = Pump Rate ¥ Pump Time ¥ 8.34 ¥ % Solids ¥ % VM

(18.12)

flow. Then you can use the milligrams per liter to pounds per day equation: SS Removed = mg L ¥ MGD ¥ 8.3 lb gal (18.14)

(18.13)

EXAMPLE 18.27 Problem: If 120 mg/L suspended solids are removed by a primary clarifier, how many pounds per day of suspended solids are removed when the flow is 6,230,000 gal/d?

EXAMPLE 18.25 Problem: The sludge pump operates 20 min/h. The pump delivers 20 gal/min of sludge. Laboratory tests indicate that the sludge is 5.2% solids and 66% volatile matter. How many pounds of volatile matter are transferred from the settling tank to the digester?

Solution: SS Removed (lb d ) = 120 mg L ¥ 6.25 MGD ¥ 8.34 lb gal

Solution:

= 6255 lb d

Pump Time = 20 min/h Pump Rate = 20 gal/min % Solids = 5.2% % VM= 66%

EXAMPLE 18.28 Problem:

Volume of Solids (lb d ) = 20 gal min ¥

(20

min h ¥ 24 h d ) ¥

8.34 lb gal ¥ 0.052 ¥ 0.66 = 2748 lb d

18.8.4.5.1 Percent Total Solids EXAMPLE 18.26

The flow to a secondary clarifier is 1.6 MGD. If the influent BOD concentration is 200 mg/L and the effluent BOD concentration is 70 mg/L, how many pounds of BOD are removed daily?

Solution: Calculate the milligrams per liter of BOD removed: BOD removed (lb d ) = 200 mg L ¥ 70 mg L = 130 mg L

Problem: A settling tank sludge sample is tested for solids. The sample and dish weigh 74.69 g. The dish weighs 21.2 g. After drying, the dish with dry solids now weighs 22.3 g. What is the percent total solids (% TS) of the sample?

Next calculate the pounds per day of BOD removed: BOD removed (lb d ) = 130 mg L ¥ 1.6 MGD ¥ 8.34 lb gal

Solution: Sample + Dish ¥ Dish = Sample Weight 74.69 g ¥ 21.2 g = 53.49 g Dish + Dry Solids ¥ Dish = Dry Solids Weight 22.3 g ¥ 21.2 g = 1.1 g 1.1 g 53.49 g

¥ 100% = 2%

18.8.4.6 BOD and Suspended Solids Removal To calculate the pounds of BOD or suspended solids (SS) removed each day, you need to know the milligrams per liter of BOD or suspended solids removed and the plant © 2003 by CRC Press LLC

= 1735 lb d

18.8.5 PROBLEM ANALYSIS In primary treatment (as is also clear in the operation of other unit processes), the primary function of the operator is to identify causes of process malfunctions, develop solutions, and prevent recurrence. In other words, the operator’s goal is to perform problem analysis or troubleshooting on unit processes when required and to restore the unit processes to optimal operating condition. The immediate goal in problem analysis is to solve the immediate problem. The long-term goal is to ensure that the problem does not pop up again, causing poor performance in the future.

550

Handbook of Water and Wastewater Treatment Plant Operations

In this section, we cover a few indicators and observations of operational problems with the primary treatment process. The observations presented are not all-inclusive, but highlight the most frequently confronted problems. 1. Poor suspended solids removal (primary clarifier) Causal factors: A. Hydraulic overload B. Sludge buildup in tanks and decreased volume and allows solids to scour out tanks C. Strong recycle flows D. Industrial waste concentrations E. Wind currents F. Temperature currents 2. Floating sludge Causal factors: A. Sludge becoming septic in tank B. Damaged or worn collection equipment C. Recycled waste sludge D. Primary sludge pumps malfunctions E. Sludge withdrawal line plugged F. Return of well-nitrified waste-activated sludge G. Too few tanks in service H. Damaged or missing baffles 3. Primary sludge solids concentration too low Causal factors: A. Hydraulic overload B. Overpumping of sludge C. Collection system problems D. Decreased influent solids loading 4. Septic wastewater or sludge Causal factors: A. Damaged or worn collection equipment B. Infrequent sludge removal C. Insufficient industrial pretreatment D. Septic sewage from collection system E. Strong recycle flows F. Primary sludge pump malfunction G. Sludge withdrawal line plugged H. Sludge collectors not run often enough I. Septage dumpers 5. Primary sludge solids concentrations too high Causal factors: A. Excessive grit and compacted material B. Primary sludge pump malfunction C. Sludge withdrawal line plugged D. SRT is too long E. Increased influent loadings

18.8.6 EFFLUENT

FROM

SETTLING TANKS

Upon completion of screening, degritting, and settling in sedimentation basins, large debris, grit, and many settleable materials have been removed from the wastestream. ÂŠ 2003 by CRC Press LLC

What is left is referred to as primary effluent. Usually cloudy and frequently gray in color, primary effluent still contains large amounts of dissolved food and other chemicals (nutrients). These nutrients are treated in the next step in the treatment process, secondary treatment, which is discussed in the next section. Note: Two of the most important nutrients left to remove are phosphorus and ammonia. While we want to remove these two nutrients from the wastestream, we do not want to remove too much. Carbonaceous microorganisms in secondary treatment (biological treatment) need both phosphorus and ammonia.

18.9 SECONDARY TREATMENT The main purpose of secondary treatment (sometimes referred to as biological treatment) is to provide BOD removal beyond what is achievable by primary treatment. There are three commonly used approaches, and all take advantage of the ability of microorganisms to convert organic wastes (via biological treatment) into stabilized, low-energy compounds. Two of these approaches, the trickling filter (and its variation, the RBC) and the activated sludge process, sequentially follow normal primary treatment. The third, ponds (oxidation ponds or lagoons), can provide equivalent results without preliminary treatment. In this section, we present a brief overview of the secondary treatment process followed by a detailed discussion of wastewater treatment ponds (used primarily in smaller treatment plants), trickling filters, and RBCs. We then shift focus to the activated sludge process, the secondary treatment process, which is used primarily in large installations and is the main focus of the handbook. Secondary treatment refers to those treatment processes that use biological processes to convert dissolved, suspended, and colloidal organic wastes to more stable solids that can either be removed by settling or discharged to the environment without causing harm. Exactly what is secondary treatment? As defined by the Clean Water Act (CWA), secondary treatment produces an effluent with nor more than 30 mg/L BOD and 30 mg/L TSS. Note: The CWA also states that ponds and trickling filters will be included in the definition of secondary treatment even if they do not meet the effluent quality requirements continuously. Most secondary treatment processes decompose solids aerobically, producing carbon dioxide, stable solids, and more organisms. Since solids are produced, all of the biological processes must include some form of solids removal (settling tank, filter, etc.).

Wastewater Treatment

551

Pond surface

CO2

Photosynthesis (Algae-producing oxygen)

Aerobic decomposition (bacteria producing CO2)

IN Solids

Anaerobic digestion (settled solids)

Pond bottom

FIGURE 18.3 Stabilization pond processes. (From Spellman, F.R., Spellmanâ&#x20AC;&#x2122;s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Secondary treatment processes can be separated into two large categories: fixed film systems and suspended growth systems. Fixed film systems are processes that use a biological growth (biomass or slime) that is attached to some form of media. Wastewater passes over or around the media and the slime. When the wastewater and slime are in contact, the organisms remove and oxidize the organic solids. The media may be stone, redwood, synthetic materials, or any other substance that is durable (capable of withstanding weather conditions for many years), provides a large area for slime growth and an open space for ventilation, and is not toxic to the organisms in the biomass. Fixed film devices include trickling filters and RBCs. Suspended growth systems are processes that use a biological growth that is mixed with the wastewater. Typical suspended growth systems consist of various modifications of the activated sludge process.

18.9.1 TREATMENT PONDS Wastewater treatment can be accomplished using ponds. Ponds are relatively easy to build and manage, can accommodate large fluctuations in flow, and can also provide treatment that approaches conventional systems (producing a highly purified effluent) at much lower cost. It is the cost (the economics) that drives many managers to decide on the pond option. The actual degree of treatment provided depends on the type and number of ponds used. Ponds can be used as the sole type of treatment or they can be used in conjunction with other forms of wastewater treatment (i.e., other treatment processes followed by a pond or a pond followed by other treatment processes). ÂŠ 2003 by CRC Press LLC

18.9.1.1 Types of Ponds Ponds can be classified (named) based upon their location in the system, the type wastes they receive, and the main biological process occurring in the pond. First we look at the types of ponds according to their location and the type wastes they receive: raw sewage stabilization ponds (see Figure 18.3), oxidation ponds, and polishing ponds. In the following section, we look at ponds classified by the type of processes occurring within the pond: Aerobic Ponds, anaerobic ponds, facultative ponds, and aerated ponds. 18.9.1.1.1 Ponds Based on Location and Types of Wastes They Receive The types of ponds based on location and types of wastes they receive include raw sewage stabilization ponds, oxidation ponds, and polishing ponds. 18.9.1.1.1.1

Raw Sewage Stabilization Ponds

The raw sewage stabilization pond is the most common type of pond (see Figure 18.3). With the exception of screening and shredding, this type of pond receives no prior treatment. Generally, raw sewage stabilization ponds are designed to provide a minimum of 45 d detention time and to receive no more than 30 lb of BOD /d/acre. The quality of the discharge is dependent on the time of the year. Summer months produce high BOD removal, but excellent suspended solids removals. The pond consists of an influent structure, pond berm, or walls and an effluent structure designed to permit selection of the best quality effluent. Normal operating depth of the pond is 3 to 5 ft. The process occurring in the pond involves bacteria decomposing the organics in the wastewater (aerobically and anaerobically) and algae using the products of the

552

Handbook of Water and Wastewater Treatment Plant Operations

bacterial action to produce oxygen (photosynthesis). Because this type of pond is the most commonly used in wastewater treatment, the process that occurs within the pond is described in greater detail below. When wastewater enters the stabilization pond several processes begin to occur. These include settling, aerobic decomposition, anaerobic decomposition, and photosynthesis (see Figure 18.3). Solids in the wastewater will settle to the bottom of the pond. In addition to the solids in the wastewater entering the pond, solids, which are produced by the biological activity, will also settle to the bottom. Eventually this will reduce the detention time and the performance of the pond. When this occurs (usually 20 to 30 years) the pond will have to be replaced or cleaned. Bacteria and other microorganisms use the organic matter as a food source. They use oxygen (aerobic decomposition), organic matter, and nutrients to produce carbon dioxide, water, stable solids (which may settle out), and more organisms. The carbon dioxide is an essential component of the photosynthesis process occurring near the surface of the pond. Organisms also use the solids that settled out as food material. Because the oxygen levels at the bottom of the pond are extremely low the process used is anaerobic decomposition. The organisms use the organic matter to produce gases (hydrogen sulfide, methane, etc.), which are dissolved in the water; stable solids; and more organisms. Near the surface of the pond a population of green algae will develop that can use the carbon dioxide produced by the bacterial population, nutrients, and sunlight to produce more algae and oxygen, which is dissolved into the water. The DO is then used by organisms in the aerobic decomposition process. When compared with other wastewater treatment systems involving biological treatment, a stabilization pond treatment system is the simplest to operate and maintain. Operation and maintenance activities include collecting and testing samples for DO and pH, removing weeds and other debris (scum) from the pond, mowing the berms, repairing erosion, and removing burrowing animals. Note: DO and pH levels in the pond will vary throughout the day. Normal operation will result in very high DO and pH levels because of the natural processes occurring. Note: When operating properly the stabilization pond will exhibit a wide variation in both DO and pH. This is due to the photosynthesis occurring in the system. 18.9.1.1.1.2

Oxidation Ponds

An oxidation pond, which is normally designed using the same criteria as the stabilization pond, receives flows that have passed through a stabilization pond or primary settling tank. This type of pond provides biological treatment, ÂŠ 2003 by CRC Press LLC

additional settling, and some reduction in the number of fecal coliform present. 18.9.1.1.1.3

Polishing Ponds

A polishing pond, which uses the same equipment as a stabilization pond, receives flow from an oxidation pond or from other secondary treatment systems. Polishing ponds remove additional BOD, solids and fecal coliform and some nutrients. They are designed to provide 1 to 3 d detention time and normally operate at a depth of 5 to 10 ft. Excessive detention time or too shallow a depth will result in algae growth, which increases influent, suspended solids concentrations. 18.9.1.1.2 Ponds Based on the Type of Processes Occurring within the Ponds The type of processes occurring within the pond may also classify ponds. These include the aerobic, anaerobic, facultative, and aerated processes. 18.9.1.1.2.1

Aerobic Ponds

In aerobic ponds, which are not widely used, oxygen is present throughout the pond. All biological activity is aerobic decomposition. 18.9.1.1.2.2

Anaerobic Ponds

Anaerobic ponds are normally used to treat high strength industrial wastes. No oxygen is present in the pond and all biological activity is anaerobic decomposition. 18.9.1.1.2.3

Facultative Ponds

The facultative pond is the most common type pond (based on processes occurring). Oxygen is present in the upper portions of the pond and aerobic processes are occurring. No oxygen is present in the lower levels of the pond where anoxic and anaerobic processes are occurring. 18.9.1.1.2.4

Aerated Ponds

In the aerated pond, oxygen is provided through the use of mechanical or diffused air systems. When aeration is used, the depth of the pond and the acceptable loading levels may increase. Mechanical or diffused aeration is often used to supplement natural oxygen production or to replace it. 18.9.1.2 Process Control Calculations (Stabilization Ponds) Process control calculations are an important part of wastewater treatment operations, including pond operations. More significantly, process control calculations are an important part of state wastewater licensing examinations â&#x20AC;&#x201D; you simply cannot master the licensing examinations without being able to perform the required calculations. Whenever possible, example process control problems are provided to enhance your knowledge and skills.

Wastewater Treatment

553

18.9.1.2.1 Determining Pond Area in Acres

Area (acres) =

( )

Area ft 2

43, 560 ft 2 acre

DT (d ) =

53.5 acre 0.92 ac- ft d

= 58.2 d

(18.15)

18.9.1.2.6 Hydraulic Loading in Inches per Day 18.9.1.2.2 Determining Pond Volume in Acre Feet v (acre-feet [ac-ft ]) =

( )

v ft 3

43, 560 ft 2 ac- ft

(18.16)

(Overflow Rate) Hydraulic Loading (in. d ) = Influent Flow (acre- inches d ) Pond Area (acres)

18.9.1.2.3 Determining Flow Rate in Acre Feet per Day Q, ac- ft d = Q (MGD) ¥ 3.069 ac- ft MG (18.17)

(18.20)

Population Loading ( people acre d ) = Population Served by System ( people) (18.21) Pond Area (acres)

Note: Acre-feet (ac-ft) is a unit that can cause confusion, especially for those not familiar with pond or lagoon operations. The measurement of 1 ac-ft is the volume of a box with a 1-acre top and 1 ft of depth — but the top does not have to be an even number of acres in size to use acre-feet.

Note: Population loading normally ranges from 50 to 500 people per acre.

18.9.1.2.4 Determining Flow Rate in Acre-Inches per Day

Organic loading can be expressed as pounds of BOD per acre per day (most common), pounds BOD5 per acre-foot per day, or people per acre per day.

Q (acre - inches d ) = Q (MGD) ¥

(18.18)

36.8 acre- inches MG 18.9.1.2.5 Hydraulic Detention Time in Days HDT (d ) =

Pond Volume (ac- ft ) Influent Flow (ac- ft d )

(18.19)

Note: Hydraulic detention time normally ranges from 30 to 120 d for stabilization ponds.

18.9.1.2.7 Organic Loading

Organic L, lbs BOD Acre Day = BOD, mg L ¥ Infl. flow, MGD ¥ 8.34 Pond Area, Acres

(18.22)

Note: Normal range of organic loading is 10 to 50 lb BOD /d/acre. EXAMPLE 18.30 Problem:

A stabilization pond has a volume of 53.5 ac-ft. What is the detention time in days when the flow is 0.30 MGD?

A wastewater treatment pond has an average width of 380 ft and an average length of 725 ft. The influent flow rate to the pond is 0.12 MGD with a BOD concentration of 160 mg/L. What is the organic loading rate to the pond in pounds per day per acre?

Solution:

EXAMPLE 18.29 Problem:

Determine the flow rate in acre-feet per day: Q (ac- ft d ) = 0.03 MGD ¥ 3.069 ac- ft MG = 0.92 ac- ft d

725 ft ¥ 380 ft ¥

1 acre 43, 560 ft 2

0.12 MGD ¥ 160 mg L ¥ 8.34 lb gal = 106.1 lb d 160.1 lb d

Determine the detention time:

= 6.32 acre

6.32 acre

= 25.3 lb d acre

554

Handbook of Water and Wastewater Treatment Plant Operations

Bar racks

Grit chamber

Primary sedimentaion

Grit

Sludge

Influent

Screenings

Waste sludge

Trickling filter

Cl2 or NaOCl

Settling tank

Chlorine contact tank

Effluent

Return effluent

FIGURE 18.4 Simplified flow diagram of trickling filter used for wastewater treatment. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.) Rotating arm

Influent spray

Rock bed Bed Rock Influent Underdrain system Effluent FIGURE 18.5 Schematic of cross-section of a trickling filter. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

18.9.2 TRICKLING FILTERS Trickling filters have been used to treat wastewater since the 1890s. It was found that if settled wastewater was passed over rock surfaces, slime grew on the rocks and the water became cleaner. Today we still use this principle, but in many installations we use plastic media instead of rocks. In most wastewater treatment systems, the trickling filter follows primary treatment and includes a secondary settling tank or clarifier as shown in Figure 18.4. Trickling filters are widely used for the treatment of domestic and industrial wastes. The process is a fixed film biological treatment method designed to remove BOD and suspended solids. A trickling filter consists of a rotating distribution arm that sprays and evenly distributes liquid wastewater over a circular bed of fist-sized rocks, other coarse materials, or synthetic media (see Figure 18.5). The spaces between the media allow air to circulate easily so that aerobic conditions can be maintained. The spaces also allow wastewater to trickle down through, around, and over the media. A layer of biological slime that absorbs and con© 2003 by CRC Press LLC

sumes the wastes trickling through the bed covers the media material. The organisms aerobically decompose the solids and produce more organisms and stable wastes that either become part of the slime or are discharged back into the wastewater flowing over the media. This slime consists mainly of bacteria, but it may also include algae, protozoa, worms, snails, fungi, and insect larvae. The accumulating slime occasionally sloughs off (sloughings) individual media materials (see Figure 18.6) and is collected at the bottom of the filter, along with the treated wastewater, and passed on to the secondary settling tank where it is removed. The overall performance of the trickling filter is dependent on hydraulic and organic loading, temperature, and recirculation. 18.9.2.1 Trickling Filter Definitions To clearly understand the correct operation of the trickling filter, the operator must be familiar with certain terms. The following list of terms applies to the trickling filter process.

Wastewater Treatment

555

Influent flow

Organic matter Zoogleal Slime

Media

Sloughing

Oxygen

Air

FIGURE 18.6 Filter media showing biological activities that take place on surface area. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

We assume that other terms related to other units within the treatment system (plant) are already familiar to operators: Biological towers a type of trickling filter that is very deep (10 to 20 ft). Filled with a lightweight synthetic media, these towers are also know as oxidation or roughing towers or (because of their extremely high hydraulic loading) superrate trickling filters. Biomass the total mass of organisms attached to the media. Similar to solids inventory in the acti-

vated sludge process, it is sometimes referred to as the zoogleal slime. Distribution arm the device most widely used to apply wastewater evenly over the entire surface of the media. In most cases, the force of the wastewater being sprayed through the orifices moves the arm. Filter underdrain the open space provided under the media to collect the liquid (wastewater and sloughings) and to allow air to enter the filter. It has a sloped floor to collect the flow to a central channel for removal. Hydraulic loading the amount of wastewater flow applied to the surface of the trickling filter media. It can be expressed in several ways: flow per square foot of surface per day, flow per acre per day, or flow per acre-foot per day. The hydraulic loading includes all flow entering the filter. High-rate trickling filters a classification (see Table 18.4) in which the organic loading is in the range of 25 to 100 lb BOD /1000 ft3 of media/d. The standard rate filter may also produce a highly nitrified effluent. Media an inert substance placed in the filter to provide a surface for the microorganism to grow on. The media can be field stone, crushed stone, slag, plastic, or redwood slats. Organic loading the amount of BOD or COD applied to a given volume of filter media. It does not include the BOD or COD contributed to any recirculated flow and is commonly expressed as pounds of BOD or COD per 1000 ft3 of media. Recirculation the return of filter effluent back to the head of the trickling filter. It can level flow

TABLE 18.4 Trickling Filter Classification Filter Class

Standard

Intermediate

Hydraulic Loading (gal/d/ft2) Organic Loading BOD/1000 ft3 Sloughing frequency Distribution Recirculation Media depth (ft) Media type

25–90 5–25 Seasonal Rotary No 6–8 Rock Plastic Wood Yes Yes 80–85% 80–85%

90–230 15–30 Varies Rotary fixed Usually 6–8 Rock Plastic Wood Some Variable 50–70% 50–70%

Nitrification Filter flies BOD removal TSS removal

High Rate 230–900 25–300 Continuous Rotary fixed Always 3–8 Rock Plastic Wood Some Variable 65–80% 65–80%

Super High Rate 350–2100 Up to 300 Continuous Rotary Usually Up to 40 Plastic

Limited Very few 65–85% 65–85%

Roughing >900 >300 Continuous Rotary Fixed Not usually 3–20 Rock Plastic Wood None Not usually 40–65% 40–65%

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

556

Handbook of Water and Wastewater Treatment Plant Operations

variations and assist in solving operational problems such as ponding, filter flies, and odors. Roughing filters a classification of trickling filters (see Table 18.4) in which the organic is in excess of 200 lb BOD /1000 ft3 of media/d. A roughing filter is used to reduce the loading on other biological treatment processes to produce an industrial discharge that can be safely treated in a municipal treatment facility. Sloughing the process in which the excess growths break away from the media and wash through the filter to the underdrains with the wastewater. These sloughings must be removed from the flow by settling. Staging the practice of operating two or more trickling filters in series. The effluent of one filter is used as the influent of the next. This practice can produce a higher quality effluent by removing additional BOD or COD. 18.9.2.2 Trickling Filter Equipment The trickling filter distribution system is designed to spread wastewater evenly over the surface of the entire media. The most common system is the rotary distributor, which moves above the surface of the media and sprays the wastewater on the surface. The force of the water leaving the orifices drives the rotary system. The distributor arms usually have small plates below each orifice to spread the wastewater into a fan-shaped distribution system. The second type of distributor is the fixed nozzle system. In this system, the nozzles are fixed in place above the media and are designed to spray the wastewater over a fixed portion of the media. This system is used frequently with deep bed synthetic media filters. Note: Trickling filters that use ordinary rock are normally only about 3 m in depth because of structural problems caused by the weight of rocks, which also requires the construction of beds that are quite wide (in many applications, up to 60 ft in diameter). When synthetic media is used, the bed can be much deeper. No matter which type of media is selected, the primary consideration is that it must be capable of providing the desired film location for the development of the biomass. Depending on the type of media used and the filter classification, the media may be 3 to 20 or more ft in depth. The underdrains are designed to support the media, collect the wastewater and sloughings and carry them out of the filter, and provide ventilation to the filter. Note: In order to ensure sufficient airflow to the filter, the underdrains should never be allowed to flow more than 50% full of wastewater. ÂŠ 2003 by CRC Press LLC

The effluent channel is designed to carry the flow from the trickling filter to the secondary settling tank. The secondary settling tank provides 2 to 4 h of detention time to separate the sloughing materials from the treated wastewater. Design, construction, and operation are similar to the primary settling tankâ&#x20AC;&#x2122;s. Longer detention times are provided because the sloughing materials are lighter and settle more slowly. Recirculation pumps and piping are designed to recirculate (and thus improve the performance of the trickling filter or settling tank) a portion of the effluent back to be mixed with the filter influent. When recirculation is used, pumps and metering devices must be provided. 18.9.2.3 Filter Classifications Trickling filters are classified by hydraulic and organic loading. The expected performance and the construction of the trickling filter are also determined by the filter classification. Filter classifications include: standard rate, intermediate rate, high rate, super high rate (plastic media), and roughing rate types. Standard rate, high rate, and roughing rate are the filter types most commonly used. The standard rate filter has a hydraulic loading of 25 to 90 gal/d/ft3 and a seasonal sloughing frequency. It does not employ recirculation and typically has a 80â&#x20AC;&#x201C;85% BOD removal rate and 80 to 85% TSS removal rate. The high rate filter has a hydraulic loading of 230 to 900 gal/d/ft3 and a continuous sloughing frequency. It always employs recirculation and typically has a 65 to 80% BOD removal rate and 65 to 80% TSS removal rate. The roughing filter has a hydraulic loading of >900 gal/d/ft3 and a continuous sloughing frequency. It does not normally include recirculation and typically has a 40 to 65% BOD removal rate and 40 to 65% TSS removal rate. 18.9.2.4 Standard Operating Procedures Standard operating procedures for trickling filters include sampling and testing, observation, recirculation, maintenance, and expectations of performance. Collection of influent and process effluent samples to determine performance and monitor process condition of trickling filters is required. DO, pH, and settleable solids testing should be collected daily. BOD and suspended solids testing should be done as often as practical to determine the per cent removal. The operation and condition of the filter should be observed daily. Items to observe include the distributor movement, uniformity of distribution, evidence of operation or mechanical problems, and the presence of objectionable odors. In addition to the items above the normal observation for a settling tank should also be performed.

Wastewater Treatment

557

Primary settling

Trickling filter

Secondary settling

Recirculating FIGURE 18.7 Common form of recirculation. (From Spellman, F.R., Spellmanâ&#x20AC;&#x2122;s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

Recirculation is used to reduce organic loading, improve sloughing, reduce odors, and reduce or eliminate filter fly or ponding problems. The amount of recirculation is dependent on the design of the treatment plant and the operational requirements of the process. Recirculation flow may be expressed as a specific flow rate (i.e., 2.0 MGD). In most cases, it is expressed as a ratio (e.g., 3:1, 0.5:1.0, etc). The recirculation is always listed as the first number and the influent flow listed as the second number. Note: Since the second number in the ratio is always 1.0, the ratio is sometimes written as a single number (dropping the 1.0) Flows can be recirculated from various points following the filter to various points before the filter. The most common form of recirculation removes flow from the filter effluent or settling tank and returns it to the influent of the trickling filter as shown in Figure 18.7. Maintenance requirements include lubrication of mechanical equipment, removal of debris from the surface and orifices, as well as adjustment of flow patterns and maintenance associated with the settling tank. Expected performance ranges for each classification of trickling filter. The levels of BOD and suspended solids removal are also dependent on the type of filter. 18.9.2.5 General Process Description The trickling filter process involves spraying wastewater over a solid media such as rock, plastic, or redwood slats (or laths). As the wastewater trickles over the surface of the media, a growth of microorganisms (bacteria, protozoa, fungi, algae, helminthes or worms, and larvae) develops. This growth is visible as a shiny slime very similar to the slime found on rocks in a stream. As the wastewater passes over this slime, the slime adsorbs the organic (food) matter. This organic matter is used for food by the microorganisms. At the same time, air moving through the open spaces in the filter transfers oxygen to the wastewater. This oxygen is then transferred to the slime to keep the outer layer aerobic. As the microorganisms use the food and oxygen, they produce more organisms, carbon dioxide, sulfates, nitrates, and other stable by-products; these materials are then discarded from the slime back into the wastewater flow and are carried out of the filter. The process is shown in the following equation: ÂŠ 2003 by CRC Press LLC

Organics + Organisms + O 2 = More Organisms + CO 2 + Solid Wastes

(18.23)

The growth of the microorganisms and the buildup of solid wastes in the slime make it thicker and heavier. When this slime becomes too thick, the wastewater flow breaks off parts of the slime. These must be removed in the final settling tank. In some trickling filters, a portion of the filter effluent is returned to the head of the trickling filter to level out variations in flow and improves operations (recirculation). 18.9.2.5.1 Overview and Brief Summary of Trickling Filter Process The following list provides an overview of the trickling filter process: 1. A trickling filter consists of a bed of coarse media, usually rocks or plastic, covered with microorganisms. 2. The wastewater is applied to the media at a controlled rate, using a rotating distributor arm or fixed nozzles. Organic material is removed by contact with the microorganisms as the wastewater trickles down through the media openings. The treated wastewater is collected by an underdrain system. 3. The trickling filter is usually built into a tank that contains the media. The filter may be square, rectangular, or circular. 4. The trickling filter does not provide any actual filtration. The filter media provides a large amount of surface area that the microorganisms can cling to and grow in a slime that forms on the media as they feed on the organic material in the wastewater. 5. The slime growth on the trickling filter media periodically sloughs off and is settled and removed in a secondary clarifier that follows the filter. 6. Key factors in trickling filter operation include the following concepts:

558

Handbook of Water and Wastewater Treatment Plant Operations

A. Hydraulic loading rate

(

)

Hydraulic Loading Rate gal d ft 2 = Q (gal d ) (including recirculation)

( )

Media Top Surface ft 2

B. Organic loading rate

(

)

Organic Loading Rate lb d 1000 ft 3 = BOD in Filter (lb d )

(

Media Volume 1000 ft 3

)

C. Recirculation Recirculation ( ratio) = Recirculation Flow (MGD) Average Influent Flow (MGD) 18.9.2.6 Operator Observations, Process Problems, and Troubleshooting Trickling filter operation requires routine observation, meter readings, process control sampling and testing, and process control calculations. Comparison of daily results with expected normal ranges is the key to identifying problems and appropriate corrective actions. 18.9.2.6.1 Operator Observations 1. Slime — The operator checks the thickness of slime to ensure that it is thin and uniform (normal) or thick and heavy (indicates organic overload). The operator is concerned with ensuring that excessive recirculation is not taking place and checks slime toxicity (if any). The operator is also concerned about the color of the slime. Green slime is normal, dark green or black slime indicates organic overload. Other colors may indicate industrial waste or chemical additive contamination. The operator should check the subsurface growth of the slime to ensure that it is normal (thin and translucent). If growth is thick and dark, organic overload conditions are indicated. Distribution arm operation is a system function important to slime formation. It must be checked regularly for proper operation. For example, the distribution of slime should be even and uniform. Striped conditions indicate clogged orifices or nozzles.

2. Flow — Flow distribution must be checked to ensure uniformity. If nonuniform, the arms are not level or the orifices are plugged. Flow drainage is also important. Drainage should be uniform and rapid. If not, ponding may occur from media breakdown or debris on surface. 3. Distributor — Movement of the distributor is critical to proper operation of the trickling filter. Movement should be uniform and smooth. Chattering, noisy operation may indicate bearing failure. The distributor seal must be checked to ensure there is no leakage. 4. Recirculation — The operator must check the rate of recirculation to ensure that it is within design specifications. Rates above design specifications indicate hydraulic overloading, while rates under design specifications indicate hydraulic underloading. 5. Media — The operator should check to ensure that media are uniform. 18.9.2.6.2 Process Control Sampling and Testing To ensure proper operation of the trickling filter, sampling and scheduling are important. For samples and the tests derived from the samples to be beneficial, operators must perform a variety of daily or variable tests. Individual tests and sampling may be needed daily, weekly, or monthly, depending on seasonal change. Frequency may be lower during normal operations and higher during abnormal conditions. The information gathered through collection and analysis of samples from various points in the trickling filter process is helpful in determining the current status of the process as well as identifying and correcting operational problems. The following routine sampling points and types of tests will permit the operator to identify normal and abnormal operating conditions. 1. Filter influent — Tests include DO, pH, temperature, settleable solids, BOD, suspended solids, and metals. 2. Recirculated flow — Tests include DO, pH, flow rate, and temperature. 3. Filter effluent — Tests include DO, pH, and jar tests. 4. Process effluent — Tests include DO, pH, settleable solids, BOD, and suspended solids. 18.9.2.6.3 Troubleshooting Operational Problems (Note: Much of the information in this section is based on the Environmental Protection Agency’s (EPA) Performance Evaluation and Troubleshooting at Municipal

Wastewater Treatment

Wastewater Treatment Facilities, Washington, D.C., current editions.) The following sections are not all-inclusive; they do not cover all of the operational problems associated with the trickling filter process. They do provide information on the most common operational problems. 18.9.2.6.3.1

Ponding

1. Symptoms A. Small pools or puddles of water on the surface of the media. B. Decreased performance in the removal of BOD and TSS. C. Possible odors due to anaerobic conditions in the media. D. Poor air flow through the media. 2. Causal factors A. Inadequate hydraulic loading to keep the media voids flushed clear. B. Application of high strength wastes without sufficient recirculation to provide dilution. C. Nonuniform media. D. Degradation of the media due to aging or weathering E. Medium is uniform, but is too small. F. Debris (moss, leaves, sticks) or living organisms (snails) clog the void spaces. 3. Corrective actions Corrective actions are listed in increasing impact on the quality of the plant effluent: A. Remove all leaves, sticks, and other debris from the media. B. Increase recirculation of dilute, highstrength wastes to improve sloughing to keep voids open. C. Use high-pressure stream of water to agitate and flush the ponded area. D. Rake or fork the ponded area. E. Dose the filter with chlorine solution for 2 to 4 h. The specific dose of chlorine required will depend on the severity of the ponding problem. When using elemental chlorine, the dose must be sufficient to provide a residual at the orifices of 1â&#x20AC;&#x201C;50 mg/L. If the filter is severely clogged, the higher residuals may be needed to unload the majority of the biomass. If the filter cannot be dosed by elemental chlorine, chlorinated lime or high test hypochlorite powder may be used. Dosing should be in the range of 8 to 10 lb of chlorine/1000 ft2 of media. F. If the filter design permits, the filter media can be flooded for a period of 4 h. Remember, if the filter is flooded, care must be taken

ÂŠ 2003 by CRC Press LLC

559

to prevent hydraulic overloads of the final settling tank. The trickling filter should be drained slowly at low flow periods. G. Dry the media. By stopping the flow to the filter, the slime will dry and loosen. When the flow is restarted, the loosened slime will flow out of the filter. The amount of drying time will be dependent on the thickness of the slime and the amount of removal desired. Time may range from a few hours to several days. Note: Portions of the media can be dried without taking the filter out of service by plugging the orifices that normally service the area. Note: If these corrective actions do not provide the desired improvement, the media must be carefully inspected. Remove a sample of the media from the affected area. Carefully clean it, inspect for its solidity, and determine its size uniformity (3 to 5 in.). If it is acceptable, the media must be carefully replaced. If the media appear to be decomposing or are not uniform, then they should be replaced. 18.9.2.6.3.2

Odors

Frequent offensive odors usually indicate an operational problem. These foul odors occur within the filter periodically and are normally associated with anaerobic conditions. Under normal circumstances, a slight anaerobic slime layer forms due to the inability of oxygen to penetrate all the way to the media. Under normal operation, the outer slime layers will remain aerobic, and no offensive odors are produced. 1. Causal factors A. Excessive organic loading due to poor filter effluent quality (recirculation), poor primary treatment operation, and poor control of sludge treatment process that results in high BOD recycle flows. B. Poor ventilation because of submerged or obstructed underdrains, clogged vent pipes, or clogged void spaces. C. Filter is overloaded hydraulically or organically. D. Poor housekeeping. 2. Corrective actions A. Evaluate the operation of the primary treatment process. Eliminate any short-circuiting. Determine any other actions that can be taken to improve the performance of the primary process.

560

Handbook of Water and Wastewater Treatment Plant Operations

B. Evaluate and adjust control of sludge treatment processes to reduce the BOD or recycle flows. C. Increase recirculation rate to add additional DO to filter influent. Do not increase recirculation rate if the flow rate through the underdrains would cause less than 50% open space. D. Maintain aerobic conditions in filter influent. E. Remove debris from media surface. F. Flush underdrains and vent pipes. G. Add one of the commercially available masking agents to reduce odors and prevent complaints. H. Add chlorine at a 1 to 2 mg/L residual for several hours at low flow. This will reduce activity and cut down on the oxygen demand. Chlorination only treats symptoms; a permanent solution must be determined and instituted. 18.9.2.6.3.3

High Clarifier Effluent Suspended Solids and BOD

1. Symptom A. The effluent from the trickling filter processsettling unit contains a high concentration of suspended solids. 2. Causal factors A. Recirculated flows are too high, causing hydraulic overloading of the settling tank. In multiple unit operations, the flow is not evenly distributed. B. Settling tank baffles or skirts have corroded or broken. C. Sludge collection mechanism is broken or malfunctioning. D. Effluent weirs are not level. E. Short-circuiting occurs because of temperature variations. F. Improper sludge withdrawal rate or frequency. G. Excessive solids loading from excessive sloughing. 3. Corrective actions A. Check hydraulic loading and adjust recirculated flow if hydraulic loading is too high. B. Adjust flow to ensure equal distribution. C. Inspect sludge removal equipment. Repair broken equipment. D. Monitor sludge blanket depth and sludge solids concentration; adjust withdrawal rate and/or frequency to maintain aerobic conditions in settling tank. E. Adjust effluent weir to obtain equal flow over all parts of the weir length. © 2003 by CRC Press LLC

F. Determine temperature in the clarifier at various points and depths throughout the clarifier. If depth temperatures are consistently 1 to 2°F lower than surface readings, a temperature problem exists. Baffles may be installed to help to break up these currents. G. High sloughing rates because of the biological activity or temperature changes may create excessive solids loading. An addition of 1 to 2 mg/L of cationic polymer may be helpful in improving solids capture. Remember, if polymer addition is used, solids withdrawal must be increased. H. High sloughings because of organic overloading, toxic wastes, or wide variations in influent flow are best controlled at their source. 18.9.2.6.3.4

Filter Flies

1. Symptoms A. The trickling filter and surrounding area become populated with large numbers of very small flying insects (psychoda moths). 2. Causal factors A. Poor housekeeping. B. Insufficient recirculation. C. Intermittent wet and dry conditions. D. Warm weather. 3. Corrective actions Corrective actions for filter fly problems revolve around the need to disrupt the fly’s life cycle (7 to 10 d in warm weather): A. Increase recirculation rate to obtain a hydraulic loading of at least 200 gal/d/ft2. At this rate, filter fly larvae are normally flushed out of the filter. B. Clean filter walls and remove weeds, brush, and shrubbery around the filter. This removes some of the area for fly breeding. C. Dose the filter periodically with low chlorine concentrations (less than 1 mg/L). This normally destroys larvae. D. Dry the filter media for several hours. F. Flood the filter for 24 h. G. Spray area around the filter with insecticide. Do not use insecticide directly on the media, because of the chance of carryover and unknown effects on the slime populations. 18.9.2.6.3.5

Freezing

1. Symptoms A. Decreased air temperature results in visible ice formation and decreased performance. B. Distributed wastes are in a thin film or spray. This is more likely to cause ice formation.

Wastewater Treatment

561

2. Causal factors A. Recirculation causes increased temperature drops and losses. B. Strong prevailing winds cause heat losses. C. Intermittent dosing allows water to stand too long, causing freezing. 3. Corrective actions All corrective actions are based upon a need to reduce heat loss as the wastes move through the filter. A. Reduce recirculation as much as possible to minimize cooling effects. B. Operate two stage filters in parallel to reduce heat loss. C. Adjust splash plates and orifices to obtain a coarse spray. D. Construct a windbreak or plant evergreens or shrubs in the direction of the prevailing wind. E. If intermittent dosing is used, leave dump gates open. F. Cover pump wet wells and dose tanks to reduce heat losses. G. Cover filter media to reduce heat loss. H. Remove ice before it becomes large enough to cause stoppage of arms.

Total Flow (MGD) = Influent Flow ¥

(Recirculation Rate + 1.0) EXAMPLE 18.31 Problem: The trickling filter is currently operating with a recirculation rate of 1.5. What is the total flow applied to the filter when the influent flow rate is 3.65 MGD?

Solution: Total Flow (MGD) = 3.65 MGD ¥ (1.5 + 1.0) = 9.13 MGD

18.9.2.7.2 Hydraulic Loading Calculating the hydraulic loading rate is important in accounting for both the primary effluent as well as the recirculated trickling filter effluent. Both of these are combined before being applied to the surface of the filter. The hydraulic loading rate is calculated based on the surface area of the filter. EXAMPLE 18.32 Problem:

Note: During periods of cold weather, the filter will show decreased performance. However, the filter should not be shut off for extended periods. Freezing of the moisture trapped within the media causes expansion and may cause structural damage. 18.9.2.7 Process Calculations Several calculations are useful in the operation of a trickling filter, these include: total flow, hydraulic loading, and organic loading.

A trickling filter 90-ft in diameter is operated with a primary effluent of 0.488 MGD and a recirculated effluent flow rate of 0.566 MGD. Calculate the hydraulic loading rate on the filter in units gallons per day per square foot.

Solution: The primary effluent and recirculated trickling filter effluent are applied together across the surface of the filter, therefore: 0.488 MGD + 0.566 MGD = 1.054 MGD

18.9.2.7.1 Total Flow

= 1, 054, 000 gal d

If the recirculated flow rate is given, total flow is:

Circular Surface Area = 0.785 ¥ ( Diameter )

Total Flow (MGD) = Influent Flow (MGD) +

= 0.785 ¥ (90 ft )

Recirculation Flow (MGD) Total Flow (gal d ) = Total Flow (MGD) ¥

(18.24)

= 6359 ft 2 1, 054, 000 gal d 6359 ft 2

= 165.7 gal d ft 2

18.9.2.7.3 Organic Loading Rate As mentioned earlier, trickling filters are sometimes classified by the organic loading rate applied. The organic

562

Handbook of Water and Wastewater Treatment Plant Operations

loading rate is expressed as a certain amount of BOD applied to a certain volume of media.

Zoogleal slime Oxygen

EXAMPLE 18.33

Media

Problem: A trickling filter, 50 ft in diameter, receives a primary effluent flow rate of 0.445 MGD. Calculate the organic loading rate in units of pounds of BOD applied per day per 900 ft3 of media volume. The primary effluent BOD concentration is 85 mg/L. The media depth is 9 ft.

Sloughings

Solution: 0.445 MGD ¥ 85 mg L ¥

Wastewater holding tank

8.34 lb gal = 315.5 BOD applied d Surface Area = 0.785 ¥ (Diameter ) = 0.785 ¥ (50 ft )

= 1962.5 ft 2 A¥D= v 1962.5 ft 2 ¥ 9 ft 2 = 17, 662.5

(Trickling filter volume) To determine the pounds of BOD/1000 ft3 in a volume of thousands of cubic feet, we must set up the equation as shown below: 315.5 BOD d 17, 662.5

1000 1000

Regrouping the numbers and the units together: =

315.5 lb ¥ 1000 17, 662.5

lb BOD d 1000 ft 3

= 17.9 lb BOD d 1000 ft 3

18.9.2.7.4 Settling Tanks In the operation of settling tanks that follow trickling filters, various calculations are routinely made to determine detention time, surface settling rate, hydraulic loading and sludge pumping.

18.9.3 ROTATING BIOLOGICAL CONTACTORS The RBC is a biological treatment system (see Figure 18.8) and is a variation of the attached growth idea provided by the trickling filter. Still relying on microorganisms that grow on the surface of a medium, the RBC is a fixed film biological treatment device; the basic biological process © 2003 by CRC Press LLC

Organic matter

FIGURE 18.8 Cross-section of a rotating biological contactor (RBC) treatment system. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

is similar to that occurring in the trickling filter. An RBC consists of a series of closely spaced (mounted side by side), circular, plastic (synthetic) disks that are typically about 3.5 m in diameter and attached to a rotating horizontal shaft (see Figure 18.8). Approximately 40% of each disk is submersed in a tank containing the wastewater to be treated. As the RBC rotates, the attached biomass film (zoogleal slime) that grows on the surface of the disk moves into and out of the wastewater. While submerged in the wastewater, the microorganisms absorb organics; while they are rotated out of the wastewater, they are supplied with needed oxygen for aerobic decomposition. As the zoogleal slime reenters the wastewater, excess solids and waste products are stripped off the media as sloughings. These sloughings are transported with the wastewater flow to a settling tank for removal. Modular RBC units are placed in series (see Figure 18.9) simply because a single contactor is not sufficient to achieve the desired level of treatment; the resulting treatment achieved exceeds conventional secondary treatment. Each individual contactor is called a stage and the group is known as a train. Most RBC systems consist of two or more trains with three or more stages in each. The key advantage in using RBCs instead of trickling filters is that RBCs are easier to operate under varying load conditions, since it is easier to keep the solid medium wet at all times. The level of nitrification, which can be achieved by a RBC system, is also significant. This is especially the case when multiple stages are employed. 18.9.3.1 RBC Equipment The equipment that makes up a RBC includes the rotating biological contactor (the media: either standard or high

Wastewater Treatment

563

Cl2

Rotating biological contactors Influent

Primary settling tank

Secondary settling tanks

Effluent

Solids disposal FIGURE 18.9 Rotating biological contactor (RBC) treatment system in series. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

density), a center shaft, drive system, tank, baffles, housing or cover, and a settling tank. The rotating biological contactor consists of circular sheets of synthetic material (usually plastic) that are mounted side by side on a shaft. The sheets (media) contain large amounts of surface area for growth of the biomass. The center shaft provides the support for the disks of media and must be strong enough to support the weight of the media and the biomass. Experience has indicated a major problem has been the collapse of the support shaft. The drive system provides the motive force to rotate the disks and shaft. The drive system may be mechanical, air driven, or a combination of each. When the drive system does not provide uniform movement of the RBC, major operational problems can arise. The tank holds the wastewater where the RBC rotates. It should be large enough to permit variation of the liquid depth and detention time. Baffles are required to permit proper adjustment of the loading applied to each stage of the RBC process. Adjustment can be made to increase or decrease the submergence of the RBC. RBC stages are normally enclosed in some type of protective structure (cover) to prevent loss of biomass due to severe weather changes (snow, rain, temperature, wind, sunlight, etc.). In many instances this housing greatly restricts access to the RBC. The settling tank is provided to remove the sloughing material created by the biological activity and is similar in design to the primary settling tank. The settling tank provides 2- to 4-h detention times to permit settling of lighter biological solids. 18.9.3.2 RBC Operation During normal operation, operator vigilance is required to observe the RBC movement, slime color, and appearance. If the unit is covered, observations may be limited to that portion of the media, which can be viewed through the access door. Sampling and testing should be conducted daily for DO content and pH. BOD and suspended solids testing should also be accomplished to aid in assessing performance. © 2003 by CRC Press LLC

18.9.3.3 RBC: Expected Performance The RBC normally produces a high quality effluent with BOD at 85 to 95% and suspended solids removal at 85 to 95%. The RBC treatment process may also significantly reduce (if designed for this purpose) the levels of organic nitrogen and ammonia nitrogen. 18.9.3.4 Operator Observations, Process Problems, and Troubleshooting Rotating biological filter operation requires routine observation, process control sampling and testing, troubleshooting, and process control calculations. Comparison of daily results with expected normal ranges is the key to identifying problems and appropriate corrective actions. 18.9.3.4.1 Operator Observations Note: If the RBC is covered, observations may be limited to the portion of the media that can be viewed through the access door. 1. Rotation — The operator routinely checks the operation of the RBC to ensure that smooth, uniform rotation is occurring (normal operation). Erratic, nonuniform rotation indicates a mechanical problem or uneven slime growth. If no movement is observed, mechanical problems or extreme excess of slime growth are indicated. 2. Slime color and appearance — Slime color and appearance can indicate process condition. Gray, shaggy slime growth on the RBC indicates normal operation. Reddish brown or golden brown shaggy growth indicates normal during nitrification. A very dark brown, shaggy growth (with worms present) indicates a very old slime. White chalky growth indicates high influent sulfur or sulfide levels. No visible slime growth to the RBC indicates a severe pH or temperature change.

564

Handbook of Water and Wastewater Treatment Plant Operations

18.9.3.4.2 Process Control Sampling and Testing For process control, the RBC process does not require large amounts of sampling and testing to provide the information required. The frequency for performing suggested testing depends on available resources and variability of process. Frequency may be lower during normal operation and higher during abnormal conditions. The following routine sampling points and types of tests will permit the operator to identify normal and abnormal operating conditions: 1. RBC train influent — Tests include pH, temperature, settleable solids, BOD, suspended solids, and metals. 2. RBC — Test includes speed of rotation. 3. RBC train effluent — Tests include DO, pH, jar tests. 4. Process effluent — Tests include DO, pH, settleable solids, BOD, and suspended solids 18.9.3.4.3 Troubleshooting Operational Problems (Note: Much of the information in this section is based on material provided by EPA in Performance Evaluation and Troubleshooting at Municipal Wastewater Treatment Facilities, Washington, D.C., current edition.) The following sections are not all-inclusive; they do not cover all of the operational problems associated with the rotating biological contactor process. They do provide information on the most common operational problems. 18.9.3.4.3.1

White Slime

1. Symptom A. White slime on most of the disk area. 2. Causal factors A. High hydrogen sulfide in influent B. Septic influent C. First stage overloaded 3. Corrective actions A. Aerate RBC or plant influent. B. Add sodium nitrate or hydrogen peroxide to influent. C. Adjust baffles between stages 1 and 2 to increase fraction of total surface area in first stage. 18.9.3.4.3.2

Excessive Sloughing

1. Symptom A. Loss of slime. 2. Causal factors A. Excessive pH variance. B. Toxic influent. 3. Corrective actions A. Implement and enforce pretreatment program. B. Install pH control equipment. C. Equalize flow to acclimate organisms. © 2003 by CRC Press LLC

18.9.3.4.3.3

RBC Rotation

1. Symptom A. RBC rotation is uneven. 2. Causal factors A. Mechanical growth. B. Uneven growth. 3. Corrective actions A. Repair mechanical problem. B. Increase rotational speed. C. Adjust baffles to decrease loading. D. Increase sloughing. 18.9.3.4.3.4

Solids

1. Symptom A. Solids accumulating in reactors. 2. Causal factors A. Inadequate pretreatment. 3. Corrective actions A. Identify and correct grit removal problem. B. Identify and correct primary settling problem. 18.9.3.4.3.5

Shaft Bearings

1. Symptom A. Shaft bearings running hot or failing. 2. Causal factor A. Inadequate maintenance. 3. Corrective action A. Follow manufacturer’s recommendations. 18.9.3.4.3.6

Drive Motor

1. Symptom A. Drive motor running hot. 2. Causal factors A. Inadequate maintenance. B. Improper chain drive alignment. 3. Corrective actions A. Follow manufacturer’s recommendations. B. Adjust alignment. 18.9.3.5 RBC: Process Control Calculations Several process control calculations may be useful in the operation of a RBC. These include soluble BOD, total media area, organic loading rate, and hydraulic loading rate. Settling tank calculations and sludge pumping calculations may be helpful for evaluation and control of the settling tank following the RBC. 18.9.3.5.1 RBC: Soluble BOD The soluble BOD concentration of the RBC influent can be determined experimentally in the laboratory or it can be estimated using the suspended solids concentration and the K factor. The K factor is used to approximate the BOD (particulate BOD) contributed by the suspended matter. The K factor must be provided or determined experimentally

Wastewater Treatment

565

in the laboratory. The K factor for domestic wastes normally ranges from 0.5 to 0.7. Soluble BOD 5 = Total BOD 5 -

turer must provide the individual stage areas (or the total train area) because physical determination of this would be extremely difficult.

(18.25)

(K Factor ¥ Total Suspended Solids) EXAMPLE 18.34 Problem: The suspended solids concentration of a wastewater is 250 mg/L. If the normal K value at the plant is 0.6, what is the estimated particulate BOD concentration of the wastewater?

Total Area = 1st Sage Area + 2nd Stage Area + º + nth Stage Area

(18.26)

18.9.3.5.3 RBC: Organic Loading Rate If the soluble BOD concentration is known, the organic loading on a RBC can be determined. Organic loading on a RBC based on soluble BOD concentration can range from 3 to 4 lb/d/1000 ft2. EXAMPLE 18.36

Solution:

Problem:

The K value of 0.6 indicates that about 60% of the suspended solids are organic suspended solids (particulate BOD): 250 mg L ¥ 0.6 = 150 mg L particulate BOD

An RBC has a total media surface area of 102,500 ft2 and receives a primary effluent flow rate of 0.269 MGD. If the soluble BOD concentration of the RBC influent is 159 mg/L, what is the organic loading rate in pounds per 1000 ft2?

Solution:

EXAMPLE 18.35 Problem: An RBC receives a flow of 2.2 MGD with a BOD content of 170 mg/L and suspended solids concentration of 140 mg/L. If the K value is 0.7, how many pounds of soluble BOD enter the RBC daily?

Solution: Total BOD = Particulate BOD + Soluble BOD 170 mg L = 140 mg L ¥ 0.7 + x mg L 170 mg L = 98 mg L + x mg L

0.269 MGD ¥ 159 mg L ¥ 356.7 lb d 102, 500 ft

8.34 lb 1 gal

1000 ( number ) 1000 ( unit )

= 356.7 lb d = 3.48 lb d 1000 ft 2

18.9.3.5.4 RBC: Hydraulic Loading Rate The manufacturer normally specifies the RBC media surface area and the hydraulic loading rate is based on the media surface area (usually in square feet). Hydraulic loading on a RBC can range from 1 to 3 gal/d/ft2. EXAMPLE 18.37

170 mg L - 98 mg L = x x = 72 mg L soluble BOD Now the pounds per day of soluble BOD may be determined: mg L soluble BOD ¥ MGD Flow ¥ 8.34 lb gal = lb d 72 mg L ¥ 2.2 MGD ¥ 8.34 lb gal = 1321 lb d

Problem: An RBC treats a primary effluent flow rate of 0.233 MGD. What is the hydraulic loading rate in gal/d/ft2 if the media surface area is 96,600 ft2?

Solution: 233, 000 gal d 96, 600 ft 2

= 2.41 gal d ft 2

soluble BOD

18.9.3.5.2 RBC: Total Media Area Several process control calculations for the RBC use the total surface area of all the stages within the train. As was the case with the soluble BOD calculation, plant design information or information supplied by the unit manufac© 2003 by CRC Press LLC

18.10 ACTIVATED SLUDGE The biological treatment systems discussed to this point (ponds, trickling filters, and RBCs) have been around for years. The trickling filter, for example, has been around and successfully used since the late 1800s. The problem

566

Handbook of Water and Wastewater Treatment Plant Operations

Aeration tank

Settling tank

Air

Activated sludge FIGURE 18.10 The activated sludge process. (From Spellman, F.R., Spellmanâ&#x20AC;&#x2122;s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

with ponds, trickling filters, and RBCs is that they are temperature sensitive and remove less BOD. In addition, trickling filters cost more to build than the activated sludge systems that were later developed. Note: Although trickling filters and other systems cost more to build than activated sludge systems, it is important to point out that activated sludge systems cost more to operate because of the need for energy to run pumps and blowers. As shown in Figure 18.10, the activated sludge process follows primary settling. The basic components of an activated sludge sewage treatment system include an aeration tank and a secondary basin, settling basin, or clarifier (see Figure 18.10). Primary effluent is mixed with settled solids recycled from the secondary clarifier and is then introduced into the aeration tank. Compressed air is injected continuously into the mixture through porous diffusers located at the bottom of the tank, usually along one side. Wastewater is fed continuously into an aerated tank, where the microorganisms metabolize and biologically flocculate the organics. Microorganisms (activated sludge) are settled from the aerated mixed liquor under quiescent conditions in the final clarifier and are returned to the aeration tank. Left uncontrolled, the number of organisms would eventually become too great; therefore, some must periodically be removed (wasted). A portion of the concentrated solids from the bottom of the settling tank must be removed from the process (waste activated sludge). Clear supernatant from the final settling tank is the plant effluent.

18.10.1 ACTIVATED SLUDGE TERMINOLOGY To better understand the discussion of the activated sludge process presented in the following sections, you must understand the terms associated with the process. Some of these terms have been used and defined earlier in the text, but we list them here again to refresh your memory. ÂŠ 2003 by CRC Press LLC

Review these terms and remember them. They are used throughout the discussion. Absorption taking in or reception of one substance into the body of another by molecular or chemical actions and distribution throughout the absorber. Activated to speed up reaction. When applied to sludge, it means that many aerobic bacteria and other microorganisms are in the sludge particles. Activated sludge a floc or solid formed by the microorganisms. It includes organisms, accumulated food materials, and waste products from the aerobic decomposition process. Activated sludge process a biological wastewater treatment process in which a mixture or influent and activated sludge is agitated and aerated. The activated sludge is subsequently separated from the treated mixed liquor by sedimentation and is returned to the process as needed. The treated wastewater overflows the weir of the settling tank in which separation from the sludge takes place. Adsorption the adherence of dissolved, colloidal, or finely divided solids to the surface of solid bodies when they are brought into contact. Aeration mixing air and a liquid by one of the following methods: spraying the liquid in the air, diffusing air into the liquid, or agitating the liquid to promote surface adsorption of air. Aerobic a condition in which free or dissolved oxygen is present in the aquatic environment. Aerobic organisms must be in the presence of DO to be active. Bacteria single-cell plants that play a vital role in stabilization of organic waste. Biochemical oxygen demand (BOD) a measure of the amount of food available to the microorganisms in a particular waste. It is measured by the amount of dissolved oxygen used up during a

Wastewater Treatment

specific time period (usually 5 d, expressed as BOD5). Biodegradable from “degrade” (to wear away or break down chemically) and “bio” (by living organisms). Put it all together, and you have a substance, usually organic, that can be decomposed by biological action. Bulking a problem in activated sludge plants that results in poor settleability of sludge particles. Coning a condition that may be established in a sludge hopper during sludge withdrawal, when part of the sludge moves toward the outlet while the remainder tends to stay in place. Development of a cone or channel of moving liquids surrounded by relatively stationary sludge. Decomposition generally, in waste treatment, decomposition refers to the changing of waste matter into simpler, more stable forms that will not harm the receiving stream. Diffuser a porous plate or tube through which air is forced and divided into tiny bubbles for distribution in liquids. Commonly made of carborundum, aluminum, or silica sand. Diffused air aeration a diffused air activated sludge plant takes air, compresses it, then discharges the air below the water surface to the aerator through some type of air diffusion device. Dissolved oxygen (DO) atmospheric oxygen dissolved in water or wastewater. Note: The typical required DO for a well-operated activated sludge plant is between 2.0 and 2.5 mg/L. Facultative facultative bacteria can use either molecular (dissolved) oxygen or oxygen obtained from food materials. In other words, facultative bacteria can live under aerobic or anaerobic conditions. Filamentous bacteria organisms that grow in thread or filamentous form. Food-to-microorganisms ratio (F:M ratio) a process control calculation used to evaluate the amount of food (BOD or COD) available per pound of mixed liquor volatile suspended solids. Food Microorganism

BOD lb d

MLVSS (lb) Q ( MGD) ¥ BOD ( mg L ) ¥ 8.34 lb gal

v (MG ) ¥ MLVSS ( mg L ) ¥ 8.34 lb gal

567

solids available per pound of mixed liquor suspended solids. Mean cell residence time (MCRT) the average length of time mixed liquor suspended solids particle remains in the activated sludge process. This is usually written as MCRT and may also be referred to as sludge retention rate. MCRT (d ) =

Solids in Activated Sludge Process (lb) Solids Removed from Process ( lb d )

Mixed liquor the contribution of return activated sludge and wastewater (either influent or effluent) that flows into the aeration tank. Mixed liquor suspended solids (MLSS) t h e s u s pended solids concentration of the mixed liquor. Many references use this concentration to represent the amount of organisms in the liquor or the amount of organisms in the activated sludge process. Mixed liquor volatile suspended solids (MLVSS) the organic matter in the mixed liquor suspended solids. This can also be used to represent the amount of organisms in the process. Nematodes microscopic worms that may appear in biological waste treatment systems. Nutrients substances required to support plant organisms. Major nutrients are carbon, hydrogen, oxygen, sulfur, nitrogen, and phosphorus. Protozoa single-cell animals that are easily observed under the microscope at a magnification of 100¥. Bacteria and algae are prime sources of food for advanced forms of protozoa. Return activated sludge (RAS) the solids returned form the settling tank to the head of the aeration tank. Rising sludge rising sludge occurs in the secondary clarifiers or activated sludge plant when the sludge settles to the bottom of the clarifier, is compacted, and then rises to the surface in relatively short time. Rotifiers multicellular animals with flexible bodies and cilia near their mouths used to attract food. Bacteria and algae are their major source of food. Secondary treatment a wastewater treatment process used to convert dissolved or suspended materials into a form that can be removed. Settleability a process control test used to evaluate the settling characteristics of the activated sludge. Readings taken at 30 to 60 min are used to calculate the settled sludge volume and the sludge volume index.

568

Handbook of Water and Wastewater Treatment Plant Operations

Settled sludge volume (SSV) the volume of milligrams per liter (or percent) occupied by an activated sludge sample after 30 or 60 minutes of settling. Normally written as SSV with a subscript to indicate the time of the reading used for calculation (SSV30 or SSV60). Shock load the arrival at a plant of a waste toxic to organisms in sufficient quantity or strength to cause operating problems, such as odor or sloughing off of the growth of slime on the trickling filter media. Organic overloads also can cause a shock load. Sludge volume index (SVI) a process control calculation used to evaluate the settling quality of the activated sludge. Requires the SSV30 and mixed liquor suspended solids test results to calculate. Sludge Vol. Index (SVI), mL g =

(30 min settled vol.,

mL L )(1000 mg g) Mixed Liquor Suspended Solids, mg L

Solids material in the solid state. The different types of solids include: Dissolved solids present in solution. Solids that will pass through a glass fiber filter. Fixed also known as the inorganic solids. The solids that are left after a sample is ignited at 550°C for 15 min. Floatable solids that will float to the surface of still water, sewage, or other liquid. Usually composed of grease particles, oils, light plastic material, etc. Also called scum. Nonsettleable finely divided suspended solids that will not sink to the bottom in still water, sewage, or other liquid in a reasonable period, usually 2 h. Non-settleable solids are also known as colloidal solids. Suspended solids that will not pass through a glass fiber filter. Total solids in water, sewage, or other liquids. It includes the suspended solids and dissolved solids. Volatile organic solids. Measured as the solids that are lost on ignition of the dry solids at 550°C. Waste activated sludge (WAS) t h e s o l i d s b e i n g removed from the activated sludge process.

18.10.2 ACTIVATED SLUDGE PROCESS: EQUIPMENT The equipment requirements for the activated sludge process are more complex than other processes discussed. Equipment includes an aeration tank, aeration, systemsettling tank, return sludge, and waste sludge. © 2003 by CRC Press LLC

18.10.2.1 Aeration Tank The aeration tank is designed to provide the required detention time (depends on the specific modification) and ensure that the activated sludge and the influent wastewater are thoroughly mixed. Tank design normally attempts to ensure no dead spots are created. 18.10.2.2 Aeration Aeration can be mechanical or diffused. Mechanical aeration systems use agitators or mixers to mix air and mixed liquor. Some systems use a sparge ring to release air directly into the mixer. Diffused aeration systems use pressurized air released through diffusers near the bottom of the tank. Efficiency is directly related to the size of the air bubbles produced. Fine bubble systems have a higher efficiency. The diffused air system has a blower to produce large volumes of low pressure air (5 to 10 psi), air lines to carry the air to the aeration tank, and headers to distribute the air to the diffusers that release the air into the wastewater. 18.10.2.3 Settling Tank Activated sludge systems are equipped with plain settling tanks designed to provide 2 to 4 h HDT. 18.10.2.4 Return Sludge The return sludge system include pumps, a timer or variable speed drive to regulate pump delivery and a flow measurement device to determine actual flow rates. 18.10.2.5 Waste Sludge In some cases, the WAS withdrawal is accomplished by adjusting valves on the return system. When a separate system is used it includes pumps, a timer or variable speed drive, and a flow measurement device.

18.10.3 OVERVIEW OF ACTIVATED SLUDGE PROCESS The activated sludge process is a treatment technique in which wastewater and reused biological sludge full of living microorganisms are mixed and aerated. The biological solids are then separated from the treated wastewater in a clarifier and are returned to the aeration process or wasted. The microorganisms are mixed thoroughly with the incoming organic material, and they grow and reproduce by using the organic material as food. As they grow and are mixed with air, the individual organisms cling together (flocculate). Once flocculated, they more readily settle in the secondary clarifiers. The wastewater being treated flows continuously into an aeration tank where air is injected to mix the wastewater

Wastewater Treatment

with the returned activated sludge and to supply the oxygen needed by the microbes to live and feed on the organics. Aeration can be supplied by injection through air diffusers in the bottom of tank or by mechanical aerators located at the surface. The mixture of activated sludge and wastewater in the aeration tank is called the mixed liquor. The mixed liquor flows to a secondary clarifier where the activated sludge is allowed to settle. The activated sludge is constantly growing, and more is produced than can be returned for use in the aeration basin. Some of this sludge must be wasted to a sludge handling system for treatment and disposal. The volume of sludge returned to the aeration basins is normally 40 to 60% of the wastewater flow. The rest is wasted.

18.10.4 ACTIVATED SLUDGE PROCESS: FACTORS AFFECTING OPERATION A number of factors affect the performance of an activated sludge system. These include the following: 1. 2. 3. 4. 5. 6. 7. 8.

Temperature Return rates Amount of oxygen available Amount of organic matter available pH Waste rates Aeration time Wastewater toxicity

To obtain the desired level of performance in an activated sludge system, a proper balance must be maintained between the amount of food (organic matter), organisms (activated sludge), and oxygen (DO). The majority of problems with the activated sludge process result from an imbalance between these three items. To fully appreciate and understand the biological process taking place in a normally functioning activated sludge process, the operator must have knowledge of the key players in the process: the organisms. This makes a certain amount of sense when you consider that the heart of the activated sludge process is the mass of settleable solids formed by aerating wastewater containing biological degradable compounds in the presence of microorganisms. Activated sludge consists of organic solids plus bacteria, fungi, protozoa, rotifers, and nematodes. 18.10.4.1 Growth Curve To understand the microbiological population and its function in an activated sludge process, the operator must be familiar with the microorganism growth curve (see Section 11.12.5, Figure 11.15). © 2003 by CRC Press LLC

569

In the presence of excess organic matter, the microorganisms multiply at a fast rate. The demand for food and oxygen is at its peak. Most of this is used for the production new cells. This condition is known as the log growth phase (see Figure 11.15). As time continues, the amount of food available for organisms declines. Floc begins to form, while the growth rate of bacteria and protozoa begins to decline. This is referred to as the declining growth phase (see Figure 11.15). The endogenous respiration phase occurs as the food available becomes extremely limited and the organism mass begins to decline (see Figure 11.15). Some of the microorganisms may die and break apart, releasing organic matter that can be consumed by the remaining population. The actual operation of an activated-sludge system is regulated by three factors: (1) the quantity of air supplied to the aeration tank, (2) the rate of activated-sludge recirculation, and (3) the amount of excess sludge withdrawn form the system. Sludge wasting is an important operational practice because it allows the operator to establish the desired concentration of MLSS, F:M ratio, and sludge age. Note: Air requirements in an activated sludge basin are governed by (1) BOD loading and the desired removal effluent, (2) volatile suspended solids concentration in the aerator, and (3) suspended solids concentration of the primary effluent.

18.10.5 ACTIVATED SLUDGE FORMATION The formation of activated sludge is dependent on three steps. The first step is the transfer of food from wastewater to organism. Second is the conversion of wastes to a usable form. Third is the flocculation step. 1. Transfer — Organic matter (food) is transferred from the water to the organisms. Soluble material is absorbed directly through the cell wall. Particulate and colloidal matter is adsorbed to the cell wall, where it is broken down into simpler soluble forms and absorbed through the cell wall. 2. Conversion — Food matter is converted to cell matter by synthesis and oxidation into end products such as CO2, H2O, NH3, stable organic waste, and new cells. 3. Flocculation — Flocculation is the gathering of fine particles into larger particles. This process begins in the aeration tank and is the basic mechanism for removal of suspended matter in the final clarifier. The concentrated bio-floc that settles and forms the sludge blanket in the secondary clarifier is known as activated sludge.

570

Handbook of Water and Wastewater Treatment Plant Operations

18.10.6 ACTIVATED SLUDGE: PERFORMANCECONTROLLING FACTORS To maintain the working organisms in the activated sludge process, the operator must ensure that a suitable environment is maintained by being aware of the many factors influencing the process and by monitoring them repeatedly. Control is defined as maintaining the proper solids (floc mass) concentration in the aerator for the incoming water (food) flow by adjusting the return and waste sludge pumping rate and regulating the oxygen supply to maintain a satisfactory level of DO in the process. 18.10.6.1 Aeration The activated sludge process must receive sufficient aeration to keep the activated sludge in suspension and to satisfy the organism oxygen requirements. Insufficient mixing results in dead spots, septic conditions, and loss of activated sludge. 18.10.6.2 Alkalinity The activated sludge process requires sufficient alkalinity to ensure that pH remains in the acceptable range of 6.5 to 9.0. If organic nitrogen and ammonia are being converted to nitrate (nitrification), sufficient alkalinity must be available to support this process as well.

Note: The activity level of bacteria within the activated sludge process increases with rise in temperature. 18.10.6.6 Toxicity Sufficient concentrations of elements or compounds that enter a treatment plant that have the ability to kill the microorganisms (the activated sludge) are known as toxic waste (shock level). Common to this group are cyanides and heavy metals. Note: A typical example of a toxic substance added by operators is the uninhabited use of chlorine for odor control or control of filamentous organisms (prechlorination). Chlorination is for disinfection. Chlorine is a toxicant and should not be allowed to enter the activated sludge process; it is not selective with respect to type of organisms damaged or killed. It may kill the organisms that should be retained in the process as workers. However, chlorine is very effective in disinfecting the plant effluent after treatment by the activated sludge process. 18.10.6.7 Hydraulic Loading

The pH of the mixed liquor should be maintained within the range of 6.5 to 9.0 (ideally 6.0 to 8.0). Gradual fluctuations within this range will normally not upset the process. Rapid fluctuations or fluctuations outside this range can reduce organism activity.

Hydraulic loading is the amount of flow entering the treatment process. When compared with the design capacity of the system, it can be used to determine if the process is hydraulically overloaded or underloaded. If more flow is entering the system than it was designed to handle, the system is hydraulically overloaded. If less flow is entering the system than it was designed to handle, the system is hydraulically underloaded. Generally, the system is more affected by overloading than by underloading. Overloading can be caused by stormwater, infiltration of groundwater, excessive return rates, or many other causes. Underloading normally occurs during periods of drought or in the period following initial start-up when the plant has not reached its design capacity. Excess hydraulic flow rates through the treatment plant will reduce the efficiency of the clarifier by allowing activated sludge solids to rise in the clarifier and pass over the effluent weir. This loss of solids in the effluent degrades effluent quality and reduces the amount of activated sludge in the system, reducing process performance.

18.10.6.5 Temperature

18.10.6.8 Organic Loading

As temperature decreases, activity of the organisms will also decrease. Cold temperatures also require longer recovery time for systems that have been upset. Warm temperatures tend to favor denitrification and filamentous growth.

Organic loading is the amount of organic matter entering the treatment plant. It is usually measured as BOD. An organic overload occurs when the amount of BOD entering the system exceeds the design capacity of the system. An organic underload occurs when the amount of BOD

18.10.6.3 Nutrients The microorganisms of the activated sludge process require nutrients (nitrogen, phosphorus, iron, and other trace metals) to function. If sufficient nutrients are not available, the process will not perform as expected. The accepted minimum ratio of carbon to nitrogen, phosphorus, and iron is 100 parts carbon to 5 parts nitrogen, 1 part phosphorus, and 0.5 parts iron. 18.10.6.4 pH

ÂŠ 2003 by CRC Press LLC

Wastewater Treatment

entering the system is significantly less than the design capacity of the plant. Organic overloading may occur when the system receives more waste than it was designed to handle. It can also occur when an industry or other contributor discharges more wastes to the system than originally planned. Wastewater treatment plant processes can also cause organic overloads returning high-strength wastes from the sludge treatment processes. Regardless of the source, an organic overloading of the plant results in increased demand for oxygen. This demand may exceed the air supply available from the blowers. When this occurs, the activated sludge process may become septic. Excessive wasting can also result in a type of organic overload. The food available exceeds the number of activated sludge organisms, resulting in increased oxygen demand and very rapid growth. Organic underloading may occur when a new treatment plant is initially put into service. The facility may not receive enough waste to allow the plant to operate at its design level. Underloading can also occur when excessive amounts of activated sludge are allowed to remain in the system. When this occurs, the plant will have difficulty in developing and maintaining a good activated sludge.

18.10.7 ACTIVATED SLUDGE MODIFICATIONS First developed in 1913, the original activated sludge process has been modified over the years to provide better performance for specific operating conditions or with different influent waste characteristics. 18.10.7.1 Conventional Activated Sludge 1. Employing the conventional activated sludge modification requires primary treatment. 2. Conventional activated sludge provides excellent treatment, but large aeration tank capacity is required, and construction costs are high. 3. In operation, initial oxygen demand is high. The process is also very sensitive to operational problems (e.g., bulking). 18.10.7.2 Step Aeration 1. Step aeration requires primary treatment. 2. It provides excellent treatment. 3. Operation characteristics are similar to conventional. 4. It distributes organic loading by splitting influent flow. 5. It reduces oxygen demand at the head of the system. 6. It reduces solids loading on settling tank. ÂŠ 2003 by CRC Press LLC

571

18.10.7.3 Complete Mix 1. May or may not include primary treatment. 2. Distributes waste, return, and oxygen evenly throughout tank. 3. Aeration may be more efficient. 4. Maximizes tank use. 5. Permits a higher organic loading. Note: During the complete mix, activated sludge process organisms are in declining phase on growth curve. 18.10.7.4 Pure Oxygen 1. 2. 3. 4. 5. 6. 7.

Requires primary treatment. Permits higher organic loading. Uses higher solids levels. Operates at higher F/M ratios. Uses covered tanks. Potential safety hazards (pure oxygen). Oxygen production is expensive.

18.10.7.5 Contact Stabilization 1. Contact stabilization does not require primary treatment. 2. During operation, organisms collect organic matter (during contact). 3. Solids and activated sludge are separated from flow via settling. 4. Activated sludge and solids are aerated for 3 to 6 h (stabilization). Note: Return sludge is aerated before it is mixed with influent flow. 5. The activated sludge oxidizes available organic matter. 6. While the process is complicated to control, it requires less tank volume than other modifications and can be prefabricated as a package unit for flows of 0.05 to 1.0 MGD. 7. A disadvantage is that common process control calculations do not provide usable information. 18.10.7.6 Extended Aeration 1. Does not require primary treatment. 2. Used frequently for small flows such as schools and subdivisions. 3. Uses 24-h aeration. 4. Produces low BOD effluent. 5. Produces the least amount of WAS.

572

Handbook of Water and Wastewater Treatment Plant Operations

TABLE 18.5 Activated Sludge Modifications Parameter

Conventional

Aeration time (h)

4–8

Settling time (h) Return rate (% of influent flow) MLSS (mg/L)

2–4 25–100 1500–4000

DO (mg/L) SSV30 (mL/L) F:M ratio (lb BOD5/lb MLVSS) MCRT (whole system [d]]) % Removal BOD5 % Removal TSS Primary treatment

1–3 400–700 02–0.5 5–15 85–95% 85–95% Yes

Contact Stabilization

Extended Aeration

0.5–1.5 (contact) 3–6 (reaeration) 2–4 25–100 1000–3000 3000–8000 1–3 400–700 (contact) 0.2–0.6 (contact) N/A 85–95% 85–95% No

Oxidation Ditch

2–4 25–100 2000–6000

1–3 400–700 0.05–0.15 20–30 85–95% 85–95% No

N/A = not available. Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

6. Process is capable of achieving 95% or more removals of BOD. 7. Can produce effluent low in organic and ammonia nitrogen.

oxygen required is dependent on the influent food (BOD), the activity of the activated sludge, and the degree of treatment desired. 18.10.8.3 pH

18.10.7.7 Oxidation Ditch 1. Does not require primary treatment. 2. The oxidation ditch process is similar to the extended aeration process. Table 18.5 lists the process parameters for each of the four most commonly used activated sludge modifications.

18.10.8 ACTIVATED SLUDGE: PROCESS CONTROL PARAMETERS In operating an activated sludge process, the operator must be familiar with the many important process control parameters that must be monitored frequently and adjusted occasionally to maintain optimal performance. 18.10.8.1 Alkalinity Monitoring alkalinity in the aeration tank is essential to control of the process. Insufficient alkalinity will reduce organism activity and may result in low effluent pH and, in some cases, extremely high chlorine demand in the disinfection process. 18.10.8.2 Dissolved Oxygen The activated sludge process is an aerobic process that requires some DO be present at all times. The amount of © 2003 by CRC Press LLC

Activated sludge microorganisms can be injured or destroyed by wide variations in pH. The pH of the aeration basin will normally be in the range of 6.5 to 9.0. Gradual variations within this range will not cause any major problems; rapid changes of one or more pH units can have a significant impact on performance. Industrial waste discharges, septic wastes, or significant amounts of stormwater flows may produce wide variations in pH. pH should be monitored as part of the routine process control-testing schedule. Sudden changes or abnormal pH values may indicate an industrial discharge of strongly acidic or alkaline wastes. Because these wastes can upset the environmental balance of the activated sludge, the presence of wide pH variations can result in poor performance. Processes undergoing nitrification may show a significant decrease in effluent pH. 18.10.8.4 Mixed Liquor Suspended Solids, Mixed Liquor Volatile Suspended Solids, and Mixed Liquor Total Suspended Solids The MLSS or MLVSS can be used to represent the activated sludge or microorganisms present in the process. Process control calculations, such as sludge age and SVI, cannot be calculated unless the MLSS is determined. Adjust the MLSS and MLVSS by increasing or decreasing the waste sludge rates.

Wastewater Treatment

573

Suspended solids in

Suspended solids out Settling tank

Sludge solids out

Suspended solids in

Sludge solids out Suspended solids out

FIGURE 18.11 Settling tank mass balance. (From Spellman, F.R., Spellmanâ&#x20AC;&#x2122;s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

The mixed liquor total suspended solids (MLTSS) is an important activated sludge control parameter. To increase the MLTSS, for example, the operator must decrease the waste rate or increase the MCRT. The MCRT must be decreased to prevent the MLTSS from changing when the number of aeration tanks in service is reduced. Note: In performing the Gould sludge age test, assume that the source of the MLTSS in the aeration tank is influent solids.

the solids become very light and will not settle quickly enough to be removed in the secondary clarifier. 18.10.8.7 Temperature Because temperature directly affects the activity of the microorganisms, accurate monitoring of temperature can be helpful in identifying the causes of significant changes in organization populations or process performance. 18.10.8.8 Sludge Blanket Depth

18.10.8.5 Return Activated Sludge Rate and Concentration The sludge rate is a critical control variable. The operator must maintain a continuous return of activated sludge to the aeration tank or the process will show a drastic decrease in performance. If the rate is too low, solids remain in the settling tank, resulting in solids loss and a septic return. If the rate is too high, the aeration tank can become hydraulically overloaded, causing reduced aeration time and poor performance. The return concentration is also important because it may be used to determine the return rate required to maintain the desired MLSS. 18.10.8.6 Waste Activated Sludge Flow Rate Because the activated sludge contains living organisms that grow, reproduce, and produce waste matter, the amount of activated sludge is continuously increasing. If the activated sludge is allowed to remain in the system too long, the performance of the process will decrease. If too much activated sludge is removed from the system, ÂŠ 2003 by CRC Press LLC

The separation of solids and liquid in the secondary clarifier results in a blanket of solids. If solids are not removed from the clarifier at the same rate they enter, the blanket will increase in depth. If this occurs, the solids may carry over into the process effluent. The sludge blanket depth may be affected by other conditions, such as temperature variation, toxic wastes, or sludge bulking. The best sludge blanket depth is dependent upon such factors as hydraulic load, clarifier design, and sludge characteristics (see Figure 18.11). The best blanket depth must be determined on an individual basis by experimentation. Note: In measuring sludge blanket depth, it is general practice to use a 15 to 20 ft long clear plastic pipe marked at 6-in. intervals, the pipe is equipped with a ball valve at the bottom.

18.10.9 OPERATIONAL CONTROL LEVELS (Note: Much of the information in this section is based on Activated Sludge Process Control, Part II, 2nd ed., Virginia Water Control Board, 1990.) The operator has two methods available to operate an activated sludge system. The operator can wait until the

574

Handbook of Water and Wastewater Treatment Plant Operations

process performance deteriorates and make drastic changes, or the operator can establish normal operational levels and make minor adjustments to keep the process within the established operational levels.

frequently, and controlled by either local ordinance or by implementation of a pretreatment program.

Note: Control levels can be defined as the upper and lower values for a process control variable that can be expected to produce the desired effluent quality.

Process sidestreams are flows produced in other treatment processes that must be returned to the wastewater system for treatment prior to disposal. Examples of process sidestreams include the following:

While the method will guarantee that plant performance will always be maintained within effluent limitations, the second method has a much higher probability of achieving this objective. This section discusses methods used to establish normal control levels for the activated sludge process. Several major factors should be considered when establishing control levels for the activated sludge system. These include the following: 1. 2. 3. 4. 5.

Influent characteristics Industrial contributions Process sidestreams Seasonal variations Required effluent quality

18.10.9.3 Process Sidestreams

1. Thickener supernatant 2. Aerobic and anaerobic digester supernatant 3. Liquids removed by sludge dewatering processes (filtrate, centrate, and subnate) 4. Supernatant from heat treatment and chlorine oxidation sludge treatment processes Testing these flows periodically to determine both their quantity and strength is important. In many treatment systems, a significant part of the organic and/or hydraulic loading for the plant is generated by sidestream flows. The contribution of the plant sidestream flows can significantly change the operational control levels of the activated sludge system. 18.10.9.4 Seasonal Variations

18.10.9.1 Influent Characteristics Influent characteristics were discussed earlier. A major area to consider when evaluating influent characteristics is the nature and volume of industrial contributions to the system. Waste characteristics (BOD, solids, pH, metals, toxicity, and temperature), volume, and discharge pattern (continuous, slug, daily, weekly, etc.) should be evaluated when determining if a waste will require pretreatment by the industry or adjustments to operational control levels.

Seasonal variations in temperature, oxygen solubility, organism activity, and waste characteristics may require several normal control levels for the activated sludge process. For example, during cold months of the year, aeration tank solids levels may have to be maintained at significantly higher level than are required during warm weather. Likewise, the aeration rate may be controlled by the mixing requirements of the system during the colder months and by the oxygen demand of the system during the warm months.

18.10.9.2 Industrial Contributions

18.10.9.5 Control Levels at Start-Up

One or more industrial contributors produce a significant portion of the plant loading (in many systems). Identifying and characterizing all industrial contributors is important. Remember that the volume of waste generated may not be as important as the characteristics of the waste. Extremely high-strength wastes can result in organic overloading and poor performance because of insufficient nutrient availability. A second consideration is the presence of materials that even in small quantities are toxic to the process microorganisms or create a toxic condition in the plant effluent or plant sludge. Industrial to a biological treatment system should be thoroughly characterized prior to acceptance, monitored

Control levels for an activated sludge system during startup are usually based upon design engineer recommendations or information available from recognized reference sources. Although these levels provide a starting point, you should recognize that both the process control parameter sensitivity and control levels should be established on a plant-by-plant basis. During the first 12 months of operation, you should evaluate all potential process control options, to determine the following:

ÂŠ 2003 by CRC Press LLC

1. Sensitivity to effluent quality changes 2. Seasonal variability 3. Potential problems

Wastewater Treatment

18.10.10 OPERATOR OBSERVATIONS: INFLUENT AND AERATION TANK Wastewater operators are required to monitor or make certain observations of treatment unit processes to ensure optimum performance and make adjustments when required. In monitoring the operation of an aeration tank, the operator should look for three physical parameters (turbulence, surface foam and scum, and sludge color and odor), that aid in determining how the process is operating and indicate if any operational adjustments should be made. This information should be recorded each time operational tests are performed. We summarize aeration tank and secondary settling tank observations in the following sections. Remember that many of these observations are very subjective and must be based upon experience. Plant personnel must be properly trained on the importance of ensuring that recorded information is consistent throughout the operating period.

575

18.10.10.1.3

Sludge Color and Odor

Though not as reliable an indicator of process operations as foam, sludge colors and odor are also useful indicators. Colors and odors that are important include the following: 1. Chocolate brown or earthy odor — Indicates normal operation. 2. Light tan or brown or no odor — Indicates sand and clay from infiltration or inflow. Adjustment: Extremely young sludge, decrease wasting. 3. Dark brown or earthy odor — Indicates old sludge and high solids. Adjustment: Increase wasting. 4. Black color or rotten egg odor — Indicates septic conditions, low DO concentration, and an airflow rate that is too low. Adjustment: Increase aeration. 18.10.10.1.4

Mixed Liquor Color

A light chocolate brown mixed liquor color indicates a well-operated activated sludge process.

18.10.10.1 Visual Indicators: Influent and Aeration Tank

18.10.10.2 Final Settling Tank (Clarifier) Observations

18.10.10.1.1

Settling tank observations include flow pattern (normally uniform distribution), settling, amount and type of solids leaving with the process effluent (normally very low) and the clarity or turbidity of the process effluent (normally very clear). Observations should include the following conditions:

Turbulence

Normal operation of an aeration basin includes a certain amount of turbulence. This turbulent action is required to ensure a consistent mixing pattern. Whenever excessive, deficient or nonuniform mixing occurs, adjustments may be necessary to airflow, or diffusers may need cleaning or replacement. 18.10.10.1.2

Surface Foam and Scum

The type, color and amount of foam or scum present may indicate the required wasting strategy to be employed. Types of foam include the following: 1. Fresh, crisp, white foam — Moderate amounts of crisp, white foam are usually associated with activated sludge processes producing an excellent final effluent. Adjustment: None, normal operation. 2. Thick, greasy, dark tan foam — A thick, greasy dark tan or brown foam or scum normally indicates an old sludge that is overoxidized, has a high mixed liquor concentration, and has a waste rate that is too high. Adjustment: Indicates old sludge, more wasting required. 3. White billowing foam — Large amounts of a white, soap suds-like foam indicate a very young, underoxidized sludge. Adjustment: Young sludge, less wasting required. © 2003 by CRC Press LLC

1. Sludge bulking — Occurs when solids are evenly distributed throughout the tank and leaving over the weir in large quantities. 2. Sludge Solids Washout — Sludge blanket is down, but solids are flowing over the effluent weir in large quantities. Control tests indicate good quality sludge. 3. Clumping — Large clumps or masses of sludge (several inches or more) rise to the top of the settling tank. 4. Ashing — Fine particles of gray to white material flowing over the effluent weir in large quantities. 5. Straggler floc — Small, almost transparent, very fluffy, buoyant solids particles (1/8 to 1/4 in. diameter rising to the surface). Usually is accompanied by a very clean effluent. New growth is usually most noted in the early morning hours. Sludge age is slightly below optimum. 6. Pin floc — Very fine solids particles (usually less than 1/32 in. diameter) suspended throughout lightly turbid liquid. Usually the result of an overoxidized sludge.

576

Handbook of Water and Wastewater Treatment Plant Operations

18.10.11 PROCESS CONTROL TESTING AND SAMPLING

but normal is from 1 to 3 mg/L. If the system contains too little DO, the process will become septic. If it contains too much DO, energy and money is wasted.

The activated sludge process generally requires more sampling and testing to maintain adequate process control than any of the other unit processes in the wastewater treatment system. During periods of operational problems, both the parameters tested and the frequency of testing may increase substantially. Process control testing may include the following:

18.10.11.1.4 Settled Sludge Volume (Settleability) SSV is determined at specified times during sample testing. Both 30- and 60-min observations are used for control. Subscript numbers indicates settling time (e.g., SSV30 and SSV60). The test is performed on aeration tank effluent sample.

1. Settleability testing to determine the settled sludge volume 2. Suspended solids testing to determine influent and MLSS 3. RAS solids and WAS concentrations 4. Determination of the volatile content of the mixed liquor suspended solids 5. DO and pH of the aeration tank 6. BOD and COD of the aeration tank influent and process effluent 7. Microscopic evaluation of the activated sludge to determine the predominant organism. The following sections describe most of the common process control tests. 18.10.11.1 Aeration Influent Sampling 18.10.11.1.1 pH pH is tested daily with a sample taken from the aeration tank influent and process effluent. pH is normally close to 7.0 (normal) with the best pH range from 6.5 to 8.5 (6.5 to 9.0 is satisfactory). A pH of less than 9.0 may indicate toxicity from an industrial waste contributor. A pH of greater than 6.5 may indicate loss of flocculating organisms, potential toxicity, industrial waste contributor, or acid storm flow. Keep in mind that the effluent pH may be lower because of nitrification. 18.10.11.1.2 Temperature Temperature is important because it forecasts the following: 1. Temperature increases: A. Organism activity increases B. Aeration efficiency decreases C. Oxygen solubility decreases 2. Temperature decreases: A. Organism activity decreases B. Aeration efficiency increases C. Oxygen solubility increases 18.10.11.1.3 Dissolved Oxygen The content of DO in the aeration process is critical to performance. DO should be tested at least daily (peak demand). Optimum is determined for individual plants, ÂŠ 2003 by CRC Press LLC

SSV =

Milliliters of Settled Sludge (1000 mL L ) Milliliters of Sample (18.27)

%SSV =

Milliliters of Settled Sludge ÂĽ 100 Milliliters of Sample

(18.28)

Under normal conditions, sludge settles as a mass, producing clear supernatant with SSV60 in the range of 400 to 700 mL/L. When higher values are indicated, this may indicate excessive solids (old sludge) and bulking conditions. Rising solids (if sludge is well oxidized) may rise after 2 or more hours. However, rising solids in less than 1 h indicates a problem. Note: Running the settleability test with a diluted sample can assist in determining if the activated sludge is old (too many solids) or bulking (not settling). Old sludge will settle to a more compact level when diluted. 18.10.11.1.4.1

Centrifuge Testing

The centrifuge test provides a quick, relatively easy control test for the solids level in the aerator, but does not usually correlate with MLSS results. Results are directly affected by variations in sludge quality. 18.10.11.1.5 Alkalinity Alkalinity is essential to biological activity. Nitrification needs 7.3-mg/L alkalinity per milligrams per liter or TKN. 18.10.11.1.6 Biochemical Oxygen Demand Testing showing an increase in BOD indicates increased organic loading; a decrease in BOD indicates decreased organic loading. 18.10.11.1.7 Total Suspended Solids An increase in TSS indicates an increase in organic loading; a decrease TSS indicates a decrease in organic loading. 18.10.11.1.8 Total Kjeldahl Nitrogen TKN determination is required to monitor process nitrification status and to determine alkalinity requirements. 18.10.11.1.9 Ammonia Nitrogen Determination of ammonia nitrogen is required to monitor process nitrification status.

Wastewater Treatment

577

TABLE 18.6 Process Condition vs. Organisms Present and Population Process Condition

Organism Population

Poor BOD and TSS removal No floc formation Very cloudy effluent

Predominance of amoeba and flagellates Mainly dispersed bacteria A few ciliates present

Poor quality effluent Dispersed bacteria Some floc formation Cloudy effluent

Predominance of amoeba and flagellates Some free-swimming ciliates

Satisfactory effluent Good floc formation Good settleability Good Clarity

Predominance of free-swimming ciliates Few amoeba and flagellates

High-quality effluent Excellent floc formation Excellent Settleability High effluent clarity

Predominance of stalked ciliates Some free-swimming ciliates A few rotifers A few flagellates

Effluent High TSS and Low BOD High settled sludge volume Cloudy effluent

Predominance of rotifers Large numbers of stalked ciliates A few free-swimming ciliates No flagellates

Source: Spellman, F.R., Spellmanâ&#x20AC;&#x2122;s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

18.10.11.1.10 Metals Metal contents are measured to determine toxicity levels. 18.10.11.2 Aeration Tank 18.10.11.2.1 pH Normal pH range in the aeration tank is 6.5 to 9.0 pH decreases indicate process sidestreams or insufficient alkalinity available. 18.10.11.2.2 Dissolved Oxygen Normal DO range in an aeration tank is 1 to 3 mg/L. DO level decreases may indicate increased activity, increased temperature, increased organic loading, or decreased MLSS or MLVSS. An increase in DO could be indicative of decreased activity, decreased temperature, decreased organic loading, increased MLSS or MLVSS, or influent toxicity. 18.10.11.2.3 Dissolved Oxygen Profile All DO profile readings should be less than 0.5 mg/L. Readings of greater than 0.5 mg/L indicate inadequate aeration or poor mixing. 18.10.11.2.4 Mixed Liquor Suspended Solids The range of MLSS is determined by the process modification used. When MLSS levels increase, more solids, organisms, and an older, more oxidized sludge are typical. ÂŠ 2003 by CRC Press LLC

18.10.11.2.5

Microscopic Examination

The activated sludge process can not operate as designed without the presence microorganisms. Microscopic examination of an aeration basin sample, which determines the presence and the type of microorganisms, is important. Different species prefer different conditions; therefore, the presence of different species can indicate process conditions. Note: It is important to point out that during microscopic examination, identifying of all organisms present is not required, but identification of the predominant species is required. Table 18.6 lists process conditions indicated by the presence and population of certain microorganisms. 18.10.11.2.5.1

Interpretation

Routine process control identification can be limited to the general category of organisms present. For troubleshooting more difficult problems, a more detailed study of organism distribution may be required (the knowledge required to perform this type of detailed study is beyond the scope of this text). The major categories of organisms found in the activated sludge are:

578

Handbook of Water and Wastewater Treatment Plant Operations

1. Protozoa 2. Rotifers 3. Filamentous organisms Note: Bacteria are the most important microorganisms in the activated sludge. They perform most of the stabilization or oxidation of the organic matter and are normally present in extremely large numbers. They are not, however, normally visible with a conventional microscope operating at the recommended magnification and are not included in the Table 18.6 list of indicator organisms. Note: The presence of free-swimming and stalked ciliates, some flagellates, and rotifers in mixed liquor indicate a balanced, properly settling environment. Protozoa Protozoa are secondary feeders in the activated sludge process (secondary as feeders, but nonetheless definitely important to the activated sludge process). Their principal function is to remove (eat or crop) dispersed bacteria and help to produce a clear process effluent. To help gain an appreciation for the role of protozoa in the activated sludge process consider the following explanation. The activated sludge process is typified by the successive development of protozoa and mature floc particles. This succession can be indicated by the presence of the type of dominant protozoa present. At the start of the activated process (or recovery from an upset condition), the amoebas dominate. Note: Amoebas have very flexible cell walls and move by shifting fluids within the cell wall. They predominate during process start-up or during recovery from severe plant upsets. As the process continues uninterrupted or without upset, small populations of bacteria begin to grow in logarithmic fashion; as the population increases, they develop into mixed liquor. When this occurs, the flagellates dominate. Note: Flagellated protozoa typically have a single hair-like flagella or tail that they use for movement. The flagellate predominates when the MLSS and bacterial populations are low and organic load is high. As the activated sludge gets older and denser, the flagellates decrease until they are seldom used. When the sludge attains an age of about 3 d, lightly dispersed floc particles (flocculation grows fine solids into larger, more settleable solids) begin to form and bacteria increase. At this point, free-swimming ciliates dominate. ÂŠ 2003 by CRC Press LLC

Note: The free-swimming ciliated protozoa have hairlike projections (cilia) that cover all or part of the cell. The cilia are used for motion and create currents that carry food to the organism. The free-swimming ciliates are sometimes divided into two subcategories: free swimmers and crawlers. The free swimmers are usually seen moving through the fluid portion of the activated sludge, while the crawlers appear to be walking or grazing on the activated sludge solids. The free-swimming ciliated protozoa usually predominate when a large number of dispersed bacteria are present that can be used as food. Their predominance indicates a process nearing optimum conditions and effluent quality. The process continues with floc particles beginning to stabilize, taking on irregular shapes, and starting to show filamentous growth. At this stage, the crawling ciliates dominate. Eventually, mature floc particles develop and increase in size, and large numbers of crawling and stalked ciliates are present. When this occurs, the succession process has reached its terminal point. The succession of protozoan and mature floc particle development just described details the occurrence of phases of development in a step-by-step progression. Protozoan succession is also based on other factors, including DO and food availability. Probably the best way to understand protozoan succession based on DO and food availability is to view the wastewater treatment plantâ&#x20AC;&#x2122;s aeration basin as a stream within a container. The saprobity system classifies the various phases of the activated sludge process in relation to the self-purification process that takes place in a stream. With this system, a clear relationship between the two processes based on available DO and food supply is evident. Any change in the relative numbers of bacteria in the activated sludge process has a corresponding change to microorganismâ&#x20AC;&#x2122;s population. Decreases in bacteria increase competition between protozoa and result in secession of dominant groups of protozoa. The degree of success or failure of protozoa to capture bacteria depends on several factors. Those with more advanced locomotion capability are able to capture more bacteria. Individual protozoan feeding mechanisms are also important in the competition for bacteria. At the beginning of the activated sludge process, amoebas and flagellates are the first protozoan groups to appear in large numbers. They can survive on smaller quantities of bacteria because their energy requirements are lower than other protozoan types. Because few bacteria are present, competition for dissolved substrates is low. As the bacteria population increases, these protozoa are not able to compete for available food. This is when the next group of protozoa (the free-swimming protozoa) enters the scene.

Wastewater Treatment

Free-swimming protozoa take advantage of the large populations of bacteria because they are better equipped with food-gathering mechanisms than the amoebas and flagellates. The free swimmers are important for their insatiable appetites for bacteria and are also important in floc formation. Secreting polysaccharides and mucoproteins that are absorbed by bacteria — which make the bacteria sticky through biological agglutination (biological gluing together) — allows them to stick together and, more importantly, to stick to floc. Large quantities of floc are prepared for removal form secondary effluent and are either returned to aeration basins or wasted. The crawlers and stalked ciliates succeed the free swimmers. Note: Stalked ciliated protozoa are attached directly to the activated sludge solids by a stalk. In some cases, the stalk is rigid and fixed in place, while in others, the organism can more (contract or expand the stalk) to change its position. The stalked ciliated protozoa normally have several cilia that are used to create currents that carry bacteria and organic matter to them. The stalked ciliated protozoa predominate when the dispersed bacteria population decreases and does not provide sufficient food for the free swimmers. Their predominance indicates a stable process, operating at optimum conditions. The free swimmers are replaced in part because the increasing level of mature floc retards their movement. Additionally, the type of environment that is provided by the presence of mature floc is more suited to the needs of the crawlers and stalked ciliates. The crawlers and stalked ciliates also aid in floc formation by adding weight to floc particles, enabling removal. Rotifers Rotifers are a higher life form normally associated with clean, unpolluted waters. Significantly larger than most of the other organisms observed in activated sludge, rotifers can use other organisms, as well as organic matter, as their food source. Rotifers are usually the predominant organism; the effluent will usually be cloudy (pin of ash floc) and will have very low BOD. Filamentous Organisms Filamentous organisms (bacteria, fungi, etc.) occur whenever the environment of the activated sludge favors their predominance. They are normally present in small amounts and provide the basic framework for floc formation. When the environmental conditions (i.e., pH, nutrient levels, DO, etc.) favor their development, they become the predominant organisms. When this occurs, they restrict settling, and the condition known as bulking occurs. © 2003 by CRC Press LLC

579

Note: Microorganism examination of activated sludge is a useful control tool. In attempting to identify the microscopic contents of a sample, the operator should try to identify the predominant groups of organisms. Note: During microscopic examination of the activated sludge, a predominance of amoebas indicates that the activated sludge is very young. 18.10.11.3 Settling Tank Influent 18.10.11.3.1

Dissolved Oxygen

The DO level of the activated sludge-settling tank should be 1 to 3 mg/L; lower levels may result in rising sludge. 18.10.11.3.2

Normal pH range in an activated sludge-settling tank should be maintained between 6.5 to 9.0. Decreases in pH may indicate alkalinity deficiency. 18.10.11.3.3

Alkalinity

A lack of alkalinity in an activated sludge-settling tank will prevent nitrification. 18.10.11.3.4

Total Suspended Solids

MLSS sampling and testing is required for determining solids loading, mass balance, and return rates. 18.10.11.3.5

Settled Sludge Volume (Settleability)

SSV is determined at specified times during sample testing. Thirty- and 60-minute observations: 1. Normal operation — When the process is operating property, the solids will settle as a “blanket” (a mass), with a crisp or sharp edge between the solids and the liquor above. The liquid over the solids will be clear, with little or no visible solids remaining in suspension. Settled sludge volume at the end of 30 to 60 min will be in the range of 400 to 700 mL. 2. Old or overoxidized activated sludge — When the activated sludge is overoxidized, the solids will settle as discrete particles. The edge between the solids and liquid will be fuzzy, with a large number of visible solids (pin floc, ash floc, etc.) in the liquid. The settled sludge volume at the end of 30 or 60 min will be greater than 700 mL. 3. Young or under-oxidized activated sludge — When the activated sludge is under-oxidized, the solids settle as discrete particles, and the boundary between the solids and the liquid is poorly defined. Large amounts of small visible solids are suspended in the liquid. The settled

580

Handbook of Water and Wastewater Treatment Plant Operations

sludge volume after 30 to 60 minutes will usually be less than 400 mL. 4. Bulking activated sludge — When the activated sludge is experiencing a bulking condition, very little or no settling is observed. SSV =

Milliliters of Settled Sludge 1000 mL (18.29) Milliliters of Sample

%SSV =

Milliliters of Settled Sludge ¥ 100 Milliliters of Sample

(18.30)

Note: Running the settleability test with a diluted sample can assist in determining if the activated sludge is old (too many solids) or bulking (not settling). Old sludge will settle to a more compact level when diluted. 18.10.11.3.6

Flow

Monitoring flow in settling tank influent is important for determination of mass balance. 18.10.11.3.7

Jar Tests

Jar tests are performed as required on settling tank influent and are beneficial in determining the best flocculant aid and appropriate doses to improve solids capture during periods of poor settling. 18.10.11.4 Settling Tank 18.10.11.4.1

Sludge Blanket Depth

As mentioned, sludge blanket depth refers to the distance from the surface of the liquid to the solids-liquid interface. It can also refer to the thickness of the sludge blanket as measured from the bottom of the tank to the solids-liquid interface. Part of the operator’s sampling routine, this measurement is taken directly in the final clarifier. Sludge blanket depth is dependent upon hydraulic load, return rate, clarifier design, waste rate, sludge characteristics, and temperature. If all other factors remain constant, the blanket depth will vary with the amount of solids in the system and the return rate; it will vary throughout the day. Note: Depth of sludge blanket provides an indication of sludge quality; it is used as a trend indicator. Many factors affect test result. 18.10.11.4.2

Suspended Solids and Volatile Suspended Solids

Suspended solids and volatile suspended solids concentrations of the mixed liquor (MLSS), RAS, and WAS are routinely sampled and tested because they are critical to process control. © 2003 by CRC Press LLC

18.10.11.5 Settling Tank Effluent 18.10.11.5.1

Biochemical Oxygen Demand and Total Suspended Solids BOD and TSS testing is conducted variably (daily, weekly, and monthly). Increases indicate treatment performance is decreasing; decreases indicate treatment performance is increasing. 18.10.11.5.2 Total Kjeldahl Nitrogen TKN sampling and testing is variable. An increase in TKN indicates nitrification is decreasing; a decrease in TKN indicates nitrification is increasing. 18.10.11.5.3 Nitrate Nitrogen Nitrate nitrogen sampling and testing are variable. Increases in nitrate nitrogen indicate nitrification is increasing or industrial contribution of nitrates. A decrease indicates reduced nitrification. 18.10.11.5.4 Flow Settling tank effluent flow is sampled and tested daily. Results are required for several process control calculations. 18.10.11.6 Return Activated Sludge and Waste Activated Sludge 18.10.11.6.1

Total Suspended Solids and Volatile Suspended Solids TSS and total volatile suspended solids concentrations of the mixed liquor (MLSS), RAS, and WAS are routinely sampled (using either grab or composite samples) and tested, because they are critical to process control. The results of the suspended and volatile suspended tests can be used directly or to calculate such process control figures as MCRT or ratio F/M ratio. In most situations, increasing the MLSS produces an older, denser sludge, while decreasing MLSS produces a younger, less dense sludge. Note: Control of the sludge wasting rate by constant MLVSS concentration involves maintaining a certain concentration of volatile suspended solids in the aeration tank. Note: The activated sludge aeration tank should be observed daily. Included in this daily observation should be a determination of the type and amount of foam, mixing uniformity, and color. 18.10.11.6.2 Flow Test the flow of RAS daily. Test results are required to determine mass balance and for control of sludge blanket, MLSS, and MLVSS. For WAS, flow is sampled and tested whenever sludge is wasted. Results are required to determine mass balance and to control solids level in process.

Wastewater Treatment

18.10.12 PROCESS CONTROL ADJUSTMENTS In the routine performance of their duties, wastewater operators make process control adjustments to various unit processes, including the activated sludge process. In the following a summary is provided of the process controls available for the activated sludge process and the result that will occur from adjustment of each. 1. Process control: Return rate A. Condition: Return rate is too high. Results: 1. Hydraulic overloading of aeration and settling tanks. 2. Reduced aeration time. 3. Reduced settling time. 4. Loss of solids over time. B. Condition: Return rate is too low. Results: 1. Septic return. 2. Solids buildup in settling tank. 3. Reduced MLSS in aeration tank. 4. Loss of solids over weir. 2. Process control: Waste rate A. Condition: Waste rate is too high. Results: 1. Reduced MLSS. 2. Decreased sludge density. 3. Increased SVI. 4. Decreased MCRT. 5. Increased F/M Ratio. B. Condition: Waste rate is too low. Results: 1. Increased MLSS. 2. Increased sludge density. 3. Decreased SVI. 4. Increased MCRT. 5. Decreased F/M ratio. 3. Process control: Aeration rate A. Condition: Aeration rate is too high. Results: 1. Wasted energy. 2. Increased operating cost. 3. Rising solids. 4. Breakup of activated sludge. B. Condition: Aeration rate is too low. Results: 1. Septic aeration tank. 2. Poor performance. 3. Loss of nitrification. ÂŠ 2003 by CRC Press LLC

581

18.10.13 TROUBLESHOOTING OPERATIONAL PROBLEMS The most important dual function performed by the wastewater operator is the identification of process control problems and implementing the appropriate actions to correct the problems. In this section, typical aeration system operational problems are listed with their symptoms, causes, and the appropriate corrective actions required to restore the unit process to a normal or optimal performance level. 1. Symptom 1: The solids blanket is flowing over the effluent weir (classic bulking). Settleability test shows no settling. A. Cause: Organic overloading. Corrective action: Reduce organic loading. B. Cause: Low pH. Corrective action: Add alkalinity. C. Cause: Filamentous growth. Corrective action: Add nutrients. Add chlorine or peroxide to return. D. Cause: Nutrient deficiency. Corrective action: Add nutrients. E. Cause: Toxicity. Corrective action: Identify source. Implement pretreatment. F. Cause: Overaeration. Corrective action: Reduce aeration during low flow periods. 2. Symptom 2: Solids settled properly in settleability test, but large amounts of solids are lost over effluent weir. A. Cause: Billowing solids due to short-circuiting. Corrective action: Identify short circuiting cause and eliminate if possible. 3. Symptom 3: Large amounts of small pinhead sized solids are leaving the settling tank. A. Cause: Old sludge. Corrective action: Reduce sludge age (gradual change is best). Increase waste rate. B. Cause: Excessive turbulence. Corrective Action: Decrease turbulence (adjust aeration during low flows). 4. Symptom 4: Large amount of light floc (low BOD and high solids) leaving settling tank. A. Cause: Extremely old sludge. Corrective action: Reduce age. Increase waste. 5. Symptom 5: Large amounts of small translucent particles (1/16 to 1/8 in.) are leaving the settling tank. A. Cause: Rapid solids growth. Corrective action: Increase sludge age. B. Cause: Slightly young activated sludge. Corrective action: Decrease waste.

582

Handbook of Water and Wastewater Treatment Plant Operations

6. Symptom 6: Solids are settling properly, but rise to surface within a short time. Many small (1/4 in.) to large (several feet) clumps of solids on surface of settling tank. A. Cause: Denitrification. Corrective action: Increase rate of return. Adjust sludge age to eliminate nitrification. B. Cause: Overaeration. Corrective action: Reduce aeration. 7. Symptom 7: RAS has a rotten egg odor. A. Cause: Return is septic. Corrective action: Increase aeration rate B. Cause: Return rate is too low. Corrective action: Increase rate of return. 8. Symptom 8: Activated sludge organisms die during a short time. A. Cause: Influent contained toxic material. Corrective action: Isolate activated sludge (if possible). Return all available solids. Stop wasting. Increase return rate. Implement pretreatment program. 9. Symptom 9: Surface of aeration tank covered with thick, greasy foam. A. Cause: Extremely old activated sludge. Corrective action: Reduce activated sludge age. Increase wasting. Use foam control sprays. B. Cause: Excessive grease and oil in system. Corrective action: Improve grease removal. Use foam control sprays. Implement pretreatment program. C. Cause: Froth forming bacteria. Corrective action: Remove froth forming bacteria. 10. Symptom 10: Large clouds of billowing white foam on the surface of the aeration tank. A. Cause: Young activated sludge. Corrective action: Increase sludge age. Decrease wasting. Use foam control sprays. B. Cause: Low solids in aeration tank. Corrective action: Increase sludge age. Decrease wasting. Use foam control sprays. C. Cause: Surfactants (detergents). Corrective action: Eliminate surfactants. Use foam control sprays. Add antifoam.

18.10.14 PROCESS CONTROL CALCULATIONS As with other wastewater treatment unit processes, process control calculations are important tools used by the operator to optimize and control process operations. In this section we review the most frequently used activated sludge calculations. © 2003 by CRC Press LLC

18.10.14.1 Settled Sludge Volume SSV is the volume that a settled activated sludge occupies after a specified time. The settling time may be shown as a subscript (i.e., SSV60 indicates the reported value was determined at 60 min). SSV can be determined for any time interval; the most common values are the 30-min reading (SSV30) and 60-min reading (SSV60). The settled sludge volume can be reported as milliliters of sludge per liter of sample or as a percent of SSV. SVI (mL L ) =

SSV (mL L ) Sample Volume ( L )

(18.31)

Note: 1,000 mL = 1 L Sample Volume (l) =

Sample Volume (mL) (18.32) 1000 ml L

% Settled Sludge Volume = Settled Sludge Volum, MI ¥ 100 Sample Volume, MI

(18.33)

EXAMPLE 18.38 Problem: Using the information provided in the table, calculate the SSV30 and the % SSV60. Time

Milliliters

Start 15 min 30 min 45 min 60 min

2500 2250 1800 1700 1600

Solution: SSV30 =

1800 mL

% SSV60 =

2.5 L

= 720 mL L

1600 mL ¥ 100 2500 mL

= 64%

18.10.14.2 Estimated Return Rate There are many different methods available for estimation of the proper return sludge rate. A simple method described in the Operation of Wastewater Treatment Plants, Field Study Program (1986) — developed by the California State University, Sacramento — uses the %SSV60. This value can provide an approximation of the appropriate RAS rate. The results of this calculation can then be adjusted based upon sampling and visual observations to develop the optimum return sludge rate:

Wastewater Treatment

583

Note: The %SSV60 must be converted to a decimal percent and total flow rate (wastewater flow and current return rate in million gallons per day must be used). Estimated Return Rate (MGD) = [Influent Flow (MGD) + Curent Return Flow (MGD)] ¥ %SSV60

EXAMPLE 18.40 Problem: The SSV30 is 365 mL/L and the MLSS is 2365 mg/L. What is the SVI?

Solution: Sludge Volume Index =

This equation: 1. Assumes %SSV60 is representative. 2. Assumes return rate, in per cent equals %SSV60. 3. Actual return rate is normally set slightly higher to ensure organisms are returned to the aeration tank as quickly as possible. The rate of return must be adequately controlled to prevent the following: A. Aeration and settling hydraulic overloads. B. Low MLSS levels in the aerator. C. Organic overloading of aeration. D. Solids loss due to excessive sludge blanket depth. EXAMPLE 18.39

2365 mg L

= 154.3

In this example, SVI equals 154.3. What does this mean? It means is that the system is operating normally with good settling and low effluent turbidity. We know this because we compare the result with the parameters listed below to obtain the expected condition (the result). Expected Condition (Indications in Parentheses)

SVI Value Less than 100 100–200 Greater than 250

Problem: The influent flow rate is 4.2 MGD and the current return activated sludge flow rate is 1.5 MGD. The SSV60 is 38%. Based upon this information what should be the return sludge rate in million gallons per day?

365 MI L ¥ 1000

Old sludge — possible pin floc (effluent turbidity increasing) Normal operation — good Settling (low effluent turbidity) Bulking sludge — poor settling (high effluent turbidity)

The SVI is best used as a trend indicator to evaluate what is occurring compared to previous SVI values. Based upon this evaluation, the operator may determine if the SVI trend is increasing or decreasing (refer to the chart below).

Solution: Estimated Return Rate (MGD) = [ 4.2 MGD + 1.5 MGD] ¥ 0.38

SVI Value

Result

Adjustment

Increasing

Sludge is becoming less dense Sludge is either younger or bulking Sludge will settle more slowly Sludge will compact less Sludge is becoming denser Sludge is becoming older Sludge will settle more rapidly Sludge will compact more with no other process changes No changes indicated Sludge should continue to have Its current characteristics

Decrease waste Increase return rate

= 2.2 MGD

18.10.14.3 Sludge Volume Index

Decreasing

SVI is a measure of the settling quality (a quality indicator) of the activated sludge. As the SVI increases the sludge settles slower, does not compact as well, and is likely to result in an increase in effluent suspended solids. As the SVI decreases the sludge becomes denser, settling is more rapid, and the sludge becomes older. SVI is the volume in milliliters occupied by 1 g of activated sludge. SSV, (milliliters per liter) and the MLSS (milligrams per liter) are required for this calculation: Sludge Volume Index (SVI) = SSV, MI L ¥ 1000 MLSS mg L © 2003 by CRC Press LLC

(18.34)

Holding constant

Increase waste rate Decrease return rate

18.10.14.4 Waste Activated Sludge The quantity of solids removed from the process as WAS, is an important process control parameter that operators need to be familiar with. More importantly, operators must also know how to calculate it and can do so with the following equation:

584

Handbook of Water and Wastewater Treatment Plant Operations

Waste (lb d ) = WAS Concentration (mg L ) ¥ WAS Flow (MGD) ¥ 8.34 lb MG mg L

(18.35)

EXAMPLE 18.41 Problem: The operator wastes 0.44 MGD of activated sludge. The WAS has a solids concentration of 5540 mg/L. How many pounds of WAS are removed from the process?

Primary effluent flow Primary effluent BOD Primary effluent TSS Effluent flow Effluent BOD Effluent TSS

2.5 MGD 145 mg/L 165 mg/L 2.2 MGD 22 mg/L 16 mg/L

Aeration volume Settling volume MLSS MLVSS % Waste Volatile Desired F:M

0.65 MG 0.30 MG 3,650 mg/L 2,550 mg/L 71% 0.3

Solution: F M Ratio =

Solution:

145 mg L ¥ 2.2 MGD ¥ 8.34 lb mg L MG 2550 mg L ¥ 0.65 MG ¥ 8.34 lb mg L MG

= 0.19 lb BOD lb MLVSS

Waste (lb d ) = 5540 mg L ¥ 0.44 MGD ¥ 8.34 lb MG mg L = 20, 329.6 lb d

Note: If the MLVSS concentration is not available, it can be calculated if %VM of the MLSS is known (see Equation 18.37): MLVSS (mg L ) = MLSS ¥ %VM (decimal) (18.37)

18.10.14.5 Food to Microorganism Ratio (F:M Ratio) The F:M ratio is a process control calculation used in many activated sludge facilities to control the balance between available food materials (BOD or COD) and available organisms (MLVSS). The COD test is sometimes used, because the results are available in a relatively short period of time. To calculate the F:M ratio, the following information is required and Equation 18.46 is used:

Note: The F value in the F:M ratio for computing loading to an activated sludge process can be either BOD or COD. Remember that the reason for sludge production in the activated sludge process is to convert BOD to bacteria. One advantage of using COD over BOD for analysis of organic load is that COD is more accurate. EXAMPLE 18.43

1. 2. 3. 4.

Aeration Aeration Aeration Aeration

tank tank tank tank

influent flow rate (MGD) influent BOD or COD (mg/L) MLVSS (mg/L) volume (MG)

F M Ratio =

(18.36)

Prim. Eff. COD BOD mg L ¥ Flow MGD ¥ 8.34 lb mg L MG MLVSS mg L ¥ Aerator Volume, MG ¥ 8.34 lb mg L MG

Typical F/M ratio for activated sludge processes is shown in the chart below: Process

lb BOD/lb MLVSS

lb COD/lb MLVSS

0.2–0.4 0.2–0.6 0.05–0.15 0.05–0.15 0.25–1.0

0.5–1.0 0.5–1.0 0.2–0.5 0.2–0.5 0.5–2.0

Conventional Contact stabilization Extended aeration Oxidation ditch Pure oxygen

EXAMPLE 18.42 Problem: Given the following data, what is the F:M ratio?

Problem: The aeration tank contains 2985 mg/L of MLSS. Laboratory tests indicate the MLSS is 66% volatile matter. What is the MLVSS concentration in the aeration tank?

Solution: MLVSS ( mg L ) = 2985 mg L ¥ 0.66 = 1970 mg L

18.10.14.5.1 F:M Ratio Control Maintaining the F:M ratio within a specified range can be an excellent control method. Although the F:M ratio is affected by adjustment of the return rates, the most practical method for adjusting the ratio is through waste rate adjustments. Increasing the rate will decrease the MLVSS and increase the F:M ratio. Decreasing the waste rate will increase the MLVSS and decrease the F:M ratio 18.10.14.5.2 Establishing Desired F:M Levels The desired F:M ratio must be established on a plant-byplant basis. Comparison of F:M ratios with plant effluent quality is the primary means to identify the most effective range for individual plants, when the range of F:M values that produce the desired effluent quality is established.

Wastewater Treatment

585

18.10.14.5.3 Required MLVSS Quantity The pounds of MLVSS required in the aeration tank to achieve optimum F:M ratio can be determined from the average influent food (BOD or COD) and the desired F:M ratio: MLVSS ( lb ) =

(18.38)

Primary Effluent of BOD or COD ( mg L ) ¥ Q ( MGD) ¥ 8.34 lb gal Desired F M Ratio ( lb BOD or COD lb MLVSS)

The required pounds of MLVSS determined by this calculation can then be converted to a concentration value with the following equation: MLVSS (mg L ) =

(18.39)

Desired MLVSS (lb) Aeration Volume (MG) ¥ 8.34 lb mg L MG

If the desired MLVSS is greater than the actual MLVSS, wasting is stopped until the desired level is achieved. Practical considerations require that the required waste quantity be converted to a required volume to waste per day. This is accomplished by converting the waste pounds to flow rate in million gallons per day or gallons per minute. Waste, MGD =

(18.41)

Waste Volatile, lb d [Waste Volatile Concentration, mg L ¥ 8.34 lb gal] Note: When F:M ratio is used for process control, the volatile content of the waste activated sludge should be determined. EXAMPLE 18.45

EXAMPLE 18.44

Problem:

Given the following information, determine the required waste rate in gallons per minute to maintain an F:M ratio of 0.17 lb COD/lb MLVSS:

The aeration tank influent flow is 4.0 MGD, and the influent COD is 145 mg/L. The aeration tank volume is 0.65 MG. The desired F:M ratio is 0.3 lb COD/lb MLVSS.

1. How many pounds of MLVSS must be maintained in the aeration tank to achieve the desired F:M ratio? 2. What is the required concentration of MLVSS in the aeration tank?

Primary effluent COD Primary effluent flow MLVSS Aeration tank volume Waste volatile concentration

140 mg/L 2.2 MGD 3549 mg/L 0.75 MG 4440 mg/L (volatile solids)

Solution: Solution: 1. MLVSS (lb) =

145 mg L ¥ 4.0 MGD ¥ 8.34 lb gal 0.3 lb COD lb MLVSS

16, 124 MLVSS

Required MLVSS, lb

[0.65 MG ¥ 8.34 lb gal]

= 2.974 mg L MLVSS

18.10.14.5.4

Calculating Waste Rates Using F:M Ratio Maintaining the desired F:M ratio is accomplished by controlling the MLVSS level in the aeration tank. This may be accomplished by adjustment of return rates, but the most practical method is by proper control of the waste rate: Waste Volume of Solids (lb d ) = Actual MLVSS (lb) - Desired MLVSS (lb) © 2003 by CRC Press LLC

= 3.549 mg L ¥ 0.75 MG ¥ 8.34 lb gal = 22, 199 lb

= 16, 124 lb MLVSS MLVSS, mg L =

Actual MLVSS (lb)

140 mg L ¥ 2.2 MGD ¥ 8.34 lb gal 0.17 lb COD lb MLVSS

= 15.110 lb MLVSS Waste, lb d = 22, 199 lb - 15, 110 lb = 7, 089 lb Waste, MGD =

[4440

Waste, gpm =

7, 089 lb d

mg L ¥ 8.34 lb gal]

= 0.19 MGD

0.19 MGD ¥ 1, 000, 000 gpd MGD

(18.40) = 132 gpm

1440 min d

586

Handbook of Water and Wastewater Treatment Plant Operations

18.10.14.6 Mean Cell Residence Time (MCRT)

Note: You should remember the following important process control parameters:

MCRT (sometimes called SRT) is a process control calculation used for activated sludge systems. The MCRT calculation illustrated in Example 18.46 uses the entire volume of the activated sludge system (aeration and settling). Equation 18.52 is used to calculate the MCRT: MCRT ( d ) =

1. To increase F:M, decrease MCRT. 2. To increase MCRT, decrease waste rate. 3. MCRT is increased, MLTSS and 30-min setting increases. 4. Return sludge rate has no impact on MCRT. 5. MCRT has no impact on F:M change when the number of aeration tanks in service is reduced.

(18.42)

MLSS ( mg L ) ¥ [ Aeration Volume ( MG ) + Clarifier Volume ( MG )] ¥ 8.34 lb mg L MG

[ WAS ( mg L ) ¥ WAS Flow (MGD) ¥ 8.34

[TSSout (mg

lb mg L MG ] +

L ) ¥ Q ( MGD) ¥ 8.34 lb mg L MG

]

18.10.14.6.2

Typical MCRT Values

The following chart lists the various aeration process modifications and associated MCRT values.

Note: MCRT can be calculated using only the aeration tank solids inventory. When comparing plant operational levels to reference materials, it is important to determine which calculation the reference manual uses to obtain its example values. Other methods are available to determine the clarifier solids concentration. The simplest method assumes that the average suspended solids concentration is equal to the aeration tank’s solids concentration.

Process Conventional Step aeration Contact stabilization (contact) Extended aeration Oxidation ditch Pure oxygen

18.10.14.6.3

Problem: Given the following data, what is the MCRT? 4.2 MGD 135 mg/L 150 mg/L 4.2 MGD 22 mg/L 10 mg/L

Aeration volume Settling volume MLSS Waste rate Waste concentration Desired MCRT

1.20 MG 0.60 MG 3,350 mg/L 0.080 MGD 6100 mg/L 8.5 d

Control Values for MCRT

18.10.14.6.4

Waste Quantities and Requirements

MCRT for process control requires the determination of the optimum range for MCRT values. This is accomplished by comparison of the effluent quality with MCRT values. When the optimum MCRT is established, the quantity of solids to be removed (wasted) is determined by

( )

MCRT =

5–15 5–15 5–15 20–30 20–30 8–20

Control values for the MCRT are normally established based on effluent quality. Once the MCRT range required to produce the desired effluent quality is established, it can be used to determine the waste rate required to maintain it.

EXAMPLE 18.46

Influent flow Influent BOD Influent TSS Effluent flow Effluent BOD Effluent TSS

MCRT (d)

Waste Quantity lb d =

3350 mg L ¥ [1.2 MGD + 0.6 MG] ¥ 8.34 lb mg L MG [6100 mg L ¥ 0.08 ¥ 8.34 lb mg L MG] +

[10

mg L ¥ 4.2 MGD ¥ 8.34 lb mg L MG]

(

[

( )] ¥

( )

8.34 lb mg L MG

(18.43)

()

Desired MCRT d

= 11.4 d

18.10.14.6.1

)

MLSS mg L ¥ Aeration Volume MG + Clarifier Volume MG

Mean Cell Residence Time Control

Because it provides an accurate evaluation of the process condition and takes all aspects of the solids inventory into account, the MCRT is an excellent process control tool. Increases in the waste rate will decrease the MCRT, as will large losses of solids over the effluent weir. Reductions in waste rate will result in increased MCRT values. © 2003 by CRC Press LLC

[

TSS out

(mg L) ¥ Q (MGD) ¥ 8.34 lb mg

L MG

]

EXAMPLE 18.47 Problem: Given the following data, determine the waste rate to maintain an MCRT of 8.6 d:

Wastewater Treatment

587

MLSS Aeration volume Clarifier volume Effluent TSS Effluent flow

Waste (MGD) =

3400 mg/L 1.4 MG 0.5 MG 10 mg/L 5.0 MGD

Waste (gal min ) =

Waste Quantity (lb d ) = 5848 lb 8.6 d

mg L ¥ 5.0 MGD ¥ 8.34 lb mg L MG]

18.10.14.6.4.1

Waste Rate in Million Gallons/Day

When the quantity of solids to be removed from the system is known, the desired waste rate in million gallons per day can be determined. The unit used to express the rate (MGD, gal/d, and gal/min) is a function of the volume of waste to be removed and the design of the equipment. Waste (MGD) = Waste Pounds d WAS Concentrations, mg L ¥ 8.34

(18.44)

Waste (gal min) = Waste (MGD) ¥ 1, 000, 000 gal d MGD 1440 min d

(18.45)

EXAMPLE 18.48 Problem: Given the following data, determine the required waste rate to maintain an MCRT of 8.8 d: MLSS Aeration volume Clarifier volume Effluent TSS Effluent flow Waste concentrations

2500 mg/L 1.2 MG 0.2 MG 11 mg/L 5.0 MGD 6000 mg/L

(18.46) Solution:

1440 min d

2500 mg L ¥ [1.2 MG + 0.2 MG ] ¥ 8.34 lb mg L MG 8.8 d mg L ¥ 5.0 MGD ¥ 8.34 lb mg L MG ]

= 3317 lb d - 459 lb d = 2858 lb d

Mass balance is based upon the fact that solids and BOD are not lost in the treatment system. In simple terms, the mass balance concept states that what comes in must equal waste that goes out. The concept can be used to verify operational control levels and determine if potential problems exist within the plant’s process control monitoring program. Note: If influent values and effluent values do not correlate within 10 to 15%, it usually indicates either a sampling or testing error or a process control discrepancy. Mass balance procedures for evaluating the operation of a settling tank and a biological process are described in this section. Operators should recognize that although the procedures are discussed in reference to the activated sludge process, the concepts can be applied to any settling or biological process. 18.10.14.7.1

Mass Balance: Settling Tank Suspended Solids

The settling tank mass balance calculation assumes that no suspended solids are produced in the settling tank. Any settling tank operation can be evaluated by comparing the solids entering the unit with the solids leaving the tank as effluent suspended solids or as sludge solids (see Figure 18.11). If sampling and testing are accurate and representative, and process control and operation are appropriate, the quantity of suspended solids entering the settling tank should equal (±10%) the quantity of suspended solids leaving the settling tanks as sludge, scum, and effluent total suspended solids. Note: In most instances, the amount of suspended solids leaving the process as scum is so small that it is ignored in the calculation. 18.10.14.7.1.1

Waste Quantity ( lb d )

[10

0.57 MGD ¥ 1, 000, 000 gal d MGD

18.10.14.7 Mass Balance

3400 ( mg L ) ¥ [1.4 MG + 0.5 MG] ¥ 8.34 lb mg L MG

= 0.057 MGD

= 40 gal min

Solution:

[10

2858 lb d 6000 mg l ¥ 8.34 lb gal

Mass Balance Calculation

TSSin (lb d ) = TSSin ( mg L ) ¥ Q ( MGD) ¥ 8.34 lb mg L MG

TSSout (lb d ) = TSSout (mg L ) ¥ Q (MGD) ¥ 8.34 lb mg L MG

588

Handbook of Water and Wastewater Treatment Plant Operations

Sludge Solids (lb d ) =

Solution:

Sludge Pumped (gal) ¥ %Solids ¥ 8.34 lb mg L MG

= 2445 mg L ¥ 2.6 MGD ¥ 8.34 lb mg L MG

%Mass Balance =

[TSS

(lb) - (TSSout (lb) + Sludge Solids (lb))] ¥ 100 TSSin (lb)

18.10.14.7.1.2

Explanation of Results

1. The mass balance is ±15% or less — The process is considered to be in balance. Sludge removal should be adequate with the sludge blanket depth remaining stable. Sampling is considered to be producing representative samples that are being tested accurately. 2. The mass balance is greater than ±15% — Indicates that more solids are entering the settling tank than are being removed. Sludge blanket depth should be increasing, effluent solids may also be increasing, and effluent quality in decreasing. If changes described are not occurring, the mass balance may indicate that sample type, location, times, or procedures and/or testing procedures are not producing representative results. 3. If the mass balance is greater than 15%, it indicates that fewer solids are entering the settling tank than are being removed. Sludge blanket depth should be decreasing; sludge solids concentration may also be decreasing. This could adversely impact sludge treatment processes. If changes described are not occurring, the mass may indicate that sample type, location, times, or procedures and/or testing procedures are not producing representative results. EXAMPLE 18.49 Problem: Given the following data, determine the solids mass balance for the settling tank: Process Influent Effluent Return

Solids in (lb d )

Extended Aeration (No Primary) Flow TSS Flow TSS Flow TSS

2.6 MGD 2445 mg/L 2.6 MGD 17 mg/L 0.5 MGD 8470 mg/L

= 53, 017 lb d Solids out (lb d ) = 17 mg L ¥ 2.6 MGD ¥ 8.34 lb mg L MG = 369 lb d Sludge Solids out (lb d ) = 8470 mg L ¥ 0.5 MGD ¥ 8.34 lb mg L MG = 35, 320 lb d %Mass Balance =

[53, 017 lb d - (396 lb d + 35, 320 lb d)] ¥ 100 53, 017 lb d

= 32.7% The mass balance indicates:

1. The sampling point or collection procedure or laboratory procedure is producing inaccurate data upon which to make process control decisions. 2. More solids are entering the settling tank each day than are being removed. This should result in either: A. A solids buildup in the settling tank. B. A loss of solids over the effluent weir. Investigate further to determine the specific cause of the imbalance.

18.10.14.7.2 Mass Balance: Biological Process Solids are produced whenever biological processes are used to remove organic matter from wastewater (see Figure 18.11). Mass balance for an aerobic biological process must take into account both the solids removed by physical settling processes and the solids produced by biological conversion of soluble organic matter to insoluble suspended matter or organisms. Research has shown that the amount of solids produced per pound of BOD removed can be predicted based upon the type of process being used. Although the exact amount of solids produced can vary from plant to plant, research has developed a series of K factors that can be used to estimate the solids production for plants using a particular treatment process. These average factors provide a simple method to evaluate the effectiveness of a facility’s process control program.

Wastewater Treatment

589

current operating conditions, that waste rate must be to maintain the current solids level.

TABLE 18.7 Conversion Factors K Process

lb Solids/lb BOD Removed

Primary Activated sludge with primary Activated sludge without primary Conventional Step feed Extended aeration Oxidation ditch Contact stabilization Trickling filter Rotating Biological contactor

Waste Rate, MGD =

1.7 0.7

Solids Produced, lb d

(18.44)

(Waste Concentration ¥ 8.34 lb gal)

EXAMPLE 18.50 0.85 0.85 0.65 0.65 1.0 1.0 1.0

Problem: Given the following data, determine the mass balance of the biological process and the appropriate waste rate to maintain current operating conditions:

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

The mass balance also provides an excellent mechanism to evaluate the validity of process control and effluent monitoring data generated. Table 18.7 lists average K factors in pounds of solids produced per pound of BOD removed for selected processes.

Process Influent

Effluent

Waste

Extended Aeration (No Primary) Flow BOD TSS Flow BOD TSS Flow TSS

1.1 220 240 1.5 18 22 24,000 8710

MGD mg/L mg/L MGD mg/L mg/L gal/d mg/L

18.10.14.7.2.1 Conversion Factor

Conversion factors depend on the activated sludge modification involved. Factors generally range from 0.5 to 1.0 lb of solids/lb BOD removed (see Table 18.7). 18.10.14.7.2.2 Mass Balance Calculation

BOD out ( lb ) = BOD out ( mg L ) ¥ Q ( MGD) ¥ 8.34 lb mg L MG

]

Solids Produced ( lb d ) = BOD in ( lb ) - BOD out ( lb) ¥ K TSSout ( lb d ) = TSSout ( mg L ) ¥ Q ( MGD) ¥ 8.34 lb mg L MG Waste ( lb d ) = Waste ( mg L ) ¥ Q ( MGD) ¥ 8.34 lb mg L MG Solids Removed ( lb d ) = TSSout ( lb d ) + Waste ( lb d ) %Mass Balance =

8.34 lb mg L MG = 165 lb d BOD Removed (lb d ) = 2018 lb d - 165 lb d = 1853 lb d Solids Produced (lb d ) = 1853 lb d ¥ 0.65 lb lb BOD Solids Out (lb d ) = 22 mg L ¥ 1.1 MGD ¥ 8.34 lb mg L MG

(18.47)

18.10.14.7.2.3 Explanation of Results

If the mass balance is ±15%, the process sampling and testing and process control are within acceptable levels. If the balance is greater than15%, investigate further to determine if the discrepancy represents a process control problem or is the result of nonrepresentative sampling and inaccurate testing. 18.10.14.7.2.4 Sludge Waste Based upon Mass Balance

BODout (lb d ) = 18 mg L ¥ 1.1 MGD ¥

= 1204 lb d

[Solids Produced - Solids Removed] ¥ 100 Solids Produced

8.34 lb mg L MG = 2018 lb d

BOD in ( lb ) = BOD in ( mg L ) ¥ Q ( MGD) ¥ 8.34 lb mg L MG

[

BOD in (lb d ) = 220 mg L ¥ 1.1 MGD ¥

= 202 lb d Sludge Out (lb d ) = 8710 mg L ¥ 0.024 MGD ¥ 8.34 lb mg L MG = 1743 lb d Solids Removed (lb d ) = 292 lb d + 1743 lb d = 1945 lb d %Mass Balance =

[1204 lb d - 1945 lb d] ¥ 100

= 62%

1204 lb d

590

Handbook of Water and Wastewater Treatment Plant Operations

The mass balance indicates: 1. The sampling points, collection methods, and laboratory testing procedures are producing nonrepresentative results. 2. The process is removing significantly more solids than is required. Additional testing should be performed to isolate the specific cause of the imbalance. To assist in the evaluation, the waste rate based upon the mass balance information can be calculated: Waste (gal d ) =

Solids Produced (lb d )

Waste TSS ( mg L ) ¥ 8.34

Using this equation results in the following: Waste (gal d ) =

1204 (lb d ) ¥ 1, 000, 000 8710 mg L ¥ 8.34

= 16, 575 gal d

18.10.15 SOLIDS CONCENTRATION: SECONDARY CLARIFIER The solids concentration in the secondary clarifier can be assumed to be equal to the solids concentration in the aeration tank effluent. It may also be determined in the laboratory using a core sample taken from the secondary clarifier. The secondary clarifier solids concentration can be calculated as an average of the secondary effluent suspended solids and the RAS suspended solids concentration.

18.10.16 ACTIVATED SLUDGE PROCESS RECORD KEEPING REQUIREMENTS Wastewater operators soon learn that record keeping is a major requirement and responsibility of their jobs. Records are essential for process control, providing information on the cause of problems, providing information for making seasonal changes, and compliance with regulatory agencies. Records should include sampling and testing data, process control calculations, meter readings, process adjustments, operational problems and corrective actions taken, and process observations.

18.11 DISINFECTION OF WASTEWATER Like drinking water, liquid wastewater effluent is disinfected. Unlike drinking water, wastewater effluent is disinfected not to directly (direct end-of-pipe connection) protect a drinking water supply, but instead is treated to protect public health in general. This is particularly important when the secondary effluent is discharged into a body of water used for swimming or for a downstream water supply. © 2003 by CRC Press LLC

In the treatment of water for human consumption, treated water is typically chlorinated (although ozonation is also currently being applied in many cases). Chlorination is the preferred disinfection in potable water supplies because of chlorine’s unique ability to provide a residual. This chlorine residual is important because when treated water leaves the waterworks facility and enters the distribution system, the possibility of contamination is increased. The residual works to continuously disinfect water right up to the consumer’s tap. In this section, we discuss basic chlorination and dechlorination. In addition, we describe UV irradiation, ozonation, bromine chlorine, and no disinfection. Keep in mind that much of the chlorination material presented here is similar to the information presented in Chapter 17, Water Treatment Operations and Unit Processes.

18.11.1 CHLORINE DISINFECTION Chlorination for disinfection, as shown in Figure 18.1, follows all other steps in conventional wastewater treatment. The purpose of chlorination is to reduce the population of organisms in the wastewater to levels low enough to ensure that pathogenic organisms will not be present in sufficient quantities to cause disease when discharged. Note: Chlorine gas is heavier than air (vapor density of 2.5). Exhaust from a chlorinator room should be taken from floor level. Note: The safest action to take in the event of a major chlorine container leak is to call the fire department. Note: You might wonder why it is that chlorination of critical waters such as natural trout streams is not normal practice. This practice is strictly prohibited because chlorine and its by-products (i.e., chloramines) are extremely toxic to aquatic organisms. 18.11.1.1 Chlorination Terminology Remember that there are several terms used in discussion of disinfection by chlorination. Because it is important for the operator to be familiar with these terms, we repeat key terms again. Chlorine a strong oxidizing agent that has strong disinfecting capability. A yellow-green gas that is extremely corrosive and is toxic to humans in extremely low concentrations in air. Contact time the length of time the time the disinfecting agent and the wastewater remain in contact. Demand the chemical reactions that must be satisfied before a residual or excess chemical will appear.

Wastewater Treatment

Disinfection the selective destruction of disease-causing organisms. All the organisms are not destroyed during the process. This differentiates disinfection from sterilization, which is the destruction of all organisms. Dose the amount of chemical being added in milligrams per liter. Feed rate the amount of chemical being added in pounds per day. Residual the amount of disinfecting chemical remaining after the demand has been satisfied. Sterilization the removal of all living organisms. 18.11.1.2 Wastewater Chlorination: Facts and Process Description 18.11.1.2.1 Chlorine Facts 1. Elemental chlorine (Cl2 — gaseous) is a yellowgreen gas, 2.5 times heavier than air. 2. The most common use of chlorine in wastewater treatment is for disinfection. Other uses include odor control and activated sludge bulking control. Chlorination takes place prior to the discharge of the final effluent to the receiving waters (see Figure 18.1). 3. Chlorine may also be used for nitrogen removal through a process called breakpoint chlorination. For nitrogen removal, enough chlorine is added to the wastewater to convert all the ammonium nitrogen gas. To do this, approximately 10 mg/L of chlorine must be added for every 1 mg/L of ammonium nitrogen in the wastewater. 4. For disinfection, chlorine is fed manually or automatically into a chlorine contact tank or basin, where it contacts flowing wastewater for at least 30 min to destroy disease-causing microorganisms (pathogens) found in treated wastewater. 5. Chorine may be applied as a gas, a solid, or liquid hypochlorite form. 6. Chorine is a very reactive substance. It has the potential to react with many different chemicals (including ammonia), as well as with organic matter. When chlorine is added to wastewater, several reactions occur: A. Chlorine will react with any reducing agent (i.e., sulfide, nitrite, iron, and thiosulfate) present in wastewater. These reactions are known as chlorine demand. The chlorine used for these reactions is not available for disinfection. B. Chlorine also reacts with organic compounds and ammonia compounds to form chlororganics and chloramines. Chloramines are © 2003 by CRC Press LLC

591

part of the group of chlorine compounds that have disinfecting properties and show up as part of the chlorine residual test. C. After all of the chlorine demands are met, the addition of more chlorine will produce free residual chlorine. Producing free residual chlorine in wastewater requires very large additions of chlorine. 18.11.1.2.2 Hypochlorite Facts Although there are some minor hazards associated with its use (e.g., skin irritation, nose irritation, and burning eyes), hypochlorite is relatively safe to work with. It is normally available in dry form as a white powder, pellet or tablet, or liquid form. It can be added directly using a dry chemical feeder or it can be dissolved and fed as a solution. Note: In most wastewater treatment systems, disinfection is accomplished by means of combined residual. 18.11.1.2.3 Process Description Chlorine is a very reactive substance. Chlorine is added to wastewater to satisfy all chemical demands (i.e., to react with certain chemicals such as sulfide, sulfite, ferrous iron, etc.). When these initial chemical demands have been satisfied, chlorine will react with substances, such as ammonia, to produce chloramines and other substances that, although not as effective as chlorine, have disinfecting capability. This produces a combined residual, which can be measured using residual chlorine test methods. If additional chlorine is added, free residual chlorine can be produced. Due to the chemicals normally found in wastewater, chlorine residuals are normally combined rather than free residuals. Control of the disinfection process is normally based upon maintaining total residual chlorine (TRC) of at least 1.0 mg/L for a contact time of at least 30 min at design flow. Note: Residual level, contact time, and effluent quality affect disinfection. Failure to maintain the desired residual levels for the required contact time will result in lower efficiency and increased probability that disease organisms will be discharged. Based on water quality standards, total residual limitations on chlorine are: 1. Fresh water — Less than 11 ppb total residual chlorine. 2. Estuaries — Less than 7.5 ppb for halogen produced oxidants. 3. Endangered species — Use of chlorine is prohibited.

592

Handbook of Water and Wastewater Treatment Plant Operations

18.11.1.3 Chlorination Equipment 18.11.1.3.1 Hypochlorite Systems Depending on the form of hypochlorite selected for use, special equipment that controls the addition of hypochlorite to the wastewater is required. Liquid forms require the use of metering pumps, which can deliver varying flows of hypochlorite solution. Dry chemicals require the use of a feed system designed to provide variable doses of the form used. The tablet form of hypochlorite requires the use of a tablet chlorinator designed specifically to provide the desired dose of chlorine. The hypochlorite solution or dry feed systems dispenses the hypochlorite, which is then mixed with the flow. The treated wastewater then enters the contact tank to provide the required contact time. 18.11.1.3.2 Chlorine Systems Because of the potential hazards associated with the use of chlorine, the equipment requirements are significantly greater than those associated with hypochlorite use. The system most widely used is a solution feed system. In this system, chlorine is removed from the container at a flow rate controlled by a variable orifice. Water moving through the chlorine injector creates a vacuum that draws the chlorine gas to the injector and mixes it with the water. The chlorine gas reacts with the water to form hypochlorous and hydrochloric acid. The solution is then piped to the chlorine contact tank and dispersed into the wastewater through a diffuser. Larger facilities may withdraw the liquid form of chlorine and use evaporators (heaters) to convert to the gas form. Small facilities will normally draw the gas form of chlorine from the cylinder. As gas is withdrawn liquid will be converted to the gas form. This requires heat energy and may result in chlorine line freeze-up if the withdrawal rate exceeds the available energy levels. 18.11.1.4 Chlorination: Operation In both the hypochlorite and chlorine systems normal operation requires adjustment of feed rates to ensure the required residual levels are maintained. This normally requires chlorine residual testing and adjustment based upon the results of the test. Other activities include the removal of accumulated solids from the contact tank, collection of bacteriological samples to evaluate process performance, and maintenance of safety equipment (respirator air pack, safety lines, etc.). Hypochlorite operation may also include makeup solution (solution feed systems), adding powder or pellets to the dry chemical feeder or tablets to the tablet chlorinator. Chlorine operations include the adjustment of chlorinator feed rates, inspection of mechanical equipment, testing for leaks using an ammonia swab (white smoke indicates leaks), changing containers (requires more than one person for safety), and adjusting the injector water feed rate when required. © 2003 by CRC Press LLC

Chlorination requires routine testing of plant effluent for TRC and may also require the collection and analysis of samples to determine the fecal coliform concentration in the effluent. 18.11.1.5 Troubleshooting Operational Problems Operational problems with the plant’s disinfection process occasionally develop. The wastewater operator must not only be able to recognize these problems, but also correct them. For proper operation, the chlorination process requires routine observation, meter readings, process control and testing, and various process control calculations. Comparison of daily results with expected normal ranges is the key to identifying problems during the troubleshooting process and taking appropriate corrective actions (if required). In this section, we review normal operational and performance factors. We point out the various problems that can occur with the plant’s disinfection process, the causes, and the corrective actions that should be taken. 18.11.1.5.1 Operator Observations The operator should consider the following items: 1. Flow distribution — The operator monitors the flow to ensure that it is evenly distributed between all units in service, and that the flow through each individual unit is uniform, with no indication of short-circuiting. 2. Contact tank — The contact tanks or basins must be checked to ensure that no excessive accumulation of scum is on the surface, no indication of solids accumulation is on the bottom, and mixing appears to be adequate. 3. Chlorinator — The operator should check to ensure that there is no evidence of leakage, operating pressure or vacuum is within specified levels, current chlorine feed settling is within expected levels, in-line cylinders have sufficient chlorine to ensure continuous feed, and the exhaust system is operating as designed. 18.11.1.5.1.1

Factors Affecting Performance

Operators must be familiar with those factors that affect chlorination performance. Any item that interferes with the chlorine reactions or increases the demand for chlorine can affect performance and may produce nondisinfectant products. We discuss the main factors affecting chlorination performance below: 1. Effluent quality — Poor quality effluents have higher chlorine demands. In addition, high concentrations of solids prevent chlorine-organism

Wastewater Treatment

contact, and incomplete nitrification can cause extremely high chlorine demand. 2. Mixing — In order to be effective, chlorine must be in contact with the organisms. Poor mixing results in poor chlorine distribution. Installing of baffles and using a high length-towidth ratio will improve mixing and contact. 3. Contact time — The chlorine disinfection process is time dependent. As the contact time decreases, process effectiveness decreases. A minimum of 30 min of contact must be available at design flow. 4. Residual levels — The chlorine disinfection process is TRC dependent. The concentration of residual must be sufficient to ensure the desired reactions occur. At the design contact time, the required minimum TRC concentration is 1.0 mg/L. 18.11.1.5.1.2

Process Control Sampling and Testing

To ensure proper operation of the chlorination process, the operator must perform process control testing for the chlorination process. (Note: The process performance evaluation is based on the bacterial content (fecal coliform) of the final effluent.) Process control testing consists of performing a total chlorine residual test on chlorine contact effluent. The frequency of the testing is specified in the plant permit. The normal expected range of results is also specified in the plant permit. 18.11.1.5.1.3

593

Troubleshooting

The following sections present common operational problems, symptoms, casual factors, and corrective actions associated with chlorination system use in wastewater treatment. 1. Symptom 1: Coliform count fails to meet required standards for disinfection. A. Cause: Inadequate chlorination equipment capacity. Corrective action: Replace equipment as necessary to provide treatment based on maximum flow through the pipe. B. Cause: Inadequate chlorine residual control. Corrective action: Use chlorine residual analyzer to monitor and control chlorine dosage automatically. C. Cause: Short circuiting in chlorine contact chamber. Corrective action: Install baffling in the chlorine contact chamber. Install mixing device in chlorine contact chamber. D. Cause: Solids buildup in contact chamber. Corrective action: Clean contact chamber. © 2003 by CRC Press LLC

E. Cause: Chlorine residual is too low. Corrective action: Increase contact time or increase chlorine feed rate. Symptom 2: Low chlorine gas pressure at the chlorinator. A. Cause: Insufficient number of cylinders connected to the system. Corrective action: Connect enough cylinders to system so that feed rate does not exceed recommended withdrawal rate for cylinders. B. Cause: Stoppage or restriction of flow between cylinders and chlorinator. Corrective action: Disassemble chlorine header system at point where cooling begins, locate stoppage, and clean with solvent. Symptom 3: No chlorine gas pressure at the chlorinator. A. Cause: Chlorine cylinders empty or not connected to the system. Corrective action: Connect cylinders or replace empty cylinders. B. Cause: Plugged or damaged pressure reducing valve. Corrective action: Repair reducing valve after shutting cylinder valves and decreasing gas in the header system. Symptom 4: Chlorinator will not feed any chlorine. A. Cause: Pressure reducing valve in chlorinator is dirty. Corrective action: Disassemble chlorinator and clean valve stem and seat. Precede valve with filter or sediment trap. B. Cause: Chlorine cylinder is hotter than chlorine control apparatus (chlorinator). Corrective action: Reduce temperature in cylinder area; do not connect a new cylinder, which has been sitting in the sun. Symptom 5: Chlorine gas escaping from the chlorine pressure reducing valve (CPRV). A. Cause: Main diaphragm of CPRV has ruptured. Corrective action: Disassemble valve and diaphragm. Inspect chlorine supply system for moisture intrusion. Symptom 6: Inability to maintain chlorine feed rate without icing of chlorine system. A. Cause: Insufficient evaporator capacity. Corrective action: Reduce feed rate to 75% of evaporator capacity. If this eliminates problem, then main diaphragm of CPRV is ruptured. B. Cause: External CPRV cartridge is clogged. Corrective action: Flush and clean cartridge.

594

Handbook of Water and Wastewater Treatment Plant Operations

7. Symptom 7: Chlorinator system is unable to maintain sufficient water bath temperature to keep external CPRV open. A. Cause: Heating element malfunction. Corrective action: Remove and replace heating element. 8. Symptom 8: Inability to obtain maximum feed rate from chlorinator. A. Cause: Inadequate chlorine gas pressure. Corrective action: Increase pressure and replace empty or low cylinders. B. Cause: Water pump injector clogged with deposits. Corrective action: Clean injector parts using muriatic acid. Rinse parts with fresh water and place back in service. C. Cause: Leak in vacuum relief valve. Corrective action: Disassemble vacuum relief valve and replace all springs. D. Cause: Vacuum leak in joints, gaskets, tubing, etc. in chlorinator system. Corrective action: Repair all vacuum leaks by tightening joints, replacing gaskets, and replacing tubing and compression nuts. 9. Symptom 9: Inability to maintain adequate chlorine feed rate. A. Cause: Malfunction or deterioration of chlorine water supply pump. Corrective action: Overhaul pump (if turbine pump is used, try closing valve to maintain proper discharge pressure). 10. Symptom 10: Chlorine residual too high in plant effluent to meet requirements. A. Cause: Chlorine residual too high Corrective action: Install dechlorination facilities. 11. Symptom 11: Wide variation in chlorine residual produced in the effluent. A. Cause: Chlorine flow proportion meter capacity inadequate to meet plant flow rates. Corrective action: Replace with higher capacity chlorinator meter. B. Cause: Malfunctioning controls. Corrective action: Call manufacturer technical representative. C. Cause: Solids settled in chlorine contact chamber. Corrective action: Clean chlorine contact tank. D. Cause: Flow proportioning control device not zeroed or spanned correctly Corrective action: Rezero and span the device in accordance with manufacturerâ&#x20AC;&#x2122;s instructions. ÂŠ 2003 by CRC Press LLC

12. Symptom 12: Unable to obtain chlorine residual. A. Cause: High chemical demand. Corrective action: Locate and correct the source of the high demand. B. Cause: Test interference. Corrective action: Add sulfuric acid to samples to reduce interference. 13. Symptom 13: Chlorine residual analyzer, recorder, and controller does not control chlorine residual properly. A. Cause: Electrodes fouled. Corrective action: Clean electrodes. B. Cause: Loop time is too long. Corrective actions: Reduce control loop time by: 1. Moving the injector closer to the point of application. 2. Increasing the velocity in the sample line to the analyzer. 3. Moving the cell closer to the sample point. 4. Moving the sample point closer to the point of application. C. Cause: Insufficient potassium iodide being added for the amount of residual being measured. Corrective action: Adjust potassium iodide feed to correspond with the chlorine residual being measured. D. Cause: Buffer additive system is malfunctioning. Corrective action: Repair buffer additive system. E. Cause: Malfunctioning of analyzer cell. Corrective action: Call authorized service personnel to repair electrical components. F. Cause: Poor mixing of chlorine at point of application. Corrective action: Install mixing device to cause turbulence at point of application. G. Cause: Rotameter tube range is improperly set. Corrective action: Replace rotameter with a proper range of feed rate. 18.11.1.6 Dechlorination The purpose of dechlorination is to remove chlorine and reaction products (chloramines) before the treated wastestream is discharged into its receiving waters. Dechlorination follows chlorination, usually at the end of the contact tank to the final effluent. Sulfur dioxide gas, sodium sulfate, sodium metabisulfate, or sodium bisulfates are the chemicals used to dechlorinate. No matter which chemical is used to dechlorinate, its reaction with chlorine is instantaneous.

Wastewater Treatment

18.11.1.7 Chlorination Environmental Hazards and Safety Chlorine is an extremely toxic substance that can cause severe damage when released to the environment. For this reason, most state regulatory agencies have established a chlorine water quality standard (e.g., in Virginia, 0.011 mg/L in fresh waters for TRC and 0.0075 mg/L for chlorine produced oxidants in saline waters). Studies have indicated that quantities above these levels chlorine can reduce shellfish growth and destroy sensitive aquatic organisms. This standard has resulted in many treatment facilities being required to add an additional process to remove the chlorine prior to discharge. As mentioned, the process, known as dechlorination, uses chemicals that react quickly with chlorine to convert it to a less harmful form. Elemental chlorine is a chemical with potentially fatal hazards associated with it. For this reason many different state and federal agencies regulate the transport, storage, and use of chlorine. All operators required to work with chlorine should be trained in proper handling techniques. They should also be trained to ensure that all procedures for storage transport, handling and use of chlorine are in compliance with appropriate state and federal regulations. 18.11.1.8 Chlorine: Safe Work Practice Because of the inherent dangers involved with handling chlorine, each facility using chlorine (for any reason) should ensure that a written safe work practice is in place and is followed by plant operators. A sample safe work practice for handling chlorine is provided below. WORK: CHEMICAL HANDLING: CHLORINE Practice 1. Plant personnel must be trained and instructed on the use and handling of chlorine, chlorine equipment, chlorine emergency repair kits, and other chlorine emergency procedures. 2. Use extreme care and caution when handling chlorine. 3. Lift chlorine cylinders only with an approved and load-tested device. 4. Secure chlorine cylinders into position immediately. Never leave a cylinder suspended. 5. Avoid dropping chlorine cylinders. 6. Avoid banging chlorine cylinders into other objects. 7. Store chlorine 1-ton cylinders in a cool dry place away from direct sunlight or heating units. Railroad tank cars are direct sunlight compensated. 8. Store chlorine 1-ton cylinders on their sides only (horizontally). ÂŠ 2003 by CRC Press LLC

595

9. Do not stack unused or used chlorine cylinders. 10. Provide positive ventilation to the chlorine storage area and chlorinator room. 11. Always keep chlorine cylinders at ambient temperature. Never apply direct flame to a chlorine cylinder. 12. Use the oldest chlorine cylinder in stock first. 13. Always keep valve protection hoods in place until the chlorine cylinders are ready for connection. 14. Except to repair a leak, do not tamper with the fusible plugs on chlorine cylinders. 15. Wear a self-contained breathing apparatus (SCBA) whenever changing a chlorine cylinder and have at least one other person with a standby SCBA unit outside the immediate area. 16. Inspect all threads and surfaces of a chlorine cylinder, and have at least one other person with a standby SCBA unit outside the immediate area. 17. Use new lead gaskets each time a chlorine cylinder connection is made. 18. Use only the specified wrench to operate chlorine cylinder valves. 19. Open chlorine cylinder valves slowly (no more than one full turn). 20. Do not hammer, bang, or force chlorine cylinder valves under any circumstances. 21. Check for chlorine leaks as soon as the chlorine cylinder connection is made. Leaks are checked for by gently expelling ammonia mist from a plastic squeeze bottle filled with approximately 2 oz of liquid ammonia solution. Do not put liquid ammonia on valves or equipment. 22. Correct all minor chlorine leaks at the chlorine cylinder connection immediately. 23. Except for automatic systems, draw chlorine from only one manifolded chlorine cylinder at a time. Never simultaneously open two or more chlorine cylinders connected to a common manifold pulling liquid chlorine. Two or more cylinders connected to a common manifold pulling gaseous chlorine are acceptable. 24. Wear SCBA and chemical protective clothing covering face, arms, and hands before entering an enclosed chlorine area to investigate a chlorine odor or chlorine leak (two-person rule required). 25. Provide positive ventilation to a contaminated chlorine atmosphere before entering whenever possible. 26. Have at least two personnel present before entering a chlorine atmosphere. One person should enter the chlorine atmosphere, and the other should observe in the event of an emergency. Never enter a chlorine atmosphere unattended.

596

27.

28.

29.

30. 31.

32.

33.

34.

35. 36. 37.

38.

39.

40.

Handbook of Water and Wastewater Treatment Plant Operations

Remember that the Occupational Safety and Health Administration (OSHA) mandates that only fully qualified Level III hazardous material (HAZMAT) responders are authorized to aggressively attack a HAZMAT leak such as chlorine. Use supplied-air-breathing equipment when entering a chlorine atmosphere. Never use canister-type gas masks when entering a chlorine atmosphere. Ensure that all supplied air-breathing apparatuses have been properly maintained in accordance with the plant’s SCBA inspection guidelines as specified in the plant’s respiratory protection program. Stay upwind from all chlorine leak danger areas unless involved with making repairs. Look to plant windsocks for wind direction. Contact trained plant personnel to repair chlorine leaks. Roll uncontrollable leaking chlorine cylinders so that the chlorine escapes as a gas, not as a liquid. Stop leaking chlorine cylinders or leaking chlorine equipment (by closing off valves if possible) prior to attempting repair. Connect uncontrollable leaking chlorine cylinders to the chlorination equipment and feed the maximum chlorine feed rate possible. Keep leaking chlorine cylinders at the plant site. Chlorine cylinders received at the plant site must be inspected for leaks prior to taking delivery from the shipper. Never ship a leaking chlorine cylinder back to the supplier after it has been accepted (bill of lading has been signed by plant personnel) from the shipper. Instead, repair or stop the leak first. Keep moisture away from a chlorine leak. Never put water onto a chlorine leak. Call the fire department or rescue squad if a person is incapacitated by chlorine. Administer cardiopulmonary resuscitation (use barrier mask if possible) immediately to person who has been incapacitated by chlorine. Breathe shallow rather than deep if exposed to chlorine without the appropriate respiratory protection. Place a person who does not have difficulty breathing and is heavily contaminated with chlorine into a deluge shower. Remove their clothing under the water and flush all body parts that were exposed to chlorine. Flush eyes contaminated with chlorine with copious quantities of lukewarm running water for at least 15 min.

41. Drink milk if throat is irritated by chlorine. 42. Never store other materials in chlorine cylinder storage areas. Substances like acetylene and propane are not compatible with chlorine. 18.11.1.9 Chlorination Process Calculations There are several calculations that may be useful in operating a chlorination system. Many of these calculations are discussed and illustrated in this section. 18.11.1.9.1 Chlorine Demand Chlorine demand is the amount of chlorine in milligrams per liter that must be added to the wastewater to complete all of the chemical reactions that must occur prior to producing a residual: Chlorine Demand = Chlorine Dose (mg L ) Chlorine Residual (mg L )

(18.48)

EXAMPLE 18.51 Problem: The plant effluent currently requires a chlorine dose of 7.1 mg/L to produce the required 1.0 mg/L chlorine residual in the chlorine contact tank. What is the chlorine demand in milligrams per liter?

Solution: Chlorine Demand ( mg L ) = 7.1 mg L - 1.0 mg L = 6.1 mg L

18.11.1.9.2 Chlorine Feed Rate Chlorine feed rate is the amount of chlorine added to the wastewater in pounds per day: Chlorine Feed Rate = Dose (mg L ) ¥ Q (MGD) ¥ 8.34 lb mg L MG

(18.49)

EXAMPLE 18.52 Problem: The current chlorine dose is 5.55 mg/L. What is the feed rate in pounds per day if the flow is 22.89 MGD?

Solution: Chlorine Feed Rate (lb d ) = 5.55 mg L ¥ 22.89 MGD ¥ 8.34 lb mg L MG = 1060 lb d

Wastewater Treatment

597

18.11.1.9.3 Chlorine Dose Chlorine dose is the concentration of chlorine being added to the wastewater. It is expressed in milligrams per liter: Dose (mg L ) =

Chlorine Feed Rate (lb d ) (18.50) Q (MGD) ¥ 8.34 lb mg L MG

EXAMPLE 18.55 Problem: The laboratory reports that the chlorine dose required to maintain the desired residual level is 8.5 mg/L. Today’s flow rate is 3.25 MGD. The hypochlorite powder used for disinfection is 70% available chlorine. How many pounds of hypochlorite must be used?

EXAMPLE 18.53 Solution:

Problem: Three hundred twenty pounds of chlorine are added per day to a wastewater flow of 5.6 MGD. What is the chlorine does in milligrams per liter?

Solution:

Hypochlorite Quantity (lb d ) =

8.5 mg L ¥ 3.25 MGD ¥ 8.34 lb mg L MG 0.70

= 329 lb d

Dose ( mg L ) =

320 lb d 5.6 MGD ¥ 8.34 lb mg L MG

18.11.1.9.6 Required Quantity of Liquid Hypochlorite

= 6.9 mg L

18.11.1.9.4 Available Chlorine When hypochlorite forms of chlorine are used, the available chlorine is listed on the label. In these cases, the amount of chemical added must be converted to the actual amount of chlorine using the following calculation: Available Chlorine = Amount of Hypochlorite ¥ % Available Chlorine (18.51)

Use Equation 18.65 to calculate the required quantity of liquid hypochlorite: Hypochlorite Quantity (gal d ) = Required Chlorine Dose (mg L ) ¥ Q (MGD) ¥ 8.34 lb mg L MG %Available Chlorine ¥ 8.34 lb gal ¥

(18.53)

Hypochlorite Solution Specific Gravity

EXAMPLE 18.54 EXAMPLE 18.56 Problem: The calcium hypochlorite used for chlorination contains 62.5% available chlorine. How many pounds of chlorine are added to the plant effluent if the current feed rate is 30 lb of calcium hypochlorite per day?

Problem: The chlorine dose is 8.8 mg/L and the flow rate is 3.28 MGD. The hypochlorite solution is 71% available chlorine and has a specific gravity of 1.25. How many pounds of hypochlorite must be used?

Solution: Hypochlorite Quantity (gal d )

Available Chlorine = 30 lb ¥ 0.625 = 18.75 lb Chlorine

18.11.1.9.5 Required Quantity of Dry Hypochlorite Use Equation 18.64 to determine the amount of hypochlorite needed to achieve the desired dose of chlorine: Hypochlorite Quantity ( lb d ) =

(18.52)

Required Chlorine Dose ( mg L ) ¥ Q ( MGD) ¥ 8.34 lg mg L MG %Available Chlorine

8.8 mg L ¥ 3.28 MGD ¥ 8.34 lb mg L MG 0.71 ¥ 8.34 lb gal ¥ 1.25

= 32.5 gal d

18.11.1.9.7 Chlorine Ordering Because disinfection must be continuous, the supply of chlorine must never be allowed to run out. The following calculation provides a simple method for determining when additional supplies must be ordered. The process consists of three steps:

598

Handbook of Water and Wastewater Treatment Plant Operations

1. Adjust the flow and use variations if projected changes are provided. 2. If an increase in flow or required dosage is projected, the current flow rate or dose must be adjusted to reflect the projected change. 3. Use the following equation: Projected Flow = Current Flow (MGD) ¥

[1.0 + %Change] Projected Dose = Current Dose (mg L ) ¥

(18.54)

[1.0 + %Change] EXAMPLE 18.57 Problem: Based on available information for the past 12 months, the operator projects that the effluent flow rate will increase by 7.5% during the next year. If the average daily flow has been 4.5 MGD, what will be the projected flow for the next 12 months?

Solution: Projected Flow (MGD) = 4.5 MGD ¥ [1.0 + 0.075] = 4.84 MGD

EXAMPLE 18.58 Problem: The plant currently uses 90 lb of chlorine/d. The town wishes to order enough chlorine to supply the plant for 3 months (assume 31 d/month). How many pounds of chlorine should be ordered to provide the needed supply?

Solution: Determine the amount of chlorine required for a given period: Chlorine Required = Feed Rate (lb d ) ¥ Number of Days Required = 90 lb d ¥ 124 d = 11, 160 lb

Note: In some instances, projections for flow or dose changes are not available, but the plant operator may wish to include an extra amount of chlorine as a safety factor. This safety factor can be stated as a specific quantity or as a percentage © 2003 by CRC Press LLC

of the projected usage. Safety factor as a specific quantity can be expressed as follows: Total Required Cl2 = Chlorine Required ( lb) + Safety Factor

Note: Chlorine is only shipped in full containers. Unless you specifically ask for the amount of chlorine actually required or used during a specified period, all decimal parts of a cylinder are rounded up to the next highest number of full cylinders.

18.11.2 UV IRRADIATION Although ultraviolet disinfection was recognized as a method for achieving disinfection in the late nineteenth century, its application virtually disappeared with the evolution of chlorination technologies. However, in recent years, there has been resurgence in its use in the wastewater field, largely as a consequence of concern for discharge of toxic chlorine residual. Even more recently, UV has gained more attention because of the tough new regulations on chlorine use imposed by both OSHA and EPA. Because of this relatively recent increased regulatory pressure, many facilities are actively engaged in substituting chlorine for other disinfection alternatives. UV technology has made many improvements, making UV attractive as a disinfection alternative. UV light has very good germicidal qualities and is very effective in destroying microorganisms. It is used in hospitals, biological testing facilities, and many other similar locations. In wastewater treatment, the plant effluent is exposed to ultraviolet light of a specified wavelength and intensity for a specified contact period. The effectiveness of the process is dependent upon 1. UV light intensity 2. Contact time 3. Wastewater quality (turbidity) The Achilles’ heel of UV for disinfecting wastewater is turbidity. If the wastewater quality is poor, the ultraviolet light will be unable to penetrate the solids and the effectiveness of the process decreases dramatically. For this reason, many states limit the use of UV disinfection to facilities that can reasonably be expected to produce an effluent containing less than or equal to 30 mg/L of BOD and TSS. In the operation of UV systems, UV lamps must be readily available when replacements are required. The best lamps are those with a stated operating life of at least 7500 h that do not produce significant amounts of ozone or hydrogen peroxide. The lamps must also meet technical

Wastewater Treatment

specifications for intensity, output, and arc length. If the UV light tubes are submerged in the wastestream, they must be protected inside quartz tubes. These tubes not only protect the lights, but also make cleaning and replacement easier. Contact tanks must be used with UV disinfection. They must be designed with the banks of UV lights in a horizontal position that is either parallel or perpendicular to the flow or with banks of lights placed in a vertical position perpendicular to the flow. Note: The contact tank must provide a minimum of 10 sec of exposure time. We stated earlier that turbidity problems have been the main hinderance with using UV in wastewater treatment. If turbidity is UV’s Achilles’ heel, then the need for increased maintenance (as compared to other disinfection alternatives) is the toe of the same foot. UV maintenance requires that the tubes be cleaned on a regular basis or as needed. In addition, periodic acid washing is also required to remove chemical buildup. In operating UV disinfection systems, routine monitoring is required. Monitoring to check on bulb burnout, buildup of solids on quartz tubes, and UV light intensity is required. Note: UV light is extremely hazardous to the eyes. Never enter an area where UV lights are in operation without proper eye protection. Never look directly into the UV light.

18.11.3 OZONATION Ozone is a strong oxidizing gas that reacts with most organic and many inorganic molecules. It is produced when oxygen molecules separate, collide with other oxygen atoms, and form a molecule consisting of three oxygen atoms. For high-quality effluents, ozone is a very effective disinfectant. Current regulations for domestic treatment systems limit use of ozonation to filtered effluents unless the system’s effectiveness can be demonstrated prior to installation. Note: Effluent quality is the key performance factor for ozonation. For ozonation of wastewater, the facility must have the capability to generate pure oxygen along with an ozone generator. A contact tank with greater than or equal to 10-min contact time at design average daily flow is required. Off-gas monitoring for process control is also required. In addition, safety equipment capable of monitoring ozone in the atmosphere and a ventilation system capable of preventing ozone levels exceeding 0.1 ppm is required. The actual operation of the ozonation process consists of monitoring and adjusting the ozone generator and mon© 2003 by CRC Press LLC

599

itoring the control system to maintain the required ozone concentration in the off-gas. The process must also be evaluated periodically using biological testing to assess its effectiveness. Note: Ozone is an extremely toxic substance. Concentrations in air should not exceed 0.1 ppm. It also has the potential to create an explosive atmosphere. Sufficient ventilation and purging capabilities should be provided. Ozone has certain advantages over chlorine for disinfection of wastewater: (1) it increases DO in the effluent, (2) it has a briefer contact time, (3) it has no undesirable effects on marine organisms, and (4) it decreases turbidity and odor.

18.11.4 BROMINE CHLORIDE Bromine chloride is a mixture of bromine and chlorine. It forms hydrocarbons and hydrochloric acid when mixed with water. Bromine chloride is an excellent disinfectant that reacts quickly and normally does not produce any long-term residuals. Note: Bromine chloride is an extremely corrosive compound in the presence of low concentrations of moisture. The reactions occurring when bromine chloride is added to the wastewater are similar to those occurring when chlorine is added. The major difference is the production of bromamine compounds rather than chloramines. The bromamine compounds are excellent disinfectants, but are less stable and dissipate quickly. In most cases, the bromamines decay into other, less toxic compounds rapidly and are undetectable in the plant effluent. The factors that affect performance are similar to those affecting the performance of the chlorine disinfection process. Such factors as effluent quality and contact time have a direct impact on the performance of the process.

18.11.5 NO DISINFECTION In a very limited number of cases, treated wastewater discharges without disinfection is permitted. These are approved on a case-by-case basis. Each request must be evaluated based upon the point of discharge, the quality of the discharge, the potential for human contact, and many other factors.

18.12 ADVANCED WASTEWATER TREATMENT Advanced wastewater treatment is defined as the methods and processes that remove more contaminants (suspended and dissolved substances) from wastewater than are taken

600

Handbook of Water and Wastewater Treatment Plant Operations

out by conventional biological treatment. In other words, advanced wastewater treatment is the application of a process or system that follows secondary treatment or that includes phosphorus removal or nitrification in conventional secondary treatment. Advanced wastewater treatment is used to augment conventional secondary treatment because secondary treatment typically removes only between 85 and 95% of the BOD and TSS in raw sanitary sewage. Generally, this leaves 30 mg/L or less of BOD and TSS in the secondary effluent. To meet stringent water-quality standards, this level of BOD and TSS in secondary effluent may not prevent violation of water-quality standards â&#x20AC;&#x201D; the plant may not make permit. Thus, advanced wastewater treatment is often used to remove additional pollutants from treated wastewater. In addition to meeting or exceeding the requirements of water-quality standards, treatment facilities use advanced wastewater treatment for other reasons as well. For example, conventional secondary wastewater treatment is sometimes not sufficient to protect the aquatic environment. This is the case when periodic flow events occur in a stream; the stream may not provide the amount of dilution of effluent needed to maintain the necessary DO levels for aquatic organism survival. Secondary treatment has other limitations. It does not significantly reduce the effluent concentration of nitrogen and phosphorus (important plant nutrients) in sewage. If discharged into lakes, these nutrients contribute to algal blooms and accelerated eutrophication (lake aging). Also, the nitrogen in the sewage effluent may be present mostly in the form of ammonia compounds. If in high enough concentration, ammonia compounds are toxic to aquatic organisms. Yet another problem with these compounds is that they exert a nitrogenous oxygen demand in the receiving water as they convert to nitrates. This process is called nitrification. Note: The term tertiary treatment is commonly used as a synonym for advanced wastewater treatment. These two terms do not have precisely the same meaning. Tertiary suggests a third step that is applied after primary and secondary treatment. Advanced wastewater treatment can remove more than 99% of the pollutants from raw sewage and can produce an effluent of almost potable (drinking) water quality. However, advanced treatment is not free. The cost of advanced treatment for operation and maintenance as well as for retrofit of present conventional processes is very high (sometimes doubling the cost of secondary treatment). A plan to install advanced treatment technology calls for careful study; the benefit-to-cost ratio is not always big enough to justify the additional expense. ÂŠ 2003 by CRC Press LLC

Even considering the expense, application of some form of advanced treatment is not uncommon. These treatment processes can be physical, chemical, or biological. The specific process used is based upon the purpose of the treatment and the quality of the effluent desired.

18.12.1 CHEMICAL TREATMENT The purpose of chemical treatment is to remove: 1. 2. 3. 4. 5.

BOD TSS Phosphorus Heavy metals Other substances that can be chemically converted to a settleable solid

Chemical treatment is often accomplished as an addon to existing treatment systems or by means of separate facilities specifically designed for chemical addition. In each case, the basic process necessary to achieve the desired results remains the same: 1. Chemicals are thoroughly mixed with the wastewater. 2. The chemical reactions that occur form solids (coagulation). 3. The solids are mixed to increase particle size (flocculation). 4. Settling and filtration (separation) remove the solids. The specific chemical used depends on the pollutant to be removed and the characteristics of the wastewater. Chemicals may include the following: 1. 2. 3. 4. 5. 6.

Lime Alum (aluminum sulfate) Aluminum salts Ferric or ferrous salts Polymers Bioadditives

18.12.1.1 Operation, Observation, and Troubleshooting Procedures Operation and observation of performance of chemical treatment processes are dependent on the pollutant being removed and process design. Operational problems associated with chemical treatment processes used in advanced treatment usually revolve around problems with floc formation, settling characteristics, removal in the settling tank, and sludge (in settling tank) turning anaerobic.

Wastewater Treatment

To correct these problems, the operator must be able to recognize the applicable problem indicators through proper observation. Below we list common indicators and observations of operational problems, along with the applicable causal factors and corrective actions: 1. Poor floc formation and settling characteristics. A. Causal factors: 1. Insufficient chemical dispersal during rapid mix. 2. Excessive detention time in rapid mix. 3. Improper coagulant dosage. 4. Excessive flocculator speed. B. Corrective actions (where applicable): 1. Increase speed of rapid mixer. 2. Reduce detention time to15â&#x20AC;&#x201C;60 sec. 3. Correct dosage (determine by jar testing). 4. Reduce flocculator speed. 2. Good floc formation, poor removal in settling tank. A. Causal factors: 1. Excessive velocity between flocculation and settling. 2. Settling tank operational problem. B. Corrective action: 1. Reduce velocity to acceptable range 3. Settling tank sludge is turning anaerobic. A. Causal factors: 1. A sludge blanket has developed in settling tank. 2. Excessive organic carryover from secondary treatment. B. Corrective actions: 1. Increase sludge withdrawal to eliminate blanket. 2. Correct secondary treatment operational problems.

18.12.2 MICROSCREENING Microscreening (also called microstraining) is an advanced treatment process used to reduce suspended solids. The microscreens are composed of specially woven steel wire fabric mounted around the perimeter of a large revolving drum. The steel wire cloth acts as a fine screen, with openings as small as 20 mm (or millionths of a meter) that are small enough to remove microscopic organisms and debris. The rotating drum is partially submerged in the secondary effluent, which must flow into the drum then outward through the microscreen. As the drum rotates, captured solids are carried to the top where a high-velocity water spray flushes them into a hopper or backwash tray mounted on the hollow axle of the drum. Backwash solids are recycled to plant influent for treatment. These units ÂŠ 2003 by CRC Press LLC

601

have found greatest application in treatment of industrial waters and final polishing filtration of wastewater effluents. Expected performance for suspended solids removal is 95 to 99%, but the typical suspended solids removal achieved with these units is about 55%. The normal range is from 10 to 80%. According to Metcalf & Eddy2, the functional design of the microscreen unit involves the following considerations: 1. The characterization of the suspended solids with respect to the concentration and degree of flocculation 2. The selection of unit design parameter values that will not only ensure capacity to meet maximum hydraulic loadings with critical solids characteristics, but also provide desired design performance over the expected range of hydraulic and solids loadings 3. The provision of backwash and cleaning facilities to maintain the capacity of the screen. 18.12.2.1 Operation, Observation, and Troubleshooting Procedures Microscreen operators typically perform sampling and testing on influent and effluent TSS and monitor screen operation to ensure proper operation. Operational problems generally consist of gradual decrease in throughput rate, leakage at ends of the drum, reduced screen capacity, hot or noisy drive systems, erratic drum rotation, and sudden increases in effluent solids: 1. Decrease in throughput rate (from slime growth). A. Causal factors: 1. Inadequate cleaning. 2. Spray nozzles plugged. B. Corrective actions (where applicable): 1. Increase backwash pressure (60 to 120 psi). 2. Add hypochlorite upstream of the unit. 3. Unclog nozzles. 2. Decreased performance from leakage at ends of the drum. A. Causal factor: 1. Defective or leaking units. B. Corrective actions: 1. Tighten tension on sealing bands. 2. Replace sealing bands if excessive tension is required. 3. Screen capacity is reduced after shutdown period. A. Causal factor: 1. Screen is fouled. B. Corrective actions: 1. Clean screen prior to shutdown. 2. Clean screen with hypochlorite.

602

Handbook of Water and Wastewater Treatment Plant Operations

4. Drive System is running hot or noisy. A. Causal factor: 1. Inadequate lubrication. B. Corrective action: 1. Fill to specified level with recommended oil. 5. Erratic drum rotation. A. Causal factors: 1. Improper drive belt adjustment. 2. Drive belts are worn out. B. Corrective actions: 1. Adjust tension to specified level. 2. Replace drive belts. 6. Sudden increase in effluent solids. A. Causal factors: 1. Hole in screen fabric. 2. Screws that secure fabric are loose. 3. Solids collection trough is overflowing. B. Corrective actions (where applicable): 1. Repair fabric. 2. Tighten screws. 3. Reduce microscreen influent flow rate. 7. Decreased screen capacity after high-pressure washing. A. Causal factor: 1. Iron or manganese oxide film on fabric. B. Corrective action: 1. Clean screen with inhibited acid cleaner. Follow manufacturerâ&#x20AC;&#x2122;s instruction.

18.12.3 FILTRATION The purpose of filtration processes used in advanced treatment is to remove suspended solids. The specific operations associated with a filtration system are dependent on the equipment used. A general description of the process follows. 18.12.3.1 Filtration Process Description Wastewater flows to a filter (gravity or pressurized). The filter contains single, dual, or multimedia. Wastewater flows through the media, which removes solids. The solids remain in the filter. Backwashing the filter as needed removes trapped solids. Backwash solids are returned to the plant for treatment. Processes typically remove 95 to 99% of the suspended matter. 18.12.3.2 Operation, Observation, and Troubleshooting Procedures Operators routinely monitor filter operation to ensure optimum performance and to detect operational problems based on indication or observation of equipment malfunction or process suboptimal performance. We discuss operational problems typically encountered in the list that follows:

ÂŠ 2003 by CRC Press LLC

1. High effluent turbidity. A. Causal factors: 1. Filter requires backwashing. 2. Prior chemical treatment inadequate. B. Corrective actions (where applicable): 1. Backwash unit as soon as possible. 2. Adjust/control chemical dosage properly. 2. High head loss through the filter. A. Causal factor: 1. Filter requires backwashing. B. Corrective action: 1. Backwash unit as soon as possible. 3. High head loss through unit right after backwashing. A. Causal factors: 1. Backwash cycle was insufficient. 2. Surface scour or wash arm inoperative. B. Corrective actions (where applicable): 1. Increase backwash time. 2. Repair air scour or surface scrubbing arm. 4. Backwash water requirement exceeds 5%. A. Causal factors: 1. Excessive solids in filter influent. 2. Excessive filter aid dosage. 3. Surface washing or air scour not operating. 4. Surface washing or air scour not operated long enough during backwash cycle. 5. Excessive backwash cycle used. B. Corrective actions (where applicable) 1. Improve treatment prior to filtration. 2. Reduce control or filter aid dose rates. 3. Repair mechanical problem. 4. Increase surface wash cycle time. 5. Adjust backward cycle length. 5. Filter surface clogging. A. Causal factors: 1. Inadequate prior treatment (single media filters). 2. Excessive filter aid dosage (dual or mixed media filters). 3. Inadequate surface wash cycle. 4. Inadequate backwash cycle. B. Corrective actions (where applicable): 1. Improve prior treatment. 2. Replace single media with dual or mixed media. 3. Reduce or eliminate filter aid. 4. Provide adequate surface wash cycle. 5. Provide adequate backwash cycle. 6. Short filter runs. A. Causal factor: 1. High head loss. B. Corrective actions:

Wastewater Treatment

10.

11.

1. Improve prior treatment. 2. Replace single media with dual or mixed media. 3. Reduce or eliminate filter aid. 4. Provide adequate surface wash cycle. 5. Provide adequate backwash cycle. Filter effluent turbidity increases rapidly. A. Causal factors: 1. Inadequate filter aid dosage. 2. Filter aid system mechanical failure. 3. Filter aid requirement has changed. B. Corrective actions (where applicable): 1. Increase chemical dosage. 2. Repair feed system. 3. Adjust filter aid dose rate (do jar test). Mud Ball Formation. A. Causal factors: 1. Inadequate backwash flow rate. 2. Inadequate surface wash. B. Corrective actions (where applicable): 1. Increase backwash flow to specified levels. 2. Increase surface wash cycle. Gravel displacement. A. Causal factor: 1. Air is entering the underdrains during backwash cycle. B. Corrective actions: 1. Control backwash volume. 2. Control backwash water head. 3. Replace media (severe displacement). Medium is lost during backwash cycle. A. Causal factors: 1. Excessive backwash flows. 2. Excessive auxiliary scour. 3. Air attached to filter media, causing it to float. B. Corrective actions (where applicable): 1. Reduce backwash flow rate. 2. Stop auxiliary scour several minutes before end of backwash cycle. 3. Increase backwash frequency to prevent bubble displacement and maintain maximum operating water depth above filter surface. Filter backwash cycle not effective during warm weather. A. Causal factor: 1. Decreased water viscosity due to higher temperatures. B. Corrective action: 1. Increase backwash rate until required bed expansion is achieved.

ÂŠ 2003 by CRC Press LLC

603

12. Air binding causes premature head loss increase. A. Causal factors: 1. Air bubble produced by exposing an influent containing high dissolved oxygen levels to less than atmospheric pressure. 2. Pressure drops occurring during changeover to backwash cycle. B. Corrective actions (where applicable): 1. Increase backwash frequency. 2. Maintain maximum operating water depth.

18.12.4 BIOLOGICAL NITRIFICATION Biological nitrification is the first basic step of biological nitrification-denitrification. In nitrification, the secondary effluent is introduced into another aeration tank, trickling filter, or biodisc. Because most of the carbonaceous BOD has already been removed, the microorganisms that drive in this advanced step are the nitrifying bacteria nitrosomonas and nitrobacter. In nitrification, the ammonia nitrogen is converted to nitrate nitrogen, producing a nitrified effluent. At this point, the nitrogen has not actually been removed, only converted to a form that is nontoxic to aquatic life and that does not cause an additional oxygen demand. The nitrification process can be limited (performance affected) by alkalinity (requires 7.3 parts alkalinity to 1.0 part ammonia nitrogen), pH, DO availability, toxicity (ammonia or other toxic materials), and process MCRT (SRT). As a general rule, biological nitrification is more effective and achieves higher levels of removal during the warmer times of the year. 18.12.4.1 Operation, Observation, and Troubleshooting Procedures Ensuring the nitrification process performs as per design requires the operator to monitor the process and make routine adjustments. The loss of solids from settling tank, RBC, or from a trickling filter are common problems that the operator must be able to identify as well as to take proper corrective actions. In these instances, the operator needs to be familiar with activated sludge system, RBC, or trickling filter operations. The operator must also be familiar with other nitrification operational problems and must be able to take the proper corrective actions. We list typical nitrification operational problems and recommended corrective actions below: 1. pH decreases with loss of nitrification. A. Causal factors: 1. Insufficient alkalinity available for process. 2. Acid wastes in process influent.

604

Handbook of Water and Wastewater Treatment Plant Operations

B. Corrective actions (where applicable): 1. If process alkalinity is less than 30 mg/L, add lime or sodium hydroxide to process influent. 2. Identify source and control of acid wastes. 2. Incomplete nitrification. A. Causal factors: 1. Process is DO. 2. Process is temperature limited. 3. Influent nitorgen loading has increased. 4. Low nitrifying bacteria population in process. 5. Peak hourly ammonium concentrations exceed available oxygen supplies. B. Corrective actions (where applicable): 1. Increase process aeration rate. 2. Decrease process nitrogen loading. 3. Increase nitrifying bacteria population. 4. Put additional units in service. 5. Modify operation to increase nitrogen removal. 6. Decrease wasting or solids loss. 7. Add settled raw sewage to nitrification unit to increase biological solids. 8. Increase oxygen supply. 9. Install flow equalization to minimize peaks. 3. SVI of nitrification sludge is very high (>250). A. Causal factor: 1. Nitrification is occurring in the first stage (BOD removal sludge). B. Corrective actions (where applicable): 1. Transfer sludge from first to second stages. 2. Operate first stage at lower MCRT or SRT.

18.12.5 BIOLOGICAL DENITRIFCATION Biological denitrification removes nitrogen from the wastewater. When bacteria come in contact with a nitrified element in the absence of oxygen, they reduce the nitrates to nitrogen gas, which escapes the wastewater. The denitrification process can be done in either an anoxic activated sludge system (suspended growth) or in a column system (fixed growth). The denitrification process can remove up to 85% or more of nitrogen. After effective biological treatment, little oxygen demanding material is left in the wastewater when it reaches the denitrification process. The denitrification reaction will only occur if an oxygen demand source exists when no DO is present in the wastewater. An oxygen demand source is usually added to reduce the nitrates quickly. The most common demand source added is soluble BOD or methanol. Approximately 3 mg/L of methanol is added for every 1 mg/L of nitrate-nitrogen. ÂŠ 2003 by CRC Press LLC

Suspended growth denitrification reactors are mixed mechanically, but only enough to keep the biomass from settling without adding unwanted oxygen. Submerged filters of different types of media may also be used to provide denitrification. A fine media downflow filter is sometimes used to provide both denitrification and effluent filtration. A fluidized sand bed where wastewater flows upward through a media of sand or activated carbon at a rate to fluidize the bed may also be used. Denitrification bacteria grow on the media. 18.12.5.1 Observation, Operation, and Troubleshooting Procedures In operation of a denitrification process, operators monitor performance by observing various parameters. Parameters or other indicators and observations that demonstrate process malfunction or suboptimal performance indicate the need for various corrective actions. We discuss several of these indicators of poor process performance, their causal factors, and corrective actions in the sections that follow. 1. Process effluent: sudden increase in BOD. A. Causal factor: 1. Excessive methanol or other organic matter present. B. Corrective actions (as required): 1. Reduce methanol addition. 2. Install automated methanol control system. 3. Install aerated stabilization unit for removal of excess methanol. 2. Sudden increase in effluent nitrate concentration. A. Causal factors: 1. Inadequate methanol control. 2. Denitrification pH is outside 7.0 to 7.5 range required for process. 3. Loss of solids from denitrification process due to pump failure. 4. Excessive mixing introducing DO. B. Corrective actions (where applicable): 1. Identify and correct control problem. 2. Correct pH problem in nitrification process. 3. Adjust pH at process influent. 4. Correct denitrification sludge return. 5. Increase denitrification sludge waste rate. 6. Decrease denitrification sludge waste rate. 7. Transfer sludge from carbonaceous units to denitrification unit. 8. Reduce mixer speed. 9. Remove some mixers from service. 3. High head loss (packed bed nitrification). A. Causal factors: 1. Excessive solids in unit. 2. Nitrogen gas accumulating in unit.

Wastewater Treatment

B. Corrective action: 1. Backwash unit 1 to 2 min and return to service. 4. Out of service packed bed unit binds on start-up. A. Causal factor: 1. Solids have floated to top during shut down. B. Corrective action: 1. Backwash units before removing from service and immediately before placing in service.

18.12.6 CARBON ADSORPTION The main purpose of carbon adsorption used in advanced treatment processes is the removal of refractory organic compounds (non-BOD) and soluble organic material that are difficult to eliminate by biological or physical or chemical treatment. In the carbon adsorption process, wastewater passes through a container filled either with carbon powder or carbon slurry. Organics adsorb onto the carbon (i.e., organic molecules are attracted to the activated carbon surface and are held there) with sufficient contact time. A carbon system usually has several columns or basins used as contactors. Most contact chambers are either open concrete gravity-type systems or steel pressure containers applicable to either upflow or downflow operation. With use, carbon loses its adsorptive capacity. The carbon must then be regenerated or replaced with fresh carbon. As head loss develops in carbon contactors, they are backwashed with clean effluent in much the same way the effluent filters are backwashed. Carbon used for adsorption may be in granular or powdered form. Note: Powdered carbon is too fine for use in columns. It is usually added to the wastewater and later removed by coagulation and settling. 18.12.6.1 Operation, Observation, and Troubleshooting Procedures In operation of a carbon adsorption system for advanced wastewater treatment, operators are primarily interested in monitoring the system to prevent excessive head loss, reduce levels of hydrogen sulfide in the carbon contactor, ensure that the carbon is not fouled, and ensure corrosion of metal parts and damage to concrete in contactors is minimal: 1. Excessive head loss. A. Casual factors: 1. Highly turbid influent. 2. Growth and accumulation of biological solids in unit. 3. Excessive carbon fines due to deterioration during handling. 4. Inlet or outlet screens plugged. ÂŠ 2003 by CRC Press LLC

605

B. Corrective actions (where applicable): 1. Backwash unit vigorously. 2. Correct problem in prior treatment steps. 3. Operate as an expanded upflow bed to remove solids continuously. 4. Increase frequency of backwashing for downflow beds. 5. Improve soluble BOD removal in prior treatment steps. 6. Remove carbon from unit and wash out fines. 7. Replace carbon with harder carbon. 8. Backflush screens. 2. Hydrogen sulfide is in carbon contactor. A. Causal factors: 1. Low or no DO and nitrate in contactor influent. 2. High influent BOD concentrations 3. Excessive detention time in carbon contactor B. Corrective actions (where applicable): 1. Add air, oxygen, or sodium nitrate to unit influent. 2. Improve soluble BOD removal in prior treatment steps. 3. Precipitate sulfides already formed with iron on chlorine. 4. Reduce detention time by removing contactors from service. 5. Backwash units more frequently and more violently, using air scour or surface wash. 3. Large decrease in COD removed or pounds of carbon regenerated. A. Causal factor: 1. Carbon is fouled and losing efficiency. B. Corrective action: 1. Improve regeneration process performance. 4. Corrosion of metal parts or damage to concrete in contactors. A. Causal factors: 1. Hydrogen sulfide in carbon contactors. 2. Holes in protective coatings exposed to dewatered carbon. B. Corrective actions: 1. Add air, oxygen, or sodium nitrate to unit influent. 2. Improve soluble BOD removal in prior treatment steps. 3. Precipitate sulfides already formed with iron on chlorine. 4. Reduce detention time by removing contactors from service.

606

Handbook of Water and Wastewater Treatment Plant Operations

18.12.7.1 Types or Modes of Land Application

5. Backwash units more frequently and more violently, using air scour or surface wash. 6. Repair protective coatings.

18.12.7 LAND APPLICATION The application of secondary effluent onto a land surface can provide an effective alternative to the expensive and complicated advanced treatment methods discussed previously and the biological nutrient removal (BNR) system discussed briefly in Section 18.12.8. A high-quality polished effluent (i.e., effluent with high levels of TSS, BOD, phosphorus, and nitrogen compounds as well as refractory organics are reduced) can be obtained by the natural processes that occur as the effluent flows over the vegetated ground surface and percolates through the soil. Limitations are involved with land application of wastewater effluent. For example, the process needs large land areas. Soil type and climate are also critical factors in controlling the design and feasibility of a land treatment process.

Three basic types or modes of land application or treatment are commonly used: irrigation (slow rate), overland flow, and infiltration-percolation (rapid rate). The basic objectives of these types of land applications and the conditions under which they can function vary. In irrigation (also called slow rate), wastewater is sprayed or applied (usually by ridge-and-furrow surface spreading or by sprinkler systems) to the surface of the land. Wastewater enters the soil. Crops growing on the irrigation area utilize available nutrients. Soil organisms stabilize organic content of the flow. Water returns to the hydrologic (water) cycle through evaporation or by entering the surface water or groundwater (see Figure 18.12A). The irrigation land application method provides the best results (compared with the other two types of land application systems) with respect to advanced treatment levels of pollutant removal. Not only are suspended solids and BOD significantly reduced by filtration of the wastewater, but also biological oxidation of the organics in the top few inches of soil occurs. Nitrogen is removed primarily by crop uptake, and phosphorus is removed by adsorption within the soil.

A. Applied wastewater

Evapotranspiration

Percolation B.

Applied wastewater

Grass and vegetative litter Evapotranspiration Sheet flow

Slope 2 to 8%

Runoff collection

C. Applied wastewater

Evaporation

Percolation FIGURE 18.12 Land application. (From Spellman, F.R., Spellmanâ&#x20AC;&#x2122;s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

ÂŠ 2003 by CRC Press LLC

Wastewater Treatment

Irrigation expected performance levels are: 1. 2. 3. 4. 5.

BOD — 98% Suspended solids — 98% Nitrogen — 85% Phosphorus — 95% Metals — 95%

The overland flow mode of land application used for water purification is accomplished by physical, chemical, and biological processes as the wastewater flows in a thin film down the relatively impermeable surface. In the process, wastewater sprayed over sloped terraces flows slowly over the surface. Soil and vegetation remove suspended solids, nutrients, and organics. A small portion of the wastewater evaporates. The remainder flows to collection channels. Collected effluent is discharged to surface waters (see Figure 18.2B). Overflow flow expected performance levels are: 1. 2. 3. 4. 5.

BOD — 92% Suspended solids — 92% Nitrogen — 70 to 90% Phosphorus — 40 to 80% Metals — 50%

In the infiltration-percolation (rapid rate) land application process, wastewater is sprayed/pumped to spreading basins (a.k.a. recharge basins or large ponds). Some wastewater evaporates. The remainder percolates/infiltrates into soil. Solids are removed by filtration. Water recharges the groundwater system. Most of the effluent percolates to the groundwater; very little of it is absorbed by vegetation (see Figure 18.2C). The filtering and adsorption action of the soil removes most of the BOD, TSS, and phosphorous from the effluent; however, nitrogen removal is relatively poor. Infiltration-percolation expected performance levels are: 1. 2. 3. 4. 5.

BOD — 85 to 99% Suspended solids — 98% Nitrogen — 0 to 50% Phosphorus — 60 to 95% Metals — 50 to 95%

18.12.7.1.1 Operation, Observation, and Troubleshooting Procedures Performance levels are dependent on the land application process used. To be effective, operators must monitor the operation of the land application process employed. Experience has shown that these processes can be very effective, but problems exist when the flow contains potentially toxic materials that may become concentrated in the crops being grown on land. Along with this problem, other prob© 2003 by CRC Press LLC

607

lems are common, including ponding, deterioration of distribution piping systems, malfunctioning sprinkler heads, waste runoff, irrigated crop die-off, poor crop growth, and too much flow rate: 1. In irrigated areas, water is ponding. A. Causal factors: 1. Excessive application rate. 2. Inadequate drainage because of groundwater levels. 3. Damaged drainage wells. 4. Inadequate well withdrawal rates. 5. Damaged drain tiles. 6. Broken pipe in distribution system. B. Corrective actions (where applicable): 1. Reduce application rate to acceptable level. 2. Irrigate in portions of site where groundwater is not a problem. 3. Store wastewater until condition is corrected. 4. Repair drainage wells. 5. Increase drainage well pumping rates. 6. Repair damaged drain tiles. 7. Repair pipe. 2. Deterioration of distribution piping. A. Causal factors: 1. Effluent remains in pipe for long periods. 2. Different metals used in same line. B. Corrective actions (where applicable): 1. Drain pipe after each use. 2. Coat steel valves. 3. Install cathodic or anodic protection. 3. No flow from source sprinkler nozzles. A. Casual factor: 1. Nozzles clogged. B. Corrective action: 1. Repair or replace screen on irrigation pump inlet. 4. Wastes are running off irrigation area. A. Causal factors: 1. High sodium adsorption ratio has caused clay soil to become impermeable. 2. Solids seal soil surface. 3. Application rate is greater than soil infiltration rate. 4. Break in distribution piping. 5. Soil permeability has decreased because of continuous application of wastewater. 6. Rain has saturated the soil. B. Corrective actions (where applicable): 1. Feed calcium and magnesium to maintain a sodium adsorption ratio of less than 9. 2. Strip crop area.

608

Handbook of Water and Wastewater Treatment Plant Operations

3. Reduce application rate to acceptable level. 4. Repair system. 5. Allow 2- to 3-d rest period between each application. 6. Store wastewater until soil has drained. Irrigated crop is dead. A. Causal factors: 1. Too much or not enough water has been applied. 2. Wastewater contains toxic materials in toxic concentrations. 3. Excessive insecticide or herbicide applied. 4. Inadequate drainage has flooded root zone of crop. B. Corrective actions (where applicable): 1. Adjust application rate to appropriate level. 2. Eliminate source of toxicity. 3. Apply only as permitted or directed. Poor crop growth. A. Causal factors: 1. Too little nitrogen or phosphorus. 2. Timing of nutrient applications does not coincide with plant nutrient need. B. Corrective actions (where applicable): 1. Increase application rate to supply nitrogen and phosphorus. 2. Augment nitrogen and phosphorus of wastewater with commercial fertilizer applications. 3. Adjust application schedule to match crop need. Irrigation pump has normal pressure, but above average flow rate. A. Causal factors: 1. Broken main, riser, or lateral. 2. Leaking gasket. 3. Sprinkler head or nozzle is missing. 4. Too many distribution laterals are in service at one time. B. Corrective actions (where applicable): 1. Locate and repair problems. 2. Locate and replace defective gasket. 3. Correct valving to adjust number of laterals in service. Irrigation pump has above average pressure, but below average flow. A. Causal factor: 1. Blockage in system. B. Corrective action: 1. Locate and correct blockage. Irrigation pump has below average pressure and flow rate

ÂŠ 2003 by CRC Press LLC

10.

11.

12.

13.

A. Causal factors: 1. Worn impeller. 2. Partially clogged pump inlet screen. B. Corrective actions (where applicable): 1. Replace impeller. 2. Clean screen. Excessive erosion occurring. A. Causal factors: 1. Excessive application rates. 2. Inadequate crop coverage. B. Coverage actions (where applicable): 1. Reduce application rate. Odor complaints. A. Causal factors: 1. Wastes are turning septic during transport to treatment or irrigation site. 2. Storage reservoirs are septic. B. Corrective actions (where applicable): 1. Aerate or chemically treat wastes during transport. 2. Install cover over discharge point. Collect and treat gases before release. 3. Improve pretreatment. 4. Aerate storage reservoirs. Center pivot irrigation rigs stuck in mud. A. Causal factors: 1. Excessive application rates. 2. Improper rig or tires. 3. Poor drainage. B. Corrective actions: 1. Reduce application rate. 2. Install tire with higher flotation capabilities. Nitrate in groundwater near irrigation site is increasing. A. Causal factors: 1. Nitrogen application rate does not balance with crop need. 2. Applications are occurring during dormant periods. 3. Crop is not being properly harvested and removed. B. Corrective actions (where applicable): 1. Change to crop with higher nitrogen requirement. 2. Adjust schedule to apply only during active growth periods. 3. Harvest and remove crop as required.

18.12.8 BIOLOGICAL NUTRIENT REMOVAL Recent experience has shown that BNR systems are reliable and effective in removing nitrogen and phosphorus. The process is based upon the principle that under specific

Wastewater Treatment

conditions, microorganisms will remove more phosphorus and nitrogen than is required for biological activity. Several patented processes are available for this purpose. Performance depends on the biological activity and the process employed.

18.13 SOLIDS (SLUDGE OR BIOSOLIDS) HANDLING The wastewater treatment unit processes described to this point remove solids and BOD from the wastestream before the liquid effluent is discharged to its receiving waters. What remains to be disposed of is a mixture of solids and wastes, called process residuals; they are more commonly referred to as sludge or biosolids. Note: Sludge is the commonly accepted name for wastewater solids. If wastewater sludge is used for beneficial reuse (e.g., as a soil amendment or fertilizer), it is commonly called biosolids. The most costly and complex aspect of wastewater treatment can be the collection, processing, and disposal of sludge. This is the case because the quantity of sludge produced may be as high as 2% of the original volume of wastewater, depending somewhat on the treatment process being used. Because sludge can be as much as 97% water content and the cost of disposal will be related to the volume of sludge being processed, one of the primary purposes or goals (along with stabilizing it so it is no longer objectionable or environmentally damaging) of sludge treatment is to separate as much of the water from the solids as possible. Sludge treatment methods may be designed to accomplish both of these purposes. Sludge treatment methods are generally divided into three major categories: thickening, stabilization, and dewatering. Many of these processes include complex sludge treatment methods (i.e., heat treatment, vacuum filtration, incineration and others).

18.13.1 SLUDGE: BACKGROUND INFORMATION When we speak of sludge or biosolids, we are speaking of the same substance or material; each is defined as the suspended solids removed from wastewater during sedimentation and concentrated for further treatment and disposal or reuse. The difference between the terms sludge and biosolids is determined by the way they are managed. (Note: The task of disposing, treating or reusing wastewater solids is called sludge or biosolids management.) Sludge is typically seen as wastewater solids that are disposed. Biosolids is the same substance managed for reuse, commonly called beneficial reuse (e.g., for land application as a soil amendment, such as biosolids compost).

ÂŠ 2003 by CRC Press LLC

609

Note that even as wastewater treatment standards have become more stringent because of increasing environmental regulations, the volume of wastewater sludge has also increased. Note also that before sludge can be disposed of or reused, it requires some form of treatment to reduce its volume, stabilize it, and inactivate pathogenic organisms. Sludge forms initially as a 3 to 7% suspension of solids; with each person typically generating about 4 gal of sludge per week, the total quantity generated each day, week, month, and year is significant. Because of the volume and nature of the material, sludge management is a major factor in the design and operation of all water pollution control plants. Note: Wastewater solids account for more than half of the total costs in a typical secondary treatment plant. 18.13.1.1 Sources of Sludge Wastewater sludge is generated in primary, secondary, and chemical treatment processes. In primary treatment, the solids that float or settle are removed. The floatable material makes up a portion of the solid waste known as scum. Scum is not normally considered sludge; however, it should be disposed of in an environmentally sound way. The settleable material that collects on the bottom of the clarifier is known as primary sludge. Primary sludge can also be referred to as raw sludge because it has not undergone decomposition. Raw primary sludge from a typical domestic facility is quite objectionable and has a high percentage of water â&#x20AC;&#x201D; two characteristics that make handling difficult. Those solids not removed in the primary clarifier are carried out of the primary unit. These solids are known as colloidal suspended solids. The secondary treatment system (i.e., trickling filter, activated sludge, etc.) is designed to change those colloidal solids into settleable solids that can be removed. Once in the settleable form, these solids are removed in the secondary clarifier. The sludge at the bottom of the secondary clarifier is called secondary sludge. Secondary sludges are light and fluffy and more difficult to process than primary sludges. In short, secondary sludges do not dewater well. The addition of chemicals and various organic and inorganic substances prior to sedimentation and clarification may increase the solids capture and reduce the amount of solids lost in the effluent. This chemical addition results in the formation of heavier solids that trap the colloidal solids or convert dissolved solids to settleable solids. The resultant solids are known as chemical sludges. As chemical usage increases, so does the quantity of sludge that must be handled and disposed. Chemical sludges can be very difficult to process; they do not dewater well and contain lower percentages of solids.

610

Handbook of Water and Wastewater Treatment Plant Operations

TABLE 18.8 Typical Water Content of Sludges Water Treatment Process Primary Sedimentation Trickling Filter Humus (low rate) Humus (high rate) Activated sludge

% Moisture of Sludge Generated

Water/lb Sludge Solids

93 97 99

13.3 32.3 99

Source: U.S. Environmental Protection Agency, Operational Manual: Sludge Handling and Conditioning, EPA-430/9–78–002, 1978.

18.13.1.2 Sludge Characteristics The composition and characteristics of sewage sludge vary widely and can change considerably with time. Notwithstanding these facts, the basic components of wastewater sludge remain the same. The only variations occur in quantity of the various components as the type of sludge and the process from which it originated changes. The main component of all sludges is water. Prior to treatment, most sludges contain 95 to +99% water (see Table 18.8). This high water content makes sludge handling and processing extremely costly in terms of both money and time. Sludge handling may represent up to 40% of the capital cost and 50% of the operation cost of a treatment plant. As a result, the importance of optimum design for handling and disposal of sludge cannot be overemphasized. The water content of the sludge is present in a number of different forms. Some forms can be removed by several sludge treatment processes, allowing the same flexibility in choosing the optimum sludge treatment and disposal method. The various forms of water and their approximate percentages for a typical activated sludge are shown in Table 18.9. The forms of water associated with sludges are: Free water water that is not attached to sludge solids in any way. This can be removed by simple gravitational settling. Floc water water that is trapped within the floc and travels with them. Its removal is possible by mechanical dewatering. Capillary water water that adheres to the individual particles and can be squeezed out of shape and compacted. Particle water water that is chemically bound to the individual particles and can’t be removed without inclination. © 2003 by CRC Press LLC

TABLE 18.9 Distribution of Water in an Activated Sludge Water Type Free Water Floc Water Capillary Water Particle Water Solids Total

% Volume 75 20 2 2.5 0.5 100

Source: U.S. Environmental Protection Agency, Operational Manual: Sludge Handling and Conditioning, EPA-430/9–78– 002, 1978.

From a public health view, the second and probably more important component of sludge is the solids matter. Representing from 1 to 8% of the total mixture, these solids are extremely unstable. Wastewater solids can be classified into two categories based on their origin: organic and inorganic. Organic solids in wastewater, are materials that are or were at one time alive and that will burn or volatilize at 550°C after 15 minutes in a muffle furnace. The percent of organic material within a sludge will determine how unstable it is. The inorganic material within a sludge will determine how stable it is. The inorganic solids are those solids that were never alive and will not burn or volatilize at 550°C after 15 minutes in a muffle furnace. Inorganic solids are generally not subject to breakdown by biological action and are considered stable. Certain inorganic solids, however, can create problems when related to the environment (e.g., heavy metals such as copper, lead, zinc, mercury, and others). These can be extremely harmful if discharged. Organic solids may be subject to biological decomposition in either an aerobic or anaerobic environment. Decomposition of organic matter (with its production of objectionable by-products) and the possibility of toxic organic solids within the sludge compound the problems of sludge disposal. Note: Before moving on to a discussion of the fundamentals of sludge treatment methods, it is important to begin by covering sludge pumping calculations. It is important to point out that it is difficult (if not impossible) to treat the sludge unless it is pumped to the specific sludge treatment process.

Wastewater Treatment

611

18.13.1.3 Sludge Pumping Calculations

EXAMPLE 18.60

While on shift, wastewater operators are often called upon to make various process control calculations. An important calculation involves sludge pumping. The sludge pumping calculations the operator may be required to make during plant operations (and should be known for licensure examinations) are covered in this section.

Problem: What is the pump operating time?

Solution: Pump Operating Time = 15 min h ¥ 24 c d

18.13.1.3.1 Estimating Daily Sludge Production The calculation for estimation of the required sludgepumping rate provides a method to establish an initial pumping rate or to evaluate the adequacy of the current withdrawal rate: Est. Pump Rate = gpm

(18.55)

(Influent TSS Conc. - Effluent TSS Conc.) ¥ Flow ¥ 8.34

= 360 min d

18.13.1.3.3 Sludge Pumped in Gallons per day Use Equation 18.57 to calculate the amount of sludge pumped in gallons per day: Sludge Pumped (gal d ) = Operating Time (min d ) ¥ Pump Rate (gal min) (18.57)

% Solids in Sludge ¥ 8.34 lb gal ¥ 1440 min d

EXAMPLE 18.59

EXAMPLE 18.61

Problem:

The sludge withdrawn from the primary settling tank contains 1.4% of solids. The unit influent contains 285 mg/L TSS and the effluent contains 140 mg/L TSS. If the influent flow rate is 5.55 MGD, what is the estimated sludge withdrawal rate in gallons per minute (assuming the pump operates continuously)?

What is the amount of sludge pumped in gallons per day?

Solution: Sludge Pumped (gal d ) = 360 min d ¥ 120 gal min = 43, 200 gal d

Solution:

(285 mg Sludge Rate, gpm =

L - 140 mg L ) ¥ 5.55 ¥ 8.34

0.014 ¥ 8.34 lb gal ¥ 1440 min d

= 40 gpm

18.13.1.3.4 Sludge Pumped in Pounds per Day Use Equation 18.58 to calculate the amount of sludge pumped in pounds per day: Sludge Pumped (lb d ) = Sludge Pumped (gal d ) ¥

The following chart is used for Examples 18.60 to 18.65.

8.34 lb gal

(18.58)

EXAMPLE 18.62 Operating time Frequency Pump rate Solids Volatile matter

15 min/c 24 c/d 120 gal/min 3.7% 66%

Problem: What is the amount of sludge pumped in pounds per day?

Solution: 18.13.1.3.2 Sludge Pumping Time The sludge pumping time is the total time the pump operates during a 24-h period in minutes: Pump Operating Time = Time c (min) ¥ Frequency (c d ) © 2003 by CRC Press LLC

Sludge Pumped (lb d ) = 43, 200 gal d ¥ 8.34 lb gal = 360, 300 lb d

18.13.1.3.5 Solids Pumped in Pounds per Day (18.56)

Use Equation 18.59 to calculate the amount of sludge pumped in pounds per day:

612

Handbook of Water and Wastewater Treatment Plant Operations

Solids Pumped (lb d ) = Sludge Pumped (lb d ) ¥ % Solids

(18.59)

EXAMPLE 18.65 Problem: Use Equation 18.61 to calculate the amount of (1) solids and (2) volatile matter removed in pounds per day.

EXAMPLE 18.63

Solution:

Problem: What is the amount of solid pumped in pounds per day?

1. Amount of solids removed in pounds per day: Solids (lb d ) = 15 min c ¥ 24 c d ¥ 120 gal min ¥

Solution:

8.34 ¥ 0.0370

Solids Pumped (lb d ) = 360, 300 lb d ¥ 0.0370

= 13, 331 lb d

2. Amount of volatile solids in pounds per day:

18.13.1.3.6 Volatile Matter Pumped in Pounds per Day Use Equation 18.60 to calculate the amount of volatile matter pumped in pounds per day: VM (lb d ) = Solids Pumped (lb d ) ¥ %VM (18.60) EXAMPLE 18.64 Problem: What is the amount of volatile matter pumped in pounds per day?

VM (lb d ) = 15 min c ¥ 24 c d ¥ 120 gal min ¥ 8.34 ¥ 0.0370 ¥ 0.66 = 8798 lb d

18.13.1.3.7 Sludge Production in Pounds per Million Gallons A common method of expressing sludge production is in pounds of sludge per million gallons of wastewater treated: Sludge (lb MG ) =

Solution: VM (lb d ) = 13, 331 lb d ¥ 0.66

EXAMPLE 18.66

= 8.798 lb d

Problem:

If you wish to calculate the pounds of solids or the pounds of volatile solids removed per day, the individual equations demonstrated above can be combined into a single calculation: Solids (lb d ) = Pump Time ( min c) ¥ Frequency (c d ) ¥ Rate (gal min ) ¥ 8.34 lb gal ¥ %Solids VM (lb d ) = time ( min c) ¥ Frequency (c d ) ¥ Rate (gal min ) ¥ 8.34 ¥ %Solids ¥ %VM

Total Sludge Production (lb) (18.62) Total Wastewater Flow (MG )

Records show that the plant has produced 85,000 gal of sludge during the past 30 d. The average daily flow for this period was 1.2 MGD. What was the plant’s sludge production in pounds per million gallons?

Solution: Sludge (lb MG ) =

85, 000 gal ¥ 8.34 lb gal 1.2 MGD ¥ 30 d

= 19, 692 lb MG

(18.61) 18.13.1.3.8 Sludge Production in Wet Tons per Year Sludge production can also be expressed in terms of the amount of sludge (water and solids) produced per year. This is normally expressed in wet tons per year:

Wastewater Treatment

(

613

)

(18.63)

Sludge wet tons year =

(

)

(

)

Sludge Production lb MG ¥ Average Daily Flow MGD ¥ 365 d year 2000 lb ton

EXAMPLE 18.67 Problem: The plant is currently producing sludge at the rate of 16,500 lb/MG. The current average daily wastewater flow rate is 1.5 MGD. What will be the total amount of sludge produced per year in wet tons per year?

Solution: Sludge (wet tons year ) =

16, 500 lb MG ¥ 1.5 MGD ¥ 365 d year 2000 lb ton

= 4517 wet tons year

18.13.1.4 Sludge Treatment: An Overview The release of wastewater solids without proper treatment could result in severe damage to the environment. We must have a system to treat the volume of material removed as sludge throughout the system. Release without treatment would defeat the purpose of environmental protection. A design engineer can choose from many processes when developing sludge treatment systems. No matter what the system or combination of systems chosen, the ultimate purpose will be the same: the conversion of wastewater

sludges into a form that can be handled economically and disposed of without damaging the environment or creating nuisance conditions. Leaving either condition unmet will require further treatment. The degree of treatment will generally depend on the proposed method of disposal. Sludge treatment processes can be classified into a number of major categories. In this handbook, we discuss the processes shown in Figure 18.13: thickening, digestion (or stabilization), de-watering, incineration, and land application. Each of these categories has then been further subdivided according to the specific processes that are used to accomplish sludge treatment. As mentioned, the importance of adequate, efficient sludge treatment cannot be overlooked when designing wastewater treatment facilities. The inadequacies of a sludge treatment system can severely affect a plant’s overall performance capabilities. The inability to remove and process solids as fast as they accumulate in the process can lead to the discharge of large quantities of solids to receiving waters. Even with proper design and capabilities in place, no system can be effective unless it is properly operated. Proper operation requires proper operator performance. Proper operator performance begins and ends with proper training.

18.13.2 SLUDGE THICKENING The solids content of primary, activated, trickling-filter, or even mixed sludge (i.e., primary plus activated sludge) varies considerably, depending on the characteristics of the sludge. Note that the sludge removal and pumping facilities and the method of operation also affect the solids

Thickening

Digestion

Dewatering

Solids handling

Land application

Incineration

FIGURE 18.13 Major solids handling processes. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

614

Handbook of Water and Wastewater Treatment Plant Operations

content. Sludge thickening (or concentration) is a unit process used to increase the solids content of the sludge by removing a portion of the liquid fraction. By increasing the solids content, more economical treatment of the sludge can be effected. Sludge thickening processes include: 1. Gravity thickeners 2. Flotation thickeners 3. Solids concentrators 18.13.2.1 Gravity Thickening Gravity thickening is most effective on primary sludge. In operation, solids are withdrawn from primary treatment (and sometimes secondary treatment) and pumped to the thickener. The solids buildup in the thickener forms a solids blanket on the bottom. The weight of the blanket compresses the solids on the bottom and “squeezes” the water out. By adjusting the blanket thickness, the percent of solids in the underflow (solids withdrawn from the bottom of the thickener) can be increased or decreased. The supernatant (clear water) that rises to the surface is returned to the wastewater flow for treatment. Daily operations of the thickening process include pumping, observation, sampling and testing, process control calculations, maintenance and housekeeping. Note: The equipment employed in thickening depends on the specific thickening processes used. Equipment used for gravity thickening consists of a thickening tank, which is similar in design to the settling tank used in primary treatment. Generally the tank is circular and provides equipment for continuous solids collection. The collector mechanism uses heavier construction than a settling tank’s because the solids being moved are more concentrated. The gravity thickener pumping facilities (i.e., pump and flow measurement) are used for withdrawal of thickened solids. Performance of gravity thickeners (i.e., the solids concentrations achieved) typically results in producing 8 to 10% solids from primary underflow, 2 to 4% solids from waste activated sludge, 7 to 9% solids from trickling filter residuals, and 4 to 9% from combined primary and secondary residuals. The performance of gravity thickening processes depends on various factors, including: 1. 2. 3. 4. 5. 6.

Type of sludge Condition of influent sludge Temperature Blanket depth Solids loading Hydraulic loading

7. Solids retention time 8. HDT 18.13.2.2 Flotation Thickening Flotation thickening is used most efficiently for waste sludges from suspended-growth biological treatment process, such as the activated sludge process. In operation, recycled water from the flotation thickener is aerated under pressure. During this time the water absorbs more air than it would under normal pressure. The recycled flow together with chemical additives (if used) is mixed with the flow. When the mixture enters the flotation thickener, the excess air is released in the form of fine bubbles. These bubbles become attached to the solids and lift them toward the surface. The accumulation of solids on the surface is called the float cake. As more solids are added to the bottom of the float cake, it becomes thicker and water drains from the upper levels of the cake. The solids are then moved up an inclined plane by a scraper and discharged. The supernatant leaves the tank below the surface of the float solids and is recycled or returned to the wastestream for treatment. Flotation thickener performance is typically 3 to 5% solids for WAS with polymer addition and 2 to 4% solids without polymer addition. The flotation thickening process requires pressurized air, a vessel for mixing the air with all or part of the process residual flow, a tank for the flotation process to occur, solids collector mechanisms to remove the float cake (solids) from the top of the tank, and accumulated heavy solids from the bottom of the tank. Since the process normally requires chemicals be added to improve separation, chemical mixing equipment, storage tanks, and metering equipment to dispense the chemicals at the desired dose are required. The performance of dissolved air-thickening process depends on various factors that include: 1. 2. 3. 4. 5.

Bubble size Solids loading Sludge characteristics Chemical selection Chemical dose

18.13.2.3 Solids Concentrators Solids concentrators (belt thickeners) usually consist of a mixing tank, chemical storage and metering equipment, and a moving porous belt. In operation, the process residual flow is chemically treated and then spread evenly over the surface of the moving porous belt. As the flow is carried down the belt (similar to a conveyor belt) the solids are mechanically turned or agitated and water drains through the belt. This process is primarily used in facilities where space is limited.

Wastewater Treatment

18.13.2.3.1 Operation, Observation, and Troubleshooting Procedures As with other unit treatment processes, proper operation of sludge thickeners depends on operator observation. The operator must make routine adjustment of sludge addition and withdrawal rates to achieve desired blanket thickness. Sampling and analysis of influent sludge, supernatant, and thickened sludge are also required. If possible, sludge addition and withdrawal should be continuous to achieve optimum performance. Mechanical maintenance is also required. Expected performance ranges for gravity and dissolved air flotation thickeners are listed below: 1. 2. 3. 4.

Primary sludge — 8 to 19% solids WAS — 2 to 4% solids Trickling filter sludge — 7 to 9% solids Combined sludges — 4 to 9% solids

Typical operational problems with sludge thickeners include odors, rising sludge, thickened sludge below desired solids concentration, a dissolved air concentration that is too low, an effluent flow containing excessive solids, and torque alarm conditions. Gravity Thickener 1. Odors and rising sludge. A. Causal factors: 1. Sludge withdrawal rate is too low. 2. Overflow rate is too low. 3. Septicity in the thickener. B. Corrective actions (where applicable): 1. Increase sludge withdrawal rate. 2. Increase influent flow rate. 3. Add chlorine, permanganate, or peroxide to influent. 2. Thickened sludge is below desired solids concentration. A. Causal factors: 1. Overflow rate is too high. 2. Sludge withdrawal rate is too high. 3. Short-circuiting. B. Corrective actions (where applicable): 1. Decrease influent sludge flow rate. 2. Decrease pump rate for sludge withdrawal. 3. Identify cause and correct. 3. Torque alarm is activated. A. Causal factors: 1. Heavy sludge accumulation. 2. Collector mechanism is jammed. B. Corrective actions (where applicable): 1. Agitate sludge blanket to decrease density. 2. Increase sludge withdrawal rate. 3. Attempt to locate and remove obstacle. 4. Dewater tank and remove obstacle. © 2003 by CRC Press LLC

615

Dissolved Air Flotation Thickener 1. Float solids concentration is too low. A. Causal factors: 1. Skimmer speed is too high. 2. Unit is overloaded. 3. Insufficient polymer dose. 4. Excessive air-to-solids ratio. 5. Low dissolved air levels. B. Corrective actions (where applicable): 1. Adjust skimmer speed to permit concentration to occur. 2. Stop sludge flow through unit or purge with recycles flow. 3. Determine proper chemical dose and adjust. 4. Reduce airflow to pressurization tank. 5. Identify malfunction and correct. 2. Dissolved air concentration is too low. A. Causal factor: 1. Mechanical malfunction. B. Corrective action: 1. Identify cause and correct. 3. Effluent (subnatant) flow contains excessive solids. A. Causal factors: 1. Unit is overloaded. 2. Chemical dose is too low. 3. Skimmer is not operating. 4. Low air-to-solids ratio. 5. Solids buildup in thickener. B. Corrective actions (where applicable): 1. Turn off sludge flow. 2. Purge unit with recycle. 3. Determine proper chemical dose and blow. 4. Turn skimmer on. 5. Adjust skimmer speed. 6. Increase airflow to pressurization system. 7. Remove sludge from tank. 18.13.2.3.2 Process Calculations (Gravity and Dissolved Air Flotation) Sludge thickening calculations are based on the concept that the solids in the primary or secondary sludge are equal to the solids in the thickened sludge. Assuming a negligible amount of solids are lost in the thickener overflow, the solids are the same. Note that the water is removed to thicken the sludge and results in higher percent solids. 18.13.2.3.2.1

Estimating Daily Sludge Production

Equation 18.76 provides a method to establish an initial pumping rate or to evaluate the adequacy of the current pump rate:

616

Handbook of Water and Wastewater Treatment Plant Operations

Surf. Loading, gal d ft 2 =

(

)

Solids Loading Rate lb d ft 2 =

Sludge Applied to the Thickener, gpd (18.64) Thickener Area, ft 2

(18.66)

%Sludge Solids ¥ Sludge Flow (gal d ) ¥ 8.34 lb gal Thickener Area ft 2

( )

EXAMPLE 18.68 EXAMPLE 18.70 Problem: The sludge withdrawn from the primary settling tank contains 1.5% of solids. The unit influent contains 280 mg/L TSS, and the effluent contains 141 mg/L. If the influent flow rate is 5.55 MGD, what is the estimated sludge withdrawal rate in gallons per minute (assuming the pump operates continuously)?

Problem: The thickener influent contains 1.6% of solids. The influent flow rate is 39,000 gal/d. The thickener is 50 ft in diameter and 10 ft deep. What is the solid loading in pounds per day?

Solution:

Solution: Surface Loading =

(

Solids Loading Rate lb d ft 2

32, 000 gpd 0.785 ¥ 70 ft ¥ 70 ft

= 8.32 gpd ft 2 18.13.2.3.2.2

0.016 ¥ 39, 000 gal d ¥ 8.34 lb gal 0.785 ¥ 50 ft ¥ 50 ft

= 2.7 lb d ft 2

Surface Loading Rate

Surface loading rate (surface settling rate) is hydraulic loading — the amount of sludge applied per square foot of gravity thickener:

(

)

18.13.2.3.2.4

The concentration factor (CF) represents the increase in concentration resulting from the thickener:

Surface Loading Rate gal d ft 2 = Sludge Applied to the Thickener (gal d ) (18.65) Thickener Area ft 2

( )

Concentration Factor

CF =

Thickened Sludge Concentration (%) Influent Sludge Concentration (%)

(18.67)

EXAMPLE 18.71 EXAMPLE 18.69

Problem:

Problem: The 70-ft diameter gravity thickener receives 32,000 gal/d of sludge. What is the surface loading in gallons per square foot per day?

Solution:

(

Surface Loading Rate gal d ft 2 =

)

CF =

7.7% 3.5%

= 2.2

32, 000 gal d 0.785 ¥ 70 ft ¥ 70 ft

= 8.32 gal d ft 2 18.13.2.3.2.3

The influent sludge contains 3.5% solids. The thickened sludge solids concentration is 7.7%. What is the concentration factor?

Solids Loading Rate

The solids loading rate is the pounds of solids per day being applied to 1 ft2 of tank surface area. The calculation uses the surface area of the bottom of the tank. It assumes the floor of the tank is flat and has the same dimensions as the surface. © 2003 by CRC Press LLC

18.13.2.3.2.5

Air-to-Solids Ratio

The air-to-solids ratio is the ratio between the pounds of solids entering the thickener and the pounds of air being applied: Air:Solids Ratio =

(

(18.68)

)

(

Air Flow ft 3 min ¥ 0.075 lb ft 3

)

Sludge Flow (gal min) ¥ %Solids ¥ 8.34 lb gal

Wastewater Treatment

617

EXAMPLE 18.72 Problem: The sludge pumped to the thickener is 0.85% solids. The airflow is 13 ft3/min. What is the air-to-solids ratio if the current sludge flow rate entering the unit is 50 gal/min?

Air:Solids Ratio =

13 ft min ¥ 0.075 lb ft

50 gal min ¥ 0.0085 ¥ 8.34 lb gal

= 0.28 18.13.2.3.2.6

Recycle Flow in Percent

The amount of recycle flow expressed as a percent: Recycle Flow (%) = Recycle Flow Rate (gal min) ¥ 100 Sludge Flow (gal min)

(18.69)

EXAMPLE 18.73 Problem: The sludge flow to the thickener is 80 gal/min. The recycle flow rate is 140 gal/min. What is the recycle flow?

Solution: Recycle Flow (%) =

140 gal min ¥ 100 80 gal min

= 175%

18.13.3 SLUDGE STABILIZATION The purpose of sludge stabilization is to reduce volume, stabilize the organic matter, and eliminate pathogenic organisms to permit reuse or disposal. The equipment required for stabilization depends on the specific process used. Sludge stabilization processes include: 1. 2. 3. 4. 5. 6. 7.

Aerobic digestion Anaerobic digestion Composting Lime stabilization Wet air oxidation (heat treatment) Chemical oxidation (chlorine oxidation) Incineration

18.13.3.1 Aerobic Digestion Equipment used for aerobic digestion consists of an aeration tank (digester) which is similar in design to the aeration tank used for the activated sludge process. Either © 2003 by CRC Press LLC

Parameter Detention time (d) Volatile solids loading lb/ft3/d DO (mg/L) pH Volatile Solids Reduction

Solution:

TABLE 18.10 Aerobic Digester Normal Operating Levels Normal Levels 10–20 0.1–0.3 1.0 5.9–7.7 40–50%

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

diffused or mechanical aeration equipment is necessary to maintain the aerobic conditions in the tank. Solids and supernatant removal equipment is also required. In operation, process residuals (sludge) are added to the digester and aerated to maintain a DO concentration of 1.0 mg/L. Aeration also ensures that the tank contents are well mixed. Generally, aeration continues for approximately 20 d retention time. Aeration is periodically stopped and the solids are allowed to settle. Sludge and the clear liquid supernatant are withdrawn as needed to provide more room in the digester. When no additional volume is available, mixing is stopped for 12 to 24 h before solids are withdrawn for disposal. Process control testing should include alkalinity, pH, percent solids, percent volatile solids for influent sludge, supernatant, digested sludge, and digester contents. Normal operating levels for an aerobic digester are listed in Table 18.10. A typical operational problem associated with an aerobic digester is pH control. For example, when pH drops, this may indicate normal biological activity or low influent alkalinity. This problem is corrected by adding alkalinity (lime, bicarbonate, etc.). 18.13.3.1.1 Process Control Calculations: Aerobic Digester Wastewater operators (who operate aerobic digesters) are required to make certain process control calculations. Moreover, licensing examinations typically include aerobic digester problems for determining volatile solids loading, digestion time, digester efficiency, and pH adjustment. These process control calculations are explained in the following sections: 18.13.3.1.1.1

Volatile Solids Loading

Volatile solids loading for the aerobic digester is expressed in pounds of volatile solids entering the digester per day per cubic foot of digester capacity:

618

Handbook of Water and Wastewater Treatment Plant Operations

(

)

the calculation used to determine percent volatile matter reduction is more complicated:

Volatile Solids Loading lb d ft 3 = Volatile Solids Added (lb d )

(18.70)

( )

% Matter Reduction =

Digester Volume ft 3

(

)

% Volatile Matter - % Volatile Matterout ¥ 100 in % Vol. Matterin - (% Vol. Matterin ¥ % Vol. Matterout )] [

EXAMPLE 18.74 Problem: The aerobic digester is 25 ft in diameter and has an operating depth of 24 ft. The sludge added to the digester daily contains 1350 lb of volatile solids. What is the volatile solids loading in pounds per day per cubic foot?

Solution:

EXAMPLE 18.76 Problem: Using the digester data provided below, determine the percent volatile matter reduction for the digester. Data:

(

Volatile Solids Loading lb d ft =

)

Raw sludge volatile matter = 71% Digested sludge volatile matter = 53%

1350 lb d 0.785 ¥ 25 ft ¥ 25 ft ¥ 24 ft %VM Reduction =

= 0.11 lb d ft 3 18.13.3.1.1.2

(18.72)

= 53.9% or 54%

Digestion Time

Digestion time is the theoretical time the sludge remains in the aerobic digester: Digestion Time (d ) =

[0.71 - 0.53] ¥ 100 [0.71 - (0.71 ¥ 0.53)]

Digester Volume (gal) (18.71) Sludge Added (gal d )

2. Moisture reduction — Use Equation 18.73 to calculate percent moisture reduction: %Moisture Reduction =

(18.73)

[%Moisture - %Moisture ] ¥ 100 [%Moisture - (%Moisture ¥ %Moisture )] out

EXAMPLE 18.75

Problem: Digester volume is 240,000 gal. Sludge is being added to the digester at the rate of 13,500 gal/d. What is the digestion time in days?

out

EXAMPLE 18.77 Problem: Using the digester data provided below, determine the %moisture reduction for the digester (Note: %Moisture = 100% – Percent Solids):

Solution: Digestion Time (d ) = 18.13.3.1.1.3

240, 000 gal

13, 500 (gal d )

= 17.8 d

Digester Efficiency

To determine digester efficiency or the percentage of reduction, a two-step procedure is required. The percent volatile matter reduction must first be calculated and then the percent moisture reduction:

Solution: Raw Sludge Digested Sludge

%Solids %Moisture %Solids %Moisture

%Moisture Reduction =

1. Percent volatile matter reduction —Because of the changes occurring during sludge digestion,

6% 94% (100% – 6%) 15% 85% (100% – 15%)

[0.94 - 0.85] ¥ 100 [0.94 - (0.94 ¥ 0.85)]

= 64%

Wastewater Treatment

18.13.3.1.1.4

619

pH Adjustment

Occasionally, the pH of the aerobic digester will fall below the levels required for good biological activity. When this occurs, the operator must perform a laboratory test to determine the amount of alkalinity required to raise the pH to the desired level. The results of the lab test must then be converted to the actual quantity of chemical (usually lime) required by the digester: Chem. Required, lbs =

Chemical Used in Lab Test, mg Sample Volume, Liters

absence of oxygen and is slower than aerobic digestion. The advantage of anaerobic digestion is that only a small percentage of the wastes are converted into new bacterial cells. Most of the organics are converted into carbon dioxide and methane gas. Note: In an anaerobic digester, the entrance of air should be prevented because of the potential for air mixed with the gas produced in the digester that could create an explosive mixture.

18.13.3.2 Anaerobic Digestion

Equipment used in anaerobic digestion includes a sealed digestion tank with either a fixed or a floating cover (see Figure 18.14), heating and mixing equipment, gas storage tanks, solids and supernatant withdrawal equipment, and safety equipment (e.g., vacuum relief, pressure relief, flame traps, explosion proof electrical equipment). In operation, process residual (thickened or unthickened sludge) is pumped into the sealed digester. The organic matter digests anaerobically by a two-stage process. Sugars, starches, and carbohydrates are converted to volatile acids, carbon dioxide, and hydrogen sulfide. The volatile acids are then converted to methane gas. This operation can occur in a single tank (single stage) or in two tanks (two stages). In a single-stage system, supernatant and digested solids must be removed whenever flow is added. In a two-stage operation, solids and liquids from the first stage flow into the second stage each time fresh solids are added. Supernatant is withdrawn from the second stage to provide additional treatment space. Solids are periodically withdrawn for dewatering or disposal. The methane gas produced in the process may be used for many plant activities.

Anaerobic digestion is the traditional method of sludge stabilization. It involves using bacteria that thrive in the

Note: The primary purpose of a secondary digester is to allow for solids separation.

¥ Dig. Vol, MG ¥ 8.34 lb gal (18.74) EXAMPLE 18.78 Problem: The lab reports that it took 225 mg of lime to increase pH of a 1-L sample of the aerobic digester contents to pH 7.2. The digester volume is 240,000 gal. How many pounds of lime will be required to increase the digester pH to 7.2?

Solution: Chemical Required =

225 mg ¥ 240, 000 gal ¥ 3.785 L gal 1 L ¥ 454 g ¥ 1000 mg g

= 450 lb

Gas dome Floating cover Scum layer

Supernatant

Gas Biosolids inlet

Digesting biosolids Recirc to heater Digested biosolids Stabilized thickened biosolids

FIGURE 18.14 Floating cover anaerobic digester. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

620

Handbook of Water and Wastewater Treatment Plant Operations

TABLE 18.11 Anaerobic Digester — Sludge Parameters Raw Sludge Solids <4% Solids

4–8% Solids >8% Solids

Impact Loss of alkalinity; decreased SRT; increased heating requirements; decreased volatile acids-alkalinity ratio Normal operation Poor mixing; organic overloading; decreased volatile acids-alkalinity ratio

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

Various performance factors affect the operation of the anaerobic digester. For example, percent volatile matter in raw sludge, digester temperature, mixing, volatile acids-alkalinity ratio, feed rate, percent solids in raw sludge and pH are all important operational parameters that the operator must monitor. Along with being able to recognize normal and abnormal anaerobic digester performance parameters, wastewater operators must also know and understand normal operating procedures. Normal operating procedures include sludge additions, supernatant withdrawal, sludge withdrawal, pH control, temperature control, mixing, and safety requirements. Important performance parameters are listed in Table 18.11. 18.13.3.2.1 Sludge Additions Sludge must be pumped (in small amounts) several times each day to achieve the desired organic loading and optimum performance. Note: Keep in mind that in fixed cover operations additions must be balanced by withdrawals. If not, structural damage occurs. 18.13.3.2.2 Supernatant Withdrawal Supernatant withdrawal must be controlled for maximum sludge retention time. When sampling, sample all drawoff points and select level with the best quality. 18.13.3.2.3 Sludge Withdrawal Digested sludge is withdrawn only when necessary. Always leave at least 25% seed. 18.13.3.2.4 pH Control pH should be adjusted to maintain 6.8 to 7.2 pH by adjusting feed rate, sludge withdrawal, or alkalinity additions. Note: The buffer capacity of an anaerobic digester is indicated by the volatile acid/alkalinity relationship. Decreases in alkalinity cause a corresponding increase in ratio. © 2003 by CRC Press LLC

18.13.3.2.5 Temperature Control If the digester is heated, the temperature must be controlled to a normal temperature range of 90 to 95∞F. Never adjust the temperature by more than 1∞F per day. 18.13.3.2.6 Mixing If the digester is equipped with mixers, mixing should be accomplished to ensure organisms are exposed to food materials. 18.13.3.2.7 Safety Anaerobic digesters are inherently dangerous; several catastrophic failures have been recorded. To prevent such failures, safety equipment such as pressure relief and vacuum relief valves, flame traps, condensate traps, and gas collection safety devices are installed. It is important that these critical safety devices be checked and maintained for proper operation. Note: Because of the inherent danger involved with working inside anaerobic digesters, they are automatically classified as permit-required confined spaces. All operations involving internal entry must be made in accordance with OSHA’s confined space entry standard. 18.13.3.2.8 Process Control Monitoring, Testing, and Troubleshooting During operation, anaerobic digesters must be monitored and tested to ensure proper operation. Testing should be accomplished to determine supernatant pH, volatile acids, alkalinity, BOD or COD, total solids and temperature. Sludge (in and out) should be routinely tested for percent solids and percent volatile matter. Normal operating parameters are listed in Table 18.12. 18.13.3.2.9 Anaerobic Digester: Troubleshooting As with all other unit processes, the wastewater operator is expected to recognize problematic symptoms with anaerobic digesters and effect the appropriate corrective actions. Symptoms, causes, and corrective actions are discussed below. 1. Symptom 1: Digester gas production is reduced, pH drops below 6.8, and volatile acids-alkalinity ratio increases. A. Causes: 1. Digester souring. 2. Organic overloading. 3. Inadequate mixing. 4. Low alkalinity. 5. Hydraulic overloading. 6. Toxicity. 7. Loss of digestion capacity. B. Corrective actions: 1. Add alkalinity (digested sludge, lime, etc.). 2. Improve temperature control.

Wastewater Treatment

621

TABLE 18.12 Anaerobic Digester: Normal Operating Ranges Parameter

Normal Range

Sludge retention time Heated Unheated Volatile solids loading Operating temperature Heated Unheated Mixing Heated (primary) Unheated (secondary) %Methane in gas %Carbon dioxide in gas pH Volatile acids-alkalinity ratio Volatile solids reduction Moisture reduction

30–60 d 180+ d 0.04–0.1 lb VM/day/ft3 90–95∞F Varies with season

3. Improve mixing. 4. Eliminate toxicity. 5. Clean digester. 2. Symptom 2: Gray foam oozing from digester. A. Cause: 1. Rapid gasification. 2. Foam producing organisms present. 3. Foam producing chemical present. B. Corrective actions: 1. Reduce mixing. 2. Reduce feed rate. 3. Mix slowly by hand. 4. Clean all contaminated equipment. Anaerobic Digester: Process Control Calculations

Process control calculations involved with anaerobic digester operation include determining the required seed volume, volatile acid-alkalinity ratio, SRT, estimated gas production, volatile matter reduction, and percent moisture reduction in digester sludge. Examples on how to make these calculations are provided in the following sections. 18.13.3.2.10.1

Required Seed Volume in Gallons

Operating Condition

Volatile Acids-Alkalinity Ratio £0.1 0.1–0.3 ≥0.5 ≥0.8

Optimum Acceptable range %Carbon dioxide in gas increases pH decreases

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

EXAMPLE 18.79 Yes No 60– 72% 28–40% 6.8–7.2 £0.1 40–60% 40–60%

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

18.13.3.2.10

TABLE 18.13

Problem: The new digester requires a 25% seed to achieve normal operation within the allotted time. If the digester volume is 266,000 gal, how many gallons of seed material will be required?

Solution: Seed Volume (gal) = 266, 000 ¥ 0.25 = 66, 500 gal 18.13.3.2.10.2

Volatile Acids-Alkalinity Ratio

The volatile acids-alkalinity ratio can be used to control operation of an anaerobic digester: Acids:Alkalinity Ratio = Volatile Acids Concentration Alkalinity Concentration

(18.76)

EXAMPLE 18.80 Problem: The digester contains 240 mg/L volatile acids and 1860 mg/L alkalinity. What is the volatile acids-alkalinity ratio? Acids: Alkalinity Ratio =

240 mg L 1860 mg L

= 0.13

Increases in the ratio normally indicate a potential change in the operation condition of the digester as shown in Table 18.13. 18.13.3.2.10.3

Sludge Retention Time

SRT is the length of time the sludge remains in the digester: SRT (d ) = (18.77) Digester Volume (gal) Sludge Volume Added per Day (gal d )

622

Handbook of Water and Wastewater Treatment Plant Operations

EXAMPLE 18.81

Data:

Problem:

Raw sludge volatile matter = 74% Digested sludge volatile matter = 55%

Sludge is added to a 525,000-gal digester at the rate of 12,250 gal/d. What is the sludge retention time?

%VM Reduction =

Solution: SRT (d ) = 18.13.3.2.10.4

525, 000 gal 12, 250 gal d

18.13.3.2.10.6

= 42.9 d

The rate of gas production is normally expressed as the volume of gas (ft3) produced per pound of volatile matter destroyed. The total cubic feet of gas a digester will produce per day can be calculated by:

)

Percent Moisture Reduction in Digested Sludge

%Moisture Reduction =

(

)

(18.78)

out

Problem: The digester receives 11,450 lb of volatile matter per day. The volatile matter reduction achieved by the digester is 52%. The rate of gas production is 11.2 ft3 of gas per pound of volatile matter destroyed. What is the estimated gas production per day?

EXAMPLE 18.84

Using the digester data provided below, determine the percent moisture reduction and percent volatile matter reduction for the digester (Note: %Moisture = 100% – Percent Solids):

Solution: Raw sludge %solids = 6% Digested sludge %solids = 14%

Solution:

(

%Moisture Reduction =

)

Gas Production ft d = 11, 450 lb d ¥ 0.52 ¥ 11.2 ft 3 lb

Percent Volatile Matter Reduction

Because of the changes occurring during sludge digestion, the calculation used to determine percent volatile matter reduction is more complicated: (18.79)

% Reduction =

(% Volatile Matter - % Volatile Matter ) ¥ 100 [% Volatile Matter - (% Volatile Matter ¥ % Volatile Matter )] in

out

EXAMPLE 18.83 Problem: Using the digester data provided below, determine the percent volatile matter reduction for the digester.

[0.94 - 0.86] ¥ 100

[0.94 - (0.94 ¥ 0.86)]

= 61%

18.13.3.3 Other Sludge Stabilization Processes

= 66, 685 ft 3 d 18.13.3.2.10.5

out

Problem:

EXAMPLE 18.82

(18.80)

[%Moisture - %Moisture ] ¥ 100 [%Moisture - (%Moisture ¥ %Moisture )] in

Gas Production ft 3 d = VM in (lb d ) ¥ %VM Reduction ¥ Production Rate ft 3 lb

= 57%

Use Equation 18.92 to calculate the percent moisture reduction in digested sludge:

Estimated Gas Production

(

[0.74 - 0.55] ¥ 100

[0.74 - (0.74 ¥ 0.55)]

Along the aerobic and anaerobic digestion, other sludge stabilization processes include composting, lime stabilization, wet air oxidation, and chemical (chlorine) oxidation. These other stabilization processes are briefly described in this section. 18.13.3.3.1 Composting The purpose of composting sludge is to stabilize the organic matter, reduce volume, and eliminate pathogenic organisms. In a composting operation, dewatered solids are usually mixed with a bulking agent (i.e., hardwood chips) and stored until biological stabilization occurs. The composting mixture is ventilated during storage to provide sufficient oxygen for oxidation and to prevent odors. After the solids are stabilized, they are separated from the bulking agent. The composted solids are then stored for curing and applied to farmlands or other beneficial uses. Expected performance of the composting operation for

Wastewater Treatment

both percent volatile matter reduction and percent moisture reduction ranges from 40 to 60%+. 18.13.3.3.2 Lime Stabilization In lime stabilization, process residuals are mixed with lime to achieve a pH of 12. This pH is maintained for at least 2 h. The treated solids can then be dewatered for disposal or directly land applied. 18.13.3.3.3 Thermal Treatment Thermal treatment (or wet air oxidation) subjects sludge to high temperature and pressure in a closed reactor vessel. The high temperature and pressure rupture the cell walls of any microorganisms present in the solids and causes chemical oxidation of the organic matter. This process substantially improves dewatering and reduces the volume of material for disposal. It also produces a very high strength waste, which must be returned to the wastewater treatment system for further treatment. 18.13.3.3.4 Chlorine Oxidation Chlorine oxidation also occurs in a closed vessel. In this process, chlorine (100 to 1000 mg/L) is mixed with a recycled solids flow. The recycled flow and process residual flow are mixed in the reactor. The solids and water are separated after leaving the reactor vessel. The water is returned to the wastewater treatment system and the treated solids are dewatered for disposal. The main advantage of chlorine oxidation is that it can be operated intermittently. The main disadvantage is production of extremely low pH and high chlorine content in the supernatant. 18.13.3.3.5 Stabilization: Operation and Performance Depending on the stabilization process employed, the operational components vary. In general, operations include pumping, observations, sampling and testing, process control calculations, maintenance, and housekeeping. Performance of the stabilization process will also vary with the type of process used. Stabilization processes can generally produce 40 to 60% reduction of both volatile matter (organic content) and moisture.

18.13.4 SLUDGE DEWATERING Digested sludge removed from the digester is still mostly liquid. Sludge dewatering is used to reduce volume by removing the water to permit easy handling and economical reuse or disposal. Dewatering processes include sand drying beds, vacuum filters, centrifuges, filter presses (belt and plate), and incineration. 18.13.4.1 Sand Drying Beds Drying beds have been used successfully for years to dewater sludge. Composed of a sand bed (consisting of a gravel base, underdrains, and 8 to 12 in. of filter grade © 2003 by CRC Press LLC

623

sand), drying beds include an inlet pipe, splash pad containment walls, and a system to return filtrate (water) for treatment. In some cases, the sand beds are covered to provide drying solids protection from the elements. In operation, solids are pumped to the sand bed and allowed to dry by first draining off excess water through the sand and then by evaporation. This is the simplest and cheapest method for dewatering sludge. No special training or expertise is required. There is a downside; drying beds require a great deal of manpower to clean beds, they can create odor and insect problems, and they can cause sludge buildup during inclement weather. 18.13.4.1.1 Performance Factors In sludge drying beds, various factors affect the length of time required to achieve the desired solids concentrations. The major factors and their impact on drying bed performance include the following: 1. Climate — Drying beds in cold or moist climates will require significantly longer drying time to achieve an adequate level of percent solids concentrations in the dewatered sludge. 2. Depth of applied sludge — The depth of the sludge drawn onto the bed has a major impact on the required drying time. Deeper sludge layers require longer drying times. Under ideal conditions, a well-digested sludge drawn to a depth of approximately 8 in. will require approximately 3 weeks to reach the desired 40 to 60% solids. 3. Type of sludge applied — The quality and solids concentration of the drying media will affect the time requirements. 4. Bed cover — Covered-drying beds prevent rewetting of the sludge during storm events. In most cases, this reduces the average drying time required to reach the desired solids levels. 18.13.4.1.2 Operation, Observation, and Troubleshooting Procedures Although drying beds involve two natural processes — drainage and evaporation — that normally work well enough on their own, a certain amount of preparation and operator attention is still required to maintain optimum drying performance. For example, in the preparation stage, all debris is removed from the raked and leveled media surface. Then, all openings to the bed are sealed. After the bed is properly prepared, the sludge lines are opened, and sludge is allowed to flow slowly onto the media. The bed is filled to desired operating level (8 to 12 in.). The sludge line is closed and flushed, and the bed drain is opened. Water begins to drain. The sludge remains on the media until the desired percent solids (40 to 60%) is achieved. Later, the sludge is removed. In most operations,

624

Handbook of Water and Wastewater Treatment Plant Operations

manual removal is required to prevent damage to the underdrain system. The sludge is disposed of in an approved landfill or by land application as a soil conditioner. 18.13.4.1.3 Operational Problems In the operation of a sludge drying bed, the operator observes the operations and looks for various indicators of operational problems and makes process adjustments as required: 1. Sludge takes a long time to dewater. A. Causal factors: 1. Applied sludge is too deep. 2. Sludge was applied to a dirty bed. 3. The drain system is plugged or broken. 4. Insufficient design capacity. 5. Inclement weather or poor drying conditions. B. Corrective actions (where applicable): 1. Allow bed to dry to minimum acceptable % solids and remove. 2. Use described procedure below to determine appropriate sludge depth. a. Clean bed and apply smaller depth of sludge (i.e., 6 to 8 in.). b. Measure the decrease in depth (drawdown) at the end of 3 days of drying. c. Use a sludge depth equal to twice the 3-d drawdown depth for future applications. 3. After sludge has dried, remove sludge and 0.5 to 1.0 in. of sand. Add clean sand. 4. Allow sludge to dry to minimum allowable percent solids and remove. 5. Use external water source (with back flow prevention) to slowly flush underdrains. 6. Repair or replace underdrains as required. 7. Prevent damage to underdrains by draining during freezing weather. 8. Use polymer to increase bed performance. 9. Cover or enclose the beds. 2. Influent sludge is very thin. A. Causal factor: 1. Coning is occurring in the digester. B. Corrective action: 1. Reduce rate of sludge withdrawal. 3. Sludge feed lines plug frequently. A. Causal factor: 1. Solids or grit is accumulating in the line. B. Corrective actions: 1. Open lines fully at the start of each withdrawal cycle. 2. Flush lines at the end of each withdrawal cycle. ÂŠ 2003 by CRC Press LLC

4. Flies breeding in the drying sludge. A. Causal factors: 1. Inadequately digested sludge. 2. Natural insect reproduction. B. Corrective actions: 1. Break sludge crust and apply a larvicide (borax). 2. Use insecticide (if approved) to remove adult insects. 3. Remove sludge as soon as possible. 5. Objectionable odors are present when sludge is applied to bed. A. Causal factor: 1. Raw or partially digested sludge is being applied to the bed. B. Corrective actions: 1. Add lime to the sludge to control odors and potential insect and rodent problems. 2. Remove the sludge as quickly as possible. 3. Identify and correct the digester problem. 18.13.4.2 Rotary Vacuum Filtration Rotary vacuum filters have also been used for many years to dewater sludge. The vacuum filter includes filter media (belt, cloth or metal coils), media support (drum), vacuum system, chemical feed equipment, and conveyor belts to transport the dewatered solids. In operation, chemically treated solids are pumped to a vat or tank in which a rotating drum is submerged. As the drum rotates, a vacuum is applied to the drum. Solids collect on the media and are held there by the vacuum as the drum rotates out of the tank. The vacuum removes additional water from the captured solids. When solids reach the discharge zone, the vacuum is released and the dewatered solids are discharged onto a conveyor belt for disposal. The media are then washed prior to returning to the start of the cycle. 18.13.4.2.1 Types of Rotary Vacuum Filters The three principal types of rotary vacuum filters are rotary drum, coil, and belt. The rotary drum filter consists of a cylindrical drum rotating partially submerged in a vat or pan of conditioned sludge. The drum is divided length-wise into a number of sections that are connected through internal piping to ports in the valve body (plant) at the hub. This plate rotates in contact with a fixed valve plate with similar parts that are connected to a vacuum supply, a compressed air supply, and an atmosphere vent. As the drum rotates, each section is connected to the appropriate service. The coil type vacuum filter uses two layers of stainless steel coils arranged in corduroy fashion around the drum. After a dewatering cycle, the two layers of springs leave the drum bed and are separated from each other so that

Wastewater Treatment

the cake is lifted off the lower layer and is discharged from the upper layer. The coils are then washed and reapplied to the drum. The coil filter is used successfully for all types of sludges; sludges that have extremely fine particles or are resistant to flocculation de-water poorly with this system. The media on a belt filter leave the drum surface at the end of the drying zone and passes over a small diameter discharge roll to aid cake discharge. Washing of the media occurs next. Then the media are returned to the drum and to the vat for another cycle. This type of filter normally has a small-diameter curved bar between the point where the belt leaves the drum and the discharge roll. This bar primarily aids in maintaining belt dimensional stability. 18.13.4.2.1.1

Filter Media

Drum and belt vacuum filters use natural or synthetic fiber materials. On the drum filter, the cloth is stretched and secured to the surface of the drum. In the belt filter, the cloth is stretched over the drum and through the pulley system. The installation of a blanket requires several days. The cloth will (with proper care) last several hundred to several thousand hours. The life of the blanket depends on the cloth selected, the conditioning chemical, backwash frequency, and cleaning (i.e., acid bath) frequency. 18.13.4.2.1.2

Filter Drum

The filter drum is a maze of pipe work running from a metal screen and wooden skeleton and connecting to a rotating valve port at each end of the drum. The drum is equipped with a variable speed drive to turn the drum from 1/8 to 1 r/min. Normally, solids pickup is indirectly related to the drum speed. The drum is partially submerged in a vat containing the conditioned sludge. Normally, submergence is limited to 1/5 or less of filter surface at a time. 18.13.4.2.1.3

Chemical Conditioning

Sludges that are dewatered using vacuum filtration are normally chemically conditioned just prior to filtration. Sludge conditioning increases the percentage of solids captured by the filter and improves the de-watering characteristics of the sludge. Conditional sludge must be filtered as quickly as possible after chemical addition to obtain these desirable results. 18.13.4.2.2 Operation, Observation, and Troubleshooting Procedures In operation, the rotating drum picks up chemically treated sludge. A vacuum is applied to the inside of the drum to draw the sludge onto the outside of the drum cover. This porous outside cover or filter medium allows the filtrate or liquid to pass through into the drum and the filter cake (dewatered sludge) to stay on the medium. In the cake release or discharge mode, slight air pressure is applied to the drum interior. Dewatered solids are lifted from the medium and scraped off by a scraper blade. Solids drop ÂŠ 2003 by CRC Press LLC

625

onto a conveyor for transport for further treatment or disposal. The filtrate water is returned to the plant for treatment. While in operation, the operator observes drum speed, sludge pickup, filter cake thickness and appearance, chemical feed rates, sludge depth in vat, and overall equipment operation. Sampling and testing are routinely performed on influent sludge solids concentration, filtrate BOD and solids, and sludge cake solids concentration. We cover the indicators and observations of vacuum filter operational problems and causal factors, along with recommended corrective actions in the following list: 1. High solids in filtrate. A. Causal factors: 1. Improper coagulant dosage. 2. Filter media binding. B. Corrective actions (where applicable): 1. Adjust coagulant dosage. 2. Recalibrate coagulant feeder. 3. Clean synthetic cloth with steam and detergent. 4. Clean steel coil with acid bath. 5. Clean cloth with water or replace cloth. 2. Thin filter cake and poor dewatering. A. Causal factors: 1. Filter media binding. 2. Improper chemical dosage. 3. Inadequate vacuum. 4. Drum speed is too high. 5. Drum submergence is too low. B. Corrective actions (where applicable): 1. Clean synthetic cloth with steam and detergent. 2. Clean steel cloth with acid bath. 3. Clean cloth with water or replace cloth. 4. Adjust coagulant dosage. 5. Recalibrate coagulant feeder. 6. Repair vacuum system. 7. Reduce drum speed. 8. Increase drum submergence. 3. Vacuum pump stops. A. Causal factors: 1. Power to drive motor is off. 2. Lack of seal water. 3. Broken drive belt. B. Corrective action (where applicable): 1. Reset heater, breaker, etc. and restart. 2. Starts seal water flow. 3. Replace drive belt. 4. Drum stops rotating. A. Causal factor: 1. Power to drive motor is off. B. Corrective action: 1. Reset heater, breaker, etc. and restart 5. Receiver vibrating.

626

Handbook of Water and Wastewater Treatment Plant Operations

A. Causal factors: 1. Filtrate pump is clogged. 2. Loose bolts and gasket around inspection plate. 3. Worn ball check valve in filtrate pump. 4. Air leaks in suction line. 5. Dirty drum face. 6. Seal strips are missing. B. Corrective actions (where applicable): 1. Clear pump. 2. Tighten bolts and gasket. 3. Replace ball check. 4. Seal leaks. 5. Clean face with pressure hose. 6. Replace missing seal strips. High vat level. A. Causal factors: 1. Improper chemical conditioning. 2. Feed Rate is too high. 3. Drum speed is too slow. 4. Filtrate pump is off or clogged. 5. Drain line is plugged. 6. Vacuum pump has stopped. 7. Seal strips are missing. B. Corrective actions: 1. Change coagulant dosage. 2. Reduce feed rate. 3. Increase drum speed. 4. Turn on or clean pump. 5. Clean drain line. 6. Replace seal strips. Low vat level. A. Causal factors: 1. Feed rate is too low. 2. Vat drain valve is open. B. Corrective actions (where applicable): 1. Increase feed rate. 2. Close vat drain valve. Vacuum pump is drawing high amperage. A. Causal factors: 1. Filtrate pump is clogged. 2. Improper chemical conditioning. 3. High vat level. 4. Cooling water flow to vacuum pump is too high. B. Corrective actions (where applicable): 1. Clear pump clog. 2. Adjust coagulant dosage. 3. Decrease cooling water flow rate. Scale buildup on vacuum pump seals. A. Causal factor: 1. Hard, unstable water. B. Corrective action: 1. Add sequestering agent.

18.13.4.2.3 Process Control Calculations Probably the most frequent calculation vacuum filter operators have to make is for determining filter yield. Example 18.85 illustrates how this calculation is made. 18.13.4.2.3.1

Filter Yield: Vacuum Filter

EXAMPLE 18.85 Problem: Thickened thermally conditioned sludge is pumped to a vacuum filter at a rate of 50 gal/min. The vacuum area of the filter is 12 ft wide with a drum diameter of 9.8 ft. If the sludge concentration is 12%, what is the filter yield in pounds per hour per square foot? Assume the sludge weighs 8.34 lb/gal.

Solution: Calculate the filter surface area: Area of a cylinder side = 3.14 ¥ Diameter ¥ Length = 3.14 ¥ 9.8 ft ¥ 12 ft = 369.3 ft 2 Calculate the pounds of solids per hour: 50 gal min ¥ 60 min h ¥ 8.34 lb gal ¥ 0.12 = 3002.4 lb h Divide the two results: 3002.4 lb h 369.3 ft 2

= 8.13 lb h ft 2

18.13.4.3 Pressure Filtration Pressure filtration differs from vacuum filtration in that the liquid is forced through the filter media by a positive pressure instead of a vacuum. Several types of presses are available, but the most commonly used types are plate and frame presses and belt presses. Filter presses include the belt or plate and frame types. The belt filter includes two or more porous belts, rollers, and related handling systems for chemical makeup and feed. It also includes supernatant and solids collection and transport (see Figure 18.15). The plate and frame filter consists of a support frame, filter plates covered with porous material, hydraulic or mechanical mechanism for pressing plates together, and related handling systems for chemical makeup and feed. It also includes supernatant and solids collection and transport.

Wastewater Treatment

627

Polymer Sludge

Disposal Water Return for treatment FIGURE 18.15 Belt filter press. (From Spellman, F.R., Spellmanâ&#x20AC;&#x2122;s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

In the plate and frame filter, solids are pumped (sandwiched) between plates. Pressure (200 to 250 psi) is applied to the plates and water is squeezed from the solids. At the end of the cycle, the pressure is released and as the plates separate the solids drop out onto a conveyor belt for transport to storage or disposal. Performance factors for plate and frame presses include feed sludge characteristics, type and amount of chemical conditioning, operating pressures, and the type and amount of precoat. In operation, the belt filter uses a coagulant (polymer) mixed with the influent solids. The chemically treated solids are discharged between two moving belts. First water drains from the solids by gravity. The two belts then move between a series of rollers, and pressure squeezes additional water out of the solids. The solids are then discharged onto a conveyor belt for transport to storage or disposal. Performance factors for the belt press include sludge feed rate, belt speed, belt tension, belt permeability, chemical dosage, and chemical selection. Filter presses have lower operation and maintenance costs than vacuum filters or centrifuges. They typically produce a good quality cake and can be batch operated. The downside is that construction and installation costs are high. Moreover, chemical addition is required and the presses must be operated by skilled personnel. 18.13.4.3.1 Operation, Observation, and Troubleshooting Procedures Most plate and filter press operations are partially or fully automated. Operation consists of observation, maintenance, and sampling and testing. Operation of belt filter presses consists of preparation of conditioning chemicals, chemical feed rate adjustments, sludge feed rate adjustments, belt alignment, belt speed and belt tension adjustments, sampling and testing, and maintenance. ÂŠ 2003 by CRC Press LLC

We include common operational problems, causal factors, and recommended corrective actions for the plate press and belt filter press in the following list. Plate Press 1. Plates fail to seal. A. Causal factors: 1. Poor alignment. 2. Inadequate shimming. B. Corrective actions (where applicable): 1. Realign parts. 2. Adjust shimming of stay bosses. 2. Cake discharge is difficult. A. Causal factors: 1. Inadequate precoat. 2. Improper conditioning. B. Corrective actions (where applicable): 1. Increase precoat and feed at 25 to 40 psig. 2. Change conditioner type or dosage (use filter leaf test to determine). 3. Filter cycle times are excessive. A. Causal factors: 1. Improper conditioning. 2. Feed solids are low. B. Corrective Actions (where applicable): 1. Change chemical dosage. 2. Improve thickening operation. 4. Filter cake sticks to conveyors. A. Causal factor: 1. Improper conditioning chemical or dosage. B. Corrective action: 1. Increase inorganic conditioner dose. 5. Precoat pressures gradually increase. A. Causal factors: 1. Improper sludge conditioning. 2. Improper precoat feed.

628

Handbook of Water and Wastewater Treatment Plant Operations

3. Filter media plugged. 4. Calcium buildup in media. B. Corrective actions (where applicable): 1. Change chemical dosage 2. Decrease feed for a few cycles and optimize. 3. Wash filter media. 4. Wash media with inhibited hydrochloric acid. Frequent media binding. A. Causal factors: 1. Inadequate precoat. 2. Initial feed rate too high (no precoat). B. Corrective actions (where applicable): 1. Increase precoat. 2. Reduce feed rate or develop initial cake slowly. Excessive moisture in cake. A. Causal factors: 1. Improper conditioning. 2. Filter cycle too short. B. Corrective actions (where applicable): 1. Change chemical dosage. 2. Lengthen filter cycle. Sludge blowing out of press. A. Causal factor: 1. Obstruction between plates. B. Corrective Action: 1. Shut down feed pump, hit press closure drive, restart feed pump, and clean after cycle. Plate Press: Leaks around Lower Faces of Plates A. Causal factor: 1. Wet cake soiling media on lower faces. B. Corrective actions (where applicable): 1. Change chemical dosage. 2. Lengthen filter cycle.

Belt Press 1. Filter cake discharge is difficult. A. Causal factors: 1. Wrong conditioning chemical selected. 2. Improper chemical dosage. 3. Changing sludge characteristics. 4. Wrong application point. B. Corrective actions (where applicable): 1. Change conditioning chemical. 2. Adjust chemical dosage. 3. Change chemical or sludge 4. Adjust application point. 2. Sludge leaking from belt edges.

ÂŠ 2003 by CRC Press LLC

A. Causal factors: 1. Excessive belt tension. 2. Belt speed too low. 3. Excessive sludge feed rate. B. Corrective actions (where applicable): 1. Reduce belt tension. 2. Increase belt speed. 3. Reduce sludge feed rate. 3. Excessive moisture in filter cake. A. Causal factors: 1. Improper belt speed or drainage time. 2. Wrong conditioning chemical. 3. Improper chemical dosage. 4. Inadequate belt washing. 5. Wrong belt weave or material. B. Corrective actions (where applicable): 1. Adjust belt speed. 2. Change conditioning chemical. 3. Adjust chemical dosage. 4. Clear spray nozzles or adjust sprays. 5. Replace belt. 4. Excessive belt wear along edges. A. Causal factors: 1. Roller misalignment. 2. Improper belt tension. 3. Tension or alignment in control system. B. Corrective actions (where applicable): 1. Correct roller alignment. 2. Correct tension. 3. Repair tracking and alignment system controls. 5. Belt shifts or seizes. A. Causal factors: 1. Uneven sludge distribution. 2. Inadequate or uneven belt washing. B. Corrective actions: 1. Adjust feed for uniform sludge distribution. 2. Clean and adjust belt-washing sprays. 18.13.4.3.2 Process Control Calculations: Filter Presses As part of the operating routine for filter presses, operators are called upon to make certain process control calculations. The process control calculation most commonly used in operating the belt filter press determines the hydraulic loading rate on the unit. The most commonly used process control calculation used in operation of plate and filter presses determines the pounds of solids pressed per hour. Both of these calculations are demonstrated below.

Wastewater Treatment

18.13.4.3.2.1

629

Hydraulic Loading Rate: Belt Filter Press

EXAMPLE 18.86 Problem: A belt filter press receives a daily sludge flow of 0.30 gal. If the belt is 60 in. wide, what is the hydraulic loading rate on the unit in gallons per minute for each foot of belt width?

Solution: 0.30 MGD ¥ 1, 000, 000 gal MG

= 208.3 gal min

1440 min d 60 in. ¥

1 ft 12 in.

208.3 gal 5 ft 18.13.4.3.2.2

= 5 ft = 41.7 gal min ft

Pounds of Solids Pressed per Hour: Plate and Frame Press

EXAMPLE 18.87 Problem: A plate and frame filter press can process 850 gal of sludge during its 120-min operating cycle. If the sludge concentration is 3.7%, and if the plate surface area is 140 ft2, how many pounds of solids are pressed per hour for each square foot of plate surface area?

Solution: 850 gal ¥ 0.037 ¥ 8.34 lb gal = 262.3 lb 262.3 lb 120 min

60 min 1h

131.2 lb h 140 ft 2

= 131.2 lb h = 0.94 lb h ft 2

18.13.4.4 Centrifugation Centrifuges of various types have been used in dewatering operations for at lease 30 years and appear to be gaining in popularity. Depending on the type of centrifuge used and the centrifuge pumping equipment for solids feed and centrate removal, chemical makeup and feed equipment and support systems for removal of dewatered solids are required. 18.13.4.4.1 Operation, Observation, and Troubleshooting Procedures Generally, the centrifuge spins at a very high speed when operating. The centrifugal force it creates throws the solids out of the water. Chemically conditioned solids are pumped into the centrifuge. The spinning action throws the solids to the outer wall of the centrifuge. The centrate (water) flows inside the unit to a discharge point. The solids held against the outer wall are scraped to a discharge point by an internal scroll moving slightly faster or slower than the centrifuge speed of rotation. In the operation of the continuous feed, solids bowl, conveyor type centrifuge (this is the most common type currently used), and other commonly used centrifuges, solid and liquid separation occurs as a result of rotating the liquid at high speeds to cause separation by gravity. In the solid bowl type, the solid bowl has a rotating unit with a bowl and a conveyor (see Figure 18.16). The unit has a conical section at one end that acts as a drainage device. The conveyor screw pushes the sludge solids to outlet ports and the cake to a discharge hopper. The sludge slurry enters the rotating bowl through a feed pipe leading into the hollow shaft of the rotating screw conveyor. The sludge is distributed through ports into a pool inside the rotating bowl. As the liquid sludge flows through the hollow shaft toward the overflow device, the fine solids settle to the wall of the rotating bowl. The screw conveyor pushes the solids to the conical section where the solids are forced out of the water and the water drains back in the pool. The expected percent solids for centrifuge dewatered sludges is in the range of 10 to 15%. The expected Screw conveyor

Centrate discharge

Inlet

Dam

Rotating bowl

Solids discharge

FIGURE 18.16 Centrifuge. (From Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.)

630

Handbook of Water and Wastewater Treatment Plant Operations

TABLE 18.14 Expected Percent Solids for Centrifuge Dewatered Sludges Type of Sludge

%Solids

Raw sludge Anaerobic digestion Activated sludge Heat treated

25–35% 15–30% 8–10% 30–50%

Source: Spellman, F.R., Spellman’s Standard Handbook for Wastewater Operators, Vol. 1, Technomic Publ., Lancaster, PA, 1999.

performance is depended on the type of sludge being dewatered, as shown in Table 18.14. Centrifuge operation is dependent upon various performance factors: 1. Bowl design (length-diameter ratio and flow pattern) 2. Bowl speed 3. Pool volume 4. Conveyor design 5. Relative conveyor speed 6. Type and condition of sludge 7. Type and amount of chemical conditioning 8. Operating pool depth 9. Relative conveyor speed (if adjustable) Centrifuge operators often find that the operation of centrifuges can be simple, clean, and efficient. In most cases, chemical conditioning is required to achieve optimum concentrations. Operators soon discover that centrifuges are noisemakers; units run at very high speed and produce high-level noise, which can cause loss of hearing with prolonged exposure. When working in an area where a centrifuge is in operation, special care must be taken to provide hearing protection. Actual operation of a centrifugation unit requires the operator to perform the following tasks: 1. Control and adjust chemical feed rates. 2. Observe unit operation and performance. 3. Control and monitor centrate returned to treatment system. 4. Perform required maintenance as outlined in the manufacturer’s technical manual. The centrifuge operator must be trained to observe and recognize (as with other unit processes) operational problems that may occur with centrifuge operation. We cover several typical indicators and observations of centrifuge problems, along with causal factors and suggested © 2003 by CRC Press LLC

corrective actions (troubleshooting procedures) in the following sections. 1. Poor centrate clarity. A. Causal factors: 1. Feed rate is too high. 2. Wrong plate dam position. 3. Worn conveyor flights. 4. Speed is too high. 5. High feed sludge solids concentration. 6. Improper chemical conditioning. B. Corrective actions (where applicable): 1. Adjust sludge feed rate. 2. Increase pool depth. 3. Repair or replace conveyor. 4. Change pulley setting to obtain lower speed. 5. Dilute sludge feed. 6. Adjust chemical dosage. 2. Solids cake is not dry enough. A. Causal factors: 1. Feed rate is too high. 2. Wrong plate dam position. 3. Speed is too low. 4. Excessive chemical conditioning. 5. Influent is too warm. B. Corrective actions (if applicable): 1. Reduce sludge feed rate. 2. Decrease pool depth to increase dryness. 3. Change pulley setting to obtain higher speed. 4. Adjust chemical dosage. 5. Reduce influent temperature. 3. Torque control keeps tripping. A. Causal factors: 1. Feed rate is too high. 2. Feed solids concentration is too high. 3. Foreign material (i.e., tramp iron) in machine. 4. Gear unit is misaligned. 5. Gear unit has mechanical problem. B. Corrective actions (where applicable): 1. Reduce flows. 2. Dilute flows. 3. Remove conveyor or clear foreign materials. 4. Correct gear unit alignment. 5. Repair gear unit. 4. Excess vibration. A. Causal factors: 1. Improper lubrication. 2. Improper adjustment of vibration isolators. 3. Discharge funnels are contacting centrifuge.

Wastewater Treatment

4. Portion of conveyor flights may be plugged (causing an imbalance). 5. Gear box improperly aligned. 6. Pillow box bearings are damaged. 7. Bowl is out of balance. 8. Parts are not tightly assembled. 9. Uneven wear on conveyor. B. Corrective actions (where applicable): 1. Lubricate according to manufacturer’s instructions. 2. Adjust isolators. 3. Reposition slip joints at funnels. 4. Flush centrifuge. 5. Align gearbox. 6. Replace bearings. 7. Return rotating parts to factory for rebalancing 8. Tighten parts. 9. Resurface and rebalance. Sudden increase in power consumption. A. Causal factors: 1. Contact between bowl exterior and accumulated solids in case. 2. Effluent pipe is plugged. B. Corrective actions (where applicable): 1. Apply hard surfacing to areas with wear. 2. Clear solids discharge. Gradual increase in power consumption. A. Causal factor: 1. Conveyor blade wear. B. Corrective action: 1. Replace blades. Spasmodic surging of solids discharge. A. Causal factors: 1. Pool depth too low. 2. Conveyor helix is rough. 3. Feed pipe too near drainage deck. 4. Excessive vibration. B. Corrective Actions (where applicable): 1. Increase pool depth. 2. Refinish conveyor blade area. 3. Move feed pipe to effluent end. Centrifuge shuts down or will not start. A. Causal factors: 1. Blown fuses. 2. Overload relay is tripped. 3. Motor overheated or thermal protectors are tripped. 4. Torque control is tripped. 5. Vibration switch is tripped. B. Corrective actions (where applicable): 1. Replace fuses. 2. Flush centrifuge and reset relay. 3. Flush centrifuge and reset thermal protectors.

631

18.13.4.5 Sludge Incineration Not surprisingly, incinerators produce the maximum solids and moisture reductions. The equipment required depend on whether the unit is a multiple hearth or fluidbed incinerator. Generally, the system will require a source of heat to reach ignition temperature, solids feed system and ash handling equipment. It is important to note that the system must also include all required equipment (e.g., scrubbers) to achieve compliance with air pollution control requirements. In operation, solids are pumped to the incinerator. The solids are dried and ignited (burned). As they burn the organic matter is converted to carbon dioxide and water vapor and the inorganic matter is left behind as ash or fixed solids. The ash is then collected for reuse of disposal. 18.13.4.5.1 Process Description The incineration process first dries then burns the sludge. The process involves the following steps: 1. The temperature of the sludge feed is raised to 212°F. 2. Water evaporates from the sludge. 3. The temperature of the water vapor and air mixture increases. 4. The temperature of the dried sludge volatile solids raises to the ignition point. Note: Incineration will achieve maximum reductions if sufficient fuel, air, time, temperature, and turbulence are provided. 18.13.4.5.2 Incineration Processes 18.13.4.5.2.1

Multiple Hearth Furnace

The multiple hearth furnace consists of a circular steel shell surrounding a number of hearths. Scrappers (rabble arms) are connected to a central rotating shaft. Units range from 4.5 to 21.5 ft in diameter and have from 4 to 11 hearths. In operation, dewatered sludge solids are placed on the outer edge of the top hearth. The rotating rabble arms move them slowly to the center of the hearth. At the center of the hearth, the solids fall through ports to the second level. The process is repeated in the opposite direction. Hot gases generated by burning on lower hearths dry the solids. The dry solids pass to the lower hearths. The high temperature on the lower hearths ignites the solids. Burning continues to completion. Ash materials discharge to lower cooling hearths where they are discharged for disposal. Air flowing inside center column and rabble arms continuously cools internal equipment. 18.13.4.5.2.2

Fluidized Bed Furnace

The fluidized bed incinerator consists of a vertical circular steel shell (reactor) with a grid to support a sand bed and an air system to provide warm air to the bottom of the

632

Handbook of Water and Wastewater Treatment Plant Operations

sand bed. The evaporation and incineration process takes place within the super-heated sand bed layer. In operation, air is pumped to the bottom of the unit. The airflow expands (fluidizes) the sand bed inside. The fluidized bed is heated to its operating temperature (1200 to 1500°F). Auxiliary fuel is added when needed to maintain operating temperature. The sludge solids are injected into the heated sand bed. Moisture immediately evaporates. Organic matter ignites and reduces to ash. Residues are ground to fine ash by the sand movement. Fine ash particles flow up and out of the unit with exhaust gases. Ash particles are removed using common air pollution control processes. Oxygen analyzers in the exhaust gas stack control the airflow rate. Note: Because these systems retain a high amount of heat in the sand, the system can be operated as little as four hours per day with little or no reheating. 18.13.4.5.3 Operation, Observation, and Troubleshooting Procedures The operator of an incinerator monitors various performance factors to ensure optimal operation. These performance factors include feed sludge volatile content, feed sludge moisture content, operating temperature, sludge feed rate, fuel feed rate, and air feed rate. Note: To ensure that the volatile material is ignited, the sludge must be heated between 1400 and 1700°F. In ensuring operating parameters are in the correct range, the operator monitors and adjusts sludge feed rate, airflow, and auxiliary fuel feed rate. All maintenance conducted on an incinerator should be in accordance with manufacturer’s recommendations. 18.13.4.5.3.1

Operational Problems

The operator of a multiple hearth or fluidized bed incinerator must be able to recognize operational problems using various indicators and observations. We discuss these indicators and observations, causal factors, and recommended corrective actions in the following list: Multiple Hearths 1. Incinerator temperature is too high. A. Causal factors: 1. Excessive fuel feed rate. 2. Greasy solids. 3. Thermocouple has burned out. B. Corrective actions (where applicable): 1. Decrease fuel feed rate. 2. Reduce sludge feed rate. 3. Increase air feed rate. 4. Replace thermocouple.

2. Furnace temperature is too low. A. Causal factors: 1. Moisture content of the sludge has increased. 2. Fuel system malfunction. 3. Excessive air feed rate. 4. Flame is out. B. Corrective actions (where applicable): 1. Increase fuel feed rate until dewatering operation improves. 2. Establish proper fuel feed rate. 3. Decrease air feed rate. 4. Increase sludge feed rate. 5. Relight furnace. 3. Oxygen content of stack gas is too high. A. Causal factors: 1. Sludge feed rate is too low. 2. Sludge feed system blockage. 3. Air feed rate is too high. B. Corrective actions (where applicable): 1. Increase sludge feed rate. 2. Clear any feed system blockages. 3. Decrease air feed rate. 4. Oxygen content of stack gas is too low. A. Causal factors: 1. Volatile or grease content of the sludge has increased. 2. Air feed rate is too low. B. Corrective actions (where applicable): 1. Increase air feed rate. 2. Decrease sludge feed rate. 3. Increase air feed rate. 5. Furnace refractories have deteriorated. A. Causal factor: 1. Rapid start-up or shutdown of furnace. B. Corrective actions: 1. Repair furnace refractories. 2. Follow specified start-up or shutdown procedures. 6. Unusually high cooling effect. A. Causal factor: 1. Air leak. B. Corrective action: 1. Locate and repair leak. 7. Short hearth life. A. Causal factor: 1. Uneven firing. B. Corrective action: 1. Fire hearths equally on both sides. 8. Center shaft shear pin failure. A. Causal factors: 1. Rabble arm is dragging on hearth. 2. Debris is caught under the arm. B. Corrective actions (where applicable):

Wastewater Treatment

10.

11.

12.

13.

14.

15.

1. Adjust rabble arm to eliminate rubbing. 2. Remove debris. Scrubber temperature is too high. A. Causal factor: 1. Low water flow to scrubber. B. Corrective action: 1. Adjust water flow to proper level. Stack gas temperatures are too low. A. Causal factors: 1. Inadequate fuel feed supply. 2. Excessive sludge feed rate. B. Corrective actions (where applicable): 1. Increase fuel feed rate. 2. Decrease sludge feed rate. Stack gas temperatures are too high. A. Causal factors: 1. Sludge has higher volatile content (heat value). 2. Excessive fuel feed rate. B. Corrective actions (where applicable): 1. Increase air feed rate. 2. Decrease sludge feed rate. 3. Decrease fuel feed rate. Furnace burners are slagging up. A. Causal factor: 1. Burner design. B. Corrective action: 1. Replace burners with newer designs that reduce slagging. Rabble arms are dropping. A. Causal factors: 1. Excessive hearth temperatures. 2. Loss of cooling air. B. Corrective actions (where applicable): 1. Maintain temperatures within proper range. 2. Discontinue injection of scum into the hearth. 3. Repair cooling air system immediately. Excessive air pollutants are in stack gas. A. Causal factors: 1. Incomplete combustion (insufficient air). 2. Air pollution control malfunction. B. Corrective actions (where applicable): 1. Raise air-fuel ratio. 2. Repair or replace broken equipment. Flashing or explosions. A. Causal factor: 1. Scum or grease additions. B. Corrective action: 1. Remove scum or grease before incineration.

ÂŠ 2003 by CRC Press LLC

633

Fluidized Beds 1. Bed temperature is falling. A. Causal factors: 1. Inadequate fuel supply. 2. Excessive sludge feed rate. 3. Excessive sludge moisture levels. 4. Excessive air flow. B. Corrective actions (where applicable): 1. Increase fuel supply. 2. Repair fuel system malfunction. 3. Decrease sludge feed rate. 4. Correct sludge de-watering process problem. 5. Decrease airflow rate. 2. Low (<3%) oxygen in exhaust gas. A. Causal Factors: 1. Low air flow rate. 2. Fuel feed rate is too high. B. Corrective actions (where applicable): 1. Increase blower air feed rate. 2. Reduce fuel feed rate. 3. Excessive (>6%) oxygen in exhaust gas. A. Causal factor: 1. Sludge feed rate is too low. B. Corrective actions (where applicable): 1. Increase sludge feed rate. 2. Adjust fuel feed rate to maintain steady bed temperature. 4. Erratic bed depth on control panel. A. Causal factor: 1. Bed pressure taps are plugged with solids. B. Corrective actions (where applicable): 1. Tap a metal rod into pressure tap pipe when the unit is not in operation. 2. Apply compressed air to pressure tap while the unit is in operation (follow manufacturerâ&#x20AC;&#x2122;s safety guidelines). 5. Preheat burner fails and alarm sounds. A. Causal factors: 1. Pilot flame is not receiving fuel. 2. Pilot flame is not receiving spark. 3. Defective pressure regulators. 4. Pilot flame ignites, but flame scanner malfunctions. B. Corrective actions (where applicable): 1. Correct fuel system problem. 2. Replace defective part. 3. Replace defective regulators. 4. Clear scanner sight glass. 5. Replace defective scanner. 6. Bed temperature is too high. A. Causal factors:

634

Handbook of Water and Wastewater Treatment Plant Operations

1. Bed gun fuel feed rate is too high. 2. Grease or high organic content in sludge (high heat value). B. Corrective Actions (where applicable): 1. Reduce bed gun fuel feed rate. 2. Increase airflow rate. 3. Decrease sludge fuel rate. 7. Bed temperature reads off scale. A. Causal factor: 1. Thermocouple has burned out. B. Corrective action: 1. Replace thermocouple. 8. Scrubber inlet shows high temperature. A. Causal factors: 1. No water flowing in scrubber. 2. Spray nozzles are plugged. 3. Ash water not recirculating. B. Corrective actions (where applicable): 1. Open valves to provide water. 2. Correct system malfunction to provide required pressure. 3. Clear nozzles and strainers. 4. Repair or replace recirculation pump. 5. Unclog scrubber discharge line. 9. Poor bed fluidization. A. Causal factor: 1. Sand leakage through support plate during shutdown. B. Corrective actions (where applicable): 1. Clear wind box. 2. Clean wind box at least once per month. 18.13.4.6 Land Application of Biosolids The purpose of land application of biosolids is to dispose of the treated biosolids in an environmentally sound manner by recycling nutrients and soil conditioners. In order to be land applied, wastewater biosolids must comply with state and federal biosolids management and disposal regulations. Biosolids must not contain materials that are dangerous to human health (i.e., toxicity, pathogenic organisms, etc.) or dangerous to the environment (i.e., toxicity, pesticides, heavy metals, etc.). Treated biosolids are land applied by either direct injection or application and plowing in (incorporation). 18.13.4.6.1 Process Control: Sampling and Testing Land application of biosolids requires precise control to avoid problems. The quantity and the quality of biosolids applied must be accurately determined. For this reason, the operator’s process control activities include biosolids sampling and testing functions. Biosolids sampling and testing includes determination of percent solids, heavy metals, organic pesticides and © 2003 by CRC Press LLC

herbicide, alkalinity, total organic carbon, organic nitrogen, and ammonia nitrogen. 18.13.4.6.2 Process Control Calculations Process control calculations include determining disposal cost, plant available nitrogen (PAN), application rate (dry tons and wet tons per acre), metals loading rates, maximum allowable applications based upon metals loading, and site life based on metals loading. 18.13.4.6.2.1

Disposal Cost

The cost of disposal of biosolids can be determined by the following equation: Cost = Wet Tons Biosolids Produced Year ¥ % Solids ¥ Cost Dry Ton

(18.81)

EXAMPLE 18.88 Problem: The treatment system produces 1925 wet tons of biosolids for disposal each year. The biosolids are 18% solids. A contractor disposes of the biosolids for $28.00 per dry ton. What is the annual cost for sludge disposal?

Solution: Cost = 1925 wet tons year ¥ 0.18 ¥ $28.00 dry ton = $9702 18.13.4.6.2.2

Plant Available Nitrogen

One factor considered when land applying biosolids is the amount of nitrogen in the biosolids available to the plants grown on the site. This includes ammonia nitrogen and organic nitrogen. The organic nitrogen must be mineralized for plant consumption. Only a portion of the organic nitrogen is mineralized per year. The mineralization factor (f1) is assumed to be 0.20. The amount of ammonia nitrogen available is directly related to the time elapsed between applying the biosolids and incorporating (plowing) the sludge into the soil. We provide volatilization rates based upon this example below:

[

PAN (lb dry ton ) = (Organic Nitrogen ( mg kg) ¥ f1 ) +

(Ammonia Nitrogen (mg kg) ¥ V )] ¥ 1

0.002 lb dry ton

(18.82)

where f1 = Mineral rate for organic nitrogen (assume 0.20) V1 = Volatilization rate ammonia nitrogen V1 = 1.00 if biosolids are injected

Wastewater Treatment

635

V1 = 0.85 if biosolids are plowed in within 24 h V1 = 0.70 if biosolids are plowed in within 7 d EXAMPLE 18.89

(18.85)

Metal Concentration (mg kg) ¥ 0.002 lb dry ton ¥ Applied Rate (dry tons acre)

Problem: The biosolids contain 21,000 mg/kg of organic nitrogen and 10,500 mg/kg of ammonia nitrogen. The biosolids are incorporated into the soil within 24 h after application. What is the PAN per dry ton of solids?

Solution: PAN (lb dry ton ) = [(21, 000 mg kg ¥ 0.20) + (10, 500 ¥ 0.85)] ¥ 0.002 = 26.3 lb PAN dry ton 18.13.4.6.2.3

Loading Rate (lb acre) =

EXAMPLE 18.91 Problem: The biosolids contain 14 mg/kg of lead. Biosolids are currently being applied to the site at a rate of 11 dry tons/acre. What is the metals loading rate for lead in pounds per acre?

Solution: Loading Rate (lb acre) = 14 mg kg ¥ 0.002 lb dry ton ¥ 11 dry tons

Application Rate Based on Crop Nitrogen Requirement

In most cases, the application rate of domestic biosolids to crop lands will be controlled by the amount of nitrogen the crop requires. The biosolids application rate based upon the nitrogen requirement is determined by the following: 1. Using an agriculture handbook to determine the nitrogen requirement of the crop to be grown 2. Determining the amount of sludge in dry tons required to provide this much nitrogen

= 0.31 lb acre 18.13.4.6.2.5

Maximum Allowable Applications Based upon Metals Loading

If metals are present, they may limit the total number of applications a site can receive. Metals loading are normally expressed in terms of the maximum total amount of metal that can be applied to a site during its use: Applications =

Dry tons acre = Plant Nitrogen Requirement (lb acre) (18.83) PAN (lb dry ton)

(18.86)

Maximum Allowable Cumulative Load for the Metal ( lb acre) Metal Loading ( lb acre application )

EXAMPLE 18.92 EXAMPLE 18.90

Problem:

Problem: The crop to be planted on the land application site requires 150 lb of nitrogen per acre. What is the required biosolids application rate if the PAN of the biosolids is 30 lb/dry ton?

The maximum allowable cumulative lead loading is 48.0 lb/acre. Based upon the current loading of 0.35 lb/acre, how many applications of biosolids can be made to this site?

Solution:

Solution: Dry tons acre =

Applications =

150 lb nitrogen acre 30 lb dry ton

(18.84)

= 137 applications

= 5 dry tons acre 18.13.4.6.2.4

Metals Loading

48 lb acre 0.35 lb acre application

18.13.4.6.2.6

Site Life Based on Metals Loading

The maximum number of applications based upon metals loading and the number of applications per year can be used to determine the maximum site life:

636

Handbook of Water and Wastewater Treatment Plant Operations

Site Life (years) = (18.87) Maximum Allowable Applications Number of Applications Planned Year EXAMPLE 18.93 Problem: Biosolids is currently applied to a site twice annually. Based upon the lead content of the biosolids, the maximum number of applications is determined to be 135 applications. Based upon the lead loading and the application rate, how many years can this site be used?

Solution: Site Life ( years) =

135 applications 2 applications year

= 68 years

Note: When more than one metal is present, the calculations must be performed for each metal. The site life would then be the lowest value generated by these calculations.

18.14 PERMITS, RECORDS, AND REPORTS Permits, records, and reports play a significant role in wastewater treatment operations. In fact, in regards to the permit, one of the first things any new operator quickly learns is the importance of â&#x20AC;&#x153;making permitâ&#x20AC;? each month. In this section, we briefly cover National Pollutant Discharge Elimination System (NPDES) permits and other pertinent records and reports the wastewater operator must be familiar with. Note: The discussion that follows is general in nature; it does not necessarily apply to any state in particular, but instead is an overview of permits, records, and reports that are an important part of wastewater treatment plant operations. For specific guidance on requirements for your locality, refer to your state water control board or other authorized state agency for information. In this handbook, the term board signifies the state-reporting agency.

18.14.1 DEFINITIONS There are several definitions that should be discussed prior to discussing the permit requirements for records and reporting: Average daily limitation the highest allowable average over a 24-h period, calculated by adding all ÂŠ 2003 by CRC Press LLC

of the values measured during the period and dividing the sum by the number of values determined during the period. Average hourly limitation the highest allowable average for a 60-min period, calculated by adding all of the values measured during the period and dividing the sum by the number of values determined during the period. Average monthly limitation the highest allowable average over a calendar month, calculated by adding all of the daily values measured during the month and dividing the sum by number of daily values measured during the month. Average weekly limitation the highest allowable average over a calendar week, calculated by adding all of the daily values measured during the calendar week and dividing the sum by the number of daily values determined during the week. Daily discharge the discharge of a pollutant measured during a calendar day or any 24-h period that reasonably represents the calendar for the purpose of sampling. For pollutants with limitations expressed in units of weight, the daily discharge is calculated as the total mass of the pollutant discharged over the day. For pollutants with limitations expressed in other units, the daily discharge is calculated as the average measurement of the pollutant over the day. Discharge monitoring report forms used in the reporting of self-monitoring results of the permittee. Discharge permit State Pollutant Discharge Elimination System (state-PDES) permit that specifies the terms and conditions under which a point source discharge to state waters is permitted. Effluent limitation any restriction by the state board on quantities, discharge rates, or concentrations of pollutants that are discharged from point sources into state waters. Maximum daily discharge the highest allowable value for a daily discharge. Maximum discharge the highest allowable value for any single measurement. Minimum discharge the lowest allowable value for any single measurement. Point source any discernible, defined, and discrete conveyance, including but not limited to any pipe, ditch, channel, tunnel, conduit, well, discrete fissure, container, rolling stock, vessel, or other floating craft, from which pollutants are or may be discharged. This definition does not include return flows from irrigated agricultural land.

Wastewater Treatment

637

18.14.2 NPDES PERMITS In the U.S., all treatment facilities that discharge to state waters must have a discharge permit issued by the state water control board or other appropriate state agency. This permit is known on the national level as the National NPDES permit and on the state level as the state-PDES permit. The permit states the specific conditions that must be met to legally discharge treated wastewater to state waters. The permit contains general requirements (applying to every discharger) and specific requirements (applying only to the point source specified in the permit). A general permit is a discharge permit that covers a specified class of dischargers. It is developed to allow dischargers with the specified category to discharge under specified conditions. All discharge permits contain general conditions. These conditions are standard for all dischargers and cover a broad series of requirements. Read the general conditions of the treatment facility’s permit carefully. Permittees must retain certain records. These records include:

Monitoring: 5. 1. Date, time, and exact place of sampling or measurements 2. Names of the individuals performing sampling or measurement 3. Dates and times analyses were performed 4. Names of the individuals who performed the analyses 5. Analytical techniques or methods used 6. Observations, readings, calculations, bench data, and results 7. Instrument calibration and maintenance 8. Original strip chart recordings for continuous monitoring 9. Information used to develop reports required by the permit 10. Data used to complete the permit application Note: All records must be kept at least 3 years (longer at the request of the state board). 18.14.2.1 Reporting In general, reporting must be made under the following conditions and situations (requirements may vary depending upon state regulatory body with reporting authority): 1. Unusual or extraordinary discharge reports — The board must be notified by telephone within © 2003 by CRC Press LLC

24 h of occurrence and submit written report within 5 d. The report must include: A. Description of the non-compliance and its cause. B. Noncompliance dates, times, and duration. C. Steps planned or taken to reduce or eliminate occurrence. D. Steps planned or taken to prevent reoccurrence. Anticipated noncompliance — The board must be notified at least 10 d in advance of any changes to the facility or activity that may result in noncompliance. Compliance schedules — Compliance or noncompliance with any requirements contained in compliance schedules must be reported no later than 14 d following scheduled date for completion of the requirement. 24-h Reporting — Any noncompliance that may adversely affect state waters or may endanger public health must be reported orally with 24 h of the time the permittee becomes aware of the condition. A written report must be submitted within 5 d. Discharge monitoring reports (DMRs) — These reports consist of self-monitoring data generated during a specified period (normally 1 month). When completing the DMR, remember: A. More frequent monitoring must be reported. B. All results must be used to complete reported values. C. Pollutants monitored by an approved method, but not required by the permit must be reported. D. No empty blocks on the form should be left blank. E. Averages are arithmetic unless noted otherwise. F. Appropriate significant figures should be used. G. All bypasses and overflows must be reported. H. The licensed operator must sign the report. I. Responsible official must sign the report. J. Department must receive by the 10th of the following month.

18.14.2.2 Sampling and Testing The general requirements of the permit specify minimum sampling and testing that must be performed on the plant discharge. The permit will also specify the frequency of sampling, sample type, and length of time for composite samples. Unless a specific method is required by the permit, all sample preservation and analysis must be in compliance

638

Handbook of Water and Wastewater Treatment Plant Operations

with the requirements set forth in the Code of Federal Regulations, Guidelines Establishing Test Procedures for the Analysis of Pollutants Under the Clean Water Act (40 CFR 136). Note: All samples and measurements must be representative of the nature and quantity of the discharge. 18.14.2.3 Effluent Limitations The permit sets numerical limitations on specific parameters contained in the plant discharge. Limits may be expressed as:

18.14.2.8 Reporting Calculations Failure to accurately calculate report data will result in violations of the permit. The basic calculations associated with completing the DMR are covered below. 18.14.2.8.1 Average Monthly Concentration The average monthly concentration (AMC) is the average of the results of all tests performed during the month: AMC (mg L ) =

Â Test + Test 1

1. 2. 3. 4. 5. 6. 7. 8. 9.

Average monthly quantity (kg/d) Average monthly concentration (mg/L) Average weekly quantity (kg/d) Average weekly concentration (mg/L) Daily quantity (kg/d) Daily concentration (mg/L) Hourly average concentration (mg/L) Instantaneous minimum concentration (mg/L) Instantaneous maximum concentration (mg/L)

18.14.2.4 Compliance Schedules The facility may require additional construction or other modifications to fully comply with the final effluent limitations. If this is the case, the permit will contain a schedule of events to be completed to achieve full compliance.

Any special requirements or conditions set for approval of the discharge will be contained in this section. Special conditions may include: 1. Monitoring required to determine effluent toxicity 2. Pretreatment program requirements 18.14.2.6 Licensed Operator Requirements The permit will specify, based on the treatment system complexity and the volume of flow treated, the minimum license classification required to be the designated responsible charge operator. 18.14.2.7 Chlorination or Dechlorination Reporting Several reporting systems apply to chlorination or chlorination followed by dechlorination. It is best to review this section of the specific permit for guidance. Contact the appropriate state regulatory agency for any needed clarification. © 2003 by CRC Press LLC

+ Test 3 + K + Test n

(18.88)

where N = tests during a month. 18.14.2.8.2 Average Weekly Concentration (AWC) The average weekly concentration (AWC) is the results of all the tests performed during a calendar week. A calendar week must start on Sunday and end on Saturday and be completely within the reporting month. A weekly average is not computed for any week that does not meet these criteria: AWC (mg L ) =

Â Test + Test 1

18.14.2.5 Special Conditions

+ Test 3 + K + Test n

(18.89)

where N = tests during a calendar week. 18.14.2.8.3 Average Hourly Concentration The average hourly concentration (AHC) is the average of all test results collected during a 60-min period: AHC (mg L ) =

Â Test + Test 1

+ Test 3 + K + Test n

(18.90)

where N = tests during a 60-min period. 18.14.2.8.4 Daily Quantity Daily quantity (DQ) is the quantity of a pollutant in kilograms per day discharged during a 24-h period: DQ ( kg d ) = Concentration (mg L ) ¥ Q (MGD) ¥ 3.785 kg MG mg L

(18.91)

Wastewater Treatment

639

18.14.2.8.5 Average Monthly Quantity Average monthly quantity (AMQ) is the average of all the individual daily quantities determined during the month: AMQ ( kg d ) =

Â DQ + DQ 1

(18.92)

+ DQ 3 + º + DQ n

18.15 CHAPTER REVIEW QUESTIONS AND PROBLEMS 18.1. 18.2. 18.3. 18.4. 18.5. 18.6.

where N = tests during a month. 18.14.2.8.6 Average Weekly Quantity The average weekly quantity (AWQ) is the average of all the daily quantities determined during a calendar week. A calendar week must start on Sunday and end on Saturday and be completely within the reporting month. A weekly average is not computed for any week that does not meet these criteria:

18.8.

18.9.

18.10.

AWQ ( kg d ) =

Â DQ + DQ

18.7.

(18.93)

+ DQ 3 + º + DQ n N

18.13.

where N = tests during a calendar week.

18.14.

18.14.2.8.7 Minimum Concentration The minimum concentration is the lowest instantaneous value recorded during the reporting period. 18.14.2.8.8 Maximum Concentration Maximum concentration is the highest instantaneous value recorded during the reporting period. 18.14.2.8.9 Bacteriological Reporting Bacteriological reporting is used for reporting fecal coliform test results. To make this calculation the geometric mean calculation is used and all monthly geometric means are computed using all the test values. Note that weekly geometric means are computed using the same selection criteria discussed for average weekly concentration and quantity calculations. The easiest method used in making this calculation requires a calculator, which can perform logarithmic (log) or Nth root functions: Geometric Mean =

18.11. 18.12.

(18.94)

18.15. 18.16.

18.17.

18.18.

18.19.

18.20.

18.21.

Who must sign the DMR? What does the COD test measure? Give three reasons for treating wastewater. Name two types of solids based on physical characteristics. Define organic and inorganic. Name four types of microorganisms that may be present in wastewater. When organic matter is decomposed aerobically, what materials are produced? Name three materials or pollutants, which are not removed by the natural purification process. What are the used water and solids from a community that flow to a treatment plant called? Where do disease-causing bacteria in wastewater originate? What does the term pathogenic mean? What is wastewater called that comes from the household? What is wastewater called that comes from industrial complexes? The lab test indicates that a 500-g sample of sludge contains 22 g of solids. What are the percent solids in the sludge sample? The depth of water in the grit channel is 28 in. What is the depth in feet? The operator withdraws 5250 gal of solids from the digester. How many pounds of solids have been removed? Sludge added to the digester causes a 1920–ft3 change in the volume of sludge in the digester. How many gallons of sludge have been added? The plant effluent contains 30 mg/L solids. The effluent flow rate is 3.4 MGD. How many pounds per day of solids are discharged? The plant effluent contains 25 mg/L BOD. The effluent flowrate is 7.25 MGD. How many kilograms per day of BOD are being discharged? The operator wishes to remove 3280 lb/d of solids from the activated sludge process. The waste activated sludge concentration is 3250 mg/L. What is the required flow rate in million gallons per day? The plant influent includes an industrial flow that contains 240 mg/L BOD. The industrial flow is 0.72 MGD. What is the population equivalent for the industrial contribution in people per day?

640

Handbook of Water and Wastewater Treatment Plant Operations

18.22. The label of hypochlorite solution states that the specific gravity of the solution is 1.1288. What is the weight of 1 gal of the hypochlorite solution? 18.23. What must be done to the cutters in a comminutor to ensure proper operation? 18.24. What is grit? Give three examples of material which is considered to be grit. 18.25. The plant has three channels in service. Each channel is 2 ft wide and has a water depth of 3 ft. What is the velocity in the channel when the flow rate is 8 MGD? 18.26. The grit from the aerated grit channel has a strong hydrogen sulfide odor upon standing in a storage container. What does this indicate and what action should be taken to correct the problem? 18.27. What is the purpose of primary treatment? 18.28. What is the purpose of the settling tank in the secondary or biological treatment process? 18.29. The circular settling tank is 90 ft in diameter and has a depth of 12 ft. The effluent weir extends around the circumference of the tank. The flow rate 2.25 MGD. What is the detention time in hours, surface loading rate in gallons per day per square foot and weir overflow rate in gallons per day per foot? 18.30. Give three classifications of ponds based upon their location in the treatment system. 18.31. Describe the processes occurring in a raw sewage stabilization pond (facultative). 18.32. How do changes in the season affect the quality of the discharge from a stabilization pond? 18.33. What is the advantage of using mechanical or diffused aeration equipment to provide oxygen? 18.34. Name three classifications of trickling filters. Identify the classification that produces the highest quality effluent. 18.35. Microscopic examination reveals a predominance of rotifers. What process adjustment does this indicate is required? 18.36. Increasing the wasting rate will __________ the MLSS, ______________ the return concentration, ______________ the MCRT, __________ the F:M ratio, and __________ the SVI. 18.37. The plant currently uses 45.8 lb of chlorine per day. Assuming the chlorine usage will increase by 10% during the next year, how many 2000-lb cylinders of chlorine will be needed for the year (365 days)? 18.38. The plant has 6 2000-lb cylinders on hand. The current dose of chlorine being used to disinfect the effluent is 6.2 mg/L. The averÂŠ 2003 by CRC Press LLC

18.39.

18.40.

18.41.

18.42.

18.43.

18.44. 18.45. 18.46. 18.47. 18.48. 18.49.

18.50. 18.51.

18.52. 18.53. 18.54.

age effluent flow rate is 2.25 MGD. Allowing 15 days for ordering and shipment, when should the next order for chlorine be made? The plant feeds 38 lb of chlorine per day and uses 150-lb cylinders. Chlorine use is expected to increase by 11% next year. The chlorine supplier has stated that the current price of chlorine ($0.170/lb) will increase by 7.5% next year. How much money should the town budget for chlorine purchases for the next year (365 days)? The sludge pump operates 30 min every 3 h. The pump delivers 70 gal/min. If the sludge is 5.1% solids and has a volatile matter content of 66%, how many pounds of volatile solids are removed from the settling tank each day? The aerobic digester has a volume of 63,000 gal. The laboratory test indicates that 41 mg of lime were required to increase the pH of a 1-L sample of digesting sludge from 6 to the desired 7.1. How many pounds of lime must be added to the digester to increase the pH of the unit to 7.4? The digester has a volume of 73,500 gal. Sludge is added to the digester at the rate of 2750 gal/d. What is the SRT in days? The raw sludge pumped to the digester contains 72% volatile matter. The digested sludge removed from the digester contains 48% volatile matter. What is the percent volatile matter reduction? The acronym NPDES stands for ______________________________. How can primary sludge be freshened going into a gravity thickener? A neutral solution has a pH value of _______. Why is the seeded BOD test required for some samples? What is the foremost advantage of the COD over the BOD? High mixed liquor concentration is indicated by a ____________________ aeration tank foam. What typically happens to the activity level of bacteria when the temperature is increased? List three factors other than food that affects the growth characteristics of activated sludge. What are the characteristics of facultative organisms? BOD measures the amount of ___________ material in wastewater. The activated sludge process requires ______________________ in the aeration tank to be successful.

Wastewater Treatment

18.55. The activated sludge process can not be successfully operated with a ________ clarifier. 18.56. The activated biosolids process can successfully remove ___________ BOD. 18.57. Successful operation of a complete mix reactor in the endogenous growth phase is ______________. 18.58. The bacteria in the activated biosolids process are either __________ or ___________. 18.59. Step feed activated biosolids processes have ___________ mixed liquor concentrations in different parts of the tank. 18.60. An advantage of contact stabilization compared to complete mix is ___________ aeration tank volume. 18.61. Increasing the _____________ of wastewater increases the BOD in the activated biosolids process. 18.62. Bacteria need phosphorus to successfully remove _________ in the activated biosolids process. 18.63. The growth rate of microorganisms is controlled by the _________ ratio. 18.64. Adding chlorine just before the __________ can control alga growth. 18.65. What is the purpose of the secondary clarifier in an activated biosolids process? 18.66. The ____________ growth phase should occur in a complete mix activated biosolids process. 18.67. The typical DO value for activated biosolids plants is between ______ and ______ mg/L. 18.68. In the activated biosolids process, what change would an operator normally expect to make when the temperature decreases from 25°C to15°C? 18.69. In the activated biosolids process, what change must be made to increase the MLVSS? 18.70. In the activated biosolids process, what change must be made to increase the F:M? 18.71. What does the Gould sludge age assume to be the source of the MLVSS in the aeration tank? 18.72. What is one advantage of complete mix over plug flow? 18.73. The grit in the primary sludge is causing excessive wear on primary treatment sludge pumps. The plant uses an aerated grit channel. What action should be taken to correct this problem? 18.74. When the MCRT increases, the MLSS concentration in the aeration tank ___________. 18.75. Exhaust air from a chlorine room should be taken from where? 18.76. If chlorine costs $0.21/lb, what is the daily cost to chlorinate a 5-MGD flow rate at chlorine feed rate of 2.6 mg/L? © 2003 by CRC Press LLC

641

18.77. What is the term that describes a normally aerobic system from which the oxygen has temporarily been depleted? 18.78. The ratio that describes the minimum amount of nutrients theoretically required for an activated sludge system is 100:5:1. What are the elements that fit this ratio? 18.79. A flotation thickener is best used for what type of sludge? 18.80. True of false: Drying beds are an example of a sludge stabilization process. 18.81. The minimum flow velocity in collection systems should be ___________________. 18.82. What effect will the addition of chlorine, acid, alum, carbon dioxide, or sulfuric acid have on the pH of wastewater? 18.83. An amperometric titrater is used to measure ___________________. 18.84. The normal design detention time for primary clarifier is _________________. 18.85. The volatile acids-alkalinity ratio in an anaerobic digester should be approximately ________. 18.86. The surface loading rate in a final clarifier should be approximately _____________. 18.87. In a conventional effluent chlorination system the chlorine residual measured is mostly in the form of ___________. 18.88. For a conventional activated biosolids process, the Food:Microorganism (F:M) ratio should be in the range of ___ to ____. 18.89. Denitrification in a final clarifier can cause clumps of sludge to rise to the surface. The sludge flocs attach to small sticky bubbles of _________ gas. 18.90. An anaerobic digester is covered and kept under positive pressure to do what? 18.91. During the summer months, the major source of oxygen added to a stabilization pond is ___________. 18.92. Which solids cannot be removed by vacuum filtration? 18.93. The odor recognition threshold for H2S is reported to be as low as:

REFERENCES 1. Metcalf & Eddy, Inc. Wastewater Engineering: Treatment, Disposal, Reuse, 3rd ed., McGraw-Hill, New York, 1991, pp. 29–31. 2. Metcalf & Eddy, Inc., Wastewater Engineering: Treatment, Disposal, Reuse, 3rd ed., McGraw-Hill, New York, 1991.