Nireas vol 2 part 1 final b by GEORGE DIAMANDIS

ELEARNI NG FOR THE OPERATORS OFWASTEWATER TREATMENT

VOLUME 2

MAIN WASTEWATER TREATMENTPROCESSES 2.1-2.2-2.3

NIREAS VOLUME 2 [2.1-2.2-2.3]

2. MAIN WASTEWATER TREATMENT PROCESSES 2.1 Introduction 2.1.1 An Overiew

2.2 Preliminary Treatment 2.2.1 Screens-Microscreens 2.2.2 Flow Measurement 2.2.3 Flow equalization 2.3.1 Introduction 2.3.1.1 Process Description 2.3.2.2 Two-Story (Imhoff) Tank2.3.2.3 Plain Settling Tanks (Clarifiers) 2.3.3 Operator Observations 2.3.3.1 Primary Clarification: Normal Operation 2.3.3.2 Operational Parameters for Primary Clarification 2.3.3.3 Process Control Calculations

NIREAS VOLUME 2 [2.1-2.2-2.3]

2. MAIN WASTEWATER TREATMENT PROCESSES

2.1 Introduction explaining the phases of treatment (primary, secondary, etc.), contributing the normative definitions 2.1.1 An Overiew Every community produces both liquid and solid wastes and air emissions. The liquid waste --- wastewater --- is essentially the water supply of the community after it has been used in a variety of applications (see Fig. 2.1). From the standpoint of sources of generation, wastewater may be defined as a combination of the liquid or water-carried wastes removed from residences, institutions, and commercial and industrial establishments, together with such groundwater, surface water, and stormwater as may be present. Fig.2.1.1 Schematic diagram of a wastewater infrastructure

Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003

NIREAS VOLUME 2 [2.1-2.2-2.3]

When untreated wastewater accumulates and is allowed to go septic, the decomposition of the organic matter it contains will lead to nuisance conditions including the production of malodorous gases. In addition, untreated wastewater contains numerous pathogenic microorganisms that dwell in the human intestinal tract. Wastewater also contains nutrients, which can stimulate the growth of aquatic plants, and may contain toxic compounds or compounds that potentially may be mutogenic or carcinogenic. For these reasons, the immediate and nuisance-free removal of wastewater from its sources of generation, followed by treatment, reuse, or dispersal into the environment is necessary to protect public health and the environment. Wastewater engineering is that branch of environmental engineering in which the basic principles of science and engineering are applied to solving the issues associated with the treatment and reuse of wastewater. The ultimate goal of wastewater engineering is the protection of public health in a manner commensurate with environmental, economic, social, and political concerns. To protect public health and the environment, it is necessary to have knowledge of (1) constituents of concern in wastewater, (2) impacts of these constituents when wastewater is dispersed into the environment, (3) transformation and long-term fate of these constituents in treatment processes, (4) treatment methods that can be used to remove or modify the constituents found in wastewater, and (5) methods for beneficial use or disposal of solids generated by the treatment systems. To provide an initial perspective on the field of wastewater engineering, common terminology is first defined followed by (1) a discussion of the issues that need to be addressed in the planning and design of wastewater management systems and (2) the current status and new directions in wastewater engineering. Objectives of wastewater treatment Wastewater treatment is very necessary for the above-mentioned reasons. It is more vital for the: Reduction of biodegradable organic substances in the environment: organic substances with carbon, nitrogen, phosphorus and sulphur in organic matter needs to be broken down by oxidation into gases which is either released or remains in solution. Reduction of nutrient concentration in the environment: nutrients such as nitrogen and phosphorous from wastewater in the environment enrich water bodies or render it

NIREAS VOLUME 2 [2.1-2.2-2.3]

eutrophic leading to the growth of algae and other aquatic plants. These plants deplete oxygen in water bodies and this hampers aquatic life. Elimination of pathogens: organisms that cause disease in plants, animals and humans are called pathogens. They are also known as micro-organisms because they are very small to be seen with the naked eye. Examples of micro-organisms include bacteria (e.g. vibro cholerae), viruses (e.g. enterovirus, hepatits A & E virus), fungi (e.g. candida albicans), protozoa (e.g entamoeba hystolitica, giardia lamblia) and helminthes (e.g. schistosoma mansoni, asaris lumbricoides). These micro-organisms are excreted in large quantities in faeces of infected animals and humans (Awuah and Amankwaa-Kuffuor, 2002). Recycling and Reuse of water: Water is a scarce and finite resource which is often taken for granted. In the last half of the 20th century, population has increased resulting in pressure on the already scarce water resources. Urbanization has also changed the agrarian nature of many areas. Population increase means more food has to be cultivated for the growing population and agriculture as we know is by far the largest user of available water which means that economic growth is placing new demands on available water supplies. The temporal and spatial distribution of water is also a major challenge with groundwater. From: Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003

1-1

TERMINOLOGY

In the literature, and in governmental regulations, a variety of terms have been used for individual constituents of concern in wastewater. The terminology used commonly for key concepts and terms in the field of wastewater management is summarized in Table 1-1. In some cases, confusion and undue negative perceptions arise with the use of the terms contaminants, impurities, and pollutants, which are often used interchangeably. To avoid confusion, the term constituent is used in this text in place of these terms to refer to an individual compound or element, such as ammonia nitrogen. The term characteristic is used to refer to a group of constituents, such as physical or biological characteristics.

NIREAS VOLUME 2 [2.1-2.2-2.3]

The term “sludge” has been used for many years to signify the residuals produced in wastewater treatment. In 1994, the Water Environment Federation adopted a policy defining “biosolids” as a primarily organic, solid wastewater treatment product that can be recycled beneficially. In this policy, “solids” are defined as the residuals that are derived from the treatment of wastewater. Solids that have been treated to the point at which they are suitable for beneficial use are termed “biosolids.” In this text, the terms of solids and biosolids are used extensively, but “sludge” continues to be used, especially in cases where untreated solid material and chemical residuals are referenced.

Table 2.1.1 Terminology commonly used in the field of wastewater engineering a Term

Definition

_ Biosolids

Primarily an organic, semi-solid wastewater product that remains after solids are stabilized biologically or chemically and are suitable for beneficial use.

Class A biosolidsb

Biosolids in which the pathogens (including enteric viruses, pathogenic bacteria, and viable helminth ova) are reduced below current detectable levels.

Class B biosolidsb

Biosolids in which the pathogens are reduced to levels that are unlikely to pose a threat to public health and the environment under specific use conditions. Class B biosolids cannot be sold or given away in bags or other containers or applied on lawns or home gardens.

Characteristics (wastewater)

General classes of wastewater constituents such as physical, chemical, biological, and biochemical.

Composition

The makeup of wastewater, including the physical, chemical, and biological constituents

Constituentsc

Individual components, elements, or biological entities such as suspended solids or ammonia nitrogen.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Contaminants

Constituents added to the water supply through use.

Disinfection

Destruction of disease-causing microorganisms by physical or chemical means.

Effluent

The liquid discharged from a processing step.

Nonpoint sources

Sources of pollution that originate from multiple sources over a relatively large area.

Impurities

Constituents added to the water supply through use.

Nutrient

An element that is essential for the growth of plants and animals. Nutrients in wastewater, usually nitrogen and phosphorus, may cause unwanted algal and plant growths in lakes and streams.

Parameter

A measurable factor such as temperature.

Point sources

Pollutional loads discharged at a specific location from pipes, outfalls, and conveyance methods from either municipal wastewater treatment plants or industrial waste treatment facilities.

Pollutants

Constituents added to the water supply through use. Continued on following page

NIREAS VOLUME 2 [2.1-2.2-2.3] Table 2.1.1 Continued from previous page

Term

Definition

Reclamation

Treatment of wastewater for subsequent reuse application or the act of reusing treated wastewater.

Recycling

The reuse of treated wastewater and biosolids for beneficial purposes

Repurification

Treatment of wastewater to a level suitable for a variety of applications including indirect or direct potable reuse.

Reuse

Beneficial use of reclaimed or repurified wastewater or stabilized biosolids.

Sludge

Solids removed from wastewater during treatment. Solids that are treated further are termed biosolids.

Solids

Material is removed from wastewater by gravity separation (by clarifiers, thickeners, and lagoons) and is the solid residue from dewatering operations.

_________________________________________________________ _____________________________ a

Adapted, in part, from Crites and Tchobanoglous (1998)

U.S. EPA, 1999

To avoid confusion the term â&#x20AC;&#x153;constituentsâ&#x20AC;? will be used in this text in place of contaminants, impurities, and pollutants.

Table 2.1.2 Levels of wastewater treatmenta Treatment level

Description

Preliminary

Removal of wastewater constituents such as rags, sticks, floatables, grit, and grease that may cause maintenance or operational problems with the treatment operations, processes, and ancillary systems.

Primary

Removal of a portion of the suspended solids and organic

NIREAS VOLUME 2 [2.1-2.2-2.3] matter from the wastewater.

Advanced primary

Enhanced removal of suspended solids and organic matter from the wastewater. Typically accomplished by chemical addition or filtration.

Secondary

Removal of biodegradable organic matter (in solution or suspension) and suspended solids. Disinfection is also typically included in the definition of conventional secondary treatment.

Secondary with nutrient removal

Removal of biodegradable organics, suspended solids, and nutrients (nitrogen, phosphorus, or both nitrogen and phosphorus).

Disinfection

Destruction of disease-causing microorganisms by physical or chemical means. Tertiary

Removal of residual suspended solids (after secondary treatment), usually by granular medium filtration or microscreens. Disinfection is also typically a part of tertiary treatment. Nutrient removal is often included in this definition.

Advanced

Removal of dissolved and suspended materials remaining after normal biological treatment when required for various water reuse applications.

Adapted, in part, from Crites and Tchobanoglous (1998)

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig. 2.1.2 Unit Operations and processes in a conventional wastewater plant.a Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003

What process should we select to remove pollutants from the wastewater?

Fig. 2.1.3 AFlow Sheet of the Operations and processes in a conventional wastewater plant.b b

Operation of Wastewater Treatment Plants, 7th edition,2008

NIREAS VOLUME 2 [2.1-2.2-2.3]

Table 2.1.3. Unit operations and processes used to remove constituents found in wastewater Constituent

Unit operation or process

See Chap.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Suspended solids

Screening Grit removal Sedimentation and high-rate clarification Hgh-rate clarification Flotation Chemical precipitation Depth filtration Surface filtration

Biodegradable organics

Aerobic suspended growth variations Aerobic attached growth variations Anaerobic suspended growth variations Anaerobic attached growth variations Lagoon variations Physical-chemical systems Chemical oxidation Advanced oxidation Membrane filtration

Nutrients Nitrogen

Chemical oxidation (breakpoint chlorination) Suspended-growth nitrification and denitrification variations Fixed-film nitrification and denitrification variations Air stripping Ion exchange

Phosphorus

Chemical treatment Biological phosphorus removal

NIREAS VOLUME 2 [2.1-2.2-2.3]

Nitrogen and phosphorus

Pathogens

Biological nutrient removal variations

Chlorine compounds Chlorine dioxide Ozone Ultraviolet (UV) radiation

Colloidal and dissolved solids

Membranes Chemical treatment Carbon adsorption Ion exchange

Volatile organic compounds

Air stripping Carbon adsorption Advanced oxidation

Odors

Chemical scrubbers Carbon adsorption Biofilters Compost filters

NIREAS VOLUME 2 [2.1-2.2-2.3]

ASSIGNMENTS SECTION

ASSIGNMENTS SECTION 1

QUESTIONS Write your answers:

2.1A Where do the disease-causing bacteria in wastewater come from?

2.1B What is the term that means "disease-causing

2.1C What is the most frequently used means of disinfecting treated wastewater?

2.1D What is the purpose of preliminary treatment?

2.1E What is the primary treatment?

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.1F What is the tertiary treatment?

2.1G Report methods or processes to remove Suspended Solids from the wastewater

2.1H Report methods or processes to remove Biodegradable organics from the wastewater

2.1I

What means Reduction of nutrient concentration in the environment?

2.1J Report methods or processes to remove Nitrogen from the wastewater

2.1K Report methods or processes to remove Phosphorous from the wastewater

NIREAS VOLUME 2 [2.1-2.2-2.3]

SUGGESTED ANSWARS: 2.1A Disease-causing bacteria in wastewater come from the body wastes of humans who have a disease. 2.1B Pathogenic means disease-causing. 2.1C Chlorination is the most frequently used means of disinfecting treated wastewater. 2.1D The purpose of preliminary treatment the removal of wastewater constituents such as rags, sticks, floatables, grit, and grease that may cause maintenance or operational problems with the treatment operations, processes 2.1E The primary treatment is the removal of a portion of the suspended solids and organic matter from the wastewater. 2.1F The tertiary treatment is the removal of residual suspended solids (after secondary treatment), usually by granular medium filtration or microscreens. Disinfection is also typically a part of tertiary treatment. Nutrient removal is often included in this definition. 2.1G Screening Grit removal, Sedimentation and high-rate clarification, High-rate clarification Flotation,Chemical precipitation, Depth filtration, Surface filtration

2.1H Aerobic suspended growth variations, Aerobic attached growth variations, Anaerobic suspended growth variations , Anaerobic attached growth variations, Lagoon variations, Physical-chemical systems, Chemical oxidation, Advanced oxidation, Membrane filtration 2.1I The nutrients such as nitrogen and phosphorous from wastewater in the environment enrich water bodies or render it eutrophic leading to the growth of algae and other aquatic plants. These plants deplete oxygen in water bodies and this hampers aquatic life.

2.1J Chemical oxidation (breakpoint chlorination), Suspended-growth nitrification and denitrification variations, Fixed-film nitrification and denitrification variations, Air stripping, Ion exchange 2.1K Two methods (mainly): Chemical treatment, Biological phosphorus removal

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.2.1 Screens-Microscreens SCREENING (Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003) The first unit operation generally encountered in wastewater treatment plants is screening. A screen is a device with openings, generally of uniform size, that is used to retain solids found in the influent wastewater to the treatment plant or in combined wastewater collection systems subject to overflows, especially from stormwater. The principal role of screening is to remove coarse materials from the flow stream that could (1) damage subsequent process equipment, (2) reduce overall treatment process reliability and effectiveness, or (3) contaminate waterways. Fine screens are sometimes used in place of or following coarse screens where greater removals of solids are required to (1) protect process equipment or (2) eliminate materials that may inhibit the beneficial reuse of biosolids. The application of screening devices needs to consider all aspects of screenings removal, transport, and disposal. Considerations include (1) the degree of screenings removal required because of potential effects on downsteam processes, (2) health and safety of the operators as screenings contain pathogenic organisms and attract insects, (3) odor potential, and (4) requirements for handling, transport and disposal, i.e., removal of organics (by washing) and reduced water content (by pressing), and (5) disposal options. Thus, an integrated approach is required to achieve effective screenings management. Classification of Screens Two general types of screens, coarse screens and fine screens, are used in preliminary treatment of wastewater. Coarse screens have clear openings ranging from 6 to 150 mm (0.25 to 6 in); fine screens have clear openings less than 6 mm (0.25 in). Microscreens, which generally have screen openings less than 50 Âľm, are used principally in removing fine solids from treated effluents.

NIREAS VOLUME 2 [2.1-2.2-2.3]

The screening element may consist of parallel bars, rods or wires, grating, wire mesh, or perforated plate, and the openings may be of any shape but generally are circular or rectangular slots. A screen composed of parallel bars or rods is often called a "bar rack" or a coarse screen and is used for the removal of coarse solids. Fine screens are devices consisting of perforated plates, wedgewire elements, and wire cloth that have smaller openings. The materials removed by these devices are known as screenings. Coarse Screens (Bar Racks) In wastewater treatment, coarse screens are used to protect pumps, valves, pipelines, and other appurtenances from damage or clogging by rags and large objects. Industrial waste treatment plants may or may not need them, depending on the character of the wastes. According to the method used to clean them, coarse screens are designated as either hand-cleaned or mechanically cleaned. Hand Cleaned Coarse Screens Hand cleaned coarse screens are used frequently ahead of pumps in small wastewater pumping stations and sometimes used at the headworks of small-to-medium size wastewater treatment plants. Oftentimes they are used for standby screening in bypass channels for service during high flow periods, when mechanically cleaned screens are being repaired, or in the event of a power failure. Normally, mechanically cleaned screens are provided in lieu of hand-cleaned screens to minimize manual labor required to clean the screens and to reduce flooding due to clogging. A perforated drainage plate should be provided at the top of the rack where the rakings may be stored temporarily for drainage. The screen channel should be designed to prevent the accumulation of grit and other heavy materials in the channel ahead of the screen and following it. The channel floor should be level or should slope downward through the screen without pockets to trap solids. Fillets may be desirable at the base of the sidewalls. The channel preferably should have a straight approach, perpendicular to the bar screen, to promote uniform distribution of screenable

NIREAS VOLUME 2 [2.1-2.2-2.3]

solids throughout the flow and on the screen. Typical design information for hand and mechanically cleaned bar screens is provided in Table 2.2.1. Table 2.2.1. Typical design information for hand and mechanically cleaned bar screens

Mechanically Cleaned Bar Screens The design of mechanically cleaned bar screens has evolved over the years to reduce the operating and maintenance problems and to improve the screenings removal capabilities. Many of the newer designs include extensive use of corrosion resistant materials including stainless steel and plastics. Mechanically cleaned bar screens are divided into four principal types: (1) chain-driven, (2) reciprocating rake, (3) catenary, and (4) continuous belt. Cable-driven bar screens were used extensively in the past, but largely have been replaced in wastewater applications by the other types of screens. Typical design information for mechanically cleaned is also included in Table 2.2.1. Examples of the different types of mechanically cleaned bar screens are shown on Fig. 2.2.2 and the advantages and disadvantages of each type are presented in table 2.2.2.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig.2.2.2. Typical mechanically cleaned coarse (a) front-cleaned, frontreturn chain-driven, (b) reciprocating rake, (c) catenary, and (d) continuous belt. adapted from Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003 Chain-driven screens Chain-driven mechanically cleaned bar screens can be divided into categories based on whether the screen is raked to clean from the front (upstream) side or the back (downstream) side and whether the rakes return to the bottom of the bar screen from the front or back. Each type has its advantages and disadvantages although the general mode of operation is similar. In general, front cleaned, front return screens (see Fig. 2.2.2a) are more efficient in terms of retaining captured solids, but they are less rugged and are susceptible to jamming by solids that collect at the base of the rake. Front cleaned, front return screens are seldom used for plants serving combined sewers where large objects can jam the rakes. In front

NIREAS VOLUME 2 [2.1-2.2-2.3]

cleaned, back return screens, the cleaning rakes return to the bottom of the bar screen on the downstream side of the screen, pass under the bottom of the screen, and clean the bar screen as the rake rises. The potential for jamming is minimized, but a hinged plate, which is also subject to jamming, is required to seal the pocket under the screen. In back cleaned screens, the bars protect the rake from damage by the debris. However, a back-cleaned screen is more susceptible to solids carryover to the downstream side, particularly as rake wipers wear out. The bar rack of the back cleaned, back return screens is less rugged than the other types because the top of the rack is unsupported so the rake tines can pass through. Most of the chain operated screens share the disadvantage of submerged sprockets that require frequent operator attention and are difficult to maintain. Additional disadvantages include the adjustment and repair of the heavy chains, and the need to dewater the channels for inspection and repair of submerged parts. Reciprocating rake (climber) screen The reciprocating rake type bar screen (see Fig2.2.2b) imitates the movements of a person raking the screen. The rake moves to the base of the screen, engages the bars, and pulls the screenings to the top of the screen where they are removed. Most screen designs utilize a cog-wheel drive mechanism for the rake. The drive motors are either submersible electric or hydraulic type. A major advantage is that all parts requiring maintenance are above the water line and can be easily inspected and maintained without dewatering the channel. The front cleaned, front return feature minimizes solids carryover. The screen uses only one rake instead of multiple rakes that are used with other types of screens. As a result, the reciprocating rake screen may have limited capacity in handling heavy screenings loads, particularly in deep channels where a long "reach" is necessary. The high overhead clearance required to accommodate the rake mechanism can limit its use in retrofit applications. Catenary screen A catenary screen is a type of front cleaned, front return chain-driven screen, but it has no submerged sprockets. In the catenary screen (see Fig. 2.2.2c), the rake is held against the rack by the weight of the

NIREAS VOLUME 2 [2.1-2.2-2.3]

chain. If heavy objects become jammed in the bars, the rakes pass over them instead of jamming. The screen, however, has a relatively large â&#x20AC;&#x153;footprintâ&#x20AC;? and thus requires greater space for installation. Continuous belt screen The continuous belt screen is a relatively new development for use in screening applications in the United States. It is a continuous, self-cleaning screening belt that removes fine and coarse solids (see Fig. 2.2.2d). A large number of screening elements (rakes) are attached to the drive chains; the number of screening elements depends on the depth of the screen channel. Because the screen openings can range from 0.5 to 30 mm (0.02 to 1.18 in), it can be used either as a coarse or fine screen. Hooks protruding from the belt elements are provided to capture large solids such as cans, sticks, and rags. The screen has no submerged sprocket Table 2.2.2. Advantages and disadvantages of various types of bar screens Type of screen Change-driven screen Front clean/back return

Front clean/front return

Advantages

Disadvantages

Multiple cleaning elements (short cleaning cycle)

Unit has submerged moving parts that require channel dewatering for maintenance

Used for heavy duty applications

Less efficient screenings removal, i.e., carryover of residual screenings to screened wastewater channel

Multiple cleaning elements (short cleaning cycle)

Very little screenings carryover

Unit has submerged moving parts that require channel dewatering for maintenance Submerged moving parts (chains, sprockets, and shafts) are subject to fouling

NIREAS VOLUME 2 [2.1-2.2-2.3] Heavy objects may cause rake to jam

Back clean/back return

Reciprocating rake

Multiple cleaning elements (short cleaning cycle)

Unit has submerged moving parts that require channel dewatering for maintenance

Submerged moving parts (chains, sprockets, and shafts) are protected by bar rack

Long rake teeth are susceptible to breakage

No submerged moving parts; maintenance and repairs can be done above operating floor Can handle large objects (bricks, tires, etc.) Effective raking of screenings and efficient discharge of screenings

Unaccounted for high channel water level can submerge rake motor and cause motor burn-out Requires more headroom than other screens Long cycle time; raking capacity may be limiting

Relatively low operating and maintenance costs

Grit accumulation in front of bar may impede rake movement

Some susceptibility to screenings carryover

Stainless steel construction Relatively high cost due to reduces corrosion stainless steel construction High flow capacity Catenary

Sprockets are not submerged; most maintenance can be done above the operating floor

Because design relies on weight of chain for engagement of rakes with bars, chains are very heavy and difficult to handle

Required headroom is relatively low

Because of the angle of inclination of the screen (45 to 75-deg), screen has a large footprint

Multiple cleaning elements (short cleaning cycle)

Misalignment and warpage can occur when rakes are jammed

NIREAS VOLUME 2 [2.1-2.2-2.3] Can handle large objects

May emit odors because of open design

Very little screenings carryover Continuous belt

Most maintenance can be done above operating floor

Overhaul or replacement of the screening elements is a time- consuming and expensive operation

Unit is difficult to jam

Table 2.2.3 Main types of screening devices for wastewater preliminary treatment

NIREAS VOLUME 2 [2.1-2.2-2.3]

Table 2.2.4. Typical data on the removal of BOD and TSS with fine screens used to replace primary sedimentationa

Type of screen Fixed parabolic Rotary drum

Size of openings in mm 0.0625 1.6 0.01 0.25

Percent removal BOD TSS 5-20 5-30 25-50 25-45

a The actual removal achieved will depend on the nature of the wastewater collection system and the wastewater travel time.

Fig. 2.2.3. Typical fine screens: (a) static wedgewire, (b) drum, and (c) step. In step screens, screenings are moved up the screen by means of movable and fixed vertical plates. adapted from Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003

NIREAS VOLUME 2 [2.1-2.2-2.3]

adapted from Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003 Design of Screens Fine Screens The applications for fine screens range over a broad spectrum; uses include preliminary treatment (following coarse bar screens), primary treatment (as a substitute for primary clarifiers), and treatment of combined sewer overflows. Fine screens can also be used to remove solids from primary effluent that could cause clogging problems in trickling filters. Screens for Preliminary and Primary Treatment Fine screens used for preliminary treatment are of the (1) static (fixed), (2) rotary drum, or (3) step type. Typically, the openings vary from 0.2 to 6 mm (0.01 to 0.25 in). Examples of fine screens are illustrated on Fig. 2.2.3., descriptive information is provided in Table 2.2.3, and additional information is given below. In many cases, application of fine screens is limited to plants where headloss through the screens is not a problem.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fine screens may be used to replace primary treatment at small wastewater treatment plants, up to 0.13 to m3/s (3 Mgal/d) in design capacity. Typical removal rates of BOD and TSS are reported in Table 5-5. Stainless steel mesh or special wedge-shaped bars are used as the screening medium. Provision is made for the continuous removal of the collected solids, supplemented by water sprays to keep the screening medium clean. Headloss through the screens may range from about 0.8 to 1.4 m (2.5 to 4.5 ft). Static wedgewire screens Static wedgewire screens (see Fig. 2.2.3a) customarily have 0.2 to 1.2 mm (0.01 to 0.06 in) clear openings and are designed for flowrates of about 400 to 1200 L/m2 â&#x20AC;˘ min (10 to 30 gal/ft2 â&#x20AC;˘ min) of screen area. Headloss ranges from 1.2 to 2 m (4 to 7 ft). The wedgewire medium consists of small, stainless steel wedge-shaped bars with the flat part of the wedge facing the flow. Appreciable floor area is required for installation and the screens must be cleaned once or twice daily with high-pressure hot water, steam, or degreaser to remove grease buildup. Static wedgewire screens are generally applicable to smaller plants or for industrial installations. Drum screens For the drum type screen (see Fig. 2.2.3b), the screening or straining medium is mounted on a cylinder that rotates in a flow channel. The construction varies, principally with regard to the direction of flow through the screening medium. The wastewater flows either into one end of the drum and outward through the screen with the solids collection on the interior surface, or into the top of the unit and passing through to the interior with solids collection on the exterior. Internally fed screens are applicable for flow ranges of 0.03 to 0.8 m3/s (0.7 to 19 Mgal/d) per screen, while externally fed screens are applicable for flowrates less than 0.13 m3/s (3 Mgal/d (Laughlin & Roming, 1993). Drum screens are available in various sizes, from 0.9 to 2 m (3 to 6.6 ft) in diameter and from 1.2 to 4 m (4 to 13.3 ft) in length. Step screens Step screens are widely used in Europe. The design consists of two step-shaped sets of thin vertical plates, one fixed and one movable (see Fig. 2.2.3c). The fixed and movable step plates alternate across the width of an open channel and together form a single screen face. The

NIREAS VOLUME 2 [2.1-2.2-2.3]

movable plates rotate in a vertical motion. Through this motion, solids captured on the screen face are automatically lifted up to the next fixed step landing, and are eventually transported to the top of the screen where they are discharged to a collection hopper. The circular pattern of the moving plates provides a self-cleaning feature for each step. Normal ranges of openings between the screen plates are 3 to 6 mm (0.12 or 0.24 in), however, openings as small as 1 mm (0.04 in) are available. Solids trapped on the screen also create a â&#x20AC;&#x153;filter matâ&#x20AC;? that enhances solids removal performance. In addition to wastewater screening, step screens can be used for removal of solids from septage, primary sludge, or digested biosolids. Microscreens (used mainly for Primary effluent, secondary effluent and from stabilizationpond effluent) In wastewater pretreatment can be use for packed bed filters (preliminary and primary treatment at the same time, equivalent with a septic tank pretreatment). For raw wastewater can remove even 90% of the suspended solids and more than 50% of total BOD5 !!!. Microscreening involves the use of variable low-speed (up to 4 r/min), continuously backwashed, rotating-drum screens operating under gravity-flow conditions (see Fig 2.2.4). The filtering fabrics have openings of 10 to 35 mm and are fitted on the drum periphery. The wastewater enters the open end of the drum and flows outward through the rotating-drum screening cloth. The collected solids are backwashed by high-pressure jets into a trough located within the drum at the highest point of the drum. (The cloth medium surface filters can be used in advanced wastewater treatment.) The principal applications for microscreens are to remove suspended solids from Primary, secondary effluent and from stabilization-pond effluent. Typical suspended-solids removal achieved with microscreens ranges from 10 to 80 percent, with an average of 55 percent. Problems encountered with microscreens include incomplete solids removal and inability to handle solids fluctuations. Reducing the rotating speed of the drum and less frequent

NIREAS VOLUME 2 [2.1-2.2-2.3]

flushing of the screen have resulted in increased removal efficiencies but reduced capacity. The functional design of a microscreen involves (1) characterizing the suspended solids with respect to the concentration and degree of flocculation, (2) selecting design parameters that will not only assure sufficient capacity to meet maximum hydraulic loadings with critical solids characteristics but also meet operating performance requirements over the expected range of hydraulic and solids loadings, and (3) providing backwash and cleaning facilities to maintain the capacity of the screen. Typical design information for microscreens is presented in Table 2.2.5. Because of the variable performance of microscreens, pilot-plant studies are recommended, especially if the units are to be used to remove solids from stabilization-pond effluent, which may contain significant amounts of algae.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig.2.2.4. Microscreens used in wastewater treatment as a replacement for primary treatment: (a) disk type with stainless-steel fabric and (b) drum type with wedgewire screen. The size of the openings on both screens is 250 mm Table 2.2.5. Typical design information for microscreens used for screening secondary settled effluent a

NIREAS VOLUME 2 [2.1-2.2-2.3]

Adapted in part from Tchobanoglous, 1988.

Rotary Drum Fine Screen The wastewater flows into and through the open end of the inclined screen basket with the floating and suspended materials being retained by the screen basket. Blinding of the screen surface generates an additional filtering effect so that suspended materials can be retained that are smaller than the bar or perforation of the screen basket. The basket starts to rotate when a certain upstream water level is exceeded due to screen surface blinding. The rotating screen drum lifts the screenings and drops them into the centrally arranged trough. Screenings removal is supported by a scraper brush and spray bar. A screw conveyor in the trough rotates and transports the screenings through an inclined pipe. The conveying screw transports, dewaters and compacts the screenings, without odour nuisance, and discharges them into the customer's container or a subsequent conveying unit. The Rotary Drum Fine Screen / Perforated Plate Screen RPPS is completely made of stainless steel and is acid treated in a pickling bath. They are either

NIREAS VOLUME 2 [2.1-2.2-2.3]

installed directly into channels or are supplied as tank-mounted units, with an inclination of 35째. Available common bar spacing (0.5 - 6 mm) or plate perforation (1 - 6 mm) and the drum diameter (up to 3000 mm) many flow rates can be realised.

Fig.2.2.5. Rotary Drum Fine Screen

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig. 2.2.6 Typical device used for compacting screenings.

Screenings Characteristics and Quantities Screenings are the material retained on bar racks and screens. The smaller the screen opening, the greater will be the quantity of collected screenings. While no precise definition of screenable material exists, and no recognized method of measuring quantities of screenings is available, screenings exhibit some common properties. Screenings Retained on Coarse Screens. Coarse screenings, collected on coarse screens of about 12 mm (0.5 in) or greater spacing, consist of debris such as rocks, branches, pieces of lumber, leaves, paper, tree roots, plastics, and rags. Organic matter can collect as well. The accumulation of oil and grease can be a serious problem, especially in cold climates. The quantity and characteristics of screenings collected for disposal vary, depending on the type of bar screen, the size of the bar screen opening, the type of sewer system, and the geographic location. Typical data on the characteristics and quantities of coarse screenings to be expected at wastewater-treatment plants served by conventional gravity sewers are reported in Table 2.2.6. Combined storm and sanitary collection systems may produce volumes of screenings several times the amounts produced by separate systems. The quantities of screenings have also been observed to vary widely, ranging from large quantities during the â&#x20AC;&#x153;first flushâ&#x20AC;? to diminishing amounts as the wet weather flows persist. The quantities of screenings removed from combined sewer flows are reported to range from 3.5 to 84 L/1000 m3 of flow (0.5 to 11.3 ft3/Mgal) (WEF, 1998b). Table 2.2.6 Typical information on the characteristics and quantities of screenings removed from wastewater with coarse screens

NIREAS VOLUME 2 [2.1-2.2-2.3]

adapted from Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003 Table 2.2.7 Typical information on the characteristics and quantities of screenings removed from wastewater with fine bar and rotary-drum screens

Screenings Retained on Fine Screens. Fine screenings consist of materials that are retained on screens with openings less than 6 mm (0.25 in). The materials retained on fine screens include small rags, paper, plastic materials of various types, razor blades, grit, undecomposed food waste, feces, etc. Compared to coarse screenings, the specific weight of the fine screenings is slightly lower and the moisture content is slightly higher (see Table 2.2.7). Because putrescible matter, including fecal material, is contained within screenings, they must be handled and disposed of properly. Fine screenings contain substantial grease and scum, which require similar care, especially if odors are to be avoided. Screenings Handling, Processing, and Disposal. In mechanically cleaned screen installations, screenings are discharged from the screening unit directly into a screenings grinder, a pneumatic ejector, or a container for disposal; or onto a conveyor for transport to a screenings compactor or

NIREAS VOLUME 2 [2.1-2.2-2.3]

collection hopper. Belt conveyors and pneumatic ejectors are generally the primary means of mechanically transporting screenings. Belt conveyors offer the advantages of simplicity of operation, low maintenance, freedom from clogging, and low cost. Belt conveyors give off odors and may have to be provided with covers. Pneumatic ejectors are less odorous and typically require less space; however, they are subject to clogging if large objects are present in the screenings. Screenings compactors can be used to dewater and reduce the volume of screenings (see Fig. 2.2.6 ). Such devices, including hydraulic ram and screw compactors, receive screenings directly from the bar screens and are capable of transporting the compacted screenings to a receiving hopper. Compactors can reduce the water content of the screenings by up to 50 percent and the volume by up to 75 percent. As with pneumatic ejectors, large objects can cause jamming, but automatic controls can sense jams, automatically reverse the mechanism, and actuate alarms and shut down equipment. Means of disposal of screenings include (1) removal by hauling to disposal areas (landfill) including codisposal with municipal solid wastes, (2) disposal by burial on the plant site (small installations only), (3) incineration either alone or in combination with sludge and grit (large installations only), and (4) discharge to grinders or macerators where they are ground and returned to the wastewater. The first method of disposal is most commonly used. In some states, screenings are required to be lime stabilized for the control of pathogenic organisms before disposal in landfills. Grinding the screenings and returning them to the wastewater flow shares many of the disadvantages cited under comminution, as discussed in the following section.

COARSE SOLIDS REDUCTION As an alternative to coarse bar screens or fine screens, comminutors and macerators can be used to intercept coarse solids and grind or shred them in the screen channel. High-speed grinders are used in conjunction with mechanically cleaned screens to grind and shred screenings that are removed from the wastewater. The solids are cut up into a smaller, more

NIREAS VOLUME 2 [2.1-2.2-2.3]

uniform size for return to the flow stream for subsequent removal by downstream treatment operations and processes. Comminutors, macerators, and grinders can theoretically eliminate the messy and offensive task of screenings handling and disposal. The use of comminutors and macerators is particularly advantageous in a pumping station to protect the pumps against clogging by rags and large objects and to eliminate the need to handle and dispose of screenings. They are particularly useful in cold climates where collected screenings are subject to freezing. There is a wide divergence of views, however, on the suitability of using devices that grind and shred screenings at wastewater treatment plants. One school of thought maintains that once coarse solids have been removed from wastewater, they should not be returned, regardless of the form. The other school of thought maintains that once cut up, the solids are more easily handled in the downstream processes. Shredded solids often present downstream problems, particularly with rags and plastic bags, as they tend to form rope-like strands. Rag and plastic strands can have a number of adverse impacts, such as clogging pump impellers, sludge pipelines, and heat exchangers, and accumulating on air diffusers and clarifier mechanisms. Plastics and other non-biodegradable material may also adversely affect the quality of biosolids that are to be beneficially reused. Approaches to using comminutors, macerators, and grinders are applicable in many retrofit situations. Examples of retrofit applications include plants where a spare channel has been provided for the future installation of a duplicate unit or in very deep influent pumping stations where the removal of screenings may be too difficult or costly to achieve. Comminutors Comminutors are used most commonly in small wastewater treatment plants, less than 0.2 m3/s (5 Mgal/d). Comminutors are installed in a wastewater flow channel to screen and shred material to sizes from 6 to 20 mm (0.25 to 0.77 in) without removing the sshredded solids from the flow stream. A typical comminutor uses a stationary horizontal screen concave to the direction of flow (see Fig. 2.2.7). A rotating or oscillating arm that contains cutting teeth meshes with the screen. The cutting teeth and the shear bars cut coarse

NIREAS VOLUME 2 [2.1-2.2-2.3]

material. The small sheared particles pass through the screen and into the downstream channel. Comminutors may create string of material, namely rags, that can could downstream treatment equipment. Because of operating problems and high maintenance with comminutors, newer installations use generally a type of screening device or a macerator described below.

Figure 2.2.7.. Typical comminutor used for particle size reduction of solids. Macerators Macerators are slow-speed grinders that typically consist of two sets of counter-rotating assemblies with blades (see Fig. 5-9a). The assemblies are mounted vertically in the flow channel. The blades or teeth on the rotating assemblies have a close tolerance that effectively chops material as it passes through the unit. The chopping action reduces the potential for producing ropes of rags or plastic that can collect on downstream equipment. Macerators can be used for in pipeline installations to shred solids, particularly ahead of wastewater and sludge pumps, or in channels at smaller wastewater treatment plants. Sizes for pipeline applications typically range from 100 to 400 mm (4 to 16 in) in diameter. Another type of macerator used in channel applications is a moving, linked screen that allows wastewater to pass through the screen while diverting screenings to a grinder located at one side of the channel (see Fig. 2.2.8b). Standard sizes of this device are available for use

NIREAS VOLUME 2 [2.1-2.2-2.3]

in large channels ranging from widths of 750 to 1,800 mm (30 to 72-in) and depths of 750 to 2,500 mm (30 to 100 in). The headloss is lower than that of the units with counter rotating blades shown on Fig. 2.2.8a.

Figure 2.2.8. Typical macerators: (a) schematic of in-channel type slowspeed grinder/macerator, (b) view of a macerator mounted in an open channel, and (c) schematic of linked-screen macerator. Grinders High-speed grinders, typically referred to as hammermills, receive screened materials from bar screens. The materials are pulverized by a high-speed rotating assembly that cuts the materials passing through the unit. The cutting or knife blades force screenings through a stationary grid or louver that encloses the rotating assembly. Wash water is typically used to keep the unit clean and to help transport materials back to the wastewater stream. Discharge from the grinder can be located either upstream or downstream of the bar screen. Design Considerations

NIREAS VOLUME 2 [2.1-2.2-2.3]

Comminuting and macerating devices may be preceded by grit chambers to prolong the life of the equipment and to reduce the wear on the cutting surfaces.

Comminutors should be constructed with a bypass arrangement

so that a manual bar screen is used in case flowrates exceed the capacity of the comminutor or when there is a power or mechanical failure. Stop gates and provisions for dewatering the channel should also be included to facilitate maintenance. Headloss through a comminutor usually ranges from 0.1 to 0.3 m (4 to 12 in), and can approach 0.9 m (3 ft) in large units at maximum flowrates.

In cases where a comminutor or macerator precedes grit chambers, the cutting teeth are subject to high wear and require frequent sharpening or replacement. Units that use cutting mechanisms ahead of the screen grid should be provided with rock traps in the channel upstream of the comminutor to collect material that could jam the cutting blade. Because these units are complete in themselves, no detailed design is necessary. Manufacturers' data and rating tables for these units should be consulted for recommended channel dimensions, capacity ranges, headloss, upstream and downstream submergence, and power requirements. Because manufacturers' capacity ratings are usually based on clean water, the ratings should be decreased by approximately 80 percent to account for partial clogging of the screen.

2.2.2 Flow Measurement (*) The control and monitoring of flows and levels in the wastewater treatment industry involve the measurement of water, biological sludge, solid and liquid additives, and reagent flows. This section discusses methods of flow detection followed by a summary of wastewater-related level detection techniques. (*) data from Environmental Engineerâ&#x20AC;&#x2122;s handbook,Ch7 Wastewater Treatment, Liu&Liptak, CRC, USA 1999

NIREAS VOLUME 2 [2.1-2.2-2.3]

Flow Sensors for the Wastewater Industry Flow detection applications in the wastewater treatment industry include the measurement of large flows in partially filled pipes using weirs, flumes, or ultrasonic sensors. When water is flowing in regular pipelines, magnetic flowmeters, venturi tubes, flow nozzles, and pitot tubes are the usual sensors. In smaller pipelines, orifice plates, vortex flowmeters, or variable area flowmeters are used. For sludge services, doppler-type ultrasonic and magnetic flowmeter (provided with electrode cleaners), V-cone detector, and segmental wedge-type detector can be used. Gas, liquid, or solid additives can be charged by Coriolis mass flowmeters (gas or liquid), metering pumps, turbine or positive displacement meters (liquids), variable-area flowmeters (gas or liquid), or gravimetric feeders (solids). Table 2.2.8 summarizes flowmeter features and capabilities. The following sections provide a brief summary of the features and capabilities of the flowmeters used in the wastewater treatment industry.

Table 2.2.8. Orientation Table For Flow Sensors

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.2.2.1 Magnetic Flowmeters Design Pressure Varies with pipe size. For a 4 in (100 mm) unit, the maximum pressure is 285 psig (20 bars); special units are available with pressure ratings up to 2500 psig (172 bars). Design Temperature Up to 250°F (120°C) with Teflon liners and up to 360°F (180°C) with ceramic liners. Materials Of Construction Liners: ceramics, fiberglass, neoprene, polyurethene, rubber, Teflon, vitreous enamel, and Kynar; Electrodes: platinum, Alloy 20, Hastelloy C, stainless steel, tantalum, titanium, tungsten carbide, Monel, nickel, and platinum-alumina cermet. Type Of Flow Detected Volumetric flow of conductive liquids, including slurries and corrosive or abrasive materials. Minimum Conductivity Required The majority of designs require 1 to 5 mS/cm. Some probe types require more. Special designs can operate at 0.05 or 0.1 mS/cm. Flow Ranges From 0.01 to 100,000 gpm (0.04 to 378,000 liters per minute (lpm)). Size Ranges From 0.1 to 96 in (2.5 mm to 2.4 m) in diameter. Velocity Ranges 0–0.3 to 0–30 ft/sec (0–0.1 to 0–10 m/sec). Error (Inaccuracy) 61% of actual flow with pulsed direct current (dc) units within a range of up to 10:1 if flow velocity exceeds 0.5 ft/sec (0.15 m/sec), 61% to 62% full-scale with alternating current (ac) excitation.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Cost The probe designs are least expensive, at a cost of about 1500€. A 1-in (25mm) ceramic tube unit can be obtained for under 2000 €. A 1-in (25-mm) metallic wafer unit can be obtained for under 3000. An 8-in (200-mm) flanged meter that has a Teflon liner and stainless electrodes and is provided with 4 to 20 mA dc out-put, grounding ring, and calibrator costs about 8000 €. The scanning magmeter probe used in open-channel flow scanning costs about 10,000 €. Magnetic flowmeters use Faraday’s Law of electromagnetic induction for measuring flow. Faraday’s Law states that when a conductor moves through a magnetic field of given strength, a voltage level is produced in the conductor that depends on the relative velocity between the conductor and the field. This concept is used in electric generators. Faraday foresaw the practical application of the principle to flow measurement because many liquids are adequate electrical conductors. In fact, he attempted to measure the flow velocity of the Thames River using this principle. He failed because his instrumentation was not adequate, but 150 years later, the principle is successfully applied in magnetic flowmeters. Designs And Applications Magnetic flowmeters are available in conventional (see Figure 2.2.9), ceramic (see Figure ......), and probe (see Figure .......) constructions. Most liquids or slurries are adequate electrical conductors to be measured by electromagnetic flowmeters. If the liquid conductivity is equal to 20 mS per cm or greater, most conventional magnetic flowmeters can be used. Special designs are available to measure the flow of liquids with threshold conductivities as low as 0.1 mS.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig. 2.2.9. The short-form magnetic flowmeter. The probe-type magnetic flowmeter.

Magnetic flowmeters are not affected by viscosity or consistency (referring to Newtonian and nonNewtonian fluids, respectively). Changes in the flow profile due to changes in Reynolds numbers or upstream piping do not greatly affect the performance of magnetic flowmeters. The voltage generated is the sum of the incremental voltages across the entire area between the electrodes, resulting in a measure of the average fluid velocity. Nevertheless, the meter should be installed with five diameters of straight pipe before and three diameters of straight pipe following the meter. Magnetic flowmeters are bidirectional. Manufacturers offer converters with output signals for both direct and reverse flows. The magnetic flowmeter must be full to assume accurate measurement. If the pipe is only partially full, the electrode voltage, which is proportional to the fluid velocity, is still multiplied with the full cross section, and the reading will

NIREAS VOLUME 2 [2.1-2.2-2.3]

be high. Similarly, if the liquid contains entrained gases, the meter measures them as liquid, the reading will be high. The meterâ&#x20AC;&#x2122;s electrodes must remain in electrical contact with the fluid being measured and should be installed in the horizontal plane. In applications where a buildup or coating occurs on the inside wall of the flowmeter, periodic flushing or cleaning is recommended. Special meters for measuring sewage sludge flow are designed to prevent the buildup and carbonizing of sludge on the meter electrodes. They use selfheating to elevate the metering body temperature to prevent sludge and grease accumulation. Advantages Magnetic flowmeters have the following advantages: 1. The magnetic flowmeter has no obstructions or moving parts. Flowmeter pressure loss is no greater than that of the same length of pipe. Pumping costs are thereby minimized. 2. Electric power requirements can be low, particularly with the pulsed dc types. Electric power requirements as low as 15 or 20 W are common. 3. The meters are suitable for most acids, bases, waters, and aqueous solutions because the lining materials are not only good electrical insulators but are also corrosion-resistant. Only a small amount of electrode metal is required, and stainless steel, Alloy 20, the Hastelloys, nickel, Monel, titanium, tantalum, tungsten carbide, and even platinum are all available. 4. The meters are widely used for slurry services not only because they are obstructionless but also because some of the liners, such as polyurethane, neoprene, and rubber, have good abrasion or erosion resistance. 5. The meters are capable of handling extremely low flows. Their minimum size is less than A k in (3.175 mm) inside diameter. The meters are also suitable for high volume flow rates with sizes as large as 10 ft (3.04 m). 6. The meters can be used as bidirectional meters. Limitations Magnetic flowmeters do have some specific application limitations: 1. The meters work only with conductive fluids. Pure substances, hydrocarbons, and gases cannot be measured. Most acids, bases, water, and aqueous solutions can be measured. 2. The conventional meters are relatively heavy, especially in larger sizes. Ceramic and probe-type units are lighter.

NIREAS VOLUME 2 [2.1-2.2-2.3]

3. Electrical installation care is essential. 4. The price of magnetic flowmeters ranges from moderate to expensive. Their corrosion resistance, abrasion resistance, and accurate performance over wide turn-down ratios can justify the cost. Ceramic and probe-type units are less expensive. 5. Periodically checking the zero on ac-type magnetic flowmeters requires block valves on either side to bring the flow to zero and keep the meter full. Cycled dc units do not have this requirement. 2.2.2.2 Orifices Design Pressure For plates, limited by the readout device only; integral orifice transmitter to 1500 psig (10.3 MPa) Design Temperature Function of the associated readout system when the differential pressure unit must operate at the elevated temperature. For the integral orifice transmitter, the standard range is 220 to 250째F (229 to 121째C). Sizes Maximum size is the pipe size. Fluids Liquids, vapors, and gases Flow Range From a few cc/min using integral orifice transmitters to any maximum flow; limited only by pipe size Materials Of Construction No limitation on plate materials. Integral orifice transmitter wetted parts can be obtained in steel, Stainless Steel, Monel, Nickel, And Hastelloy. Inaccuracy The orifice plate, if the bore diameter is correctly calculated and prepared, can be accurate to 60.25 to 60.5% of the actual flow. When a conventional d/p cell is used to detect the orifice differential, that adds a 60.1 to 60.3% of the fullscale error. The error contribution of smart d/p cells is only 0.1% of the actual span.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Intelligent D/P Cells Inaccuracy of 60.1%, rangeability of 40:1, the built-in proportional integral and derivative (PID) algorithm Rangeability If rangeability is defined as the flow range within which the combined flow measurement error does not exceed 61% of the actual flow, then the rangeability of conventional orifice installations is 3:1. When intelligent transmitters with automatic switching capability between the high and low spans are used, the rangeability can approach 10:1. and 2000 €. The cost of d/p transmitters ranges from 900 € to 2000 €, depending on type and intelligence. Cost A plate only is 50€ to 300€, depending on size and materials. For steel orifice flanges from 2 to 12 in (50 to 300 mm), the cost ranges from 200€ to 1000€. For flanged meter runs in the same size range, the cost ranges from 400€ to 3000€. The cost of electronic or pneumatic integral orifice transmitters is about 1500€ .

The orifice plate, when installed in a pipeline, causes an increase in flow velocity and a corresponding decrease in pressure. The flow pattern shows an effective decrease in the cross-section beyond the orifice plate, with a maximum velocity and minimum pressure at the vena contracta (see Figure 2.2.10). This location can be from .35 to .85 pipe diameters downstream from the orifice plate depending on the b ratio and the Reynolds number. This flow pattern and the sharp leading edge of the orifice plate (see Figure 2.2.10) that produces it are important. The sharp edge results in an almost pure line contact between the plate and the effective flow, with negligible fluid-to-metal friction drag at this boundary. Any nicks, burrs, or rounding of the sharp edge can result in large measurement errors.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig. 2.2.10 Pressure profile through an orifice plate and the different methods of detecting the pressure drop.

Fig. 2.2.11 Flow measurement devices.

When differential pressure is measured at a location close to the orifice plate, friction effects between the fluid and the pipe wall upstream and downstream from the orifice are minimized so that pipe roughness has a minimum effect. Fluid viscosity, as reflected in the Reynolds number, has a considerable

NIREAS VOLUME 2 [2.1-2.2-2.3]

influence, particularly at low Reynolds numbers. Since the formation of the vena contracta is an inertial effect, a decrease in the ratio of inertial to frictional forces (decrease in Reynolds number), and the corresponding change in flow profile, results in less constriction of flow at the vena contracta and an increase of the flow coefficient. In general, the sharp edge orifice plate should not be used at pipe Reynolds numbers under 10,000. The minimum recommended Reynolds number varies from 10,000 to 15,000 for 2-in (50mm) through 4-in (102-mm) pipe sizes for b ratios up to 0.5 and from 20,000 to 45,000 for higher b ratios. The Reynolds number requirement increases with pipe size and b ratio and can range up to 200,000 for pipes 14 in (355 mm) and larger. Maximum Reynolds numbers can be 106 for 4-in (102-mm) pipe and 107 for larger sizes. Wastewater Applications If the water is dirty, containing solids or sludge, the pressure taps must be protected by clean water purging or by use of chemical seals and the orifice plates should be the segmental or eccentric orifice type (see Figure 2.2.13). Annular orifices and V-cone meters are also applicable to dirty services. Because the pressure recovery of orifices is low, they are not recommended to measure larger flows.

Pitot Tubes Types A. Standard, single-port B. Multiple-opening, averaging C. Area averaging for ducts Applications Liquids, gases, and steam Operating Pressure Permanently installed carbon or stainless steel units can operate at up to 1400 psig (97 bars) at 100°F (38°C) or 800 psig (55 bars) at approximately 700°F (371°C). The pressure rating of retractable units is a function of the isolating valve. Operating Temperature Up to 750°F (399°C) in steel and 850°F (454°C) in stainless steel construction when permanently installed

NIREAS VOLUME 2 [2.1-2.2-2.3]

Flow Ranges Generally 2-in (50-mm) pipes or larger; no upper limit Materials Of Construction Brass, steel, and stainless steel Minimum Reynolds Number Range from 20,000 to 50,000 Rangeability Same as orifice plates Straight-Run Requirements Downstream of valve or two elbows in different planes, 25â&#x20AC;&#x201C;30 pipe diameters upstream and 5 downstream; if straightening vanes are provided, 10 pipe diameters upstream and 5 downstream Inaccuracy For standard industrial units: 0.5 to 5% of full scale. Full-traversing Pitot Venturis under National-Bureau-of-Standards-type laboratory conditions can give 0.5% of the actual flow error. Industrial Pitot Venturis must be individually calibrated to obtain 1% of range performance. Inaccuracy of individually calibrated multipleopening averaging pitot tubes is claimed to be 2% of the range when the Reynolds numbers exceed 50,000. Area-averaging duct units are claimed to be between 0.5 and 2% of the span. The error of the d/p cell is additional to the errors listed.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig. 2.2.12 Schematic diagram of an industrial device (Pitot tube) for sensing static and dynamic pressures in a flowing fluid.

Costs A 1-in-diameter averaging pitot tube in stainless steel costs 750 € if fixed and 1400 € if retractable for hot-tap installation. The cost usually doubles if the pitot tube is calibrated. Hastelloy units for smokestack applications can cost 2000 € or more. A local pitot indicator costs 400 €; a d/p transmitter suited for pitot applications with 4 to 20 mA dc output costs about 1000 €. While pitot sensors are low- accuracy and low-rangeability detectors, they do have a place in wastewater treatment- related flow measurement. Pitot tubes should be used when the measurement is not critical, the water is reasonably clean, and a low cost measurement is needed. These sensors can be inserted in the pipe without shutdown and can also be removed for periodic cleaning while the pipe is in use. Segmental Wedge Flowmeters Applications Clean, viscous liquids or slurries and fluids with solids Sizes 1- to 12-in (25.4- to 305-mm) diameter pipes

NIREAS VOLUME 2 [2.1-2.2-2.3]

Designs For smaller sizes (1 and 1.5 in), the wedge can be integral; for larger pipes, remote seal wedges are used with calibrated elements. Wedge Opening Height From 0.2 to 0.5 of the inside pipe diameter Pressure Drops 25 to 200 in H2O (6.2 to 49.8 kPa) Materials Of Construction Carbon or stainless steel element; stainless or Hastelloy C seal; special wedge materials like tungsten carbide are available. Design Pressure 300 to 1500 psig (20.7 to 103 bars) with remote seals Design Temperature 240 to 700째F (240 to 370째C) but also used in high-temperature processes up to 850째F (454째C) Inaccuracy The elements are individually calibrated; the d/p cell error contribution to the total measurement inaccuracy is 0.25% of full scale. The error over a 3:1 flow range is usually not more than 3% of the actual flow. Cost A 3-in (75-mm) calibrated stainless steel element with two stainless steel chemical tees and an electronic d/p transmitter provided with remote seals is about $3500. The segmental wedge flow element provides a flow opening similar to that of a segmental orifice, but flow obstruction is less abrupt (more gradual), and its sloping entrance makes the design similar to the flow tube family. It is primarily used on slurries. Its main advantage is its ability to operate at low Reynolds numbers. While the square root relationship between the flow and pressure drop in sharp-edged orifices, venturis, or flow nozzles requires a Reynolds number above 10,000, segmental wedge flowmeters require a Reynolds number of only 500 or 1000. For this reason the segmental wedge flowmeter can measure flows at low flow velocities and when process fluids are viscous. In that respect, it is similar to conical or quadrant edge orifices.

NIREAS VOLUME 2 [2.1-2.2-2.3]

For pipe sizes under 2 in (50 mm), the segmental wedge flow element is made by a V-notch cut into the pipe and a solid wedge welded accurately in place (see Figure 7.6.10). In sizes over 2 in, the wedge is fabricated from two flat plates that are welded together before insertion into the spool piece. On clean services, regular pressure taps are located equidistant from the wedge (see Figure 7.6.10), while on applications where the process fluid contains solids in suspension, chemical tees are added upstream and downstream of the wedge flow element. The chemical seal element is flush with the pipe, eliminating pockets and making the assembly self-cleaning. The seals are made of corrosion-resistant materials and are also suited for high-temperature services. Some users have reported applications on processes at 3000 psig (210 bars) and 850째F (454째C).

Fig. 2.2.13 The segmental wedge flowmeter designed for clean fluid service. Purge Flowmeter One variety of variable-area flowmeters is the purge flowmeter (see Figure 2.2.14). The features and characteristics of these instruments are summarized next.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig. 2.2.14 A purge flow regulator consisting of a glass tube rotameter, an inlet needle valve, and a differential pressure regulator. (Reprinted, from Krone America Inc.)

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig. 2.2.15. Variable-area flowmeters. The area open to flow is changed by the flow itself in a variable-area flowmeter. Either gravity or spring action can be used to return the float or vane as flow drops.

2.2.3 Flow equalization The variations of influent wastewater flowrate and characteristics at wastewater treatment facilities were discussed in Chap.1.3. Flow equalization is a method used to overcome the operational problems caused by flowrate variations, to improve the performance of the downstream processes, nd to reduce the size and cost of downstream treatment facilities. Description/Application Flow equalization simply is the damping of flowrate variations to achieve a constant or nearly constant flowrate and can be applied in a number of different situations, depending on the characteristics of the collection system. The principal applications are for the equalization of (1) dry-weather flows to reduce peak flows and loads, (2) wet-weather flows in sanitary collection systems experiencing inflow and infiltration, or (3) combined stormwater and sanitary system flows. The application of flow equalization in wastewater treatment is illustrated in the two flow diagrams given on Fig. 2.2.12. In the in-line arrangement (Fig. 2.2.12a), all of the flow passes through the equalization basin. This arrangement can be used to achieve a considerable amount of constituent concentration and flowrate damping. In the off-line arrangement (Fig. 2.2.12b), only the flow above some predetermined flow limit is diverted into the equalization basin. Although pumping requirements are minimized in this arrangement, the amount of constituent concentration damping is considerably reduced. Off-line equalization is sometimes used to capture the "first flush" from combined collection systems.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Figure 2.2.1. Typical wastewater-treatment plant flow diagram incorporating flow equalization: (a) in-line equalization and (b) off-line equalization. Flow equalization can be applied after grit removal, after primary sedimentation, and after secondary treatment where advanced treatment is used. The principal benefits that are cited as deriving from application of flow equalization are: (1) biological treatment is enhanced, because shock loadings are eliminated or can be minimized, inhibiting substances can be diluted, and pH can be stabilized; (2) the effluent quality and thickening performance of secondary sedimentation tanks following biological treatment is improved through improved consistency in solids loading; (3) effluent filtration surface area requirements are reduced, filter performance is improved, and more uniform filter-backwash cycles are possible by lower hydraulic loading; and (4) in chemical treatment, damping of mass loading improves chemical feed control and process reliability. Apart from improving the performance of most treatment operations and processes, flow equalization is an attractive option for upgrading the performance of overloaded treatment plants. Disadvantages of flow equalization include (1) relatively large land areas or sites are needed, (2) equalization facilities may have to be covered for odor control near residential areas, (3) additional operation and maintenance is required, and (4) capital cost is increased.

NIREAS VOLUME 2 [2.1-2.2-2.3]

More Informationâ&#x20AC;Ś. Design Considerations The design of flow equalization facilities is concerned with the following questions: 1. Where in the treatment process flowsheet should the equalization facilities be located? 2. What type of equalization flowsheet should be used, in-line or off-line? 3. What is the required basin volume? 4. What are the features that should be incorporated into design? 5. How can the deposition of solids and potential odors be controlled? Location of Equalization Facilities. The best location for equalization facilities must be determined for each system. Because the optimum location will vary with the characteristics of the collection system and the wastewater to be handled, land requirements and availability, and the type of treatment required, detailed studies should be performed for several locations throughout the system. Where equalization facilities are considered for location adjacent to the wastewater-treatment plant, it is necessary to evaluate how they could be integrated into the treatment process flowsheet. In some cases, equalization after primary treatment and before biological treatment may be appropriate. Equalization after primary treatment causes fewer problems with solids deposits and scum accumulation. If flow-equalization systems are to be located ahead of primary settling and biological systems, the design must provide for sufficient mixing to prevent solids deposition and concentration variations, and aeration to prevent odor problems. Volume Requirements for the Equalization Basin. The volume required for flowrate equalization is determined by using an inflow cumulative volume diagram in which the cumulative inflow volume is plotted versus the time of day. The average daily flowrate, also plotted on the same diagram, is the straight line drawn from the origin to the endpoint of the diagram. In practice, the volume of the equalization basin will be larger than that theoretically determined to account for the following factors: 1. Continuous operation of aeration and mixing equipment will not allow complete drawdown, although special structures can be built. 2. Volume must be provided to accommodate the concentrated plant recycle streams that are expected, if such flows are returned to the equalization basin (a practice that is not recommended unless the basin is covered because of the potential to create odors). 3. Some contingency should be provided for unforeseen changes in diurnal flow.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Although no fixed value can be given, the additional volume will vary from 10 to 20 percent of the theoretical value, depending on the specific conditions.

More information in p.333-344, in Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003

2.3.1 Introduction The purpose of primary treatment (primary sedimentation or primary clarification) is to remove settleable organic and floatable solids. Normally, each primary clarification unit can be expected to remove 90 to 95% settleable solids, 40 to 60% total suspended solids, and 25 to 35% biochemical oxygen demand (BOD5). Note: Performance expectations for settling devices used in other areas of plant operation are normally expressed as overall unit performance rather than settling unit performance. 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. This chapter focuses on primary treatment, or primary clarification, which achieves primary settling through the use of large basins under relatively quiescent conditions. Within these basins, mechanical scrapers collect the primary settled solids into a hopper from which 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. d

NIREAS VOLUME 2 [2.1-2.2-2.3]

Figure 2.3.1a Typical wastewater treatment plant with primary settling (sedimentation) and recycling of the biosolids: 2.3.1.1 Process Description In primary sedimentation, wastewater enters a settling tank or basin. Velocity is reduced to approximately 0,3 m/min or 1 foot per minute (fpm). 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 moves on to the next step in the treatment process. Detention time, temperature, tank design, and condition of the equipment control the efficiency of the process.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Figure 2.3.1b Typical wastewater treatment plant (flow sheet) WITH primary settling (sedimentation) and/or flotation.

Overview of Primary Treatment Primary treatment reduces the organic loading on downstream treatment processes by removing a large amount of settleable, suspended, and floatable materials.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Primary treatment reduces the velocity of the wastewater through a clarifier to approximately 0,3 m/min ( or 1 to 2 fpm), so settling and flotation can take place. Slowing the flow enhances removal of suspended solids in wastewater. Primary settling tanks remove floated grease and scum, remove the settled sludge solids, and collect them for pumped transfer to disposal or further treatment. The clarifiers 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 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. 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. Factors affecting primary clarifier performance include: Rate of flow through the clarifier Wastewater characteristics (strength, temperature, amount and type of industrial waste, and density, size, and shape of the particles) Performance of pretreatment processes Nature and amount of any wastes recycled to the primary clarifier 2.3.1.2 Clarifier Operation Calculations Key factors in primary clarifier operation include the following concepts: Retention Time (hr) = Volume (m3) Ď&#x2021; 24 hr/day Flow (m3/d)

(2.3.1)

3 Surface Loading Rate (m3/m2) = Q (m /d) Surface Area (m2)

(2.3.2)

Solids Loading Rate (kg/day/m2) =

Solids into Clarifier (kg/day)

(2.3.3)

Surface Area (m2)

Weir Overflow Rate (m3/linear m) =

Q (m3/d)........ Weir Length (linear m)

(2.3.4)

2.3.2 Types of Sedimentation Tanks adapted from Spellman's Standard Handbook for Wastewater Operators, 2nd edition, Frank R. Spellman, 2011, USA Sedimentation equipment includes septic tanks, two-story tanks (Imhoff tanks), and plain settling tanks or clarifiers. All three devices may be used for primary treatment, while plain settling tanks are normally used for secondary or advanced wastewater treatment processes. 2.3.2. 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 hr or more) but do not separate

NIREAS VOLUME 2 [2.1-2.2-2.3]

decomposing solids from the wastewater flow. When the tank becomes full, solids will be discharged with the flow. The process is suitable for small facilities (e.g., schools, motels, homes), but, due to the long detention times and lack of control, it is not suitable for larger applications.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Fig. 2.3.2. Septic tank configurations. A. Typical household septic tank; B. Typical large institutional septic tank with dosing siphon. For large fields, uniform distribution is obtained by periodic flooding of the field followed by periodic drying. Dosing tanks are used to flood these fields; they collect the sewage, and automatic bell siphons or pumps transport the waste to the field. Adapted from Environmental Engineerâ&#x20AC;&#x2122;s handbook,Ch7 Wastewater Treatment, Liu&Liptak, CRC, USA 1999 The minimum effective tank capacity should be as follows: a) for flows up to 1500 gpd, the capacity the daily sewage flow; b) for flows in excess of 1500 gpd, the volume V in gallons can be calculated from the following equation: V = 1125 + 0.75 X Q (2.3.5) where: Q = The daily sewage flow (maximum value) 2.3.2.2 Two-Story (Imhoff) Tank The two-story or Imhoff tank is similar to a septic tank with regard to 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 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

NIREAS VOLUME 2 [2.1-2.2-2.3]

result of the solids decomposition are released through the gas vents running along each side of the settling compartment.

Fig. 2.3.3 Imhoff tank configuration. Adapted from Environmental Engineerâ&#x20AC;&#x2122;s handbook,Ch7 Wastewater Treatment, Liu&Liptak, CRC, USA 1999 IMHOFF TANK DESIGN Surface loading of the settling zone should be 600 gpd per ft2 with detention times of 1 to 2 hr and velocities below 0.75 in per sec. The effective settling zone depth should be about 7 ft and its length can be from 25 to 50 ft. The gas-vent and scum area should be 20% of the total surface area. Total depths average around 30 ft (see Figure 7.20.4). Septic tanks are suitable only for isolated facilities with low waste flows where the soil can be used as an absorption field. Their use should be avoided except when an alternative is not available and the site conditions are favorable. The operation of Imhoff tanks is not complex. They are less efficient than settling basins and heated- ludge digestion tanks. The newer treatment methods offer more efficient alternatives to Imhoff tanks, but in small treatment units, they do provide efficient solids separation without mechanical or electrical equipment. â&#x20AC;&#x201D;R.

2.3.2.3 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 hr (2-hr average). Sludge removal is accomplished frequently on either a continuous or an intermittent basis. Continuous removal requires additional sludge treatment processes to remove the excess water resulting from removal of sludge

NIREAS VOLUME 2 [2.1-2.2-2.3]

containing less than 2 to 3% solids. Intermittent sludge removal requires that 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, which prevent floatable solids (scum) from leaving the tank; scum troughs; scum collectors; effluent troughs; and effluent weirs require frequent cleaning to prevent heavy biological growth 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's operations and maintenance (O&M) manual. Process control sampling and testing are used to evaluate the performance of the settling process. Settleable solids, dissolved oxygen, pH, temperature, total suspended solids, and BODrâ&#x20AC;&#x17E; as well as sludge solids and volatile matter, testing is routinely accomplished.

Figure 2.3.4 Typical rectangular primary sedimentation tank: (a) plan (b) section. (adapted from Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003)

NIREAS VOLUME 2 [2.1-2.2-2.3]

Table 2.3.1 Typical design information for primary sedimentation tanks (adapted from Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003)

Table 2.3.2 Typical dimensional data for rectangular and circular sedimentation tanks used for primary treatment of wastewater (adapted from Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4th edition, 2003)

2.3.3 Operator Observations

NIREAS VOLUME 2 [2.1-2.2-2.3]

Before identifying a primary treatment problem and proceeding with appropriate troubleshooting effort, the operator must be cognizant of what constitutes "normal" operation (is the system operating as per design or is there a problem?). Several important items of normal operation can have a strong impact on performance. The following sections discuss the important operational parameters and "normal" observations. 2.3.3.1 Primary Clarification: Normal Operation Again, as mentioned earlier, in primary clarification, wastewater enters a settling tank or basin. Velocity is reduced to approximately 1 foot per minute. 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 moves on to the next step in the treatment process. Detention time, temperature, tank design, and condition of the equipment control the efficiency of the process. 2.3.3.2 Operational Parameters for Primary Clarification Flow distribution—Normal flow distribution is indicated by the flow to each in-service unit being equal and uniform. There is no indication of shortcircuiting. The surface-loading rate is within design specifications. Weir condition—Weirs are level, flow over the weir is uniform, and the weir overflow rate is within design specifications. Scum removal—The surface is free of scum accumulations, and the scum removal does not operate continuously. Sludge removal—No large clumps of sludge appear on the surface, the system operates as designed, the pumping rate is controlled to prevent coning or buildup, and the sludge blanket depth is within desired levels. Performance—The unit is removing expected levels of BOD5, total suspended solids, and settleable solids. Unit maintenance—Mechanical equipment is maintained in accordance with planned schedules, and 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: pH (6.5 to 9.0) Dissolved oxygen (<1.0 mg/L) Temperature (varies with climate and season) Settleable solids (influent, 5 to 15 mL/L; effluent, 0.3 to 5 mL/L) BOD5 (influent, 150 to 400 mg/L; effluent, 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 of these tests should be performed periodically to provide reference information for evaluation of performance.

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.3.3.3 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: • 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. Percent Removal The expected ranges of percent removal for a primary clarifier (for are: Settleable solids 90-95% Suspended solids 40-60% BOD5 25-35% Detention Time The primary purpose of primary settling is to remove settleable solids. This is accomplished by slowing the flow down to approximately o,3 m/min (1 fpm). The flow at this velocity will stay in the primary tank from 1.5 to 2.5 hr. The length of time the water stays in the tank is called the hydraulic detention time. Surface Loading Rate (Surface Settling Rate) and Surface Overflow Rate Flow (m /d) . Surface Loading Rate (m/d) = Settling Tank Area (m2) 3

(2.3.6)

Problem: A settling tank is 40 m in diameter, and flow to the unit is 16000 m3/d. What is the surface loading rate (Surface Settling Rate) in (m3/ m2/d ) m/d ? Solution: 16000 m3/d Surface Loading Rate = ---------- ---- --------------- = 50,96 m/d (m3/ m2/d ) 0.785 χ 20 m χ 20 m

NIREAS VOLUME 2 [2.1-2.2-2.3]

Problem: A circular clarifier has a diameter of 16 m. If the primary effluent flow is 8.000 m3/d, what is the surface overflow rate in gpd/ft2? Solution: 8000 m3/d Surface Overflow Rate = --------- ---- --------------- = 159,24 m/d (m3/ m2/d ) 0.785 χ 8 m χ 8 m Weir Overflow Rate 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 120-240 m2/d (m3/ d / m) or 10,000 to 20,000 gpd/ft are used in the design of a settling tank. Example Problem: The circular settling tank is 30 m in diameter and has a weir along its circumference. The effluent flow rate is 12.000 m3/d. What is the weir overflow rate in gallons per day per meter of weir? Solution: 12000 m3/d Weir Overflow Rate (m3/d/m = - ---- --------------- = 127.4 m2/d (m3/ m/d ) 3,14 x 30 m Sludge Pumping Determination of sludge pumping (the quantity of solids and volatile solids removed from the sedimentation tank) provides the accurate information required for process control of the sedimentation process. Solids (kg/day) = Pump Rate χ Pump Time χ 1000 kg/m3 (2.3.7)

χ % Solids

Volatile Matter (kg/day) = Pump Rate χ Pump Time χ % Solids χ % Volatile Matter (2.3.7) Example Problem: The sludge pump operates 20 minutes per hour. The pump delivers 5 m3/h of sludge. Laboratory tests indicate that the sludge is 5.2% solids and 66% volatile matter. How many kg of volatile matter are transferred from the settling tank to the digester? Solution:

NIREAS VOLUME 2 [2.1-2.2-2.3]

Pump time = 20 min/hr , Pump rate = 5 m3/h Percent solids = 5.2% , Percent volatile matter = 66% Volatile matter = 5 m3/h χ1000 Lt/m3 x (20 /60) hr χ 24 hr/day χ 0.052 χ 0.66 = 1371.7 kg/day Percent Total Solids (%TS) (weight to weight units) Example Problem: A settling tank sludge sample is tested for solids. The sample and dish weighed 74.69 g. The dish alone weighs 21.2 g. After drying, the dish with dry solids weighed 22.3 g. What is the percent total solids (%TS) of the sample? Solution: Dry solids + dish = 22.3 g Dry solids weight = 22.3 g - 21.2 g = 1.1 g Sample + dish = 74.69 g Sample weight = 74.69 g - 1.2 g = 53.49 g (1.1 g)/(53.49 g) χ 100% = 2%

BOD and SS Removal (kg/day) To calculate the pounds of biochemical oxygen demand (BOD) or suspended solids removed each day, we need to know the mg/L BOD or SS removed and the plant flow. Then, we can use the following equation: SS Removed (2.3.7)

(kg/day)

(mg/L)

m3/d

Problem: If 120 mg/L suspended solids are removed by a primary clarifier, how many kg/day suspended solids are removed when the flow is 23000 m3/d?. Assume mg/L= g/m3 Solution: SS Removed = 120 mg/L χ 23000 m3/d x 0.001kg/g = 2760 kg/day Example

NIREAS VOLUME 2 [2.1-2.2-2.3]

Problem: The flow to a secondary clarifier is 6000 m3/d. If the influent BOD concentration is 200 mg/L and the effluent BOD concentration is 70 mg/L, how many kgs of BOD are removed daily? Solution: BOD Removed = 200 mg/L - 70 mg/L = 130 mg/L, After calculating mg/L BOD removed, calculate kg/day BOD removed: BOD Removed = 130 g/m3 Ď&#x2021; 6000 m3/d x 0.001kg/g = 780 kg/day 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. 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), discussed in the next chapter. Note: Two of the most important nutrients left to remove are phosphorus and ammonia. Although 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.

more ............ Sedimentation Tank Performance The efficiency of sedimentation basins with respect to the removal of BOD and TSS is reduced by (1) eddy currents formed by the inertia of the incoming fluid, (2) windinduced circulation cells formed in uncovered tanks, (3) thermal convection currents, (4) cold or warm water causing the formation of density currents that move along the bottom of the basin and warm water rising and flowing across the top of the tank, and (5) thermal stratification in hot arid climates (Fair and Geyer, 1954). Factors that affect performance are considered in the following discussion. BOD and TSS Removal. Typical performance data for the removal of BOD and TSS in primary sedimentation tanks, as a function of the detention time and constituent concentration, are presented on Fig. 2.3.9 . The curves shown on Fig. 2.3.9 are derived from observations of the performance of actual sedimentation tanks. The curvilinear relationships in the figure can be modeled as rectangular hyperbolas using the following relationship (Crites and Tchobanoglous, 1998).

NIREAS VOLUME 2 [2.1-2.2-2.3]

(2.3.8)

Figure 2.3.9. 5â&#x20AC;&#x201C;46 Typical BOD and TSS removal in primary sedimentation tanks. (Greeley, 1938.) Typical values for the empirical constants in Eq. (2.3.8) at 20Â°C are as follows: Item a b BOD 0.018 - 0.020 TSS 0.0075 - 0.014 A fact that is often overlooked in sedimentation tank performance is the change in the wastewater characteristics that occurs through the sedimentation process. Larger, more slowly biodegradable suspended solids settle first, leaving a more volatile fraction in suspension that remains in the primary tank effluent. The strict use of removal curves, such as those given on Fig. 2.3.9, does not account for the transformation in wastewater characteristics that actually occurs. Where possible for domestic wastewater, primary tank influent and effluent should be characterized to determine concentration and composition of the constituents. Such characterization is important when determining the organic loading required to be treated by the succeeding biological treatment units.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Short Circuiting and Hydraulic Stability. In an ideal sedimentation basin (see Fig. 2.3.10a), a given block of entering water should remain in the basin for the full detention time. Unfortunately, in practice sedimentation basins are seldom ideal and considerable short circuiting will be observed for one or more of the reasons cited above. To determine if short circuiting exists and to what extent, tracer studies, should be performed. Time-concentration curves should be developed for analysis. If in the repeated tests the timeconcentration curves are similar, then the basin is stable. If the timeconcentration curves are not repeatable, the basin is unstable and the performance of the basin will be erratic (Fair and Geyer, 1954). The method of influent flow distribution, as discussed above, will also affect short circuiting. Temperature Effects. Temperature effects can be significant in sedimentation basins. It has been shown that a 1째 Celsius temperature differential between the incoming wastewater and the wastewater in the sedimentation tank will cause a density current to form (see Figs. 2.3.10b and c). The impact of the temperature effects on performance will depend on the material being removed and its characteristics. Wind Effects. Wind blowing across the top of open sedimentation basins can cause circulation cells to form (see Fig. 2.3.10 d). When circulation cells form, the effective volumetric capacity of the basin is reduced. As with temperature effects, the impact of the reduced volume on performance will depend on the material being removed and its characteristics. Design Considerations If all solids in wastewater were discrete particles of uniform size, uniform density, uniform specific gravity, and uniform shape, the removal efficiency of these solids would be dependent on the surface area of the tank and time of detention. The depth of the tank would have little influence, provided that horizontal velocities would be maintained below the scouring velocity. However, the solids in most wastewaters are not of such regular character but are heterogeneous in nature, and the conditions under which they are present range from total dispersion to complete flocculation. Design parameters for sedimentation are considered below. Typical design data for sedimentation tanks are presented in Tables 2.3.1 & 2.3.2 . Additional details on the analysis and design of sedimentation tanks may be found in WPCF, 1985. .Detention Time. The bulk of the finely divided solids reaching primary sedimentation tanks is incompletely flocculated but is susceptible to flocculation. Flocculation is aided by eddying motion of the fluid within the tanks and proceeds through the coalescence of fine particles, at a rate that is a function of their concentration and of the natural ability of the particles to coalesce upon collision. As a general rule, coalescence of a suspension of solids becomes more complete as time elapses, thus, detention time is a consideration in the design of sedimentation tanks. The mechanics of flocculation are such, however, that as the time of sedimentation increases, less and less coalescence of remaining particles occurs.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Figure 2.3.10 5–47 Typical flow patterns observed in rectangular sedimentation tanks: (a) ideal flow, (b) effect of density flow or thermal stratification (water in tank is warmer than influent), (c) effect of thermal stratification (water in tank is colder than influent), and (d) formation of winddriven circulation cell. (Crites and Tchobanoglous, 1998.) Normally, primary sedimentation tanks are designed to provide 1.5 to 2.5 h of detention based on the average rate of wastewater flow. Tanks that provide shorter detention periods (0.5 to 1 h), with less removal of suspended solids, are sometimes used for preliminary treatment ahead of biological treatment units. In cold climates, increases in water viscosity at lower temperatures retard particle settling in clarifiers and reduce performance at wastewater temperatures below 20°C (68°F). A curve showing the increase in detention time necessary to equal the detention time at 20°C is presented on Fig. 2.3.11 (WPCF, 1985). For wastewater having a temperature of 10°C, for example, the detention period is 1.38 times that required at 20°C to achieve the same efficiency. Thus, in cold climates, safety factors should be considered in clarifier design to ensure adequate performance. Surface Loading Rates. Sedimentation tanks are normally designed on the basis of a surface loading rate (commonly termed “overflow rate”) expressed as cubic

NIREAS VOLUME 2 [2.1-2.2-2.3]

Figure 2.3.11 Curve of the increase in detention time required at cooler temperatures to achieve the same sedimentation performance as achieved at 20째C. meters per square meter of surface area per day, m3/m2_d (gallons per square foot of surface area per day, gal/ft2_d). The selection of a suitable loading rate depends on the type of suspension to be separated. Typical values for various suspensions are reported in Table 2.3.1. Designs for municipal plants must also meet the approval of state regulatory agencies, many of which have adopted standards for surface loading rates that must be followed. When the area of the tank has been established, the detention period in the tank is governed by water depth. Overflow rates in current use result in nominal detention periods of 2.0 to 2.5 h, based on average design flow. The effect of the surface loading rate and detention time on suspended solids removal varies widely depending on the character of the wastewater, proportion of settleable solids, concentration of solids, and other factors. It should be emphasized that overflow rates must be set low enough to ensure satisfactory performance at peak rates of flow, which may vary from over 3 times the average flow in small plants to 2 times the average flow in large plants . Weir Loading Rates. In general, weir loading rates have little effect on the efficiency of primary sedimentation tanks and should not be considered when reviewing the appropriateness of clarifier design. For general information purposes only, typical weir loading rates are given in Table 2.3.1.

NIREAS VOLUME 2 [2.1-2.2-2.3]

EXAMPLE 5â&#x20AC;&#x201C;10 Design of a Primary Sedimentation Basin The average flowrate at a small municipal wastewater-treatment plant is 20,000 m3/d. The highest observed peak daily flowrate is 50,000 m3/d. Design rectangular primary clarifiers with a channel width of 6 m (20 ft). Use a minimum of two clarifiers. Calculate the scour velocity, to determine if settled material will become resuspended. Estimate the BOD and TSS removal at average and peak flow. Use an overflow rate of 40 m3/m2/d at average flow (see Table 2.3.1) and a side water depth of 4 m (13.1 ft). Solution 1. Calculate the required surface area. For average flow conditions, the required area is:

2. Determine the tank length.

However, for the sake of convenience, the surface dimensions will be rounded to 6 m by 42 m. 3. Compute the detention time and overflow rate at average flow. Using the assumed sidewater depth of 4 m,

4. Determine the detention time and overflow rate at peak flow.

5. Calculate the scour velocity below, using the following values:

NIREAS VOLUME 2 [2.1-2.2-2.3]

Cohesion constant k =0.05 Specific gravity s= 1.25 Acceleration due to gravity g =9.81 m/s2 Diameter of particles d =100 Îźm =100 x10-6 m Darcy-Weisbach friction factor f= 0.025

6. Compare the scour velocity calculated in the previous step to the peak horizontal velocity (the peak flow divided by the cross-sectional area through the flow passes). The peak flow horizontal velocity through the settling tank is

The horizontal velocity value, even at peak flow, is substantially less than the scour velocity. Therefore, settled matter should not be resuspended.

7. Use Eq. (5â&#x20AC;&#x201C;45) and the accompanying coefficients to estimate the removal rates for BOD and TSS at average and peak flow. a. At average flow:

b. At peak flow:

HIGH-RATE CLARIFICATION

NIREAS VOLUME 2 [2.1-2.2-2.3]

High-rate clarification employs physical/chemical treatment and utilizes special flocculation and sedimentation systems to achieve rapid settling. The essential elements of high-rate clarification are enhanced particle settling and the use of inclined plate or tube settlers. Advantages of high-rate clarification are (1) units are compact and thus reduce space requirements, (2) start-up times are rapid (usually less than 30 min) to achieve peak efficiency, and (3) a highly clarified effluent is produced. Enhanced particle flocculation and highrate clarification applications are discussed in this section. Enhanced Particle Flocculation Enhanced particle flocculation has been used in Europe for more than 15 years but has only been introduced relatively recently in the United States. In its most basic form, enhanced particle flocculation involves the addition of an inert ballasting agent (usually silica sand or recycled chemically conditioned sludge) and a polymer to a coagulated and partially flocculated suspension. The polymer appears to coat the ballasting particles and forms the â&#x20AC;&#x153;glueâ&#x20AC;? that binds the chemical floc to the ballasted particles . After contact with the ballasting agent, the mixture is stirred gently in a maturation tank that allows the floc particles to grow. The particles grow as the larger, faster settling particles overtake and collide with slower-settling particles . The velocity gradient G for flocculation is important as a high gradient will cause a breakdown in the floc particles, and insufficient agitation will inhibit floc formation. Velocity gradients for enhanced particle settling of wastewater generally range from 200 to 400 s_1. Analysis of Ballasted Particle Flocculation and Settling The settling velocity of the ballasted particle is increased, when compared to an unballasted floc particle, by (1) increasing the density of the particle, (2) decreasing the coefficient of drag and increasing the Reynolds number, and (3) decreasing the shape factor through the formation of more dense spherical-shaped particles . The ballasted floc particles appear to be more spherical than the floc particles alone. In effect, ballasted flocculent particles settle with a velocity closer to that of a discrete particle than that of flocculent particles that have very high shape factors. Table 2.3.3. Summary of features of high-rate clarification processes

NIREAS VOLUME 2 [2.1-2.2-2.3]

Table 2.3.4. Ranges of overflow rates and BOD and TSS removals from high-rate clarification processes treating wet-weather flows

NIREAS VOLUME 2 [2.1-2.2-2.3]

Figure 2.3.12 High-rate clarification processes: (a) ballasted flocculation, (b) lamella plate clarification, and (c) dense-sludge.

FLOTATION Flotation is a unit operation used to separate solid or liquid particles from a liquid phase. Separation is brought about by introducing fine gas (usually air) bubbles into the liquid phase. The bubbles attach to the particulate matter, and the buoyant force of the combined particle and gas bubbles is great enough to cause the particle to rise to the surface. Particles that have a higher density than the liquid can thus be made to rise. The rising of particles with lower density than the liquid can also be facilitated (e.g., oil suspension in water). In wastewater treatment, flotation is used principally to remove suspended matter and to concentrate biosolids (see Chap. 14). The principal advantages of flotation over sedimentation are that very small or light particles that settle slowly can be removed more completely and in a shorter time. Once the particles have been floated to the surface, they can be collected by a skimming operation. Description The present practice of flotation as applied to wastewater treatment is confined to the use of air as the flotation agent. Air bubbles are added or caused to form by (1) injection of air while the liquid is under pressure, followed by release of the pressure (dissolved-air flotation), and (2) aeration at atmospheric pressure (dispersed-air flotation). In these systems,

NIREAS VOLUME 2 [2.1-2.2-2.3]

the degree of removal can be enhanced through the use of various chemical additives. In municipal wastewater treatment, dissolved-air flotation is frequently used, especially for thickening of waste biosolids. Dissolved-Air Flotation. In dissolved-air flotation (DAF) systems, air is dissolved in the wastewater under a pressure of several atmospheres, followed by release of the pressure to the atmospheric level (see Fig. 5â&#x20AC;&#x201C;53). In small pressure systems, the entire flow may be pressurized by means of a pump to 275 to 350 kPa (40 to 50 lb/in2 gage) with compressed air added at the pump suction (see Fig. 5â&#x20AC;&#x201C;53a). The entire flow is held in a retention tank under pressure for several minutes to allow time for the air to dissolve. It is then admitted through a pressure-reducing valve to the flotation tank where the air comes out of solution in very fine bubbles.

NIREAS VOLUME 2 [2.1-2.2-2.3]

Figure 2.3.13. Schematic of dissolved-air flotation systems: (a) without recycle in which the entire flow is passed through the pressurizing tank and (b) with recycle in which only the recycle flow is pressurized. The pressurized flow is mixed with the influent before being released into the flotation tank. In the larger units, a portion of the DAF effluent (15 to 120 percent) is recycled, pressurized, and semisaturated with air (Fig. 2.3.13b). The recycled flow is mixed with the unpressurized main stream just before admission to the flotation tank, with the result that the air comes out of solution in contact with particulate matter at the entrance to the tank. Pressure types of units have been used mainly for the treatment of industrial wastes and for the concentration of solids.

more ... EXAMPLE: Flotation Thickening of Activated-Sludge Mixed Liquor Design a flotation thickener without and with pressurized recycle to thicken the solids in activated-sludge mixed liquor from 0.3 to about 4 percent. Assume that the following conditions apply: 1. Optimum A/S ratio=0.008 mL/mg 2. Temperature =20째C 3. Air solubility =18.7 mL/L 4. Recycle-system pressure= 275 kPa 5. Fraction of saturation = 0.5 6. Surface-loading rate =8 L/m2_min 7. Sludge flowrate = 400 m3/d

NIREAS VOLUME 2 [2.1-2.2-2.3]

Comment Alternatively, the recycle flowrate could have been set and the pressure determined. In an actual design, the costs associated with the recycle pumping, pressurizing systems, and tank construction can be evaluated to find the most economical combination.

NIREAS VOLUME 2 [2.1-2.2-2.3]

ASSIGNMENTS SECTION ASSIGNMENTS SECTION 2 QUESTIONS 2.3A What is the main difference between the sludge from primary and secondary clarifiers?

2.3B What is the main difference between the effluent from primary and secondary clarifiers?

2.3C

List ihe significant items to check before start-up of a circular clarifier.

2.3D What safety precautions should be taken during startup of a clarifier?

2.3E What happens when the flights in a rectangular clarifier are not straight across the tank?

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.3F Describe a good operational strategy for a clarifier

2.3G What types of abnormal conditions could affect clarifier performance?

2.3H What steps could be taken to improve clarifier effluent quality when excessive storm flow infiltration is a frequent problem?

2.3I

What is the suspended solids removal efficiency of a primary clarifier if the influent concentration is 300 mg/L and the effluent concentration is 120 mg/L?

2.3J List the basic laboratory tests used to determine clarifier efficiency.

2.3K

At what two points should samples be collected for measuring clarifier efficiency?

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.3L

About what percentage of settleable solids should you expect to be removed by your clarifier?

2.3M What is the suspended solids removal efficiency of a primary clarifier if the influent concentration is 300 mg/L and the effluent concentration is 120 mg/L?

2.3N

How often should sludge be removed from a clarifier?

2.3O How can you tell when to stop pumping sludge?

2.3P

How can floating material (scum) be kept from the clarifier effluent?

2.3Q What is "short-circuiting" in a clarifier?

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.3R Why is "short-circuiting" undesirable?

2.3S How can "short-circuiting" be corrected?

2.3T A circular clarifier has a diameter of 80 feet and an average depth of 10 feet. The flow of wastewater is 4.0 MGD and the suspended solids concentration is 190 mg/L. Calculate the following: 1. Detention Time, in hours 2. Weir Overflow Rate, in GPD/ft 3. Surface Loading Rate, in GPD/sq ft

2.3U A circular clarifier has a diameter of 80 feet and an average depth of 10 feet. The clarifier treats 4.0 MGD from the plant inflow plus 1.2 MGD of return sludge flow. The mixed liquor suspended solids concentration is 2,700 mg/L. Calculate the solids loading in Ibs/day/sq ft.

2.3V

What safety items should be considered when reviewing plans and specifications for clarifiers?

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.3W Would you place the flotation process BEFORE or AFTER primary sedimentation?

2.3X Give a very brief description of: 1. Colloids 2. Emulsion

2.3Y

Why is the flotation process used in some wastewater treatment plants?

NIREAS VOLUME 2 [2.1-2.2-2.3]

SUGGESTED ANSWARS: 2.3A

The main difference between the sludge from primary and secondary clarifiers is that primary sludge is usually denser than secondary sludge. Other differences are that the primary sludge is septic, odorous, unstable and needs digestion.

2.3B The main difference between the effluent from primary and secondary clarifiers is that the effluent from a secondary clarifier is normally clearer than primary effluent. 2.3C Significant check items before starting a circular clarifier include: 1. Control gates for operation 2. Clarifier tank for sand and debris 3. Collector drive mechanism for lubrication, oil level, drive alignment, and complete assembly 4. Gaskets, gears, drive chain sprockets, and drive motor for proper installation and rotation 5. Squeegee blades on the collector plows for proper distance from the floor of the tank 6. All other mechanical items below the waterline for proper installation and operation 7. Tank sumps or hoppers and return lines for debris and obstructions 8. Tank structure for corrosion, cracks, and other indications of structural failure 2.3D

Safety precautions that should be taken during startup of a clarifier include:

1. Wear a crane or a hard hat when down in the tank for protection from falling objects. 2. Keep hands away from moving equipment. 3. When working on equipment, be sure to tag and use a lockout device on the main circuit breaker and influent control gates to prevent equipment from starting unexpectedly and causing equipment damage or personal injury. 2.3E

When the flights in a rectangular clarifier are not straight across the tank, sludge will be piled higher on the trailing side or the flights will hang up and cause severe damage to the flights.

2.3F The best operational strategy for a clarifier is to develop and implement a good preventive maintenance program, to closely monitor operating conditions, and to respond to any lab results that indicate problems are developing. 2.3G Abnormal conditions that could affect clarifier performance include: 1. Toxic wastes from industrial spills or dumps 2. Storm flows and hydraulic overloads 3. Septicity from collection system problems 2.3H Steps that could be taken to improve clarifier effluent quality when excessive storm flow infiltration is a frequent problem are

NIREAS VOLUME 2 [2.1-2.2-2.3]

sealing of the sanitary sewers or use of a flow equalization basin. 2.3J

The basic laboratory tests used to determine clarifier efficiency are dissolved oxygen (DO), settleable solids, pH, temperature, BOD, suspended solids, chlorine residual (if needed), and coliform group bacteria (if needed).

2.3K

To measure clarifier efficiency, sample the influent and effluent of the clarifier.

2.3L

A clarifier should be able to remove 95 to 99 percent of settleable solids. 2.3M What is the suspended solids removal efficiency of a primary clarifier if the influent concentration is 300 mg/L and the effluent concentration is 120 mg/L? Calculate the suspended solids removal efficiency. Known Unknown Influent SS, mg/L = 300 mg/L Efficiency, % Effluent SS, mg/L = 120 mg/L Efficiency, % = (In-Out)/(In) x (100%)= = (300 mg/L-120 mg/L)/ 300 mg/L = = (0.60) (100%) = 60% Suspended Solids Removal

2.3N

Remove sludge from a clarifier often enough to prevent septic conditions or sludge gasification. The proper interval is dependent on many conditions and may vary from thirty minutes to eight hours, and as much as twenty-four hours in a few instances. Experience will dictate the proper frequency of removal. 2.3O Stop pumping sludge when it becomes thin. Thin sludge can be detected by the sound of the sludge pump, differences in sludge pumping pressure gauge readings, sludge density gauge readings, visual observation of a small quantity (gallon or less), and through a sight glass in the sludge line while the sludge is being pumped. 2.3P

Floating material (scum) can be kept from the clarifier effluent by the following methods. To collect scum, a baffle is generally provided at some location in the tank. Primary clarifiers often have a scum collection area where the scum is skimmed off by some mechanical method, usually a skimming arm or a paddle wheel. If mechanical methods are not provided, hand tools may be used, such as a skimming dipper attached to a broom handle

2.3Q

Short-circuiting occurs in a clarifier when the flow is not uniform throughout the tank. In this situation, the water flows too rapidly in one or more sections of the clarifier to allow sufficient time for settling to occur.

2.3R

Short-circuiting is undesirable because where the velocity is too high, particles will not have time to settle. Where the velocity is too low, undesirable septic conditions may develop.

NIREAS VOLUME 2 [2.1-2.2-2.3]

2.3S Short-circuiting may be corrected by installing weir plates, baffles, port openings, and by proper design of the inlet channel. 2.3T A circular clarifier has a diameter of 80 feet and an average depth of 10 feet. The flow of wastewater is 4.0 MGD and the suspended solids concentration is 190 mg/L. Calculate the following: 1. Detention Time, in hours 2. Weir Overflow Rate, in GPD/ft 3. Surface Loading Rate, in GPD/sq ft Known Diameter, ft = 80 ft Depth, ft= 10 ft Flow, MGD = 4.0 MGD SS Conc, mg/L = 190 mg/L Unknown Detention Time, in hours Weir Overflow Rate, in GPD/ft Surface Loading Rate, GPD/sq ft 1. Calculate the tank volume in cubic feet. Tank Volume,cub ft = (π/4) χ (Diameter, ft)2 χ Depth, ft = (0.785)(80 ft)2 χ 10 ft = 0.785 χ 6,400 χ 10 = 0.785 χ 64,000 = 50,240 cu ft 2. Convert the tank volume from cubic feet to gallons. Tank Volume, gal = 50,240 cu ft χ 7.5 gal/cu ft = 376,880 gal 3. Estimate the detention time in hours. Detention = Tank Volume, gal χ 24 hr/day Time, hr Flow, gal/day = 376,800 gal χ 24 hr/day 4,000,000 gal/day = 0.376800 χ 6 = 2.2608 = 2.3 hr 4. Estimate the weir overflow rate in gallons per day per foot of weir length. Weir Overflow Rate, GPD/ft = (Flow, GPD/ Length of Weir, ft) = ((4,000,000 GPD/ (3.14 x 8 0 ft)) = = 15,923 GPD/ft of Weir 5. Calculate the surface area in square feet. Surface

(π/4)(Diameter)2 Area, sq ft

= (0.785) (80 ft)2 = 0.785 χ 6,400 = 5,024 sq ft 6. Estimate the surface loading rate in gallons per day per square foot of surface area. Surface Loading (GPD/sq ft) = (Flow, GPD/ Rate, Surface Area, sq ft)

NIREAS VOLUME 2 [2.1-2.2-2.3] =

4,000,000 GPD / 5,024 sq ft = 800 GPD/sq ft (close enough)

NOTE: The suspended solids concentration of 190 mg/L was not needed to solve this problem. Try to determine the information to solve problems and forget the unimportant data. 2.3U

A circular clarifier has a diameter of 80 feet and an average depth of 10 feet. The clarifier treats 4.0 MGD from the plant inflow plus 1.2 MGD of return sludge flow. The mixed liquor suspended solids concentration is 2,700 mg/L. Calculate the solids loading in Ibs/day/sq ft.

Known 1. Diameter, ft = 80 ft 2. Depth, ft = 10 ft 3. Plant Inflow, MGD = 4.0 MGD

Unknown Solids Loading, Ib /d /

4. = 1.2 MGD 5. Return Sludge Flow, MGD 6. MLSS, mg/L = 2,700 mg/L 7. Surface Area, sq ft = 5,024 sq ft (from Problem 5.6D) 1. Calculate the solids applied in pounds per day. Solids Applied, lbs/day = Total Flow, MGD χ MLSS, mg/L χ 8.34 lbs/gal = (4.0 MGD + 1.2 MGD) χ 2,700 mg/L χ 8.34 lbs/gal = 117,094 lbs/day 2. Estimate the solids loading in pounds of solids per day per square foot of surface area. Solids Loading, Ibs/day/sq ft = Solids Applied, lbs/day / Surface Area, sq ft =117,094 lbs/day/ 5,024 sq ft = = 23.3 Ibs/day/sq ft 2.3V

Safety items that should be considered when reviewing plans and specifications for clarifiers include:

1. Clarifiers must be equipped with adequate access by stairs, ladders, ramps, catwalks, and bridges. Be sure railings meet state and OSHA requirements. 2. Catwalks and bridges must have floor plates or grates firmly secured and equipped with toeboards and nonskid surfaces. 3. Adequate lighting must be provided. 4. Launders, channels, and effluent pipelines that carry flow from the clarifier to another conduit, channel, or structure must have safety grates over the entrance. 5. In a circular clarifier, turntables, adjustable inlet deflection baffles, and return sludge control valves must have safe access.

NIREAS VOLUME 2 [2.1-2.2-2.3]

6. Adequate guards must be placed over chain drives, belts; and other moving parts. 7. Safety hooks, poles, or floats should be stationed at strategic locations near every basin to rescue anyone who falls into a basin. 8. Do not allow any pipes or conduits to cross on top of catwalks or bridges. 9. Adequate offset of drive units, motors, and other equipment must be provided to allow unobstructed access to all areas. 2.3W The flotation process should be placed AFTER primary sedimentation. 2.3X

Colloidsâ&#x20AC;&#x201D;Very small, finely divided solids (particles that do not dissolve) that remain dispersed in a liquid for a long time due to their small size and electrical charge. Emulsionâ&#x20AC;&#x201D;A liquid mixture of two or more liquid substances not normally dissolved in one another, but one liquid held in suspension in the other.

2.3Y The flotation process is used to remove colloids and emulsions.

Thi spr oj ecthasbeenf undedwi t hsuppor tf r om t heEur opeanCommi ssi on.Thi spubl i cat i on [ communi cat i on]r ef l ect st hevi ewsonl yoft he aut horandt heCommi ssi oncannotbehel d r esponsi bl ef oranyusewhi chmaybemadeof t hei nf or mat i oncont ai nedt her ei n.