ELEARNI NG FOR THE OPERATORS OFWASTEWATER TREATMENT
VOLUME 3
ODOURS’ CONTROL
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3. ODOURS’ CONTROL 3.1. The problem of the odours’ management a
The potential release of odours is a major concerns of the public relative to the implementation of modifications to existing
wastewater treatment facilities and to the construction of new facilities. Thus, the control of odours has become a major consideration in the design and operation of wastewater collection, treatment, and disposal facilities, especially with respect to the public acceptance of these facilities. In many instances, projects have been rejected because of the fear of potential odours. In view of the importance of odours in the field of wastewater management, the following discussion must deal with: (1) the types of odours encountered, (2) the sources of odours, (3) the movement of odourous gases, (4) strategies for odour control, (5) odour control methods, and (6) the design of odour control facilities. a
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Wastewater Engineering, treatment and Reuse, Metcalf & Eddy,4 edition, 2003
3.1.1 What is an odour? An odour is composed of molecules, each of which has a specific size and shape. Each of these molecules has a correspondingly sized and shaped receptor in the human nose. When a specific receptor receives a molecule, it sends a signal to the brain and the brain identifies the smell associated with that particular molecule. An odour (or odor) is a volatilized chemical compound, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also called smells, which can refer to both pleasant and unpleasant odors. Odor in a substance is due to volatile Organic compounds which evaporate and get carried away by air, moved to other places, and if unpleasant odors approaching undiluted people, then possibly creates disturbance . NEED FOR ODOR CONTROL Odor control in wastewater collection systems and at wastewater treatment plants is very important. With the increased demand for housing, collection systems are being extended further and further away from the treatment plant. Longer collection systems create longer flow times to reach the treatment plant. Increased travel times cause the wastewater to become septic and thus cause odor and corrosion problems in collection systems and treatment plants. To complicate matters, the larger buffer areas around wastewater treatment plants have all but disappeared. Land values and increased population have made it impossible to continue to have large buffer areas around most plants. As homes and businesses become neighbors to existing plants, what was a minor odor problem now becomes a major problem. No longer can even the smallest trace of odor exist without complaints from neighbors. Thus, preventing the emission of odors has become a prime operating consideration.
more information‌. The terms fragrance, scent, and aroma are used primarily by the food and cosmetic industry to describe a pleasant odor, and are sometimes used to refer to perfumes. In contrast, malodorous, stench, reek, and stink are used specifically to describe unpleasant odors. The study of odors is a growing field but is a complex and difficult one. The human olfactory system can detect many thousands of scents based on only very minute airborne concentrations of a chemical. The sense of smell of many animals is even better. Some fragrant flowers give off odor plumes that move downwind and are detectable by bees more than a kilometer away. The study of odors can also get complicated because of the complex chemistry taking place at the moment of a smell sensation. For example iron metal objects are perceived to have an odor when touched although iron vapor pressure is negligible. According to a 2006 study this smell is the result of aldehyde sand ketones released from the human skin on contact with ferrous ions that are formed in the sweat-mediated corrosion of iron. The same chemicals are also associated with the smell of blood as ferrous iron in blood on skin produces the same reaction.
3.1.2 Types and composition of odours For humans, the importance of odors at low concentrations is related primarily to the psychological stress the odors cause, rather than to the harm they do to the body. The principal types of odors encountered in wastewater management facilities are reported in Table 3-1. With few exceptions, odorous compounds typically contain either sulfur or nitrogen. The characteristic odor of organic compounds containing sulfur is that of decayed organic material. Of the odorous compounds reported in Table 3-1a, the rotten egg smell of hydrogen sulfide is the odor encountered most commonly in wastewater management facilities.
Substance
Remarks
Typicalb Threshold Odor, ppm
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Allcyl Mercaptan Ammonia Benzyl Mercaptan Chlorine Chlorophenol Crotyl Mercaptan Dimethyl Sulfide Diphenyl Sulfide Ethyl Mercaptan Ethyl Sulfide Hydrogen Sulfide Methyl Mercaptan Methyl Sulfide Pyridine Skatole Sulfur Dioxide Thiocresol Thiophenol
Very disagreeable, garlic-like Sharp, pungent Unpleasant
0.00005
Pungent, irritating Medicinal Skunk
0.010 0.00018 0.000029
Decayed vegetables
0.0001
Unpleasant
0.000048
Odor of decayed cabbage Nauseating Rotten egg
0.00019
Decayed cabbage
0.0011
Decayed vegetables
0.0011
0.037 0.00019
0.00025 0.00047
Disagreeable, irritating 0.0037 Fecal, nauseating 0.0012 Pungent, irritating 0.009 Rancid, skunk-like Putrid, garlic-like
0.0001 0.000062
__________________________________________________________________________________________________________ a
MOR 11. Chapter 27, "Odor Control," Water Pollution Control Federation, Washington, DC, 1976. b Various references will list slightly different threshold odor concentrations. Table33. 1a. Odor Characteristicsa
3.2 Odors’ Generation
In order to control odors more effectively, an understanding of odor generation is needed. Understanding the problem and the causes will lead to a more effective solution. a) Biological Generation of Odors The principal source of odor generation is the production of inorganic (no or one carbon atom in formula, H2S) and organic (more than one carbon atom in formula, C8H7N) gases by microorganisms in the collection system and treatment processes. Odors also may be produced when odor-containing or odor-generating materials are discharged into the collection system by industries and businesses. The main concerns of operators are the inorganic gases hydrogen sulfide (H2S) and ammonia (NH3). These two gases give off the most offensive odors. As little as 0.5 ppb (parts of gas per billion parts of air) of hydrogen sulfide can be detected by the human nose and cause odor complaints. Hydrogen sulfide has an extremely offensive smell and has the odor produced by rotten eggs. Ammonia has a very sharp, pungent smell and also is very offensive. Other inorganic gases found in wastewater treatment plants are: carbon dioxide (CO2), methane (CH4), nitrogen (N2), oxygen (O2), and hydrogen (H2). Normally found in nature, these gases are the products of normal respiration and biological activity of plants and animals and are not odorous. Organic gases usually are formed in the collection system and in the treatment plant by the anaerobic decomposition of nitrogen and sulfur compounds. Organic gases also can derive their odors from industrial sources. Examples of organic gases found around treatment plants are MERCAPTANS1 INDOLE2 and SKATOLE3. These odorous compounds contain nitrogen or sulfur-bearing organic compounds. In the normal biological oxidation of organic matter, the microorganisms remove hydrogen atoms from the organic compounds. In the process, the microorganisms use the bound sources of oxygen to gain energy. The hydrogen atoms are then transferred through a series of reactions that are sometimes called "hydrogen transfer" or "dehydrogenation."
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1
Mercaptams (mer-CAP-tans). Compounds containing sulfur that have an extremely offensive skunk-like odor; also sometimes described as smelling like garlic or onions. 2 Indole (IN-dole). An organic compound (C8H7N) containing nitrogen that has an ammonia odor. 3 Skatole (SKAY-tole). An organic compound (C9H9N) that contains nitrogen and has a fecal odor.
more information…. CHEMICAL REACTIONS DESCRIBING ODORS' PRODUCTION The following reactions illustrate the role of the hydrogen atom in the formation of both odorless and odorous compounds or end products.
AEROBIC REACTION
O2 (g) + 4 H+(aq) +4 e- 2 H2O (l) Molecular
Water
Oxygen
(Odorless)
ANOXIC REACTION
2 N03- (aq) + 12 H+(aq) +10 e- N2 (g) Nitrate
+ 6 Η2Ο (l)
Nitrogen Gas (Odorless)
AΝAEROBIC REACTIONS
CO2 (g) + 8H+(aq) +8 e- CH4 (g) + 2 Η2Ο (l) Carbon
Methane Gas
Dioxide
(Relatively Odorless)
SO4 2- (aq) + 10 H+ (aq) +8 e- H2S (g) + 4 Η2Ο (l) Sulfate
Oxidized Organics
Hydrogen Sulfide Gas (Odorous)
+ nH+ Lower Organics (Odorous)
α
Table 3.1b Odor thresholds of odorous compounds and their character (more detailed information for odorous compounds)
Odorous
Chemical
Molecular
Odor
weight
threshold ,
compound
formula
(gmol-1)
ppmvb Characteristic odor
Ammonia
NH3
17.0
Pungent, irritating 46.8
Chlorine
Cl2
71.0
0.314 Pungent, suffocating
Chlorophenol
ClC6H4OH
128.56
0.000 Medicinal odor 18
Crotyl
CH3—CH=CH—
mercaptan
CH2—SH
Dimethyl sulfide CH3—S —CH3
90.19
0.000 Skunklike 029
62
0.000 Decayed 1
Diphenyl sulfide (C6H5)2S
186
cabbage
0.004 Unpleasant 7
Ethyl
CH3CH2—SH
62
0.000 Decayed
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mercaptan Ethyl sulfide
19 (C2H5)2SH
90.21
cabbage
0.000 Nauseating odor 025
Hydrogen
H2S
34
0.000 Rotten eggs
sulfide Indole
47 C8H6NH
117
0.000 Fecal, nauseating 1
Methyl amine
CH3NH2
31
Putrid, fishy 21.0
Methyl
CH3SH
48
0.002 Decayed
mercaptan Skatole
1 C9H9N
131
cabbage
0.019 Fecal
odor,
nauseating Sulfur dioxide
SO2
64.07
Thiocresol
CH3—C6H4—SH
Trimethyl amine
(
0.009 Pungent, irritating
124
0.000 Skunklike, rancid 062
CH3)3N
59
0.000 Pungent, fishy 4
"Adapted in part from Patterson et al. (1984), U.S. EPA (1985), and WEF (1998). bParts
per million by volume.
SULFIDE FORMS (Fig. 3.1) below pH of 5
neutral pH of 7
pHof 9 to 11
pH of 14
hydrogen sulfide (H2S) gas, 100%
H2S, 50% HS-, 50%
HS- ion, 100%
S2- ion, 90%
Fig.3.1 Effect of pH on Hydrogen Sulfide – Sulfide equilibrium
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3.3 Sources of Odours (in the wastewater management systems) The principal sources of odors in wastewater management facilities and the relative potential for release of odor are presented in Table 32. Minimization of odors from these sources is the concern of odor management. Wastewater Collection Systems The potential for odor release from collection systems is high. The principal sources of odorous compounds in collection systems are from (1) the biological conversion, under anaerobic conditions, of organic matter containing nitrogen and sulfur, and (2) the discharge of industrial wastewater that may contain odorous compounds or compounds that may react with compounds in the wastewater to produce odorous compounds. Odorous gases released to the sewer atmosphere can accumulate and be released at air release valves, cleanouts, and access ports (i.e., manholes). Wastewater Treatment Facilities In considering the potential for the generation and release of odors from treatment plants, it is common practice to consider the liquid and solids processing facilities separately. The headworks and preliminary treatment operations have the highest potential for release of odor, especially for treatment plants that have long collection systems where anaerobic conditions can be created (see Fig. 3-1). Side stream discharges including return flows from filter backwashing and from sludge and biosolids processing facilities are often a major source of odors, especially where these flows are allowed to discharge freely into a control structure or mixing chamber. Sludge And Biosolids Handling Facilities Typically, the most significant source of odors at wastewater treatment plants are sludge thickening facilities, anaerobic digesters, and sludge load-out facilities. The highest potential for odor release occurs when unstabilized sludge is handled (e.g., turned, spread, or stored).
Îą
Table 3.2. Sources of odor in wastewater management systems
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a
Wastewater Engineering, treatment and
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Reuse, Metcalf & Eddy,4 edition, 2003
3.4. Measurement of Odours In order to control odors effectively, the operator should know where odors originate and the cause. Odor detection in the past has been very unscientific because it relied on the human sense of smell. While our noses are more sensitive than most instruments or detection devices, each person has a different tolerance level for various odors. Occasionally, what smells good to one individual smells bad to another. There are several methods today, used to measure the odours Odors can be detected, measured, and identified by several methods. Gas detection devices can be used to detect the presence of specific gases that cause odors. Today odors can be measured by the use of an OLFACTOMETER (a device used to measure odors in the field by diluting odors with odor-free air), an ODOR PANEL (a group of people used to measure odors) or possibly by analytic testing. The olfactometer can measure odors in the field by diluting the odors with odor-free air. The number of dilutions required to reduce an odor to its Minimum Detectable THRESHOLD ODOR Concentration (MDTOC) provides a quantitative measure of the concentration or strength of an odor. The odor concentration is reported as the number of dilutions to "MDTOC." The results are reproducible within reasonable levels but there is the possibility of individual interpretation MDTOC is the minimum odor of a gas or water sample that can just be detected after successive dilutions with odorless gas or water. also called ODOR THRESHOLD. The most common method used to evaluate odor nuisances is the odor panel. This method of odor measurement involves a group of people (usually eight or more) in the evaluation of odors. The panel members are given diluted odorous gas samples and are asked to indicate whether they can or cannot detect an odor at various dilutions. The dilution(s) at which the odor is detected may be used to assess its strength or "threshold odor concentration." Analytic testing to identify relative components of a gas sample is also an option in the odor identification process. Gas chromatography (GC) and mass spectrometry (MS) have been used for this purpose. However, this quantitative analysis may be difficult to interpret because the threshold concentrations of some odors are below the GC/MS detection level. Certain types of odors have significant effects on people and animals. These odors have a major health and economic impact on those affected. For these reasons, the identification of odors is important. Once the odor has been identified, the solutions can be studied. The following are some facts that can help in odor classification: 1. Almost all individuals have a sense of smell. 2. Individuals respond differently to the same odor. 3. Some odors are objectionable and others are pleasant. 4. Odors travel great distances with the direction of the wind. 5. Small concentrations of odors can be offensive. 6. Similar compounds do not have the same odor. 7. The human nose rapidly becomes fatigued (insensitive) to odors. The best that most operators can do when recording odors is to classify the odor in some reasonable fashion. Sometimes a person not working in a plant will have to identify odors because an operators nose can become insensitive. Usually offensive odors can be classified into the following groups: (1) Ammonia, (2) Decayed cabbage, (3) Decayed flesh, (4) Fecal, (5) Fishy, (6) Garlic, (7) Medicinal, (Describing characteristics of odors can be combined the above groups).
more information‌ ODOR CONCENTRATİON
(8) Rotten egg, (9) Skunk
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The measurement of odor concentration is the most widespread method to quantify odors. The method is based on dilution of an odor sample to the odor threshold (the point at which the odor is only perceptible to 50 % of the test panel). The numerical value of the odor concentration is equal to the dilution factor that is necessary to reach the odor threshold. Its unit is the European Odor Unit, OUE. Therefore, the odor concentration at the odor threshold is 1 OUE by definition. To establish the odor concentration, an olfactometer is used which employs a panel of test persons. A diluted odorous mixture and an odor-free gas (as a reference) are presented from sniffing ports to a group of panelists. In comparing the gases emitted from each port, the panelists are asked to report the presence of odor. The gas-diluting ratio is then decreased by a factor of 1.4 or two (i.e. the concentration is increased accordingly). The panelists are asked to repeat their judgment. This continues for a number of dilution levels. The responses of the panelists over a range of dilution settings are used to calculate the concentration of the odor in terms of European Odor Units (OUE/m3). Olfactometers
Fig. 3.2 Olfactometers Odor intensity can be divided into the following categories according to intensity: 0 - no odor 1 - very weak (odor threshold) 2 - weak 3 - distinct 4 - strong 5 - very strong 6 - intolerable This method is most often applied by having a dilution series tested by a panel of independent observers who have been trained to differentiate intensity . It is generally accepted that the extent of objection and reaction to odor by neighbors is highly variable. The reaction can be based on previous experience, relationship to the odor-producing enterprise and the sensitivity of the individual. Weather (temperature, humidity, wind direction) affects the volatility of compounds, preventing or enhancing movement into the gaseous phase where an odor can be dispersed downwind. Most of us will accept even a strong odor for a short period of time, provided we don’t have to smell it often. But we have a threshold for the frequency and duration of the odor above which our tolerance is exceeded, and we view the odor as a nuisance. These thresholds, however, are person-specific. While it is the frequency and duration of an odor that often triggers a nuisance complaint, odor measurement procedures typically focus on the first three traits (intensity, offensiveness and character). From a human health standpoint, exposure time is an essential measure in predicting any negative effects that may occur and this encompasses frequency and duration as well as concentration (intensity). As a result, regulatory procedures often include concentration, frequency and duration as part of the compliance protocol.
more information‌. Defining odor An odorant is a substance capable of eliciting an olfactory response, whereas odor is the sensation resulting from stimulation of the olfactory organs. Odor threshold is a term used to identify the concentration at which animals respond 50 percent of the time to repeated presentations of an odorant being tested. Most often, however, odor threshold is used to describe the detection threshold, which identifies the concentration at which 50 percent of a human panel can identify the presence of an odor or odorant without characterizing the stimulus. The recognition threshold is the concentration at which 50 percent of the panel can identify the odorant or odor.
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Although the detection threshold concentrations of substances that evoke a smell are low, oftentimes in the parts per billion (ppb) or parts per trillion (ppt) range, a concentration only 10 to 50 times above the detection threshold value often is the maximum intensity that can be detected by humans. This is in contrast to other sensory systems where maximum intensities are many more multiples of threshold intensities. For example, the maximum intensity of sight is about 500,000 times that of the threshold intensity and a factor of one trillion is observed for hearing. For this reason, smell is often concerned with identifying the presence or absence of odor rather than with quantifying intensity or concentration. Perception of a mixture of odorants, such as those in livestock odor, is very different from how each chemical would be perceived independently. Odorants can act as additive agents, counteractants, masking agents or be synergistic in nature. The combination of two odorants can have an odor equal to that of either one of the components, have an odor less than that of one of the components, have an odor equal to the sum of the components or even have an odor greater than the sum of the components. This makes odor quantification and characterization a challenging process. Odor can be evaluated subjectively in terms of intensity (strength) or in terms of quality (offensiveness). Odor quality is evaluated by describing the odor or comparing the sample odor to familiar odors. Evaluation of odor quality is difficult because of the challenges that come with trying to describe odors. Challenges with current methods of measurement Challenges with current methodology include the use of humans for assessment. Work has shown that the same panelist’s response from one day to the next can vary by as much as three-fold, possibly due to health or mood of the individual. Variability in the sensitivity of the individual conducting the evaluation and odor fatigue are further concerns commonly addressed in procedural protocol. Odor fatigue is a temporary condition where a person becomes acclimated to an odorant or odor to the point that they are no longer aware the odor is present. An example would be when you walk into a barbecue restaurant and by the time you leave you are unaware of the aroma that attracted you in the door. Onsite methods are complicated by the influence visual perception might have in an evaluation (smelling with your eyes, so to speak). Each of us has a unique odor acuity. While methods try to minimize panelist variation, the difference in sense of smell from one person is another consideration in human assessment methods. The measurement of odor concentration by dilution is more direct and objective than that of odor quality or intensity. However, each of the above procedures requires the use of the human nose as a detector, so not one is completely objective. Use of a forced-choice method in which a panelist must simply identify the presence or absence of an odor, is generally a better method than ranking, as the human nose cannot distinguish small differences between levels of intensity.
ODOR TYPE This is a verbal characterization of the sensed odor by the test person, such as disgusting, caustic, ruffling, etc. There are no more applications needed than a test person to run this method. The evaluation of the odor type could be an emission or an immission method. It has a great impact on evaluating the source of the odor emission. SAMPLÄ°NG TECHNÄ°QUE There are two main odor sampling techniques, the direct odor sampling and the indirect odor sampling technique Direct odor sampling Air will be sampled at the source and fed straight into the olfactometer for assessment by an odor panel. The following problems can be associated with this technique: Odor panel members need to be seated in an odor neutral environment, thus they need to be housed in a separate area. This is difficult to achieve when assessing odor released from, for example factories, where the odor can be emitted from a stack on the end of a production line. This means that the odor sample collected needs to be transported from the stack to the unit where the odor panel sits. This can sometimes be on the other side of the factory plant. The sample then must therefore pass through a very long sample line to the olfactometer. This can have influences on the sample quality, can have potential air blockages due to water condensation or other operational procedures. Therefore most odor annoyance assessment companies use the indirect air sampling method. Indirect odor sampling Indirect odor sampling is done with the use of odor (air) sampling bags, which are made from an odor neutral material e.g. Teflon. The odor sample bags are connected to an air sampling line which is then, for example, hooked up to a stack. The air stream is then sampled and stored in the odor sample bag and can then be analyzed in a suitable environment (e.g. in an odor laboratory). The indirect method is used to sample a wide variety of odor sources. From stacks on the end of a factory line, water surfaces or ambient air surroundings. Each odor source has its own set of problems when sampled; these problems need to be overcome in order to collect a representative sample of the odor source. The following problems can be encountered:
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High lethal gas concentrations Sometimes odor sources emit a high concentration of gases that are lethal to man. These samples must be diluted to a safe level, before being presented to the odor panel. This pre-dilution can be done in a stack-dilution probe, by the addition of an inert gas or on a dilution device for example an extra olfactometer. Odor measurement techniques • Dilution-to-threshold methods Dilution-to-threshold techniques dilute an odor sample with odorless air at a number of levels and the dilution series is presented in ascending order of odor concentration. From one level to the next, the dilution decreases and the amount of odorous air increases. The first few levels include the sample diluted with a large amount of odorless air so evaluation can begin below the threshold of detection. Preferably, multiple presentations (two odorless air samples and the diluted odor sample) are made at each level of dilution. When a forced-choice method is used, a panelist, typically trained to conduct these evaluations, must identify the presentation that is different from the others at each level, even if it is a guess. This permits use of all the data. The threshold of detection is the dilution level at which the panelist can determine a difference between the diluted and the odorless samples. After the detection threshold is reached, the panelist continues the evaluation at the next level or two to be certain the identification was not made by chance. • Ranking methods Odor can be evaluated using panelists to rank samples, a procedure in which an arbitrary scale is used to describe either the intensity or offensiveness of an odor. Typically, a scale of 0 to 10 is used, with 0 indicating no odor or not offensive and 10 representing a very intense or offensive odor. Such methods use either odor adsorbed onto cotton or a liquid sample that has been diluted. Manure can be diluted with water to a range of concentrations and then evaluated by a panel. One study, for example, diluted stored dairy manure with water to create five dilution levels. For each level, two blank samples of water and one diluted manure sample were presented in flasks that had been painted black to avoid bias based on appearance of the diluted manure. Panelists evaluated the samples in an ascending series; the dilution decreased and odor increased from one level to the next. At each dilution level, panelists identified the flask in each set of three that contained the odorous sample (forced-choice). A separate study analyzed panelist variability when this procedure was used and observed that each panel member had a distinct and repeatable odor probability distribution. • Referencing methods This method uses different amounts of 1-butanol as a standard to which sample odor intensity is compared, again using a human panel. The range of 1-butanol concentrations is often from 0 to 80 ppm. As the concentration of butanol is changed, the sample odor is compared to the butanol to determine at what concentration of butanol the sample’s intensity is equivalent. The use of butanol as a reference standard is widely accepted as common practice in Europe and has been incorporated into portable and laboratory scale instrumentation. Most of the methods currently used in the United States employ butanol as a means of assessing panelist suitability rather than as the sole means of determining an odor’s strength or acceptability. Emerging methods Efforts are underway across the United States to develop evaluation methods that can be used on-site and without the influence of human subjectivity, with the goal of providing an objective and affordable means of quantifying odors. • Surrogate compounds Odors from livestock facilities contain hundreds of different compounds, all interacting with each other and their environment in additive and non-additive (counteractant, masking) manners. From the standpoint of odor control, it is desirable to know which compounds are most important in defining an odor, so those few compounds can be targeted with control strategies. Compounds that have been well-correlated to odor measures in studies led by Iowa State University and elsewhere and might be useful as surrogates in determining odor, include volatile fatty acids and phenolics. In order to identify and quantify the constituents of odor, gas chromatography-mass spectrometry (GC/MS) is most frequently employed. Samples are commonly trapped (adsorbed) onto some type of sorbent material that concentrates compounds of interest then quantified by GC/MS. Concentrations of identified compounds and the interactions of the identified compounds are mathematically correlated to odor measurements made using traditional methods, most commonly the dilution-to-threshold methods. Interpretation of the results is complicated because odors that are equal in concentration may not be equal in offensiveness or intensity. Furthermore, two odors of equal concentration may be perceived as having different intensities. While GC/MS is frequently used to identify and quantify odorous compounds and the use of surrogate compounds is an objective method, this approach does not represent the experience of odor sensation as perceived by humans. Efforts to combine both instrumental and human methods are under development. Electronic nose Electronic nose analysis with a sensor array is a potential technology for odor evaluation. To date, relatively little research has been conducted with electronic noses in the area of agricultural manure odors. The electronic nose has been developed in an attempt to mimic the human sense of smell and is frequently used in the food, beverage, and perfume industries for product development and quality
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control. The sensor array of an electronic nose detects the chemicals that humans perceive as odors and records numerical results. The instrument will generate a different pattern of response for different types of samples. Commercially available electronic noses have 32, 64, or 128 sensors. Each sensor has an individual characteristic response, and some of the sensors overlap and are sensitive to similar chemicals, as are the receptors in the human nose. A single sensor is partially responsive to a broad range of chemicals and more responsive to a narrow range of compounds. Multiple sensors in a single instrument provide for responsive to a great number and many types of chemicals, with certain sensors that mix being moderately to extremely sensitive to speciďŹ c compounds. The technology is relatively new to the agricultural industry, although the potential for application is certainly great. Recent work demonstrated that an electronic nose can distinguish between pig and chicken slurry and between emissions from swine and dairy facilities because the sensor response patterns between the comparisons were different. At the current point of development, the electronic nose appears to be less sensitive than olfactometry measures, though sensor improvements occur routinely. Sensor selection is critical from both the standpoint of sensitivity to compounds that contribute to the offensive odors (malodor) as well as response and durability of the sensors in humid environments. Problems Where the E-Nose Can Help The electronic nose is best suited for matching complex samples with subjective end points such as odor or flavor. For example, when has milk turned sour? Or, when is a batch of coffee beans optimally roasted? The electronic nose can match a set of sensor responses to a calibration set produced by the human taste panel or olfactory panel routinely used in food science. The electronic nose is especially useful where consistent product quality has to be maintained over long periods of time, or where repeated exposure to a sample poses a health risk to the human olfactory panel. Although the electronic nose is also effective for pure chemicals, conventional methods are often more practical.
Problems That the E-Nose Does Best Identification of spilled chemicals in commerce (for U.S. Coast Guard). Quality classification of stored grain. Water and wastewater analysis. Identification of source and quality of coffee. Monitoring of roasting process. Rancidity measurements of olive oil (due to accumulation of short-chain aldehydes). Detection and diagnosis of pulmonary infections. Diagnosis of ulcers by breath tests. Freshness of fish. Process control of cheese, sausage, beer, and bread manufacture. Bacterial growth on foods such as meat and fresh vegetables.
Figure 3.3: Compare Human nose and Electronic nose
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Figure 3.4: An Electronic nose
Conclusions Odor measurement is a complicated task. While a number of methods are available, none are without drawbacks. However, dilution-tothreshold methods are the most widely accepted methods at the current time.
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Figure 3.4: A curious cat with sensible nose!!!!!
3.5. Specific topics for the working place and neighbours Odor Complaints Periodically, all wastewater treatment plants will cause some odors. These will be detected by the public and must be handled by the operator. All complaints should be answered promptly and courteously. The public pays for your services and indirectly is your boss. When responding to odor complaints, maintain a positive attitude. Beginning a conversation with a negative attitude will quickly upset the public. Even if you cannot detect the odor when you answer a complaint, that does not mean that the odor was not there or is not there now. Your nose may not be as sensitive as the nose of the person filing the complaint. Also, your nose may be accustomed to the smell and may no longer be able to detect the offensive odor. The greatest complication develops if you do not properly handle the problem. If the public unites against the plant and becomes very odor conscious, even the slightest odor can cause an uproar. Remember that the person filing the complaint called because of a problem. You must be a diplomatic listener. Invite those who have complained to visit the plant and offer them a tour. While you are showing them around the plant, they may indicate to you where the odor that is bothering them is the strongest. This information may help you identify and control the cause of the odor problem. Whenever an odor complaint is investigated, a record should be made of the visit and the important facts should be recorded according a form (Figure 3.6). Investigations in the neighborhood near the location of an odor complaint can be very helpful. Odors can be coming from a nearby sewer, storm drain, trash pile, home plumbing problem, or dead animal. If an odor complaint is repeated and the source cannot be located, consider sending personnel to the site during the time of day when odors are a problem to determine the source.
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Fig. 3.6 Odor Complaint Form
Movement of Odors from Wastewater Treatment Facilities a
Under quiescent meteorological conditions, odorous gases that develop at treatment facilities tend to hover over the point of
generation (e.g., sludge thickening facilities, sludge storage lagoons), because the odorous gases are more dense than air. Depending on the local meteorological conditions, it has been observed that odors may be measured at undiluted concentrations at great distances from the point of generation. The following events appear to happen (1) in the early morning hours, under quiescent meteorological conditions, a cloud of odors will develop over the wastewater treatment unit prone to the release of odors and (2) the concentrated cloud of odors can then be transported (i.e., pushed along), without breaking up, over great distances by the weak early morning breezes, as they develop. In some cases, odors have been detected at distances of up to 25 km from their source. This transport phenomenon has been termed the puff movement of odors (Tchobanoglous and Schroeder, 1985). The puff movement of odors was first described by Wilson (1975). The most common method used to mitigate the effects of the odor puff is to install barriers to induce turbulence, thus breaking up and dispersing the cloud of concentrated odors.
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Buffer distance Treatment process unit
ft
m
Sedimentation tank
400
125
Trickling filter
400
125
Aeration tank Aerated lagoon
500
150
1000
300
500
150
1000
300
500
150
Sludge digester (aerobic or anaerobic) Sludge-handling units Open drying beds
Table 3-5
Covered drying beds
400
125
Sludge-holding tank
1000
300
buffer distances from
Sludge-thickening tank
1000
300
treatment units for
Vacuum filter
Suggested minimum
odor containmenta,b
500
150
Wet air oxidation
1500
450
Effluent recharge bed
800
250
Open
500
150
Enclosed
200
75
Open
300
100
Enclosed
200
75
Denitrification
300
100
Polishing lagoon
500
150
Land disposal
500
150
Secondary effluent filters
Advanced wastewater treatment Tertiary effluent filters
a
Source: New York State Department of Environmental Conservation.
b
Actual buffer
distance requirements will depend on local conditions.
3.6. Treatment alternatives:
3.6.1 Strategies for Odor Management. Strategies for the management and control of odors are presented and discussed below. An overview of some of the methods used to control and treat odorous gases is presented in the following section. Where there are chronic odor problems at treatment facilities, approaches to solving these problems may include (1) control of odor-causing wastewaters discharged to the collection system and treatment plant that creates odor problems, (2) control of odors generated in the wastewater-collection system, (3) control of odors generated in wastewater treatment facilities, (4) installation of odor containment and treatment facilities, (5) application of chemicals to the liquid (wastewater) phase, (6) use of odor masking and neutralizing agents, (7) use of gas-phase turbulence, inducing structures and facilities, and (8) establishment of buffer zones. In Table 3-3 are listed the Odor containment alternatives for the control of odor emissions from wastewater management facilities. Table 3-3 Odor containment alternatives for the control of odor emissions from wastewater management facilities
Facility (sources)
Suggested control strategies
Wastewater sewers
Seal existing access ports (i.e., manholes). Eliminate the use of structures that create turbulence and enhance volatilization. Ventilate sewer reaches that develop positive pressure to odor control facilities
Containment and precautions have less cost and impacts for the society
a
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Sewer appurtenances
Isolate and cover existing appurtenances
Pump stations
Vent odorous gases from wetwell to treatment unit. Use variable-
Bar racks
speed pumps to reduce the size of the wetwell Cover existing units. Reduce headloss through bar racks
Comminutors
Cover existing units. Use in-line enclosed comminutors
Parshall flume Grit chamber
Cover existing units. Use alternative measuring device Cover existing aerated grit chambers. Reduce turbulence in conventional horizontal-flow grit chambers; cover if necessary. Avoid the use of aerated grit chambers
Equalization basins Primary and secondary sedimentation tanks Biological treatment
Cover existing units. Use submerged mixers, and reduce airflow Cover existing units. Replace conventional overflow weirs with submerged weirs Cover existing units. Use submerged mixers and reduce aeration rate
Sludge thickeners
Cover existing units
Transfer channels
Use enclosed transfer channels
14
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Control of Discharges to Collection System. The elimination and or control of wastewater discharges containing odorous compounds to the collection system can be accomplished by (1) the adoption of more stringent waste discharge ordinances and enforcement of their requirements, (2) requiring pretreatment of industrial wastewater, and (3) providing flow equalization at the source to eliminate slug discharges of wastewater. Odor Control In Wastewater Collection Systems. The release of odors from the liquid phase in wastewater collection systems can be limited by (1) maintaining aerobic conditions through the addition of hydrogen peroxide, pure oxygen, or air at critical locations in the collection system and to long force mains (see Fig. 3-2), (2) controlling anaerobic microbial growth by disinfection or pH control, (3) oxidizing odorous compounds by chemical addition, (4) design of the wastewater collection system to minimize the release of odors due to turbulence, and (5) off-gas treatment at selected locations. Odor Control In Wastewater Treatment Facilities. With the proper attention to design details, such as the use of submerged inlets and weirs, the elimination of hydraulic jumps in influent piping and channels, the elimination of physical conditions leading to the formation of turbulence, proper process loadings, containment of odor sources, off-gas treatment, and good housekeeping, the routine development of odors at treatment plants can be minimized. It must also be recognized, however, that odors will occasionally develop. When they do, it is important that immediate steps be taken to control them. Often, this will involve operational changes or the addition of chemicals, such as chlorine, hydrogen peroxide, lime, or ozone.
3.6.2 Odor Treatment Methods The general classification of odor treatment methods is presented in Table 3-4, along with typical applications in wastewater management. Odor treatment methods are either designed to treat the odor-producing compounds in the wastewater stream, or to treat the foul air. The majority of the methods in Table 3-4 are meant to treat the foul air (i.e., the gas phase). As noted above, to control the release of odorous gases from treatment facilities, it has become more common to cover wastewater treatment processes (see Fig. 3-1). The principal methods used to treat odorous gases include the use of (1) chemical scrubbers, (2) activated carbon absorbers, (3) vapor phase biological treatment processes (i.e., compost filters), (4) treatment in conventional biological treatment processes, and (5) thermal processes. Each of these methods is discussed below. The specific method of odor control and treatment that should be applied will vary with local conditions. However, because odor control measures are expensive, the cost of making process changes or modifications to the facilities to eliminate odor development should always be evaluated and compared to the cost of various alternative odor control measures before their adoption is suggested.
Selection and Design of Odor Control Facilities The following steps are involved in the selection and design of odor control and treatment facilities. 1. Determine the characteristics and volumes of the gas to be treated, 2. Define the exhaust requirements for the treated gas, 3. Evaluate climatic and atmospheric conditions, 4. Select one or more odor control and treatment technologies to be evaluated, 5. Conduct pilot tests to determine design criteria and performance, 6. life cycle economic analysis Many of the chemical odor control technologies are supplied as complete packages, designed to meet a given performance specification. The analysis of chemical scrubbers and the design of biofilters is considered in the following discussion.
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Table 3-4 Methods used to treat odorous gases found in wastewater management facilities a
Method
Description and/or application
Chemical methods Chemical oxidation
Oxidizing the odor compounds in wastewater is one of the most common methods used to achieve odor control. Chlorine, ozone, hydrogen peroxide, and potassium permanganate are among the oxidants that have been used. Chlorine also limits the development of a slime layer
Chemical precipitation
Chemical precipitation refers to the precipitation of sulfide with metallic salts, especially iron
Neutralizing agents
Compounds that can be sprayed or atomized in fine mists to chemically react with, neutralize, and/or dissolve odorous compounds
Scrubbing with various alkalies
Odorous gases can be passed through specially designed chemical scrubbing towers to remove odors. If the level of carbon dioxide is high, costs may be prohibitive
Thermal oxidation b Oxygen injection
Combustion of off-gases at temperatures from 800 to 1400°C will eliminate odors. Lower The injection of oxygen (either air or pure oxygen) into the wastewater to control the development of anaerobic conditions has proved to be effective
“Physical” methods Adsorption on activated carbon Adsorption on sand,
Odorous gases can be passed through beds of activated carbon to remove odors. Carbon regeneration can be used to reduce costs. Additional details may be found in Chap. 11 Odorous gases can be passed through sand, soil, or compost beds. Odorous gases from
soil, or compost beds
pumping stations may be vented to the surrounding soils or to specially designed beds containing sand or soils. Odorous gases collected from treatment units may be passed through compost beds
Dilution with
Gases can be mixed with fresh air sources to reduce the odor unit values. Alternatively,
odor-free air
gases can be discharged through tall stacks to achieve atmospheric dilution and dispersion
Masking agents
Perfume scents can be sprayed in fine mists near offending process units to overpower or mask objectionable odors. In some cases, the odor of the masking agent is worse than the original odor. Masking agents should not be confused with neutralizing agents described below
Scrubbing towers
Odorous gases can be passed through specially designed gas scrubbing towers to remove odors
Turbulence-inducing
Use of wind breaks, such as high fences and trees, and propeller fans
facilities
Biological methods Activated-sludge aeration tanks
Odorous gases can be combined with the process air for activated-sludge aeration tanks to remove odorous compounds
Biotrickling filters
Specially designed biotrickling filters can be used to remove odorous compounds biologically. Typically, the filters are filled with plastic packing of various types on which biological growths can be maintained
Compost filters
Gases can be passed through biologically active beds of compost to remove odors
Sand and soil filters
Gases can be passed through biologically active beds of compost to remove odors
Trickling filters
Odorous gases can be passed through existing trickling filters to remove odorous compounds
as “Physical” are defined the methods without using strong chemical reactants. b
a
Oxygen injection can be considered more a chemical than a physical method
Adapted in part from U.S. EPA (1985).
Design and operational changes that can be instituted can include (1) minimization of free-fall turbulence by controlling water levels, (2) reduction of overloading of plant processes, (3) increasing the aeration rate in biological treatment processes, (4) increasing the plant treatment capacity by operating standby process units, (5) reducing solids inventory and sludge backlog, (6) increasing the frequency of pumping of sludge and scum, (7) adding chlorinated dilution water to sludge thickeners, (8) controlling the release of aerosols, (9) increasing the frequency of disposal of grit and screenings, and (10) cleaning odorous accumulations more frequently. Odor Containment and Treatment. In cases where the treatment facilities are close to developed areas, it has become common practice to cover treatment units such as the headworks (see Fig. 3-1b), primary clarifiers (see Figs. 3-1a and 3-1b), trickling filters (see Figs. 3-1c and 3-1d), sludge thickeners (see Fig 3-1e), sludge processing facilities, and sludge load out facilities (see Fig. 3-1f). Where covers are used, the trapped gases must be collected and treated. The specific method of treatment, as discussed above, will depend on the characteristics of the odorous compounds. Typical odor containment and process alternatives for the control of the emission of odors from wastewater management facilities are reported in Table 3-2.
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Figure 3.7. Typical odor containment facilities at wastewater treatment plants: (a) covered primary sedimentation tanks, (b) covered primary sedimentation tanks with odor-control facilities, (c) covered trickling filter, (d) view inside covered trickling filter; the large pipe shown overhead is used to remove odorous gases for treatment in a chemical scrubber, (e) covered sludge thickener, and (f) enclosed sludgeloadout facility.
Figure 3.8. Typical uses of commercial oxygen in wastewater-collection systems for odor control: (a) sidestream oxygenation and reinjection of wastewater into a gravity sewer, (b) injection of oxygen into a hydraulic fall, and (c) injection of oxygen into two-phase flow in force main. (From Speece
et al., 1990.)
a
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3.6.2.1 Chemical methods (Chemical Additions to Wastewater for Odor Control) . Odors can be eliminated in the liquid phase through the addition of a variety of chemicals to achieve (1) chemical oxidation, (2) chemical precipitation, and (3) pH control. The most common oxidizing chemicals that can be added to wastewater include oxygen, air, chlorine, sodium hypochlorite, potassium permanganate, hydrogen peroxide, ozone, and ferric salts. While all of these compounds will oxidize hydrogen sulfide (H2S) and other odorous compounds, their use is complicated by the chemical matrix in which the odorous gases exist. The only way to establish the required chemical dosages for the removal of chemical compounds
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is through bench or pilot scale testing. Odorous compounds can also be reduced by precipitation. For example, ferrous chloride and ferrous sulfate can be used for the control of H2S odors by precipitation of the sulfide ion as ferrous sulfide. As with the oxidation reactions, the required chemical dosage can only be determined through bench or pilot scale testing. The release of H2S can also be controlled by increasing the pH value of the wastewater. Increasing the pH of the wastewater results in reduced bacterial activity and also shifts the equilibrium so that the sulfide ion is present as HS-. With most of the odor control methods involving the addition of chemicals to wastewater, some residual product is formed that must ultimately be dealt with. Because of the uncertainties associated with chemical addition, this method of odor management is not used commonly. Additional details on chemical addition may be found in Rafson (1998). (1) Chemical oxidation (Chemical Treatment of Odors in Wastewater) (1a)Chlorination Chlorination is one of the oldest and most effective methods used for odor control. Chlorine is used in the disinfection process and is readily available at the wastewater treatment plant. Because of this availability, chlorine is frequently used to control odors. Chlorine is a very reactive chemical and, therefore, oxidizes many compounds in wastewater. The reaction between chlorine and hydrogen sulfide and ammonia has been studied by many researchers.
more information…. The reaction between chlorine and hydrogen sulfide is: H2S (g) + 4 Cl2 (g)+ 4 H2O (l) H2SO4 (aq)+ 8 HCl (aq) The reaction of ammonia with chlorine is: NH3 (g) + Cl2 (g) NH2CI (aq) + HCI (aq)
(monochloramine, NH2CI)
NH2CI (aq) + Cl2 (g) NHCl2 (aq) + HCI (aq) (dichloramine, NHCl2) NHCI2 (aq) + Cl2 (g) NCI3 (g) + HCI (aq)
(trichloramine, NCl3)
The most important roles that chlorine plays in controlling odors are to: (1) inhibit the growth of slime layers in sewers, (2) destroy bacteria that convert sulfate to sulfide, and (3) destroy hydrogen sulfide at the point of application. This control requires less chemical than trying to oxidize the odor once formed. This means that the chlorine should be added in the collection system ahead of the plant. Odors are not always removed by the use of chlorine. The reaction of chlorine with certain chemicals can cause a more odorous gas. One example is the reaction of chlorine with PHENOLs (Organic compounds that arc derivatives of benzene) to form chlorophenol, a medicinalsmelling substance. Experience has shown that a dosage as high as 12 to 1 of chlorine to dissolved sulfide (12 mg/Z. chlorine per each 1 mg/L sulfide) may be needed to control the generation of hydrogen sulfide in sewers. Do not determine the chlorine dose on the basis of the concentration ol H2S in the sewer atmosphere. Use the procedure in "STANDARD METHODS" and determine the chlorine consumed during a five-minute contact period. The correct dosage will yield a measurable chlorine residual at the end of five minutes, which is sufficient time for all the chlorine-sulfide reactions to go to completion. Sodium hypochlorite has been used like chlorine to control odors. The chemical reactions with other substances are very similar. (1b) Hydrogen Peroxide For a number of years, hydrogen peroxide (H2O2) has been used as an oxidant to control odors. Hydrogen peroxide reacts in three possible ways to control odors.
more information…. 1. Oxidant action: Oxidizes the compound to a non-odorous state. An example of this is the conversion of hydrogen sulfide to sulfate compounds. H2S (g) + H2O2 (g) H2O (l)+ SO4-2,(aq) sulfate compounds (main sulphur product) In actual practice, a dose of 2:1 to 4:1 of H2O2 to S2- is needed for control. 2. Oxygen producing: Acts to prevent the formation of odor compounds. This is accomplished by keeping the system aerobic.
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3. Bactericidal to the sulfate-reducing bacteria: Kills the bacteria that produce odors. Without biological activity, odors will not be generated. This high dose of H2O2 is probably not economically feasible. Advantages of hydrogen peroxide use include its effectiveness as an oxidant, its ability to inhibit the regeneration of sulfate-reducing microorganisms, and the lack of toxic by-products. Disadvantages of hydrogen peroxide include its inability to treat ammonia or odorous organics, the contact time required for effective odor control (15 minutes to 2 hours), and its high cost.
(1c) Oxygen and Aeration Oxygen has been used for odor control with a great deal of success. The most common practice with oxygen is to use air to aerate the wastewater and try to keep the wastewater aerobic. The transfer of oxygen to the wastewater will increase its OXIDATION-REDUCTION POTENTIAL (ORP/} and thus reduce the formation of odorous gases. With more oxygen in the wastewater, the ORP is increased, and the sulfate ion is not used as an oxygen source; therefore the odor is reduced. Upstream aeration will cause hydrogen sulfide to be stripped out (carried out by the air) of the liquid, if it is present, and thus reduce the release of odors at the plant when water flows over weirs or other locations of high turbulence. STRIPPED ODORS 10 may be collected from above the surface where aeration takes place and be treated. If these odors are not properly handled, localized corrosion and odor problems can result. Use of high-purity oxygen to maintain aerobic conditions in force mains can be a very effective way to control odors. However, three conditions must be met in order to successfully use this method: The wastewater and oxygen must be thoroughly mixed. The force main must have a continuous uphill grade from the point of application. There must be adequate pressure within the main (typically greater than 15 psi gauge pressure) to force the oxygen into solution. (1d) Ozone Ozone is a powerful oxidizing agent that effectively removes odors. Ozone has limited use because an effective concentration may be too costly to use at large treatment plants. Ozone works well when used to remove odors from air collected over sources of odors. An advantage of ozone is the fact that there have been no known deaths resulting from the use of ozone. Ozone can cause irritation of the nose and throat at a concentration of 0.1 ppm, but some people can smell ozone at concentrations around 0.01 to 0.02 ppm. Another advantage of ozone is that you can manufacture what you need at the plant site and do not have to handle bulky containers. Ozone is not available in containers because it is relatively unstable and cannot be stored. (1e) Chromate Chromate ions can effectively inhibit the sulfate reduction to sulfide. However, this method introduces heavy metals into the sludge and wastewater, and this may cause an even more offensive odor. Heavy metal ions, such as chromate, cause serious toxic conditions that limit their usefulness. (1f) Metallic Ion Certain metallic ions (mainly zinc) have been used to form precipitates with sulfide compounds. These precipitates are insoluble and have a toxic effect on biological processes such as sludge digestion. Therefore, this process has its limitations. (1g) Nitrate Compounds The first chemicals used in the anaerobic breakdown of wastes are nitrate ions. If enough nitrate ions are present, the sulfate ions will not be broken down. The cost of this type of treatment to halt hydrogen sulfide production is very high and, at present, is not practical. (1h) pH Control (Continuous) Increasing the pH of the wastewater is an effective odor control method for hydrogen sulfide. By increasing the pH above 9, biological slimes and sludge growth are inhibited. This, in turn, halts sulfide production. Also, any sulfide present will be in the form of the HS- ion or S2- ion (above pH 11), rather than as H2S gas, which is formed and released at low pH values. (1j) pH Control (Shock Treatment) Short-term, high pH (greater than 12.5) slug dosing with sodium hydroxide (NaOH) is effective in controlling sulfide generation for periods of up to a month or more depending on temperature and sewer conditions. At pH 13 the HS- ion and S2- ion are approximately of equal concentration and no H2S is present. Care must be exercised in selecting the length of dosing so that downstream treatment plant biological systems will not be seriously impaired. (1k) Thermal Processing Three thermal processing techniques have been used: (1) thermal oxidation, (2) direct combustion, and (3) catalytic oxidation. The oxidation of methane (CH4) can be used to illustrate the basic principle of all three thermal processes. CH4 (g) + 2O2 (g) ď‚Ž CO2 (g) + 2H2O (l) + heat
(3-7)
If the gas to be combusted does not liberate enough heat to sustain the combustion process, it is usually necessary to use an external fuel source such as fuel oil, natural gas, or propane. Unfortunately, because of the low concentrations of odorous combustible gases in most
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waste streams, sustainable thermal oxidation is seldom possible, and large amounts of fuel are typically required to maintain the combustion temperatures needed to eliminate odors. (1l) Thermal oxidation involves preheating the odorous gases before passing them into the combustion chamber so that complete oxidation can be achieved. Combustion occurs at temperatures in the range from 425 to 760°C (800 to 1,400°F). Thermal oxidation is used, more commonly, for concentrated waste streams.
Figure 3-12 Regenerative Thermal Oxidizers (RTO’s)
(1m) Direct combustion involves the flaring of odorous gases. Depending on the design of the combustion facility, incomplete combustion can occur due to variations in gas flow. For this method of odor control to be sustainable, the waste gas must typically contain 50 percent of the fuel value of the gas stream to be combusted. (1n) Catalytic oxidation is a flameless oxidation process that occurs at 310 to 425°C (600 to 800°F) in the presence of a catalyst. Common catalysts include platinum, palladium, and rubidium. The decrease in temperature as compared to complete thermal oxidation reduces the energy requirements significantly. However, because the catalysts can become fouled, the gas to be oxidized must not contain particulate material or constituents that will result in a residue. (1o) Other new alternatives Fig.3-13
shows a typical oxygen ionization system installation. The
modular ionization units are installed in the supply air ductwork. Ion tubes in the ionization units generate a high voltage electrical discharge that strips ions from oxygen. Both negative and positive ions are formed. These ions are very small and create oxygen clusters. These ionized oxygen clusters attract and react with the odorous molecules in the air. Applicable Treatment Processes: Enclosed rooms in pump stations or treatment plants where working conditions are a concern due to odor and/or hydrogen sulfide concentrations. Typical Design Criteria: Air flow 12 air changes per hour in rooms with open tanks. Number of ionization units +/- one unit per 4,000 – 7,000 cu ft (dependent on odor concentrations) Supply air velocity in duct 16 – 30 ft/sec Power consumption approx. 50 watts per 4,000 – 5,000 cu ft of volume treated Fig.3-14. The UV – Oxidation process is a new odor control technology in the wastewater treatment plant industry, although it has a successful history in removing volatile organic compounds (VOC’s) from air in other industrial applications (primarily aircraft industry paint booths). Applicable Treatment Processes: All liquid treatment plant processes, sludge thickening, sludge dewatering. Typical Design Criteria: Air flow rates dependent on volumes to be treated, H2S removal efficiency> 99%, VOC removal efficiency 50 – 85%
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(2) Use of Odor Masking And Neutralization. On occasion, chemicals have been added to wastewater to mask an offensive odor with a less offensive odor. Masking chemicals are based on essential oils with the most common aromas being vanilla, citrus, pine, or floral (Williams, 1996). Typically, enough masking chemical is added to wastewater to overpower the offensive odor. Masking chemicals, however, do not modify or neutralize the offensive odors. Neutralization involves finding masking odors that can be combined with the odorous gases in the vapor state so that the combined gases cancel each other’s odor or produce an odor of lower intensity. Although odor masking and neutralization are viable options for short-term management of odor problems, the key to long-term odor management is to identify the source of the odors and implement corrective measures. Use Of Turbulence Inducing Structures And Facilities For Odor Dispersion In a number of wastewater treatment plants, physical facilities used to induce turbulence have been constructed specifically for the purpose of gas phase odor reduction. The high barrier fence (3.7 m (12 ft) shown in Fig. 3-9 surrounds sludge storage lagoons. Operationally, any odorous gases that develop under quiescent conditions over the lagoons are diluted as they move away from the storage lagoons, due to the local turbulence induced by the barrier. Trees are also used commonly to dilute odorous gases by inducing turbulence (i.e., the formation of eddies) and mixing. Trees are also known to help purify the air as a result of respirometric activity. .
Figure 3.9 High barrier fence placed around sludgeholding lagoons to induce air turbulence and mixing, and thus limit the release of concentrated odors off the treatmentplant site.
Use Of Buffer Zones The use of buffer zones can also help in reducing the impact of odors on developed areas. Typical buffer zone distances used by regulatory agencies are presented in Table 3-5. If buffer zones are used, odor studies should be conducted that identify the type and magnitude of the odor source, meteorological conditions, dispersion characteristics, and type of adjacent development. Trees that grow rapidly are often planted at the periphery of the buffer zones to further reduce the impact of odors.
3.6.2.2 Physical methods 1) Chemical Scrubbers: The basic design objective of a chemical scrubber is to provide contact between air, water, and chemicals (if used) to provide oxidation or entrainment of the odorous compounds. The odorous compounds are absorbed into the scrubber liquid, where they are oxidized and/or removed from the scrubber as an overflow or blowdown stream. Applicable Treatment Processes: All liquid treatment plant processes, pump stations, sludge thickening, sludge dewatering.
more information‌. The principal wet scrubber types, as shown in Fig. 3-6, include single-stage counter-current packed towers, counter-current spray chamber absorbers, and crossflow scrubbers. In most single-stage scrubbers, such as shown in Fig. 3-7, the scrubbing fluid (usually sodium hypochlorite) is recirculated. The commonly used oxidizing scrubbing liquids are chlorine (particularly sodium hypochlorite) and potassium
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Figure 3-10. Typical wet-scrubber systems for odor control: (a) countercurrent packed tower, (b) spray chamber absorber, and (c) cross-flow scrubber.
a
Typical Design Criteria:
Air flow velocity < 8.5 feet/sec (500 fpm), Detention time (in packing): 1.5 – 2 sec, Packing depth : 6 – 10 feet (dependent on contaminant loading), H2S removal efficiency: 99% Major Design Considerations: Contaminants to be removed: Chemical treatment selection is based on contaminants to be removed. Hydrogen sulfide may be treated using a solution of sodium hypochlorite and sodium hydroxide. Ammonia may be treated using a dilute solution of sulfuric acid. VOC’s are not effectively treated in wet chemical scrubber systems. Number of stages required: A multiple stage scrubber system may be required for removal of multiple contaminants such as hydrogen sulfide and ammonia. Multiple stages may also be used for treatment of very high contaminant concentrations. Two and three stage systems are common. Liquid solution controls: The liquid solution must be properly monitored and controlled to ensure proper pH and ORP levels are maintained in the scrubber solution for efficient operation. Liquid levels must also be monitored and controlled. Packing material: Packing should possess high surface area to volume ratio and possess shapes and sizes to provide a tortuous path for adequate detention time, while minimizing pressure drop. Materials vary dependent on contaminants being treated and the temperature of the air. a
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Figure 3-11. Typical sodium hypochlorite scrubber used to treat the odors from the trickling filters shown in Fig. 3-3 c and d.
Read more about the chemical reactions, occurred in a chemical scrubber. Typical simplified scrubbing reactions with chlorine, hypochlorite, and hydrogen peroxide are as follows. With chlorine H2S (g) + 4Cl2 (g) + 4H2O (l) H2SO4 (aq) + 8HCl (aq) (34.06)
(4 x 70.91)
H2S (g) + Cl2 (g) S° (s) + 2HCl (aq) (34.06)
(3-1)
(3-2)
(70.91)
With hypochlorite solution H2S (g) + 4NaOCl (aq) + 2NaOH (aq) Na2SO4 (aq) + 2H2O (l) + 4NaCl (l) (34.06)
(4 x 74.45)
H2S (g) + NaOCl (aq) S° (s) + NaCl (aq) + H2O (l) (34.06)
(3-3)
(3-4)
(74.45)
With hydrogen peroxide H2S (g) + H2O2 (g) S° (s) + 2H2O (l), pH < 8.5 (34.06)
(3-5)
(34.0)
The reaction given by Eq. (3-1) occurs when excess chlorine is added. If chlorine is added slowly under controlled conditions, the reaction given by Eq. (3-2) occurs. In the reaction given by Eq. (3-1), 8.33 mg/L of chlorine is required per mg/L of hydrogen sulfide, or 8.86 mg/L if the hydrogen sulfide is expressed as sulfide. The pH range for this reaction is 6 to 9, with 6 being optimum. In addition, 10.0 mg/L of alkalinity as CaCO3 will be required for each mg/L of H2S removed. In practice, the required chlorine dosage for the reaction given by Eq (31) will vary from about 9 to 10 mg/L per mg/L of H2S. For the reaction given by Eq. (3-2), 2.08 mg/L of chlorine is required per mg/L of hydrogen sulfide. The pH range for reaction given by Eq. (3-2) is 5 to 9, with 9 being optimum. In addition, 2.0 mg/L of alkalinity as CaCO 3 will be required for each mg/L of H2S removed. In most scrubbing operations, the objective is to convert the sulfide to sulfate. In the reactions given by Eqs. (3-3) and (3-4) , 8.74 and 2.19 mg/L of sodium hypochlorite are required per mg/L of hydrogen sulfide, respectively. In practice, the required sodium hypochlorite dosage for the reaction given by Eq (3-3) will vary from 8 to 10 mg/L per mg/L of H2S. In the reaction given in Eq. (3-5), 1.0 mg/L of hydrogen peroxide is required for each mg/L of sulfide expressed as hydrogen sulfide. In practice, the required dosage can vary from 1 to 4 mg/L per mg/L of H2S. Because the systems used to carry out the reactions defined by Eqs. (3-1) through (3-5) are complex, especially where competing reactions may occur, some experimentation will be required to establish the correct dosage Hypochlorite scrubbers can be expected to remove oxidizable odorous gases when other gas concentrations are minimal. Typical removal efficiencies for single-stage scrubbers are reported in Table 3-6. In cases where the concentrations of odorous components in the exhaust gas from the scrubbers are above desirable levels, multistage scrubbers (see Fig. 3-7) are often used. In the three-stage scrubber shown in Fig. 3-7, the first stage is a pretreatment stage used to raise the pH so that a portion of the odorous gases (e.g., hydrogen sulfide) are
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reduced before treatment with chlorine in the second and third stages. The reaction that occurs in the first-stage of a three stage unit can be represented as follows: H2S (g) + 2NaOH (aq) Na2S (aq) + 2H2O (l)
(3-6)
To reduce maintenance problems due to precipitation, it is recommended that a low hardness (less than 50 mg/L as CaCO3) be used for the makeup water.
2) Activated Carbon Adsorbers Activated carbon adsorbers are commonly used for odor control (see Fig. …….). Activated carbon has a complex pore structure with a very large surface area. Odorous compounds are transferred from the air being treated to the surface of the carbon as the air is forced through the carbon bed. (fig. 3.8)
Figure 3-8
Activated carbon absorbers
more information…. a
There is a physical attraction between the compound and the carbon once contact is made which causes a bonding. The compounds will
continue to adsorb onto the surface of the carbon until all of the pore space in the carbon is used up. The rate of adsorption for different constituents or compounds will depend on the nature of the constituents or compounds being adsorbed (nonpolar versus polar). It has also been found that the removal of odors depends on the concentration of the hydrocarbons in the odorous gas. Typically, hydrocarbons are adsorbed preferentially before polar compounds such as H2S are removed (Note activated carbon is nonpolar). Thus, the composition of the odorous gases to be treated must be known if activated carbon is to be used effectively. Because the life of a carbon bed is limited, carbon must be regenerateda or replaced regularly for continued odor removal. To prolong the life of the carbon, two-stage systems have been used, with the first stage being a wet scrubber followed by activated carbon adsorption. a a
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Carbon Regeneration And Reactivation Economical application of activated carbon depends on an efficient means of regenerating and
reactivating the carbon after its adsorptive capacity has been reached. Regeneration is the term used to describe all of the processes that are used to recover the adsorptive capacity of the spent carbon, exclusive of reactivation, including: (1) chemicals to oxidize the adsorbed material, (2) steam to drive off the adsorbed material, (3) solvents, and (4) biological conversion processes. Typically some of the adsorptive capacity of the carbon (about 4 to 10 percent) is also lost in the regeneration process, depending on the compounds being adsorbed and the regeneration method used (Crittenden, 2000). In some applications, the capacity of the carbon following regeneration has remained essentially the same for years. A major problem with the use of powdered activated carbon is that the methodology for its regeneration is not well defined. The use of powdered activated carbon produced from recycled solid wastes may obviate the need to regenerate the spent carbon, and may be more economical. Additional details on carbon reactivation and regeneration may be found in Sontheimer et al. (1988). Reactivation of granular carbon involves essentially the same process used to create the activated carbon from virgin material. Spent carbon is reactivated in a furnace by oxidizing the adsorbed organic material and, thus, removing it from the carbon surface. The following series of events occurs in the reactivation of spent activated carbon: (1) the carbon is heated to drive off the absorbed organic material (i.e., absorbate), (2) in the process of driving off the absorbed material some new compounds are formed that remain on the surface of the carbon, and (3) the final step in the reactivation process is to burn off the new compounds that were formed when the absorbed material was burned off. With effective process control, the adsorptive capacity of reactivated carbon will be essentially the same as that of the virgin carbon (Crittenden, 2000). For planning purposes, it is often assumed that a loss of 2 to 5 percent will occur in the reactivation process. It is important to note that most other losses of carbon occur through attrition due to mishandling. For example, right angle
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bends in piping cause attrition through abrasion and impact. The type of pumping facilities used will also affect the amount of attrition. In general, a 4 to 8 percent loss of carbon is assumed, due to handling. Replacement carbon must be available to make up the loss. 3) Vapor Phase Biological Treatment Processes Biological Odor Control Systems Biological odor control systems rely on the biological oxidation of odorous air. Odorous air has been fed to activated sludge systems, passed through trickling filters, and used as source air for dedicated biological towers. Biological odor removal systems are not particularly effective in the removal of organic odors and relatively few of these systems are in use today. However, given adequate contact time and balanced environmental conditions, they have proven effective in removing a variety of inorganic compounds from process foul air streams. Advantages of biological odor removal systems include their simple operation, lack of chemical usage, and their ability to treat high volumes of gas economically. Disadvantages include the space required for system installation, limited gas transfer capability, media fouling, the need for balanced environmental conditions, and their sometimes questionable reliability. The two principal biological processes used for the treatment of odorous gases present in the vapor phase are: (1) biofilters and (2) biotrickling filters (Eweis et al., 1998). Biofilters Biofilters are packed bed filters. In open biofilters (see Fig. 3-10a), the gases to be treated move upward through the filter bed. In closed biofilters (see Fig. 3-10b), the gases to be treated are either blown or drawn through the packing material. As the odorous gases move through the packing in the biofilter, two processes occur simultaneously: sorption (i.e., absorption/adsorption) and bioconversion. Odorous gases are absorbed into the moist surface biofilm layer and the surfaces of the biofilter packing material. Microorganisms, principally bacteria, actinomycetes, and fungi, attached to the packing material, oxidize the absorbed/adsorbed gases and renew the treatment capacity of the packing material. Moisture content and temperature are important environmental conditions that must be maintained to optimize microorganism activity (Williams and Miller, 1992a; Yang and Allen, 1994; commonly, one drawback is the large surface Eweis et al., 1998). Although compost biofilters are used commonly, one drawback is the large surface area (footprint) required for these units. a
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Figure 3-10 Typical packed-bed biofilters: (a) openbed type and (b) enclosed type.
Figure 3-11 Typical packed-bed Biofilter open-bed type .
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Figure 3-12 a,b Typical packed-bed biofilters enclosed type.
4) Biotrickling Filters (or Biotrickling Scrubers) Biotrickling filters are essentially the same as biofilters with the exception that moisture is provided continuously by applying (typically spraying) a liquid (e.g., treated effluent) over the packing (see Fig. 3-10b). The liquid is recirculated and nutrients are often added. Because water is lost in the gas leaving the filter, makeup water must be provided. Similarly because of the accumulation of salts in the recycled water, a blowdown stream is required. Compost is not a suitable packing material for biotrickling filters because water will accumulate within the compost thereby limiting the free movement of air within the filter. Typical packing materials include Pall rings, Raschig rings, and granular activated carbon (Eweis et al., 1998). Advantages of Biotrickling Scrubbers compared to other odor control technologies include: Proven effectiveness in many wastewater treatment applications, H2S removal efficiencies typically greater than 99%, Can provide VOC treatment with higher EBRTs, Provides good pH and acid control, even with high H2S concentrations, Lower operating and maintenance costs, compared to chemical treatment technologies , Ease of operation and maintenance, compared to chemical scrubbers, No ongoing chemical costs, storage or handling required, Small footprint requirement, Long media life between replacements, Biological air treatment is more environmentally friendly and safer than chemical treatment Disadvantages of Biotrickling Scrubbers compared to other odor control technologies include: Higher capital costs, especially compared to carbon absorbers and chemical feed systems, Requires biological acclimation period for effective treatment, Difficulty in maintaining biological growth when inlet loadings are low, Removal efficiencies may be reduced when inlet loadings are wide ranging, especially when base loadings are low, May require ongoing nutrient addition, Tower heights of larger systems can be aesthetically displeasing in some locations, A little slower to react to quick, extreme, loading variations than chemical scrubbers or carbon adsorption. May react a little more quickly than biofiltration
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Applicable Treatment Processes: All wastewater treatment plant processes, pump stations, sludge thickening, sludge dewatering. a
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Figure 3-12, a,b,c,d, Typical packed-bed Biotrickling filters:
more information…. Typical Design Criteria: Surface Loading (Wood-Chip media) : 3 – 4 cfm per sf media Surface Loading (Soil media) : 2 cfm per sf media, Inorganic media: 10-12 cfm/ft2, Media Bed Depth (Wood-chip/Soil media) : 3-5 ft, Media Bed Depth (Inorganic, synthetic media): 5-6 ft. Detention Time (Wood-chip/Soil media): 45-120 seconds Detention Time (Inorganic, synthetic media) : 20 – 30 seconds Pressure Drop : 6” – 12” H2S removal efficiency >99% Odor removal efficiency >85% (4a)Design Considerations for Odor Control Biofilters Important design considerations for biofilters include: (1) the type and composition of the packing material, (2) facilities for gas distribution, (3) maintenance of moisture within the biofilter, and (4) temperature control. Each of these topics is considered below. Design and operational parameters are presented and discussed following the discussion of the above topics. (4b) Packing Material The requirements for the packing used in biofilters are: (1) sufficient porosity and near-uniform particle size, (2) particles with large surface areas and significant pH-buffering capacities, and (3) the ability to support a large population of microflora (WEF, 1995). Packing materials used in biofilters include compost, peat, and a variety of synthetic mediums. Although soil and sand have been used in the past, they are less used today because of excessive headloss and clogging problems (Bohn and Bohn, 1988). Bulking materials used to maintain the porosity of compost and peat biofilters include perlite, Styrofoam pellets, wood chips, bark, and a variety of ceramic and plastic materials. A typical recipe for a compost biofilter is as follows (Schroeder, 2001): Compost = 50 percent by volume Bulking agent = 50 percent by volume (some constructors in Europe give bulking material content until 80%) 1 meq CaCO3 (50 mg)/kg of packing material by weight Optimal physical characteristics of a packing material include a pH between 7 and 8, airfilled pore space between 40 and 80 percent, and organic matter content of 35 to 55 percent (Williams and Miller, 1992a). When compost is used, additional compost must be added
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periodically to account for the loss due to biological conversion. Bed depths of up to 1.8 m (6.0 ft) have been used. However, because most of the removal takes place in the first 20 percent of the bed, the use of deeper beds is not recommended. (4b) Gas Distribution An important design feature of a biofilter is the method used to introduce the gas to be treated. The most commonly used gas distribution systems include: (1) perforated pipes, (2) prefabricated underdrain systems, and (3) plenums. Perforated pipes are usually placed in a gravel layer below the compost (see Fig. 15-12). Where perforated pipes are used, it is important to size the pipe so that it performs as a reservoir and not a manifold (Crites and Tchobanoglous, 1998). A variety of ]
Figure 3-13 Definition Sketch for open biofilters: (a) open bed and (b) trench type prefabricated under drain systems are available which allow for the movement of gas upward through the compost bed and allow for the collection of drainage. Air plenums are used to equalize the air pressure to achieve uniform flow upward through the compost bed. The height of air plenums will typically vary from 200 to 500 mm. (4c) Moisture Control Perhaps the most critical item in the successful operation of a biofilter is to maintain the proper moisture within the filter bed. If the moisture content is too low biological activity will be reduced. If the moisture content is too high, the flow of air will be restricted and anaerobic conditions may develop within the bed. Also, biofilters tend to dry out unless moisture or humidity is added. The optimal moisture content is between about 50 to 65 percent defined as follows. mass of water Moisture content, % x 100 mass of water mass of dry packing
(3-6)
Moisture can be supplied by adding water to the top of the bed (usually by spraying) or by humidifying the incoming gas in a humidification chamber. The relative humidity of the gas entering the biofilter should be 100 percent at the operating temperature of the biofilter (Eweis et al., 1998). In biotrickling filters the liquid application rate is typically about 0.75 to 1.25 m 3/m2• d.
(4d) Temperature Control The operating temperature range for biofilters is between 15 and 45°C, with the optimal range being between 25 to 35°C. In cold climates, biofilters must be insulated and the incoming gas must be heated. Where the incoming gas is warmer, it may have to be cooled before being introduced to the biofilter. Operation at higher temperatures (e.g., 45 to 60°C) is often possible, as long as the temperature remains relatively constant.
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more information….
Table 3-6 Parameters used for the design and analysis of bulk media filtersa
Design And Operating Parameters For Biofilters The sizing of biofilters is typically based on a consideration of the gas residence time in the bed, the unit air loading rate, and the constituent elimination capacity.
more information…. Additional details on biofilters may be found in van Lith (1989), Allen and Yang, (1991, 1992), WEF (1995), Eweis et al., (1998), and Devinny et al. (1999). Terms that will be encountered in the literature and the relationships commonly used to describe the performance of bulk media filters are summarized in Table 3-6. The empty bed residence time (EBRT) [see Eq. (3-7)], used to define the relationship between the volume of the contactor and the volumetric gas flow rate, is similar to Eq. used for the analysis of activated carbon systems. The true residence time is determined by incorporating the porosity, [see Eq. (15-7)]. Surface and volumetric mass loading rates are often used to define the operation of bulk media filters. The elimination capacity, as given by Eq. (15-16), is used to compare the performance of different odor control systems. The residence time for foul air from wastewater treatment facilities is typically between 15 and 60 seconds and surface loading rates have ranged up to 120 m3/m2 • min for H2S concentrations up to 20 mg/L. Constituent elimination rates are determined experimentally and are usually reported as a function of the constituent loading rate (e.g., mg H2S/m3•h). An essentially linear, 1 to 1, constituent elimination rate up to a critical loading rate has been observed for hydrogen sulfide and other odorous compounds . Yang and Allen (1994) have reported a linear 1 to 1 elimination rate for H2S, with loading rates for compost filters up to a maximum value of about 130 g S/m3•h beyond which
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the elimination rate becomes essentially constant at a rate of 130 g S/m3•h with increased loading. It should be noted that H2S is eliminated easily as it passes through a biofilter.
Table 3-7: Typical design criteria for biofilters
Typical design criteria for biofilters are presented in Table 3-7. Typical biofilters are shown in Fig. 3-11. Some states regulate the design of compost biofilters including loading rates, biofilter emission rates, odor sampling procedures, and setbacks from property lines. A typical odor emission limit at the surface of the biofilter is 50 dilutions to threshold (Finn and Spencer, 1997). The design of a compost biofilter for the elimination of hydrogen sulfide is illustrated in Example 3-1. EXAMPLE 3-1
Design of a compost biofilter for odor control
Determine the size of compost biofilter needed to scrub the air from a 100 m3 enclosed volume using the design criteria given in Table 3-7. Also estimate the mass of the buffer compound needed to neutralize the acid formed as a result of treatment within the filter. Assume 12 air changes per hour are needed. Assume a bed porosity of 40 percent. Will the volume selected be adequate if the air contains 40 ppm of H2S in addition to other odorous constituents? Assume an elimination rate of 65 gS/m3 • h, which incorporates a factor of safety of 2 as compared to the maximum rate given in Table 3-7. The temperature of the air is 20°C.
Solution 1.
Estimate the air flow to be scrubbed. Flow = volume/time Flow = 100 m3 x 12 changes per hour = 1,200 m3/h
2.
Select a loading rate from Table 15-9; use 90 m3/m2 • h.
3.
Select a filter bed depth from Table 15-9; use 1.0 m
4.
Calculate the area needed for the filter bed. Area = gas flow/ loading rate Area = (1200 m3/h)/ (90 m3/m2 • h)
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Area = 13.3 m2 5.
Check the empty bed residence time using Eq. (15-9)
Vf (13.3 m2 )(1 m) EBRT = Q 1,200 m3 / h = 0.011 h = 39.9 s (OK 39.9 s > 30 s) 6.
Determine whether the volume of the biofilter determined in Step 5 is adequate to treat the H2S. a. Determine the concentration of H2S in g/m3 using Eq. (2-45). From Example 15-1 the volume of gas occupied by one mole of a gas at a temperature of 20°C and a pressure of 1.0 atm is 24.1 L. Thus, the concentration of H2S is: 40L 3 34.08 g / mole H2 S g / m3 10 6 L 3 24.1 x 10 3 m 3 / mole of H S 2
= 0.057 g/m3 b. Determine the mass loading rate of S in g S/h
1,200 m3 0.057 g 32 g M s h m 3 34.08 g
= 64.2 g S/h c. Determine the required volume assuming an elimination rate of 65 gS/m3 • h. V
(64.2 gS / h) (65 g S / m 3 • h)
3
0.99 m
Because the volume of the bed (13.3 m3) is significantly greater than the required volume, the removal of H2S will not be an issue. 7.
Determine the mass of the buffer compound needed to neutralize the acid formed as a result of treatment within the filter a. Determine the mass of H2S in kg applied per year.
H2S, kg / yr =
(1,200 m3 / h) (0.057 g / m 3 )(24 h / d)(365 d / yr) 599.2 kg / yr (1000 g / kg)
b. Determine the mass of buffer compound required. Assume the following equation applies. H2S (g) + Ca(OH)2 (s) + 2O2 (g) CaSO4 (aq) + 2H2O (l) 34.06
74.08
Thus, about 2.05 kg of Ca(OH)2 will be required per kg of H2S. If the compost biofilter has a useful life of two years, then a total of 2,457 kg of Ca(OH)2 equivalent will be required to be added to the bed. Typically, 1.25 to 1.5 times as much are added. The buffer compound is mixed in with the compost and the bulking agent.
Comment Based on the results of the computation carried out in Step 6, it is clear why compost and soil filters are so effective in the elimination of H2S.
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(4e) Conventional Biological Treatment Processes The ability of microorganisms to oxidize hydrogen sulfide and other similar odorous compounds dissolved in the liquid under aerobic conditions is the basic concept used for the treatment of odors in liquid based systems. The two principal types of conventional liquid based systems used in wastewater treatment plants are the activated sludge process and the trickling filter process. Activated sludge diffusion is a relatively common technology that involves collecting odorous air, directing it to the suction side of aeration blowers, and diffusing it into activated sludge basins. The odors are removed by a combination of mechanisms including absorption, adsorption, condensation, and biological oxidation in the basins. A major concern with this method of odor management is the high rate of corrosion in the air piping and blowers that occurs due to the presence of moist air containing hydrogen sulfide. The ability to transfer the odorous gaseous compounds to the liquid phase is also of concern. With conventional uncovered trickling filters the major issues are how to transfer the air containing the odorous compounds to the trickling filter and how to avoid the release of untreated odorous compounds to the atmosphere. To control the release of odorous compounds, existing trickling filters that are to be used for odor control must be covered (see Fig. 3-3c). Figure 3-11 FRP Fan with inlet filter and moisture trap, a system for Activated sludge diffusion
more information…. Design Considerations For Chemical Scrubbers Most chemical scrubbers are supplied as a complete unit (see Fig. 3-11). Typical design factors for chemical scrubbers are presented in Table ……. Determination of the chemical requirements for odor scrubbing is illustrated in Example 3-1.
EXAMPLE 3-1: Chemical requirements for odor scrubbing Hydrogen sulfide is to be scrubbed from a waste air stream using chlorine. Determine the chemical (i.e., chlorine and caustic) and water requirements for the following conditions: 1. Waste air stream flowrate = 1,000 m3/min 2. H2S concentration in waste stream = 20 ppmv at 20°C 3. Specific weight of air = 0.0118 kN/m3 at 20°C 4. Density of air = 1.204 kg/m3 at 20°C (see Appendix B-1) 5.
Assume liquid to gas ratio for scrubber = 1.75
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6. Density of 50 percent NaOH solution = 1.52 kg/L
Solution 1.
Determine the volume of gas occupied by one mole of a gas at a temperature of 20°C and a pressure of 1.0 atm using Eq. (2-44)
V =
V =
2.
nRT P (1 mol)(0.082056 atm •L / g - mole •K)(273.15 + 20) 1.0 atm
24.055 L, use 24.1L
Estimate chlorine requirement. a.
Determine the amount of H2S that must be treated per day. Using Eq. (2-45) convert the H2S concentration from ppmv to
g/m3 20 m 3 34.08 g / mole H2 S 3 20 ppmv g / m3 10 6 m 3 24.1 x 10 3 m 3 / mole of H S 28.3x 10 2 3
-3
(1,000 m / min) x (28.3 x 10
b.
3
3
g/ m ) x (1440 min/ d) x (1 kg/ 10 g) = 40.8 kg / d
Estimate the chlorine dose. From Eq. (15-1), 8.33 mg/L of chlorine are required per mg/L of sulfide, expressed as hydrogen
sulfide [see Eq.
(15-1)]
Cl2 required per day = (40.8 kg/d) x (8.33) = 339.9 kg/d 3.
Estimate the water requirement for the scrubbing tower a.
Determine the mass air flowrate (1,000 m3/min) x (1.204 kg/m3) = 1,204 kg/min
b.
Determine the water flowrate 1,204 kg/min x 1.75 = 2,107 kg/min = 2.1 m3/min
4.
Determine the amount of sodium hydroxide (caustic) that must be added to replace the alkalinity consumed in the reaction. a.
From the reaction given by Eq. (15-1), 10.0 mg/L of alkalinity as CaCO3 will be required for each mg/L of H2S removed.
b.
Determine the amount of alkalinity required Alk = 40.8 kg/d x 10 = 408 kg/d as CaCO3
c.
Determine the volume of caustic required. The amount of caustic per liter is: NaOH = 1.0 L x 1.52 kg/L x 0.50 = 0.76 kg/L
Volume of NaOH =
(408 kg / d) (0.76kg / L)
536.8 L / d
Comment The water requirement for the scrubbing tower will be specified initially by the scrubber supplier and field adjusted based on the results of pilot plant studies and past operating experience.
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ASSIGMNENTS SECTION
QUESTIONS: 1a) Why is wastewater tending to become more septic and thus causing odor and corrosion problems?
1b) How are odors produced?
1c) What are the main inorganic gases of concern to operators?
1d) What is the order in which microorganisms break down compounds containing oxygen in nature?
1e) What is the major source of inorganic, odor-producing sulfate compounds found in collection systems and treatment plants?
1f) Hydrogen sulfide causes problems at what pH range?
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2a) How can odors be measured?
2b) List as many groups or types of odors or smells as you can recall.
2c) When investigating an odor complaint, why might you be unable to detect an odor that is disturbing to the person complaining?
4a) How is oxygen used to control odors?
4b)
What is a limitation of using metallic ions to precipitate sulfide?
4c)
How can pH adjustment control odors from hydrogen sulfide?
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5a) How are off gases and foul air treated in a biological odor removal tower?
5b) How is the filter feed spread over the media?
5c)
Why should the pH of the spray water not be allowed to drop below 6.0?
5d) How can the pH of the spray water be increased if the pH becomes too low?
5e) Why must caution be used when applying chlorinated secondary effluent to a biological odor removal tower?
6a) How can odors in air be treated?
6b) When operating a chemical scrubber unit using a brine solution, how would you determine if the rectifier output is set properly or is set too high or too low?
6c) What are the advantages of a chemical mist odor control system?
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6d)
How does a packed bed wet scrubber system differ from the chemical mist scrubber?
6e) What is a solid that is used in an adsorption process to remove odors from air?
6f) What is a secondary benefit derived from regenerating carbon?
6g) What are two methods for regenerating carbon in place?
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SUGGESTED ANSWERS: 1a) Wastewater is tending to become more septic, and thus causing odor and corrosion problems, because collection systems are being extended farther and farther away from treatment plants. 1b) The principal source of odor generation is the production of inorganic and organic gases by microorganisms. Odors also may be produced when odor-containing or odor-generating materials are discharged into the collection system by industries and businesses. 1c) The main inorganic gases of concern to operators are hydrogen sulfide (H2S) and ammonia (NH3). 1d) The order in which microorganisms break down compounds containing oxygen in nature is: molecular oxygen (free dissolved oxygen), nitrate, sulfate, oxidized organics, and carbon dioxide. 1e) The major source of inorganic, odor-producing sulfate compounds found in collection systems and treatment plants is sulfate compounds from the public water supply and from industrial sources. 1f) Hydrogen sulfide causes problems at lower (acidic) pH ranges. At a pH below 5, all sulfide is present in the gaseous H2S form; most of it can be released from wastewater and may cause odors, corrosion, explosive conditions, and respiratory problems. 2a) Odors can be measured by the use of an olfactometer, an odor panel, or possibly by analytic testing. 2b) Usually odors can be classified in the following groups: Ammonia Decayed cabbage Decayed flesh Fecal Fishy Garlic Medicinal Rotten egg Skunk 2c) You might not be able to detect an odor that is disturbing to a person complaining because: Your nose may not be as sensitive as the nose of the person complaining. Your nose may be accustomed to the smell and may no longer be able to detect the offensive odor. 4a) Oxygen is used to control odors by aerating wastewater and attempting to keep it aerobic. Also, aeration can strip odors out of wastewater. High-purity oxygen may also be used to maintain aerobic conditions in force 4b) A limitation of using metallic ions to precipitate sulfide is the toxic effect of the precipitates on biological processes such as sludge digestion. 4c) Odors can be controlled by increasing the pH. At pH levels above 9.0, biological slimes and sludge growth are inhibited. Also, any sulfide present will be in the form of HS- ion or S2- ion, rather than as H2S gas, which is formed and released at low pH values. 5a) Off gases and foul air are treated in a biological odor removal tower by passing this air up through the filter media where the odors are oxidized to an acceptable odor level and discharged to the atmosphere at the top of the tower. 5b) The filter feed is spread over the media by the use of spray nozzles. 5c) The pH of the spray water must be maintained above 6.0 so it will not cause corrosion damage. 5d) The pH of the spray water can be increased by the addition of caustic soda or an appropriate compatible chemical. 5e) Chlorinated secondary effluents may contain enough residual chlorine to be toxic to the biomass or they may not contain enough BOD to support the biomass. Odor removal efficiency may decline sharply. 6a) Odors in air can be treated by masking and counter-action, combustion, absorption, adsorption, and ozonation. 6b) If the rectifier output is set too high, a hypochlorite odor (smell of household bleach) is detectable. If the output is set too low, an undesirable odor is detectable. No odors are detectable if the rectifier is set properly. 6c) Advantages of a chemical mist system are: the capacity for accommodating high flow rates; low pressure drop; high transfer efficiency; and no chemical regeneration. 6d) A packed bed wet scrubber system differs from the chemical mist scrubber in its use of packing material in the contact chamber and recirculation of the scrubbing chemical. 6e) Activated carbon is a solid that is used to remove odors from air by the adsorption process. 6f) A secondary benefit derived from regenerating carbon is the conservation of landfill area. 6g) Two methods for regenerating carbon in place are the use of hot air or chemicals (sodium hydroxide)
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