Bulking Sludge. In many cases MLSS with poor settling characteristics has developed into what is known as a bulking sludge condition, which defines a condition in the activated-sludge clarifier that can cause high effluent suspended solids and poor treatment performance. In a bulking sludge condition, the MLSS floc does not compact or settle well, and floc particles are discharged in the clarifier effluent with good settling sludge, sludge levels may be as low as 10 to 30 cm at the bottom of the clarifier. In extreme bulking sludge conditions, the sludge blanket cannot be contained and large quantities of MLSS are carried into the system effluent, potentially resulting in violation of permit requirements, inadequate disinfection, and clogging of effluent filters Two principal types of sludge bulking problems have been identified. One type, filamentous bulking, is caused by the growth of filamentous organisms or organisms that can grow in a filamentous form under adverse conditions, and is the predominant form of bulking th other type of bulking, viscous bulking, is caused by an excessive amount of extracellular biopolymer, which pre sludge with a slimy, jellylike consistency(Wanner, 1994) As the biopolymers are hydrophilic, water-retentive. and this condition resultant sludge has a low density with low settling velocities and poor ction. Viscous bulking is usually found with nutrient-limite In with high amount of rbCOD Bulking sludge problems due to growt more common In filamentous growt Figure 8-11 bacteria form filaments of single-cell organisms that attach end-to-end. and 以四四 Boc,.tl, oe pride by l on the filaments normally protrude out solved oxgen miaregnisaie w hot Corey o b wld g n iwm f cotone saby mecom bunda of the sludge floc. This structure,in contrast to the preferred dense floc with good settling properties, has an increased surface area to mass ratio, which results in poor settling On Fig. 7-8, a good settling, dense nonfilamentous floc is contrasted to floc containing filamentous growth Many types of filamentous bacteria exist, and means have been developed for the identification and classification of filamentous bacteria found commonly in activated-sludge systems(Eikelboom, 2000) The classification system is based on morphology(size and shape of cells, length and shape of filaments staining responses, and cell inclusions. Common filamentous organisms are summarized in Table 7-3, along with the operating conditions that favor their growth Sludge bulking can be caused by a variety of associated with each of these categories are identified in Table zx nd operational issues. Individual items Tab. 7-3 Filamentous organisms foundin activated sludge andassociated process conditions Filament bype identified Sphaerotilus natans, Halsicomenobacter hydross Low dissolved parv M.parvicella, types 0o4t, 0092, 0675. 185 O2IN ida limicola Complete-mix reactor conditions Beggiatoa, Thiothrix spp. types 021N, 0914 S. natons, Thiothrix pe 021N, possible H. hydrosis, types 0041, 0675 ow pH, nutrient deficiency Tab. 7-4 Factors that affea sludge bulking 7-11
7-11 Bulking Sludge. In many cases MLSS with poor settling characteristics has developed into what is known as a bulking sludge condition, which defines a condition in the activated-sludge clarifier that can cause high effluent suspended solids and poor treatment performance. In a bulking sludge condition, the MLSS floc does not compact or settle well, and floc particles are discharged in the clarifier effluent. With good settling sludge, sludge levels may be as low as 10 to 30 cm at the bottom of the clarifier. In extreme bulking sludge conditions, the sludge blanket cannot be contained and large quantities of MLSS are carried into the system effluent, potentially resulting in violation of permit requirements, inadequate disinfection, and clogging of effluent filters. Two principal types of sludge bulking problems have been identified. One type, filamentous bulking, is caused by the growth of filamentous organisms or organisms that can grow in a filamentous form under adverse conditions, and is the predominant form of bulking that occurs. The other type of bulking, viscous bulking, is caused by an excessive amount of extracellular biopolymer, which produces a sludge with a slimy, jellylike consistency (Wanner, 1994). As the biopolymers are hydrophilic, the activated sludge is highly water-retentive, and this condition is referred to as hydrous bulking. The resultant sludge has a low density with low settling velocities and poor compaction. Viscous bulking is usually found with nutrient-limited systems or in a very high loading condition with wastewater having a high amount of rbCOD. Bulking sludge problems due to the growth of filamentous bacteria are more common. In filamentous growth, bacteria form filaments of single-cell organisms that attach end-to-end, and the filaments normally protrude out of the sludge floc. This structure, in contrast to the preferred dense floc with good settling properties, has an increased surface area to mass ratio, which results in poor settling. On Fig. 7-8, a good settling, dense nonfilamentous floc is contrasted to floc containing filamentous growth. Many types of filamentous bacteria exist, and means have been developed for the identification and classification of filamentous bacteria found commonly in activated-sludge systems (Eikelboom, 2000). The classification system is based on morphology (size and shape of cells, length and shape of filaments), staining responses, and cell inclusions. Common filamentous organisms are summarized in Table 7-3, along with the operating conditions that favor their growth. Sludge bulking can be caused by a variety of factors, including wastewater characteristics, design limitations, and operational issues. Individual items associated with each of these categories are identified in Table 7-4. Fig. 7-8 Examples of good and poor settling floc particles: (a)nonfilamentous good Settling floc; (b)floc particles bridged by filamentous microbes; (c)floc Particles with limited filamentous microbes and secondary form; (d)filaments extending from floc causing poor settling; (e)Thiothrix filaments with sulfur granules; (f)type 1701 filamentous microbes observed under low dissolved oxygen Tab. 7-3 Filamentous organisms found in activated sludge and associated process conditions Tab. 7-4 Factors that affect sludge bulking
Activated-sludge reactor operating conditions (low DO, low F/M, and complete-mix operation) wastewater characteristics variations in flowrate Variations in composition development of filamentous populations. One of the kinetic features of filamentous Hapticity that relates to these conditions is that Nutrient content they are very competitive at low Nature of waste components substrate concentrations whether it be organic substrates, DO, or nutrients Thus, lightly loaded complete-mix Short circuiting (aeration tanks and clarifiers) activated-sludge systems or low DO Clarifier design(sludge colection and removal) (<0.5 mg/L) operating conditions imited return sludge pumping capacity provide an environment more ssve dissolved oxygen favorable to filamentous bacteria Insuficient nutrients than to the desired floc- formins Low F/M bacteria Insufficient soluble BOD Filamentous bacteria such as Beggiatoa and Thiothrix grow well on hydrogen sulfide and reduced substrates, respectively, that would be found in septic wastewaters (Wanner, 1994). When the influent wastewater contains fermentation products such as volatile fatty acids and reduced sulfur compounds(sulfides and thiosulfate), Thiothrix can proliferate. Prechlorination of tl wastewaters has been done in some cases to prevent their growth. Besides causing bulking problems in activated-sludge systems, Beggiatoa and Thiothrix can create problems in fixed-film systems, including trickling filters and rotating biological contactors In the control of bulking, where a number of variables are possible causes, a checklist of items to investigate is valuable. The following items are recommended: (1)wastewater characteristics,(2) dissolved oxygen content, (3)process loading,(4)return and waste sludge pumping rates, (5)internal plant overloading, and(6) clarifier operation One of the first steps to be taken when sludge settlor characteristics change is to view the mixed liquor under the microscope to determine what type of microbial growth changes or floc structure changes can be related to the development of bulking sludge. A reasonable quality phase-contrast microscope with magnification up to 1000 times(oil immersion)is necessary to view the filamentous bacteria structure and size Wastewater Characteristics. The nature of the components found in wastewater or the absence of certain components, such as trace elements, can lead to the development of a bulked sludge. If it is known that industrial wastes are being introduced into the system either intermittently or continuously, the en and phosphorus in the waste should be checked first. because limitations of both or either are known to favor bulking. Nutrient deficiency is a classic problem in the treatment of industrial wastewaters containing high levels of carbonaceous BOD. Wide fluctuations in pH are also known to be detrimental in plants of conventional design. Variations in organic waste loads due to batch-type operations can also lead to bulking and should be checked Dissolved Oxygen Concentration. Limited dissolved oxygen has been noted more frequently than any other cause of bulking. If the problem is due to limited oxygen, it can usually be confirmed by operating the aeration equipment at full capacity by decreasing the system SrT, if possible, to reduce the oxygen demand. The aeration -equipment should have adequate capacity to maintain at least 2 mg/L of dissolved oxygen in the aeration tank under normal loading conditions. If 2 mg/L of Process Loading/Reactor Configuration. The aeration Sri should be checked to make sure that it is within the range of generally accepted values. In many cases, complete-mix systems with long SRTs and sequent low F/M ratios experience filamentous growths. In such systems, the filamentous organisms are more competitive for substrate. Laboratory research and full-scale investigations have led to activated-sludge design configurations that provide conditions favoring the dominance of floc-forming bacteria over filamentous organisms (Jenkins et al., 1993). Reactors in series with various types of environmental conditions, i.e., aerobic, anoxic, and anaerobic, are generally used to augment or replace a complete-mix reactor. The series configurations are called selector processes because they provide conditions that cause selection of floc-forming bacteria in lieu of filamentous organisms as the dominant population 7-12
7-12 Activated-sludge reactor operating conditions (low DO, low F/M, and complete-mix operation) clearly have an effect on the development of filamentous populations. One of the kinetic features of filamentous organisms that relates to these conditions is that they are very competitive at low substrate concentrations whether it be organic substrates, DO, or nutrients. Thus, lightly loaded complete-mix activated-sludge systems or low DO (<0.5 mg/L) operating conditions provide an environment more favorable to filamentous bacteria than to the desired floc-forming bacteria. Filamentous bacteria such as Beggiatoa and Thiothrix grow well on hydrogen sulfide and reduced substrates, respectively, that would be found in septic wastewaters (Wanner, 1994). When the influent wastewater contains fermentation products such as volatile fatty acids and reduced sulfur compounds (sulfides and thiosulfate), Thiothrix can proliferate. Prechlorination of the wastewaters has been done in some cases to prevent their growth. Besides causing bulking problems in activated-sludge systems, Beggiatoa and Thiothrix can create problems in fixed-film systems, including trickling filters and rotating biological contactors. In the control of bulking, where a number of variables are possible causes, a checklist of items to investigate is valuable. The following items are recommended: (1) wastewater characteristics, (2) dissolved oxygen content, (3) process loading, (4) return and waste sludge pumping rates, (5) internal plant overloading, and (6) clarifier operation. One of the first steps to be taken when sludge settling characteristics change is to view the mixed liquor under the microscope to determine what type of microbial growth changes or floc structure changes can be related to the development of bulking sludge. A reasonable quality phase-contrast microscope with magnification up to 1000 times (oil immersion) is necessary to view the filamentous bacteria structure and size. Wastewater Characteristics. The nature of the components found in wastewater or the absence of certain components, such as trace elements, can lead to the development of a bulked sludge. If it is known that industrial wastes are being introduced into the system either intermittently or continuously, the quantity of nitrogen and phosphorus in the wastewater should be checked first, because limitations of both or either are known to favor bulking. Nutrient deficiency is a classic problem in the treatment of industrial wastewaters containing high levels of carbonaceous BOD. Wide fluctuations in pH are also known to be detrimental in plants of conventional design. Variations in organic waste loads due to batch-type operations can also lead to bulking and should be checked. Dissolved Oxygen Concentration. Limited dissolved oxygen has been noted more frequently than any other cause of bulking. If the problem is due to limited oxygen, it can usually be confirmed by operating the aeration equipment at full capacity by decreasing the system SRT, if possible, to reduce the oxygen demand. The aeration-equipment should have adequate capacity to maintain at least 2 mg/L of dissolved oxygen in the aeration tank under normal loading conditions. If 2 mg/L of oxygen cannot be maintained, installation of improvements to the existing aeration system may be required. Process Loading/Reactor Configuration. The aeration SRT should be checked to make sure that it is within the range of generally accepted values. In many cases, complete-mix systems with long SRTs and subsequent low F/M ratios experience filamentous growths. In such systems, the filamentous organisms are more competitive for substrate. Laboratory research and full-scale investigations have led to activated-sludge design configurations that provide conditions favoring the dominance of floc-forming bacteria over filamentous organisms (Jenkins et al., 1993). Reactors in series with various types of environmental conditions, i.e., aerobic, anoxic, and anaerobic, are generally used to augment or replace a complete-mix reactor. The series configurations are called selector processes because they provide conditions that cause selection of floc-forming bacteria in lieu of filamentous organisms as the dominant population
Internal Plant Overloading. To avoid internal plant overloading recycle loads should be controlled so they are not returned to the plant flow during times of peak hydraulic and organic loading Examples of recycle loads are centrate or filtrate from sludge dewatering operations and supernatant from sludge digesters Clarifier Operation. The operating characteristics of the clarifier may also affect characteristics. Poor settling is often a problem in center-feed circular tanks where sludge is removed fro the tank directly under the point where the mixed liquor enters. Sludge may actually be retained in the tank for many hours rather than the desired 30 min and cause localized septic conditions. If this is the case then the design is at fault and changes must be made in the inlet feed well and sludge withdrawal Temporary Control Measures. In an emergency situation or while the aforementioned factors are being investigated, chlorine and hydrogen peroxide may be used to provide temporary help. Chlorination of return sludge has been practiced quite extensively as a means of controlling bulking A typical design for a low(5 to 10 h)I system uses 0.002 to 0.008 kg of chlorine per kg mLSS. d (Jenkins et al., 1993) Although chlorination is effective in controlling bulking caused by filamentous growths, it is ineffective when bulking is due to light floc containing bound water. Chlorination normally results in the production of a turbid effluent until such time as the sludge is free of the filamentous forms. chlorination of a nitrifying sludge will also produce a turbid effluent because of the death of the nitrifying organisms. The use of chlorine also raises issues about the formation of trihalomethanes and other compounds with potential health and environmental effects. Hydrogen peroxide has also been used in the control of filamentous organisms in bulking sludge. Dosage of hydrogen peroxide and treatment time depend on the extent of the filamentous development ising Sludge. Occasionally, sludge that has good settling characteristics will be observed to rise or float to the surface after a relatively short settling period. The most common cause of this phenomenon is denitrification, in which nitrites and nitrates in the wastewater are converted to nitrogen gas. As nitrogen gas is formed in the sludge layer, much of it is trapped in the sludge mass. If enough gas is formed, the sludge mass becomes buoyant and rises or floats to the surface. Rising sludge can be differentiated from bulking sludge by noting the presence of small gas bubbles attached to the floating solids and the presence of more floating sludge on the secondary clarifier surface. Rising sludge is common in short SRT systems, where the temperature encourages the initiation of nitrification, and the mixed liquor is very active due to the low sludge age Rising sludge problems may be overcome by()increasing the return activated-sludge withdrawal rate from the clarifier to reduce the detention time of the sludge in the clarifier, (2) decreasing the rate of flow of aeration liquor into the offending clarifier if the sludge depth cannot be reduced by increasing the return activated-sludge withdrawal rate, (3) where possible, increasing the speed of the sludge-collecting mechanism in the settling tanks, and (4)decreasing the srt to bring the activated sludge out of nitrification. For warm climates where it is very difficult to operate at a low enough SrT to limit nitrification, an anoxic/aerobic process is preferred to denitrification to prevent rising sludge and to improve sludge settling characteristics Nocardia Foam. Two bacteria genera, Nocardia and Microthrix parvicella, are associated with extensive foaming in activated-sludge processes. These organisms have hydrophobic cell surfaces and attach to air bubbles, where they stabilize the bubbles to cause foam. The organisms can be found at high concentrations in the foam above the mixed liquor. Both types of bacteria can be identified under microscopic examination. Nocardia has a filamentous structure. and the filaments are very short and are ontained within the floc particles Microthrix parvicella has thin filaments extending from the floc particles. Foaming on an activated-sludge basin and a microscopic view of Nocardia are shown on Fig 7-9 Fig. 7-9 The foam is thick. has a brown color. and can build up in thickness of 0.5 (b)microscopic observation of gram-stained Nocardia filaments The foam production can occur with both diffused and mechanical aeration but is more pronounced with diffused aeration and with higher air flowrates. problems of nocardia foaming in the activated sludge can 7-13
7-13 Internal Plant Overloading. To avoid internal plant overloading, recycle loads should be controlled so they are not returned to the plant flow during times of peak hydraulic and organic loading. Examples of recycle loads are centrate or filtrate from sludge dewatering operations and supernatant from sludge digesters. Clarifier Operation. The operating characteristics of the clarifier may also affect sludge settling characteristics. Poor settling is often a problem in center-feed circular tanks where sludge is removed from the tank directly under the point where the mixed liquor enters. Sludge may actually be retained in the tank for many hours rather than the desired 30 min and cause localized septic conditions. If this is the case, then the design is at fault, and changes must be made in the inlet feed well and sludge withdrawal equipment. Temporary Control Measures. In an emergency situation or while the aforementioned factors are being investigated, chlorine and hydrogen peroxide may be used to provide temporary help. Chlorination of return sludge has been practiced quite extensively as a means of controlling bulking. A typical design for a low (5 to 10 h) τ system uses 0.002 to 0.008 kg of chlorine per kg MLSS.d (Jenkins et al., 1993). Although chlorination is effective in controlling bulking caused by filamentous growths, it is ineffective when bulking is due to light floc containing bound water. Chlorination normally results in the production of a turbid effluent until such time as the sludge is free of the filamentous forms. Chlorination of a nitrifying sludge will also produce a turbid effluent because of the death of the nitrifying organisms. The use of chlorine also raises issues about the formation of trihalomethanes and other compounds with potential health and environmental effects. Hydrogen peroxide has also been used in the control of filamentous organisms in bulking sludge. Dosage of hydrogen peroxide and treatment time depend on the extent of the filamentous development. Rising Sludge. Occasionally, sludge that has good settling characteristics will be observed to rise or float to the surface after a relatively short settling period. The most common cause of this phenomenon is denitrification, in which nitrites and nitrates in the wastewater are converted to nitrogen gas. As nitrogen gas is formed in the sludge layer, much of it is trapped in the sludge mass. If enough gas is formed, the sludge mass becomes buoyant and rises or floats to the surface. Rising sludge can be differentiated from bulking sludge by noting the presence of small gas bubbles attached to the floating solids and the presence of more floating sludge on the secondary clarifier surface. Rising sludge is common in short SRT systems, where the temperature encourages the initiation of nitrification, and the mixed liquor is very active due to the low sludge age. Rising sludge problems may be overcome by (1) increasing the return activated-sludge withdrawal rate from the clarifier to reduce the detention time of the sludge in the clarifier, (2) decreasing the rate of flow of aeration liquor into the offending clarifier if the sludge depth cannot be reduced by increasing the return activated-sludge withdrawal rate, (3) where possible, increasing the speed of the sludge-collecting mechanism in the settling tanks, and (4) decreasing the SRT to bring the activated sludge out of nitrification. For warm climates where it is very difficult to operate at a low enough SRT to limit nitrification, an anoxic/aerobic process is preferred to denitrification to prevent rising sludge and to improve sludge settling characteristics. Nocardia Foam. Two bacteria genera, Nocardia and Microthrix parvicella, are associated with extensive foaming in activated-sludge processes. These organisms have hydrophobic cell surfaces and attach to air bubbles, where they stabilize the bubbles to cause foam. The organisms can be found at high concentrations in the foam above the mixed liquor. Both types of bacteria can be identified under microscopic examination. Nocardia has a filamentous structure, and the filaments are very short and are contained within the floc particles. Microthrix parvicella has thin filaments extending from the floc particles. Foaming on an activated-sludge basin and a microscopic view of Nocardia are shown on Fig. 7-9. The foam is thick, has a brown color, and can build up in thickness of 0.5 to 1 m. The foam production can occur with both diffused and mechanical aeration but is more pronounced with diffused aeration and with higher air flowrates. Problems of Nocardia foaming in the activated sludge can Fig. 7-9 Nocardia foam: (a)example of foam on an aeration tank; (b)microscopic observation of gram-stained Nocardia filaments
also lead to foaming in anaerobic and aerobic digesters that receive the waste-activated sludge. Nocardia growth is common where surface scum is trapped in either the aeration basin or secondary clarifiers Aeration basins that are baffled with flow from one cell to the next occurring under the baffles, instead of over the top, encourage Nocardia growth and foam collection Methods that can be used to control Nocardia include(1)avoiding trapping foam in the secondar treatment process,(2) avoiding the recycle of skimmings into the secondary treatment process, and (3) oSing chlorine spray on the surface of the Nocardia foam. The use of a selector design may help to scourage Nocardia foaming, but significant foaming has been observed with anoxic/aerobic processes The addition of a small concentration of cationic polymer has controlling Nocardia foaming(Shao et al., 1997) The presence of Nocardia has also been associated with the presence of Nocardia-Microthrix with fats and edible oils in wastewater. Reducing the oil and grease content from discharges to the collection system from restaurants, truck stops, and meatpacking facilities by effective degreasing Activated-Sludge selector Processes In the above discussion, problems caused by ed sudge to nuisance microorganisms in activated sludge were bacteria on sludge settling characteristics and the potential of sludge bulking when the filamentous bacteria are present in high numbers. Prior to the 19705 configurations:(a)/aerobic;(b)high F/; ilamentous bulking was considered nevitable consequence of activated-sludge treatment, but work by Chudoba et al(1973)with staged versus complete-mix activated-sludge reactors led to the concept that reactor configuration designs, now termed selectors, could be used to control filamentous bulking and improve sludge-settling characteristics The concept of a selector is the use of a specific bioreactor design that favors the growth of floc-forming acteria instead of filamentous bacteria to provide an activated sludge with better settling and thickening properties. The high substrate concentration in the selector favors the growth of nonfilamentous A selector is a small tank (20 to 60 min contact time)or a series of tanks in which the incom ing wastewater is mixed with return sludge under aerobic, anoxic, and anaerobic conditions. Various types of selectors are shown on Fig. 7-10. The selector reactor precedes the activated-sludge aeration tank and may be designed as a separate reaction stage for a complete-mix reactor(see Fig. 7-10a)or as individual compartments in a plug-flow system(see Fig. 7-10b and c). Sequencing batch reactors may also be perated to employ the selector concept. The goal in the selector is to have most of the rbCOD consumed by the floc-forming bacteria Because the particulate degradable Cod decomposes at a much slower rate and will be present in the aeration tank, the rbCoD must be utilized for the benefit of the floc-forming bacteria. Selector designs are based on either kinetic or metabolic mechanisms. The kinetics-based selector designs are called high F/M selectors, and the metabolic-based selectors are either anoxic or anaerobic processes Kinetics-Based Selector. Selector designs based on biokinetic mechanisms provide for reactor substrate concentrations that result in faster substrate uptake by the floc-forming bacteria. While filamentous bacteria are more efficient for substrate utilization at low substrate concentrations, the floc-forming bacteria have higher growth rates at high soluble substrate concentrations. A series of reactors at relatively low t values(minutes)is used to provide high soluble substrate concentrations, in contrast to feeding influent wastewater to aeration tanks with t values on the order of hours for three reactors in series the following COD F/M ratios, based on the: influent to flowrate and COD concentration, are recommended First reactor, 12 g COD/g MLSS. d Second reactor, 6 g COD/g MLSS.d 7-14
7-14 also lead to foaming in anaerobic and aerobic digesters that receive the waste-activated sludge. Nocardia growth is common where surface scum is trapped in either the aeration basin or secondary clarifiers. Aeration basins that are baffled with flow from one cell to the next occurring under the baffles, instead of over the top, encourage Nocardia growth and foam collection. Methods that can be used to control Nocardia include (1) avoiding trapping foam in the secondary treatment process, (2) avoiding the recycle of skimmings into the secondary treatment process, and (3) using chlorine spray on the surface of the Nocardia foam. The use of a selector design may help to discourage Nocardia foaming, but significant foaming has been observed with anoxic/aerobic processes. The addition of a small concentration of cationic polymer has been used with some success for controlling Nocardia foaming (Shao et al., 1997). The presence of Nocardia has also been associated with the presence of Nocardia-Microthrix with fats and edible oils in wastewater. Reducing the oil and grease content from discharges to the collection system from restaurants, truck stops, and meatpacking facilities by effective degreasing processes can help control potential Nocardia problems. Activated-Sludge Selector Processes In the above discussion, problems caused by nuisance microorganisms in activated sludge were presented, including the effect of filamentous bacteria on sludge settling characteristics and the potential of sludge bulking when the filamentous bacteria are present in high numbers. Prior to the 1970s, f ilamentous bulking was considered an i nevitable consequence of activated-sludge treatment, but work by Chudoba et al. (1973) with staged versus complete-mix activated-sludge reactors led to the concept that reactor configuration designs, now termed selectors, could be used to control filamentous bulking and improve sludge-settling characteristics. The concept of a selector is the use of a specific bioreactor design that favors the growth of floc-forming bacteria instead of filamentous bacteria to provide an activated sludge with better settling and thickening properties. The high substrate concentration in the selector favors the growth of nonfilamentous organisms. A selector is a small tank (20 to 60 min contact time) or a series of tanks in which the incoming wastewater is mixed with return sludge under aerobic, anoxic, and anaerobic conditions. Various types of selectors are shown on Fig. 7-10. The selector reactor precedes the activated-sludge aeration tank and may be designed as a separate reaction stage for a complete-mix reactor (see Fig. 7-10a) or as individual compartments in a plug-flow system (see Fig. 7-10b and c). Sequencing batch reactors may also be operated to employ the selector concept. The goal in the selector is to have most of the rbCOD consumed by the floc-forming bacteria. Because the particulate degradable COD decomposes at a much slower rate and will be present in the aeration tank, the rbCOD must be utilized for the benefit of the floc-forming bacteria. Selector designs are based on either kinetic or metabolic mechanisms. The kinetics-based selector designs are called high F/M selectors, and the metabolic-based selectors are either anoxic or anaerobic processes. Kinetics-Based Selector. Selector designs based on biokinetic mechanisms provide for reactor substrate concentrations that result in faster substrate uptake by the floc-forming bacteria. While filamentous bacteria are more efficient for substrate utilization at low substrate concentrations, the floc-forming bacteria have higher growth rates at high soluble substrate concentrations. A series of reactors at relatively low τ values (minutes) is used to provide high soluble substrate concentrations, in contrast to feeding influent wastewater to aeration tanks with τ values on the order of hours. For three reactors in series, the following COD F/M ratios, based on the: influent to flowrate and COD concentration, are recommended. . First reactor, 12 g COD/g MLSS.d . Second reactor, 6 g COD/g MLSS.d Fig. 7-10 Typical selector configurations: (a)anaerobic/aerobic; (b)high F/M; (c)anoxic selector
Third reactor, 3 g COD/g mlss.d The F/M ratio is calculated for the first reactor using the volume and mlss concentration at that and the influent wastewater flowrate and COd concentration The f/M value shown for the second includes the volume of the first and second reactor and the applied loading as the product of the flowrate and COD concentration Ibertson(1987)recommended a similar approach based on a BOD F/M loading of 3 to 5 g BOD/g MLSS in the first reactor, with the second and third reactors being equal to and twice the first reactor volume, respectively. Albertson further notes that if the loading to the first reactor is too high(F/M>8 g BOD/g MLSS. d), a viscous, nonfilamentous-type bulking can develop. The kinetic concept of a high F/M selector suggests that it be aerobic, and high DO concentrations are needed to maintain an aer obic floc (6 to 8 mg/L). In many cases, such high DO concentrations are not practical or provided, and the staged selector design(described above) is operated at a low to zero DO concentration so that a metabolic selector mechanism is involved A sequencing batch reactor(SBR) can also act as a very effective high F/M selector, depending on the wastewater strength and feeding strategy. For high-strength wastewaters with a relatively large fraction of the SBr volume occupied by the influent wastewater, a high initial F/M ratio can occur. The subsequent reaction by the batch process is equal to that for a plug-flow reactor. Metabolic-Based Selector. With biological nutrient-removal processes, improved sludge-settling characteristics, and, in many cases, minimal filamentous bacteria growth has been observed. The anoxic or anaerobic metabolic conditions used in these processes favor growth of the floc-forming bacteria. The filamentous bacteria cannot use nitrate or nitrite for an electron acceptor, thus yielding a significant advantage to denitrifying floc-forming bacteria. Similarly, the filamentous bacteria do not store polyphosphates and thus cannot consume acetate in the anaerobic contact zone in biologica phosphorus-removal designs, giving an advantage for substrate uptake and growth to the phosphorus-storing bacteria. In some wastewater-treatment facilities (Seattle and San Francisco,for example), an anaerobic selector has been used before the aeration tank in low SrT activated-sludge systems designed for BOD removal, even though phosphorus removal is not required Where nitrification is used and phosphorus removal is not required, anoxic selectors(either the stage high F/M gradient or the single-stage designs) have been used. For the high F/M anoxic or selectors, the resultant mixed-liquor SvI may be in the range of 65 to 90 mL/g, and for single-tank anoxic selectors, SVI values in the range of 100 to 120 mL/g are more commonly obtained The use of selector designs in activated sludge is more common because of the many advantages derived from the minimal investment in a relatively small reactor volume. By improving sludge settling, the activated-sludge treatment capacity may be increased, as higher MLSS concentrations are usually possible The hydraulic capacity of the secondary clarifiers is also increased 7-4 Processes for bod removal and nitrification Process design considerations For BOD removal and nitrification processes, the rbCOD concentration is important for evaluating the oxygen demand profiles for plug-flow, staged, and batch-fed processes. The effect of nb VSS concentration in the influent will be significant in process sludge production and aeration volume requirements In the following paragraphs, three activated-sludge process design examples are provided to demonstrate application of these fundamental principles to BOD removal and nitrification processes omplete-Mix Activated-Sludge Process In a typical complete-mix activated-sludge(CMAS) process, effluent from the primary sedimentation tank and recycled return activated sludge are introduced typically at several points in the reactor. Because the tank contents are thoroughly mixed, the organic load, oxygen demand, and substrate concentration are uniform throughout the entire aeration tank and the f/m ratio is low Care should be taken to assure that the CMAS reactor is well mixed and that influent feed and effluent with-drawn points are selected to prevent short-circuiting of untreated or partially treated wastewater. The complete-mix reactor is usually configured in square, rectangular, or round shapes. Tank dimensions 7-15
7-15 . Third reactor, 3 g COD/g MLSS.d The F/M ratio is calculated for the first reactor using the volume and MLSS concentration at that reactor and the influent wastewater flowrate and COD concentration. The F/M value shown for the second reactor includes the volume of the first and second reactor and the applied loading as the product of the influent flowrate and COD concentration. Albertson (1987) recommended a similar approach based on a BOD F/M loading of 3 to 5 g BOD/g MLSS in the first reactor, with the second and third reactors being equal to and twice the first reactor volume, respectively. Albertson further notes that if the loading to the first reactor is too high (F/M > 8 g BOD/g MLSS.d), a viscous, nonfilamentous-type bulking can develop. The kinetic concept of a high F/M selector suggests that it be aerobic, and high DO concentrations are needed to maintain an aerobic floc (>6 to 8 mg/L). In many cases, such high DO concentrations are not practical or provided, and the staged selector design (described above) is operated at a low to zero DO concentration so that a metabolic selector mechanism is involved. A sequencing batch reactor (SBR) can also act as a very effective high F/M selector, depending on the wastewater strength and feeding strategy. For high-strength wastewaters with a relatively large fraction of the SBR volume occupied by the influent wastewater, a high initial F/M ratio can occur. The subsequent reaction by the batch process is equal to that for a plug-flow reactor. Metabolic-Based Selector. With biological nutrient-removal processes, improved sludge-settling characteristics, and, in many cases, minimal filamentous bacteria growth has been observed. The anoxic or anaerobic metabolic conditions used in these processes favor growth of the floc-forming bacteria. The filamentous bacteria cannot use nitrate or nitrite for an electron acceptor, thus yielding a significant advantage to denitrifying floc-forming bacteria. Similarly, the filamentous bacteria do not store polyphosphates and thus cannot consume acetate in the anaerobic contact zone in biological phosphorus-removal designs, giving an advantage for substrate uptake and growth to the phosphorus-storing bacteria. In some wastewater-treatment facilities (Seattle and San Francisco, for example), an anaerobic selector has been used before the aeration tank in low SRT activated-sludge systems designed for BOD removal, even though phosphorus removal is not required. Where nitrification is used and phosphorus removal is not required, anoxic selectors (either the staged high F/M gradient or the single-stage designs) have been used. For the high F/M anoxic or anaerobic selectors, the resultant mixed-liquor SVI may be in the range of 65 to 90 mL/g, and for single-tank anoxic selectors, SVI values in the range of 100 to 120 mL/g are more commonly obtained. The use of selector designs in activated sludge is more common because of the many advantages derived from the minimal investment in a relatively small reactor volume. By improving sludge settling, the activated-sludge treatment capacity may be increased, as higher MLSS concentrations are usually possible. The hydraulic capacity of the secondary clarifiers is also increased. 7-4 Processes for BOD Removal and Nitrification Process Design Considerations For BOD removal and nitrification processes, the rbCOD concentration is important for evaluating the oxygen demand profiles for plug-flow, staged, and batch-fed processes. The effect of nbVSS concentration in the influent will be significant in process sludge production and aeration volume requirements. In the following paragraphs, three activated-sludge process design examples are provided to demonstrate application of these fundamental principles to BOD removal and nitrification processes. Complete-Mix Activated-Sludge Process In a typical complete-mix activated-sludge (CMAS) process, effluent from the primary sedimentation tank and recycled return activated sludge are introduced typically at several points in the reactor. Because the tank contents are thoroughly mixed, the organic load, oxygen demand, and substrate concentration are uniform throughout the entire aeration tank and the F/M ratio is low. Care should be taken to assure that the CMAS reactor is well mixed and that influent feed and effluent with-drawn points are selected to prevent short-circuiting of untreated or partially treated wastewater. The complete-mix reactor is usually configured in square, rectangular, or round shapes. Tank dimensions depend mainly on the size, type, and mixing pattern of the aeration equipment