7 Suspended Growth Biological Treatment Processes 7-1 Introduction to the Activated-sludge Process Historical Development The activated-sludge process is now used routinely for biological treatment of municipal and industrial wastewaters. The antecedents of the activated-sludge process date back to the early 1880s to the work of Dr. Angus Smith, who investigated the aeration of wastewater in tanks and the hastening of the oxidation of the organic matter. The aeration of wastewater was studied subsequently by a number of investigators, and in 1910 Black and Phelps reported that a considerable reduction in putrescibility could be secured by forcing air into wastewater in basins. In experiments conducted at the Lawrence Experiment Station during 1912 and 1913 by Clark and Gage with aerated wastewater, growths of organisms could be cultivated in bottles and in tanks partially filled with roofing slate spaced about 25 mm(I in) apart and would greatly increase the degree of purification obtained( Clark and Adams, 1914). The results of the work at the Lawrence Experiment Station were so striking that knowledge of them led Dr. G J. Fowler of the University of Manchester, England to suggest experiments along similar lines be conducted at the Manchester Sewage Works where Ardern and Lockett carried out valuable research on the subject. During the course of their experiments, Ardern and Lockett found that the sludge played an important part in the esults obtained by aeration, as announced in their paper of May 3 1914 (Ardern and Ratum activated sludge Lockett, 1914). The activated sludge by Ardern and Lockett because it involved the activated mass of microorganisms capable of aerobic stabilization of Retum activated sludge material in wastewater (Metcalf Eddy 930) Fig. 7-I Description of basic basic activated-sludge treatment Te sequence illustrated on Fig 7-la and b. consists of the follow typical activated-sludge processes with different types of reactors: (o) schematic flow diagram of plug flow process and view of plug flow reactor, (b) schematic flow diagram of complete - mix process and view of complete-mix activated-sludge reactor, and component (c) schematic diogram of sequencing batch reactor process and view of sequencing batch reactor. /From H. D Stensel) 1) reactor in which the microorganisms responsible for treatment are kept in suspension and aerated ;(2) 7-1
7-1 7 Suspended Growth Biological Treatment Processes 7-1 Introduction to the Activated-Sludge Process Historical Development The activated-sludge process is now used routinely for biological treatment of municipal and industrial wastewaters. The antecedents of the activated-sludge process date back to the early 1880s to the work of Dr. Angus Smith, who investigated the aeration of wastewater in tanks and the hastening of the oxidation of the organic matter. The aeration of wastewater was studied subsequently by a number of investigators, and in 1910 Black and Phelps reported that a considerable reduction in putrescibility could be secured by forcing air into wastewater in basins. In experiments conducted at the Lawrence Experiment Station during 1912 and 1913 by Clark and Gage with aerated wastewater, growths of organisms could be cultivated in bottles and in tanks partially filled with roofing slate spaced about 25 mm (1 in) apart and would greatly increase the degree of purification obtained (Clark and Adams, 1914). The results of the work at the Lawrence Experiment Station were so striking that knowledge of them led Dr. G. J. Fowler of the University of Manchester, England to suggest experiments along similar lines be conducted at the Manchester Sewage Works where Ardern and Lockett carried out valuable research on the subject. During the course of their experiments, Ardern and Lockett found that the sludge played an important part in the results obtained by aeration, as announced in their paper of May 3, 1914 (Ardern and Lockett, 1914). The process was named activated sludge by Ardern and Lockett because it involved the production of an activated mass of microorganisms capable of aerobic stabilization of organic material in wastewater (Metcalf & Eddy, 1930). Fig. 7-1 Description of Basic Process By definition, the basic activated-sludge treatment process, as illustrated on Fig. 7-la and b, consists of the following three basic component s: (1) a reactor in which the microorganisms responsible for treatment are kept in suspension and aerated; (2)
liquid-solids separation, usually in a sedimentation tank; and(3)a recycle system for returning solids removed from the liquid-solids separation unit back to the reactor. Numerous process configurations have e of the activated-sludge process is t he nction with phys. in sedimentation tanks. In ical and nemical processes treatment of wastewater, and posttreatment, including imary ent at removing settleable soluble, colloidal, and particulate ,and for biological ater from smaller-sized communities less operator-intensive he world that have hot climates eapplications, e used, including sequencing batch ed since its early conception for higher-quality effluents from nitrogen removal, and/or biological with large length considering the evolution of the activated-slud wastewater ndustrial wastes to domestic ne f the toxic effects of some of the ume allowed for greater d activate to be single-stage, complete-mix activated-slu he CMAS process ha ank and second stage for n ditch uring th f the tank substances 1 basin. The plug- eration s, so that the concentration of the reactor length. Although process configurations employing long, narrow tanks are commonly referred to as plug-flow processes, in reality, true plug flow does not exist 7-2
7-2 liquid-solids separation, usually in a sedimentation tank; and (3) a recycle system for returning solids removed from the liquid-solids separation unit back to the reactor. Numerous process configurations have evolved employing these components. An important feature of the activated-sludge process is the formation of flocculent settleable solids that can be removed by gravity settling in sedimentation tanks. In most cases, the activated-sludge process is employed in conjunction with physical and chemical processes that are used for the preliminary and primary treatment of wastewater, and posttreatment, including disinfection and possibly filtration. Historically, most activated-sludge plants have received wastewaters that were pretreated by primary sedimentation, as shown on Fig. 7-la and b. Primary sedimentation is most efficient at removing settleable solids, whereas the biological processes are essential for removing soluble, colloidal, and particulate (suspended) organic substances; for biological nitrification and denitrification; and for biological phosphorus removal. For applications such as treating wastewater from smaller-sized communities, primary treatment is often not used as more emphasis is placed on simpler and less operator-intensive treatment methods. Primary treatment is omitted frequently in areas of the world that have hot climates where odor problems from primary tanks and primary sludge can be significant. For these applications, various modifications of conventional activated-sludge processes are used, including sequencing batch reactors, oxidation ditch systems, aerated lagoons, or stabilization ponds. Evolution of the Activated-Sludge Process A number of activated-sludge processes and design configurations have evolved since its early conception as a result of (1) engineering innovation in response to the need for higher-quality effluents from wastewater treatment plants; (2) technological advances in equipment, electronics, and process control; (3) increased understanding of microbial processes and fundamentals; and (4) the continual need to reduce capital and operating costs for municipalities and industries. With greater frequency, activated-sludge processes used today may incorporate nitrification, biological nitrogen removal, and/or biological phosphorus removal. These designs employ reactors in series, operated under aerobic, anoxic, and anaerobic conditions, and may use internal recycle pumps and piping. Since the process came into common use in the early 1920s and up until the late 1970s, the type of activated-sludge process used most commonly was the one in which a plug-flow reactor with large length to width ratios (typically > 10:1) was used (see Fig. 7-la). In considering the evolution of the activated-sludge process, it is important to note that the discharge of industrial wastes to domestic wastewater collection systems increased in the late 1960s. The use of a plug-flow process became problematic when industrial wastes were introduced because of the toxic effects of some of the discharges. The complete-mix reactor was developed, in part, because the larger volume allowed for greater dilution and thus mitigated the effects of toxic discharges. The more common type of activated-sludge process in the 1970s and early 1980s tended to be single-stage, complete-mix activated-sludge (CMAS) processes (see Fig. 7-lb), as advanced by McKinney (1962). In Europe, the CMAS process has not been adopted generally as ammonia standards have become increasingly stringent. For some nitrification applications, two-stage systems (each stage consisting of an aeration tank and clarifier) were used with the first stage designed for BOD removal, followed by a second stage for nitrification. Other activated-sludge processes that have found application include the oxidation ditch (1950s), contact stabilization (1950s), Krause process (1960s), pure oxygen activated sludge (1970s), Orbal process (1970s), deep shaft aeration (1970s), and sequencing batch reactor process (1980). With the development of simple inexpensive program logic controllers (PLCs) and the availability of level sensors and automatically operated valves, the sequencing batch reactor (SBR) process (see Fig. 7-1c) became more widely used by the late 1970s, especially for smaller communities and industrial installations with intermittent flows. In recent years, however, SBRs are being used for large cities in some parts of the world. The SBR is a fill-and-draw type of reactor system involving a single complete-mix reactor in which all steps of the activated-sludge process occur. Mixed liquor remains in the reactor during all cycles, thereby eliminating the need for separate sedimentation tanks. In comparing the plug-flow (Fig. 7-la) and complete-mix activated-sludge (CMAS) (Fig. 7-1 b) processes, the mixing regimes and tank geometry are quite different. In the CMAS process, the mixing of the tank contents is sufficient so that ideally the concentrations of the mixed-liquor constituents, soluble substances (i.e., COD, BOD, NH4-N), and colloidal and suspended solids do not vary with location in the aeration basin. The plug-flow process involves relatively long, narrow aeration basins, so that the concentration of soluble substances and colloidal and suspended solids varies along the reactor length. Although process configurations employing long, narrow tanks are commonly referred to as plug-flow processes, in reality, true plug flow does not exist
Activated-sludge process designs before and until the late 1970s generally involved the configurations own on Fig. 7-la and b. However, with interest in biological nutrient removal, staged reactor designs onsisting of complete-mix reactors in series have been developed(see Fig. 7-2) Some of the stages are not aerated(anaerobic or anoxic stages)and internal recycle flows may be used For nitrification, intemal recycle a staged aerobic Aerobic AnoxicAerobic rovide more use of the total reactor volume single-stage vre Recent process Bardenpho process with stog Developments reactors for biological nitroge As noted above removal: (a) schematic diagram of staged process numerous nd (b view of a staged modifications of Palmetto FL. the first of its ype in the United States.(From activated-sludge H D Sensei evolved in the last 10 to 20 years, aimed principally at effective and efficient removal of nitrogen and phosphorus Because of the development of improved membrane design, principally for water treatment applications, membrane technology has found ing application for enhanced solids separation for water reuse, and more recently for use in suspended growth reactors for wastewater treatment. Membrane biological reactors(MBRs) may change the look of wastewater-treatment facilities in the future. Because the design and operation of the activated-sludge process is becoming more complex, computer modeling is an increasingly important tool to incorporate the large number of components and reactions necessary to evaluate activated-sludge performance 7-2 Wastewater Characterization Activated-sludge process design requires determining (1) the aeration basin volume, (2) the amount of Idge production, (3)the amount of oxygen needed, and(4)the effluent concentration of important parameters. To design an activated-sludge treatment process properly, characterization of the wastewater perhaps the most critical step in the process. For biological nutrient-removal processes, wastewater characterization is essential for predicting performance. Wastewater characterization is an important element in the evaluation of existing facilities for optimizing performance and available treatment capacity. Flow characterization is also important including diurnal, seasonal, and wet-weather flow variations Without comprehensive wastewater characterization, facilities may either be under- or overdesigned, resulting in inadequate or inefficient treatment Key wastewater Constituents for Process Design Carbonaceous Constituents. Carbonaceous constituents measured by BOD or COD analyses are critical to the activated-sludge process design. Higher concentrations of degradable COd or BOD result in (1)a larger aeration basin volume, (2)more oxygen transfer needs, and (3) greater sludge production While the bod has been the common parameter to characterize carbonaceous material in wastewater, COD is becoming more common. By using a COD mass balance, the fate of carbonaceous material between the amount oxidized and the amount incorporated into cell mass is followed more easily. The various forms of the COd in wastewater are shown on Fig. 7-3 Unlike BoD. some F.7-3 Total COD portion of the COD Fractionation of COD in is not biodegradable the COD fractions so the Cod is ed in the detailed asign of activated. sludge processes biodegradable and nonbiodegrad ColloidalParticulate
7-3 Activated-sludge process designs before and until the late 1970s generally involved the configurations shown on Fig. 7-1a and b. However, with interest in biological nutrient removal, staged reactor designs consisting of complete-mix reactors in series have been developed (see Fig. 7-2). Some of the stages are not aerated (anaerobic or anoxic stages) and internal recycle flows may be used. For nitrification, a staged aerobic reactor design may also be used to provide more efficient use of the total reactor volume than a single-stage CMAS process. Recent Process Developments As noted above, numerous modifications of the activated-sludge process have evolved in the last 10 to 20 years, aimed principally at effective and efficient removal of nitrogen and phosphorus. Because of the development of improved membrane design, principally for water treatment applications, membrane technology has found increasing application for enhanced solids separation for water reuse, and more recently for use in suspended growth reactors for wastewater treatment. Membrane biological reactors (MBRs) may change the look of wastewater-treatment facilities in the future. Because the design and operation of the activated-sludge process is becoming more complex, computer modeling is an increasingly important tool to incorporate the large number of components and reactions necessary to evaluate activated-sludge performance. 7-2 Wastewater Characterization Activated-sludge process design requires determining (1) the aeration basin volume, (2) the amount of sludge production, (3) the amount of oxygen needed, and (4) the effluent concentration of important parameters. To design an activated-sludge treatment process properly, characterization of the wastewater is perhaps the most critical step in the process. For biological nutrient-removal processes, wastewater characterization is essential for predicting performance. Wastewater characterization is an important element in the evaluation of existing facilities for optimizing performance and available treatment capacity. Flow characterization is also important including diurnal, seasonal, and wet-weather flow variations. Without comprehensive wastewater characterization, facilities may either be under- or overdesigned, resulting in inadequate or inefficient treatment. Key Wastewater Constituents for Process Design Carbonaceous Constituents. Carbonaceous constituents measured by BOD or COD analyses are critical to the activated-sludge process design. Higher concentrations of degradable COD or BOD result in (1) a larger aeration basin volume, (2) more oxygen transfer needs, and (3) greater sludge production. While the BOD has been the common parameter to characterize carbonaceous material in wastewater, COD is becoming more common. By using a COD mass balance, the fate of carbonaceous material between the amount oxidized and the amount incorporated into cell mass is followed more easily. The various forms of the COD in wastewater are shown on Fig. 7-3. Unlike BOD, some portion of the COD is not biodegradable, so the COD is divided into biodegradable and nonbiodegradable Fig. 7-3
concentrations. The next level of interest is how much of the cod in each of these categories is dissolved or soluble, and how much is particulate, comprised of colloidal and suspended solids. The nonbiodegradable soluble COD(nbsCOD) will be found in the activated-sludge effluent, and nonbiodegradable particulates will contribute to the total sludge production Because the nonbiodegradable particulate COD(nbpCOD)is organic material, it will also contribute to the VSs concentration of the wastewater and mixed liquor in the activated-sludge process, and is referred to here as the nonbiodegradable volatile suspended solids(nbVSS). The influent wastewater will also contain nonvolatile influent suspended solids that add to the MLSS concentration in the activated-sludge process. These solids are influent inert TSS (iTSS)and can be quantified by the difference in influent wastewater TSS and VSS concentrations For biodegradable COD, understanding the fractions that are measured as soluble, soluble readily biodegradable COd(rbCOD), and particulate is extremely important for activated -sludge process design. The rbCod portion is quickly assimilated by the biomass, while the particulate and colloidal COD must first be dissolved by extracellular enzymes and are thus assimilated at much slower rates Tab. 7-I Biological Effect of rbCOD processes affected by Activated-sludge aeration For plug How or staged aeration zones, there will be a Monk with a readily biodegradable hic igher fraction of rbCoD in the inn COD(rbCOD)concentration Biological nitrogen removal For the preanoxic tank, there will be a higher in influent wastewater influent COD. Can result in smaller anoxic tank volu Biological phosphorus rem Greater infuent rbCoD concentration results in a greater amount of biological phosphorus removal Activated-sludge selector coD fraction of rbCoD in influent COD provides more greater impact on improving sludge volume index(SVI) The rbCoD fraction of the Cod has a direct effect on the activated-sludge biological kinetics and process performance Process applications where the rbCoD concentration affects the process design and erformance are summarized in Table 7-1 The rbCoD consists of complex soluble COd that can be fermented to volatile fatty acids(VFAs)in the influent wastewater. Wastewaters that are more septic, for example, from collection systems in warm climates with minimal slopes, will contain higher concentrations of VF Nitrogenous Constituents. The total Kjeldahl nitrogen(TKN) is a measure of the sum of the ammonia and organic nitrogen About 60 to 70 percent of the influent tKn concentration will be as NH4-N, which is readily available for bacterial synthesis and nitrification. Organic nitrogen is present in both soluble and particulate forms, and some portion of each of these is nonbiodegradable. The particulate degradable organic nitrogen will be removed more slowly than the soluble degradable organic nitrogen because a hydrolysis reaction is necessary first. The nondegradable organic nitrogen is assumed to be about 6 percent of the nondegradable vss as Cod in the influent wastewater( Grady et al., 1999). The particulate nondegradable nitrogen will be captured in the activated-sludge floc and exit in the waste sludge, but the oluble nondegradable nitrogen will be found in the secondary clarifier effluent. The soluble nondegradable nitrogen contributes to the effluent total nitrogen concentration and is a small fraction of the influent wastewater TKn concentration(<3 percent). The soluble nondegradable organic nitrogen concentration in domestic wastewater typically ranges from l to 2 mg/L as N Alkalinity. Alkalinity concentration is an important wastewater characteristic that affects the performance of biological nitrification processes. Adequate alkalinity is needed to achieve complete nitrification Measurement Methods for Wastewater Characterization Readily Biodegradable COD. The rbCoD concentration is either determined from a biological response or estimated by a physical separation technique In the biological response method the oxygen uptake rate (OUR) is followed and recorded with time after mixing the wastewater sample with an acclimated activated-sludge sample. The wastewater may be preaerated so that upon contact with th activated sludge a high DO concentration is present to allow an immediate measurement of the OUR. The wastewater sample and activated sludge are mixed in a batch reactor with separate aeration and mixing 7-4
7-4 concentrations. The next level of interest is how much of the COD in each of these categories is dissolved or soluble, and how much is particulate, comprised of colloidal and suspended solids. The nonbiodegradable soluble COD (nbsCOD) will be found in the activated-sludge effluent, and nonbiodegradable particulates will contribute to the total sludge production. Because the nonbiodegradable particulate COD (nbpCOD) is organic material, it will also contribute to the VSS concentration of the wastewater and mixed liquor in the activated-sludge process, and is referred to here as the nonbiodegradable volatile suspended solids (nbVSS). The influent wastewater will also contain nonvolatile influent suspended solids that add to the MLSS concentration in the activated-sludge process. These solids are influent inert TSS (iTSS) and can be quantified by the difference in influent wastewater TSS and VSS concentrations. For biodegradable COD, understanding the fractions that are measured as soluble, soluble readily biodegradable COD (rbCOD), and particulate is extremely important for activated-sludge process design. The rbCOD portion is quickly assimilated by the biomass, while the particulate and colloidal COD must first be dissolved by extracellular enzymes and are thus assimilated at much slower rates. The rbCOD fraction of the COD has a direct effect on the activated-sludge biological kinetics and process performance.Process applications where the rbCOD concentration affects the process design and performance are summarized in Table 7-1. The rbCOD consists of complex soluble COD that can be fermented to volatile fatty acids (VFAs) in the influent wastewater. Wastewaters that are more septic, for example, from collection systems in warm climates with minimal slopes, will contain higher concentrations of VFAs. Nitrogenous Constituents. The total Kjeldahl nitrogen (TKN) is a measure of the sum of the ammonia and organic nitrogen. About 60 to 70 percent of the influent TKN concentration will be as NH4-N, which is readily available for bacterial synthesis and nitrification. Organic nitrogen is present in both soluble and particulate forms, and some portion of each of these is nonbiodegradable. The particulate degradable organic nitrogen will be removed more slowly than the soluble degradable organic nitrogen because a hydrolysis reaction is necessary first. The nondegradable organic nitrogen is assumed to be about 6 percent of the nondegradable VSS as COD in the influent wastewater (Grady et al., 1999). The particulate nondegradable nitrogen will be captured in the activated-sludge floc and exit in the waste sludge, but the soluble nondegradable nitrogen will be found in the secondary clarifier effluent. The soluble nondegradable nitrogen contributes to the effluent total nitrogen concentration and is a small fraction of the influent wastewater TKN concentration (<3 percent). The soluble nondegradable organic nitrogen concentration in domestic wastewater typically ranges from 1 to 2 mg/L as N. Alkalinity. Alkalinity concentration is an important wastewater characteristic that affects the performance of biological nitrification processes. Adequate alkalinity is needed to achieve complete nitrification. Measurement Methods for Wastewater Characterization Readily Biodegradable COD. The rbCOD concentration is either determined from a biological response or estimated by a physical separation technique. In the biological response method the oxygen uptake rate (OUR) is followed and recorded with time after mixing the wastewater sample with an acclimated activated-sludge sample. The wastewater may be preaerated so that upon contact with the activated sludge a high DO concentration is present to allow an immediate measurement of the OUR. The wastewater sample and activated sludge are mixed in a batch reactor with separate aeration and mixing. Tab. 7-1 Biological processes affected by readily biodegradable COD(rbCOD) concentration in influent wastewater
An idealized example of the OUR response for a wastewater sample using an activated sludge containing nitrifying bacteria is shown on Fig 7-4. The OUR versus time can be divided into four areas, which can be used to determine the oxygen consumed for the reaction indicated by the area. Area A is the oxygen used for rbCod degradation area ogent nitrification, area C for particulate COD degradation. and area D for endogenous en A represents rh暑30B COD angen demand Nitrogen Compounds. For the nitrogen compounds, the soluble organic nitrogen concentration is of interest fre 0 nitrogen concentration. A filtered sample from the plant effluent or from a bench-scale treatability reactor can be used to determine the total effluent soluble organic nitrogen concentration by the difference between the tKn concentration of the filtered sample and the effluent NH4-N concentration Recycle flows and loadings The impact of recycle flows must also be quantified and included in defining the influent wastewater characteristics to the activated-sludge process. The possible sources of recycle flows supernatant flows(if settling and decanting are practiced in the digestion operation), recycle of centrate or filtrate from solids dewatering equipment, backwash water from effluent filtration processes, and water from odor-control scrubbers. Depending on the source, a significant BOD, TSS, and NH4-N load may be added to the influent wastewater. Compared to untreated wastewater or primary clarifier effluent, the BOD/VSS ratio is often much lower for recycle streams. In addition, a significant NH-N load can be returned to the influent wastewater from anaerobic digestion-related processes. Concentrations of NH4-N in the range of 1000 to 2000 mg/L are possible in centrate or filtrate from the dewatering of anaerobically digested solids. Thus, the ammonia load from a return flow of about one-half percent of the influent flow can increase the influent TKN load to the activated-sludge process by 10 to 20 percent. The return solids load from effluent polishing filters can be estimated by a mass balance on solids removed across the filtration process, and thus released in the backwash water flow. In all cases, a mass balance for flow and mport fo constituents, inc haws ond ads to the nitrogse -s mpoe nds end phosphorus should be done to 7-3 Fundamentals of Process Analysis and Control The purpose of this section is to introduce (1) the basic considerations involved in process design, (2) process control measures, (3)operating problems associated with the activated-sludge process, and (4) activated-sludge selector processes Process Design Considerations In the design of the activated-sludge process, consideration must be given to(I) selection of the reactor type,(2)applicable kinetic relationships, (3) solids retention time and loading criteria to be used,(4) sludge production, (5)oxygen requirements and transfer, (6) nutrient requirements, (7) other chemical requirements, 8)settling characteristics of biosolids, (9)use of selectors, and (10)effluent characteristics Selection of Reactor Type. Important factors that must be considered in the selection of reactor types for the activated-sludge process include(1) the effects of reaction kinetics, (2) oxygen transfer requirements, (3)nature of the wastewater, (4)local environmental conditions, (5) presence of toxic or inhibitory substances in the influent wastewater,(6)costs, and(7) expansion to meet future treatment Selection of Solids Retention Time and Loading Criteria. Certain design and operating parameters e process from another. The common parameters used are the solids retention time(SrT), the food to biomass(F/M ratio(also known as food to microorganism ratio), and the volumetric organic loading rate. While the Srt is the basic design and operating parameter, the F/M ratio and volumetric loading rate provide values that are useful for comparison to historical data and typical observed operating conditions Solids Retention Time. The SRT, in effect, represents the average period of time during which the
7-5 An idealized example of the OUR response for a wastewater sample using an activated sludge containing nitrifying bacteria is shown on Fig. 7-4. The OUR versus time can be divided into four areas, which can be used to determine the oxygen consumed for the reaction indicated by the area. Area A is the oxygen used for rbCOD degradation, area B for zero-order nitrification, area C for particulate COD degradation, and area D for endogenous decay Nitrogen Compounds. For the nitrogen compounds, the soluble organic nitrogen concentration is of interest from the standpoint of its effect on the effluent total nitrogen concentration. A filtered sample from the plant effluent or from a bench-scale treatability reactor can be used to determine the total effluent soluble organic nitrogen concentration by the difference between the TKN concentration of the filtered sample and the effluent NH4-N concentration. Recycle Flows and Loadings The impact of recycle flows must also be quantified and included in defining the influent wastewater characteristics to the activated-sludge process. The possible sources of recycle flows include digester supernatant flows (if settling and decanting are practiced in the digestion operation), recycle of centrate or filtrate from solids dewatering equipment, backwash water from effluent filtration processes, and water from odor-control scrubbers. Depending on the source, a significant BOD, TSS, and NH4-N load may be added to the influent wastewater. Compared to untreated wastewater or primary clarifier effluent, the BOD/VSS ratio is often much lower for recycle streams. In addition, a significant NH4-N load can be returned to the influent wastewater from anaerobic digestion-related processes. Concentrations of NH4-N in the range of 1000 to 2000 mg/L are possible in centrate or filtrate from the dewatering of anaerobically digested solids. Thus, the ammonia load from a return flow of about one-half percent of the influent flow can increase the influent TKN load to the activated-sludge process by 10 to 20 percent. The return solids load from effluent polishing filters can be estimated by a mass balance on solids removed across the filtration process, and thus released in the backwash water flow. In all cases, a mass balance for flow and important constituents, such as BOD, TSS/VSS, nitrogen compounds, and phosphorus should be done to account for all contributing flows and loads to the activated-sludge process. 7-3 Fundamentals of Process Analysis and Control The purpose of this section is to introduce (1) the basic considerations involved in process design, (2) process control measures, (3) operating problems associated with the activated-sludge process, and (4) activated-sludge selector processes. Process Design Considerations In the design of the activated-sludge process, consideration must be given to (1) selection of the reactor type, (2) applicable kinetic relationships, (3) solids retention time and loading criteria to be used, (4) sludge production, (5) oxygen requirements and transfer, (6) nutrient requirements, (7) other chemical requirements, (8) settling characteristics of biosolids, (9) use of selectors, and (10) effluent characteristics. Selection of Reactor Type. Important factors that must be considered in the selection of reactor types for the activated-sludge process include (1) the effects of reaction kinetics, (2) oxygen transfer requirements, (3) nature of the wastewater, (4) local environmental conditions, (5) presence of toxic or inhibitory substances in the influent wastewater, (6) costs, and (7) expansion to meet future treatment needs. Selection of Solids Retention Time and Loading Criteria. Certain design and operating parameters distinguish one activated-sludge process from another. The common parameters used are the solids retention time (SRT), the food to biomass (F/M) ratio (also known as food to microorganism ratio), and the volumetric organic loading rate. While the SRT is the basic design and operating parameter, the F/M ratio and volumetric loading rate provide values that are useful for comparison to historical data and typical observed operating conditions. Solids Retention Time. The SRT, in effect, represents the average period of time during which the Fig. 7-4 Idealized oxygen uptake rate(OUR) in aerobic batch test for a mixture of influent wastewater and activated-sludge mixed liquor. Area A represents rb COD oxygen demand