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
sludge has remained in the system. SrT is the most critical parameter for activated-sludge design as Srt affects the treatment process performance, aeration tank volume, sludge production, and oxygen requirements. For BOD removal, SRT values may range from 3 to 5 d, depending on the mixed-liquor temperature. At 18 to 25 C an SRT value close to 3 d is desired where only BOD removal is required. To limit nitrification, some activated-sludge plants have been operated at SRT values of l d or less. At 10C SRI values of 5 to 6 d are common for BOD removal only. Temperature and other factors that affect Srt in various treatment applications are summarized in Table 7-2 SRT ranges for activated-sludge Removal of soluble BOD in domes -2 d Develop flocculent biomass for treating dustrial wastewater 3-18 Temperature/ compounds Biological phosphorus removal Stabilization of activated sludge 20-40 Degradation of xenobiotic compounds For nitrification design, a safety factor is used to increase the Srt above that calculated based on nitrification kinetics and the required effluent NH4-N concentration. A factor of safety is used for two reasons:(1)to allow flexibility for operational variations in controlling the Srt, and (2) to provide for dditional nitrifying bacteria to handle peak TKN loadings. The influent TKN concentration and mass loading can vary throughout the day(a peak to average tKn loading of 1.3 to 1.5 is not unusual, depending on plant size)and can also be affected by return flows from digested and dewatered biosolids processing. By increasing the design SRT, the inventory of nitrifying bacteria is increased to meet the NH4-N concentration at the peak load so that the effluent NH4-n concentration requirement is achieved Food to Microorganism Ratio. A process parameter commonly used to characterize process designs and operating conditions is the food to microorganism(biomass)ratio(F/M). Typical values for the BOD F/M ratio reported in the literature vary from 0.04 g substrate/g biomass. d for extended aeration processes to 1.0 g/gd for high rate processes. The BOd F/M ratio is usually evaluated for systems that were designed based on SrT to provide a reference point to previous activated-sludge design and operating Volumetric Organic Loading Rote. The volumetric organic loading rate is defined as the amount of BOd or COD applied to the aeration tank volume per day, Organic loadings, expressed in kg bOd o COD/m.d, may vary from 0.3 to more than 3. 0. Higher volumetric organic loadings generally result higher required oxygen transfer rates per unit volume for the aeration system Sludge Production. The design of the sludge-handling and disposal/reuse facility depends on the prediction of sludge production for the activated-sludge process. Sludge will accumulate in the activated-sludge process if it cannot be processed fast enough by an undersized sludge-handling facility Eventually, the sludge inventory capacity of the activated-sludge sy stem will be exceeded and excess solids will exit in the secondary clarifier effluent, potentially violating discharge limits. The sludge production relative to the amount of Bod removed also affects the aeration tank size Two methods are used to determine sludge production. The first method is based on an estimate of ar bserved sludge production yield from published data from similar facilities, and the second is based on process design in 000 which wastewate characterization is done and the various sources of sludge production accounted for. For en wastewater. the 04060811.5234567101520304050 Figure 8-7 Net solids production vs. solids retention time (SRn) and temperoture: (o) with primary treatment and (b)without primary treame
7-6 sludge has remained in the system. SRT is the most critical parameter for activated-sludge design as SRT affects the treatment process performance, aeration tank volume, sludge production, and oxygen requirements. For BOD removal, SRT values may range from 3 to 5 d, depending on the mixed-liquor temperature. At 18 to 25。C an SRT value close to 3 d is desired where only BOD removal is required. To limit nitrification, some activated-sludge plants have been operated at SRT values of 1 d or less. At 10。 C, SRT values of 5 to 6 d are common for BOD removal only. Temperature and other factors that affect SRT in various treatment applications are summarized in Table 7-2. For nitrification design, a safety factor is used to increase the SRT above that calculated based on nitrification kinetics and the required effluent NH4-N concentration. A factor of safety is used for two reasons: (1) to allow flexibility for operational variations in controlling the SRT, and (2) to provide for additional nitrifying bacteria to handle peak TKN loadings. The influent TKN concentration and mass loading can vary throughout the day (a peak to average TKN loading of 1.3 to 1.5 is not unusual, depending on plant size) and can also be affected by return flows from digested and dewatered biosolids processing. By increasing the design SRT, the inventory of nitrifying bacteria is increased to meet the NH4-N concentration at the peak load so that the effluent NH4-N concentration requirement is achieved. Food to Microorganism Ratio. A process parameter commonly used to characterize process designs and operating conditions is the food to microorganism (biomass) ratio (F/M). Typical values for the BOD F/M ratio reported in the literature vary from 0.04 g substrate/g biomass.d for extended aeration processes to 1.0 g/g.d for high rate processes. The BOD F/M ratio is usually evaluated for systems that were designed based on SRT to provide a reference point to previous activated-sludge design and operating performance. Volumetric Organic Loading Rote. The volumetric organic loading rate is defined as the amount of BOD or COD applied to the aeration tank volume per day, Organic loadings, expressed in kg BOD or COD/m3 .d, may vary from 0.3 to more than 3.0. Higher volumetric organic loadings generally result in higher required oxygen transfer rates per unit volume for the aeration system. Sludge Production. The design of the sludge-handling and disposal/reuse facility depends on the prediction of sludge production for the activated-sludge process. Sludge will accumulate in the activated-sludge process if it cannot be processed fast enough by an undersized sludge-handling facility. Eventually, the sludge inventory capacity of the activated-sludge system will be exceeded and excess solids will exit in the secondary clarifier effluent, potentially violating discharge limits. The sludge production relative to the amount of BOD removed also affects the aeration tank size. Two methods are used to determine sludge production. The first method is based on an estimate of an observed sludge production yield from published data from similar facilities, and the second is based on the actual activated-sludge process design in which wastewater characterization is done and the various sources of sludge production are considered and accounted for. For a given wastewater, the Tab. 7-2 Typical minimum SRT ranges for activated-sludge treatment
Yobs value will vary depending on whether the substrate is defined as BOD, bCoD, or COD Observed volatile suspended solids yield values, based on BOD, are illustrated on Fig. 7-5 The observed yield decreases as the Srt is increased due to biomass loss by more endogenous respiration. The yield is lower with increasing temperature as a result of a higher endogenous respiration rate at higher temperature. The yield is higher when no primary treatment is used, as more nbvss remains in the influent wastewater. The total mass of dry solids wasted/day includes TSS and not just VSS. The TSSI includes the VSS plus inorganic solids Oxygen Requirements. The oxygen required for the biodegradation of carbonaceous material determined from a mass balance using the bcod concentration of the wastewater treated and the amount of biomass wasted from the system per day. If all of the bCod were oxidized to COz, H20, and NH3, the oxygen demand would equal the bCOD concentration. However, bacteria oxidize a portion of the bCoD to provide energy and use the remaining portion of the bCod for cell growth. Oxygen is also consumed for endogenous respiration, and the amount will depend on the system SRT. For a given SrT, a mas balance on the system can be done where the bCod removal equals the oxygen used plus the biomass VSS remaining in terms of an oxygen equivalent. The oxygen requirements for BOd removal without imary treatment and nitrification can be computed. As an approximation, for BOD removal only, the oxygen requirement will vary from 0.90 to 1. 3 kg O2/kg BOD removed for SRTs of 5 to 20 d, respectively(WEF, 1998) NOx is the amount of Tkn oxidized to nitrate. A nitrogen mass balance for the system that accounts for the influent TKN, nitrogen removed for biomass synthesis, and unoxidized effluent nitrogen is done to determine NOx. The nitrogen mass balance is based on the assumption of 0. 12 g N/g biomass(CsH7NO2 for biomass) Nutrient Requirements. If a biological system is to function properly, nutrients must be available in adequate amounts. Using the formula CsH7NO2, for the composition of cell biomass, about 12. 4 percent by weight of nitrogen will be required. The phosphorus requirement is usually assumed to be about one-fifth of the nitrogen value. These are typical values, not fixed quantities, because it has been shown that the percentage distribution of nitrogen and phosphorus in cell tissue varies with the system SRT and environmental conditions. The amount of nutrients required can be estimated based on the daily biomass production rate. It should be noted that nutrient limitations can occur when the concentrations of nitrogen and phosphorus are in the range of 0. 1 to 0.3 mg/L. As a general role, for SRT values greater than 7 d, about 5 g nitrogen and 1 g phosphorus will be required per 100 g of Bod to provide an excess of nutrients Other Chemical Requirements. In addition to the nutrient requirements, alkalinity is a major chemical requirement needed for nitrification. The amount of alkalinity required for nitrification, taking into account cell growth, is about 7.07 g CaCO3/g NH4-N. In addition to the alkalinity required for nitrification dditional alkalinity must be available to maintain the ph in the range from 6.8 to 7.4. Typically the amount of residual alkalinity required to maintain pH near a neutral point(ie, pH= 7) is between 70 and 80 mg/L as CaCO3 Mixed-Liquor Settling Characteristics. Clarifier design must provide adequate clarification of the effluent and solids thickening for the activated-sludge solids. In the design of installations where sludge characteristics are not known, data from other installations must be assumed or experience of the designer with similar suspended growth processes must be utilized Two commonly used measures developed to quantify the settling characteristics of activated sludge are the sludge volume index(SVi)and the zone settling rate(WEF, 1998 ). The Svi is the volume of l g of sludge after 30 min of settling. The Svi is determined by placing a mixed-liquor sample in a I-to 2-L cylinder and measuring the settled volume after 30 min and the corresponding sample Mlss concentration. For example, a mixed-liquor sample with a 3000 mg/L TSS concentration that settles to a volume of 300 mL in 30 min in a l-L cylinder would have an SV of 100 mL/g. A value of 100 mL/g is considered a good settling sludge (SVI values below 100 are desired). SVI values above 150 are typically associated with filamentous growth( Parker et al., 2001) Because the SVI test is empirical, it is subject to significant errors. For ample, if sludge with a concentration of 10,000 mg/L did not settle all after 30 min the svi value would be 100. To avoid erroneous results and to allow for a meaningful comparison of svI results for different sludges, the diluted SVI(DS VI) test has been used (Jenkins et al., 1993). In Jenkins's analysis, the sludge sample is diluted with process effluent until the settled volume after 30 min is 250 mL/L or less. The standard SVI test is then followed with this sample 7-7
7-7 Yobs value will vary depending on whether the substrate is defined as BOD, bCOD, or COD. Observed volatile suspended solids yield values, based on BOD, are illustrated on Fig. 7-5. The observed yield decreases as the SRT is increased due to biomass loss by more endogenous respiration. The yield is lower with increasing temperature as a result of a higher endogenous respiration rate at higher temperature. The yield is higher when no primary treatment is used, as more nbVSS remains in the influent wastewater.The total mass of dry solids wasted/day includes TSS and not just VSS. The TSSI includes the VSS plus inorganic solids. Oxygen Requirements. The oxygen required for the biodegradation of carbonaceous material is determined from a mass balance using the bCOD concentration of the wastewater treated and the amount of biomass wasted from the system per day. If all of the bCOD were oxidized to CO2, H20, and NH3, the oxygen demand would equal the bCOD concentration. However, bacteria oxidize a portion of the bCOD to provide energy and use the remaining portion of the bCOD for cell growth. Oxygen is also consumed for endogenous respiration, and the amount will depend on the system SRT. For a given SRT, a mass balance on the system can be done where the bCOD removal equals the oxygen used plus the biomass VSS remaining in terms of an oxygen equivalent. The oxygen requirements for BOD removal without nitrification can be computed. As an approximation, for BOD removal only, the oxygen requirement will vary from 0.90 to 1.3 kg O2/kg BOD removed for SRTs of 5 to 20 d, respectively (WEF, 1998). NOx is the amount of TKN oxidized to nitrate. A nitrogen mass balance for the system that accounts for the influent TKN, nitrogen removed for biomass synthesis, and unoxidized effluent nitrogen is done to determine NOx. The nitrogen mass balance is based on the assumption of 0.12 g N/g biomass (C5H7NO2 for biomass). Nutrient Requirements. If a biological system is to function properly, nutrients must be available in adequate amounts. Using the formula C5H7NO2, for the composition of cell biomass, about 12.4 percent by weight of nitrogen will be required. The phosphorus requirement is usually assumed to be about one-fifth of the nitrogen value. These are typical values, not fixed quantities, because it has been shown that the percentage distribution of nitrogen and phosphorus in cell tissue varies with the system SRT and environmental conditions. The amount of nutrients required can be estimated based on the daily biomass production rate. It should be noted that nutrient limitations can occur when the concentrations of nitrogen and phosphorus are in the range of 0.1 to 0.3 mg/L. As a general role, for SRT values greater than 7 d, about 5 g nitrogen and 1 g phosphorus will be required per 100 g of BOD to provide an excess of nutrients. Other Chemical Requirements. In addition to the nutrient requirements, alkalinity is a major chemical requirement needed for nitrification. The amount of alkalinity required for nitrification, taking into account cell growth, is about 7.07 g CaCO3/g NH4-N. In addition to the alkalinity required for nitrification, additional alkalinity must be available to maintain the pH in the range from 6.8 to 7.4. Typically the amount of residual alkalinity required to maintain pH near a neutral point (i.e., pH ≈ 7) is between 70 and 80 mg/L as CaCO3. Mixed-Liquor Settling Characteristics. Clarifier design must provide adequate clarification of the effluent and solids thickening for the activated-sludge solids. In the design of installations where sludge characteristics are not known, data from other installations must be assumed or experience of the designer with similar suspended growth processes must be utilized. Two commonly used measures developed to quantify the settling characteristics of activated sludge are the sludge volume index (SVI) and the zone settling rate (WEF, 1998). The SVI is the volume of 1 g of sludge after 30 min of settling. The SVI is determined by placing a mixed-liquor sample in a 1- to 2-L cylinder and measuring the settled volume after 30 min and the corresponding sample MLSS concentration. For example, a mixed-liquor sample with a 3000 mg/L TSS concentration that settles to a volume of 300 mL in 30 min in a 1-L cylinder would have an SVI of 100 mL/g. A value of 100 mL/g is considered a good settling sludge (SVI values below 100 are desired). SVI values above 150 are typically associated with filamentous growth (Parker et al., 2001 ). Because the SVI test is empirical, it is subject to significant errors. For example, if sludge with a concentration of 10,000 mg/L did not settle at all after 30 min, the SVI value would be 100. To avoid erroneous results and to allow for a meaningful comparison of SVI results for different sludges, the diluted SVI (DSVI) test has been used (Jenkins et al., 1993). In Jenkins's analysis, the sludge sample is diluted with process effluent until the settled volume after 30 min is 250 mL/L or less. The standard SVI test is then followed with this sample. Fig. 7-5 Net solids production vs. solids retention time(SRT) and temperature:(a)with primary treatment and (b)without primary treatment
Many SVI tests at waste water treatment plants are done in a 2-L settleometer that has a larger diameter than 1-or 2-L graduated cylinders(see Fig. 7-6). To eliminate well effects on solids settling in a small-diameter test apparatus,use of a slow-speed stirring device is encouraged(Wahlberg and Keinath 1988). The test is called a stirred SVI when a stirring device is used(see Standard Methods, WEF, 1998) The stirred sVi test is used frequently in Europe Secondary Clarification. Several approaches are used in the design of secondary clarification facilities The approach used most commonly is to base the design on a consideration of the surface overflow rate and the solids loading rate. Because steady-state operations seldom occur due to fluctuations in wastewater flowrate, return activated-sludge flowrate, and mLss concentrations, attention to the occurrence of peak events and use of safety factors are important design considerations Overflow rates are based on wastewater flowrates instead of on the mixed-liquor flowrates because the overflow rate is equivalent to an upward flow velocity. The return sludge flow is drawn off the bottom of he tank and does not contribute to the upward flow velocity. Selection of a surface overflow rate is influenced by the target effluent requirements and the need to provide consistent process performance The solids loading rate on an activated-sludge settling tank may be computed by dividing the total solids applied by the surface area of the tank The commonly used units for SlR are kilograms per square meter per hour(kg/m"h). If peak flowrates are of short duration, average 24-h values may govern; if peaks are of long duration, peak values should be assumed to govern to prevent the solids from overflowing the tank In effect, the solids loading rate represents a characteristic value for the suspension under consideration. In a settling tank of fixed surface area, the effluent quality will deteriorate if the solids loading is increased beyond the characteristic value for the suspension. Higher rates should not be used for design without extensive experimental work covering all seasons and operating'variables While the surface overflow rate has been the historical clarifier design parameter, the solids loading rate is considered by some to be the limiting parameter that affects the effluent concentration. Parker et al. (2001) have shown that with a proper hydraulic design and management of solids in the sedimentation tank, the overflow rate has little or no effect on the effluent quality over a wide range of overflow rates, and the design can be based on the solids loading rates. Wahlberg(1995) supports Parker's position and, based on the evaluation of secondary clarifier performance for a number of facilities, found no effect of using surface overflow rates up to 3. 4 m/h Use of Selectors. Because solids separation is one of the most important aspects of biological wastewater treatment, a biological selector(a small contact tank) is often incorporated in the design to limit the growth of organisms that do not settle well. Selectors are naturally incorporated into the biological nitrogen- and phosphorus. removal processes described. For BOD removal only or BOD removal and nitrification processes, an appropriate selector desig n be added before the activated-sludge aeration basin Effluent Characteristics. The major parameters of interest that determine effluent quality from biological treatment processes consist of organic compounds, suspended solids, and nutrients as indicated by the following four constituents 1. Soluble biodegradable organics a. Organics that escaped biological treatment 2.. Cellular components(result of cell death or ly)ological degradation of the waste b. Organics formed as intermediate products in the Biomass produced during treatment that escaped separation in the final settling tank b. Colloidal organic solids in the plant influent that escaped treatment and separation a. Contained in biomass in effluent suspended solids b, Soluble nitrogen as NH4-N, NO3-N, N2-0, and organic N c. Soluble orthophosphates 4. Nonbiodegradable organics a. Those originally present in the influent b. Byproducts of biological degradation In a well-operating activated-sludge process treating domestic wastes with an SrT->4 d, the solub carbonaceous BOD of a filtered sample is usually less than 3.0 mg/L. With a proper secondary clarifier design and good settling sludge, the effluent suspended solids may be in the range of 5 to 15 mg/L 7-8
7-8 Many SVI tests at wastewater treatment plants are done in a 2-L settleometer that has a larger diameter than 1- or 2-L graduated cylinders (see Fig. 7-6). To eliminate well effects on solids settling in a small-diameter test apparatus, use of a slow-speed stirring device is encouraged (Wahlberg and Keinath, 1988). The test is called a stirred SVI when a stirring device is used (see Standard Methods, WEF, 1998). The stirred SVI test is used frequently in Europe. Secondary Clarification. Several approaches are used in the design of secondary clarification facilities. The approach used most commonly is to base the design on a consideration of the surface overflow rate and the solids loading rate. Because steady-state operations seldom occur due to fluctuations in wastewater flowrate, return activated-sludge flowrate, and MLSS concentrations, attention to the occurrence of peak events and use of safety factors are important design considerations. Overflow rates are based on wastewater flowrates instead of on the mixed-liquor flowrates because the overflow rate is equivalent to an upward flow velocity. The return sludge flow is drawn off the bottom of the tank and does not contribute to the upward flow velocity. Selection of a surface overflow rate is influenced by the target effluent requirements and the need to provide consistent process performance. The solids loading rate on an activated-sludge settling tank may be computed by dividing the total solids applied by the surface area of the tank. The commonly used units for SLR are kilograms per square meter per hour (kg/m2 .h). If peak flowrates are of short duration, average 24-h values may govern; if peaks are of long duration, peak values should be assumed to govern to prevent the solids from overflowing the tank. In effect, the solids loading rate represents a characteristic value for the suspension under consideration. In a settling tank of fixed surface area, the effluent quality will deteriorate if the solids loading is increased beyond the characteristic value for the suspension. Higher rates should not be used for design without extensive experimental work covering all seasons and operating 'variables. While the surface overflow rate has been the historical clarifier design parameter, the solids loading rate is considered by some to be the limiting parameter that affects the effluent concentration. Parker et al. (2001) have shown that with a proper hydraulic design and management of solids in the sedimentation tank, the overflow rate has little or no effect on the effluent quality over a wide range of overflow rates, and the design can be based on the solids loading rates. Wahlberg (1995) supports Parker's position and, based on the evaluation of secondary clarifier performance for a number of facilities, found no effect of using surface overflow rates up to 3.4 m/h. Use of Selectors. Because solids separation is one of the most important aspects of biological wastewater treatment, a biological selector (a small contact tank) is often incorporated in the design to limit the growth of organisms that do not settle well. Selectors are naturally incorporated into the biological nitrogen- and phosphorus. removal processes described. For BOD removal only or BOD removal and nitrification processes, an appropriate selector design can be added before the activated-sludge aeration basin. Effluent Characteristics. The major parameters of interest that determine effluent quality from biological treatment processes consist of organic compounds, suspended solids, and nutrients as indicated by the following four constituents: 1. Soluble biodegradable organics a. Organics that escaped biological treatment b. Organics formed as intermediate products in the biological degradation of the waste c. Cellular components (result of cell death or lysis) 2. Suspended organic material a. Biomass produced during treatment that escaped separation in the final settling tank b. Colloidal organic solids in the plant influent that escaped treatment and separation 3. Nitrogen and phosphorus a. Contained in biomass in effluent suspended solids b. ,Soluble nitrogen as NH4-N, NO3-N, N2-O, and organic N c. Soluble orthophosphates 4. Nonbiodegradable organics a. Those originally present in the influent b. Byproducts of biological degradation In a well-operating activated-sludge process treating domestic wastes with an SRT -> 4 d, the soluble carbonaceous BOD of a filtered sample is usually less than 3.0 mg/L. With a proper secondary clarifier design and good settling sludge, the effluent suspended solids may be in the range of 5 to 15 mg/L. Fig. 7-6 Field test for determining sludge volume index(SVI)
Process control To maintain high levels of treatment performance with the activated-sludge process under a wide range of operating conditions, special attention must be given to process control. The principal approaches to process control are(1)maintaining dissolved oxygen levels in the aeration tanks, (2)regulating the amount of return activated sludge(RAS), and(3)controlling the waste-activated sludge (WAS). The parameter used most commonly for controlling the activated-sludge process is SrT. The mixed-liquor upended solids(MLSS) concentration may also be used as a control parameter. Return activated sludge is important in maintaining the MLSS concentration and controlling the sludge blanket level in the secondary clarifier. The waste activated-sludge flow from the recycle line is selected usually to maintain the desired SRT. Oxygen uptake rates(OURs) are also measured as a means of monitoring and controlling the activated-sludge process. Routine microscopic observations are important for monitoring the microbial haracteristics and for early detection of changes that might negatively impact sludge settling and process rformance Dissolved Oxygen Control. Theoretically, the amount of oxygen that must be transferred in the aeration tanks equals the amount of oxygen required by the microorganisms in the activated-sludge system oxidize the organic material. In practice, the transfer efficiency of oxygen for gas to liquid is relatively low so that only a small amount of oxygen supplied is used by the microorganisms. When oxygen limits the growth of microorganisms, filamentous organisms may predominate and the settleability and qual ity of the activated sludge may be poor. In general, the dissolved oxygen concentration in the aeration tank should be maintained at about 1.5 to 2 mg/L in all areas of the aeration tank. Higher DO concentrations (2.0 mg/L)may improve nitrification rates in reactors with high BOD loads. Values above 4 mg/L do not improve operations significantly, but increase the aeration costs considerably Return Activated-Sludge Control. The purpose of the return of activated sludge is to maintain a sufficient concentration of activated sludge in the aeration tank so that the required degree of treatment can be obtained in the time interval desired. the return of activated sludge from the final clarifier to the inlet of the aeration tank is the essential feature of the process. Ample return sludge pump capacity should be provided and is important to prevent the loss of sludge solids in the effluent The solids form a sludge blanket in the bottom of the clarifier, which can vary in depth with flow and solids loadings variations to the clarifier. At transient peak flows, less time for sludge thickening is available so that the sludge blanket depth increases. Sufficient return sludge pumping capacity is needed, along with sufficient clarifier deptl (3.7 to 5.5 m), to maintain the blanket below the effluent weirs. Return sludge pumping rates of 50 to 75 percent of the average design wastewater flowrate are typical, and the design average capacity is typically of 100 to 150 percent of the average design flowrate. Return sludge concentrations from secondary clarifiers range typically from 4000 to 12,000 mg/L (WEF, 1998) Several techniques are used to calculate the desirable return sludge flowrate. Common control strategies for determining the return sludge flowrate are based on maintaining either a target MLSS level in the aeration tanks or a given sludge blanket depth in the final clarifiers. The most commonly used techniques to determine return sludge flowrate are(1) settleability, (2) sludge blanket level control, (3) secondary clarifier mass balance, and(4)aeration tank mass balance Settleability. Using the settleability test, the return sludge ate is set so that the flowrate approximately equal to the percentage ratio of the volume by the settleable solids from the aeration tank effluent to the volume of the clarified liquid(supernatant) after sealing for 30 min in a 1000-mL graduated cylinder. This ratio should not be less than 15 percent at any time. For example, if the settleable solids occupied a volume of 275 mL after 30 min of settling, the percentage volume would be equal to 38 percent [(275 mL/725 mL)* 100). If the plant flow were 2 m /s, the return sludge rate should be038×2m/s=0.76m3/s Sludge Blanket Level. With the sludge blanket level control method, an optimum sludge blanket level is maintained in the clarifiers. The optimum level is determined by experience and is a balance between settling depth and sludge storage. The optimum depth of the sludge blanket usually ranges between 0.3 and 0.9 m. The sludge blanket method of control requires considerable operator attention because of the diurnal flow and sludge production variations and changes in the settling characteristics of the sludge Several methods are available to detect the sludge blanket levels, including withdrawing samples using air-lift pumps, gravity-flow tubes, portable sampling pumps, and core samplers, or using sludge-supernatant interface detector
7-9 Process Control To maintain high levels of treatment performance with the activated-sludge process under a wide range of operating conditions, special attention must be given to process control. The principal approaches to process control are (1) maintaining dissolved oxygen levels in the aeration tanks, (2) regulating the amount of return activated sludge (RAS), and (3) controlling the waste-activated sludge (WAS). The parameter used most commonly for controlling the activated-sludge process is SRT. The mixed-liquor suspended solids (MLSS) concentration may also be used as a control parameter. Return activated sludge is important in maintaining the MLSS concentration and controlling the sludge blanket level in the secondary clarifier. The waste activated-sludge flow from the recycle line is selected usually to maintain the desired SRT. Oxygen uptake rates (OURs) are also measured as a means of monitoring and controlling the activated-sludge process. Routine microscopic observations are important for monitoring the microbial characteristics and for early detection of changes that might negatively impact sludge settling and process performance. Dissolved Oxygen Control. Theoretically, the amount of oxygen that must be transferred in the aeration tanks equals the amount of oxygen required by the microorganisms in the activated-sludge system to oxidize the organic material. In practice, the transfer efficiency of oxygen for gas to liquid is relatively low so that only a small amount of oxygen supplied is used by the microorganisms. When oxygen limits the growth of microorganisms, filamentous organisms may predominate and the settleability and quality of the activated sludge may be poor. In general, the dissolved oxygen concentration in the aeration tank should be maintained at about 1.5 to 2 mg/L in all areas of the aeration tank. Higher DO concentrations (>2.0 mg/L) may improve nitrification rates in reactors with high BOD loads. Values above 4 mg/L do not improve operations significantly, but increase the aeration costs considerably. Return Activated-Sludge Control. The purpose of the return of activated sludge is to maintain a sufficient concentration of activated sludge in the aeration tank so that the required degree of treatment can be obtained in the time interval desired. The return of activated sludge from the final clarifier to the inlet of the aeration tank is the essential feature of the process. Ample return sludge pump capacity should be provided and is important to prevent the loss of sludge solids in the effluent. The solids form a sludge blanket in the bottom of the clarifier, which can vary in depth with flow and solids loadings variations to the clarifier. At transient peak flows, less time for sludge thickening is available so that the sludge blanket depth increases. Sufficient return sludge pumping capacity is needed, along with sufficient clarifier depth (3.7 to 5.5 m), to maintain the blanket below the effluent weirs. Return sludge pumping rates of 50 to 75 percent of the average design wastewater flowrate are typical, and the design average capacity is typically of 100 to 150 percent of the average design flowrate. Return sludge concentrations from secondary clarifiers range typically from 4000 to 12,000 mg/L (WEF, 1998). Several techniques are used to calculate the desirable return sludge flowrate. Common control strategies for determining the return sludge flowrate are based on maintaining either a target MLSS level in the aeration tanks or a given sludge blanket depth in the final clarifiers. The most commonly used techniques to determine return sludge flowrate are (1) settleability, (2) sludge blanket level control, (3) secondary clarifier mass balance, and (4) aeration tank mass balance. Settleability. Using the settleability test, the return sludge pumping rate is set so that the flowrate is approximately equal to the percentage ratio of the volume occupied by the settleable solids from the aeration tank effluent to the volume of the clarified liquid (supernatant) after sealing for 30 min in a 1000-mL graduated cylinder. This ratio should not be less than 15 percent at any time. For example, if the settleable solids occupied a volume of 275 mL after 30 min of settling, the percentage volume would be equal to 38 percent [(275 mL / 725 mL) * 100]. If the plant flow were 2 m3 /s, the return sludge rate should be 0.38 × 2 m3 /s = 0.76 m3 /s. Sludge Blanket Level. With the sludge blanket level control method, an optimum sludge blanket level is maintained in the clarifiers. The optimum level is determined by experience and is a balance between settling depth and sludge storage. The optimum depth of the sludge blanket usually ranges between 0.3 and 0.9 m. The sludge blanket method of control requires considerable operator attention because of the diurnal flow and sludge production variations and changes in the settling characteristics of the sludge. Several methods are available to detect the sludge blanket levels, including withdrawing samples using air-lift pumps, gravity-flow tubes, portable sampling pumps, and core samplers, or using sludge-supernatant interface detectors
Mass-Balance lysis. The sludge pumping rate determined by making mass-balance the settling tank or the aeration tank. The Definition sketch for suspended mass balance for retum shdge control: (a)secondary darifier and the two mass-balance on tank analyses are illustrated on Fig. the sludge-blanket level in the settling tank remains constant and that the solids in the effluent from the tting tank are negligible, the mass balance around the settling tank shown on Fig. 7-7a is as follows The required ras pumping rate can also be estimated by performing a mass balance around the aeration tank(see Fig. 7-7b). The solids entering the tank will equal the solids leaving the tank if new cell growth can be considered negligible. Under conditions such as high organic loadings, this assumption may be incorrect. Solids enter the aeration tank in the return sludge and in the influent to the secondary process. Sludge Wasting, To maintain a given SRT, the excess activated sludge produced each day must be wasted. The most common practice is to waste sludge from the return sludge line because rAs is more concentrated and requires smaller waste sludge pumps. The waste sludge can be discharged to the primary sedimentation tanks for co-thickening, to thickening tanks, or to other sludge -thickening facilities. An alternative method of wasting sometimes used is withdrawing mixed liquor directly from the aeration tank or the aeration tank effluent pipe where the concentration of solids is uniform. The waste mixed liquor can then be discharged to a sludge-thickening tank or to the primary sedimentation tanks where it mixes and settles with the untreated primary sludge Oxygen Uptake Rates. Microorganisms in the activated-sludge process use oxygen as they consume the substrate. The rate at which they use oxygen, known as the oxygen uptake rate(OUR), is a measure of the biological activity and loading on the aeration tank. Values for the OUR are obtained by performing a series of DO measurements over a period of time, and the measured results are conventionally reposed as 2/L. min or mg O2/Lh. Oxygen uptake is most valuable for plant operations when combined with VSS data The combination of OUR with MLVsS yields a value termed the specific oxygen uptake rate(SOUR) or respiration rate. The SOUR is a measure of the amount of oxygen used by microorganisms and is reported as mg O2/g MLVSS h. It has been shown that the mixed liquor SOUR and the final effluent COD can be correlated thereby allowing predictions of final effluent quality to be made during transient loading conditions Changes in SOUR values may also be used to assess the presence of toxic or inhibitory substances in the influent wastewater about the condition of the microbial popule opic observations provide valuable monitoring information Microscopic Observations. Routine micros tion in the activated -sludge process. Specific information gathered includes changes in floc size and density the status of filamentous organism growth in the floc the presence of Nocardia bacteria, and the type and abundance of higher life-forms such as protozoans and rotifers. Changes in these characteristics can provide an indication of the changes in the wastewater characteristics or of an operational problem. a decrease in the protozoan population may be indicative of DO limitations, operation at a lower SRT inhibitory substances in the wastewater. Early detection of filamentous or Nocardia growth will allow time for corrective action to be taken to minimize potential problems associated with excessive growth of these organisms. Procedures may be followed to identify the specific type of filamentous organism, which may help identify an opeating or design condition that encourages their growth Jenkins et al., 1993) Operational Problems The most common problems encountered in the operation of an activated-sludge plant are bulking sludge, rising sludge, and Nocardia foam. Because few plants have escaped these problems, it is appropriate to discuss their nature and methods for their control 7-10
7-10 Mass-Balance Analysis. The return sludge pumping rate may also be determined by making a mass-balance analysis around either the settling tank or the aeration tank. The appropriate limits for the two mass-balance analyses are illustrated on Fig. 7-7. Assuming the sludge-blanket level in the settling tank remains constant and that the solids in the effluent from the settling tank are negligible, the mass balance around the settling tank shown on Fig. 7-7a is as follows: The required RAS pumping rate can also be estimated by performing a mass balance around the aeration tank (see Fig. 7-7b). The solids entering the tank will equal the solids leaving the tank if new cell growth can be considered negligible. Under conditions such as high organic loadings, this assumption may be incorrect. Solids enter the aeration tank in the return sludge and in the influent to the secondary process. Sludge Wasting. To maintain a given SRT, the excess activated sludge produced each day must be wasted. The most common practice is to waste sludge from the return sludge line because RAS is more concentrated and requires smaller waste sludge pumps. The waste sludge can be discharged to the primary sedimentation tanks for co-thickening, to thickening tanks, or to other sludge-thickening facilities. An alternative method of wasting sometimes used is withdrawing mixed liquor directly from the aeration tank or the aeration tank effluent pipe where the concentration of solids is uniform. The waste mixed liquor can then be discharged to a sludge-thickening tank or to the primary sedimentation tanks where it mixes and settles with the untreated primary sludge. Oxygen Uptake Rates. Microorganisms in the activated-sludge process use oxygen as they consume the substrate. The rate at which they use oxygen, known as the oxygen uptake rate (OUR), is a measure of the biological activity and loading on the aeration tank. Values for the OUR are obtained by performing a series of DO measurements over a period of time, and the measured results are conventionally reposed as mg O2/L.min or mg O2/L.h. Oxygen uptake is most valuable for plant operations when combined with VSS data. The combination of OUR with MLVSS yields a value termed the specific oxygen uptake rate (SOUR) or respiration rate. The SOUR is a measure of the amount of oxygen used by microorganisms and is reported as mg O2/g MLVSS.h. It has been shown that the mixed liquor SOUR and the final effluent COD can be correlated, thereby allowing predictions of final effluent quality to be made during transient loading conditions. Changes in SOUR values may also be used to assess the presence of toxic or inhibitory substances in the influent wastewater. Microscopic Observations. Routine microscopic observations provide valuable monitoring information about the condition of the microbial population in the activated-sludge process. Specific information gathered includes changes in floc size and density the status of filamentous organism growth in the floc, the presence of Nocardia bacteria, and the type and abundance of higher life-forms such as protozoans and rotifers. Changes in these characteristics can provide an indication of the changes in the wastewater characteristics or of an operational problem. A decrease in the protozoan population may be indicative of DO limitations, operation at a lower SRT inhibitory substances in the wastewater. Early detection of filamentous or Nocardia growth will allow time for corrective action to be taken to minimize potential problems associated with excessive growth of these organisms. Procedures may be followed to identify the specific type of filamentous organism, which may help identify an opeating or design condition that encourages their growth (Jenkins et al., 1993). Operational Problems The most common problems encountered in the operation of an activated-sludge plant are bulking sludge, rising sludge, and Nocardia foam. Because few plants have escaped these problems, it is appropriate to discuss their nature and methods for their control. Fig. 7-7 Definition sketch for suspended mass balance for return sludge control: (a)secondary clarifier and (b)aeration tank