7 Suspended Growth Biological Treatment Processes 7-1 Introduction to the Activated-Sludge Process Historical Development Experiments conducted at the.

7 Suspended Growth Biological Treatment 7 Suspended Growth Biological Treatment ProcessesProcesses

7-1 Introduction to the Activated-Sludge 7-1 Introduction to the Activated-Sludge ProcessProcess

Historical DevelopmentHistorical Development Experiments Experiments conducted at the Lawrence conducted at the Lawrence

Experiment Station during 1912 and 1913 by Experiment Station during 1912 and 1913 by Clark and GageClark and Gage

With air input into wastewater, growths of With air input into wastewater, growths of organisms could be cultivated in bottles and in organisms could be cultivated in bottles and in tanks partially filled with roofing slate spaced tanks partially filled with roofing slate spaced about 25 mm (1 in) apart and would greatly about 25 mm (1 in) apart and would greatly increase the degree of purification.increase the degree of purification.

Lockett found that Lockett found that the sludge played an important the sludge played an important part. part.

The basic activated-sludge treatment process, as The basic activated-sludge treatment process, as illustrated on illustrated on Fig. 7-la and bFig. 7-la and b, consists of the , consists of the following three basic components: following three basic components: (1) a reactor in (1) a reactor in which the microorganisms responsible for which the microorganisms responsible for treatment are kept in suspension and aerated; (2) treatment are kept in suspension and aerated; (2) liquid-solids separation, usually in a sedimentation liquid-solids separation, usually in a sedimentation tank; and (3) a recycle system for returning solids tank; and (3) a recycle system for returning solids removed from the liquid-solids separation unit removed from the liquid-solids separation unit back to the reactor. back to the reactor.

Fig. 7-1Fig. 7-1Description of Basic ProcessDescription of Basic Process

An important feature of the activated-sludge proceAn important feature of the activated-sludge process is the ss is the formation of flocculent settleable solids .formation of flocculent settleable solids .

For these applications, For these applications, various modifications of covarious modifications of conventional activated-sludge processes are used, incnventional activated-sludge processes are used, including sequencing batch reactors, oxidation ditch sluding sequencing batch reactors, oxidation ditch systems, aerated lagoons, or stabilization ponds.ystems, aerated lagoons, or stabilization ponds.

Evolution of the Activated-Sludge ProcessEvolution of the Activated-Sludge Process

A number of activated-sludge A number of activated-sludge processes and design processes and design configurationsconfigurations have evolved since its early conception as a have evolved since its early conception as a result of result of (1) engineering innovation in response to the need (1) engineering innovation in response to the need for higher-quality effluents from wastewater treatment for higher-quality effluents from wastewater treatment plants; (2) technological advances in equipment, plants; (2) technological advances in equipment, electronics, and process control; (3) increased electronics, and process control; (3) increased understanding of microbial processes and fundamentals; understanding of microbial processes and fundamentals; and (4) the continual need to reduce capital and operating and (4) the continual need to reduce capital and operating costs.costs.

The use of a The use of a plug-flow processplug-flow process became problemati became problematic when industrial wastes were introduced because c when industrial wastes were introduced because of the toxic effects of some of the discharges.of the toxic effects of some of the discharges.

With the development of simple inexpensive With the development of simple inexpensive progrprogram logic controllers (PLCs)am logic controllers (PLCs) and the availability of and the availability of level sensorslevel sensors and and automatically operated valvesautomatically operated valves, th, the sequencing batch reactor (e sequencing batch reactor (SBRSBR) process (see Fig. ) process (see Fig. 7-1c) became more widely used by the late 1970s, 7-1c) became more widely used by the late 1970s, especially for smaller communitiesespecially for smaller communities . Some of the s . Some of the stages are not aerated (anaerobic or anoxic stages) atages are not aerated (anaerobic or anoxic stages) and internal recycle flows may be used.nd internal recycle flows may be used.

Recent Process Developments Recent Process Developments

As noted above, numerous modifications of As noted above, numerous modifications of the activated-sludge process have evolved the activated-sludge process have evolved in the last 10 to 20 years, aimed principally in the last 10 to 20 years, aimed principally at at effective and efficient removal of effective and efficient removal of nitrogen and phosphorus.nitrogen and phosphorus.

7-2 Wastewater Characterization7-2 Wastewater Characterization

Activated-sludge process design requires Activated-sludge process design requires determining determining (1) the aeration basin volume, (1) the aeration basin volume, (2) the amount of sludge production, (3) the (2) the amount of sludge production, (3) the amount of oxygen needed, and (4) the amount of oxygen needed, and (4) the effluent concentration of important effluent concentration of important parameters. parameters.

7-3 Fundamentals of Process Analysis and Control7-3 Fundamentals of Process Analysis and Control

Important factors that must be considered Important factors that must be considered in the selection of reactor types for the in the selection of reactor types for the activated-sludge process include activated-sludge process include (1) the (1) the effects of reaction kinetics, (2) oxygen effects of reaction kinetics, (2) oxygen transfer requirements, (3) nature of the transfer requirements, (3) nature of the wastewater, (4) local environmental wastewater, (4) local environmental conditions, (5) presence of toxic or conditions, (5) presence of toxic or inhibitory substances in the influent inhibitory substances in the influent wastewater, (6) costs, and (7) expansion to wastewater, (6) costs, and (7) expansion to meet future treatment needs.meet future treatment needs.

Selection of Reactor TypeSelection of Reactor Type

Selection of Solids Retention Time and Loading Selection of Solids Retention Time and Loading Criteria Criteria

The common parameters used are the solids retention time The common parameters used are the solids retention time ((SRTSRT), the food to biomass (), the food to biomass (F/MF/M) ratio (also known as foo) ratio (also known as food to microorganism ratio), and the volumetric organic loadd to microorganism ratio), and the volumetric organic loading rate(ing rate(LvLv). ).

Solids Retention TimeSolids Retention Time

The SRT, in effect, represents the average period of time dThe SRT, in effect, represents the average period of time during which the sludge has remained in the system. SRT is uring which the sludge has remained in the system. SRT is the most critical parameter for activated-sludge design as the most critical parameter for activated-sludge design as SSRT affects the treatment process performance, aeration tanRT affects the treatment process performance, aeration tank volume, sludge production, and oxygen requirementsk volume, sludge production, and oxygen requirements. .

Sludge ProductionSludge Production

Sludge will accumulate in the activated-sludge proSludge will accumulate in the activated-sludge process if it cannot be processed fast enough by an uncess if it cannot be processed fast enough by an undersized sludge-handling facility. dersized sludge-handling facility.

The observed yield decreases as the SRT is increasThe observed yield decreases as the SRT is increased due to ed due to biomass loss by more endogenous respirbiomass loss by more endogenous respirationation. The yield is higher when no primary treatme. The yield is higher when no primary treatment is used, as more nbVSS remains in the influent nt is used, as more nbVSS remains in the influent wastewater(wastewater(A-BA-B). ).

Oxygen RequirementsOxygen Requirements

As an approximation, for BOD removal only, the oxygen rAs an approximation, for BOD removal only, the oxygen requirement will vary from 0.90 to 1.3 kg Oequirement will vary from 0.90 to 1.3 kg O22/kg BOD remo/kg BOD remo

ved for SRTs of 5 to 20 d .ved for SRTs of 5 to 20 d .

Nutrient Requirements Nutrient Requirements

Using the formula CUsing the formula C55HH77NONO22, for the composition of cell bio, for the composition of cell bio

mass, about 12.4 percent by weight of nitrogen will be reqmass, about 12.4 percent by weight of nitrogen will be required. The phosphorus requirement is usually assumed to buired. The phosphorus requirement is usually assumed to be about one-fifth of the nitrogen value. As a general role, fe about one-fifth of the nitrogen value. As a general role, for SRT values greater than 7 d, about 5 g nitrogen and 1 g or SRT values greater than 7 d, about 5 g nitrogen and 1 g phosphorus will be required per 100 g of BOD to provide aphosphorus will be required per 100 g of BOD to provide an excess of nutrients.n excess of nutrients.

Other Chemical RequirementsOther Chemical Requirements

The amount of The amount of alkalinityalkalinity required for nitrification, taking required for nitrification, taking into account cell growth, is about 7.07 g CaCOinto account cell growth, is about 7.07 g CaCO33/g NH/g NH44-N. -N.

In addition to the alkalinity required for nitrification, In addition to the alkalinity required for nitrification, additional alkalinity must be available to maintain the pH additional alkalinity must be available to maintain the pH in the range from 6.8 to 7.4. Typically the amount of in the range from 6.8 to 7.4. Typically the amount of residual alkalinity required to maintain pH near a neutral residual alkalinity required to maintain pH near a neutral point (i.e., pH ≈ 7) is between 70 and 80 mg/L as CaCOpoint (i.e., pH ≈ 7) is between 70 and 80 mg/L as CaCO33..

Mixed-Liquor Settling CharacteristicsMixed-Liquor Settling Characteristics Clarifier design must provide Clarifier design must provide adequate clarification of the adequate clarification of the

effluent and solids thickening for the activated-sludge solieffluent and solids thickening for the activated-sludge solids. ds.

Two commonly used measures developed to quantify the sTwo commonly used measures developed to quantify the settling characteristics of activated sludge are the sludge volettling characteristics of activated sludge are the sludge volume index (SVI) and the zone settling rate .ume index (SVI) and the zone settling rate .

The The SVISVI is determined by placing a mixed-liquor sample in is determined by placing a mixed-liquor sample in a 1- to 2-L cylinder and measuring the settled volume after a 1- to 2-L cylinder and measuring the settled volume after 30 min and the corresponding sample MLSS concentration.30 min and the corresponding sample MLSS concentration. For example, a mixed-liquor sample with a 3000 mg/L TS For example, a mixed-liquor sample with a 3000 mg/L TSS concentration that settles to a volume of 300 mL in 30 mS 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 valin 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 vue of 100 mL/g is considered a good settling sludge (SVI values below 100 are desired). SVI values above 150 are typalues below 100 are desired). SVI values above 150 are typically associated with ically associated with filamentous growthfilamentous growth..

Use of SelectorsUse of Selectors

Because solids separation is one of the most Because solids separation is one of the most important aspects of biological wastewater important aspects of biological wastewater treatment, treatment, a biological selector (a small contact a biological selector (a small contact tank) is often incorporated in the design to limit tank) is often incorporated in the design to limit the growth of organisms that do not settle well.the growth of organisms that do not settle well. An appropriate selector design can be added An appropriate selector design can be added before the activated-sludge aeration basin. before the activated-sludge aeration basin.

Process ControlProcess Control

To maintain high levels of treatment performance To maintain high levels of treatment performance The principal approaches to process control are (1) The principal approaches to process control are (1) maintaining dissolved oxygen levels in the maintaining dissolved oxygen levels in the aeration tanks, (2) regulating the amount of return aeration tanks, (2) regulating the amount of return activated sludge (RAS), and (3) controlling the activated sludge (RAS), and (3) controlling the waste-activated sludge (WAS).waste-activated sludge (WAS).

The parameter used most commonly for The parameter used most commonly for controlling the activated-sludge process is SRT. controlling the activated-sludge process is SRT.

Dissolved Oxygen Control.Dissolved Oxygen Control.

When oxygen limits the growth of microorganisms,When oxygen limits the growth of microorganisms, filamentous organisms may predominate and the s filamentous organisms may predominate and the settleability and quality of the activated sludge may ettleability and quality of the activated sludge may be poor. be poor.

In general, the dissolved oxygen concentration in tIn general, the dissolved oxygen concentration in the aeration tank should be maintained at about he aeration tank should be maintained at about 1.5 1.5 to 2 mg/L in all areas of the aeration tank. to 2 mg/L in all areas of the aeration tank.

Return Activated-Sludge ControlReturn Activated-Sludge Control

Return sludge concentrations from secondary clarifiers ranReturn sludge concentrations from secondary clarifiers range typically from 4000 to 12,000 mg/L .ge typically from 4000 to 12,000 mg/L .

Settleability Settleability

If the settleable solids occupied a volume of 275 mL after 3If the settleable solids occupied a volume of 275 mL after 30 min of settling, the percentage volume would be equal to 0 min of settling, the percentage volume would be equal to 38 percent [(275 mL / 725 mL) * 100]. If the plant flow we38 percent [(275 mL / 725 mL) * 100]. If the plant flow were 2 mre 2 m33/s, the return sludge rate should be 0.38 × 2 m/s, the return sludge rate should be 0.38 × 2 m33/s = 0./s = 0.76 m76 m33/s./s.

Sludge Blanket Level Sludge Blanket Level

The optimum level is determined by experience The optimum level is determined by experience and is a balance between settling depth and sludge and is a balance between settling depth and sludge storage. The optimum depth of the sludge blanket storage. The optimum depth of the sludge blanket usually ranges between 0.3 and 0.9 m. usually ranges between 0.3 and 0.9 m.

Considerations:Considerations: (1) diurnal flow variations (1) diurnal flow variations (2) sludge production variations(2) sludge production variations (3)changes in the settling characteristics of the (3)changes in the settling characteristics of the

sludge. sludge.

Sludge WastingSludge Wasting

An alternative method of wasting An alternative method of wasting sometimes used is withdrawing mixed sometimes used is withdrawing mixed liquor directly from the aeration tank or the liquor directly from the aeration tank or the aeration tank effluent pipeaeration tank effluent pipe . .

Oxygen Uptake Rates Oxygen Uptake Rates

OUR or respiration rate used to assess the OUR or respiration rate used to assess the presence of toxic or inhibitory substances in presence of toxic or inhibitory substances in the influent wastewater.the influent wastewater.

Microscopic ObservationsMicroscopic Observations

Specific information gathered includesSpecific information gathered includes changes in floc size and density changes in floc size and density status of filamentous organism growth in the flocstatus of filamentous organism growth in the floc presence of presence of NocardiaNocardia bacteria, bacteria, type and abundance of higher life-forms such as protozoatype and abundance of higher life-forms such as protozoa

ns and rotifers. ns and rotifers.

A decrease in the protozoan population may be indicative of A decrease in the protozoan population may be indicative of DO limitations, operation at a lower SRT inhibitory substanDO limitations, operation at a lower SRT inhibitory substances in the wastewaterces in the wastewater. .

Operational Problems Operational Problems

The The most common problemsmost common problems encountered in the op encountered in the operation of an activated-sludge plant are eration of an activated-sludge plant are bulking slubulking sludge, rising sludge, and dge, rising sludge, and NocardiaNocardia foam foam. .

Bulking SludgeBulking Sludge

In a bulking sludge condition, the MLSS In a bulking sludge condition, the MLSS floc does floc does not compact or settle well, and floc particles are dinot compact or settle well, and floc particles are discharged in the clarifier effluent.scharged in the clarifier effluent.

The other type of bulking, viscous bulkinThe other type of bulking, viscous bulking, is caused by an excessive amount of extg, is caused by an excessive amount of extracellular biopolymer .Viscous bulking is racellular biopolymer .Viscous bulking is usually found with nutrient-limited systeusually found with nutrient-limited systems or in a very high loading condition witms or in a very high loading condition with wastewater having a high amount of rbh wastewater having a high amount of rbCOD. COD.

This structure, in contrast to the preferred deThis structure, in contrast to the preferred dense floc with good settling properties, has anse floc with good settling properties, has an increased surface area to mass ratio, whicn increased surface area to mass ratio, which results in poor settling. h results in poor settling.

The classification system is based on The classification system is based on morpmorphology (size and shape of cells, length and shology (size and shape of cells, length and shape of filaments), staining responses, and chape of filaments), staining responses, and cell inclusionsell inclusions..

One of the kinetic features of filamentous organisOne of the kinetic features of filamentous organisms that relates to these conditions is that they arms that relates to these conditions is that they are e very competitivevery competitive at at low substrate concentratiolow substrate concentrations whether it be organic substrates, DO, or nutrins whether it be organic substrates, DO, or nutrients. Thus, lightly loaded complete-mix activateents. Thus, lightly loaded complete-mix activated-sludge systems or low DO (<0.5 mg/L) operatid-sludge systems or low DO (<0.5 mg/L) operating conditions provide an environment more favong conditions provide an environment more favorable to filamentous bacteria than to the desired rable to filamentous bacteria than to the desired floc-forming bacteria.floc-forming bacteria.

When the influent wastewater contains ferWhen the influent wastewater contains fermentation products such as volatile fatty acimentation products such as volatile fatty acids(VFA) and reduced sulfur compounds (suds(VFA) and reduced sulfur compounds (sulfides and thiosulfate), lfides and thiosulfate), ThiothrixThiothrix can prolife can proliferate. rate.

View the mixed liquor under the microscopView the mixed liquor under the microscope. A reasonable quality phase-contrast micre. A reasonable quality phase-contrast microscope with magnification up to 1000 times oscope with magnification up to 1000 times (oil immersion) is necessary to view the fila(oil immersion) is necessary to view the filamentous bacteria or fungi structure and size. mentous bacteria or fungi structure and size.

Process Loading/Reactor ConfigurationProcess Loading/Reactor Configuration

In many cases, complete-mix systems with long SIn many cases, complete-mix systems with long SRTs and subsequent low F/M ratios experience filRTs and subsequent low F/M ratios experience filamentous growths. In such systems, the filamentoamentous growths. In such systems, the filamentous organisms are more competitive for substrate.us organisms are more competitive for substrate.

We can use selector processes to solve these problWe can use selector processes to solve these problems because they ems because they provide conditions that cause selprovide conditions that cause selection of floc-forming bacteria in lieu of filamentoection of floc-forming bacteria in lieu of filamentous organisms as the dominant population.us organisms as the dominant population.

Temporary Control MeasuresTemporary Control Measures

In an emergency situation or while the aforementioned factIn an emergency situation or while the aforementioned factors are being investigated, ors are being investigated, chlorine and hydrogen peroxide chlorine and hydrogen peroxide may be used to provide temporary help.may be used to provide temporary help. Chlorination of ret Chlorination of return sludge has been practiced quite extensively as a means urn sludge has been practiced quite extensively as a means of controlling bulking. of controlling bulking.

Rising Sludge Rising Sludge

The most common cause of this phenomenon is The most common cause of this phenomenon is denitrificatdenitrificationion, in which nitrites and nitrates in the wastewater are con, in which nitrites and nitrates in the wastewater are converted to nitrogen gas. If enough gas is formed, the sludge verted to nitrogen gas. If enough gas is formed, the sludge mass becomes buoyant and rises or floats to the surface. mass becomes buoyant and rises or floats to the surface.

NocardiaNocardia Foam. Foam.

Two bacteria genera, Two bacteria genera, NocardiaNocardia and and Microthrix parvicellaMicrothrix parvicella, , are associated with extensive are associated with extensive foamingfoaming in activated-sludge in activated-sludge processes. These organisms have processes. These organisms have hydrophobic cell surfacehydrophobic cell surfacess and and attach to air bubblesattach to air bubbles, where they , where they stabilize the bubblestabilize the bubbles to cause foams to cause foam. .

NocardiaNocardia has a filamentous structure, and has a filamentous structure, and the the filaments are very short and are contained wifilaments are very short and are contained within the floc particles.thin the floc particles. Microthrix parvicellaMicrothrix parvicella has has thin filamentsthin filaments extending from the floc pa extending from the floc particles. rticles.

The foam is thick, has a brown color, and caThe foam is thick, has a brown color, and can build up in thickness of 0.5 to 1 m, but is mn build up in thickness of 0.5 to 1 m, but is more pronounced with diffused aeration and wore pronounced with diffused aeration and with higher air flowrates.ith higher air flowrates.

Methods that can be used to control Methods that can be used to control NocardiaNocardia include include (1) a(1) avoiding trapping foam in the secondary treatment process, voiding trapping foam in the secondary treatment process, (2) avoiding the recycle of skimmings into the secondary tr(2) avoiding the recycle of skimmings into the secondary treatment process, and (3) using chlorine spray on the surfaceatment process, and (3) using chlorine spray on the surface of the e of the NocardiaNocardia foam. foam.

The addition of a small concentration of cationic polymer hThe addition of a small concentration of cationic polymer has been used with some success for controlling as been used with some success for controlling NocardiaNocardia f foaming .oaming . Reducing the oil and grease content from discharReducing the oil and grease content from discharges to the collection system from restaurants, truck stops, ages to the collection system from restaurants, truck stops, and meatpacking facilities by effective degreasing processes nd meatpacking facilities by effective degreasing processes can help control potential can help control potential NocardiaNocardia problems. problems.

Activated-Sludge Selector ProcessesActivated-Sludge Selector Processes

The high substrate concentration in the selector favThe high substrate concentration in the selector favors the growth of nonfilamentous organisms. ors the growth of nonfilamentous organisms.

A selector is a small tank (20 to 60 min contact tiA selector is a small tank (20 to 60 min contact time) or a series of tanks in which the incoming wasme) or a series of tanks in which the incoming wastewater is mixed with return sludge under aerobic, tewater is mixed with return sludge under aerobic, anoxic, and anaerobic conditions. anoxic, and anaerobic conditions.

The goal in the selector is to have most of the rbCThe goal in the selector is to have most of the rbCOD consumed by the floc-forming bacteria.OD consumed by the floc-forming bacteria.

With biological nutrient-removal processesWith biological nutrient-removal processes(( 脱氮除磷脱氮除磷 ), improved sludge-settling char), improved sludge-settling characteristics, and, in many cases, minimal filaacteristics, and, in many cases, minimal filamentous bacteria growth has been observed. mentous bacteria growth has been observed. The anoxic or anaerobic metabolic conditioThe anoxic or anaerobic metabolic conditions used in these processes favor growth of tns used in these processes favor growth of the floc-forming bacteria.he floc-forming bacteria. The filamentous bThe filamentous bacteria cannot use nitrate or nitrite for an eleacteria cannot use nitrate or nitrite for an electron acceptor. ctron acceptor.

Sequencing Batch Reactor ProcessSequencing Batch Reactor Process

For continuous-flow applications, at For continuous-flow applications, at least two SBR tanks must be least two SBR tanks must be provided so that one tank receives provided so that one tank receives flow while the other completes its flow while the other completes its treatment cycle.treatment cycle.

Sludge Wasting in SBRsSludge Wasting in SBRs

A unique feature of the SBR system is that tA unique feature of the SBR system is that there is no need for a return activated-sludge here is no need for a return activated-sludge (RAS) system.(RAS) system.

Because of the substrate concentration chanBecause of the substrate concentration changes with time, the substrate utilization and oges with time, the substrate utilization and oxygen demand rates change, progressing froxygen demand rates change, progressing from high to low levels. m high to low levels.

Tab. 7-6 Computation approach for the design of a SBRTab. 7-6 Computation approach for the design of a SBR

7-5 Process for Biological Nitrogen Removal7-5 Process for Biological Nitrogen Removal

Nitrogen removal is needed to prevent Nitrogen removal is needed to prevent eutrophicateutrophicationion), or for ), or for groundwater rechargegroundwater recharge or other or other reuse apreuse applicationsplications. .

Nitrogen removal can be either an integral part of tNitrogen removal can be either an integral part of the biological treatment system or an add-on procehe biological treatment system or an add-on process to an existing treatment plant.ss to an existing treatment plant.

Following the discussion of design issues, design Following the discussion of design issues, design examples are provided for examples are provided for (1) the anoxic/aerobic p(1) the anoxic/aerobic process, (2) step-feed anoxic/aerobic process, (3) inrocess, (2) step-feed anoxic/aerobic process, (3) intermittent aeration, (4) a sequencing batch reactor, termittent aeration, (4) a sequencing batch reactor, and (5) postanoxic denitrification with methanol aand (5) postanoxic denitrification with methanol addition. ddition.

First, regions of low DO or zero DO concenFirst, regions of low DO or zero DO concentration may be present within the basin as a tration may be present within the basin as a function of the mixing regime.function of the mixing regime.

Second, activated-sludge floc can contain bSecond, activated-sludge floc can contain both aerobic and anoxic zones.oth aerobic and anoxic zones.

Single-Sludge Simultaneous Nitrification DSingle-Sludge Simultaneous Nitrification Denitrificatlon (SNdN) Processesenitrificatlon (SNdN) Processes

The nitrification and denitrification rates arThe nitrification and denitrification rates are a function of the reaction kinetics, floc size a function of the reaction kinetics, floc size, floc density, floc structure, rbCOD loadine, floc density, floc structure, rbCOD loading, and bulk liquid DO concentration. g, and bulk liquid DO concentration.

Fig. 7-16 Operation of a Nitrox oxidation ditch process using intermittent aeration: (a)aerobic conditions; (b)anoxic conditions; (c)variations in ORP,DO,ammonia and nitrate

7-6 Processes for Biological Phosphorus Removal7-6 Processes for Biological Phosphorus Removal

Barnard (1974) was the first to clarify the need for Barnard (1974) was the first to clarify the need for anaerobic contacting between activated sludge and anaerobic contacting between activated sludge and influent wastewater before aerobic degradationinfluent wastewater before aerobic degradation to to accomplish biological phosphorus removal. accomplish biological phosphorus removal.

Biological Phosphorus-Removal ProcessesBiological Phosphorus-Removal Processes

The main difference between the Phoredox (A/O) The main difference between the Phoredox (A/O) process and the Aprocess and the A22O processes shown on Fig. 7-17O processes shown on Fig. 7-17a and b is that a and b is that nitrification does not occur in the Pnitrification does not occur in the Phoredox (A/O) processhoredox (A/O) process. Low operating SRTs are u. Low operating SRTs are used to prevent the initiation of nitrification. sed to prevent the initiation of nitrification.

Fig. 7-17 Fig. 7-17 Typical biological phosphorus removal processTypical biological phosphorus removal process

Process Design ConsiderationsProcess Design Considerations

The process design considerations for BPRThe process design considerations for BPR(biological phosphorus removal) processes i(biological phosphorus removal) processes include (1) wastewater characteristics, (2) annclude (1) wastewater characteristics, (2) anaerobic contact time, (3) SRT, (4) waste sluaerobic contact time, (3) SRT, (4) waste sludge processing method, and (5) chemical addge processing method, and (5) chemical addition capability.dition capability.

The conversion of rbCOD to VFAs (including acetThe conversion of rbCOD to VFAs (including acetate)occurs quickly through fermentation in the anaate)occurs quickly through fermentation in the anaerobic zone and 7 to 10 mg of acetate results in aberobic zone and 7 to 10 mg of acetate results in about 1.0 mg P removal. The more acetate, the more out 1.0 mg P removal. The more acetate, the more cell growth, and, thus, more phosphorus removal. cell growth, and, thus, more phosphorus removal.

During wet-weather conditions, especially in the During wet-weather conditions, especially in the winter, BPR may be difficult to achieve due to colwinter, BPR may be difficult to achieve due to cold, low strength wastewater that does not readily bed, low strength wastewater that does not readily become anaerobic. come anaerobic.

With continuous VFA addition, the effluent solublWith continuous VFA addition, the effluent soluble phosphorus concentration decreased from 2.5 to e phosphorus concentration decreased from 2.5 to 0.3 mg/L.0.3 mg/L.

Anaerobic Contact TimeAnaerobic Contact Time

Detention times of 0.25 to 1.0 h are adequatDetention times of 0.25 to 1.0 h are adequate for fermentation of rbCOD. Polyhydroxybe for fermentation of rbCOD. Polyhydroxybutyrate(PHB) provides energy for phosphorutyrate(PHB) provides energy for phosphorus uptake and storage. us uptake and storage.

Solids Retention TimeSolids Retention Time

First, First, the final amount of phosphorus removed is pthe final amount of phosphorus removed is proportional to the amount of biological phosphoruroportional to the amount of biological phosphorus-storing bacteria wasteds-storing bacteria wasted. .

Second, at long SRTs the biological phosphorus bSecond, at long SRTs the biological phosphorus bacteria are in a more extended endogenous phase, acteria are in a more extended endogenous phase, which will deplete more of their intracellular storawhich will deplete more of their intracellular storage products. If the intracellular glycogen(ge products. If the intracellular glycogen( 糖元糖元 ) is ) is depleted, less efficient acetate uptake and PHB stodepleted, less efficient acetate uptake and PHB storage will occur in the anaerobic contact zone, thus rage will occur in the anaerobic contact zone, thus making the overall BPR process less efficient. making the overall BPR process less efficient.

Waste Sludge ProcessingWaste Sludge Processing

Phosphorus is released when the bacteria Phosphorus is released when the bacteria that contain stored phosphorus are subject that contain stored phosphorus are subject to anaerobic conditions. to anaerobic conditions.

Anaerobic conditions in thickening and/or Anaerobic conditions in thickening and/or digestion can thus result in the release of digestion can thus result in the release of significant amounts of phosphorus.significant amounts of phosphorus.

Thickening of waste sludge by dissolved air Thickening of waste sludge by dissolved air flotation, gravity belt thickeners, or rotary-flotation, gravity belt thickeners, or rotary-drum thickeners is preferred over gravity drum thickeners is preferred over gravity thickening of waste sludge to minimize thickening of waste sludge to minimize phosphorus release. phosphorus release.

Chemical Addition CapabilityChemical Addition Capability

Phosphorus can be removed biologically to dissolved concPhosphorus can be removed biologically to dissolved concentrations as low as 0.20 to 0.30 mg/L, provided sufficient entrations as low as 0.20 to 0.30 mg/L, provided sufficient rbCOD is available. rbCOD is available.

Where there are insufficient amounts of rbCOD in the influWhere there are insufficient amounts of rbCOD in the influent wastewater, chemical addition is necessary to provide eent wastewater, chemical addition is necessary to provide enough phosphorus removal to meet the effluent discharge cnough phosphorus removal to meet the effluent discharge concentration needed. Alum or iron salts may be used and oncentration needed. Alum or iron salts may be used and may be applied at a number of locations in the liquid streamay be applied at a number of locations in the liquid stream treatment process. m treatment process.

Iron salts may be preferred in some cases over alum salts fIron salts may be preferred in some cases over alum salts for primary treatment applications, because they have the aor primary treatment applications, because they have the additional advantage of removing sulfide to help reduce ododditional advantage of removing sulfide to help reduce odors. rs.

Process ControlProcess Control

Filter backwash recycle flows should be Filter backwash recycle flows should be sent to the aerobic zone instead of the sent to the aerobic zone instead of the anaerobic or anoxic zones. Recycle streams anaerobic or anoxic zones. Recycle streams with significant concentrations of DO and with significant concentrations of DO and nitrate can have an adverse impact on nitrate can have an adverse impact on process performance. process performance.

Effluent Suspended Solids Effluent Suspended Solids The phosphorus content in the mixed-liquor The phosphorus content in the mixed-liquor

solids is greater than that from the solids is greater than that from the conventional activated-sludge process due conventional activated-sludge process due to the biological phosphorus storage.to the biological phosphorus storage.

At 3 to 6 percent phosphorus in the solids, At 3 to 6 percent phosphorus in the solids, the phosphorus contribution in an effluent the phosphorus contribution in an effluent having a TSS concentration of 10 mg/L having a TSS concentration of 10 mg/L would be 0.3 to 0.6 mg/L, values that are would be 0.3 to 0.6 mg/L, values that are significant if the effluent standard is less significant if the effluent standard is less than 1.0 mg P/L. than 1.0 mg P/L. To provide very low To provide very low effluent phosphorus concentrations, effluent effluent phosphorus concentrations, effluent filtration may be required.filtration may be required.

Methods to Improve Phosphorus-Removal Methods to Improve Phosphorus-Removal Efficiency in BPR Systems Efficiency in BPR Systems

1. Provide supplemental acetate by direct 1. Provide supplemental acetate by direct purchase or by primary sludge purchase or by primary sludge fermentation.fermentation.

2. Reduce the process SRT.2. Reduce the process SRT.

3. Add alum or iron salts in primary 3. Add alum or iron salts in primary treatment or for effluent polishing.treatment or for effluent polishing.

4. Reduce the amount of nitrate and/or 4. Reduce the amount of nitrate and/or oxygen entering the anaerobic zone.oxygen entering the anaerobic zone.

Fig. 7-18 Examples of fermentation reactors for producing vFig. 7-18 Examples of fermentation reactors for producing v

olatile fatty acids(VFAs) used for puosphorus removalolatile fatty acids(VFAs) used for puosphorus removal

A deeper depth primary clarifier design has also been proposed A deeper depth primary clarifier design has also been proposed to provide sufficient holding time for the settled primary sludge to provide sufficient holding time for the settled primary sludge for hydrolysis and acid fermentation .for hydrolysis and acid fermentation .

The additional VFA supply to the BPR procThe additional VFA supply to the BPR process decreased the average effluent phosphoress decreased the average effluent phosphorus concentration from about 1.5 to 0.5 mg/L.us concentration from about 1.5 to 0.5 mg/L. Alum was later added before the secondary Alum was later added before the secondary clarifiers at a dosage of about 8 mg/L (as alclarifiers at a dosage of about 8 mg/L (as alum) to further reduce the effluent phosphorum) to further reduce the effluent phosphorus concentration. us concentration.

With both With both prefermentation and alum additiprefermentation and alum additionon, the effluent phosphorus concentration a, the effluent phosphorus concentration averaged less than 0.20 mg/L. veraged less than 0.20 mg/L.

Fig. 7-19 Typical non-Nocardia froth on activated-sludge aeration tank sludge return channels

Wastewater normally contains soap, detergents, and other Wastewater normally contains soap, detergents, and other surfactants that produce foam when the wastewater is surfactants that produce foam when the wastewater is aeratedaerated. .

The wind may lift the froth off the tank surface The wind may lift the froth off the tank surface and blow it about, contaminating whatever it and blow it about, contaminating whatever it touches. The froth, besides being touches. The froth, besides being unsightly, is a unsightly, is a hazard to those working with it because it is very hazard to those working with it because it is very slipperyslippery, even after it collapses. In addition, , even after it collapses. In addition, once once the froth has dried, it is difficult to remove.the froth has dried, it is difficult to remove.

A commonly used system for spiral-roll tanks A commonly used system for spiral-roll tanks consists of a series of consists of a series of spray nozzlesspray nozzles mounted mounted above the surface in areas where the froth collects. above the surface in areas where the froth collects. Screened effluent or clear water is sprayed Screened effluent or clear water is sprayed through these nozzles and physically breaks down through these nozzles and physically breaks down the froth as it forms. Another approach is to meter the froth as it forms. Another approach is to meter a small quantity of a small quantity of antifoaming chemical additiveantifoaming chemical additive into the spray water.into the spray water.

Fig. 7-20 Typical circular sludge collection mechanisms: (a)suction type; (b)spiral type scrapper

Fig. 7-22 Alternative methods of flow splitting: Fig. 7-22 Alternative methods of flow splitting: (a)hydraulic symmetry; (b)flow measurement and feedback (a)hydraulic symmetry; (b)flow measurement and feedback control; (c)hydraulic split with Weirs; (d)inlet feed gate control; (c)hydraulic split with Weirs; (d)inlet feed gate controlcontrol

Fig. 7-23 Fig. 7-23 Energy-dissipating inlet devices used in circular clarifiers: Energy-dissipating inlet devices used in circular clarifiers: (a)schematic of a center column energy-dissipating inlet and flocculation (a)schematic of a center column energy-dissipating inlet and flocculation

feed well; (b)view of an energy-dissipating feed wellfeed well; (b)view of an energy-dissipating feed well

Tank Inlet DesignTank Inlet Design

Poor distribution or jetting of the tank Poor distribution or jetting of the tank influentinfluent can increase the formation of can increase the formation of density currentsdensity currents and and scouring of settled scouring of settled sludgesludge, etc. , etc.

Tank inlets should Tank inlets should dissipate influent dissipate influent energyenergy, distribute the flow , distribute the flow evenlyevenly in in horizontal and vertical directions, mitigate horizontal and vertical directions, mitigate density currents, minimize sludge blanket density currents, minimize sludge blanket disturbance, and promote flocculation. disturbance, and promote flocculation.

Fig. 7-24 Fig. 7-24 Alternative peripheral baffle arrangements: (a)StaAlternative peripheral baffle arrangements: (a)Stamford; (b)unnamed; (c)Mckinney; (d)interior troughmford; (b)unnamed; (c)Mckinney; (d)interior trough

Scum RemovalScum Removal

Typical scum-removal equipment includes beach aTypical scum-removal equipment includes beach and scraper type, rotating pipe-through skimmer, annd scraper type, rotating pipe-through skimmer, and slotted pipes. d slotted pipes.

Scum should not be returned to the plant headworScum should not be returned to the plant headworks because microorganisms responsible for foaminks because microorganisms responsible for foaming (typically g (typically NocardiaNocardia) will be recycled, causing fo) will be recycled, causing foaming problems to persist because of continuous saming problems to persist because of continuous seeding of the unwanted microorganisms. eeding of the unwanted microorganisms.

7-8 Suspended Growth Aerated Lagoons7-8 Suspended Growth Aerated Lagoons

Types of Suspended Growth Aerated LagoonsTypes of Suspended Growth Aerated Lagoons

Classified based on the manner in which the Classified based on the manner in which the solids are handled are:solids are handled are:

1. Facultative partially mixed1. Facultative partially mixed

2. Aerobic flow through with partial mixing2. Aerobic flow through with partial mixing

3. Aerobic with solids recycle and nominal 3. Aerobic with solids recycle and nominal complete mixing complete mixing

Facultative Partially Mixed LagoonFacultative Partially Mixed Lagoon

The energy input is only sufficient to The energy input is only sufficient to transfer the amount of oxygen required for transfer the amount of oxygen required for biological treatment, but is not sufficient to biological treatment, but is not sufficient to maintain the solids in suspension. maintain the solids in suspension.

Because the energy input will not maintain Because the energy input will not maintain the solids in suspension, a portion of the the solids in suspension, a portion of the incoming solids will settle along with a incoming solids will settle along with a portion of the biological solids produced portion of the biological solids produced from the conversion of the soluble organic from the conversion of the soluble organic substrate. substrate.

TemperatureTemperature

Because suspended growth aerobic flow-through Because suspended growth aerobic flow-through lagoons are often installed and operated in lagoons are often installed and operated in locations with locations with widely varying climatic conditionswidely varying climatic conditions, , the effects of temperature change must be the effects of temperature change must be considered in their design. The two most considered in their design. The two most important effects of temperature are important effects of temperature are (1) reduced (1) reduced biological activity and treatment efficiency and (2) biological activity and treatment efficiency and (2) the formation of ice.the formation of ice.

Surface aeratorsSurface aerators tend to further cool lagoons in tend to further cool lagoons in cold weather, but cold weather, but submerged diffused air systemssubmerged diffused air systems add heat to some extent. add heat to some extent.

In multiple lagoon systems, cold-weather In multiple lagoon systems, cold-weather effects can be mitigated by seasonal effects can be mitigated by seasonal changes in the method of operation. During changes in the method of operation. During the warmer months, the lagoons would be the warmer months, the lagoons would be operated in paralleloperated in parallel; in the winter, they ; in the winter, they would be would be operated in seriesoperated in series. In the winter . In the winter operating mode, the operating mode, the downstream aerators downstream aerators could be turned off and removedcould be turned off and removed, and the , and the lagoon surface is allowed to freeze. lagoon surface is allowed to freeze.

With this method of operation it is possWith this method of operation it is possible to achieve a 60 to 70 percent remoible to achieve a 60 to 70 percent removal of BOD even during the coldest wival of BOD even during the coldest winter months. Still another method that cnter months. Still another method that can be used to improve performance duran be used to improve performance during the winter months is to ing the winter months is to recycle a porecycle a portion of the solidsrtion of the solids removed by settling. removed by settling.

Mixing RequirementsMixing Requirements The threshold energy input valueThe threshold energy input value for the sus for the sus

pension of biosolids is about 1.5 to 1.75 kWpension of biosolids is about 1.5 to 1.75 kW/l0/l033 m m33. For depths greater than 3.7m, aerato. For depths greater than 3.7m, aerators with draft tubes may be considered to prers with draft tubes may be considered to prevent solids deposition.vent solids deposition.

The The hydraulic detention timehydraulic detention time for the compl for the complete-mix lagoon will typically vary from 1.5 ete-mix lagoon will typically vary from 1.5 to 3 d. The total retention time for the facultto 3 d. The total retention time for the facultative lagoons is on the order of 3d.ative lagoons is on the order of 3d.

7-9 Biological Treatment with Membrane Separation7-9 Biological Treatment with Membrane Separation

Membrane biological reactors (Membrane biological reactors (MBRMBRs), conss), consisting of a biological reactor (bioreactor witisting of a biological reactor (bioreactor with suspended biomass and solids separation h suspended biomass and solids separation by by microfiltration membranesmicrofiltration membranes with nominal with nominal pore sizes ranging from 0.1 to 0.4 pore sizes ranging from 0.1 to 0.4 μμm, are fim, are finding many applications in wastewater treatnding many applications in wastewater treatment. ment.

The ability of MBR to eliminate secondary clarificThe ability of MBR to eliminate secondary clarification and operate at higher MLSS concentration pration and operate at higher MLSS concentration provides the following ovides the following advantagesadvantages: ( 1 ) higher volu: ( 1 ) higher volumetric loading rates and thus shorter reactor hydrametric loading rates and thus shorter reactor hydraulic retention times; ulic retention times; (2) longer SRTs resulting in l(2) longer SRTs resulting in less sludge production;ess sludge production; (3) operation at low DO co (3) operation at low DO concentrations with potential for simultaneous nitrifincentrations with potential for simultaneous nitrification-denitrification in long SRT designs; cation-denitrification in long SRT designs; (4) hig(4) high-quality effluent in terms of low turbidity bacterih-quality effluent in terms of low turbidity bacteria, TSS, and BOD; anda, TSS, and BOD; and;(5) less space required for ;(5) less space required for wastewater treatment. wastewater treatment.

DisadvantagesDisadvantages of MBRs include high capital costs, of MBRs include high capital costs, limited data on membrane life, potential high cost limited data on membrane life, potential high cost of periodic membrane replacement or scouring, hiof periodic membrane replacement or scouring, higher energy costs, and the need to control membragher energy costs, and the need to control membrane fouling.ne fouling.

Membrane bioreactor systems have Membrane bioreactor systems have two basic conftwo basic configurationsigurations: (1) the : (1) the integrated bioreactorintegrated bioreactor that uses that uses membranes immersed in the bioreactor and (2) the membranes immersed in the bioreactor and (2) the recirculated MBR in which the mixed liquor circulrecirculated MBR in which the mixed liquor circulates through a ates through a membrane module situated outside tmembrane module situated outside the bioreactor.he bioreactor.

The membranes are mounted in modules (sometimThe membranes are mounted in modules (sometimes called cassettes) that can be lowered into the bies called cassettes) that can be lowered into the bioreactor. The modules are comprised of the memboreactor. The modules are comprised of the membranes, support structure for the membranes, feed inranes, support structure for the membranes, feed inlet and outlet connections, and an overall support slet and outlet connections, and an overall support structure. tructure.

The membranes are The membranes are subjected to a vacuumsubjected to a vacuum (less than 50 kPa) that (less than 50 kPa) that draws waterdraws water (permea (permeate) through the membrane while te) through the membrane while retaining sretaining solidsolids in the reactor. To maintain TSS within in the reactor. To maintain TSS within the bioreactor and to clean the exterior of ththe bioreactor and to clean the exterior of the membranes, e membranes, compressed aircompressed air is introduced is introduced through a distribution manifold at the base othrough a distribution manifold at the base of the membrane module. As the air bubbles f the membrane module. As the air bubbles rise to the surface, rise to the surface, scouring of the membranscouring of the membrane surfacee surface occurs; the air also provides oxyge occurs; the air also provides oxygen to maintain aerobic conditions. n to maintain aerobic conditions.

Fig. 7-25 Fig. 7-25 Typical membrane bioreactors: (a)schematic of placement of Typical membrane bioreactors: (a)schematic of placement of membrane bundles in an activated Sludge reactor ; (b)membrane bundle membrane bundles in an activated Sludge reactor ; (b)membrane bundle in position to be placed in a membrane bioreactorin position to be placed in a membrane bioreactor

In the MBR system, In the MBR system, MLSS concentrations in the MLSS concentrations in the range of 8000 to 10,000 mg/Lrange of 8000 to 10,000 mg/L appear to be most appear to be most cost-effective when all factors are considered.cost-effective when all factors are considered.

The membrane The membrane flux rateflux rate, defined as the mass or , defined as the mass or volume rate of transfer through the membrane volume rate of transfer through the membrane surface [surface [in terms of L/min terms of L/m22.h.h] is an important design ] is an important design and operating parameter that affects the process and operating parameter that affects the process economics. economics.

Membrane Fouling ControlMembrane Fouling Control In the activated-sludge reactor, In the activated-sludge reactor, biomass coatsbiomass coats the the

outer layer of the membranes used in an integrated outer layer of the membranes used in an integrated MBR during effluent withdrawal. MBR during effluent withdrawal. Finer particles Finer particles may penetratemay penetrate the inner pores of the membrane, the inner pores of the membrane, causing an increase in pressure loss. causing an increase in pressure loss.

First, coarse bubble aeration is provided at the First, coarse bubble aeration is provided at the bottom of the membrane tank directly below the bottom of the membrane tank directly below the membrane fibers. The air bubbles flow upward membrane fibers. The air bubbles flow upward between the vertically oriented fibers, between the vertically oriented fibers, causing the causing the fibers to agitate against one anotherfibers to agitate against one another to provide to provide mechanical scouring. mechanical scouring.

Second, filtration is interrupted about every 15 to Second, filtration is interrupted about every 15 to 30 min and the membrane fibers are 30 min and the membrane fibers are backwashedbackwashed for 30 to 45 s.for 30 to 45 s.

Typically, a low concentration of Typically, a low concentration of chlorinechlorine (<5 mg/ (<5 mg/L) is maintained in the backflush water to inactivatL) is maintained in the backflush water to inactivate and remove microbes. Third, about three times pe and remove microbes. Third, about three times per week a strong er week a strong sodium hypochloritesodium hypochlorite solution (ab solution (about 100 mg/L) or citric acid is used in the backflusout 100 mg/L) or citric acid is used in the backflush mode for 45 min . h mode for 45 min .

The cassettes can be removed easily from the aeratThe cassettes can be removed easily from the aeration basin by an overhead hoist system for a periodion basin by an overhead hoist system for a periodic chemical-bath cleaning.ic chemical-bath cleaning.

When removed for cleaning, the cassettes are subWhen removed for cleaning, the cassettes are submerged in a high-concentration chlorine solution bmerged in a high-concentration chlorine solution bath in a separate small tank or basin. ath in a separate small tank or basin. External cleaExternal cleaningning occurs about every 3 to 6 months. occurs about every 3 to 6 months.

The combination of air scour, backflushing, and mThe combination of air scour, backflushing, and maintenance cleaning is aintenance cleaning is not completely effectivenot completely effective in in controlling membrane fouling, and the pressure drcontrolling membrane fouling, and the pressure drop across the membrane increases with time. At a op across the membrane increases with time. At a maximum operating pressure drop of ≈60 kPa, the maximum operating pressure drop of ≈60 kPa, the membranes are removed from the aeration basin fmembranes are removed from the aeration basin for a recovery cleaning .or a recovery cleaning .