Env.Eng.Dep.-Operation of Treatment Plant

1. Operation of Treatment Plant A.SUNA ERSES YAY Assist.Prof.Dr erses@sakarya.edu.tr Project-Homework E-mail : wastetreamentsakarya@gmail.com Sakarya University, EngineeringFaculty, EnvironmentalEngineeringDepartment, Esentepe Campus, 54187 SAKARYA Env.Eng.Dep.-Operation of Treatment Plant

2. ACTIVATED SLUDGE ProcessandEquipmentDescription The activated-sludge process is a suspended-growth process, predominantly aerobic, thatmaintains a high microorganism population (biomass) by means of solids recycling fromthe secondary clarifier. The biomass converts biodegradable organic matter and certaininorganic compounds into new cell biomass and products of metabolism. Biomass is separatedfrom the treated wastewater in the clarifier for recycling or wasting to solidshandlingprocesses. Env.Eng.Dep.-Operation of Treatment Plant

3. Env.Eng.Dep.-Operation of TreatmentPlant

4. Intheconventionalflowsheet, influent wastewaterand recycled biomass are first combined, mixed, and aerated in a biological reactor.The contents of the biological reactor is referred to as mixed liquor and consists ofmicroorganisms; and biodegradable and nonbiodegradable suspended, colloidal, andsoluble organic and inorganic matter. Particulate matter is referred to as mixed liquorsuspended solids (MLSS) and the organic fraction is called mixed liquor volatile suspendedsolids (MLVSS). Microorganisms consist primarily of organic matter (70 to 80%) and are often measuredas MLVSS, although it must be emphasized that a fraction of the MLVSS is inertorganic matter including organisms that are no longer viable (living and actively metabolizing). Env.Eng.Dep.-Operation of TreatmentPlant

5. Wastewater components are biodegraded, sorbed, or remain untreated (recalcitrantor nondegradable) in the biological reactor. After sufficient time for appropriate biochemicalreactions, mixed liquor is transferred to a settling reactor (clarifier) to allowgravity separation of the MLSS from the treated wastewater. Settled solids are then returned(return activated sludge [RAS]) to the biological reactor to maintain a concentratedbiomass for wastewater treatment. Because microorganisms are continuously synthesizedin the process, some of the MLSS must be wasted from the system. Wasting isaccomplished by diverting a portion of the RAS or biological reactor solids (waste activatedsludge [WAS]) to solids-handling processes. Sludge wasting strategies are used toincrease, decrease, or maintain a selected biomass concentration in the system. This is aprincipal mechanism used for process control. Env.Eng.Dep.-Operation of TreatmentPlant

6. The basic activated-sludge system consists of a number of interrelated components • • A single biological reactor or multiple reactors designed for completely mixed • flow, plug flow, or intermediate patterns of flow designed to achieve carbonaceousorganic matter removal. If required, these may also provide ammonia oxidation,nitrogen removal, and phosphorus removal depending on target effluentrequirements. The primary biological reactors may be preceded by anaerated, anoxic, or anaerobic selector reactor designed to control bulking, denitrify,or select polyphosphate uptake microorganisms to promote enhanced phosphorusremoval. • An oxygen source and equipment to disperse atmospheric, pressurized, or oxygen-enriched air into the biological reactors at a rate sufficient to maintain a positivemixed liquor dissolved oxygen (DO) concentration. • • A means to appropriately mix the biological reactor contents to ensure suspensionof the MLSS without shearing the floc. • • A clarifier to separate, and possibly thicken, the MLSS from the treated wastewater. • • A method of collecting settled MLSS within the clarifier and returning it to the • biologicalreactors. • • A means of wasting MLSS from the system Env.Eng.Dep.-Operation of TreatmentPlant

7. Activated-sludge system designs are based on • the hydraulic retention time (HRT)in the biological reactor, • the amount of time biomass is retained within the system(mean cell residence time [MCRT]), • the organic loading, and • the organic load (food) tobiomass (microorganism) ratio (F:M). Env.Eng.Dep.-Operation of TreatmentPlant

8. MICROBIOLOGY AND BIOCHEMISTRY. The activated-sludge process consistsof a mixture of flocculated bacteria, fungi, protozoa, and rotifers maintained in suspensionby aeration and mechanical mixing. The primary branches of biology that arerelevant to designers and operators of activated-sludge processes are the naming andclassification of organisms (taxonomy), their metabolic activities (physiology), and theirinterrelationships with the surrounding environment (ecology). The functioning and activity of organisms (physiology) is important in determiningthe role of the consortium of organisms in the process. Classification is often based on energysource, cell carbon source, and requirement for oxygen. Organisms that use sunlightas their primary source of energy are called phototrophsand all others, using chemicals as a source of energy, are chemotrophs. Organisms that use inorganic carbon (carbondioxide, CO2) are referred to as autotrophsand those using organic carbon are heterotrophs. Env.Eng.Dep.-Operation of Treatment Plant

9. The qualitative biochemical reaction that occurs in the process may be expressed as Food + Nutrients + Organisms + Electron acceptor → New organism + products More specifically, for a chemoheterotrophic aerobic reaction that would be common inactivated sludge systems, the general (unbalanced) expression might be Organicmatter + Oxygen (O2) + Nutrients + Microbes → New microbes + CO2 + Water (H2O) For a chemoautotrophic aerobic reaction occurring in nitrifying systems, the overallexpression, which consists of a two-step process, might be Ammonium (NH4+) + O2 + CO2 + Biocarbonate (HCO3–) + Microbes → New microbes + H2O + Nitrate (NO3–) Finally, a chemoheterotrophic anoxic reaction that might take place in the clarifieror in a dedicated anoxic zone of the biological reactor once nitrate is present might beexpressed as NO3 – + Organic matter + Carbonic acid (H2CO3) +Microbes → New microbes + Nitrogen (N2) + H2O + HCO3– Env.Eng.Dep.-Operation of Treatment Plant

10. When the final electron acceptor is oxygen, the reactionis aerobic. If the final acceptor does not involve an external electron acceptor such as oxygen, the metabolism is referred to as fermentative and the organism carrying outthe reaction may be referred to as an anaerobe. Organisms may also use external, inorganicelectron acceptors; the most notable being those that use nitrates and sulfates.These reactions are anaerobic, but are often referred to as anoxic. Thus, organisms maybe classified as aerobic or anaerobic depending on the final electron acceptor. Many organismsmay be active in both the presence and absence of oxygen (facultative). Env.Eng.Dep.-Operation of TreatmentPlant

11. The reactions shown are the result of many metabolicsteps but consist of two general metabolic processes—synthesis and respiration.Synthesis is an energy-consuming reaction and results in the production of new biomass.Respiration is an energy-yielding reaction that is linked to synthesis by an important series of energy transfers Aerobicreactionsare the most efficient, resulting in high biomass yields and low-energy productsthat are highly stabilized. Anaerobic reactions are the opposite, producing low biomassyields and poorly stabilized products. Anoxic reactions fall in between, depending ontheacceptor. Env.Eng.Dep.-Operation of TreatmentPlant

12. Knowledge of the organisms present in the process and the nature of the biochemicalreactions that are occurring is important, but the interrelationships betweenorganisms and factors controlling growth and activities of the consortia (ecology) isneeded to understand system operation. The process is an open system, creating a dynamicenvironment of microorganisms. The mix of organisms found in the process isselected by the environmental conditions produced in the system and by the interactionsamong organisms. This selection, or enrichment, results in a rigorous culturethat may change rapidly as conditions change because of the rapid rates of growth ofmicroorganisms. Among the important enrichment factors found in the process arethe operational conditions such as residence time, settling, and recycling that promotebiomass separation; characteristics of the wastewater including carbon/nitrogen/phosphorus ratios and toxicity; environmental conditions including pH, temperature,DO concentration, and mixing intensity; and reactor configurations that may affect nutrient and DO concentrations or compositions. Env.Eng.Dep.-Operation of TreatmentPlant

13. Through design and operation, treatment objectives may be achieved by this fundamentalconcept of enrichment. However, not all observed enrichment reactions areconsciously selected. Bulking, or poor settling, is an example of an undesirable selectionof an organism population that continues to challenge operators. Many types ofpoor separation problems may arise. These include dispersed growth (no flocculationor deflocculation), pin floc, bulking, rising sludge blanket, and foaming or scum formation Env.Eng.Dep.-Operation of Treatment Plant

14. BASIC PROCESS GOALS The activated-sludge process may be designed and operated to remove carbonaceousbiochemical oxygen demand (CBOD), to oxidize ammonia to nitrates, to remove nitrogencompounds, or to remove phosphorus. Env.Eng.Dep.-Operation of Treatment Plant

15. CARBONACEOUS BIOCHEMICAL OXYGEN DEMAND REMOVAL. The CBOD represents all carbon-based organic matter in the wastewater that isbiodegradable, measured as BOD. It is important to note that the five-day BOD (BOD5)represents only a fraction of the biodegradable carbonaceous organic components inthe wastewater (typically, 60 to 65%). The BOD consists of both soluble (dissolved) andparticulate fractions. The soluble fraction is often consumed rapidly once in contactwith the MLSS. The particulate fraction may sorb rapidly to the biomass and degradeat a rate that depends on its composition. The biological reactor sizing depends on boththe rate of BOD uptake and the rate of its degradation. High biomass concentrationsare desirable and can result in smaller reactor sizes. However, there is an upper limit tobiomass concentration that can be achieved in a given plant based on the oxygen-transfercapacity of the system and the size of the clarifiers. Systems are occasionally designedwith oxygen-enriched air processes and oversized clarifiers to reduce biologicalreactor size, but cost considerations often dictate an optimum size of all components ofthesystem. With a typical municipal wastewater, a well-designed and operated activatedsludgesystem should achieve a CBOD effluent quality of 5 to 15 mg/L. Effluent suspendedsolids (SS) should also typically be less than 15 mg/L. Note that effluent SSmay include a significant fraction of CBOD. To achieve consistent BOD and total suspendedsolids (TSS) concentrations less than 5 mg/L, some type of tertiary treatmentwould be required. Env.Eng.Dep.-Operation of Treatment Plant

16. NITRIFICATION. The oxidation of ammonia to nitrate is primarily carried out byautotrophic bacteria in a two-step process.The ammonia-oxidizingbacteria obtains its energy by oxidizing ammonia to nitrite and the nitrite-oxidizingbacteria by oxidizing nitrite to nitrate. These reactions produce little energy; therefore,populations of nitrifiers in activated sludge are small (recall that energy is required tosynthesize biomass, in this case from inorganic carbon). It is for this reason that for nitrificationto occur the activated-sludge process must be designed and operated athigher SRTs and longer detention times to ensure that nitrifiers do not wash out of thesystem. Nitrification processes may be designed as a combined system where bothCBOD removal and ammonia oxidation can take place or in two-stage systems whereCBOD removal is achieved in the first stage and nitrification is achieved in the second. The oxygen demand for complete nitrification is high. For typical municipalwastewater facilities, nitrification will increase the required oxygenation facilities by30 to 40% of that required for CBOD removal. Nitrification will require approximately 4.6 mg oxygen/mg ammonia-nitrogenoxidized. The DO concentrations in mixedliquor affect the rate of nitrification. Nitrification rates decrease with decreases in DO.Typically, the DO concentration should range from 2 to 3 mg/L for good nitrification performance, although a minimum DO of 0.5 mg/L is acceptable under peak loadingconditions. Env.Eng.Dep.-Operation of Treatment Plant

17. Optimum growth of nitrifiers has been observed in the pH range of 6.5 to 8.0 althougheffective nitrification has been reported for systems outside this range. Duringnitrification, mineral acidity is produced (NO3–). If insufficient alkalinity is present, thesystem pH will drop and nitrification will slow. Approximately 7.1 mg of calcium carbonate(CaCO3) alkalinity is consumed per milligram of ammonia-nitrogen oxidized.A residual alkalinity of 50 to 100 mg CaCO3/L is recommended for stable operation.Supplemental alkalinity may be provided through chemical additions of lime, sodaash, ormagnesiumhydroxide. While nitrification occurs over a wide range of temperatures, a reduction in temperaturewill decrease the rate of reaction. As a result, in colder climates, MCRTs areraised to accommodate the lower nitrification rates. In warmer climates, nitrificationhas been observed at MCRT values of 3 days or less, whereas in colder climates MCRTvalues greater than 20 days may be required to achieve effective nitrification. Env.Eng.Dep.-Operation of Treatment Plant

18. NITROGEN REMOVAL—DENITRIFICATION. Denitrification is a one-stepbiological process that reduces nitrate-nitrogen to nitrogen gases. Thegases of nitrogen (N2, and other nitrogen oxides) produced in the process will be releasedfrom solution, thereby causing a reduction in system nitrogen. A number of microorganismscan affect this reaction, all readily present in most municipal wastewater.These organisms are heterotrophic, requiring organic matter for growth. They are capableof using oxygen or nitrate as their terminal electron acceptor and thermodynamicconsiderations favor oxygen. Therefore, the process of denitrification must take place inthe absence of DO and in the presence of organic matter. This is typically provided inthe activated-sludge process through the design of an anoxic zone in the biological reactorsystem. Organic carbon, such as methanol, may be added as a supplemental carbonsource, or may be provided by influent wastewater CBOD from a side stream. It shouldbe noted that denitrification may take place inadvertently in the anoxic sludge-settlingzone of the clarifier (sometimes causing rising sludge) or in the sludge return (RAS)channels. Denitrification of municipal wastewater requires that ammonia first be oxidizedto nitrate. Municipal wastewater nitrification–denitrification systems using activatedsludge are numerous. They may be single-sludge systems that incorporate CBODremoval, ammonia oxidation, and nitrate reduction in a number of steps using oneclarification process; or they may be phased systems (separate stage) with individualsludges for each of the processes. Env.Eng.Dep.-Operation of Treatment Plant

19. Denitrification processes are temperature sensitive, decreasing with decreasedtemperature. Adjustment in system MCRT may be required in colder climates to ensureadequate denitrification. The process generates alkalinity, 3.6 mg CaCO3 alkalinity/mg nitrate-nitrogen reduced. Because denitrification may reduce total process oxygenrequirements, some credit for this may be taken. Theoretically, 2.86 kg oxygendemand is satisfied per kilogram (2.86 lb/lb) of nitrate-nitrogen reduced to nitrogengas Env.Eng.Dep.-Operation of Treatment Plant

20. BIOLOGICAL PHOSPHORUS REMOVAL. In the conventional activatedsludgeprocess for municipal wastewater, the biomass will uptake phosphorus forgrowth and metabolism in a way that approximately 2% of the biological sludge masson a dry weight basis is phosphorus. Phosphorus cannot be transformed to a volatilegas, therefore its removal is achieved by sludge wasting. Thus, wasting in a conventionalplant may result in 10 to 30% removal of phosphorus. The activated-sludgeprocess may be managed, however, to select for a population of microorganisms thatwill store excessive quantities of phosphorus, in the range of 3 to 6%. Wasting of thisphosphorus-enriched sludge can result in effluent phosphorus concentrations less than 1 mg P/L. The selection process involves an anaerobic step that results in the release ofstored phosphate followed by an aerobic step in which the organisms consume largeamounts of phosphorus. In the anaerobic phase, soluble CBOD is consumed by theorganisms and stored as organic polymers as a future source of energy. The energyrequired for this storage step is provided by excess phosphorus stored as polyphosphatesin the aerobic stage. As energy is released in the anaerobic phase, the phosphatesare released to solution. Once entering the aerobic zone, energy is produced bythe oxidation of the stored organic carbon products and polyphosphate storage is initiatedbytheorganism Env.Eng.Dep.-Operation of Treatment Plant

21. Biological phosphorus removal (BPR) processes for activated-sludge systems will bedescribed below. With proper design and operation, a BPR system should produce effluentphosphorus concentrations less than 2 mg/L and often less than 1 mg/L. Becausephosphorus in the cell mass is high, careful attention must be paid to achieving low concentrationsof volatile suspended solids (VSS) in BPR system effluents Env.Eng.Dep.-Operation of Treatment Plant

22. DESCRIPTION OF FACILITIES AND EQUIPMENT USED BIOLOGICAL REACTORS. The biological reactor is the heart of the process. Airor oxygen is introduced to the aerobic zones both to provide DO for the biomass and tokeep the MLSS properly mixed throughout the reactor. The tanks are properly sized toprovide sufficient HRT for oxidation of the CBOD (and ammonia if nitrifying is a requirement)in the incoming wastewater, typically 6 to 8 hours for conventional systems,and to ensure proper flocculation of the microorganisms. Depending on the requirementsfor effluent quality, the reactor may be subdivided or compartmentalizedto achieve specific biochemical reactions (anoxic and anaerobic zones). Biological reactors are often constructed of reinforced concrete, although somepackage plants (extended air) may use steel. The reactors are typically rectangular toaccommodate common-wall construction for multiple reactors, although some installationsmay choose to use circular or oval tanks (the oxidation ditch is one example). Each reactor must be furnished with inlet and outlet gates or valves sothey can be removed from service. Proper drains or sumps should be provided forrapid dewatering (approximately 8 to 20 hours). In addition to aeration equipment, thereactors should be equipped with a froth-control system to control foaming Env.Eng.Dep.-Operation of Treatment Plant

23. AERATION SYSTEMS. The supply of oxygen to the biological reactor represents thelargest single energy consumer in the activated-sludge facility (50 to 90%). Over the years,oxygen-transfer equipment has evolved to a point where engineers have a wide selectionof efficient equipment to meet the needs of all types of facilities. Oxygen-transfer devicesare used not only to supply oxygen to the process but also to mix the aerobic compartmentsof the reactor. Typically, there are two types of aeration devices, diffusedaerationsystemsandmechanical-aerationsystems Diffused Aeration. Diffused aeration is defined as the injection of air or oxygen belowthe liquid surface. The air or oxygen is supplied by low-head blowers with pressurestypically up to 210 kPa absolute (30 psia) or 105 kPa gauge (15 psig). Diffusersare placed near the floor of the biological reactor and may be configured in a gridarrangement, along one or both longitudinal sides. Air is delivered in a piping systemfrom the blowers to downcomers that carry the air down to headers along the bottom of thereactor. Env.Eng.Dep.-Operation of Treatment Plant

24. AIR DELIVERY. The three basic components of the air-delivery system are air filtersor conditioners, blowers, and piping. Air filters remove particulates such as dust fromthe inlet air to the blowers and protect both the blowers and diffusers from mechanicaldamage or clogging. The degree of air cleaning depends on the inlet air quality, thetype of blower, and the diffusers. Mechanical Aeration. Mechanical-aeration systems include surface aeration devicesand submerged turbine aerators. Surface aerators can be grouped into four generalcategories • Radialflow, low-speedaerators, • Axialflow, high-speedaerators, • Aspiratingdevices, and • Horizontalrotors. Mixing. In the activated-sludge process mixing is important for maintaining the MLSSin suspension. In most applications, the aerators serve as both oxygen-transfer devicesandmixers. For mixing anoxic and anaerobic compartments of biological reactors, both submergedpropeller or turbine mixers have been used Env.Eng.Dep.-Operation of Treatment Plant

25. CLARIFICATION. Separation of MLSS from the liquid stream is vital to the operationand performance of activated-sludge systems. This is typically achieved by gravityseparators, although recently some work has been done with membranes. Clarificationnot only separates the MLSS but in some situations may be designed to thicken the settledsludge before returning it to the aeration process or to wasting (WAS). Clarifier shapes include rectangular, square, circular, and others, such as hexagonalor octagonal. There seems to be no observable difference in performance at averageor peak flow because of shape alone. Typically, minimum clarifier depths of 3.7 to 4.6 m (12 to 15 ft) are recommended. Deeperclarifiers may be desired for large-diameter clarifiers, but no deeper than 4.6 to 5.0 (15to 16 ft). Clarifier performance also depends on solids loading rate, surface overflowrate, inlet design, effluent weir arrangement, and settling characteristics of the MLSS. Env.Eng.Dep.-Operation of Treatment Plant

26. RETURN AND WASTE ACTIVATED SLUDGE SYSTEMS. The RAS systempumps the settled sludge, thickened as much as practical in the clarifier, from the clarifierback to the biological reactors. Most RAS stations use centrifugal pumps for this application. All activated-sludge processes must have a WAS system to remove excess biomassfrom the system. Waste sludge may be wasted from the clarifier under flow or directlyfromthebiologicalreactors. Env.Eng.Dep.-Operation of Treatment Plant

27. FACTORS AFFECTING PROCESS EFFICIENCY The factors affecting process efficiency may be categorized as environmental, designandoperational, andmaintenance. Environmental factors that affect performance include wastewater characteristics,system DO, temperature, and pH. Influent wastewater characteristics that affect theprocess include the nature of the carbonaceous organic matter and nitrogenous compounds,their biodegradability, soluble and particulate fractions, and concentration; theconcentration and availability of nutrients; the alkalinity; the flow rate and its temporalvariability; and the presence of toxic or inhibitory compounds.Design factors that affect process efficiency and operational control include reactorvolume (affecting HRT), clarifier sizing, pumping capacities of WAS and RAS systems,recycle pumping capacity, hydraulic design, and aeration system size and configuration.The operational factors include MCRT or F:M loading, DO control, RASpattern and flow, WAS rate, and recycle rates. Good performance also depends on appropriateequipment maintenance, laboratory quality control, proper sampling protocol,and adequate training of wastewater treatment plant staff Env.Eng.Dep.-Operation of Treatment Plant

28. LOADING RATES. The activated-sludge process is often classified on the basis ofloading rate. The loading rate may be expressed as a volumetric loading rate, MCRT, orF:M. Table shows a typical range of loading rates for conventional, high-rate, andlow-rate (often called extended aeration) systems. Within these three loading ranges,the reactor configuration, number of reactors, and aeration and feed patterns can be selectedto achieve the target treatment level. Hydraulically, they can be designed as eithera continuous-flow process or a batch-flow system Env.Eng.Dep.-Operation of Treatment Plant

29. Conventional systems provide BOD5 removal efficiencies of 85 to 95% and typicallycarry MLSS concentrations varying from 1000 to 3000 mg/L. At conventionalloading, some nitrification may occur, especially in warmer climates. This results inhigher than estimated oxygen demands and may cause sludge flotation in the finalclarifiers. Nitrification can be limited by decreasing the MCRT (increasing F:M). Low-rate systems are typically used for low flows and are characterized by highoxygen requirements and low sludge production rates. These systems are claimed tobe more stable and thus require less operational attention. Typically, BOD removalefficiencies range from 75 to 95% and nitrification is complete. However, these systemsmay suffer from significant excursions of effluent suspended solids because of poorflocculation (pinpointfloc) andclarifierdenitrification. High-rate systems are often used as pretreatment processes in staged biologicaltreatment systems and are also used where only carbonaceous BOD removal is required.They are characterized by low oxygen requirements and somewhat higher thannormal sludge generation compared to conventional plants. The process may produceBOD removal efficiencies over a wide range, from less than 50% to as high as 95% dependingon loading rate and waste characteristic. Sludge settling can be a problem athigh loading when flocculation does not effectively occur. Env.Eng.Dep.-Operation of Treatment Plant

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