BIOLOGICAL WASTEWATER TREATMENT PRINCIPLES GENERAL OVERVIEW
BIOLOGICAL WASTEWATER TREATMENT PRINCIPLESGENERAL OVERVIEWNatural receiving waters including rivers, streams, and tidal areas sustain abackground population of microorganisms including bacteria, fungi, and algae. Theseorganisms require energy for respiration and organic carbon food sources in order tosynthesize new cells using the following general equation:ORGANICMORECARBONMICROBES FOOD NUTRIENTS OXYGEN MICROBES DIOXIDEIn the above equation, the microbes occur naturally and consume organic foodsources that are naturally present in the water. Some of the carbon matter in the foodis biodegraded to release energy that drives the reaction. Energy is released throughthe biodegradation process by combining part of the organic food source's carboncompounds with oxygen. The byproducts of this reaction are additional microbialcells and carbon dioxide. Nutrients, primarily nitrogen and phosphorus, complete thereaction and are needed as part of the building blocks used to form new microbial celltissue.In a clean water environment, the amount of available organic food supplies arelimited. This places a restriction on how fast the above reaction proceeds and also onhow many microbes are able to grow. In general, the population of microbes in aclean water environment is restricted to a low background level by the limited amountof organic food that is present. While periodic fluctuations in the food supply maycause brief periods of population swings or declines, the number of microbes in theecosystem will achieve an equilibrium condition over time that is based on the steadystate amount of organic food present.If an artificial organic food supply is allowed to enter the ecosystem, the number ofmicrobes that can be sustained will increase in response to the added food. If theamount of additional food is large, the population of microbes may increaseexponentially and will continue to grow up to the point that the amount of food againbecomes limiting for the elevated population.Raw sewage contains a high organic carbon content that provides an excellent supplyof food for waterborne microbes. If raw sewage is discharged into a receiving water,the bacteria population will become elevated in response to the new addition of food.This causes the above biodegradation reaction to proceed rapidly and, in the process,28
to create larger amounts of new microbes and carbon dioxide. This requires that moreoxygen be available for use by the microbes. If sufficient extra food is added by thesewage discharge, the population of microorganisms and the resulting oxygenconsumption may proceed so rapidly that all of the receiving water's oxygen isdepleted.The maximum amount of dissolved oxygen present in a receiving water is a functionof temperature, atmospheric pressure, elevation, the solids content of the water, andsalinity. In any case, the saturation value of dissolved oxygen that is present isrelatively small as shown below in Table 8. At sea level and 0 C, the maximumamount of dissolved oxygen that can be saturated into solution is 14.6 mg/l. Thisvalue decreases to only 7.6 mg/l at 30 C. For this reason, there is less dissolvedoxygen available in the summer when water temperatures are warmer than during thecold winter months. Unfortunately, high water temperatures will also stimulatemicrobial activity which will cause biodegradation reaction rates to increase andoxygen depletion to occur faster. This makes summertime the most critical period formaintaining dissolved oxygen conditions in receiving waters. Table 8 also shows theeffect that salinity has on dissolved oxygen levels. As a receiving water becomeshigher in chlorides, less oxygen can be dissolved into the water.TABLE 8:SATURATION VALUES OF DISSOLVED OXYGEN IN WATERTEMPERATURE(0 C)051015202530DISSOLVED OXYGENIN FRESH WATER(mg/l)14.612.811.310.29.28.47.6DISSOLVED OXYGENIN SALT WATER(mg/l)13.011.410.19.18.37.66.9If free dissolved oxygen is present, the ecosystem is considered to be aerobic. Ifexcess raw sewage is discharged to a receiving water, the available food supply mayresult in a large microbial population that fully depletes all of the dissolved oxygen.This results in the system becoming anoxic or anaerobic. Since most fish and aquaticspecies require a minimum dissolved oxygen level of at least 5.0 mg/l to survive, thedepletion of all the dissolved oxygen is a serious environmental concern. Septicconditions also present a variety of other environmental problems including odor29
generation, acidic compound formation and pH drops, lethal gas generation, andexplosive environments.The origin of these septic system issues can be reviewed by considering the datashown on Table 9. Microbes in the ecosystem can use other oxidizing compoundsbesides oxygen in the biodegradation reaction. Other suitable oxidizing agents includenitrate, sulfates, and carbon dioxide. Bacteria prefer to use oxygen because moreenergy is released than if other compounds are used. This energy allows the bacteriato degrade the food supply more completely and at a faster rate. The carbon dioxidethat is released as a byproduct is natural to the environment and innocuous. Should allof the oxygen be depleted, other types of microbes will take over the system and useother compounds to degrade the organic matter. These alternative reactions result inless energy being released which slows down the treatment reaction rate or results inless complete treatment in the same reaction time. As shown in Table 9, some of theby-products produced by these alternative reactions are less desirable than the carbondioxide produced when oxygen is used:.TABLE 9:ORGANIC BIODEGRADATION REACTION PRODUCTSTYPE OFSYSTEMOXIDIZINGCOMPOUNDUNIT ENERGYRELEASED(Kcal/mole)BYPRODUCTSFORMEDISSUES WITHBYPRODUCTSAerobicOxygen25.3Carbon dioxideNoneAnoxicNitrate23.7Nitrogen gasRising solidsAnaerobicSulfate1.5Hydrogen sulfideOdorous,Corrosive,and ToxicAnaerobicCarbon Dioxide0.9MethaneOdorous,Explosive,and ToxicOxygen is the oxidizing agent of choice in microbial biodegradation because it resultsin high energy yields and harmless byproducts. Nitrate results in nearly the sameenergy yield, but produces nitrogen gas that can float solids in receiving waters ortreatment systems. Sulfur or carbon dioxide compounds can be used to biodegradeorganic matter under septic conditions; however, extremely low energy yields resultand hydrogen sulfide or methane gas is produced. These gases are odorous, corrosive,explosive, and toxic. They contribute to acid formation and pH reductions as well asunsafe environmental conditions. Bacteria in the ecosystem will always use oxygen30
first if it is available and, in doing so, will avoid the types of adverse byproductsshown in Table 9. Should all of the oxygen be depleted, the ecosystem will continueto biodegrade the organic food supply by converting to an anoxic or anaerobicenvironment. In these cases, adverse environmental effects will be created.The discussion on microbes up to this point has focussed on naturally occurringbacteria and other microorganisms that are simply biodegrading organic compounds.It is important to note that raw sewage discharges into a receiving water also presentadditional problems from harmful human enteric microbes, called pathogens, that canspread waterborne diseases to humans. The wastewater discharge from a communitywill contain a representative sampling of all diseases that exist within the generalpopulation of sewer users. The presence of these diseases is usually assessed bymeasuring the amount of E. coli or fecal coliform bacteria that are present in the rawsewage. These organisms serve as indicators of upstream human waste contamination.If the indicator organisms are present, it can be assumed that harmful disease causingpathogens are also present. Given the possibility of downstream human contact orshellfish contamination, the presence of pathogens in a raw sewage dischargerepresents a serious environmental health concern.As previously discussed, wastewater discharge licenses limit the amount of pollutantsthat can be discharged into the environment. Maximum discharge limits areestablished for total suspended solids (TSS), biochemical oxygen demand (BOD) andE. coli. The purpose of these limits is to accomplish the following objectives: Limit the amount of available organic matter that is discharged to thereceiving water to avoid overstimulating the growth of microorganisms inthe environment. Limit the amount of dissolved oxygen that the discharged organic matterwill deplete in the environment as it is biodegraded by naturally occurringbacteria. Disinfect the wastewater discharge to protect human health by reducing thenumber of pathogens in the water.Wastewater collection systems are designed to convey raw sewage to a centrallocation for treatment. Wastewater treatment plants are designed to process the rawsewage prior to its discharge to reduce its organic content, oxygen demand, andpathogenic content. This is accomplished by utilizing treatment processes that removethe waterborne pollutants directly (primary treatment) or that convert them intobacterial cells (secondary treatment). With biological treatment processes, the same31
organisms that occur naturally in the environment are grown under controlledconditions in the treatment plant. They are allowed to eat the organic portion of theincoming sewage which leaves clean water behind. Clean water is then dischargedinto the environment and the plant's excess cells, called sludges, are disposed of in anenvironmentally acceptable manner. By the time that the treated effluent isdischarged, it has lost its ability to serve as a food supply for the receiving water'smicrobes.A wastewater treatment plant consists of a series of unit processes that receivepolluted raw sewage directly from the sewer system and progressively clean it to apoint that it can be safely discharged to a receiving water. Figure 3 shows the typicalprogression of unit processes in a wastewater treatment plant. Raw sewage first entersthe headworks of the plant and is treated in several preliminary treatment processesthat remove debris and make the water easier to treat in subsequent downstreamprocesses. Primary treatment follows preliminary treatment and includessedimentation processes that allow the water to be held under quiescent conditions.Settleable pollutants fall to the bottom of the primary treatment reactors and formsludges. The clarified primary effluent then flows into secondary treatment wheremicrobes are grown to biodegrade non-settleable organic pollutants. Effluent from thesecondary treatment system is further treated in a disinfection process to removepathogens. Sludge residuals that form in each unit process must also be treated andproperly discharged of at licensed sludge processing facilities.PRELIMINARY TREATMENT PROCESSESPreliminary treatment systems encompass all unit processes in the headworks of atreatment plant prior to primary treatment. The purpose of these processes is to refinethe incoming wastewater’s characteristics to make the water more conducive totreatment in downstream processes. Preliminary treatment is also designed to removeundesirable pollutant constituents and debris from the influent to prevent it frominterfering with downstream treatment systems and to protect subsequent equipmentfrom damage. Typical preliminary treatment processes include screening, shredding,grit removal, equalization, and pH neutralization. Screening and shredding processeseither remove or refine incoming wastewater solids to achieve a uniform particle sizewhich can be more efficiently handled by downstream treatment systems. Poorscreening or shredding may lead to plugging problems in downstream processes,pumps, or piping. Grit removal systems remove abrasive substances from thewastewater such as sand and gravel. Inadequate grit removal can lead to excessivepump and impeller wear, equipment abrasion, pipe deterioration, and loss of availabletreatment tank volumes. Equalization is used to reduce the variability of erratic wasteloads by providing upstream storage of influent flows. Effective equalization candampen influent flow, TSS, and BOD fluctuations, minimize pH and temperature32
Figure 333
variability’s impacts on the treatment plant, and result in controlled loadings to thedownstream processes. Where influent loadings exhibit a high degree of hydraulic orpollutant variability, inadequate equalization can result in unstable downstreamperformance, particularly in biological treatment systems. Neutralization processesare used to chemically alter the influent pH by the addition of acid or alkalinecompounds and is required when the pH of wastewater is highly variable or outside arequired regulatory or process range. Inadequate neutralization can result in pHswings in downstream processes effecting their efficiency and performance. Inaddition to these systems, preliminary treatment processes also include other typicalheadworks functions such as flow measurement and wastewater sampling equipment.These systems are used to monitor the incoming waste’s characteristics for regulatoryand process control purposes.While preliminary treatment processes are conceptually simple in function andoperation, the important role that these processes play in overall treatment plantoptimization is often ignored. Since the sole purpose of preliminary treatment is tomake the wastewater easier to treat, marginal process performance of existingpreliminary systems, or the omission of critical preliminary treatment processes froma plant’s headworks, almost always results in a loss of downstream process efficiencyand stability.Preliminary treatment processes are relatively simple to operate. Key factors inkeeping typical preliminary treatment equipment operating at maximum efficiencyinclude: Grit systems should be adjusted in response to changes in the incoming flowrate. Higher wet weather flows will tend to produce more grit than lower dryweather flows. The aeration rate into an aerated grit chamber should beadjusted in response to changing flow rates and grit production amounts. Oncesettled, grit should be removed from the system frequently to prevent it frombecoming compacted or septic. Grinder equipment should be kept in a well maintained condition. Cutter andshredder blades should be kept sharp. Flow metering equipment should be frequently checked and calibrated to makesure that it is providing accurate readings. Sampling equipment should be properly maintained, frequently calibrated, andcleaned often to prevent the fouling of sample tubing that can lead tocontaminated, false samples and test results which are not representative.34
All debris removed from the headworks area, such as gravel, sand, screenings,and other materials should be disposed of frequently to prevent odors andnuisance conditions from forming in the plant.PRIMARY TREATMENT PROCESSESThe purpose of all wastewater treatment plant processes is to separate solids from rawwastewater such that the clarified flow stream may be discharged into a receivingwater with minimal adverse environmental impacts. Wastewater solids occur in avariety of forms including discrete large particles, smaller suspended solids asmeasured by the TSS test, and non-settleable colloidal or dissolved solids. Differenttreatment plant unit processes are designed to target a specific category of solids forremoval. The preliminary treatment processes previously discussed remove discrete,large solids such as debris and grit. Non-settleable, biodegradable colloidal or solublesolids are removed in secondary treatment processes, such as Brewer’s activatedsludge system, by allowing these materials to be biologically converted into microbialcells, than subsequently settled and removed in the final clarifiers.Primary treatment represents an intermediate process step between preliminary andsecondary treatment in which solids of sufficient density settle by gravity under thequiescent conditions provided in primary clarifiers. By providing a settling tank withreduced velocities, a significant portion of the waste’s influent TSS, normally fifty toseventy percent, will settle under the influence of gravity to become raw primarysludge. Since the removal of organic solids reduces the organic content of thewastewater for later bacterial biodegradation, primary treatment also reduces the BODof the influent, often by twenty-five to forty percent.A significant advantage of removing solids in a primary clarifier instead of indownstream secondary processes is that gravity separation is far less expensive thanbiological removal. In a primary clarifier, the only mechanical systems utilized arescrapers for removing settled sludge or floating solids. Secondary treatment systemsgenerally require aeration equipment, sometimes chemical feed systems, andsubstantially more process control monitoring. Solids removed as primary sludge aremuch easier to dewater than the waterlogged microbial cells that constitute secondarysludges. However, primary sludges have the potential of creating more nuisanceconditions and odors that secondary sludges since they represent raw wastewatersolids with a high organic content and without prior biological stabilization.Unlike more complex secondary treatment systems, primary treatment processes haveonly limited adjustments that can be manipulated by the operators. Once the capitalinfrastructure for the primary treatment system is in-place, the operator’s only processcontrol option is to alter the rate of sludge removal from the bottom of the clarifier. It35
is important to remove the settled sludges in a timely manner to minimize theformation of septic sludges and odorous gases.The solids removal efficiency of a primary clarifier can be measured by the followingequation:If the influent solids concentration is compared to the effluent solids concentration,efficiency of the reactor can be calculated. The efficiency of a primary clarifier iseffected by several factors including: The amount of water applied to the clarifier’s surface in gallons per day persquare foot of area. This is referred to as the clarifier’s surface overflow rateor hydraulic loading rate. As the rate of flow to the clarifier is increased, itsefficiency will be reduced. The surface overflow rate can be calculated asfollows:SOR QAwhere SOR clarifier surface overflow rate in GPD/SF (gallons perday per square foot).Q flow applied to the clarifiers in gallons per day (GPD).A area of clarifier surface on-line in square feet (SF). The length of time that the wastewater remains in the clarifier underquiescent conditions will impact its treatment performance. If the water isheld too long, it will become septic and some of the settled raw sludge mayrise again due to the formation of gas bubbles from anaerobic conditions inthe sludge blanket. If the detention time of the clarifier is too short, thepollutants will not have sufficient time to settle and will be washed throughthe primary treatment process. The detention time of a primary clarifier canbe calculated from the following equation:36
where θ detention time of the clarifier in hoursV volume of the clarifier in MG (million gallons).Q flow through the clarifier in MGD (million gallons perday). The rate at which sludge is removed from the clarifier will also impact itsefficiency. The operator must operate the raw sludge pumps in a manner thatseeks an equilibrium point at which enough holding time is maintained tothicken the settled sludge while not making the holding time so long thatseptic conditions develop. Typically, a one hour holding time is consideredsufficient for primary sludge. The sludge pump should be operated eithercontinuously or on a timer that is activated at least every hour. It isimportant to note that deep sludge blankets are subject to washout dur
that remove debris and make the water easier to treat in subsequent downstream processes. Primary treatment follows preliminary treatment and includes sedimentation processes that allow the water to be held under quiescent conditions. Settleable pollutants fall to the bottom of the primary treatment reactors and form sludges.
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water characteristics, the impact of the discharge into rivers and lakes, the design of several wastewater treatment processes and the design of the sludge treatment and disposal units. The series is comprised by the following books, namely: (1) Wastewater characteristics, treatment and disposal; (2) Basic principles of wastewater treat-
Principal Notation xv List of Acronyms and Abbreviations xvii 1 What is Domestic Wastewater and Why Treat It? 1 Origin and composition of domestic wastewater 1 Characterization of domestic wastewater 2 Wastewater collection 5 Why treat wastewater? 5 Investment in wastewater treatment 6 2 Excreta-related Diseases 8
Introduction to Wastewater Treatment Bruce J. Lesikar Professor Texas AgriLife Extension Service Overview ¾What is wastewater? ¾Why are we concerned about wastewater? ¾The big picture. ¾Goals for wastewater treatment are evolving ¾How do we implement our infrastructure? ¾Wastewater Treatment Processes - The end result is based upon your design
4 Wastewater Treatment ANNUAL REPORT 2020 Wastewater Treatment Process 1. INFLUENT PUMP STATION Wastewater from the serviced area in Thunder Bay enters the Water Pollution Control Plant at the Influent Pump Station (IPS) where five pumps are available to deliver the wastewater to the preliminary treatment process. The wastewater then flows by .
Wastewater treatment plants : wastewater resource recovery facilities ? NITROGEN and PHOSPHOROUS The process is distinguished by the fact that municipal sewage sludge from wastewater treatment plants with simultaneous phosphate elimination with iron salts could be used without any changes in the process of wastewater treatment.
biological wastewater treatment process. That is, the microorganisms that carry out the treatment are attached to a solid medium, as in trickling filter or RBC systems. By contrast, in a suspended growth biological wastewater treatment process, like the activated sludge process, the microorganisms that carry out the treatment are kept
Secondary treatment is the second stage in wastewater treatment systems in which bacteria consume the organic material in wastewater. Secondary treatment processes can remove up to 90 percent of the organic matter in wastewater using biological processes. The most common conventional methods to achieve secondary treatment are “attached growth .
