DIFFUSED AERATION DESIGN GUIDE

1 DIFFUSED AERATIONDESIGN GUIDEINTRODUCTIONT hose involved in the DESIGN of DIFFUSED AERATION equipmentfor wastewater treatment should understand the impact thatprocess type, maintenance issues and economicconsiderations can have on the selection of equipment. Likemany other engineering challenges, these factors arefrequently interrelated and trade-offs of one aspect versusanother are required for most application. This DESIGN guidepresents information that has been obtained and developedfrom a variety of sources. Some of this information hasbeen developed from actual test data, some is condensedfrom other published sources, some is based on goodengineering judgement and practical field experience. Theinformation, formulas, values and methods, etc. should beviewed as a DESIGN aid and may not be applicable in allsituations. The designer should always use goodprofessional engineering judgement for every following sections of this DESIGN GUIDE will brieflydiscuss the activated sludge process and biological treatmentoxygen demands.

2 The GUIDE provides a rational step-by-stepprocedure to convert actual oxygen requirements (AOR) tostandard oxygen requirements (SOR). It illustrates how toperform many of the oxygen transfer calculations includingapproximating aerator sizing and selection. Final equipmentsizing and configurations should be referred to the factoryfor must be provided in biological treatment systems tosatisfy several types of demands. These are referred to asactual oxygen requirements or AOR. AOR is alwaysexpressed as field conditions . Each wastewater treatmentplant has its own unique field conditions that include siteelevation, temperature, working DO level, diffusersubmergence and alpha and beta factors. The designermust use these factors to convert AOR to standard oxygenrequirements (SOR) to properly apply the aerationequipment and determine the amount of process airrequired to satisfy the biological treatment oxygen units of expression for AOR and SOR are pounds ofoxygen per day per unit volume.

3 SOR values will always belarger than AOR values. Confusion and misunderstandingcan be minimized between designer and equipment supplierif the designer expresses his desired oxygen demands interms of SOR values. If this is not possible, then clearlyidentify the oxygen demand as an AOR value and provide asmuch information as possible for the equipment supplier toassist you in making the appropriate AOR/SOR AERATION equipment manufacturers can provideinformation to engineers and designers on the oxygentransfer capability of particular equipment andconfigurations when the equipment is aerating clear tapwater. These tests, when corrected for temperature andelevation to standard conditions, become the basis fordetermining the equipment s standard oxygen requirementor SOR. Equipment manufacturers cannot guarantee theoxygen transfer capability of AERATION equipment inwastewater. Each wastewater treatment plant has its ownunique field conditions and waste type that preclude thistype of manufacturers can show engineers and designersa rational method to convert AOR to SOR and can offeradvice on the probable values used in the AOR to SORconversion.

4 However, it is the engineer s responsibility todetermine the AOR of a particular system or process andselect the appropriate conversion factors to relate AOR toSOR. Specifying an SOR value is the best way to preventconfusion and problems in the SLUDGE AND BIOLOGICAL TREATMENTA ctivated sludge AERATION tanks are the largest applicationsfor DIFFUSED AERATION equipment. These tanks and theassociated air diffusion equipment are the heart of theactivated sludge process and typically are the single largestenergy user associated with plant operations. Energy costsfor AERATION will typically be 50% to 90% of all energyconsumed at a wastewater treatment must be provided in biological wastewatertreatment systems to satisfy several types of demands. Onedemand is that associated with the oxidation of organic orcarbonaceous materials. Carbonaceous oxygen demand isassociated with two cellular functions: cell synthesis andendogenous respiration.

5 Cell synthesis carbonaceous oxygendemand occurs when organic matter is first metabolized bythe microorganisms contained in the mixed liquor. It isrelated to the oxygen required to oxidize a portion of theorganic matter to provide the energy necessary for cellsynthesis. Endogenous respiration carbonaceous oxygendemand occurs as the synthesized organisms are retained inthe treatment system and it represents the essential lifeprocesses. The net result is that increasing amounts ofoxygen are required as lower process organic loadings areused. Lower process organic loadings are characterized byoperation at a longer solids retention time (SRT) and a lowerfood-to-microorganism (F:M) is also required for biological oxidation of ammonianitrogen to nitrate nitrogen. If the process is designed andoperated in a nitrification mode, the oxygen demand due tonitrification must be included in the calculation of oxygenrequirements for the system.

6 However, nitrification may alsooccur in systems where only carbonaceous BOD removal isrequired. When the wastewater is warm, say 20 C (68 F)or above, it may not be possible to operate the treatmentsystem at a high enough loading or short SRT to preventnitrification from occurring. Under these circumstances,oxygen transfer capacity to meet this additional demandmust be provided, although not required by is also required to oxidize inorganic materials in theinfluent wastewater. A good example is hydrogen sulfide,which is oxidized chemically when brought in contact withdissolved oxygen in the biological reactor. Reactions of thistype can be quite rapid and can proceed to completeoxidation according to well-established oxygen requirements will be reduced ifdenitrification occurs in the biological treatment can occur under controlled conditions if thesystem is specifically designed with an anoxic zone fornitrogen removal.

7 Designers seldom use the potentialoxygen requirement reduction of credit when calculatingall oxygen requirements, but rather assign this luxury oxygenas an additional safety factor in the overall ACTUAL OXYGEN REQUIREMENTS -AORA number of approaches have been used to estimate theoxygen requirements caused by the biochemical oxidation oforganic matter. Many regulatory agencies specify oxygendesign criteria for various unit processes and somerequirements are probably based on various empirical orrule-of-thumb techniques. More sophisticated approachesto estimating the oxygen demand in AERATION systems maybe obtained from various computerized process , most process models either require certainvalues that need to be experimentally determined for theparticular waste or else must rely on past experience andjudgement in selecting the model values. One rationalapproach to determine the oxygen requirements in thebiological process is to total the oxygen demand due to thesources described below.

8 The summation of all of thesecontributions must be considered in the sizing of theaeration BOD LOADINGF igure 1 from the WPCF MOP 8 shows the relationshipbetween SRT and pounds of oxygen required per pound ofBOD removed at various temperatures for domesticwastewater in an activated sludge system. For typical SRTsof 5 to 10 days, the pounds of oxygen per pound of BODremoved varies, from to A value of pound ofoxygen per pound of BOD removed is commonly used. Onoccasion, some designers use a more conservative value pounds of oxygen per pound of BOD removed. Inprocesses with long detention times (more than 18 hours)and low organic loadings where excess sludge is alsooxidized in the AERATION tanks, a higher value is where higher values are justified are extendedaeration and oxidation ditches. In these cases, to pounds of oxygen per pound of BOD removedor higher is AMMONIA LOADINGThe oxidation of one pound of ammonia requires to of oxygen.

9 Typical domestic wastewater contains25-30 mg/l of ammonia. Do not underestimate the oxygendemand to oxidize the ammonia. Oxidizing 25 mg/l ofammonia is equivalent to an additional 115 mg/l of BODloading. Be award that even if a plant is not specificallydesigned to nitrify, that under favorable loading,temperature and SRT conditions, nitrification can and willoccur. This may exert a large unanticipated oxygen demandon the system and may result in process failure. Figure 2from the WPCF MOP 8 shows the relationship between SRTand nitrification SIDE STREAM LOADINGGood engineering DESIGN should analyze and account forside stream loads that are eventually returned to theaeration tanks. Generally, the most significant side streamloads will come from sludge handling or processingoperations. Although the flows may be small, the BOD maybe very high and result in significant oxygen demand appliedto the AERATION system.

10 Some sources of side stream loadingare: Septage receiving stations Filter press or vacuum filter filtrate and spray washwater Centrifuge centrate Effluent from dissolved air flotation thickeners Effluent from gravity thickeners Supernatant from aerobic or anaerobic digesters Filtrate from sand drying bed underdrains Wash water from grit dewatering screws Water from venturi scrubbers or cyclones Cooking liquors from thermal sludge processingoperations Effluent from scum or grease processing equipmentFIGURE 1 General Carbonaceous Oxidation Response of Domestic Wastewater in an Activated Sludge 1020304050 SRT (days)Carbonaceous Oxidation (O2 / CBOD5-load)30 engineering DESIGN of an AERATION system shouldinclude a rational determination of AOR taking into accountBOD loading, ammonia loading, possible nitrificationconditions and side stream loading. Further analysis of theoxygen demand and how it occurs spatially in time mayresult in minimum, average and peak values of , summer and winter operating conditions canalso be different.