Optimization of Nitrification

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    OPTIMIZATION OF NITRIFICATION I DENITRIFICATION PROCESSPERFORMANCE AND RELIABILITY AT THE BLUE PLAINS ADVANCEDWASTEWATER TREATMENT PLANTJanice Ruhl Carroll*, Paul Pitt!, Andre van Niekerk!, Allen SehlofF,Walter Bailel, Sudhir Murthl, Salil Kharkar3, Aklile Tesfaye3

    *Hazen and Sawyer, P.C., 11242 Waples Mil Road, Suite 250, Fairfax, VA 22030! Hazen and Sawyer, 2 Brown and Caldwell, 3 DC Water and Sewer AuthorityABSTRACTIn the Chesapeake Bay region, the focus on nutrient removal has heightened, to the extent thatplanning for future enhanced nutrient removal (ENR) with permit limits of 3 to 5 mg/l totalnitrogen (TN) is underway. Until such time as ENR upgrades can be made, W ASA has made acommitment to continuously meet the TN goals currently promulgated by the Chesapeake BayAgreement (CBA), as detailed in Table 1, and is working to improve nutrient removal withintheir 370 milion gallon per day (mgd) Blue Plains Advanced Wastewater Treatment Plant(A WTP).Table 1 - Current and Future CBA TN Goals:

    CBA Goals Annual AverageCurrent: Total Nitrogen (mg/L) 7.5Future: Total Nitrogen (mg/L) 3 - 5

    To continuously meet the CBA goals, and to prepare for future ENR requirements, optimizationof the nitrification/denitrification (N/DN) process to improve the performance and reliability isrequired. To improve both process performance and reliability, specific design features wereincorporated into the current N/DN upgrade project. This paper highlights ten process objectivesthat were identified to improve process performance and reliability and details the designfeatures incorporated to satisfy the process objectives. Process objectives include: providingequal flow distribution to process units, optimizing settling, optimizing wet weather operations,selectively removing foam, providing flexible aerobic and anoxic mass fractions, improvingmixing and aeration while reducing operating costs, improving control of the supplementalcarbon feed, and providing for future multi-stage denitrification.Design features incorporated to satisfy these process objectives include: positive flow splittingfor N/DN reactors and sedimentation basins, retrofits to existing mixers for anoxic and swingzones, conversion to a serpentine flow path with step feed, flexible aerobic/anoxic massfractions, a fine bubble aeration system, foam wasting stations, automated methanol feedcontrols, and provisions for a future return activated sludge (RAS) denitrification (methanol feedto RAS line and future mixer).

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    KEYWORDSNitrification, Denitrification, Nutrient Removal, Step Feed, Flow SplittingINTRODUCTIONIn the 1970's, before nutrient removal was widely employed, Blue Plains was designed as anadvanced wastewater treatment plant with separate, suspended growth systems planned forcarbonaceous BOD removal, for nitrification, and for denitrification. The nitrification facilitieswere placed on-line in the late 1970s/early 1980s; however, the denitrification-specific facilitieswere never built. In the early 1990s, denitrification was piloted at Blue Plains and from 1996 to1998 the Denitrification Demonstration Project, which included full-scale denitrification utilizingsupplemental carbon feed (methanol) for half ofthe design flow, was conducted. Based on theresults of the Denitrification Demonstration Project, additional methanol facilities were addedand full scale denitrification was brought on-line in 2000.In anticipation of more stringent, futue effuent limits for nitrogen, DC W ASA is curentlyupgrading its nitrification facilities to improve process performance and reliability within theexisting structures. This paper presents relevant information on the existing facilities, andidentifies the process objectives and design features incorporated to optimize processperformance and reliability.EXISTING NITRIFICATION FACILITIESIn order to allow for a meaningful presentation of the design features included to optimize theperformance and reliability of the N/DN process, knowledge ofthe existing facilities is required.The N/DN facilities curently in use include (Refer to Figure 1 for a plan view of the existingfacilities ):. Nitrification/Denitrification (N/DN) Reactors - Twelve reactors, arranged in parallelbanks of six (even side and odd side). Each reactor is 30 feet deep, and has a volume of4.6 milion gallons, distributed evenly amongst five stages. Flow moves through thereactors in an over/under flow pattern, as shown in Figure 2. Aeration is provided viafive 4,000 Hp three-stage, horizontally split centrifugal blowers and two sparged turbinesper stage. Air is cross-fed to the reactors, with one air header providing the air for onestage across six adjacent, parallel reactors.

    Methanol is added at the end of Stage 3, and Stages 4 and 5 (or Stage 4 only, as aminimum) are operated in an anoxic mode to enable denitrification. The mixed liquoreffluent channels are aerated.

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    .~ MIXED LIQUOR. EFLUENT CHANNEL

    MIXEDLIQUOREFLUENT .CHANNEL -'

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    -- METHANOL FEEDBUILDING

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    NOTE:ITALICIZED TEXT INDICATESSTRIICTVRES VNDERGROVND.

    FIGURE iPLAN VIEW OF EXISTING FACILITIES

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    FOR NITRIFICATIONIDENITRIFICATION MODENORMALLY ANOXIC

    r~==- NORMALLY AEROBICSECONDARY EFFLUENT

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    Fi ure 2 : Existin Nitrification/Denitrification Reactor Confi uration

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    WIT iie 2005

    Nitrification Sedimentation Basins - 28 rectangular sedimentation basins, arranged inparallel banks of 14. Each sedimentation basin has four inlet slide gates. The totalsurface area of the Nitrification Sedimentation Basins is 12.4 acres (540,000 square feet),with overflow rates in the range of 720 - 1,650 gpdlft2. Available sedimentation basinsalso include eight Dual Purpose Sedimentation Basins (DPSBs), which can be used toaugment Secondary Sedimentation or Nitrification Sedimentation Basins. Typically, fourDPSBs are used in nitrification service, which results in overflow rates in the range of610 - 1,400 gpdlft2. Sludge collection is provided via chain and flight scrapers and 42horizontal centrifugal pumps, which return sludge to the N/DN Reactors. Activatedsludge is wasted via four horizontal centrifugal pumps.

    Methanol Feed Facilties - duty and stand-by hose pumps for each reactor (24 total),seven storage tanks (four 8,700 gallon underground tanks, three 10,000 gallon above-ground tanks), day tank (1,650 gallons), transfer pumps and truck fill station. Twotrucks, containing approximately 5,000 gallons each, deliver methanol daily. Alkalinity Addition - Full-scale sodium hydroxide feed facilities were brought on-linein 1998 and consist of six 25,000 gallon storage tanks for storage of 50% sodiumhydroxide, and three metering pumps. Alkalinity addition is to a common point upstream

    of the flow split to the two parallel banks ofN/DN reactors. Alkalinity addition is notnormally required.PROCESS OBJECTIVESTen process objectives were identified as having the potential to improve process performance orreliability. Each objective and the design features included in the current upgrade project toachieve the objective are presented in the following sub-sections. In several cases, one designfeature may meet several process objectives.Provide Equal Flow Distribution to N/DN ReactorsCurrently the distribution of the main process flow, which is effuent from the SecondarySedimentation Basins, is very poor. The flow distribution to each of the six reactors in a parallelbank should be approximately 17 percent; however, the actual distribution, as determined byComputational Fluid Dynamics (CFD) modeling, varies considerably, as tabulated in Table 2and shown graphically in Figure 3. Under the ex