Optimization of Nitrification

8/3/2019 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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FIGURE iPLAN VIEW OF EXISTING FACILITIES

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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 existing reactor configuration, shown in Figure 2,the flow split to the reactors is poor, since the flow distribution is controlled by submerged weirsbetween Stages 4 and 5.The poor flow distribution causes a number of problems, including over or under feed of bothmethanol and air (oxygen). The overfeed of methanol can result in excessive consumption ofdissolved oxygen (D.O.) in the tertiary filters, and difficulties meeting effuent D.O. limits. Sincethe air is cross-fed, and there is no automatic control of aeration at the individual stages, the poorflow splitting can result in overfeed of air in some reactors, which negatively impacts thedenitrification process for that reactor. Overfeed of air also results in unnecessary blower

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Table 2 - Flow Distribution to One Parallel Bank of Reactors (Based on CFD ModelingResults)Reactor Percentage of Flow (%)

1 263 115 147 159 17

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Figure 3 - CFD Results for Flow Split to OneParallel Bank of N/DN Reactors

operating costs. Underfeed of air in some reactors can result in incomplete nitrification andhigher than desirable effluent ammonia concentrations.To provide an easy-to-control, positive flow split that is effective for both dry and wet weatheroperating scenarios, each reactor's three influent gates wil be modified to include free flowingrectangular weirs at the influent end of the reactors. Available weir lengths can be modified bylowering a slide gate to blank-off portions of the weir, in order to allow for different step feed

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percentages or modified operations for wet weather. Weir boxes for Stages lA, 1B, and 2A areshown in Figure 4. Stages 1A wil have 14 feet of weir. Stage 1B wil have 26 feet of weir, andStage 2 wil have 46 feet of weir. Moving the headloss to the upstream end of the reactor, andutilizing the headloss through the influent gates and over weirs, results in excellent distribution,as shown in Table 3, and wil improve methanol feed and aeration, and ultimately increase thereliability of nitrogen removal in the N/DN reactors. The distribution percentages cited in Table3 represent the ratio of flows between the reactor receiving the lowest flow and the reactorreceiving the highest flow, and indicates that under all flow conditions the difference betweenthe highest and lowest flows is less than 3%.Table 3 - Distribution of Secondary Effuent to N/DN Reactors

DistributionCurrent Design Design Design

Annual A vg Anual A vg Max Day Peak HourFlow (mgd) 352 373 564 755Dry Weather Conditions25% lA, 75% 2A 97.8% 98.4%50% lA, 50% IB 99.6% 99.2%50% lA, 50% 2A 99.3% 98.7%Wet Weather ConditionsAll Secondary Effluent (SE)flow to Stage 2, RAS and SE toall 12 reactors 97.2%*8 reactors receive SE and RAS(Stage 1B and 2),4 reactorsreceive RAS only 99.8% 99.1% 99.1 %

* Secondary Effuent Flow = 400 mgdMaximize Performance of Existing Sedimentation BasinsExisting sedimentation basin capacity has historically been a weak spot in terms of the reliabilityof the N/DN process. In response to this, additional sedimentation basin capacity was added asDual Purpose Sedimentation Basins in the late 1980s. The additional sedimentation basincapacity can be used to augment either the Secondary Sedimentation Basins or the NitrificationSedimentation Basins, and thus, the additional capacity is not always available. Given sitelimitations, construction of additional sedimentation basins is not feasible, and therefore,optimization of the existing sedimentation operations is required. As shown by Van Niekerk(2003), provision of step feed in the existing N/DN reactors results in lower mixed liquorsuspended solids (MLSS) concentrations and higher clarifier factors of safety for a variety ofoperating conditions while maintaining required levels of nutrient removaL. Step feed in theN/DN reactors is therefore required to improve existing settling operations.

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The existing reactor configuration (Figure 2) does not lend itself to provision of step feedwithout introducing the possibility of short-circuiting. Therefore, a step feed configuration wilbe provided by converting the reactor to a serpentine flow path, which promotes a plug flowregime, and adding baffe walls between Stages lA and 1B, 3A and 3B and 5A and 5B, as shownin Figure 4. With the conversion to step feed with a serpentine flow path and the weir boxesdescribed earlier, step feed can be controlled to Stages lA, 1B and 2A to allow for at least thefollowing operating scenarios:

. 50% to Stage lA, 50% to Stage 1B

. 25% to Stage lA, 75% to Stage 2A

. 25% to Stage lA, 25% to Stage 1B and 50% to Stage 2

. 50% to Stage lA, 50% to Stage 2

. 100% to Stage 2 (wet weather)

A limited number of additional operating scenarios (e.g. 30% to Stage 1B, 70% to Stage 2) canbe created by using combinations of available weir lengths for Stages lA, 1B and 2, so long assuffcient weir length is made available to maintain the upstream water surface elevation at anelevation that wil not submerge the upstream Secondary Sedimentation Basin effuent weirs.The inclusion of step feed and the conversion to a serpentine flow path assist in meeting severalother process objectives, as described in detail in the following sections.Optimize Wet Weather OperationsBlue Plains receives flow from a combined sewer system, and accordingly, the NPDES permitrequires that peak flows up to 740 mgd (2.0 times design flow of370 mgd) receive full treatmentfor the first four hours and flows of up to 511 mgd (1.4 times design flow of370 mgd) continueto receive full treatment for the duration ofthe wet weather event. Wet weather can potentiallyresult in a substantial shift of mixed liquor solids from the N/DN reactors to the NitrificationSedimentation Basins. This may result in solids wash-out from the sedimentation basins, whichcan be extremely detrimental to the N/DN process, or, under a worst case scenario, can result incomplete loss of nitrification. The installation of weir boxes, as well as the conversion to stepfeed with a serpentine flow path, also serve to optimize wet weather operations. Wet weatheroperations are optimized by sequentially moving towards operation in quasi-contact stabilizationor solids-holding mode, where portions of reactors, and eventually entire reactors are used forsolids retention. This limits the solids loading on the sedimentation basins. The main features ofthe proposed wet weather operations are as follows:

. Divert Secondary Effuent in excess of 400 mgd directly to Stage 2 of the N/DN reactors,utilizing the provisions made for step feed and a serpentine flow path, and continue tofeed return activated sludge (RAS) to Stage lA, which wil increase solids inventorystorage. The serpentine flow path ensures the Secondary Effuent introduced into Stage 2wil not short circuit into Stage 3, thus stil achieving reliable nitrification.

. In a peak wet weather-operating mode, the Secondary Effuent and RAS wil be directed

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to four (4) N/DN reactors on each side of the plant (8 reactors out of 12). The remainingfour reactors wil receive RAS only. The trigger point for this shift in Secondary Effluentflows depends on sludge settleability at the time of the wet weather event, and wil varyfrom 430 mgd to 460 mgd.

As shown in Figure 5, which is based on Biowin process modeling, the wet weather operatingstrategy results in a dilution of the MLSS concentration in N/DN Reactors receiving the peakSecondary Effuent flow. The N/DN Reactors that receive only RAS, experience an increasedMLSS concentration. The net result is a reduction in solids loading to the downstreamsedimentation basins, which keeps the solids loading on the sedimentation basins in theacceptable range for wet weather conditions (Figure 6) and prevents solids washout from thesedimentation basins.Provide for Flexible Aerobic and Anoxic Mass FractionsTo achieve and optimize nutrient removal throughout the year, including periods oflowtemperatures and wet weather, it is desirable to have a reactor configuration that providesflexibility in terms of aerobic and anoxic mass fractions. The configuration of the existingreactors, with only five stages, provided limited flexibility to alter the aerobic and anoxic massfractions; specifically, portions of the reactor could only be changed from anoxic to aerobicservice, or visa versa, in 20% increments. In addition, the over/under flow pattern does notadequately prevent back-mixing from aerobic zones downstream of anoxic zones, whichdecreases the efficiency of the anoxic zones.To provide increased flexibility in the aerobic and anoxic mass fractions, the reactor wil beconverted to a serpentine configuration with three swing zones (Stages 3A, 3B and 5A) andprovisions for a future swing zone (Stage 1A). To accommodate the change in the initial aerobicmass fraction, multiple methanol feed points wil be provided, as shown in Figure 4.The future swing zone wil be Stage lA, which may be utilized for RAS denitrification. Eachswing zone wil contain 10% of the reactor volume, as shown in Figure 4. To prevent back-mixing from downstream aerobic zones into upstream anoxic zones, anoxic-aerobic baffe wallswil be included between Stages 1A and 1B, 4B and 5A, and 5A and 5B. These baffle walls wilbe designed with a minimum of 0.1 feet of head loss to ensure there is no back-mixing. Theheadloss wil be achieved through use of a weir at the top of the wall, which wil allow for freesurface flow to ensure transport of foam through the reactor, and a submerged port, which wilallow for draining the reactor. To further optimize the operation of the swing zones, the zoneswil be outfitted with both mixers and fine bubble diffusers; this allows for the most efficientaeration when the stage is operated in the aerobic mode and the most efficient mixing without therisk of imparting D.O. when the stage is operated in the anoxic mode.

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Increase Mixing Effciency in Anoxic/Swing Stages

The original design for the N/DN reactors used sparged turbines to provide aeration. A sprgedturbine consists of a diffuser ring, which is mounted to the reactor floor and connected to theblower discharge piping, and a vertical shafted mixer, mounted in-line over the diffuser ring(Refer to Figure 2). The mixer blades are designed and sized to shear the air bubbles to improveoxygen transfer; mixing is a secondary function. The original sparged turbines at Blue Plainsutilize 75 Hp motors, and are located two per stage (total of 120 sparged turbines). When BluePlains converted the original Nitrification Reactors to N/DN Reactors, anoxic stages werecreated by turning off the air to the diffuser ring, and utilizing 48 of the existing mixers formixing. In addition, the sparged turbines in the initial two (aerobic) stages were retrofitted withnew 50 Hp motors and new gear boxes (total of 48 sparged turbines). Because the existingmixers are designed primarily for shearing air bubbles, they do not provide good mixing, asevidenced by the striation shown in the mixed liquor in Figure 7, and the existing mixers areexpensive to operate, as shown in Table 4. In addition, there have been concerns regarding thevibration induced by the existing mixers.Table 4 - Comparison of Power Costs for Existing and Modified Mixers

Existin2 Mixer Modified MixerHorsepower Draw 46 17Number of Mixers 48 48Power Cost $0.55/kwh $0.55/kwhAnnual Power Cost $793,000 $293,000To achieve the process objective of improving the mixing, without increasing the D.O., whilereducing the mixer power cost, the existing mixers wil be modified. Proposed modificationsinclude: reusing the existing drives, replacing the 75 Hp motors with 20 Hp motors, shorteningand reusing existing shafts and providing new hydrofoil type blades to effect mixing viadownward pumping action. Pilot testing was conducted to ensure that the proposed mixers wouldmeet the process objective. Pilot testing included retrofitting two mixers in anoxic Stage 4, andobtaining D.O., vibration and amp draw measurements. Mixer retrofits and testing wereconducted by Philadelphia Mixing Solutions, Inc. Based on visual observations, the retrofittedmixers appear to be working well; as shown in Figure 8, there is no discernable striation in themixed liquor and eddy currents are visible on the surface. D.O. testing results are shown inTable 5, and confirm that the new mixers did not increase the D.O. Power measurements showthat the mixers were drawing 17 Hp, which is inline with the power calculated for providing 0.3Hp/lOOO ft3 for good mixing (18 Hp). Vibration results confirm that the vibration of theretrofitted mixers (maximum vibration 0.069 inch/second) is lower than the vibration of theexisting mixers (maximum vibration 0.097 inch/second), and is within the recommended rangeofless than 0.3 inch/second, per ANSIIAGMA 6000-B96, Specification for Measurement ofLinear Vibration on Gear Units. Given the improvements in mixing and potential for powersavings observed, forty-six additional existing mixers wil be upgraded as part of the currentproject.

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Figure 7 - Striated Mixed Liquor in AnoxicStage 4 Mixed with Existing MixersI~. .:.- -:-

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Table 5 - D.O. Test Results from Mixer Pilot TestingDepth (feet) I D.O. (mg/L)

Samples Taken at Downstream End of Stage 310 I 1.0 I 0.90 I 0.3*20 I 1.05 I 0.92 I 0.37*Samples taken at Downstream End of Stage 410 I 0.16 I 0.15 I 0.1820 I 0.15 I 0.15 I 0.18*Sparged turbine closest to the test point in Stage 3 was not operating

Increase Aeration EffciencyAs noted previously, the existing N/DN reactors were originally designed to achieve nitrificationonly, utilizing 100% of the N/DN reactor volume and 10 sparged turbines per reactor. When thereactors were converted to N/DN reactors, only 60% of the volume was used for nitrification.Process modeling shows that the process oxygen demands begin to exceed the existing spargedturbine oxygen transfer rate in the initial aerobic stages of the reactor (1,2 and 3) under somepeak month, week, day and hour operating conditions. Under such conditions, it would benecessary to utilize more of the reactor volume (to place more sparged turbines into service, thusadding more oxygen) for nitrification, which decreases the reactor volume available fordenitrification. Given that the sparged turbines are less efficient (13 to 20% oxygen transfereffciency), which increases blower operating costs significantly, a comparison of the life cyclecosts of operating modified sparged tubines to allow for nitrification in 60% of the reactorvolume and the life cycle costs of installing and operating a fine bubble diffuser aeration systemwas developed. A summary of the analysis is presented herein.Components of the life cycle costs for modifying and continuing to operate the sparged turbinesinclude: equipment modification costs, operating costs, and maintenance costs. Costs associatedwith each component are listed in Table 7 below, and are based on a 20 year period and a 5%interest rate.

Equipment modification costs - To increase the oxygen transfer effciency of the spargedturbines, the following modifications to the existing mixers are required: Replace the existing impellers with new disc impellers, 6-bladed aqua "Ruston"type, sized to draw 75 Hp in the ungassed condition. Relocate or replace motors such that all aerobic stages have 75 Hp motors (i.e.remove and replace the 50 Hp motors installed during the previous upgrade). Replace the existing shafts with new shafts designed for the loads imposed by thenew operating conditions.

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Replace or retrofit the existing sparge rings with rings with smaller diameterholes, to increase oxygen transfer efficiency by creating smaller bubbles.The total present worth for the equipment modifications is $6.3 milion.

. Operating Costs - The power cost for operating the blowers and turbine mixers are themost substantial costs associated with operation of the sparged turbine system, therefore,the simplifying assumption that the operational costs consist only of power costs for theblowerlsparged turbine combination, was made. Table 6 summarizes the power cost forthe sparged turbine system.

Table 6 - Power Costs for Sparged Turbine System2.6 lbs 02/hp-hr

118,000 kwh165,000 kwh$9,080

. Maintenance Costs - Manufacturer's recommended routine maintenance includes a dailycheck of each sparged tubine to observe noise, vibration etc., and changing the oil ineach sparged turbine twice a year. Assuming the daily observation for each spargedturbine takes 2 minutes, with 120 sparged turbines, 4 hours per day are required forinspection. Costs associated with the oil change include the oil and labor costs. Thirtygallons of oil are required for each change, for a total of 60 gallons per sparged turbineper year. With 120 sparged turbines, and an oil cost of$250 per 55-gallon drum, the totalannual oil cost is $33,000. It was assumed that it takes two men four hours to change theoiL. For 120 sparged turbines, 1,920 man-hours are required for the two oil changes. At arate of $40 per hour, the total annual maintenance labor cost is $135,000. The total annualmaintenance cost is the sum of the oil and labor costs, which is $168,000.

Table 7 - Comparison of Life Cycle Costs for Sparged Turbines versus Fine BubbleDiffusersItem Estimated Annual Cost

Sparged Turbines Fine Bubble DiffusersDebt Service Costs N/A $1,230,000Equipment Modification Costs $500,000 N/AOperating Costs $3,314,000 $1,106,000Maintenance Costs $168,000 $6,400Equipment Replacement Costs N/A $26,000TOTAL ESTIMATED ANNAL COST $3,982,000 $2,368,000

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Operating Cost - Since the power cost for operating the blowers is by far the mostsubstantial cost associated with operation of the fine bubble diffuser system, for thepurpose of this evaluation, the simplifying assumption that the blower power costs are theonly operational cost, was made. Table 8 summarizes the power costs for the finebubble diffuser system.

Table 8 - Power Costs for Fine Bubble Diffuser SystemCalculated Standard Aeration EfficiencDail Power Re uiredDaily Power Required Adjusted for Control Variation:l 20%(Lower Variation With New Controls)Daily Power Cost $0.055/kwh

8.9 lbs 02/h -hr46,000 kwh55,000 kwh$3,030

Maintenance Costs - Assuming that the system wil be air-bumped for cleaning purposesonce per day, the annual maintenance requirements for the fine bubble diffuser systemare minimaL. Based on the manufacturer's recommendation that the reactors be drainedand the diffusers hosed down once per three years, and assuming that four reactors aresubject to the required annual maintenance per year, the annual maintenance cost isestimated to be $6,400. Note: It was assumed that washdown of each reactor would take40 man-hours (tank entr is not required).

Equipment Replacement Costs - Based on an anticipated service life of 10 years, it isassumed that complete replacement of the membranes wil be required after 10 years.Based on provision of 43,782 diffusers and a replacement cost of $4.50 per diffuser, thetotal cost to replace the diffusers in 10 years is $321,000. To accrue sufficient funds toreplace the diffusers in 10 years, the annual cost is $26,000.The estimated annual costs associated with providing process air via a fine bubble diffusersystem and a sparged turbine system indicate that operation of a fine bubble diffuser system,even with repayment ofthe initial capital expenditure, is more cost effective. The annual savingswould be approximately $1,614,000, which over the 20-year period has a present value of$20,025,000. In addition to providing cost savings, installation of a fine bubble diffuser systemhas intangible benefits, including:

By virte of providing more efficient aeration, the fine bubble diffuser system alsoprovides an "expansion" by allowing for provision of significantly more oxygen to theprocess. Aeration via fine bubble diffusers can be better controlled than aeration via spargedturbines, which allows the operation of the blowers to more closely track the processrequirements, which ultimately provides more power savings.

Based on the potential savings, fine bubble diffusers and associated modifications wil beincluded to satisfy the process objective of increasing aeration efficiency, while simultaneouslyreducing operating costs.

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Remove FoamThus far, with use of coarse bubble aeration provided by the sparged turbines, there has beenlittle foaming in the N/DN reactors. However, the current project wil include provision of finebubble diffusers in the N/DN reactors, which increases the likelihood of foaming. The currentover/under flow path through the reactor, and submerged effluent gates (refer to Figure 2)preclude transport of foam between the stages of the reactor to a central location for collectionand removaL. Design features incorporated to ensure foam removal include: conversion to aserpentine configuration with free surface ports between all stages, new effluent gates to providefor a free surface exit from the reactors to the mixed liquor effuent channel, and two new foamwasting stations. The serpentine flow path and new effuent gates are as shown in Figure 4.One foam wasting station wil be located in each mixed liquor effuent channel to collect foamgenerated in each parallel bank of reactors. The key components of the Foam Wasting Station areshown in Figure 9. The wasting stations wil utilize partially submerged baffes to direct thefoam across the channel to a downward opening weir gate and into a sump that wil becantilevered on the inside of the two southernost reactors. Rotary lobe pumps wil be used topump the foam with the waste activated sludge (WAS) to the Dissolved Air Flotation Units.Rotary lobe pumps were selected based on their advantages, which include: an ability to toleraterags and solids without a grinder, long life when operated at low speeds, quick and easyreplacement of moving parts, a small footprint (especially compared to a progressive cavitypump), good references for scum pumping applications, and an ability to break suction (bepumped dry). The controls wil be configured, through the use of two ultrasonic level sensors, toallow for two different modes of wasting. Wasting can either be 1) nearly continuous, based onmaintaining the gate elevation just below the channel water surface elevation to preferentiallywaste foam and a small quantity of mixed liquor, (gate level to be adjusted on a periodic basis totrack channel level) or 2) at periodic intervals throughout the day. Under mode 2, the gate wilnormally be fully raised, and will be lowered at intervals to effect surface wasting. Timedintervals for adjustment of the weir gate elevation are included in operating mode 1 to preventhunting and seeking ofthe gate, which could result in early bum-out of the gate actuator.Improve Methanol Feed ControlsCurrently, methanol is flow paced to each reactor. Automated monitoring of nitrate is notavailable, and thus makes process control ofthe methanol feed cumbersome, time consumingand imprecise. In addition, given the existing poor flow spit to the reactors, overfeed orunderfeed to individual reactors can occur. The overfeed of methanol leads to unnecessarychemical expenses and, since it can result in carbon bleed-through, can result in an excessiveD.O. drop in the tertiary filters, which requires additional aeration, as an additional expense.Underfeed of methanol can result in incomplete denitrification, which adversely affects meetinglow TN effuent limits.Although the modifications to the flow split previously described wil rectify the flow splittingproblems, to optimize the methanol feed process further, on-line monitoring wil be provided foreach reactor. Specifically, on-line analyzers wil be used to measure the nitrate concentration in

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each reactor, immediately upstream of the anoxic zone, and in the mixed liquor channeL. Thiswil allow for adjustment of the methanol feed to individual reactors and comparison of theprocess performance between reactors. As on-line instrumentation for nitrate is a relatively newapplication, DC W ASA wil make their decision based on a WEFT lIT A study of nitrateanalyzers that is currently under way (WERF Project Number 03-CTS-8).Provide for Future Multi-Stage DenitrificationThe current reactor configuration does not allow for multiple anoxic zones or isolation of theRAS (refer to Figure 2), which precludes separate denitrification of the RAS prior tointroduction into the aerobic zones. In addition, the current configuration includes a carbonsource only for the anoxic zone downstream of the initial aerobic zone. As lower effuentnutrient limits are promulgated within the Chesapeake Bay Region, optimization of nitrificationand denitrification wil become even more criticaL. With the conversion to a serpentine flowpath, it wil be possible to dedicate the initial half-stage (10% of the reactor volume) of thereactor to provide denitrification of the RAS. In addition, the design includes provisions forfuture installation of mixer (i.e. electrical supply and mounting platform retained intact) tooperate Stage 1A in an anoxic mode. However, without supplemental carbon feed, thedenitrification would be reliant on endogenous respiration, thus proceed slowly, and, accordingto observations by plant staff, be unreliable. Therefore, to enable future multi-stagedenitrification, methanol wil be added to the upstream end of the RAS discharge force main,which wil allow for utilization of the 0.5 MG of volume in the RAS pipe as well as the first half-stage of the reactor for supplemental RAS denitrification.Improve Mixed Liquor Distribution to the Sedimentation BasinsDC W ASA has historically had problems with the distribution of the mixed liquor to thesedimentation basins. Under design average flow conditions, sedimentation basin performanceis reported to be good with effuent TSS typically less than 10 mg/l. However, under peak flowconditions, solids loadings are higher in some basins than others, as indicated by rising sludgeblankets and periodic high effuent solids, particularly from the sedimentation basins at thedownstream end of the NSB influent channels.Each bank of 14 sedimentation basins has an aerated influent channel that is 1,100 feet long and16 feet wide, with a nominal side water depth of20 feet depending on flow. Each sedimentationbasin currently has four 3-foot x 3-foot square inlets. Assuming an equal flow distribution, atdesign peak flow (755 mgd influent, 134 mgd spent filter wash water, and 291 mgd RAS flow)the velocities in the existing channel range from 2.86 fps (upstream end) and 0.22 fps(downstream end). Velocities and headloss through the existing influent openings are 1.95 fpsand 0.16 feet, respectively. Given the length of the channel and the low influent headloss, theflow split to the sedimentation basins is poor. In addition, each NSB has 1,156 feet of V-notchweir, with minor differences in weir elevation potentially resulting in significant differences inflow distribution. To improve the flow distribution to the sedimentation basins, the samephilosophy that was applied to improve the flow distribution to the reactors (creating headloss atthe upstream end of the basins) wil be applied. Specifically, orifice plates wil be used toincrease the headloss at the influent end of the sedimentation basins. Because of the limited

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headloss available within the existing hydraulic profile, use of weirs is not possible. However,by lowering the sedimentation basin effluent weirs by three inches, and installing an orifice plate,as shown in Figure 10, in the existing stop plate grooves upstream of each influent gate, thedistribution to the sedimentation basins can be improved. The loss through the modified inletswil be 1.2 feet, at peak flow, approximately 10 times the change in the hydraulic grade along thelength of the channeL. By incorporating a significant headloss at the point of distribution (theupstream end of the basins), differences in the flow distribution caused by minor irregularities inweir elevations and changes in the hydraulic grade line in the influent channel can be mitigated.

LIFTING APPARATUS

l~-iTRIFIC~T_ION SEDIMENTATION BASINJ

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ENLARGED VIEW

ORIFICE PLATEFigure 10 : Orifce Plate at Influent to Nitrification

Sedimentation Basins

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ConclusionsFor existing wastewater treatment plants, meeting increasingly more stringent nutrient limitsrequires optimization of the existing facilities. Based on a case study of the N/DN reactors at DCWASA's Blue Plains Advanced Wastewater Treatment Plant, design features that can beincorporated into the existing structues to satisfy process objectives that improve processperformance and/or reliability are summarized in Table 9.Table 9 - Summary of Design Features Incorporated to Improve Process Performance andReliabilty

Process Objective Design Feature Included to Satisfy theProcess ObiectiveProvide equal flow distribution to N/DN Free flowing rectangular weirs in weir boxes atreactors influent end ofN/DN reactorsMaximize performance of existing Serpentine flow path with provisions forsedimentation basins controlled operation (via weir boxes) in a stepfeed mode, which reduces solids loading tosedimentation basinsOptimize wet weather operations Two-tiered operational approach to controlsolids loading on the sedimentation basins, andprevent solids washout. The two-tieredapproach initially utilizes the serpentineconfiguration and step feed weir boxes to holdsolids in Stage 1, while bypassing secondary

effuent to Stage 2. As flows continue toincrease, reactors are taken off-line to holdsolids.

Provide flexible aerobic and anoxic mass Serpentine flow path with anoxic/aerobicfractions baffe walls, and mixers and fine bubblediffusers in swing zonesIncrease mixing efficiency Retrofits for existing sparged turbines formixing, reusing existing gear boxes andportions of shaftsIncrease aeration efficiency Fine bubble diffusers in all aerobic and swingzonesRemove foam Free surface ports, new effuent gates and foamwasting stationsImprove methanol feed controls Automated monitoring of nitrate for allreactors and mixed liquor channels, withfeedback loop to trim methanol feed.

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Table 9 - Summary of Design Features Incorporated to Improve Process Performance andReliabilty (continued)

Process Objective Design Feature Included to Satisfy the ProcessObjectiveProvide for future multi-stage denitrification Provisions for use of initial 10% of the N/DNreactor volume in an anoxic mode forsupplemental RAS denitrification prior to entryinto aerobic zone, with provisions for futureinstallation of a mixer and future methanolfeed to the RAS line.Improve flow distribution to sedimentation Orifice plate at the influent ports for thebasins Nitrification Sedimentation Basins.

REFERENCESVan Niekerk, A.; Ruhl, 1.; Pitt, P.; Parker, D.; Kharkar, S.; Tesfaye, A. (2003) Upgrading of theNitrification/enitrification Facility at the Blue Plains Advanced Wastewater TreatmentPlant; WEFTEC 2003 Conference Proceedings.Metcalf and Eddy, Inc. (1991) Wastewater Engineering Treatment, Disposal and Reuse, 3rd ed.

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Optimization of Nitrification