Plant Profile

City of Bowling GreenWater Pollution Control

DivisionWastewater Treatment Plant

ImprovementsKurt A. Smith, PE

ARCADIS U.S, Inc.And

Douglas P. ClarkBowling Green WPC

Superintendent

The City of Bowling Green wasfaced with the challenges of anaging aerobic digestion systemalong with local opposition oftheir Class B biosolids landapplication program. Theseimprovements focused onproviding innovative solutionsunder some unique conditionsthat has enabled the City toproduce Class A biosolids for thesame cost of a Class B system.

Plant History

The current Bowling GreenWastewater Treatment Plant(WWTP) was placed intooperation in 1982 and is ratedby the Ohio EPA as a Class IVFacility. Treatment is achievedthrough primary, secondary andtertiary treatment processeswith aerobic digestion of wastebiosolids. The plant had acapacity of 8 MGD dry weatherflow with a peak hydrauliccapacity of 16 MGD.

Also at the pumping station is a 4MG holding tank and combinedsewer diversion chamber tohandle excess wet weather "firstflush" combined wastewater andstormwater flows. Approximately35% of the sewer system inthe City of Bowling -Green iscombined sewers.

The WWTP had served the Citywell since 1982 but eventuallybegan to show its age. In thelate 1990's the city replacedthe chlorination system withUV disinfection. In 2003 aplant study was completed thatfocused on solids handling issuesthey were experiencing. Thisarticle primarily focuses onhow the City approached andeventually addressed their solidshandling needs.

The Problem

The four existing aerobicdigesters were nearing designloadings and would experienceoccasional upsets during coldwinter months which resulted inodor problems. In addition, afternearly 20 years of operation,existing aeration equipment

All separate and combined was in need of replacement.wastewater is screened and The primary goal was clearpumped to the treatment plant - increase the solids handlingby the East Poe Road Pumping capacity of the WWTP with aStation using four variable speed, process capable of performingvertical centrifugal pumps with a effectively in a cold climate, whiletotal rated capacity of24 MGD. meeting US EPA 503 Sludge

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Regulations. Secondary goalsincluded increasing the averageday plant capacity from 8 to 10MGD, converting the aerationtanks to Biological NutrientRemoval (BNR) with anoxicselector zones, the addition ofa high efficient variable vaneblower and provide a new septagereceiving station.

Existing Conditions

The Bowling Green WWTPprocessed waste primary andsecondary sludge using a coursebubble diffused air, aerobicdigestion process.

Coarse Bubble Diffusers

The primary advantage ofthe existing aerobic digestionprocess was its relatively simpleoperation. Plant staff hadexperience with aerobic digestionat its previous facility and at thetime, electrical energy costswere relatively low since the cityoperates its own electric co-op.Near the end of its design life, theever increasing cost of electricalenergy became a significantdisadvantage. Operating neardesign loadings, the process'ssimplicity greatly limited theirability to perform during coldermonths. With the only control

Buckeye Bulletin -Winter 2008

being the amount of air applied,the operator had limited controlof the digestion process andwhen an upset became imminent,there were few options availableto avoid the problem and evenworse, limited abilities to correctthe problem, once it did occur.

During its last few years ofoperation, the WWTP was unableto achieve Class B biosolidregulations of38% volatile solidsreduction for up to six monthsduring the colder months of theyear. The cold temperatureswere suppressing the biologicalactivity of the aerobic digestionprocess.

Alternatives Evaluation

Several aerobic digestionalternatives were studied between19,99 and 2003, ranging fromtraditional Class B processessimilar to what they had, toadvanced Class A processes.During the evaluation theCity encountered significantpressure from a local faction toeliminate their Class B liquidland application program due toperceived health concerns. As aresult, the City took a second lookat a then newer 2nd generationAutothermal ThermophilicAerobic Digestion (ATAD)system that eliminated the needfor batch tanks and provided

Plant Profile

improved process control usingORP probe technology. Otheralternatives evaluated includedjet aeration, traditional ATAD(using batch tanks), dewateringwith centrifuges vs. liquidland application and landfilldisposal.

solids destruction associatedwith the Class A biosolids. TheATAD alternative also offeredsome non-tangible benefits thatincluded easing local healthconcerns and the beneficial reuseof the Class A biosolids as a topsoil product.

Opinion ofProbable TotalConstruction

Cost

DigestionAlternativeOpinion ofProbable

ConstructionCost

AverageEquivalent

AnnualCost

Aerobic DigestionAlternative

1. Jet Aeration (Class B) $592,000 $4,275,000 $7,180,000lA. Jet Aeration w/Dewatering $610,000 $3,440,000 $7,210,000(Class B)2. Jet Aeration (Class B Convertible $641,000 $4,875,000 $7,780,000to Class A)3. ATAD Liquid (Class A) $680,000 $5,125,000 $8,030,0004. ATAD w/Dewaterinz (Class A) $665,000 $5,530,000 $8,440,0005. Existing w/Dewatering to Landfill $702,000 $2,995,000 $5,900,000(Class B or less)6. 2n Generation ATAD Jet Aeration $573,000 $4,580,000 $8,230,000w/Dewatering (Class A)

Selected Plan andImprovements

Digestion System - 2nd GenerationATAD ranked highest, based onAverage Equivalent AnnualCost. Unique factors thatfavored this alternative includeda system that produced higherthan normal primary solids of4 -6%, (therefore not requiringthickening before feeding intothe ATAD reactors), reducedpower requirements and reducedhandling costs due to higher

Biosolids Centrifuge

ATAD Reactors, Cooldown Tank and Biofilter

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To provide flexibility in handlingand enable the city to marketreuse of the Class A product, a100 gpm dewatering centrifugerated for 2,100 dry lbs/hr wasincluded in the project.

Secondary Aeration Tanks -Secondary activated sludgetreatment process was changedfrom 4 tanks in parallel to 2tanks in a 3-pass plug flow mode.This reduces short circuiting andallowed for step feed operation,improving effiuent quality duringpeak flow wet weather. The newaeration tanks have a selector toimprove sludge settleability andprovisions for an anoxic zone fornitrogen removal (nitrification/denitrification).

Aeration Tanks

Blower Improvements - A newhigh efficiency 12,500 cfmvariable vane air blower wasprovided that operates at 80%efficiency between a 40 to 100%flow range. This significantlyreduced energy demand and inconjunction with the new ATADsystem it is estimated the city issaving over $60,000/year overthe old system.

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High Efficiency Blower

Septage Receiving Station - Aseptage receiving station wasprovided that increased capacity,significantly improved air quality(previous system was simpleopen, coarse grate), and includeda more user friendly unloadingstation. The City also receivedthe interest rate reductionprogram equal to the value ofthese improvements throughthe OEPA WPCLF financingprogram.

Septage Receiving Station

Effluent Limits - The increase inaverage day design flow from8 to 10 MGD for the plannedimprovements discharging toPoe Ditch, required compliancewith BADCT (Best AvailableDemonstrated ControlTechnology) for new sourceson a limited quality receivingwater. For the 2 MGD increasein capacity, the Ammonia limitwas reduced from 2.0 to 1.8 mg/l

in summer and 5.3 to 4.8 mg/l inwinter and Dissolved Oxygenlimit was increased from 5.0 to5.2 mg/l. The 30-day dischargelimits remained the same forCBOD at 10 mg/l, SuspendedSolids at 12mg/l and Phosphorusat 1.0 mg/l.

Results

After over a year of operation,the results confirm earlierprojections. The newimprovements are operating asanticipated. Some of the designinnovations ofthe 2nd GenerationATAD included using OxidationReduction Potential (ORP)probes to monitor the status ofthe Nitrification/Denitrificationcondition in the cooldown tankand control air supply to thereactors. Also, innovative heatexchangers were provided inthe cooldown tank and offgaspiping (ATAD temperatureshave reached as high as 170F), and low cost bentonite claywas used to enhance biosolidsthickening (decanting) insteadof polymers.

Dilution Dampers to Cool ATADOffgas

Buckeye Bulletin -Winter :.

Dilution Dampers to Cool ATADOffgas

Another innovation simplifiesdesign and operation by allowingthe reactors to be continuouslyfed thereby eliminating a batchtank for draw and fill whileachieving the required US EPA503 isolation time for pathogendestruction.

COSTThe entire project was constructedfor $6,703,000 and was 19%under the original proj ectconstruction budget estimate of$8,230,000. In the end, it canbe said that Bowling Green isachieving a Class A Sludge forthe Cost of Class B with thisunique system.

Startup Performance

Over the past year, the system isproducing a Class A biosolid thatis so highly desirable it is beingused as a top soil product by alocal top soil company.

Class A Product

Total solids reduction hasexceeded expectations andhas ranged between 60 - 75%.Volatile solids destruction hasbeen outstanding and has ranged

rww.ohioowea.org

Plant Profile

between 65 - 80%. Operationalcosts are also coming in aspredicted with a 50% reductionin biosolids disposal costs over

STATE CONFERENCE TEASER

A quick reminder to all of our membership concern-ing this years OWEA State Conference. The confer-ence will be held in Cleveland, Ohio starting June23, 2008 with golf and progressing through June26,2008.

A great technical program has been planned. Net-working opportunities include golf, the meet & greetand the banquet. The meet&greet will be a 3 hourcruise aboard Cleveland s Goodtime III featuring afull buffet, open bar, live music and a DJ The Ban-quet will host OWEA sfirst dinner theater in a 20 sspeakeasy (come dressed to impress). A completeitinerary will be available within the next couple ofweeks. Below is information concerning lodgingforyou early birds who want ajump start onfun andlearning.

The Hotel MarriottDowntown Cleveland at Key Center

$159/night Single or Double$20/day parking with in and out privileges

Phone: (216) 696-9200

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