Transcription
IT’S ALL IN THE MIX – DESIGN OF NEW MIXERS FORANAEROBIC SLUDGE DIGESTION FACILITIES AT ST.JOHNSBURY WWTFMeredith Zona, P.E., LEED AP BD C, StantecNaomi Johnson, P.E., Dufresne GroupJames Brimblecombe, Utility PartnersNEBRA ANNUAL CONFERENCEOCTOBER 26, 2017Image Source: Google Maps, 2012
PRESENTATION OUTLINE1 - Introduction to St. Johnsbury WWTF2 – Existing Digester Facilities3 – 2013 Digester Mixer Evaluation Study4 – Energy Evaluation of Digestion Process5 – Digester Mixer Design Considerations6 – Current Status
1 - INTRODUCTION TOST. JOHNSBURY WWTFImage Source: Google Maps, 2017
TOWN OF ST. JOHNSBURY, VTPop. 7,6002010 CensusSaint Johnsbury, VTImageSource:GoogleMaps, 2017ImageSources:GoogleMaps,2017
PLANT CHARACTERISTICS Secondary Treatment PlantOwned by Town of St. JohnsburyOperated by Utility PartnersTreats St. Johnsbury, VT WastewaterHandles Water Trt. Plant ResidualsLast Major Upgrade 1988-1990Discharges to Passumpsic River
TREATMENT PROCESS COMPONENTS– LIQUID TRAIN1Influent Lift Pumps432PreliminaryTreatment123Primary Settling54Rotating BiologicalContactors65Final Settling6Chlorination /DechlorinationImage Source: Google Maps, 2017
TREATMENT PROCESS COMPONENTS– SOLIDS TRAIN1Primary andSecondary Sludge,Scum, and Septage512Primary Digester Anaerobic Digestion3Secondary Digester Settling Gas Holder Tank44Sludge StorageTank35Land Application2Image Source: Google Maps, 2017
INFLUENT PARAMETERS2008Design2016Average Daily Flow (MGD)1.60.74Peak Daily Flow (MGD)6.96.9BOD (mg/L)251271TSS (mg/L)296298
2 – EXISTING DIGESTER FACILITIES
PRIMARY DIGESTER 45’ Diameter Dual-Deck Steel Structure Dome Surface Fixed to Concrete Tank Walls Gas Mixing System Converts Volatile Organic Solidsto Methane Gas Gas Mixing System in Constant Use
SECONDARY DIGESTER 45’ Diameter Floating Steel CoverCover Rides Up & Down on Gas BubbleSteel Rollers & Spiral Guides Align CoverDigester Serves as Settling and Gas Holding TankGas Mixing System Used Infrequently
EXISTING GAS MIXING/HEATING SYSTEM
EXISTING GAS MIXING/HEATING SYSTEMLarge Gas BubbleMixing SystemGas Drawn Off CenterWell of SecondaryDigesterGas Cleaned,Compressed, &Returned to Three, 24”Diameter Open-EndedHeating/Mixing TubesHeat ExchangerMounted on Inside ofTubes
DIGESTER SIZE & OPERATION Digester Tank Dimensions,Diam X Ht Tank Volume, Each, cf Type of Operation Internal Temperature, F45’ x 21’31,620Mesophilic95
DIGESTER OPERATING CONDITIONSDesign3,84670 Sludge Loading, lbs/day Volatile Solids (VS) In, % VS Loading to Primary Digester,lbs VS/1000 cf/day85 Tank Detention Time, Days16 Estimated VS Reduction, %552014 Avg.1,87462383760
3 – 2013 DIGESTER MIXER EVALUATION STUDY
Reasons for Evaluation Study Mixing Equipment 22 Years OldPoor Mixing CharacteristicsSolids Build-up in TankWater Residuals ImpactTanks Taken Out of Service FrequentlyGas Mixing Not Comparable to CurrentMixing Systems
Mixers EvaluatedLow Speed Mechanical MixerRooftop Mounted InternalDraft Tube Mixer,With and Without Sludge HeatExchangerLinear Motion MixerExternal Draft Tube Mixer,With and Without Sludge Heat Exchanger
Mixer Cost SummaryALTERNATIVECONSTRUCTIONCOSTANNUAL O & MCOSTPRESENT WORTHCOSTLINEAR MOTION MIXER 1,344,800 20,300 1,598,000LOW SPEED MIXER 1,234,400 22,500 1,515,000INTERNAL DRAFT TUBEMIXER 1,179,800 21,900 1,453,000INTERNAL ROOFTOPDRAFT TUBE MIXER WITHINTEGRAL HEATEXCHANGER 526,500 15,200 716,000EXTERNAL DRAFT TUBEMIXER 1,569,800 29,100 1,932,000EXTERNAL DRAFT TUBEMIXER WITH INTEGRALHEAT EXCHANGER 1,003,500 23,300 1,294,000
4 – Energy Evaluation of DigestionProcess
2015 Digestion EnergyEvaluation ReportInvestigated Digester Gas Production/Use Increased Digester Gas ProductionAlternatives Additional Energy Efficiency ImprovementAlternatives Digester Gas Use for Energy Production
DIGESTER OPERATIONS - 2014Sludge%VSRSludge bs/10³ m1.95137394239111921326235055Maximum 6.571437411067 30445615176723513In (gal) (lb/day) (days)
Increased Gas ProductionAlternatives Improve Mixing Add More Septage, Food Waste, Oil &Grease Maintain 4 – 6% TS Feed Daily Add/Withdraw Daily Similar Amount ofSludge
Energy Efficiency Improvements Improve DigesterInsulation Replace Digester Boiler Install Dual-Fuel Burner inBoiler for DomesticHeating/Hot Water
Digester Gas Use for EnergyProduction Use Digester Gas to Generate Electricity viaCombined Heat & Power System Essex Junction, VT, & Fairhaven, MA Possible Future Option for St. Johnsbury
5 – Digester Mixer DesignConsiderations
Digester Mixer DesignConsiderations Mixer Components Mixer Design Criteria Structural Analysis ofCovers Heat Exchanger Size onMixer Digester CoverModifications Internal Piping/PipingSupport Modifications Taking Digesters Offline
Mixer Components
Mixer Design Criteria Propeller Diameter Motor Size Pumping Capacity– Up/Down Draft Tube Diameters Digester Turnover Time Heat Exchange Transfer Warranty24”7.5 hp9,000 gpm27” & 28.5”30 minutes360,000 BTU/hr5 yrs.
Structural Analysis of Covers Support Added Weight of Mixer Complete Structural Analysis of Coversfor Added Load
Heat Exchanger Size Larger Heat Exchanger More Capital Cost More Efficient HeatTransfer
Cover Modifications New Pipe PenetrationsNew Cover Mounting Plates for MixersAttic InsulationFloating Cover Ballast Adjustment
Piping Changes Hot Water Piping& Pipe Supports Gas Piping,Including FlexiblePiping
Taking Digesters Off-Line Work Plan to TakeDown 1 Digester at aTime Clean Digester Remove Equipment Install Equipment Start Up Digester Repeat Sequence for2nd Digester
6 – Current Status Design/Bidding CompleteConstruction Start Date – 9/5/17Construction End Date – 9/30/18Engineer’s Estimate for Digester MixerRelated Improvements - 1,290,000
QUESTIONS?
ANAEROBIC SLUDGE DIGESTION FACILITIES AT ST. JOHNSBURY WWTF Image Source: Google Maps, 2012 . PRESENTATION OUTLINE 1 - Introduction to St. Johnsbury WWTF . TOWN OF ST. JOHNSBURY, VT Image Source: Google Maps, 2017 Saint Johnsbury, VT Image Sources: Google Maps, 2017 Pop. 7,600 2010 Census .
