Aerobic Digesters Anaerobic Digesters

6/8/2012Aerobic DigestersAnaerobic DigestersDoug Hillwww.dhillenvironmental.comPurpose of DigestionStabilize SolidsMinimize OdorsVolume ReductionImprove DewaterabilityMeet Part 503 Land Application RequirementsVector Attraction ReductionPathogens ReductionAerobic Digesters1

6/8/2012Growth Rate of usGrowthFoodConventionalTreatmentExtended AirSludge ProductionCell Residence ll GrowthHeat,NewBiomassBacterial CellAerobic ceEnergyBODCO2CellComponentsCell GrowthHeat,NewBiomassBacterial CellEndogenous Respiration – Survival ModeStored Food UsedCell Components UsedCell Growth RateReduced2

6/8/2012Endogenous Respiration– Cell oodCO2CellComponentsBacterial CellStored Food UsedCell Components UsedCell Dies and Splits Open(Aerobic Digestion)Aerobic DigestionEndogenous StabilizationHigh CRT allows for the microbes tofeed off of the cell contents of otherdying/decaying microbes underdigestion.Not all solids can be digested20 to 25% by weight inert solidsFine inorganic solids, organic solids, and cellcomponents that are not degradableAerobic DigestionADVANTAGESThe process is easy to control, easy start-upLow ammonia and BOD in return streamFew odors are experienced if properly designedand operatedExplosive gases (methane) are not produced3

6/8/2012Aerobic DigestionDISADVANTAGESAerobic digestion does NOT produce energyAerobic digestion process is energy intensiveNot usually used for primary sludge due to high O2demand and additional biomass producedTemperature variability throughout the year causesvariability in the operating performanceStabilized sludge may be difficult to dewaterAerobic DigestionOPERATIONBatch Operation:Add sludgeDigest – mix, aerateTurn off the aeration systemAllow the solids to settleDecant the clear liquidControl:D.O. (At least 1 mg/L)MixingOrganic loadingAerobic DigestionOPERATIONSolids loading is typically in the range of0.02 to 0.15 lb VSS/ft3/dayDT required depends on temperature & objectives(Part 503)Class B PathogenReduction40daysat 20oC (68oF)Vector AttractionReduction Requirements 60 days at 15oC (59oF)Min 38% VS ReductionMax SOUR 1.5 mg/h/G4

6/8/2012Aerobic DigestionOPERATIONVS reductions over 40% possible depending onsource of solids, D.O., temperature, detention timeExpect lower values if digesting only wasteactivated sludgeExpect lower values if digesting waste activatedsludge from extended aeration processExpect lower values as temperature decreasesAerobic DigestionMay have to contend with foaming Low dissolved oxygen concentrations Improper organic loading rate High oil and grease Growth of Nocardia or Microthrix ParvicellaOther filamentous populations may develop inthe aerobic digesterAutothermal Thermophilic Aerobic Digestion(ATAD)Heat released by organic decomposition duringdigestion typically sustains the thermophilic operatingtemperatures122 to 158ºFHigher loading – septage, primary, secondary sludgeMore rapid digestion process when compared toconventional aerobic digestionSolids retention time (12 to 14 days)5

6/8/2012Autothermal Thermophilic Aerobic Digestion(ATAD)Typically destroys 50 to 70 % of the VSSReduces the total solids by nearly 50%Dewaters wellPasteurized (Class A) biosolidsCity of Three Rivers, 20022.75 mgd WWTPSolids Handling ProcessesAnaerobic Digestion6

6/8/2012Anaerobic DigestionThe first anaerobic digester was built by a lepercolony in Bombay, India, in 1859Series of processes in which microorganisms breakdown biodegradable material in the absence ofoxygenAnaerobic DigestionMethane-forming bacteriaAbout 50 species of methane-forming bacteriaArchaebacteria - extremophilesOxygen-sensitiveExtreme Obligate AnaerobicKilled rapidly by relatively short timeexposures to airGeneration times3 days at 95 F50 days at 50 FAnaerobic DigestionMethane-forming bacteria are mesophiles orthermophilesMesophiles are those organisms that grow bestwithin the temperature range of 86 - 95 FThermophiles are those organisms that grow bestwithin the temperature range of 122 - 140 F7

6/8/2012Many Different Types of Anaerobic DigestersCovered anaerobic lagoon digester: Sealed withflexible cover, with methane recovered and piped tothe combustion device.Many Different Types of Anaerobic DigestersPlug flow digester: Long, narrow concrete tank witha rigid or flexible cover. Plug flow digesters are usedon farming operations to treat manure. High solids,mixed or not mixed.Many Different Types of Anaerobic DigestersComplete mix digester: Enclosed, heated tank with amechanical, hydraulic, or gas mixing system.8

6/8/2012Anaerobic DigestionJackson WWTPAnaerobic DigestionADVANTAGESRenewable EnergyProduces 12-15 ft3methane per lb VS digested65% methane yields650 Btu per cubic footDestroys Pathogenic OrganismsReduces volatile content of sludgeProduces much less biomass thanaerobic processesSludge is more easily dewatered9

6/8/2012Anaerobic DigestionDISADVANTAGESSlow start-upRequires proper start-up, close monitoringMay go SOUR if started improperly,overloaded, improper temp range, toxicityHigh BOD & P in supernatantSealed tanks – cleaning, maintenance more difficultHeating, mixing, gas collection equipment &plumbing adds cost, complexityExtreme confined space hazardProduction of explosive gasNorth Lewiston , IDJan 21, 2008“An explosion at a wastewatertreatment plant caused 3million to 5 million in damages,but no one was injured and theplant is still able to operate,authorities said.”Capt. Tom Hatley of theLewiston Fire Department saidcrews were dispatched Sundayat 7:45 a.m., minutes after whathe said sounded like a sonicboom from the city’s north end.“It pretty much rocked ourworld,” he told the LewistonTribune."A biogas plant is not 'plug and go': you feed itcarefully, you nurse it when it is not well and,most importantly, you must know what to dowhen something goes wrong."Safety issues include risk of explosion, confinedspace asphyxiation, and hydrogen sulfidepoisoning.10

6/8/2012Operation(Mesophilic)Adequate MixingTemperature (85 – 100oF)Organic Loading40 to 80 lbs VSS/day/1000 ft3Hydraulic LoadingDetention time 0.5 VA/ALK RatioGas Production30 – 35 % CO2Anaerobic DigestionClass B Pathogen ReductionMean cell residence time andtemperature shall be between15 days at 35 to 55oC (95 to131oF) and 60 days at 20oC(68oF).Vector Attraction ReductionVolatile Solids must be reducedby a minimum of 38%.Anaerobic DigestionConcernsAny amount of dissolved oxygen in an anaerobicdigester raises the ORP of the sludge anddiscourages anaerobic activitySludges and wastewaters fed to an anaerobicdigester should have no molecular oxygenWAS will slow anaerobic digestion due to high ORP11

6/8/2012Anaerobic DigestionFoamingExcessive MixingRapid breakdown of volatile acidsInconsistent loadingTemperature fluctuationsOverloadToxicityFilamentous Bacterial GrowthNocardiaMicrothrix ParvicellaAerobic DigestionAnaerobic DigestionSimple Start-upEnergy ProductionSimple OperationSludge is moreeasily dewateredBest for SecondarySludgeEnergy ConsumerBest for Primary SludgeMore Sensitive,Some Danger12

(Aerobic Digestion) Stored Food Aerobic Digestion Endogenous Stabilization High CRT allows for the microbes to feed off of the cell contents of other dying/decaying microbes under digestion. 20 to 25% by weight inert solids Fine inorganic solids, organic solids, and cell components that are not degradable Not all solids can be digested Aerobic .