Selectionofslowgrowingorganismsasameans

SelectionofslowgrowingorganismsasameansforimprovingaerobicgranularsludgestabilityM.K.deKreukandM.C.M.vanLoosdrechtDepartmentofBiotechnology,DelftUniversityofTechnology,Julianalaan67,2628BC,Delft,TheNetherlands(E-mail:[e-mail];[e-mail])AbstractRecently,severalgroupshaveshowedtheoccurrenceofaerobicgranularsludge.Theexcellentsettlingcharacteristicsofaerobicgranularsludgeallowthedesignofverycompactwastewatertreatmentplants.Inlaboratoryexperiments,highoxygenconcentrationswereneededtoobtainstablegranulation.However,inordertoobtainenergyefficientaerationandgooddenitrificationlowoxygenconcentrationswouldberequired.Fromearlierresearchonbiofilmmorphology,itwaslearnedthatslowgrowingorganismsinfluencethedensityandstabilityofbiofilmspositively.Todecreasethegrowthrateoftheorganismsintheaerobicgranules,easilydegradablesubstrate(e.g.acetate)hastobeconvertedtoslowlydegradableCODlikemicrobialstoragepolymers(e.g.PHA).Phosphateorglycogenaccumulatingbacteriaperformthisconversionstepmostefficiently.Inthispaperitisshownthattheselectionofsuchbacteriainaerobicgranulesindeedledtostablegranularsludge,evenatlowoxygenconcentrations.KeywordsAerobicgranularsludge;growthrate;oxygenconcentration;SBR;stabilityIntroductionInrecentyears,asignificantpartofresearchonwastewatertreatmenthasfocussedondevelopmentofcompactsystems.Wastewatertreatmentplants(WWTP)canbedesignedtobecompactwhenbiomassismaintainedinthesystemwithouttheuseofconventionalsettlingtanksasinflocculatedactivatedsludgesystems.ExamplesaredevelopmentsinthefieldofMembraneBio-Reactors,BiologicalAeratedFiltersandotherbiofilmsystems.Forconventionalbiofilmtechnologythesurfaceareaofthebiofilms,neededformasstransferandthereforetheconversionprocesses,islimitedbythereactorsurfacethebiomasscanattachto.Whenbiofilmsaregrowninagranularshape,amaximumsurfaceareapervol-umeofbiomasscanbeobtained.Researchshowedthatadiscontinuousinfluentfeedstimu-latedthegrowthofdenseandsmoothaerobicgranularsludge(Beunetal.,2000;EttererandWilderer,2001;Tayetal.,2001).Tobeabletoapplyaerobicgranulationtechnologyonalargescale,themostimportantconditionsfortheformationofgranuleshavetobeunderstood.Sofar,theimportanceofthefollowingconditionsatlaboratoryscalehavebeendescribedinliterature:1.Theconversionofrapidlybiodegradablesubstratesintoslowbiodegradablestoredsub-stratebyapplyingafeast/famineregime(Villaseñoretal.,2000;Beunetal.,2001);2.Theselectionofrapidsettlinggranulesbyapplyingshortsettlingtimes(Beunetal.,2000);3.Sufficienthighshearstresswithinthereactorduringaeration(Beunetal.,2000;LiuandTay,2002).Thispaperwillfocusonthefirstconditionandtheimportanceofthegrowthrateofthemicro-organismsforstablegranuleformation.Experimentsandafull-scaledesignbasedonaerobicgranularsludgetechnologyshowedthatsimultaneousnitrification/denitrificationisanimportantprocesstoobtainsuf-ficientN-removal.Simultaneousnitrification/denitrificationinaerobicgranularsludgeisbasedontheoxidationofammoniaintheaerobicouterlayerofthegranule.ProducedWaterScienceandTechnologyVol49No11–12pp9–17©IWAPublishing20049nitratecandiffusetotheinteriorofthegranule,whereitservesaselectronacceptorformaintenanceandgrowthonstoredsubstrate.Thelowertheoxygenconcentration(DO),thelowertheoxygenpenetrationdepthandthusthehigherthevolumeofbiomassinananoxicenvironment(PochanaandKeller,1999;Beunetal.,2001).Besidestheimprovednitrogenremovalfromtheinfluent,aloweroxygenconcentrationalsoleadstoabetterprocessecon-omyatlarge-scaleapplication.However,Mosquera-Corraletal.(InPreparation)showedexperimentallythatalowoxygenconcentration(DO40%ofthesaturationconcentration)incombinationwithapulsefeedofeasilydegradablesubstrate,ledtogranuleinstability,filamentousgrowthandbiomasswashout.ControlofbiofilmstabilityAsuitableparametertoenhancethecontrolofgranuleorbiofilmstabilityistheactualgrowthrateofthemicro-organismsinsidethegranule(VanLoosdrecht,1995).Picioreanuetal.(1998)linkedthebiofilmstabilitytodiffusionofthesubstratecombinedwiththegrowthrateoftheorganisms,leadingtosubstrategradientsinsidethebiofilm.Ifsubstrategradientsinsideabiofilmorgranulearesharp,heterogeneousorfloc-likebiofilmswilldevelop.Ifthegradientsaregradual,moreregularbiofilmswillbeproduced.Picioreanuetal.(1998)expressedthisinacharacteristicnumberG(growth):(1)Grepresentsparametersthathighlyaffectbiofilmandgranulestructures,namely:thecon-centrationofsolublenutrientsinthebulk,CS0;thediffusioncoefficient,DS;thebiomassmaximumdensityinthebiofilm,Cxm;themaximumspecificmicrobialgrowthrate,µm;andthebiofilmthicknessorgranuleradius,LY.AlowerGwillresultinsmootherbiofilmsorbettergranuleformation.Thismeansthatstablegranulationatalowoxygenconcentra-tionshouldoccurwhenorganismsgrowatalowrate.HeterotrophicorganismshavedecreasedgrowthrateswhentheygrowontheslowlybiodegradablestoragepolymerPHBorglycogencomparedtothegrowthoneasilybiodegradablesubstratesuchasacetateorglucose(Cartaetal.,2001).Inpreviousaerobicgranularsludgeresearch,thisphenomenonwasusedtoobtainstablegranulation.Anaero-bicpulse-feedingperiodwasapplied,inwhicht