Improvedstabilityofaerobicgranulesbyselectingslowg

JournalofBiotechnology108(2004)161–169Improvedstabilityofaerobicgranulesbyselectingslow-growingnitrifyingbacteriaYuLiu∗,Shu-FangYang,Joo-HwaTayEnvironmentalEngineeringResearchCentre,SchoolofCivilandEnvironmentalEngineering,NanyangTechnologicalUniversity,50NanyangAvenue,Singapore639798,SingaporeReceived9September2003;receivedinrevisedform28October2003;accepted18November2003AbstractThisstudyinvestigatedthefeasibilityofimprovingthestabilityofaerobicgranulesthroughselectingslow-growingnitrifyingbacteria.Forthispurpose,foursequencingbatchreactorswereoperatedatdifferentsubstrateN/CODratiosrangingfrom5/100to30/100.Resultsshowedthataerobicgranulesformedinallfourreactors,andaerobicgranulationwasagradualprocessevolvingfromthedispersedseedsludgetomatureandstablegranules,andthewholegranulationprocesscouldbedividedintothreephases,i.e.acclimationphase,granulationfollowedbygranulematuration.TheobservedgrowthrateandmeansizeofmatureaerobicgranuleswerefoundtodecreaseasthesubstrateN/CODratiowasincreased,whilenitrifyingpopulationwasenrichedmarkedlyinaerobicgranulesdevelopedathighsubstrateN/CODratios.Theenrichednitrifyingpopulationinaerobicgranuleswasresponsiblefortheobservedlowgrowthrateofaerobicgranules.ItseemscertainthatthesubstrateN/CODratioisanimportantfactorinselectingnitrifyingbacteriainaerobicgranules.Aerobicgranuleswithlowgrowthratesshowedstrongstructureandgoodsettleabilityintermsofspecificgravity,SVIandcellhydrophobicitythatfurtherleadtohighstabilityascomparedtothosehavinghighgrowthrates.Thisstudydemonstratedthattheselectionofslow-growingnitrifyingbacteriathroughcontrollingsubstrateN/CODratiowouldbeausefulstrategyforimprovingthestabilityofaerobicgranules.©2003ElsevierB.V.Allrightsreserved.Keywords:Aerobicgranules;SubstrateN/CODratio;Growthrate;Granulestability;Nitrifyingbacteria;Heterotrophicbacteria1.IntroductionMicrobialimmobilizationisaprocessofcell-to-cellinteraction,orcell-to-carriersurfaceattachmentthatinvolvesbiological,physicalandchemicalactions.Therearethreemainmembersinthefamilyofmi-crobialimmobilization,so-calledanaerobic,aerobic∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](Y.Liu).granulesandbiofilms.Amongthesethreemembers,biofilmhasthelongestresearchhistory,whilethefirstserioustechnicalreportonanaerobicgranulationap-pearedin1980(Lettingaetal.,1980).Comparedtothedevelopmentofbiofilmsandanaerobicgranula-tiontechnologies,aerobicgranulationisarecentlyde-scribedphenomenon(Morgenrothetal.,1997;Beunetal.,1999;Pengetal.,1999;Tayetal.,2001;Moyetal.,2002).Aerobicgranuleshavesomeadvantagesoverconventionalbioflocs,suchasexcellentsettleabil-ity,highbiomassretentionandresistancetoinhibitory0168-1656/$–seefrontmatter©2003ElsevierB.V.Allrightsreserved.doi:10.1016/j.jbiotec.2003.11.008162Y.Liuetal./JournalofBiotechnology108(2004)161–169andtoxiccompoundsandsoon.Itisbelievedthataerobicgranulationwouldbeanovelandpromisedbiotechnologyforhandlingindustrialandmunicipalwastewater.Itappearsfrompreviousresearchthatthestabilityofaerobicgranulesispoorerthanthatofanaerobicgranulesdevelopedinupflowanaerobicsludgeblanket(UASB)reactor(Morgenrothetal.,1997;Pengetal.,1999;ZhuandLiu,1999).Thisisprobablyduetothefactthataerobicbacteriahaveamuchhighergrowthratethananaerobicbacteria.Itcanbeanticipatedthatthepoorstabilityofaerobicgranuleswouldlimititsapplicationinwastewatertreatmentpractice.Infact,itwasfoundthatthestabilityofbiofilmiscloselyrelatedtothegrowthrateofbacteria,i.e.highergrowthrateofbacteriaresultedinaweakerstructureofbiofilm(Tijhuisetal.,1995;Kwoketal.,1998;Liu,1997).Todate,almostallresearchonaerobicgranulationhasbeenmainlyfocusedonexploitingthefeasibilityofaerobicgranulationinsequencingbatchreactors.However,thequestionofhowtoimprovethestabilityofaerobicgranulesremainsuntouched.Therefore,thisstudyattemptedtolookintomicrobialselection-basedstrategyforimprovingthestabilityofaerobicgran-ules,andfurthercontributedtoasoundunderstand-ingofcell-to-cellimmobilization.Thesewouldbeveryusefulforthedevelopmentoffull-scaleaerobicgranules-basedbioreactorforwastewatertreatment.2.MaterialsandmethodsFoursequencingbatchreactors(SBR)withaheightof80cmand6cmindiameterwereoperatedwithacycletimeof4h.Effluentwasdischargedfromthemiddleportofthereactor.Fourreactorswereinocu-latedusing650mloffreshactivatedsludgetakenfromthenitrificationunitofalocalmunicipalwastewatertreatmentplant.Syntheticsubstrateconsistsofethanolassolecarbonsource,ammoniumchloride,sodiumbicarbonateandothernecessarynutrients.Tapwaterwasusedtopreparesyntheticwastewater.Theethanolchemicaloxygendemand(COD)concentrationwasfixedat500mgl−1,whiletheammonium-nitrogenconcentrationvariedfrom25to150mgl−1infourreactors,whichgavearespectivesubstrateN/CODweightratiofrom5/100to30/100.Tosatisfythegrowthrequirementofnitrifyingbacteria,theratioofbicarbonatetoammonium-nitrogenwaskeptcon-stantatavalueof8.0mgmg−1forallthereactors.Themicronutrientsinthesyntheticwastewatercanbefoundelsewhere(LiuandCapdeville,1994).Theminimumdissolvedoxygenconcentrationsdetectedinthereactorswereabove2.