AnoxicgrowthofphosphateaccumulatingorganismsPAOs

WaterResearch36(2002)4927–4937Anoxicgrowthofphosphate-accumulatingorganisms(PAOs)inbiologicalnutrientremovalactivatedsludgesystemsZhi-rongHu*,M.C.Wentzel,G.A.EkamaDepartmentofCivilEngineering,WaterResearchGroup,UniversityofCapeTown,Rondebosch7701,Cape,SouthAfricaAbstractInthispaper,researchonthegrowthperformanceofphosphate-accumulatingorganisms(PAOs)wasconductedbasedonliteratureandexperimentalinvestigationsonbiologicalnutrientremoval(BNR)activatedsludge(BNRAS)systems.TheresearchaimsatpresentingtheoccurrenceofdenitrifyingPAOs(DPAOs),abstractinginformationonthekineticsandstoichiometryofPAOsunderanoxicconditionsanddeterminingtheconditionsthatstimulatethePAOgrowthunderanoxicconditions.TheresearchresultsindicatethatthePAOsarecapableofutilizingnitrateaselectronacceptorinsteadofoxygeninBNRASsystems,particularlyinexternalnitrificationBNRAS(ENBNRAS)systems.However,thegrowthyieldofPAOsunderanoxicconditionsshouldbereducedtoabout70%ofthatunderaerobicconditions,andfurtherthestoichiometriccoefficientforanoxicPuptakeperPHBCODutilizedshouldbereducedtoabout80%ofthatunderaerobicconditionsastheDPAOsshowasignificantlylowerBEPRperformanceandusetheinfluentRBCODless‘‘efficiently’’comparedwithaerobicPAOs(APAOs).TheresearchresultsalsoindicatethatthemajorfactorinfluencingtheoccurrenceofDPAOsandassociatedanoxicPuptakeisthenitrateloadintotheanoxicreactor,i.e.thenitrateloadshouldbelargeenoughorexceedsthedenitrificationpotentialofordinaryheterotrophicorganisms(OHOs),i.e.non-PAOorganismsintheanoxicreactortostimulateDPAOsinthesystemasthespecificdenitrificationrateofOHOs(K02OHO)issignificantlylargerthanthatofPAOs(K02PAO).Intermsofthiscompetition,ifthenitrateloadintothemainanoxicreactorislessthanthedenitrificationpotentialofOHOs,thentheOHOswilloutcompetePAOsforusingthelimitednitrate,whileifthenitrateloadinthemainanoxicreactorexceedsthedenitrificationpotentialofOHOs,thenthePAOswouldhaveopportunitiestousethe‘‘excess’’nitrateandsodevelopinthesystem.TheotherfactorsthatinfluenceDPAOsincludethesystemaerobicmassfraction,sequenceofreactorsandfrequencyofsludgealternationbetweentheaerobicandanoxicstates.AlthoughitdoesappearthatthesefactorsabovemaysignificantlyinfluencethefractionofDPAOs(ZG),thequantitativerelationshipbetweenthesefactorsandZGisnotknown,andtheexperimentalobservationsindicatethatthiswillbesystem-specific,andrequirecalibrationforeachsituation.r2002ElsevierScienceLtd.Allrightsreserved.Keywords:Denitrificationkinetics;Biologicalexcessphosphorusremoval(BEPR);Biologicalnutrientremoval(BNR);Anoxicgrowth;DenitrifyingPAOs1.IntroductionIntheUCTPHOkineticmodelforbiologicalnutrientremovalactivatedsludge(BNRAS)systems[1],onlytheaerobicgrowthofPAOsandtheirassociatedprocesseswereincludedandnottheanoxicgrowthofPAOsandassociatedprocesses,eventhoughthismodeldrewprocessandstoichiometricinformationfromthebio-chemicalmodelsforthePAObehaviouravailableatthetime[2,3]whichincludedenitrifyingPAO(DPAO)behaviour.TheDPAObehaviourwasnotincludedinUCTPHObecause:*TheexperimentaldatafromwhichUCTPHOwasdevelopedandcalibratedwerefromthelaboratory*Correspondingauthor.DepartmentofCivilandConstruc-tion,IowastateUniversity,Ames,IA50010,USA.E-mailaddress:[e-mail](Z.-R.Hu).0043-1354/02/$-seefrontmatterr2002ElsevierScienceLtd.Allrightsreserved.PII:S0043-1354(02)00186-0systemswhichexhibitedprincipallyaerobicPuptake(90%),inparticular:(i)thePAOenhancedculturesystems[4,5]and(ii)themixedcultureBNRASsystemsreceivingrealwastewaterofClaytonetal.[6].*AtthattimeinsufficientdatawereavailableonDPAObehaviourtoformulatethekineticsandstoichiometryoftheprocessesmediatedbythisgroupoforganisms.Since1992,whentheUCTPHOmodelwasdeveloped,thisphenomenonhasalsobeenreportedmorefre-quentlyinconventionalBNRASsystemsatlab-andfull-scale[7–12],particularly,intheDEPHANOXsystemswheredenitrificationbyPAOswasexploitedpurposely[13–15],andexternalnitrificationBNRAS(ENBNRAS)systemswheretheconditionsthatarecreatedfavourthedenitrificationandanoxicPuptakebyPAOs[16–19].Therefore,DPAOs(theanoxicgrowthofPAOsandassociateddenitrificationandanoxicPuptake)needtobeincludedintotheBNRkineticmodel;thisrequiresinformationonthekineticandstoichio-metricbehaviouroftheseorganisms.Therearetwoapproachestoobtainthisinformation:*ThefirstapproachistoobtaininformationonthegrowthanddeathcharacteristicsofDPAOsfromenhancedculturestudies.Wentzeletal.[4,5,20]successfullyfollowedthisapproachtodeveloptheaerobicPuptakeBEPRsectionoftheUCTPHOkineticmodel.Huetal.[17]describesanendeavourtodevelopsuchanenhancedcultureforDPAOsinacontinuous-flowBNRASsystem.*Thesecondapproachistocollectrelevantinforma-tionfromliterature,fromthisinformationhypothe-sizeonthebehaviourcharacteristicsofDPAOsandincorporatetheseinthemodel,andthenbysimula-tionobtaintheoreticalresponsesthatcanbecom-paredwiththoseobservedinliterature,inthismannertoassessinwhatmeasurethemodelisrelevant.Sincethefirstapproachprovedunsuccessful[17],thesecondapproachhadtobeadopted.Accordingly,inthispaperthestudyontheDPAObehaviourwillbeundertakentocollectsuchinformation,inparticular,inthefollowingaspects:(1)tocollect