Hydrolysisandfermentationofactivatedsludgetoenhanc

HydrolysisandfermentationofactivatedsludgetoenhancebiologicalphosphorusremovalJ.Vollertsen*,G.Petersen**andV.R.Borregaard****AalborgUniversity,SectionofEnvironmentalEngineering,Sohngaardsholmsvej57,9000Aalborg,Denmark(E-mail:[e-mail])**EnviDan,Fuglebækvej1B,2770Kastrup,Denmark***AalborgKommune,Kloakforsyningen,StigsborgBrygge5,Postboks222,9400Nørresundby,DenmarkAbstractTheconventionalmainstreamenhancedbiologicalphosphorusremoval(EBPR)processdependsonthequalityoftherawincomingwastewater.AnalternativesidestreamEBPRprocessispresented,wherethesubstratesforstoragebythepolyphosphateaccumulatingorganisms(PAOs)insteadcomefromhydrolysisofthereturnactivatedsludge.Thisprocessisstudiedinfull-scaleattwotreatmentplantsandquantifiedbymeansofphosphorusreleaseratesandreadilybiodegradableCOD(RBCOD)accumulationrates.ItwasseenthatnotonlywasasignificantamountofRBCODstoredbyPAOsbutanapproximatelyequalamountwasaccumulatedinthesidestreamhydrolysistankandmadeavailableforthesubsequentnitrogenremovalprocess.Thephosphorusreleaseofthesludgewithandwithoutadditionofdifferentsubstrateswasfurthermorestudiedinlaboratoryscale.ThestudyshowedthattheprocessispromisingandinanumberofcaseswillhavesignificantadvantagescomparedwiththeconventionalmainstreamEBPRdesign.KeywordsEBPR;enhancedbiologicalphosphorusremoval;hydrolysis;readilybiodegradableCODIntroductionTheactivatedsludgeenhancedbiologicalphosphorusremoval(EBPR)processhasbecomewidelyimplemented,astheprocesshaseconomicandenvironmentaladvantagescomparedwithchemicalprecipitation.EBPRreducesthecostforchemicalsand,moreimportantly,itreducestheamountofsurplussludgeproduced,resultinginreducedsludgemanagementcostsandreducedenvironmentalimpactsfromsludgedisposal.Avoidingorreducingthechemicalsludgeproducedbyphosphorusprecipitation,EBPRalsoallowsahigherorganicmattercontentoftheactivatedsludge.TheEBPRprocessisdependentonredoxconditionsalternatingbetweenanaerobicandaerobic/anoxicconditions.Duringanaerobicconditions,readilybiodegradableCOD(RBCOD)isstoredwithinthecellsandunderaerobic/anoxicconditionsthisstorageisuti-lised.Themicroorganismsstoreenergy-richcompoundsunderaerobic/anoxicconditionssothattheypossessanenergyreservetofuelstorageofRBCODunderanaerobicconditions.Somebacteria–polyphosphate-accumulatingmicroorganisms(PAOs)–storephos-phorus,whileotherbacteria–glycogen-accumulatingmicroorganisms(GAOs)–storeglycogen(e.g.Wentzeletal.,1986;Liuetal.,1996;Caruccietal.,1999a,b).TheRBCODintheformofVFAscanbetakenupandstoredaspolyhydroxyalkano-ates(PHAs)(e.g.Wentzeletal.,1986;Satohetal.,1996).Inaddition,Caruccietal.(1999a,b)showthatglucoseandothercarbohydratescanbedirectlytakenupduringanaerobicconditionsandstoredasglycogen.Theorganismsinvolvedcanusepolyphos-phatehydrolysisorfermentationastheenergysource.TheyobservedthatcarbohydratescouldbestoredwithoutinvolvingPAO-likeorGAO-likeorganisms,andtheorisethatthecompetitionbetweenthedifferentgroupsofmicroorganismscapableofstoringRBCODWaterScience&TechnologyVol53No12pp55–64QIWAPublishing200655doi:10.2166/wst.2006.406canexplainthehighvariabilityinP-removalratesoftenencounteredinfull-scaletreatmentplants.Also,forexample,Shintanietal.(2000)describeabacteriumcapableofanaerobicglucoseuptakeandglycogensynthesis.TheconventionalmainstreamplantlayoutforEBPRcontainsananaerobictanktowhichallincomingrawwastewaterisfed,togetherwiththeactivatedsludgereturnedfromtheclarifier(Figure1).BiomasscapableofanaerobicsubstratestorageisfavouredasittakesuptheRBCODoftheincomingwastewater,i.e.PAOs,GAOsandothertypesofsubstratestoringmicroorganismsgainacompetitiveadvantage.WithrespecttoEBPR,onlyorganismsthatusestoredpolyphosphateastheenergysourcearedesirable,astheyaccumulatephosphorusandtherebyallowthebiologicalphosphorusremovalprocesstotakeplace.OthertypesofRBCODstoringorganismsintroduceanunwelcomelossofRBCOD,andtheirpresenceresultsinanincreasedratioofRBCODtophosphorusneededtoachievebiologicalphosphorusremoval.IntheplantdesignsketchedinFigure1,theRBCODfortheEBPRissuppliedbytheincomingwastewater,andthedesignbecomesdependentonthequalityofthiswaste-water.Especiallyduringextendedperiodswithhighprecipitation,thewastewaterqualitycandeteriorateandtheresidencetimeintheanaerobiczoneofthetreatmentplantcanbecomecruciallylow,resultinginapoorefficiencyofthebiologicalphosphorusremovalprocess.WhenwastewaterqualityispoorintermsofRBCOD,substratecanbeaddedas,forexample,molasses.Fortreatmentplantswithprimarytreatment,primarysludgecanbehydrolysedanddosedasextrasubstratefortheEBPRprocess(e.g.Brinchetal.,1994;Canzianietal.,1995).Ontheotherhand,whenprimarytreatmentiscombinedwithsludgedigestiontoproducebiogas,hydrolysisofprimarysludgewilldiminishthebiogasyield.InDenmark,typicallevelsofVFAsinrawwastewaterareratherlow,andEBPRmustrelyonotherformsofRBCODandonfermentationintheanaerobictank.Forexample,forrawwastewaterenteringoneofthetreatmentplantsstudied(WWTPWest),themedianRBCODwasfoundtobe150gCODm23,whereasVFAwasaslowas1gm23(Vollertsen,2002).Asanalternativetoadditionofsubstrateandtoprimarysludgehydrolysis,nearly30Danishtreatmentpl