Optimisationofsludgelinemanagementtoenhancephospho

OptimisationofsludgelinemanagementtoenhancephosphorusrecoveryinWWTPN.Martia,J.Ferrerb,A.Secoa,A.Bouzasa,*aDepartamentoIngenierı´aQuı´mica,UniversidaddeValencia,DoctorMoliner50,46100Burjassot,Valencia,SpainbInstitutodeIngenierı´adelAguayMedioAmbiente,UniversidadPolite´cnicadeValencia,CaminodeVeras/n,46022Valencia,SpainarticleinfoArticlehistory:Received23May2008Receivedinrevisedform18July2008Accepted11August2008Publishedonline20August2008Keywords:AnaerobicdigestionPhosphorusPrecipitationSludgetreatmentStruviteWastewaterabstractThemanagementofthesludgetreatmentlinecanbeoptimizedtoreduceuncontrolledphosphorusprecipitationintheanaerobicdigesterandtoenhancephosphorusrecoveryinWWTP.Inthispaper,fouroperationalstrategies,whicharebasedonthehandlingoftheprefermentedprimarysludgeandthesecondarysludgefromanEBPRprocess,havebeentestedinapilotplant.Theseparatedormixedsludgethickening,theuseofastirredcontacttankandtheelutriationofthethickenedsludgearethemainstrategiesstudied.Boththereductionofphosphorusprecipitationinthedigesterandthesupernatantsuit-abilityforastruvitecrystallizationprocesswereassessedineachconfiguration.Themixedsludgethickeningcombinedwithahighflowrateelutriationstreamreducedthephos-phorusprecipitationinthedigesterby46%,withrespecttotheseparatesludgethickeningconfiguration(commonpracticeinWWTP).Moreover,inthisconfiguration,68%ofthesolublephosphorusinthesystemisavailableforapossiblephosphorusrecoveryprocessbycrystallization(notstudiedinthiswork).However,ahighCa/Pmolarratiowasdetectedintheresultantsupernatantwhichispointedoutasaproblemfortheefficiencyofstruvitecrystallization.ª2008ElsevierLtd.Allrightsreserved.1.IntroductionEutrophicationofwaterbodiescausedbyanincreasedinputofinorganicphosphorusisamajorworldwideproblem(de-BashanandBashan,2004).Amongthedifferentmethodsdeveloped,theenhancedbiologicalphosphorusremoval(EBPR)processappearsasafeasibletechnologytoremovephosphorusinwastewatertreatmentplants(WWTP).Incontrasttoconventionalplants,thephosphoruscontentofactivatedsludgeinEBPRprocessesreachesvaluesofupto7%(JardinandPo¨pel,1994)duetopolyphosphate(Poly-P)storagebypolyphosphateaccumulatingorganisms(PAO).Ontheotherhand,phosphorusrecoveryandreusehasbecomeimportantinrecentyearssincephosphorusisalimitedresource.Afterhumanconsumption,mostofthephosphorusremainsinwastewaters.Forthisreason,phos-phaterecoveryfromwastewatersisconsideredtobeasuit-ablemethodtopromotephosphorusrecycling.Inthesludgetreatmentline,especiallyintheanaerobicdigestionprocess,phosphorusfromPoly-Phydrolysisandfromorganicmatterdegradationisreleased,increasingtheorthophosphatecontentinthesystemand,therefore,thepotentialforphosphorusprecipitation.Theconcentrationofotherionssuchasammonium,potassiumandmagnesiumalsoincreasesinthedigester.Furthermore,thehardnessofinfluentwastewateralsocontrolsprecipitationprocessesofphosphorusinsidethedigestersinceitdeterminesCa2þandMg2þconcentrationsinthewastewater.Accordingtotheliterature(Wildetal.,1997),differentphosphatecompoundsarelikelytoprecipitateinthesludge*Correspondingauthor.Tel.:þ34963544541;fax:þ34963544898.E-mailaddress:[e-mail](A.Bouzas).Availableat–seefrontmatterª2008ElsevierLtd.Allrightsreserved.doi:10.1016/j.watres.2008.08.012waterresearch42(2008)4609–4618treatmentsystemcausingsignificantoperationalproblemsthatleadtoanincreaseinmaintenancecosts.Accumulationofstruvite(MgNH4PO4$6H2O)onpipewallsandequipmentsurfacesofanaerobicdigestionandpost-digestionprocesseshasbeenreportedasafrequentprobleminthewastewatertreatmentindustry(Ohlingeretal.,1998;ParsonsandDoyle,2004).StruviteprecipitationoccurswhenthecombinedconcentrationsofMg2þ,NH4-NandPO4-Pexceedthestruvitesolubilityproduct.SincespecificationofthesecomponentsisapHfunction,struvitesolubilityalsovarieswithpH.AspHincreases,struvitesolubilitydecreases.Therefore,anaerobicdigestionandpost-digestionprocesses,whichshowahigherpHvaluethanthepreviousstagesofthewastewatertreat-ment,aremoresusceptibletostruviteformation.Phosphorusprecipitationinthesludgelinealsodeter-minesthedissolvedphosphorusconcentrationinthereturnliquorstream,whichcausesvariationsthatmightaffecttheefficiencyoftheEBPRprocess.Therefore,inordertoassureasuccessfulnutrientremovalprocess,recirculatedliquorstothewastewatertreatmentshouldbetreated.Phosphorusrecoverybystruvitecrystallizationisoneofthetechnologiesdevelopedfortreatingsludgedigesterliquors.Theproductobtained,struvite,isavaluableslow-releasefertiliserforagricultureand,hence,aneconomicalbenefitcanbeobtained.InordertoguaranteeP-richstreamsandtherebytoobtainahighstruviteproductioninthecrys-tallizationprocess,theuncontrolledphosphorusprecipitationinthedigestershouldbereduced.Otherfactors,suchasthecalciumconcentrationinthecrystallizerinfluentstream,cansignificantlyaffecttheproductionofstruvite(Pastoretal.,2007).Therefore,suitablecharacteristicsofthesupernatantstreatedbycrystallizationshouldbeguaranteed.Asimplemethodologytoassessphosphorusprecipitationinanaerobicdigestersbas