Phosphorusrecoveryby

PhosphorusrecoverybystruvitecrystallizationinWWTPs:InfluenceofthesludgetreatmentlineoperationN.Martı´a,L.Pastorb,A.Bouzasa,*,J.Ferrerb,A.SecoaaDpto.Ingenierı´aQuı´mica,UniversidaddeValencia,DoctorMoliner50,46100Burjassot,Valencia,SpainbInstitutodeIngenierı´adelAguayMedioAmbiente(IIAMA),UniversidadPolite´cnicadeValencia,CaminodeVera,s/n46022Valencia,SpainarticleinfoArticlehistory:Received23July2009Receivedinrevisedform15December2009Accepted28December2009Availableonline7January2010Keywords:CrystallizationPhosphorusrecoverySludgetreatmentStruviteWastewaterabstractPhosphorusrecoverybystruvite(MgNH4PO4$6H2O)crystallizationisoneofthemostwidelyrecommendedtechnologiesfortreatingsludgedigesterliquorsespeciallyinwastewatertreatmentsplants(WWTP)withenhancedbiologicalphosphorusremoval(EBPR).Inthispaper,phosphorusrecoverybystruvitecrystallizationisassessedusingtherejectedliquorsresultingfromfourdifferentoperationalstrategiesofthesludgetreatmentline.Phosphorusprecipitationandrecoveryefficienciesofbetween80–90%and70–85%,respectively,wereachievedinthefourexperiments.Theprecipitatesformedweremainlystruvite,followedbyamorphouscalciumphosphateand,insomeexperiments,bycalcite.Thehighestglobalphosphorusrecoverytakingintoaccountboththesludgelineandthecrystallizerwasachievedwhenmixedthickeningandhighelutriationwerecarriedout(8.4gP/kgtreatedsludge).However,lowstruvitecontentwasobtainedinthecrystallizerwiththisoperationschemeduetothehighcalciumcontentintheelutriationstream.Therefore,ifthefinalpurposeistoobtainstruvite,thereductionoftheelutriationflowrateiswidelyrecommendedinthecaseofhighwaterhardness.ª2010ElsevierLtd.Allrightsreserved.1.IntroductionPhosphorusrecoverybystruvite(MgNH4PO4$6H2O-MAP)crystallizationisoneofthemostwidelyrecommendedtech-nologiesfortreatingsludgedigesterliquors(vonMu¨nchetal.,2001;Battistonietal.,2002;ParsonsandDoyle,2004)especiallyinwastewatertreatmentsplants(WWTP)withenhancedbiologicalphosphorusremoval(EBPR).IntheEBPRprocess,phosphatesandotherions(i.e.,Mg2þ,K2þ)aretakenupandstoredaspolyphosphates(Poly-P)insidethebacterialcells.Duringtheanaerobicdigestionprocess,thesepolyphosphatesarereleasedtotheliquidphase(Wildetal.,1997)increasingnotablythephosphate,magnesiumandpotassiumconcentration.Moreover,ammoniumconcentrationincreasessignificantlyasproteinsaredegraded,anddissolvedmagnesium,phosphorus,calciumandpotassiumconcentra-tionsincreaseduetothecelllysis.Therefore,therejectedliquorsfromdigestedsludgedewateringshowhighphos-phorus,ammoniumandmagnesiumconcentrationswhichmakethesestreamsveryappropriateforrecoveringphos-phorusasstruviteinacrystallizationprocess.ThethickenersupernatantcouldalsobeusedinthecrystallizersincePoly-Phydrolysiscantakeplaceinthegravitythickener.Otherstudieshaveshownthattheproductobtainedbystruvitecrystallizationcanbeusedasaneffectiveslowreleasefertilizerinagriculture(de-Bashanetal.,2004;Shuetal.,2006);hence,aneconomicalbenefitcanbeobtained.Theuseofstruviteasfertilizerallowsnotonlytherecoverybutalsothe*Correspondingauthor.Tel.:þ34963544541;fax:þ34963544898.E-mailaddress:[e-mail](A.Bouzas).Availableat(2010)2371–23790043-1354/$–seefrontmatterª2010ElsevierLtd.Allrightsreserved.doi:10.1016/j.watres.2009.12.043reuseofnutrients,therebyincreasingthesustainablemanagementoftheWWTP.Accumulationofstruvite(MgNH4PO4$6H2O)onpipewallsandequipmentsurfacesofanaerobicdigestionandpost-digestionprocessessignificantlyincreasesoperationalprob-lemsandmaintenancecostsofWWTPs(Ohlingeretal.,1998;ParsonsandDoyle,2004).Thisuncontrolledprecipitationreducesphosphorusconcentrationinthesludgeliquors.Therefore,inordertoguaranteephosphorusrichstreamsandtherebyobtainhighstruviteproductioninthecrystallizationprocess,theuncontrolledphosphorusprecipitationinthedigestershouldbereduced.Inapreviouswork(Martı´etal.,2008a),fouroperationalstrategiesweretestedinapilotplantinordertominimiseuncontrolledphosphorusprecipitationinthedigesterandtoincreasethephosphorustoberecovered.Inthepresentwork,phosphorusrecoveryasstruvitecrystallizationisassessedusingtherejectedliquorsresultingfromthesefouroperationalstrategies.Finally,thisstudyevaluatestheglobalphosphorusrecoverybystruvitecrystallizationinthesystemtakingintoaccountboththesludgelineandthecrystallizer.2.Materialsandmethods2.1.PilotplantdescriptionThepilotsystemusedinthisworkwascomposedofdifferentelements:aprimarysludgesettler/fermenter,abiologicalnutrientremovalsystem,ananaerobicdigesterforthestabi-lizationofprimaryandsecondarywastesludge,andacrys-tallizationreactor.AllofthemarelocatedintheCarraixetWWTP(Valencia),exceptthecrystallizer(whichislocatedintheEnvironmentalTechnologiesLaboratoryoftheUniversityofValencia).AdetaileddescriptionofthesethreepilotplantscanbefoundatBouzasetal.(2007),Garcı´a-Usachetal.(2006)andMartı´etal.(2008b),respectively.Thecrystallizationpilotplant(Fig.1)iscomposedofthecrystallizationreactor,threestainlesssteelinjectiontubesfortheinfluentandreactants,twoperistalticpumps,onemembranep