Anaerobic digestion is often used to treat or stab.docx
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Anaerobic digestion is often used to treat or stab.docx
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Anaerobicdigestionisoftenusedtotreatorstab
EXCESSSLUDGEPRODUCTIONANDCOSTSDUETOPHOSPHORUSREMOVAL
PaulEtienne*,LavalMarie-Line,SpérandioMathieu
LaboratoryofEnvironmentalProcessEngineering.DepartmentofIndustrialProcessEngineering,NationalInstituteofAppliedSciences.135AV.deRangueil31077ToulouseCedex4,France.
Email :
paul@insa-tlse.fr
ABSTRACT
Basedonadatacollectedfrom35Frenchwastewatertreatmentplantsandonpublisheddata,excesssludgeproductionandchemicalconsumptionassociatedwithPhosphorusremovalisestimatedforthethreefollowingphosphorusremovalprocesses :
chemicalprecipitation,EnhancedBiologicalPhosphorusRemovalandhybridprocess.Theinfluenceofwastewatercharacteristicsonexcesssludgeproductionareassessed.ChemicalcostsandcostsassociatedwithsludgedisposalwerecalculatedandresultsforthethreePhosphorusremovalprocessesarecompared.TheglobalcostforPhosphorusremovalisthenestimated.
Keywords:
urbanwastewatertreatment,chemicalprecipitation,Biologicalphosphorusremoval,sludgeproduction,costs
INTRODUCTION
Phosphorusisconsideredoneofthelimitingnutrientsinmostfreshwaterlakes,reservoirsandriversandsoalowPconcentrationmaycontrolalgaeboomsandeutrophication.Phosphorusinputsfrompointsourcessuchasmunicipalsewageeffluentsaremoreamenabletocontrolthanfromnon-pointsources.Therefore,regulationsforphosphorusdischargesinsensitiveareashavebeensetbytheEUUrbanWastewaterDirective(97/271/EEC).Insewagewastewater,phosphoruscomesmainlyfromhumanwastesanddetergent(about30%oftotalPinsewageinFrance[1]).Duringwastewatertreatment,partofthesolublephosphateistransferredtoasolidphase,generallyentrappedintotheorganicsludges.ThisisachievedduringnormalbiologicaldegradationprocessesbutcanalsobeachievedbyEnhancedBiologicalNutrientRemovalorbyaphysico-chemicalprocessafterchemicaladdition.Itisevidentthatphosphorusremovalincreasesthecostofwastewatertreatment.Thisisduetoinvestmentcosts,chemicalcostsandincreasedamountsofsludgetobedisposedof.
Phosphorusremovedfromwastewatercanberecycledtogetherwithsludgesforlandapplication,hencedecreasingthephosphorusinputfromfertilisers.Howevercontaminationofsludgeplacesthisdisposalrouteundermoreandmorefinancialandsocialpressure.Inaddition,theagriculturalmarketnowdemandsaconsistentandassuredquality.ThecostsforlanddisposalofsludgesisincreasingandPremovalmayresultinfurtheradditionalcostsforsludgedisposal(reductionoflandapplicationrateandfrequencies)[2].Therefore,Precoverymaybeattractiveifthesludgemasstobedisposedofissignificantlyreduced[3].
Thispaperestimatestheexcesssludgeproductionandthespecificcosts(€.Kg-1Pinfluent)relatedtoPremovalintheurbanwastewatertreatmentfield.Toreachthisobjective,wefirstexplainthehypothesesmadeforcalculations.Parameterssuchaswastewatercharacteristics,typeofchemicalsandchemicalprocessesusedforP-removal,thechemicaldosageappliedforprecipitation,etc.aregivenbasedbothonbibliographicdataandondatafrom35Frenchwastewatertreatmentplantswhereatleastan80%Premovalisachieved(fromatotalof77plantswhichrespondedtooursurvey,35achievedthislevelof80%).Basedonthedefinedvaluesfortheseparameters,theexcesssludgeproductionduetophosphorusremovalisthencalculated,consideringdifferentstrategiesforPremoval.SpecificcostsassociatedtoPremovalandalsothetotalcostinFranceisfinallyestimated.Inaddition,theimpactofphosphoruscomingfromdetergentsisdiscussed.
METHODS
Wastewatercharacteristics
InfluentwastewatercharacteristicshaveagreatimportanceonbiologicalP-removalcapacity.ThecontributionofPbypopulationequivalentwasfirstassessed.Valuesrangedbetween1.7[4,5,6]or2[7]to2.7gp.e.-1.d-1PforwastewatersinEngland.Nowak[5]observedadecreaseinthisvaluewhenindustrialwastewaterismixedwithdomestic.Oursamplesurveyseemstoconfirmthistendencywithlowervaluessuchas1.3to1.5encountered.AstudymadebyGeoplus[1]givesvaluesof1.2to1.6gcapita-1.d-1P(meanvalue1.4gCapita-1.d-1P)forhumanwastes(urine+faeces),0.3gcapita-1.d-1Pforfoodwastes,and0.75gCapita-1.d-1Pfordetergents.ThisleadstoatotalPamountofaround2.5gcapita-1.d-1P.Asimilarvalueisfoundfromoursamplesurvey.Inthisstudywehavethereforeconsideredavalueof2.5gp.e.-1.d-1PasrepresentativeofPproductioninFrance.
ForCODproductionweconsideredanaveragevalueof135gp.e.-1.d-1CODcharacterisedbyaBOD/CODof0.5.For90%CODremoval,theCODremovedwillbe120gp.e.-1.d-1CODThemeanratiosofCOD/PandBOD/Parethenaround50and25respectively.
Typeofdephosphatationprocess
ThetypeofprocessesusedandthenatureofthechemicaladdedarealsorequiredtoassesscostsassociatedwithP-removal.Datafromthesurveyarepresentedinthissection.
Figure1:
percentageofthedifferenttreatmentprocessesforP-removal.ResultsofthesurveyofFrenchWWTP(total=47).
Inthesamplesurveycarriedoutinthisstudy,theproportionofphysico-chemical,EBPRandhybridEBPR+physico-chemicalprocessesisabout47%/17%/36%(figure1).Simultaneousprecipitationrepresentsthemajorityofthephysico-chemicalprocessesusedforP-removal.
Figure2:
proportionofthedifferentphysico-chemicaltreatmentprocessesforP-removal.ResultsofthesurveyofFrenchWWTP(total=39).
Chemicaldosage
Figure3:
proportionofthedifferentchemicalagentsusedinchemicaltreatmentprocessesforP-removal.ResultsofthesurveyofFrenchWWTP(total=36).
Figure4:
proportionofchemicalagentsusedinchemicaltreatmentprocessesforP-removal.ResultsofthesurveyofFrenchWWTP(total=36).
NinetythreepercentoftheplantssurveyeduseFe-basedchemicalsofinwhichFeCl3(commercial40%ferricchloridesolution)represents70%.Thisiscertainlyduetothesimplicityofuseofthisliquidproduct.Theseobservationsleadustobaseourcalculationsofexcesssludgeproductionandcostsconsideringonlyferricchloride.AsthemolarweightofFeishigherthanthatofAl,themineralexcesssludgeproducedwillbehigher.ThepriceofaluminiumbasedproductismuchhigherthanthepriceofFe-basedproductresultinginasignificantincreaseinthespecificcost(€.kg-1P).
Figure5:
meanpriceforthedifferentchemicalsusedintheFrenchWWTPconcernedbythesurvey
Pricevariationsareimportantandcanbeattributedtomarketfluctuations,transportcostsandlocalparameters(suchasdistancefromproducerindustriesorequipmentline…).Forourcalculations,wehaveconsideredtheironbasedchemicalstohaveameanpriceof100€.t-1.
Chemicaldosage
Chemicaldosageisanimportantparameterforcostestimation.Thephysico-chemicalmechanismsofP-removalinwastewatertreatmentareverycomplex.Inthecaseofironsalts,iron(III)ionsformstrongcomplexeswithpyrophosphateandtripolyphosphates,whichareprobablyremovedbyadsorptionontoiron(III)hydroxo-phosphatesurfaces[7]Competitionbetweenhydroxylandphosphateionsforironionsatthepointofaddition,thereactionofbicarbonateionsformingironhydroxides,andtheneedtodestabiliseironphosphatesandothercolloidsprobablyaccountforthestoichiometricexcessorvariationsofironrequiredforphosphateprecipitation.WiththeobjectivesimposedbytheEUUrbanwastewaterDirective(97/271/EEC),i.e.an80%Premovalyield(basedonPcontentoftherawwastewater)or1mgtotalPintheeffluent,themolarratioFe/Premovedusedinpractisevariesbetweenlessthan0.5and2.Usingresultsobtainedatvarioustreatmentplants,[8]showedthatthelowertheFe/Pratio,thehigherthedispersionintheeffluentPconcentration.ResultsofthesamplesurveyshowedFe/Premovedvaluesrangingfrom0.8and2forwastewatertreatmentplantswhosephosphorusremovalyieldwashigherthan80%.Ithastobepointedoutthat,inourbiomassgrowthconditions,atypicalmolarratioof1moleofFe/molePremovedcorrespondstoamolarratiobasedontotalphosphorusintherawwastewaterofaround0.6moleFe/moleP.
Pre-,post-andsimultaneousprecipitationisencounteredatwastewatertreatmentplants.Thechemicaldosagerequiredishigherforpre-andpost-treatmentsthanforsimultaneousremovalprocessandgenerallylargelyexceedsthestoichiometry.ForsimultaneousprecipitationthereactiontimeforP-removalisafunctionoftheSolidsRetentionTime(SRT).InspiteofsomegoodremovalperformancesobtainedatsewageworksusingamolarFe/Pinfluentoflessthan1(around0.6-0.8),wechoseforourcalculationsforsimultaneousprecipitation,aFe/Pinfluentof0.9whichcorrespondstoFe/Premovedof1.5.
Excesssludgeproduction
Inanactivatedsludgeprocess,sludgeproductionisduetothenetgrowthofthemicroorganismstheaccumulationofrefractoryorganiccompoundsandminerals.ThelattercanbesignificantlyincreasedwhenP-removalispractised.Biologicalsludgeproductionhasbeenstudiedatourlaboratory.Theobservedyieldoforganicsludgeproductionfollowedtheclassicalrelationshipdescribedinequation1.
withY=0.44gVSS.g-1CODconsumedandKd=0.05j-1.
SoforanSRTof15days,Yobsis0.251gVSS.g-1CODconsumed.AconstantVSS/TSSof0.8isappliedgivingaTotalSolidproductionof0.31gTSS.g-1CODconsumed.
Excesssludgeproductionduetophysico-chemicalprocesses
ForanFe/Pratioof1,whichisthetheoreticalstoechiometricrequirement,itisconsideredallFeisused
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