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Treatmentofgeothermalwatersforproductionofindustrial,agriculturalordrinkingwaterDarrellL.Gallup∗ChevronCorporation,EnergyTechnologyCompany,3901BriarparkDr.,Houston,Texas77042,USAReceived14March2007;accepted16July2007Availableonline12September2007AbstractAconceptualstudyhasbeencarriedouttoconvertgeothermalwaterandcondensateintoavaluableindustrial,agriculturalordrinkingwaterresource.Laboratoryandfieldpilotteststudieswereusedfortheconceptualdesignsandpreliminarycostestimates,referredtotreatmentfacilitieshandling750kg/sofgeothermalwaterand350kg/sofsteamcondensate.Theexperimentsdemonstratedthatindustrial,agriculturalanddrinkingwaterstandardscouldprobablybemetbyadoptingcertainoperatingconditions.Sixdifferenttreatmentswereexamined.Unitprocessesforgeothermalwater/condensatetreatmentincludedesilicationofthewaterstoproducemarketableminerals,removalofdissolvedsolidsbyreverseosmosisorevaporation,removalofarsenicbyoxidation/precipitation,andremovalofboronbyvariousmethodsincludingionexchange.Thetotalprojectcostestimates,withanaccuracyofapproximately±25%,rangedfromUS$10to78millionincapitalcost,withanoperationandmaintenance(orproduct)costrangingfromUS$0.15to2.73m−3oftreatedwater.©2007CNR.PublishedbyElsevierLtd.Allrightsreserved.Keywords:Geothermalwatertreatment;Waterresources;Desilication;Arsenic;Boron1.IntroductionWiththeworldenteringanageofwatershortagesandaridfarmingland,itisincreasinglyimportantthatwefindwaysofrecyclingwastewater.Theoil,gasandgeothermalindustries,forexample,extractmassiveamountsofbrineandwaterfromthesubsurface,mostofwhichareinjectedbackintoundergroundformations.Holisticapproachestowatermanagementarebeingadoptedevermorefrequently,andproducedwaterisnowbeingconsideredasapotentialresource.Intheoilandgasarena,attemptshavebeenmadetoconvertproducedwaterfordrinkingsupplyorotherreuses(Doranetal.,1998).Turningoilfield-producedwaterintoavaluableresourceentailsanunderstandingoftheenvironmentalandeconomicimplications,andofthetechniquesrequiredtoremovedissolvedorganicandinorganiccomponentsfromthewaters.Treatmentsofgeothermalwaterandcondensateforbeneficialuse,ontheotherhand,involvetheremovalofinorganiccomponentsonly.Wehaveexploredthetechnicalandeconomicfeasibilityofreusingwatersandsteamcondensatesfromexistingandfuturegeothermalpowerplants.Producedgeothermalfluids,especiallyinaridclimates,shouldbeviewedasvaluableresourcesforindustryandagriculture,aswellasfordrinkingwatersupplies.Thispaperpresentstheresultsoflaboratoryandfieldpilotstudiesdesignedtoconvertgeothermal-producedfluidsintobeneficiallyusablewater.Thepreliminaryeconomicsofseveralwatertreatmentstrategiesarealsoprovided.2.DesignlayoutThelayoutforthetreatmentstrategies(unitsofoperation)havebeendesignedspecificallyforanominal50Mwegeothermalpowerplantlocatedinanaridclimateofthewesternhemisphere,hereafterreferredtoasthetestplant.TheaverageconcentrationofconstituentsintheproducedwaterisshowninTable1.Theamountofspentwaterfromthetestflashplantis∼750kg/s.Thepotentialamountofsteamcondensatethatcouldbeproducedattheplantis∼350kg/s.Table1includesthecompositionofthesteamcondensatederivedfromwelltests.ThesixtreatmentcasesconsideredinthestudyaregiveninTable2,togetherwithproductflowsandunitoperationsoftreatment.Fig.1providessimplifiedschematiclayoutsoftheunitoperationsforeachcase.3.EvaluationoftreatmentoptionsInthissectionthevariousoperationsconsideredforeachcasearedescribed.3.1.ArsenicremovalThetechniquesconsideredviableforremovingtracesofarsenic(As)fromcondensateorfromwaterareozoneoxidationfollowedbyironco-precipitationorcatalyzedphoto-oxidationprocesses(Khoeetal.,1997).OtherprocessesforextractingAsfromgeothermalwaters(e.g.RothbaumandAnderton,1975;UmenoandIwanaga,1998;Pascuaetal.,2007)havenotbeenconsideredinthepresentstudy.Inthecaseofthetestplant,ozone(O3)wouldbegeneratedon-siteusingparasiticpower,airandcorona-dischargeultra-violet(UV)lamps,andironintheformofferricsulfate[Fe2(SO4)3]orferricchloride(FeCl3)thatwouldbedeliveredtothegeothermalplant.Thephoto-oxidationprocessesconsistoftreatingthecondensateorwaterwithFe2+intheformofferroussulfate(FeSO4)orferrouschloride(FeCl2),orwithSO2photoabsorbers.ThelatterisgeneratedfromtheoxidationofH2Sinturbineventgas(KitzandGallup,1997).Thephoto-oxidationprocessconsistsofspargingairthroughthephoto-adsorber-treatedfluid,andthenirradiatingitwithUVlampsorexposingittosunlighttooxidizeAs3+toAs5+.IntheFephoto-oxidationmode,theFe2+isoxidizedtoFe3+,whichnotonlycatalyzestheoxidationreaction,butalsoco-precipitatestheAs.IntheSO2photo-oxidationmode,afteroxidizingtheAs,FeCl3orFe2(SO4)3isaddedtothewatertoprecipitatetheAs5+asascorodite-likemineralTable1ApproximategeothermalwaterandsteamcondensatecompositionsassumedinthestudyaTotaldissolvedsolids.Table2SummaryofthesixcasesofgeothermalfluidtreatmenttoproducemarketablewateraOntreatmentofwater,claysareproducedatarateof7.4ton/h.(FeAsO4·2H2O).Inthelaboratoryandfieldpilottests,thephoto-absorberandUVdosageswerevariedtodecreasetheAsconcentrationingeothermalfluidstobelowthedetectionlimitof2ppb(Simmonsetal.,2002).Res