67RoleofMacrophyteTyphalatifoliainaConstructedWetl

BiosystemsEngineering(2005)92(4),535–544doi:10.1016/j.biosystemseng.2005.08.007RD—RuralDevelopmentRoleofMacrophyteTyphalatifoliainaConstructedWetlandforWastewaterTreatmentandAssessmentofItsPotentialasaBiomassFuelM.P.Ciria1;M.L.Solano2;P.Soriano21BiomassProject,RenewableEnergyDepartmentCIEMAT-CEDER,Km206CarreteraN-111Madrid-Soria,42290Lubia,Soria,Spain;e-mail:[e-mail]2BiomassProject,RenewableEnergyDepartment,CIEMAT.,AvdaComplutense22,28040Madrid,Spain;e-mailofcorrespondingauthor:[e-mail](Received2December2004;acceptedinrevisedform17August2005;publishedonline10October2005)Inthelastdecade,constructedwetlandshavebeengaininginpopularityasareducedcostandlowmaintenancetechnologyfortreatingwastewaterfromsmallurbanisedareas.Inthesewetlandsatthesametimethatpollutantremovalfromwastewateroccurs,greatquantitiesofbiomassareproduced.Asaccordingtosomeauthors,plantsmustbeharvestedforthemosteffectiveremovalofpollutants,largeamountofbiomasswouldbeavailablefordifferentuses.Althoughdifferentsolutionshavebeenproposed(totransformitintocompostorfeedsupplementsforanimals),thebiomasspotentialforfuelproductionhasbeenneglected.Therefore,theobjectivesofthisworkwerefocusedtothestudyofthesuitabilityofthemacrophyteTyphalatifoliaproducedinawetlandasafuel.Maingoalswere:(1)toassesstheroleofthemacrophyte,intheremovalofbiologicaloxygendemand(BOD),chemicaloxygendemand(COD),totalsuspendedsolids(TSS),nitrogen,phosphorousandpathogensfromrawmunicipalwastewater;and(2)todeterminethethermochemicalcharacterisationofthebiomassproducedinordertoexaminethesuitabilityofthebiomassasafuel.Theconstructedwetlandconsistedoftwobeds,of40m2eachwithgravelasthesupportingmedium.Thehydraulicapplicationratewas50mmday
1.Oneofthebedswasplantedwithcattail(Typhalatifolia)andtheotheronewasusedasanunplantedcontrolbed.Withregardtowastewatertreatmentefficacy,theresultsobtainedagreewiththeimportantroleofmacrophytesformaintainingthewetlandstreatmentcapability,particularlyforsystemswithhighorganicmatterandammonia-Ncontent.Duetothehighbiomassyieldsobtainedintheplantedbed,andtothethermalbehaviourofbothcattailbiomassandtheirash,theutilisationofcattailbiomassasfuelinthermochemicalprocessescouldberecommended.r2005SilsoeResearchInstitute.AllrightsreservedPublishedbyElsevierLtd1.IntroductionConventionalwastewatertreatmentplantsinvolvelargecapitalinvestmentsandoperatingcosts,andforthatreasonthesesystemsarenotagoodsolutionforsmallvillagesthatcannotaffordsuchexpensiveconventionaltreatmentsystems.Constructedwetlandsaregaininginimportanceasaneffectivealternativeforthetreatmentofsepticeffluentsinsmallvillages.Comparedwithconventionaltreatmentsystems,wetlands:canbeestablishedinthesameplacewherethewastewaterisproduced;canbemaintainedbyrelativelyuntrainedpersonnel;haverelativelylowenergyrequirements;andarelowcostsystems.Inthewetlands,nutrientremovalfromwastewateroccursduetodifferentmechanisms:(1)plantuptake;(2)microorganismsresidingontheplantrootswhichtransformnutrients(mainlyN)intoinorganiccom-pounds(NH4+andNO3
)whicharedirectlyavailabletoplants;and(3)physicalprocesses,suchassedimentationandfiltration.Asaconsequenceofnutrientuptake,inplantedwetlandslargevolumesofbiomassareproduced(Tanner,1996;Billoreetal.,1999).Nevertheless,duringARTICLEINPRESS1537-5110/$30.00535r2005SilsoeResearchInstitute.AllrightsreservedPublishedbyElsevierLtdwintermonths,atranslocationofnutrientsfromstemstorhizomesoccurs,resultinginanincreaseofnutrientcontentinthewater(Hill&Payton,1998),moreover,accordingtoJingetal.(2001),thedeadplantsconsumeoxygenandthereforecauseanegativechemicaloxygendemand(COD)removalrate.Forthemosteffectiveremovalofpollutants,plantsshouldbeharvestedattheendofeachvegetativecycle,althoughManiosetal.(2002)suggestedthatmachineryorworkersinthewetlandbedhadadetrimentaleffectonthesubstratehydrauliccharacteristics.Propermethodsofbiomassdisposaland/orutilisationarerequired.Biomasscanbetransformedintorawmaterialforthepaperindustry,fertilizers,compostorasafeedsupplementforanimals(Badcocketal.,1983;Billoreetal.,1999;Stottmeisteretal.,2003);however,thepotentialforfuelproductionisneglected,inspiteofthefactthataquaticplantsprovideapromisingsourceofcleanenergyduetotheirhighbiomassyieldandneutralCO2balance.Itiswellknownthattheuseofbiomassasafuelforenergyproductionisrestrictedbythefactthat,asaconsequenceoffertilisation,biomasscontainsalargeamountofelementssuchasCa,K,Mg,NaandSiwithveryreactiveandproblematicbehaviour.Dependingontemperatureandredoxconditionsduringthethermo-chemicalprocesses,thoseelementscanvaporiseiftheyareinformofsimplesalts.Particularly,alkalisandalkaliearthmetalstendtoreactwithsiliconintheformofsilica(SiO2)andcreatelowmeltingpointsilicates.Thesesilicatesdepositontheheatexchangersurfaceswhichreducestheefficiencyofcombustion(Davidssonetal.,2002;Arvelakisetal.,2005).Inthecaseofaquaticplantsgrowninwetlands,noinformationabouttheircompositiononalkalineelementsiscurrentlyavailable.Forthisreason,studiesonbiomasscompositionandthermalbehaviourarenecessaryinordertoassessthesuitabilityofthebiomas