2011Componentsoffloatingemergentmacrophytetr

EcologicalEngineering37(2011)474–486ContentslistsavailableatScienceDirectEcologicalEngineeringjournalhomepage:floatingemergentmacrophytetreatmentwetlandsinfluencingremovalofstormwaterpollutantsChrisC.Tanner∗,TomR.Headley1NationalInstituteofWaterandAtmosphericResearch(NIWA),POBox11-115,Hillcrest,Hamilton3251,NewZealandarticleinfoArticlehistory:Received5August2010Receivedinrevisedform25November2010Accepted10December2010Availableonline26January2011Keywords:AquaticmacrophytesConstructedwetlandsCopperEcotechnologyFloatingtreatmentwetlandsMetalsNutrientsPhosphorusStormwaterretentionpondsSuspendedsolidsTurbidityUrbandrainageWetlandtreatmentZincabstractFloatingtreatmentwetlandsplantedwithemergentmacrophytes(FTWs)provideaninnovativeoptionfortreatingurbanstormwaters.Emergentplantsgrowonamatfloatingonthewatersurface,ratherthanrootedinthebottomsediments.Theyarethereforeabletotoleratethewidefluctuationsinwaterdepthsthataretypicalofstormwaterponds.TobetterunderstandthetreatmentcapabilitiesofFTWs,aseriesofreplicated(n=3)mesocosmexperiments(12×0.7m3tanksusing0.36m2floatingmats)wereconductedoversevendayperiodstoexaminetheinfluenceofconstituentcomponentsofFTWs(floatingmat,soilmedia,andfourdifferentemergentmacrophytespecies)forremovalofcopper,zinc,phosphorusandfinesuspendedsolids(FSS)fromsyntheticstormwater.Thepresenceofaplantedfloat-ingmatsignificantly(P0.05)improvedremovalofcopper(6-fold),finesuspendedparticles(∼3-foldreductioninturbidity)anddissolvedreactiveP(inthepresenceofFSS)comparedtothecontrol.Livingplantsprovidedalargesubmergedrootsurface-area(4.6–9.3m2ofprimaryrootsm−2mat)forbiofilmdevelopmentandplayedakeyroleintheremovalofCu,PandFSS.UptakeofCuandPintoplanttis-suesduringthetrialscouldonlyaccountforasmallfractionoftheadditionalremovalfoundintheplantedFTWs,andnon-plantedfloatingmatswithartificialrootsprovidingsimilarsurfaceareagener-allydidnotprovideequivalentbenefits.Theseresponsessuggestthatreleaseofbioactivecompoundsfromtheplantroots,orchangesinphysico-chemicalconditionsinthewatercolumnand/orsoilsintheplantedFTWsindirectlyenhancedremovalprocessesbymodifyingmetalspeciation(e.g.stimulat-ingcomplexationorflocculationofdissolvedfractions)and/orthesorptioncharacteristicsofbiofilms.Theremovalofdissolvedzincwasenhancedbytheinclusionofafloatingmatcontainingorganicsoilmedia,withreducedremovalwhenvegetatedwithallexceptoneofthetestspecies.Theresultsindi-catethatplantedFTWsarecapableofachievingdissolvedCuandZnmassremovalratesintheorderof5.6–7.7mgm−2d−1and25–104mgm−2d−1,respectively,whichcomparefavourablytoremovalratesreportedforconventionalsurfaceflowconstructedwetlandstreatingurbanstormwaters.Althoughnotdirectlymeasuredinthepresentstudy,theremovalofparticulate-boundmetalsisalsolikelytobehighgiventhattheFTWsremovedapproximately34–42%oftheturbidityassociatedwithveryfinesuspendedparticulateswithinthreedays.ThisstudyillustratesthepromiseofFTWsforstormwatertreat-ment,andsupportstheneedforlarger-scale,longer-termstudiestoevaluatetheirsustainabletreatmentperformance.©2011ElsevierB.V.Allrightsreserved.1.IntroductionUrbanwatershedsarecharacteristicallyhighlyimpervious,gen-eratinglargeflowpeaksduringstormeventsthatmobilisea“cocktail”ofparticulateanddissolvedpollutants.Conventionalurbanstormwatertreatmentdevices,suchascatch-pitsandsedi-∗Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](C.C.Tanner).1Currentaddress:HelmholtzCentreforEnvironmentalResearch(UFZ),Per-moserstrasse15,D-04318Leipzig,Germany.mentationponds,caneffectivelyremovecoarseparticulatesfromtheseflows,butarelimitedintheirabilitytoremovedissolved,colloidalandfinesuspendedmetalandnutrientfractions.Thesefinefractionshavehighermetaladsorptioncapacitythancoarserparticulates,andalongwithdissolvedformstendtobethemostbioavailableandtoxictoaquaticlife(Luoma,1983;GriffithsandTimperley,2005).Appropriatelysizedconstructedwetlandshavebeenshowntobeeffectiveatremovingawiderangeofpri-oritymetalsfromstormwatersandwastewaters(DunbabinandBowmer,1992;Carletonetal.,2001;ReddyandDeLaune,2008;KadlecandWallace,2009),andaregenerallyconsideredtobemoreeffectiveatremovingdissolvedandfineparticulatecontam-0925-8574/$–seefrontmatter©2011ElsevierB.V.Allrightsreserved.doi:10.1016/j.ecoleng.2010.12.012C.C.Tanner,T.R.Headley/EcologicalEngineering37(2011)474–486475Fig.1.Plan-view(right)andcross-section(left)ofanexperimentalfloatingtreatmentwetland.Onlyasingleplantisshownforclarity.inantfractionsthanponds(Wongetal.,2000;Bavoretal.,2001;Pontieretal.,2001).However,therootedvegetationusedincon-ventionalwetlandsystemscanonlytoleraterelativelyshallowwaterdepths(commonly0.5moverlongperiods)andshortperi-odsoftotalsubmergence,sodepths(andresidencetimesperunitarea)ofwetlandsaregenerallylessthanforstormwaterretentionponds.Floatingemergentmacrophytetreatmentwetlands(FTWs)areanoveltreatmentconceptthatemployrooted,emergentmacro-phytesgrowinginafloatingmatonthesurfaceofthewaterratherthanrootedinthesediments(FonderandHeadley,2010;HeadleyandTanner,inpress).Theplantroots