perezcalleja2017

PeriodicventingofMABRlumenallowshighremovalratesandhighgas-transferefficienciesP.Perez-Callejaa,b,M.Aybara,c,C.Picioreanud,A.L.Esteban-Garciab,K.J.Martine,R.Nerenberga,*aUniversityofNotreDame,DepartmentofCivilandEnvironmentalEngineeringandEarthSciences156FitzpatrickHall,NotreDame,IN,46556,USAbUniversityofCantabria,DepartmentofSciencesandTechniquesofWaterandtheEnvironment,UniversityofCantabria,Avda.LosCastross/n,39005,Santander,SpaincDepartmentofCivilEngineering,UniversityofConcepcion,Casilla160-C,CiudadUniversitaria,Concepcion,ChiledDepartmentofBiotechnology,FacultyofAppliedSciences,DelftUniversityofTechnology,VanderMaasweg9,2629HZ,Delft,TheNetherlandseBlackandVeatch,8400WardParkway,KansasCity,MO,64114,USAarticleinfoArticlehistory:Received8December2016Receivedinrevisedform19April2017Accepted20May2017Availableonline22May2017Keywords:Hollow-fibermembranesMBfRMABRGasback-diffusionGastransferefficiencyGastransferrateabstractThemembrane-aeratedbiofilmreactor(MABR)isanoveltreatmenttechnologythatemploysgas-supplyingmembranestodeliveroxygendirectlytoabiofilmgrowingonthemembranesurface.Whenoperatedwithclosed-endmembranes,theMABRprovides100-percentoxygentransfereffi-ciencies(OTE),resultinginsignificantenergysavings.However,closed-endMABRsaremoresensitivetoback-diffusionofinertgases,suchasnitrogen.Back-diffusionreducestheaverageoxygentransferrates(OTR),consequentlydecreasingtheaveragecontaminantremovalfluxes(J).WehypothesizedthatventingthemembranelumenperiodicallywouldincreasetheOTRandJ.Usinganexperimentalflowcellandmathematicalmodeling,weshowedthatback-diffusiongasprofilesdevelopedoverrelativelylongtimescales.Thus,veryshortventingscouldre-establishuniformgasprofilesforrelativelylongtimeperiods.Usingmodeling,wesystematicallyexploredtheeffectoftheventinginterval(timebetweenventings).Atmoderateventingintervals,openingthemembranefor20severy30min,theventingsignificantlyincreasedtheaverageOTRandJwithoutsubstantiallyimpactingtheOTEs.Whenthein-tervalwasshortenough,inthiscaseshorterthan20min,theOTRwasactuallyhigherthanforcontinuousopen-endoperation.Ourresultsshowthatperiodicventingisapromisingstrategytocombinetheadvantagesofopen-endandclosedendoperation,maximizingboththeOTRandOTE.©2017ElsevierLtd.Allrightsreserved.1.IntroductionGas-transferring,hollow-fibermembranes(HFM)arecommonlyusedtosupplygasesforenvironmental,industrialandmedicalapplications.Forexample,bundlesofHFMshavebeenusedforoxygenationofriversandwaterstreams,forbloodoxygenation,andforbioremediationofgroundwatercontaminants(Weissetal.,1998;Roggyetal.,2002;FederspielandHenchir,2004).However,anemergingapplicationisthemembrane-biofilmreactor(MBfR),whereHFMssupplygaseoussubstratestoabiofilmgrowingdirectlyonthemembrane'soutersurface(MartinandNerenberg,2012;Nerenberg,2016).Whenusedtodeliverairoroxygen,theprocessisoftenreferredtoasthemembrane-aeratedbiofilmreactor(MABR).MABRscansimultaneouslyremovebiologicalox-ygendemand(BOD),nitrify,anddenitrify(DowningandNerenberg,2008;Timberlakeetal.,1988;Hibiyaetal.,2003;Semmensetal.,2003;Teradaetal.,2003;J
acomeetal.,2006;Matsumotoetal.,2007;SyronandCasey,2008).Severalcommer-cialapplicationsareindevelopment,butveryfewfull-scaleap-plicationsexist.MABRscanbeoperatedwithclosedoropen-endedHFMs.Withclosed-endedHFMs,alltheoxygensuppliedtothemembranesisdeliveredtothebiofilm,allowing100%oxygentransferefficiencies(OTEs)(Brindleetal.,1998;Pankhaniaetal.,1999;Hibiyaetal.,2003;Teradaetal.,2003;SyronandCasey,2008;MartinandNerenberg,2012).Thiscansaveupto85%inenergycosts,*Correspondingauthor.E-mailaddresses:[e-mail](P.Perez-Calleja),[e-mail],[e-mail](M.Aybar),[e-mail](C.Picioreanu),[e-mail](A.L.Esteban-Garcia),[e-mail](K.J.Martin),[e-mail](R.Nerenberg).ContentslistsavailableatScienceDirectWaterResearchjournalhomepage:://dx.doi.org/10.1016/j.watres.2017.05.0420043-1354/©2017ElsevierLtd.Allrightsreserved.WaterResearch121(2017)349e360comparedtoconventionalactivatedsludgeprocess(Aybaretal.,2014).However,closed-endedHFMstypicallysufferfromgasback-diffusion,whereN2andotherdissolvedgasesdiffuseintothemembranelumen(Schafferetal.,1960;AhmedandSemmens,1992a).Withback-diffusion,thedistalendofthemembranemaybe“deadened,”leadingtoloweraverageoxygentransferrates(OTR)comparedtoopen-endoperation(Fig.1a).Inthispaper,weconsiderOTRtobesynonymouswiththeoxygenflux,JO2,acrossthemembrane.Withopen-endedHFMs,theintra-membranegasvelocityishighthroughoutthemembrane.Withhighvelocities,advectivemasstransportinthelumenismuchgreaterthanthediffusivetransferacrossthemembranewall.Thisresultsinmoreuniformoxygenconcentrationsinthelumen,leadingtohighaverageOTRs(Fig.1b).However,alargeamountofgasislostfromopenend.Also,thehighgasvelocityleadstogreaterfrictionalpressurelossesoccurrencealongthemembrane,resultingingreaterenergyre-quirementsandlowergaspressuresatthedistalendofthemembrane.FortheMABR,loweroverallOTRtranslatesintoloweraveragesubstrateremovalfluxes(J).ManyresearchershaveexploredwaystoimprovetheOTRofH