Effects of temperature on wastewater treatment in an affordable microbial fuel cell-adsorption hybrid system
Pei-Fang Tee
a, Mohammad Omar Abdullah
a,*, Ivy A.W. Tan
a, Mohamed A.M. Amin
a, Cirilo Nolasco-Hipolito
a, Kopli Bujang
baDepartmentofChemicalEngineeringandEnergySustainability,FacultyofEngineering,UniversitiMalaysiaSarawak(UNIMAS),94300,KotaSamarahan, Sarawak,Malaysia
bFacultyofResourceScience&Technology,UniversitiMalaysiaSarawak(UNIMAS),94300,KotaSamarahan,Sarawak,Malaysia
ARTICLE INFO Articlehistory:
Received5September2016
Receivedinrevisedform25November2016 Accepted27November2016
Availableonline27November2016 Keywords:
Temperatureeffect Hybridmicrobialfuelcell Adsorption
Bio-energygeneration Wastewatertreatment
ABSTRACT
Acosteffectivesingle-chambermicrobialfuelcell(MFC)integratedwithadsorptionsystemwastested underdifferentoperatingtemperaturestoobservepHprofiles,organics,solids,nutrients,colorand turbidityremovalandpowerdensitygeneration.Theoptimumoperatingtemperaturerangewasfound tobe25–35Cwithmajorityoftheremovalsachievedat35C.Maximumpowerdensityrecorded was746mW/m3withcoulombicefficiency(CE)of10.650.5%whenoperatedat35C.Presentstudies had successfullydemonstrated the effectiveness of ahybrid system in removing varioustypes of pollutantsinPOMEatoptimumtemperatureandabletofulfillthestringenteffluentdischargelimit.
Chemical oxygen demand (COD), total solids (TS) and turbidity removals increase linearly with temperatureswithremovalefficiencyof0.5889%C1,1.0754%C1and0.7761%C1,respectively.The temperaturecoefficient(Q10)isfoundtobe1.06,1.45and1.09,respectively.Besides,MFC-adsorption hybridsystemhaddemonstratedsuperiorstabilityoverawiderangeofoperatingtemperaturesinterms ofCODremovalascomparedtothenon-integratedMFCsystem.
ã2016ElsevierLtd.Allrightsreserved.
1.Introduction
Microbialfuelcells(MFCs)arebioelectrochemicaldevicesthat utilize bacteria as a biocatalyst to oxidize the organic matters presentinthewastewatertoproducebioenergy[1–3].MFCshave theability toremovevarioustypesofpollutants presentinthe wastewater[4,5].Theyarerenownedfortheirgreatcapabilityfor direct bio-energy generation from various biodegradable com- pounds,rangingfrompurecompoundstocomplexsubstratesin thewastewater[6].Thereweremanystudiesdoneonthetypical standalonetwo-chamberedMFCbecausesuchdesignisveryeasily constructedandtypicallysuitableforlabscalestudies.Neverthe- less, standalone treatment system has limitation such as less wastewatertreatmentefficiencyascomparedtotheMFChybrid system[7].Muchstudieshaddemonstratedtheabilityofhybrid MFCs in removing various types of pollutants present in the wastewater such as chemical oxygen demand (COD) [8–10], phosphorus [11], dye [12,13], suspended solids (SS) [10], total
nitrogen(TN)[14],ammonianitrogen(NH3N)[15,16],andsoon.
Majority of the effective pollutants removal were done by integratingMFCwithotherunitoperationorprocesses.Recently, it has been reported that greater bio-energy generation and wastewater treatment can be accomplished when MFC is hybridizedwithunitoperationsorprocessessuchasadsorption [17],membrane bioreactor[10], sequencing batchreactor [18], membrane,aerationsystem[19],anaerobicdigester[20],aerobic system [15], biofermentor [21], anaerobic fluidized bed [22], wetland [23], anaerobic fluidized membrane reactor [24] and anaerobic sludge blanket [25]. MFC could also possibly be combinedwithotherprocesssuchasnitritation-anammoxwhere the research had been greatly developing in order to remove wastewaterwhichcontenthighlevelofnitrogencontent[26,27].
Now,amongmanyinfluencingparameters,temperatureisone oftheimportantfactorswhichcanaffectthepowergenerationas wellasthewatertreatmentperformance.Thedefinitetempera- turemayaffectthebacterialkineticsandalsothetypesofbacteria thatresideintheanodicbiofilmsinthewastewaterfedsystems [28]. Thus, while bacterial growth rate and respiration can be affectedbythechangesintemperature,thecommunitydevelop- mentandstructurecanalsobecrucial[29].
*Correspondingauthor.
E-mailaddresses:[email protected],[email protected], [email protected](M.O.Abdullah).
http://dx.doi.org/10.1016/j.jece.2016.11.040 2213-3437/ã2016ElsevierLtd.Allrightsreserved.
ContentslistsavailableatScienceDirect
Journal of Environmental Chemical Engineering
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / j ec e
Thusfar,notmuchresearchhasbeendoneontheair-cathode MFC with ceramic pot as the substitute to proton exchange membrane(PEM)exceptGhadge&Ghangrekar[30]andChatterjee andGhangrekaretal.[31].Thereactorsetupsforbothstudieswere doneonstandaloneMFCandrecently,onlyonehadcometothe effort of integrating MFC with adsorption hybrid system with ceramicpotasthePEM[32].Besides,theeffectsoftemperaturein MFChybridsystem hasnever beenunderstoodespeciallywhen activatedcarbonisappliedinthedesignbecausetheperformance of both MFC and adsorption system are sensitive tooperating temperatures.
A recent study has been done by integrating MFC with adsorption system and such system has demonstrated great pollutantsremovalinthepalmoilmilleffluent(POME)suchas COD, biochemical oxygen demand (BOD), total organic carbon (TOC),TN,NH3N,totalsolids(TS),totalsuspendedsolids(TSS), totalvolatilesolids(TVS),turbidityandcolor[32].Tillnow, the studyofsuchsystemwasonlydoneatoperatingtemperatureof 251C. The effect of other operating temperature for such systemhasnotbeenstudiedintermsofwastewatertreatmentand bio-energygeneration.
MFCscanproduceenergyfromvarioussubstratesandreported inliterature.However,tothebestknowledgeoftheauthors,the
studiesoftheeffectoftemperatureshadonlybeendoneonthese few sources which includes beer brewery wastewater [33], domestic wastewater [34–36], sanitary wastewater[37], barley processing wastewater [38] and synthetic wastewater [39,40].
Municipal water treatment experiments which were done previouslyatlowtemperatures(5–15C)couldalsobeusedasa sourceofhighpotentialMFCoperationforelectricitygeneration [41]. At present, none has come to understand the effects of temperaturewithpalmoilmilleffluent(POME)asasubstrateto MFC which is knownas themajorcontributorof the pollution loadsin therivers incountriessuchasMalaysia andIndonesia.
POME is very rich in terms of organic matters and existing treatmentmethodsuchaspondingsystemrequireslongretention time and ineffective to meet the wastewater quality standard.
SincePOMEhasveryhighorganicmatterscontent,thereforeitcan beusedasasourceoffueltotheMFCforbioenergygeneration.
Theaimofthisworkwastotreatthepalmoilmilleffluentand at thesametime totaketheadvantagefromthetreatment by integratingtheMFCwithadsorptionsystemtogeneratebioenergy.
Thus,theeffectsoftemperaturesintermsofpowergeneration,pH profiles,organics,solids,nutrients,colorandturbidityremovalin theMFC-adsorptionhybridsystemwerecarriedoutsystematically andpresentedinthepresentstudy.
2.Experimental 2.1.MFCconstruction
Thesinglechamberair-cathodeMFC-adsorptionhybridsystem with the separator fabricated from ceramic material were constructed and described in Ref.[32]. The schematic diagram oftheMFC-adsorptionhybridsystemandthemembraneelectrode assemblyareasshowninFig.1.Anodecompartmentconsistsof graphite fiber brush (GFB) (Mill-Rose Co., USA) that served as current collector and employingcommercial granularactivated carbon (GAC).Priortoexperiment,theGFBwassoakedinpure acetoneovernightandheat-treatedat450C,andwashedthree timeswithdistilledwater.GACwascleanedwithdistilledwater severaltimesandovendriedtoremoveashesandotherimpurities before they were used. A specific surface area of 1000m2g1 (NantongRuibangActivatedCarbonFilterMaterialCo.Ltd,China) activated carbon fiberfelt (ACFF) was selected asthe cathodes materials, which werewrapped aroundthe ceramicpot (7mm thick)whichactedasthemediumforprotonexchange.Electro- cardiogram(ECG)gelwasappliedevenlybetweenceramicpotand ACFFinordertoincreasehydrationandcontactarea.Acopperrod wasselectedasthecircuitconnectorinthecathodecompartment.
Thereactorworkingvolumeforthissetupwas5.65L.
2.2.Experimentaldesignandoperation
TheMFC-adsorptionhybridsystemwasinoculatedwithPOME fromnearbypalmoilmillatFelcraJaya,KotaSamarahanwherethe POMEsampleswastakenfromthesecondanaerobicpondandthe samesourcewasusedasthesubstratetotheMFCswithoutany additionsofnutrientandbuffersolutionfortheanodechamber.
Wastewatersamplewaskeptinacoldroomat3Cpriortouse.The characteristic of the POME collected was analyzed and tested (Table1).
The MFC-adsorption hybrid systems were inoculated at 251C, where 50
V
of external resistance was installed inbetweentheanodeandthecathodesegment.Thesystemswere operatedatvarioustemperature(15C, 20C, 30C, 35C, 40C and55C)oncesteadyvoltagewasachievedandtheperformance werecomparedtothepreviousstudywhichwasdoneatoperating temperature of 25C [32]. The performance of the MFCs at Nomenclature
ACFF Activatedcarbonfiberfelt
AFMBR Anaerobicfluidizedbedmembranebioreactor APHA Americanpublichealthassociation
BOD Biochemicaloxygendemand
Ca Totalcoulombsbycalculatedbyintegratingcurrent overtime
CE Coulombicefficiency COD Chemicaloxygendemand
Ct Concentrationofthesolutionatanytime D1 DesignofBeheraetal.(2011)
DOE DepartmentofenvironmentMalaysia ECG Electrocardiogram
GAC Granularactivatedcarbon GFB Graphitefiberbrush I Current(A)
I-BAF Immobilizedbiologicalaeratedfilter MFC Microbialfuelcell
MBR Membranebioreactor NH4+ Ammoniumion NH3N Ammonianitrogen P Powerdensity(mW/m3) PEM Protonexchangemembrane POME Palmoilmilleffluent Pt Platinum
Q10 Temperaturecoefficient R1 Rateattemperature1 R2 Rateattemperature2 SS Suspendedsolids T1 Temperature1 T2 Temperature2 TN Totalnitrogen
TMBR Tubularmembranebioreactor TOC Totalorganiccarbon
TSS Totalsuspendedsolids TVS Totalvolatilesolids V Voltage(V)
n
Liquidvolume(L)differenttemperatureswastestedinparallelonindividualMFCsin theclosed-loopmodewithfreshPOMEusingincubators(146E, FisherScientific).Foreachtemperaturesettings,theexperiments werereplicatedwithfreshloadsofGAC.
2.3.Measurementandanalysis
ThepHwas measuredusingadigitalpHmeter(PCSTestr35, Eutech).CODwereconductedaccordingtoHACHmethod8000 which complied to the APHA standard methods with DR900 (Hach). BOD3 were carried out according to Department of Environment (DOE), Malaysia 2011 reference method where BOD was carried out for three days at 30C. TS, TSS and TVS testswereimplementedaccordingtotheAPHAstandardmethods withAPHA2540B,APHA2540DandAPHA2540E,respectively.
Turbiditywas measuredusing a turbidity meter, TN-100/T-100 (Eutech)andTOCbyusingaTOCanalyzer,TOC-LCPH(Shimadzu).
Color was conducted according to HACH method 8025 which compliedtotheAPHAstandardmethodswithDR900(Hach).TN was carried out according to the APHA 4500-Norg B method whereasNH3NwasconductedasperHACHmethod8038with
DR5000 (Hach). Experiments for each different conditionwere replicated twice. Thewastewater sampleswere taken on daily basisuntilequilibriumwasachievedandthecharacterizationwere done in triplicate for each sample obtained from the MFCs including the replicated systems. The average and standard deviation of thedata for each parameterwerethen calculated andpresented.
Voltage was continuously monitored and measured every hourly usinga multimeterwithdataloggersystem (Fluke287/
289).).Polarizationdata werecollectedbyvaryingthe external resistance(variedfrom100K
V
to10V
)withavariableresistancebox(Tenma72–7270)whenthepowerproductionwasstable.
Powerdensity,P(mWm3)wascalculatedby
P=IV/
n
(1)whereIisthecurrent,Visthevoltage,and
n
istheliquidworkingvolumeintheanodecompartment.
TheCoulombicefficiency(CE)isdescribedasthepercentageof electronsrecovered fromorganic matterversus the theoretical maximumwherebyallelectronsareusedforelectricityproduc- tion.CEwascalculatedby
CE(%)=Ca/Ct100 (2)
where Ca is the total coulombs calculated by integrating the current over time, Ct is the theoretical amount of coulombs availablefromtheoxidationofPOME.
3.Resultsanddiscussion 3.1.pHprofiles
The pH of the substrate inside the MFC-adsorption hybrid systemwasmeasuredwhenthesystemwasreadytobeoperated intheclosed-loopmode.ThepHwasmeasuredatthebeginningof theexperimentforalltheMFCsoperatedatdifferentoperating temperatureandalsothroughouttheoperationuntilequilibrium wasachieved.Threedifferenttrendscanbeobservedduringthe first 19h:the temperatures of 15C, 20C, 40C and 55C are Fig.1.(a)Schematicdiagramofthe(a)MFC-adsorptionhybridsystem(frontview)and(b)membraneelectrodeassembly(topview).
Table1
Characteristicof POME after the second anaerobicpond with the discharge standardsasperMalaysianDepartmentofEnvironment,DOE.
Parameter Concentration Unit Limit
pH 7.850.2 – 5–9
COD 8843 mg/l a
BOD3 2332 mg/l 100(20)
TS 68603 mg/l a
TSS 5393 mg/l 400
Turbidity 2032 NTU a
TOC 429.52 mg/l a
TVS 13502 mg/l a
Color 90172 Pt-Co a
TN 167.42 mg/l 200
NH3N 1212 mg/l 150
aNodischargestandard.
underonesimilartrend;25Cundersecondtrend;while30Cand 35Cunderanothertrend.Atthispoint,theMFCsoperatedatthe first trend temperatures had initial pH ranging in between 8.020.1and8.310.1.Inthesubsequent19hofoperation,the pH profile dropped to pH ranging in between 7.380.1 and 7.630.2beforegoingupagainandsettledatpH8.07,8.36,8.25 and 8.35 for MFCs operated at 15C, 20C, 40C and 55C, respectively. Whenthe operating temperature of the MFC was 25C, the corresponding pH was found to be 7.850.2 and 7.080.3 at the beginning and at the end of experiment, respectivelyand thepHdecreased to6.980.2inthefirst19h of operation. MFCs operated at 30C and 35C havingalmost
similarpHprofilewithinitialpHof8.020.2and8.070.1which settledat7.400.2and7.550.2withapHdropof0.620.1and 0.520.1,respectivelyduringthefirst19hofoperation.
Generallyfromthelowerpartofthemicrocosm,organicmatter foundinthewastewatermighthadbeenoxidizedintofattyacids undertheanaerobicconditionwhichshiftedthepHtowardsthe acidicside.WiththeabsenceofoxygenintheMFCs,pHstendtobe decreasedduetothebiodegradationoftheorganicmatterinthe wastewaterinordertoproducefattyacids[42].Thus,thisscenario explainedthegreatfallofpHinthefirst19hofoperationinMFCs operated at 30C and 35C, where the increase in microbial
Table2
PollutantsremovalefficiencyundervariousoperatingconditionsinbetweenMFC-adsorptionhybridsystemandotherMFCs.
Parameter MFC-adsorptionhybrid(Presentstudy) MFC-adsorptionhybrid[32] OtherMFCsystems
%removal %removal Refs.
COD 15C82.50.3%
20C84.20.5%
30C91.60.8%
35C93.50.6%
40C83.10.5%
55C79.20.4%
25C90.50.3% 20C62%
35C84%
40C84%
55C58.8%
30C29.49%
[43]
[44]
BOD 15C80.690.4%
20C85.00.3%
30C94.40.5%
35C95.00.3%
40C85.100.6%
55C83.340.4%
25C93.10.5% 77%(temperaturenotmentioned) [45]
TOC 100%forallthetemperatures 25C100% 85%(temperaturenotmentioned)
25C752%
[46]
[47]
30C22.82% [44]
TS 15C19.90.5%
20C27.90.8%
30C38.50.8%
35C41.40.4%
40C34.00.5%
55C29.90.8%
25C35.00.3%
TSS 15C82.30.6%
20C88.71.2%
30C97.71.0%
35C98.00.6%
40C88.80.7%
55C83.650.9%
25C96.90.2% ambient86.3%
100%(temperaturenotmentioned)
[48]
[9]
TVS 15C65.70.6%
20C76.60.5%
30C90.70.4%
35C89.50.6%
40C71.80.5%
55C65.90.3%
25C89.00.2% – –
Color 15C92.00.7%
20C95.30.8%
30C96.00.7%
35C94.00.3%
40C91.00.6%
55C90.00.5%
25C98.10.3% 292C31.67%
30C34.16%
[52]
[44]
Turbidity 15C86.60.4%
20C91.50.8%
30C98.000.7 35C93.20.5%
40C81.30.6%
55C78.20.3%
25C96.00.4% 282C96% [53]
NH3N 15C0%
20C63.50.6%
30C92.40.4%
35C93.30.8%
40C64.00.5%
55C0%
25C89.60.5% 23C30.4%
76%(Roomtemperature)
[54]
[25]
TN 15C0%
20C52.60.8%
30C98.000.7%
35C87.30.7%
40C48.00.9%
55C0%
25C84.30.4% – –
activitiesunderthesetemperatureswouldtendtogeneratemore fattyacids.
IncomparisontothepHprofilesinanon-hybriddualchamber MFCdoneinthepreviousstudy, similartrendcanbeobserved whenoperatedintherangeof8–22Cand20–35CwherethepH ofthefreshfeed(pH7.4)reachedtheminimumvalueofabout6.2 and 6.7, respectivelyafter operated for 20–25h beforethe pH startedtomoveupslowlyandfinallybecamealmostconstant[39].
Nevertheless,itisverydifficulttocomparethisperformancewith thecurrentstudybecausethepreviousstudywasdoneinawide rangeofoperatingtemperatureswhereasinthecurrentstudy,the pHprofileswasobtainedateachspecificoperatingtemperatures.
Besides,thepHprofileswerenotreportedinthepreviousstudyfor MFCoperatedatthermophilictemperatures(>40C).
3.2.Organicsandsolidsremoval
The results for all the pollutants removal efficiency under variousoperatingconditionsinbetweenMFC-adsorptionhybrid system and other MFCs were summarized in Table 2. In the previousstudy,MFC-adsorptionhybridsystemoperatedat25C hadrecordedCODreductionof90.50.3%[32].Theperformance of the COD and BOD removal percentage at various operating temperaturewerecarriedoutinthepresentstudyandresultswere asshowninFig.2a.TheoptimumCODremovalpercentagewas foundtobeat35C,followedby30Cwiththemaximumremoval of93.50.6%and91.60.8%,respectively.CODremovalpercent- ageatthermophilictemperatures(>40C)were10.430.2%and 14.370.3%lowerascomparedtotheperformanceatoptimum temperature with removal percentage of 83.10.5% and 79.20.4% for MFCs operated at 40C and 55C, respectively.
Thisscenariocanbeexplainedbythefactthattherewasverylittle mesophilicanaerobicbacteriagrowthwhichthereforeinhibited theoxidationoforganicmatterfoundinthewastewater.Likewise, CODremovalpercentagewerelesseffectiveby11.040.3%and 9.300.5%whenoperatedat15Cand20C,respectivelywhich
wasmainlyduetothelowerdegradationrateofinfluentorganic matteratlowertemperatures.Notedthattheoptimumoperating temperatureforeffectiveCODremovalinthepresentstudywas found to be at35C which is different as compared to the previousstudydoneinanon-hybriddualchamberMFC(D1)where themaximumCODremovalreportedwas84%whenoperatedat 40C[43].Thismaybeduetothepooradsorptionperformancein the MFC-adsorption hybrid system when operated at elevated temperature (40C and above). As shown in Fig. 2a, the COD removalpercentagegradientinbetween15Cand35Cinahybrid systemwas0.5889%C1ascomparedto1.4467%C1inD1(non- hybrid MFC). This scenario proved that the pollutants removal performanceofthehybridsystemwaslesssensitivetothechanges ofoperatingtemperatures.Likewise,whenthehybridsystemwas operatedatelevatedtemperatures,thegradientofCODremoval percentagewas 0.2626%C1 ascomparedto1.6800%C1forD1. ThisphenomenonhaddemonstratedtheimportanceoftheMFC- adsorption hybrid system over the standalone MFC where the integratedsystemhadprovedatremendousstabilityoverawide rangeofoperatingtemperaturesintermsofCODremovalwhere theperformancewerenotmuchaffectedbythetemperatures.This ismainlyduetothesorptioncharacteristicsandlargesurfacearea oftheGACin theanodechamber adoptedin thepresent study whichhastheabilityinremovingtheorganiccompoundsfromthe substrate.Recently,adualchamberMFCwithgraphitegranules adsorptionhybridsystemhaddemonstratedremovalof29.49%of CODwhenoperatedat30C[44].TheCODremovalefficiencywas howeverlessascomparedtopresentstudyduetothefactthatthe surfaceareaofthegraphitegranulesusedwasonly35.75m2/g, 96.4%lowercomparedtothesurfaceareaoftheGACusedinthe presentstudy.
Atthemoment,therearenostudiesdonetounderstandthe removalof BOD withMFC-adsorption hybridsystem at various operatingtemperature.Therefore,inthepresentstudy,theBOD removalpercentagewasunderstoodandpresentedinFig.2a.In thepreviousstudydonewithMFC-adsorption hybridsystem at 25C,aBODremovalof93.10.5%wasachieved[32].However,in thepresentstudy,thehighestBODremovalpercentagerecorded was95.00.3%at35C,followedby94.40.5%at30Cwhich was therefore able to achieve the stringent effluent discharge requirementsetbytheDOEfortheBODparameter,wherethefinal BOD achieved was 11.65mg/l and 12.9mg/l, respectively. In comparison to the previousstudy done in a non-hybridsemi- continuousair-cathodeMFC,suchsystemwasonlyabletoremove 77% of the BOD from landfill leachate [45]. The operating temperature was however not mentioned by the authors.
Nevertheless,bycomparingtheBODremovalefficiencyofthese two different MFCs, it can be observed that integration of adsorption system to the MFC is an added advantage to the overallBODremovalperformance.
Overall,itcanbeobservedthatregardlessofMFCsoperating temperature,theMFC-adsorptionhybridsystemhastheabilityto remove the entire TOC present in the wastewater (Fig. 2b).
Tremendous TOC removals canbe observedin the first 19hof operationforMFCsoperatedintherangeof25Cand35Cwith removalofatleast90%andabove.Inthesubsequent24h,atleast 95%ofTOCwereremovedfromthesubstrateandachieved100%of removalafteroperatedintheMFCsfor67h.ForMFCsoperatedat temperaturebelow25C,aremovalof57.50.8%and62.30.3%
was achieved at15C and 20C in the first 19hand complete removalwasachievedafter67hofoperationforbothcases.For MFCs operated at 40C and 55C, the TOC removal rate were slower as compared to other operating temperatures where completeTOCremovalswereonlyachievedafter91hand115h ofoperation, respectively.Anon-hybriddualchamber MFCwas onlyabletoremoveuptothemaximumof85%ofTOCafter96hof Fig.2.(a)CODandBODremovalpercentageatvariousoperatingtemperatureand
(b)TOCremovalefficiencyovertime.
operation [46]. On the other hand, dual reactor systems were fabricatedwith a single chamber MFC as a low-voltage power sourcetosimultaneouslyaccomplishhydrogenperoxidegenera- tion and iron (II) release for the Fenton reaction and such configurationwas abletoachieve 752%TOCremovalat25C [47].Thus,inthepresentstudy,thegreatsuperiorityofintegrating adsorptionsystem toMFC can be observed where TOC can be removedcompletelyatanyoperatingtemperatures.
Inthepreviousstudy,theremovalefficiencyforTS,TSSandTVS achieved in the same system was 35.00.3%, 96.90.2% and 89.00.2%,respectivelywhenoperatedat25C[32].Theremoval percentageofTS,TSSandTVSatvariousoperatingtemperatureis asshowninFig.3a.TheoptimumoperatingtemperatureforTSS removal was found tobe at 35C with removal percentage of 98.00.6%.However,forTVSremoval,theoptimumperformance wasfoundtobeat30Cwithremovalpercentageof90.70.4%, followedby89.50.6%at35C.ForTSremoval,theperformance wasgenerallyabout20–40%lowerascomparedtoTSSandTVS removalswherethemaximumremovalwasfoundtobeat35C withremovalpercentageof41.40.4%.Optimumtemperaturesfor solidsremovalweretypicallyrangedinbetween30Cand35C, whichcanbeexplainedbythefactthattheMFCsanaerobicanode chamberhasbettertendencytoreducesolidsproductionatsuch conditions,wherethebacterialbiomassproductionbytheMFCs systemaremuchlowerduetotheanaerobicenvironment.A90L baffled non-hybrid microbial fuel cell stacked by five stacked modules was fabricated in the previous study for brewery wastewatertreatmentatambienttemperatureand suchsystem wasabletoremoveSSasmuchas86.3%whichwasabout10%less ascomparedtothecurrentstudy[48].Nevertheless,incomparison to another study done with MFC integrated with membrane bioreactor(MBR),suchsystemwasabletoremovetheentireSS present in the wastewater [9]. Therefore, in terms of solids removal, MFC-MBR hybrid system may be a better option in comparisontoMFC-adsorptionhybridsystembecausemembrane typeofunitoperationhasbettersolidsrejectionrateascompared toadsorptionsystem.
On top of that, we had discovered that there was a linear relationship between operating temperature and pollutants removalefficiency(Fig.3b)atarateof0.5889%C1withregression (R2)of0.94forCODremovaland1.0754%C1withregression(R2) of0.96forTSremoval.Theremovalforthesetwopollutantsasa function of operating temperature were only applicable for operatingtemperaturesrangingfrom15Cto35C.Inchemistry andbiochemistry,therateofchangeofa biologicalorchemical systemasaconsequenceofthetemperatureby10Cisdefinedas below:
Q10¼ R2
R1
10=ðT2T1Þ
ð1Þ WhereQ10isthetemperaturecoefficient,R1istheremovalrateat temperature T1 (whereT1<T2), R2 is the removal rate at T2
(whereT2>T1),T1isthetemperatureatwhichtheremovalrateR1 ismeasured(whereT1<T2)andT2isthetemperatureatwhichthe removal rate R2 is measured (whereT2>T1) [49]. Typically, Q10
valueshouldfallinbetween1and3[50].TheCODandTSremoval efficiencywastakenasameasureoftherateoftheelectrochemical processeshappeningintheMFC-adsorptionhybridsystemandthe temperaturecoefficientwascalculated.Itcanbeobservedthatthe Q10 relationship between COD and TS removal efficiency with temperaturewas 1.06and 1.45, respectivelybetween15C and 35C.ThelinearfunctionoftemperatureonCODandTSinFig.3b wereshowingpositivetrendsbecausethepollutantsremovalrate increasedasafunctionofoperatingtemperatures.Theoptimum operating temperatures can be seen to be25–35C which is aboutthetemperatureofthePOME(30–35C)whenobtained freshlyfromtheoriginalsource.Therefore,MFC-adsorptionhybrid systemcanbeideallyimplementedintropicalcountrieswherethe tropical temperature of 30–35C is very close tothe optimum operatingtemperature.
3.3.Color,turbidity,NH3NandTNremovals
POME is generally associated with dark brown color. The presenceoforganiccompoundssuchasanthocyaninandcarotene pigment that were extracted from fresh fruit bunches in the sterilization processimpartsthis persistentcolortowastewater [33].Therefore,itiscrucialfordecolorizationofPOMEtohappen beforeitissuitabletobedischargedtotheriver.Inordertoachieve this,theeffectoftemperatureoncolorremovalwasinvestigatedin ordertofindtheoptimumoperatingtemperaturefortheremoval.
In thepreviousstudy donein thesameMFC-adsorptionhybrid system,aremovalof98.10.3%wasachievedwhenoperatedat temperature of 251C [32]. In the present study, it was discoveredthattherewasnotmuchchangesonthedecolorization whenoperatedinbetween15Cand35C(Fig.4).Maximumcolor removalwasachievedat25Candthedecolorizationperformance wereabout7–8%lowerthanoptimumtemperaturewhenoperated atelevatedtemperatures(>40C).Suchbehaviorcanbeattributed bythefactthatmostofthecolorremovalswereresponsiblebythe adsorptionsystemintheMFC systemwhereadsorptionprocess was more effective at lowertemperatures. The performance of colorremovalinthecurrentMFC-adsorptionhybridsystemwas typicallyattractiveascomparedtotheremovalefficiencyofanon- hybridaircathodeMFCoperatedat292Cwheretheremoval percentage recorded was only 31.67% for distillery wastewater [52]. Therefore, adsorption system played a crucial role in removingmajorityofthecolorpresentinthewastewater.
Turbidity in the wastewater is typically contributed by the presence of the suspended solids. From Fig. 4a, optimum temperaturefor effective turbidityremovalwas foundtobe at 30Cwiththeremovalpercentageof98.000.7ascomparedto Fig.3.(a)TS,TSSandTVSremovalpercentageatvariousoperatingtemperature
and(b)CODandTSremovalefficiencyasafunctionoftemperature.
96.00.4%whenoperatedat25Cinthepreviousstudy[32].As comparedtotheperformanceofthefuelcellwhichincorporated biocatalystinthesystem andoperatedat ambienttemperature (282C)withouttheapplicationofGAC[53],suchsystemwas abletoremoveturbiditymaximumupto96%,whichwas2%lessas comparedtotheMFCsysteminthecurrentstudy.Theoperating temperaturewasfoundtobeproportionaltotheturbidityremoval efficiencyatarateof0.7761%C1withregression(R2)of0.97.Such relationshipwasonlyapplicableforoperatingtemperaturerange inbetween15Cand30C.Fortemperaturemorethan35Cand above,theperformance started todecreasefrom93.20.5% at 35C–29.90.3at55C.Thetemperaturecoefficient(Q10)isfound tobe 1.09 between 15C and 30C. The linear relationship in between turbidity and operating temperature was showing positive trend because turbidity removal rate increased as a function of operating temperature with maximum removal observedat30C.
TheMFC-adsorptionhybrid systemhastheabilitytoremove NH3N and TN as much as 89.60.5% and 84.30.4%, respectivelyat the temperature of 251C as reported in the previousstudy[32].Inthepresentstudy,itcanbeobservedthat theoptimumtemperaturefor NH3Nand TNremovalswas at 35C with removal efficiency of 93.30.8% and 87.30.7%, respectively(Fig.4a).Suchscenariomaybeduetotheeffective- nessoftheammoniumion(NH4+)migrationintheanodechamber tothecathodewhichmighteventuallybecomenitrogenviafurther nitrification and denitrification reactions in the air cathode compartmentundertheoptimumtemperature.Similarphenom- enoncanalso beobserved in a non-hybriddual chamber MFC operatedatroomtemperature(23C)whereatotalof30.4%of theNH4+migratedthroughthePEMtothecathodechamberfor nitrification[54].SincetherewasaneffectiveremovalofNH3N intheanodechamberwhenoperatedat35C,TNconcentration was greatly affected as well because NH3N was one of the component measured in TN, on top of other organic nitrogen,
nitratenitrogenandnitritenitrogenpresentedinthePOME.Inthe previousstudy, an integration of dual chamber membrane-less MFCwithadsorptionhybridsystemwasfabricatedwiththeanode chamber filled with graphite granules and tested for NH3N removalinPOME[25].Suchsystemwasabletodemonstrateabout 76%ofNH3Nremovalafteroperatedatroomtemperaturefor approximately33days.TheperformanceoftheMFCsysteminthe presentstudyhashoweverdemonstratedabetterperformancein NH3Nremovalwhichmaybeduetothedifferenceintermsof anodefillerswhereGAChasbettercapabilityinremovingNH3N ascomparedtographitegranulesduetothehighsurfaceareaof GAC.Besides,theconfigurationofsingleanddoublechambersMFC mayhaveaffectedtheNH4+migrationwhere themovementof NH4+tothecathodecompartmentshouldbemoreeffectiveinthe singlechamberMFCduetothewidecathodesurfacearea.
3.4.Bioenergygeneration
All the MFC-adsorption hybrid system were inoculated at 251Cuntilastablevoltagewasachievedbeforeswitchingthem torespectiveoperatingtemperaturesof15C,20C,25C,30C, 35C, 40Cand 55Cin theclosed-loopmode.Atthefirstfew hoursofinoculation,thevoltagegeneratedwasratherlowbutwas however,startedtodemonstrateasteadyincreaseafter100hof inoculationintheMFCs.Thelow voltagegenerationduringthe firstfewhoursmightbeduetotheanodebiofilmwhichwereyetto be established and therefore limit the electron transfer to the electrodes.TheMFCswerefedwithfreshPOMEinaclosed-loop modeafter200hofinoculationattherateof6.12Lh1andCOD loadingof8843mgL1.
Inthepreviousstudy,anoperatingtemperatureof251Cwas testedinthesameMFC-adsorptionhybridsystem,fabricatedfrom lowcost,localceramicmaterialinaclosed-loopmodeandithad beenreportedthat an averagevoltageof 106mVwas achieved after 171h of operation [32]. In the present studies, the performance of the voltage generated from such system was further investigated at other operating temperatures of 15C, 20C,30C,35C,40Cand 55Candresultswereasshownin Fig.5a.
FromFig.5a,itcanbeobservedthatthevoltageweremoving downtrendespeciallyforMFCsoperatedattemperaturesof15C, 20C, 40C and 55C until a stable voltage was achieved. The average voltage recorded were 71.746.4mV, 92.484.3mV, 64.459.1mV and 62.6111.5mV, respectively. The voltage generationsattheseoperatingtemperatures werehowever20– 45%lowerascomparedtothebaselinestudiesdoneat25C.The decreasedofvoltagegenerationatlowertemperaturemaybedue toinactivemicrobialactivities[55].Therefore,suchphenomenon mayaffectthesubstrateutilizationratewhichthereforeminimal substrate is being converted into bio-energy. Low voltage generationat elevated operating temperatures (>40C)may be duetothedenaturationofenzymewhichtherefore,leadtolow voltagegeneration asa result of poorelectrons transfer tothe electrode[37].Ontopofthat,whenbacteriawasexposedtohigh temperaturescontinuously,importantelementspresentedinthe cellsuchasproteins,nucleicacidsorothertemperature-sensitive mattermaysufferedanirreversibledamagewhichthereforeaffect thecellfunction.
Operating temperatures of 30C and 35C had however achieved a more satisfactory magnitude of voltage generation withanaverageof115.688.2mVand125.699.1mV,respec- tively.Suchphenomenonmaybeduetotheincreaseinbiomass growth and substrate utilization as the temperature increases which therefore, improve the microbial activities and bacterial attachment on the electrode [56]. Thus, electrons could be Fig.4.(a)Color,turbidity,NH3NandTNremovalpercentageatvariousoperating
temperatureand(b)turbidityasremovalefficiencyasafunctionoftemperature.
transferredeffectivelytotheanodeelectrodeswhichthenhelped toincreasethevoltagegeneration.
In the previous study, the maximum power density and coulombic efficiency (CE) was approximately 55mW/m3 and 8.940.3%whenoperatedat25C[32].ThesameMFC-adsorption hybridwastestedforotheroperatingtemperatureof15C,20C, 30C, 35C, 40Cand 55Cand thefindings weretabulated in Fig.5b.ThepowerdensityandCEwerecalculatedbasedonEqs.(1) and(2).Generally,polarizationcurvesweremeasuredwhenthe cellvoltagebecomerelativelystablewhichwasafter300h.The highest power density and CE were achieved when MFC was operated at 35C with the results of 746mW/m3 and 10.650.5%, respectively. Generally, the performance at 35C was found better as compared to the MFC-adsorption hybrid systemoperatedat25Cwhichmaybeduetothefactthatthe biofilms grown at 35C are more electrochemically active as compared to those grown at 25C which in turnhelps in the transferoftheelectronstotheanodeelectrode.
3.5.ComparisonwithotherMFC-hybridsystems
MFCintegratedwithanyunitoperationsorprocesseshavethe privilegeofcombiningtheadvantagesofeachunitascomparedto thestandaloneMFC[7].SeveralstudieshavebeendoneonMFC- hybridsystemrecentlyandacomprehensivecomparisonofthese systemsweredoneintermsofoperatingtemperature,pollutant removalefficiency,MFC design,sourceofinocula,substrateand economicsfeasibilityandpresented inTable3.IntegratingMFC withmembranebioreactor(MBR)hasbeenatrendintherecent yearswhereitcanbeobservedthatsuchsystemisabletoremove
COD with about 90% or more when integrated with tubular membranebioreactor(TMBR)[9,10]andanaerobicfluidizedbed membrane bioreactor (AFMBR) [57]. Besides, the effluent SS concentrationwasnearlynilwithNH3Nremovalofmorethan 93.6%whenoperatedatroomtemperaturesranginginbetween 14Cand25C.ThereisnodoubtthatMFC-MBRhybridsystemhas theabilityinremovingorganicsandsolidsfromthewastewater.
Nevertheless, the efficiency of such system in removing other pollutantssuchascolor,turbidityandnutrientsarenotreported.
Besides,majorityoftheMFC-MBRhybridsystemrequiresaeration systemwhichthereforeincreasestheoperatingcostandmakesit impractical to scale-up. Even though no aeration system was involved in the MFC-AFMBR hybrid system, the adoption of expensiveplatinum(Pt)catalystcouldbeamajorconcerninterms ofeconomicfeasibility.
MFC integrated with adsorption can be an ideal combined system.Ontopofthetypicalbiologicalelectrochemicalprocesses that normallytakeplace inan MFC,physicochemical processes from the activated carbon happen simultaneously [32]. The activated carbon has the ability to remove a wide range of pollutantsfromthewastewaterwhichmaynotbeabletoremove throughthestandaloneMFCsystem.Therefore,anyintegrationof MFCsystemwithadsorptionunitprocessescanbeagreathybrid system toimprovethewastewaterqualitybesides derivingbio- energy from the system. Besides, the high surface area of the activatedcarboncanbeagreatplatformforbacterialattachment foramoreefficientelectrontransfertotheanodeelectronwhich improvesthebio-energygeneration.FromTable3,MFC-adsorp- tionhybridsysteminthedualchamberMFC[58]hasreporteda lowerCODandcolorremovalswhenoperatedat30Cascompared Fig.5.(a)Voltagegenerationover600hofoperation,(b)maximumpowerdensityandcoulombicefficiency(CE)asafunctionofoperatingtemperature.
tothesinglechamber,air-cathodeMFC-adsorptionhybridsystem when operated at 251C [32] and 351C (present study).
Nevertheless, dual chamber MFC-adsorption hybrid system involvesaerationsystemwhichcanbeverychallenginginterms ofscalingupandoperatingcost.Basically,thesinglechamber,air- cathodeMFC-adsorptionhybridsystemoperatedat351Chad generallydemonstratedbetterpollutantsremovalsascomparedto theoneoperatedat251Cexceptforcolorandturbidity.POME forthesestudieswastakenfromthesecondanaerobicpondwhere thesubstratetemperaturecanrangeinbetween30Cand35C.It is therefore practical for MFC-adsorption hybrid system with POME as the substrate to run at 35C for optimal pollutants removal where no further temperature adjustment is needed.
Besides,suchsystemdoesnotinvolvetheapplicationofexpensive catalystaswell asaeration systemwhich makesit possiblefor scale-upapplication.
MFC integratedwithimmobilizedbiological aeratedfilter(I- BAF) operated at 351C with POME as the substrate had generally demonstrated COD and NH3N removal which was almost comparable with the MFC-adsorption hybrid system operatedatthesameoperatingtemperature[25].Nevertheless, such removals through MFC-I-BAF hybrid system can only be achievedafteroperatedforabout33daysascomparedtoMFC- adsorption hybrid system where the same removals can be achievedwithin6days of operation. Even though the MFC-I- BAFhybrid systemwas fabricatedwithcost effectivematerials, suchsysteminvolvedtwoaerationunitswhichwasconsidernotso practicalforscale-upapplication.
4.Conclusions
Inthiswork,wehavepresentedfindingsfromMFC-adsorption hybrid system, which was operated at six different operating
temperatures(15C, 20C,30C,35C,40Cand55C)andthe performancewerecomparedtothepreviousworkdoneintermsof pHprofiles,bioelectricity generation,organics,solids,nutrients, color and turbidity removal. In the present study, it can be observed that thepollutants removalefficiencyfor POMEwith MFC-adsorption hybrid system is very much affected by the operatingtemperatures.Theoptimumtemperaturewasfoundto beat 35C wheresuch conditionhad demonstratedmaximum removalofCOD,BOD,TSS,TS,TN,NH3NandTOC.Besides,the maximumpowerdensityachievedwas746mW/m3withCEof 10.650.5% when operated at 35C. Other important findings fromthepresentstudyinclude:
Generally, MFC-adsorption hybrid system had demonstrated betterpollutantsremovalefficiencyascomparedtostandalone andothertypesofMFCs.Thismaybeduetotheapplicationof adsorptionsystemintheMFCwhichfurtherimprovedonthe wastewaterquality.
TheperformanceofCODremovalefficiencyinaMFC-adsorption hybridsystemwaslesssensitivetothechangesoftheoperating temperatureswheretheperformancewasfoundtobe80–94%
whenoperatedinbetween15and55Cascomparedto60–84%
whenoperatedwithstandaloneMFCinbetween20and55C.
Such system is feasible and ideally to be implemented in tropicalcountrieswherethetropicaltemperatureof30–35Cis veryclosetotheoptimumoperatingtemperatureasindicatedin thecurrentstudy.
Acknowledgements
ThispresentstudywassupportedbytheMinistryofHigher Education (MOHE), Malaysia under the Exploratory Research Table3
PerformanceofMFC-hybridsystemunderdifferentoperatingtemperaturesrangingfrom25Cto35C.
DualchamberMFC- adsorption[58]
Two-stageMFC- immobilized biologicalaerated filter(I-BAF)[25]
TwostageMFC-anaerobic fluidizedbedmembrane bioreactor(AFMBR)[57]
MFC-adsorption[32] MFC-adsorption(Presentwork)
Operating temperature
30C 35C 25C 25C 35C
Pollutants removed
COD71%(anode), 76%(cathode) Color73%(anode), 77%(cathode)
COD96.5%
NH3N93.6%
COD92.5%
TSS100%
COD90.5 BOD393.1 TOC100%
TSS96.9 TVS89.0 TS35.0 Color98.1 NH3N89.6 TN84.3 Turbidity96.0
COD93.6 BOD395.0 TOC100%
TSS98.0 TVS89.5 TS41.4 Color94.0 NH3N93.3 TN87.3 Turbidity93.2 Anode GACandgraphiterod MFC1Graphite
granulesand graphiterod MFC2Carbon fiberfeltandcopper wire
GFBandtitaniumcorewire GACandGFB GACandGFB
Cathode GACandgraphiterod Leadoxideelectrode withcopperwire
Carbonclothwithplatinum(Pt) catalystonwatersideandfour polytetrafluoroethylene(PTFE) layersontheairside
ACFFandcopperwire ACFFandcopperwire
PEM Glasswool Nomembrane Twolayersoftextileseparator Ceramicpot Ceramicpot
Sourceof inocula
Aerobicsludgefrom dyewastewaterplant
Anaerobicdigestion andactivatedsludge fromsewageplant
Domesticwastewaterfrom primaryclarifierofwastewater plant
POME POME
Substrate Dyewastewater POME Domesticwastewater POME POME
Other characteristic
Lowinvestmentcost, highoperatingcost duetoaeration.Two- chambersMFC.
Lowinvestment cost,highoperating costduetoaeration inbothMFCs.
HighinvestmentcostduetoPt catalystisexpensive,low operatingcostduetonoaeration involve.
Lowinvestmentcost,low operatingcostduetonoaeration andnoexpensivecatalystinvolve.
SinglechamberMFC.
Lowinvestmentcost,low operatingcostduetonoaeration andnoexpensivecatalystinvolve.
SinglechamberMFC.
