Is Biodegradability a Desirable Attribute for Discarded Solid Waste_

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IsBiodegradabilityaDesirableAttributeforDiscardedSolidWaste?PerspectivesfromaNationalLandfillGreenhouseGasInventoryModel

JamesW.Levis*andMortonA.Barlaz

DepartmentofCivil,Construction,andEnvironmentalEngineering,NorthCarolinaStateUniversity,CampusBox7908,Raleigh,NorthCarolina27695-7908,UnitedStates

SupportingInformationb

ABSTRACT:Thereisincreasinginterestintheuseofbiodegradablematerialsbecausetheyarebelievedtobe“greener”.Inalandﬁll,thesematerialsdegradeanaerobicallytoformmethaneandcarbondioxide.Thefractionofthemethanethatiscollectedcanbeutilizedasanenergysourceandthefractionofthebiogeniccarbonthatdoesnotdecomposeisstoredinthelandﬁll.Alandﬁlllife-cyclemodelwasdevelopedtorepresentthebehaviorofMSWcomponentsandnewmaterialsdisposedinalandﬁllrepresentativeoftheU.S.averagewithrespecttogascollectionandutilizationoverarangeofenvironmentalconditions(i.e.,arid,moderatewet,andbioreactor).Thebehaviorofmaterialsthatbiodegradeatrelativelyfast(foodwaste),medium(biodegradablepolymer)andslow(newsprintandoﬃcepaper)rateswasstudied.Poly(3-hydroxybutyrate-co-3-hydroxyoctanoate)(PHBO)wasselectedasillustrativeforanemergingbiodegradablepolymer.Globalwarmingpotentials(GWP)of26,720,À1000,990,and1300kgCO2ewetMgÀ1wereestimatedforMSW,foodwaste,newsprint,oﬃcepaper,andPHBO,respectivelyinanationalaveragelandﬁll.Inastate-of-the-artlandﬁllwithgascollectionandelectricitygeneration,GWP’sofÀ250,330,À1400,À96,andÀ420kgCO2ewetMgÀ1wereestimatedforMSW,foodwaste,newsprint,oﬃcepaperandPHBO,respectively.Additionalsimulationsshowedthatforahypotheticalmaterial,aslowerbiodegradationrateandalowerextentofbiodegradationimprovetheenvironmentalperformanceofamaterialinalandﬁllrepresentativeofnationalaverageconditions.

’INTRODUCTION

TheU.S.EnvironmentalProtectionAgency(EPA)estimatesthat135millionmetrictons(1t=1Mg)ofmunicipalsolidwaste(MSW)werediscardedinU.S.landﬁllsin2008.1Whileeﬀortstoreducewastegenerationandtomanagewastebyrecyclingandcompostingwillcontinue,landﬁllsremainasigniﬁcantcompo-nentofwastemanagementinfrastructure.Itisthereforeimpor-tanttounderstandtheimpactsoflandﬁlldisposalonamaterial’senvironmentalperformance.Therearecurrently503landﬁllsintheU.S.atwhichthegasisconvertedtoenergy2andanestimatedadditional545landﬁllsatwhichenergyrecoveryisviable.3Nonetheless,asaresultofgasgeneratedpriortoinstallationofgascollectionsystems,andfugitiveemissions,landﬁllsareestimatedtobethesecondlargestsourceofanthropogenicmethaneemissionsintheU.S.4Recently,eﬀortshavebeenmadetodevelopbiodegradablematerialsbecausetheyareassumedtobe“greener”alternatives.Poly(lactic)acid(PLA),whichismanufacturedfromagriculturalproductsasopposedtopetro-leum,isonesuchmaterialthathasfounduseindisposablecups,cutlery,andotherfoodserviceapplications.5Whilematerialbiodegradabilitywillreducethevolumeoccupiedinalandﬁll,anevaluationoftheenvironmentalperformanceofanewmaterialmustincludetheproduction,use,anddisposalphasesoftheproductlife-cycle.Inrecentwork,theeﬀectoftherateofmethanegenerationfromindividualMSWcomponentswascombinedwithahypotheticalscheduleforlandﬁllgascollec-tiontoillustratetheimportanceofincorporatingwaste

r2011AmericanChemicalSociety

component-speciﬁcdecayratesinanalysesofthefractionofgeneratedmethanethatiscollected.6Totheextentthatenvironmentalperformanceattheend-of-lifeisafactorinthedevelopmentandselectionofmaterialstobeusedinvariousproducts,manufacturersmusthaveanunder-standingofthenationaldisposalinfrastructureasopposedtoperformanceinaspeciﬁclandﬁll.Theobjectiveofthisstudywastodevelopandparametrizealandﬁlllife-cyclemodeltorepresentnationalaverageconditions.Themodelwasparametrizedtorepresentlandﬁllswithandwithoutgascollection,andlandﬁllsthatﬂareorusethecollectedgasbeneﬁcially.Landﬁllsoperatedunderarangeofenvironmentalconditions(i.e.,arid,moderate,wet,andbioreactor)wereconsideredandthemodelwasusedtostudythebehaviorofmaterialsthatbiodegradeatrelativelyfast(foodwaste),medium(biodegradablepolymer)andslow(newsprintandoﬃcepaper)rates.ThegoalofthisstudyistoprovideguidancetomanufacturersonenvironmentalperformanceduringlandﬁlldisposalthatreﬂectsU.S.landﬁllinfrastructure.

’MODELINGAPPROACH

Alandﬁlllife-cyclemodelwasdevelopedtoestimategreen-housegas(GHG)emissionsattributabletothedisposalof

Received:March2,2011Accepted:May13,2011Revised:May10,2011Published:May27,2011

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Table1.PropertiesforMixofU.S.LandﬁllFacilities

annualprecipitation

landﬁlltypearidmoderatewetbioreactor

adecayrate(yr)0.02a0.0380.0570.12

daaÀ1percentofwastereceived

20.028.941.110.0

bno.ofyearsgasiscollectedforenergygeneration

100765939

cpercentofgasgenerated

in100years829499.9

(cm)<51

a51102N/A

4bFromU.S.EPA,2010.Themassofwastedisposedinbioreactorlandﬁllswasassumedtobe10%.Thismasswassubtractedfromthemassdisposedinmoderateandwetlandﬁllsinequalproportions,afterwhichthefractiondisposedineachcategorywascorrected.TheoriginalmassdisposalbycategorywasadoptedfromU.S.EPA,2010.4cCriteriatoestimatethisvaluearedescribedinthetext.dJudgmentbasedonvaluesreportedinBarlazetal.,201020andTolaymatetal.,2010.21biodegradablematerialsinlandﬁlls.ThemodelwasusedtoanalyzethebehaviorofMSWandarangeofillustrativematerialsthatexhibitvaryingbiodegradationratestostudytheeﬀectofbiodegradabilityonenvironmentalperformance.BothpointestimatesandMonteCarloanalyseswerecon-ducted.ForMSW,thewastecompositionwasbasedonU.S.EPA,2009.1LandfillModel.Anestimateoftheglobalwarmingpotential(GWP)attributabletothedisposalofmaterialsinalandfillrequiresconsiderationoflandfillconstruction,operations,finalcoverplacement,gasandleachatemanagement,andlong-termmaintenanceandmonitoring(eq1).

totalGWP¼constructionCO2þoperationsCO2þfinalcoverCO2

þleachatemgmtCO2þlongtermmonitoringCO2þ25ÂfugitivemethaneÀelectricityoffsets󰀁󰀃44ÀCstoredð1Þ

Eachofthetermsineq1areinmassunits(kg).GHGemissionsassociatedwithallaspectsofthelandﬁllexceptgasmanagementandcarbonstoragehavebeenshowntobesmallrelativetotheseparameters.Assuch,emissionsforlandﬁllconstruction(1.4kgCO2eMgÀ1),operations(3.9kgCO2eMgÀ1),ﬁnalcoverplacement(1.2kgCO2eMgÀ1),leachatemanage-ment(0.31kgCO2eMgÀ1)andlong-termmaintenance(0.06kgCO2eMgÀ1)wereadoptedfromCamobreco,1999.7TheGHGemissionsandsinksassociatedwithgasmanagementandthestorageofbiogeniccarbonweredevelopedinthisstudywithcarbonstoragefactorsadoptedfromStaleyandBarlaz,2009.8LandﬁllgasgenerationwasmodeledusingaﬁrstorderdecaymodelasintheEPA’sLandGEMmodel.9Thedecayrate(k)isdependentonclimateandlandﬁlloperationstrategy(traditionalvsbioreactor).Thus,thefractionofwastedisposedinU.S.landﬁllswasdividedintothreeclimatecategoriesfortraditionallandﬁlls(arid,moderate,wet)toreﬂectdiﬀerencesinkasso-ciatedwithmoisture.Bioreactorlandﬁlls,inwhichleachateandsometimesotherliquidsarerecirculatedtoincreasek,wereconsideredasafourthcategory.ThemassoftotalwastedisposedintoeachlandﬁllcategorywasadoptedfromU.S.EPA,2010.4Table1presentstheparametersassociatedwitheachlandﬁllcategory.Themassofwastedisposedinbioreactorlandﬁllswasassumedtobe10%ofthemassdisposedinU.S.landﬁllsandthismasswassubtractedfromthemassdisposedinmoderateandwetlandﬁllsasdescribedinTable1.Ineachofthethreetraditionallandﬁllcategories,therearelandﬁllsthat(1)donotcollectgas,(2)ﬂarethegas,and(3)usethegasforenergy.Thepercentageof

wasteinlandﬁllswithgascollectionandthepercentageoftheselandﬁllswithenergyrecoverywerecalculatedusingEPAesti-matesformethanegeneration(12.4millionMg),ﬂared(3.3millionMg),andcombustedforenergy(3.3millionMg).4Basedontheassumptionthatlandﬁllscollect75%ofthegeneratedgas,theEPAGHGInventory4estimatesthat69%oflandﬁlledwastewasdisposedinlandﬁllswithgascollection(ﬂaredorconvertedtoenergy)and50%ofthatwastewasdisposedinlandﬁllswithenergyrecovery.Itwasassumedthatallbioreactorlandﬁllswereincludedinthe69%oflandﬁllsthatcollectgas,whichresultsinanestimatethat66%ofwasteintraditionallandﬁllsisdisposedinlandﬁllswithgascollection.Itisrecognizedthatthereisun-certaintyintheseestimatesandthesensitivityoftheseassump-tionsisexploredwiththeresults.

IncontrasttoLandGEM,9inwhichMSWistreatedasonesubstrate,thekandmethaneyield(L0)ofeachMSWcomponentwasmodeledseparatelytostudytheinﬂuenceofbiodegradabilityonmethanegenerationandsubsequentcollectionandemissions.Component-speciﬁcdecayrateswerecalculatedasdescribedinDelaCruzandBarlaz,2010.6Calculationofcomponent-speciﬁcdecayratesrequiresspeciﬁcationofabulkMSWdecayrateasgiveninTable1.

Forwasteinlandﬁllsthatutilizethemethanebeneﬁcially,itwasnecessarytoestimatetheperiodoverwhichtherewassuﬃcientgastooperateenergyrecoveryequipment.First,itwasassumedthatallrecoveredmethaneisconvertedtoelectricalenergyalthoughinpracticesomegasisuseddirectlyinindustrialboilersalongwithotherbeneﬁcialuses.Second,itwasassumedthatlandﬁllscouldonlygenerateelectricitywhilethegasﬂowratewasabove0.236m3sÀ1(500ft3minÀ1)at50%methane.Foreachlandﬁllcategory,thelengthoftimethatthelandﬁllgasﬂowwasabovethisthresholdwasdeterminedbymodelingmethanegenerationfora2100MgdayÀ1landﬁllthatacceptedwastefor40yearsatthedecayratesgiveninTable1.Asthedecayratedecreases,thelengthoftimeoverwhichgasgenerationisabovethe0.236m3sÀ1thresholdincreasesandofcourse,thistimewouldincreaseifthewasteacceptanceratewashigher.Allcalculationswerebasedona100yeartimehorizonatwhichpointalandﬁllwouldhaveproducedmostofitsmethane(Table1).Forlandﬁllsthatutilizethegasforenergy,thegasproducedataratelowerthantheaforementionedthresholdwasassumedtobeﬂaredbetweenthethresholdyearandyear100.Landﬁllgascollectionsystemsareinstalledinpartbasedontheageofthelandﬁllcell.Thismeansthatwasteburiedearlierinthecell’slifewillbeundergascollectionforlesstimethanwasteburiedlaterinthecell’slife.Itisthereforenecessarytotemporallyaveragethecollectioneﬃciencyforeachyearofcelloperation.

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Table2.MaterialPropertiesforFoodWaste,Newsprint,OﬃcePaper,PHBO,andMSW

materialfoodwastenewsprintoﬃcepaperPHBOMSW

acmoisturecontent(%)

7066021

ﬁelddecayrate(yrÀ1)a0.1440.0330.0290.072N/A

dmethaneyield(m3dryMgÀ1)b30074.321734167.2

carbonstoragefactor(kgCdryMgÀ1)b8042050356125

6Theﬁeld-scaledecayratesforfoodwaste,newsprint,andoﬃcepaperwereadoptedfromDelaCruzandBarlaz,2010andarebasedonadecayrateof0.04yrÀ1forMSW.ThedecayrateforPHBOwasestimatedasdescribedinthetext,andalsobasedonadecayrateof0.04yrÀ1forMSW.bThemethaneyieldsandcarbonstoragefactorsforMSW,foodwaste,newsprintandoﬃcepaper,wereadoptedfromStaleyandBarlaz,2009.8ThecorrespondingvaluesforPHBOwereestimatedasdescribedinthetext.cValueswerecalculatedastheweightedaverageofthecomponentspeciﬁcvaluesformoisturecontent,methaneyieldandcarbonstoragefactor.ComponentspeciﬁcdataandwastecompositionaregiveninSITableS1.dAbulkMSWdecayrateisnotmeaningfulbecausemethanegenerationcurvesforindividualwastecomponentsweresummed.

Gascollectionschemeswerebasedontheassumptionthatatypicalcelllifeis5yearsandthatnogascollectionisinplacefortheﬁrsttwoyearsofcelloperation(6moforbioreactors).Further,thecollectioneﬃciencypriortocellclosureandintermediatecoverinstallationis50%(i.e.,years3À5,or0.5À3yearsforabioreactor).Aftercellclosureattheendofyear5,thecollectioneﬃciencyisassumedtobe75%.Itisfurtherassumedthat10yearsafterﬁnalwasteplacement(i.e.,15yearsafterinitialwasteplacement),aﬁnalcoverisinstalledandthegascollectioneﬃciencyincreasesfrom75%to95%.Thisgascollectionsysteminstallationschedulewasusedtocalculateatemporallyaveragedgascollectioneﬃciencywhichisthevolumeofgascollecteddividedbythevolumeofgasproducedover100yearsasitappliestothe5yearsofwasteburiedinasinglelandﬁllcell.AllofthegascollectionsystemdefaultvaluescanbevariedinthemodelasdescribedwiththeResultsandDiscussion.

SomefractionoftheuncollectedmethaneisoxidizedtoCO2asitpassesthroughthelandﬁllcover.TenpercentoxidationwasassumedasrecommendedintheU.S.EPA’sAP-42database10andasusedintheU.S.GHGinventory.4Itislikelyconservativeasotherstudiesestimatemethaneoxidationof22À55%.11Whenelectricalenergyisrecovered,itisassumedtooﬀsetcoalandnaturalgasgenerationat72.5%coaland27.5%naturalgas,whichrepresentstheadjustedproportionofeachfuelonthenationalgrid.12ThisleadstoaCO2oﬀsetof1.02kgCO2ekWhÀ1.Methanewasassumedtobeconvertedtoelectricityusingaheatrateof11.6MJ/kWh,whichwasdevelopedfromvendorliterature.Finally,themassofmethanewasmultipliedby25toexpressasCO2eusingthe100yearwarmingpotential.13ModelingofIndividualWasteComponents.InadditiontoMSW,theanalysiswasconductedforfourindividualmaterialstoillustratetheeffectsofdecayrateandmethaneyieldonGHGemissionsfromwastematerials.Thefourmaterialswerefoodwaste,newsprint,officepaper,andpoly(3-hydroxybutyrate-co-3-hydroxyoctanoate)(PHBO)(C13H21O4).ThepropertiesforeachmaterialareshowninTable2.SeveralmaterialpropertiesforPHBOhadtobedevelopedincludingthemethaneyield,decayrate,andcarbonstoragefactor(CSF).ThetheoreticalmethaneyieldforPHBOwascalculatedtobe755mLgPHBOÀ1usingtheBuswellequationascitedinParkinandOwen,1986.14Thisvaluewasadjustedusingtheaveragemineralizationof45.2%measuredinareactorstudy,15resultinginaneffectiveultimateyieldof341mLgPHBOÀ1.Equation2wasusedtoestimateadecayrateforPHBOthatisapplicableatfield-scale

Table3.DecayRateForMSW,PHBO,andtheRatiobetweenthem

laboratory-scaleMSWdecayrate(yr)

reactor4areactor5reactor6average

aaaÀ1blaboratory-scalePHBOdecayrate(yrÀ1)b20.918.018.619.2

15bkphbo/kmsw1.921.701.851.83

10.910.610.110.5

ReactornumbersasassignedinFederleetal.,2002.CalculatedfromdatainFederleetal.,200215asgivenintheSI(FiguresS1ÀS8).

usinglaboratory-scaledecayratesforMSWandPHBO.

kf,phbo¼kf,MSW

k1,phbok1,MSW

ð2Þ

where,kf,phboisthefield-scaledecayrateofPHBO,kf,MSWisthefield-scaledecayrateofMSW,kl,phboisthelaboratory-scaledecayrateofPHBO,andkl,MSWisthelaboratory-scaledecayrateofMSW.Thelaboratory-scaledecayratesforPHBOandMSWwereestimatedbyregressionanalysisofthedatainFederleetal.,2002.15ForMSW,theregressionwasperformedonthelogofthedifferencebetweentotalmethaneproductionandeachgenera-tionvalue(FiguresS1ÀS4oftheSupportingInformation(SI)).ThedecayrateforPHBOwasdeterminedbyanalyzingthemineralizationrate.SincePHBOmineralizationstabilizedmuchsoonerthantotalCH4,theregressionwasonlyperformedonthedatauptoday77.9atwhichtimedecayhadessentiallyceased(SIFiguresS5ÀS8).ItshouldbenotedthatthisisanupperestimateforthedecayrateofPHBO,sincethePHBOwasgroundbeforetestingandtheothermaterialswereshreddedtoabout2Â5cm.Table3showsthelaboratorydecayratesforMSW,PHBOandtheratiobetweenthem.TheratiowasusedtoestimatethedecayrateofPHBOinactuallandﬁllenvironmentsforeachlandﬁllcategory(Table2).ACSFforPHBOwasdeterminedusingitscarboncontent(7kgCMgÀ1)andreportedmineralization(45.2%),resultinginanaverageCSFof356kgCMgPHBOÀ1witharangeof307to381basedonthereactordata.15SensitivityAnalysis.Manyoftheinputvaluesinthismodelareuncertain.Forexample,itisdifficulttoestimatethefractionofwastedisposedinlandfillswithgascollection(pointestimate69%),aswellasthefractionofthiswastethatisinlandfillswithenergyrecovery(pointestimate50%).Incontrasttothesepointestimates,datavoluntarilysubmittedtotheLandfillMethaneOutreachProgram(LMOP)database2suggestthat84%ofwaste

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isdisposedinlandfillswithgascollection,and66%ofthiswasteisinlandfillswithenergyrecovery.ThesenumbersarelikelyhigherbecausetheLMOPdatabaseconsistsoflandfillsthatvoluntarilysubmitinformation.Largerlandfillsandlandfillsthathavestate-Table4.TemporallyAveragedLandﬁllGasCollectionEﬃciencies.acollectioneﬃciency(%)

wasteage(yr)

1234567101112131415g16

aof-the-artgascollectionsystemsaremostlikelytosubmitdataandthusbeoverrepresented.

A10000iterationMonteCarloanalysiswasperformedonseveralmodelinputs.TheinputsincludedintheanalysisandthevaluesusedfortheirrespectivetriangulardistributionsaregivenintheResultsandDiscussionsection.

traditionallandﬁll

0456065707575757575757983879195

bioreactorlandﬁll

25556065707575757575757983879195

ValuerepresentsthebehaviorofanaveragemassofMSWinalandﬁllwithgascollection.ThecalculationprocedureisdescribedintheModelingApproachsection.Thesevaluesarebasedonanassumedschedulefortheinstallationofagascollectionsystem,alandﬁllcelllifeof5yearsandtheinstallationofﬁnalcover15yearsafteracellopensasdescribedinthetext.

’RESULTSANDDISCUSSION

ThecalculatedtemporallyaveragedlandﬁllgascollectioneﬃcienciesforwastedisposedintraditionalandbioreactorlandﬁllsthatcollectgasareshowninTable4.TheresultsinTable4reﬂectanaverageMgofwasteasopposedtotheﬁrstMgburied.Thus,eventhoughitwasassumedthatnogascollectionisinstalledatatraditionallandﬁllfortwoyears,wastedisposedinyeartwocomesundersomecollectionwithinayearofburial;hencethegascollectioneﬃciencyforwasteburiedinyeartwoisnonzero.

TheGHGemissionsassociatedwithfoodwaste,newsprint,oﬃcepaper,PHBO,andMSWareshowninFigure1abylandﬁllsubprocess.Temporallyaveragedcollectioneﬃciencies,deﬁnedastotalmethanecollection/totalmethaneproductionwere51,41,56,57,and49%forMSW,foodwaste,newsprint,oﬃcepaperandPHBO,respectively.Thesevaluesarerelativelylowduetotheestimatethat31%ofwasteisburiedinlandﬁllsthatdonotcollectgas.Thecollectioneﬃciencyvariesasafunctionofdecayrateasmaterialswithahigherdecayratewillproducemoregaspriortotheinstallationofgascollectionwhilewastethatdegradesmoreslowlywillhavegreatercollectioneﬃcienciessincemoreofthegaswillbeproducedaftercollectionsystemsareinplace.Methaneoxidationwasassumedtoreducefugitiveemissionsby10%inthebasecase.Biogeniccarbonstorageisalsoasigniﬁcantcomponentofthecarbonfootprint(Figure1a)whiletheenergyoﬀsetsreducetheGWPfromthefugitivemethaneemissionsby6À11%.

Figure1.GreenhousegasemissionsforeachwastecomponentandaverageMSWbyprocessandexpressedperwetMg.Thesedatarepresent(a)anationalaveragelandﬁllandthusreﬂectlandﬁllswithandwithoutgascollectionandenergyrecoveryand(b)astate-of-the-artlandﬁll.FossilCO2eemissionsfromlandﬁllconstruction,operations,closure,postclosureandleachatemanagementleadtoanadditional6.9kgCO2eMgÀ1thatisincludedinthetotalforeachofthewastestreams.

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Table5.InputsandTriangularDistributionParametersUsedintheMonteCarloAnalysis

input

wastediscardedinlandﬁllswithgascollection(%)wasteinlandﬁllswithgascollectionthatrecoverenergy(%)timeuntilﬁnalcoverisinplaceafterinitialwasteplacement(yr)gascollectioneﬃciencyunderﬁnalcover(%)oxidationrate(%)landﬁlldecayrate(yrÀ1)aridmoderatewetbioreactor

acurrent69501595100.020.0380.0570.12

minimuma60401285100.015d0.029d0.043d0.09dmaximuma84b66b209840c0.025d0.048d0.071d0.15dValuesbasedonjudgmentunlessotherwisestated.bMaximumvaluebasedonvoluntaryreportsinLMOPdatabase.2cApublishedreviewsuggestsameanof36%oxidation.11dThedecayrateswerevariedby(25%.

TheresultsinFigure1acontrastconsiderablywiththeresultsforalandﬁllthatrepresentsthestate-of-the-artasopposedtoanationalaverage.Resultsforascenarioclosertoastate-of-the-artlandﬁllarepresentedinFigure1bwhereitisassumedthatalllandﬁllshavegascollectionsystemsandconvertthemethanetoelectricalenergy.Themethanecollectionschedule,celllifeandoxidationratesremainasforFigure1a.Asexpected,collectedmethaneandtheenergyoﬀsetsincrease;withenergyoﬀsetsreducingtheCO2efromthefugitivemethaneemissionsby25À78%.

TheCO2esignatureofPHBOisinferiortothatofothermaterialsinnationalaveragelandﬁlls,butnotinstate-of-the-artlandﬁlls.ThisinferiorityinthebasecasemaybecounterintuitiveasthemethaneyieldsoffoodwasteandPHBOarecomparableonadrybasis,andthePHBOdecayrateislowerthanthatoffoodwaste(Table2).However,theresults(Figure1)areexpressedonawetbasisandthemethaneyieldsare90,70,200,and340m3wetMgÀ1forfoodwaste,newsprint,oﬃcepaperandPHBO,respectively.Thus,thehighermethaneyieldforPHBOonawetbasisincreasesCO2eforanationalaveragelandﬁll.Forastate-of-the-artlandﬁll,theCO2esignatureforPHBOissecondonlytonewsprintbecauseitsdecayrateisabouthalfthatoffoodwaste,somoreofthegasgeneratedfromPHBOcanbecollected.ItalsohasthesecondhighestCSFofanymaterialanalyzed,whichleadsittohavenetnegativeCO2e.

Resultsarepresentedbylandﬁllcategory(arid,moderate,wet,bioreactor)inSIFiguresS9ÀS12.Ingeneral,thevolumeofcollectedmethaneincreasesasthedecayratesdecreasesotheenvironmentalperformanceofwastegeneratedinaridregions,whichwasestimatedas20%ofthenationaltotal,ishighest.TheGHGperformanceofbioreactorlandﬁllsissuperiortothatofmoderateandwetlandﬁllsduetotheassumptionthatallbioreactorscollectgas.Onelimitationtothemodelingapproachisthataconstantmethaneoxidationfactorisassumedwhichsuggeststhatthemassofoxidizedmethaneincreasesasfugitiveemissionsincrease.Inreality,thefractionoftheuncollectedmethanethatisoxidizedwilldecreaseasfugitiveemissionsincreasebecauseoxygenavailabilityisalimitingfactorinmethaneoxidationandalowermethaneﬂuxtranslatestothepotentialtomeetahigherfractionofthestoichiometricoxygendemand.16AMonteCarloanalysiswasperformedbyvaryingtheparam-eterspresentedinTable5.Thecumulativedistributionfunc-tions(CDFs)fortheGHGemissionsassociatedwitheachwastecomponentandMSWdevelopedfromtheMonteCarloanalysis

Figure2.Cumulativedistributionfunctionsforeachmaterial.RangesusedforeachuncertaininputaregiveninTable5.ONP=oldnewsprint,MSW=municipalsolidwaste,FW=foodwaste,OFFP=oﬃcepaper.

arepresentedinFigure2.SummarystatisticsforeachCDFareshowninTableS2.PHBOhasthegreatestrangeamongthewastestreams(1600kgCO2e)becauseithasthehighestmethaneyieldwhichtranslatestomoreopportunityforchangesincollection,beneﬁcialuse,andoxidationtoaﬀecttheﬁnalresults.Similarly,oﬃcepaperhasthesecondhighestmethaneyieldandthesecondgreatestrange(930kgCO2e).SpearmanrankcorrelationsweredeterminedbetweenmajorinputsandthenetCO2eassociatedwithMSWdisposal(Table6).Theoxida-tionrateshowsthegreatestcorrelation,whichispartiallyduetoitslargerange.Furtherresearchintotheboundsofthisrange,andamodelingapproachthatincorporateschangesinmethaneoxidationasafunctionofthecontrollingvariables(soilmoisturecontent,temperature,porosityandmethaneﬂux)couldreducetheuncertainty.Whileworkontheseissuesisongoing,itappearsprematuretoincorporateinthissnapshotoftheU.S.landﬁllinfrastructure.Thefractionofwastedisposedinlandﬁllsthatcollectgasisthesecondmostsensitivevariablewhilethesensitivityoflandﬁlldecayratesvariesbasedontheamountofwasteineachlandﬁllcategory.Heretoo,uncertaintyexistsandwhenthedecayrateswerevariedby(50%,asopposedto(25%,theSpearmanrankcorrelationcoeﬃcientsincreasefrom0.066to0.078,0.075to0.14,0.081to0.18and0.041to0.066forarid,moderate,wet,andbioreactordecayrates,respectively.

DecayRateAnalysis.Thesignificanceofthedecayrateandmethaneyieldisfurtherillustratedbyaparametricanalysis.Ahypotheticalbiogenicpolymerconsistingof50%carboninanoxidationstateofacarbohydratewasanalyzedatfourdegreesofmineralization,withdecayratesvaryingfrom0.001to1.0yrÀ1.

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Table6.SpearmanRankCorrelationCoeﬃcientsbetweenUncertainInputsandtheNetCO2eAssociatedWithMSWDisposal

Spearman

input

Correlationoxidationrate

À0.749percentageofwastediscardedinlandﬁllsthatcollectgasÀ0.577gascollectioneﬃciencyunderﬁnalcover

À0.166timeuntilﬁnalcoverisinplaceafterﬁnalwasteplacement0.117wetlandﬁlldecayrate0.081moderatelandﬁlldecayrate

0.075percentageofwastediscardedinlandﬁllswithcollectionthatÀ0.066

recoverenergyaridlandﬁlldecayrate0.066bioreactordecayrate

0.041Table7.MaterialPropertiesforHypotheticalBiogenicPolymer

percentmineralization

L0(m3CH4MgÀ1)aCSF(kgCMgÀ1)a100465066307170331533350

500

aorganicValuesthatcarbon50%carbonwerecalculateddioxide.

oftheandforahypotheticalpolymerthatcontains50%

reactiveisincarbontheoxidationwillbeconvertedstateofacarbohydrate,tomethaneandmeaning50%toThe0%mineralizationcasesimulatesarecalcitrantbiogenicmaterial.ThemethaneyieldsandCSFsforthishypotheticalpolymeraregiveninTable7.Theresultsindicatethatdecreasedmaterialdecayratesanddecreasedmineralizationleadtode-creasedCOaveragelandfill,2e(Figure3).Theseresultssuggestthatforanationalinwhichnotallgasiscollectedandconvertedtoenergy,optimalperformancewouldbeachievedforbiogenicmaterialsthatarerecalcitrantunderanaerobicconditions.

EnvironmentalImplications.Thedescribedapproachpro-videsaframeworkforaproducertoconsidertheGHGperfor-manceofamaterialduringthedisposalphase.Theinputparameterscouldbeadjustedtoreflectregionsorcountrieswithalternatepracticesonlandfillgas.Inaddition,theanalysiscouldbeextendedtoreflectforexample,thatanestimated18.9%ofU.S.nonrecoveredMSWisdisposedbywaste-to-energy(WTE)combustion.1AnalysesofWTEhavebeenpresentedpreviously.17À19Similarly,foraproductsuchasabiodegradablebagthatisusedstrictlyforyardwaste,ascenarioinwhichasignificantpercentageoftheproductismanagedbycompostingoranaerobicdigestioncouldbedeveloped.Sotoo,thebehaviorofamaterialwheresomefractionisdiscardedaslittercouldbeconsidered.ThescenariopresentedhereisapplicabletoabiodegradablematerialthatismanagedwithMSW,themajorityofwhichisdisposedinlandfillsintheU.S.TheresultsshowthatarapidlydegradablematerialincreasesCO2erelativetoamoreslowlydegradableorrecalcitrantmaterial.

Ultimately,materialdevelopmentandselectionshouldcon-sideremissionsassociatedwithmaterialproduction,potentialdiﬀerencesintheusephasethatcouldbeattributedtothematerial,andend-of-lifemanagement.Ifemissionsforthe

Figurehypothetical3.Themineralization.

carbohydrate-basedeﬀectofdecaybiogenicrateonpolymertheGWPwithassociatedvaryinglevelswithofaproductionofabiodegradablematerialarecomparableorhigherthanemissionsassociatedwithmanufacturingamaterialfrompetroleum-basedfeedstocks,anddisposalemissionsarehigherforthebiodegradablematerialasillustratedhere,thenitishardtorationalizeasuggestionthatthebiodegradablematerialisthepreferablealternative,assumingofcourse,theavailabilityofpetroleum-basedfeedstocks.

’ASSOCIATEDCONTENT

SSupportingInformation.

Wastecomposition,summary

statisticsforMonteCarloanalysis,methaneproductiondatafromPHBObiodegradationstudy,derivationofmineralizationfactor(PHBO)anddecayrates(MSWandPHBO)basedonreactordata,andCO2eforlandﬁllsineachcategory(arid,moderate,wet,bioreactor).ThismaterialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.

’AUTHORINFORMATION

CorrespondingAuthor

*Phone:(919)515-0887;fax:(919)515-7908;e-mail:jwlevis@ncsu.edu.

’ACKNOWLEDGMENT

ThisresearchwassupportedbyProcter&Gamble.J.W.L.wassupportedinpartbyaFiessingerFellowshipfromtheEnviron-mentalResearchandEducationFoundation.’REFERENCES

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Is Biodegradability a Desirable Attribute for Discarded Solid Waste_