Availableonlineatwww.sciencedirect.com

BioresourceTechnology99(2008)72787284

Theeectoftemperaturevariationonbiomethanationathighaltitude

ReneAlvareza,b,GunnarLidenb,*aIIDEPROQ,UMSA,PlazadelObelisco1175,LaPaz,BoliviabDepartmentofChemicalEngineering,LundUniversity,P.O.Box124,22100Lund,SwedenReceived26September2006;receivedinrevisedform11December2007;accepted18December2007Availableonline8February2008

Abstract

Theaimofthecurrentstudywastoexamineeectsofdailytemperaturevariationsontheperformanceofanaerobicdigestion.Forcedsquare-wavetemperaturevariations(between11and25,15and28,and19and32C)wereimposedonabench-scaledigesterusingamixtureofllamacowsheepmanureinasemi-continuousprocess.Thevolumetricbiogasproductionrate,methaneyield,andthevol-atilesolidreductionswerecomparedwiththeresultsobtainedfromanaerobicdigestion(AD)atconstanttemperatures.Theforcedcyclicvariationsoftemperaturecausedlargecyclicvariationsintherateofgasproductionandthemethanecontent.Asmuchas9497%ofthedailybiogaswasobtainedinthe12hhalf-cycleathightemperature.Thevaluesforvolumetricbiogasproductionrateandmethaneyieldincreasedathighertemperatures.Theaveragevolumetricbiogasproductionrateforcyclicoperationbetween11and25Cwas0.22Ld1L1withayieldof0.07m3CH4kg1VSadded(VSadd),whereasforoperationbetween15and29Cthevolumetricbiogasproductionrateincreasedby25%(to0.27Ld1L1withayieldof0.08m3CH4kg1VSadd).Inthehighesttemper-atureregionafurtherincreaseof7%inbiogasproductionwasfoundandthemethaneyieldwas0.089m3CH4kg1VSadd.Theemployeddigestershowedanimmediateresponsewhenthetemperaturewaselevated,whichindicatesawell-maintainedmeta-boliccapacityofthemethanogenicbacteriaduringtheperiodoflowtemperature.Overall,periodictemperaturevariationsappeartogivelessdecreaseinprocessperformancethanapriorianticipated.2007ElsevierLtd.Allrightsreserved.

Keywords:Anaerobicdigestion;Temperature;Periodicoperation;Manure

1.Introduction

Anaerobicdigestion(AD)thatutilisesmanureforbio-gasproductionisoneofthemostpromisingusesofbio-masswastesbecauseitprovidesasourceofenergywhilesimultaneouslyresolvingecologicalandagrochemicalissues.Theanaerobicfermentationofmanureforbiogasproductiondoesnotreduceitsvalueasafertilizersupple-ment,asavailablenitrogenandothersubstancesremaininthetreatedsludge(Robertsonetal.,1975).TheuseofADiswidelydemonstratedinAsiawithseveralmillionsmall-scalebiogasplantsinChinaandIndia(Khoiyangbametal.,2004;Nazir,1991).

*Correspondingauthor.Tel.:+46462220862.E-mailaddress:Gunnar.Liden@chemeng.lth.se(G.Liden).

0960-8524/$-seefrontmatter2007ElsevierLtd.Allrightsreserved.doi:10.1016/j.biortech.2007.12.055

Anaerobicdigestionisatemperaturedependentprocess,whichisnormallyoperatedatdenedandconstanttemper-atures.Nevertheless,situationsexistinwhichreactorsaresubjecttorepeatedsuddenandabruptchangesoftemper-ature.Onfarms,bioreactorsmaybesubjectedtotempera-tureuctuationsduetolargevariationsinoutdoortemperature,especiallyinhighlandandnorthernclimates(Alvarezetal.,2006;Masseetal.,2003).Theanaerobicdigestionprocessisnormallyclassiedintothreedierenttemperatureranges,namelypsychrophilic(40C)(El-Mashadetal.,2004).Themicroorganismsinvolvedinanaerobicdigestionarecharacterizedbyanoptimaltemperatureaswellasbyanupperlimitthatwouldcauseimmediatedeathoftheconsideredgroupofbacteria(Chen,1983).Theanaerobicdigestionofmanureinconventionaltreatmenttendstohavehighprocessstability.However,

R.Alvarez,G.Liden/BioresourceTechnology99(2008)727872847279

suddenenvironmentalchanges,e.g.dramaticincreasesordropsintemperature,maycauseseveredisturbanceinallparametersoftheprocess,andtheoveralladaptationtonewstableoperationrequiresalongperiodoftime(Bousk-ovaetal.,2005;Chaetal.,1997).Theextentofthedistur-bancehasbeenrelatedtothemagnitudeofthetemperaturevariation(El-Mashadetal.,2004).Theeectofincreasingordecreasingtemperaturesfollowedbyre-establishmentoftheinitialtemperaturehasbeenassessedinsomepreviousstudies.Thesestudiesshowthatadecreaseintemperaturetypicallycauseslowerchemicaloxygendemand(COD)removaleciencies,lowerbiogasproduction,andtheaccumulationofvolatilefattyacids(VFA).Anaerobicdigestorswereinonepreviousstudyreportedtorecovertheirecienciesfullyafterthetemperaturehadbeenread-justed(AhnandForster,2002).Inanotherstudy,dierentrecoverymethodsweretestedafterthetemperatureindigestersoperatingatsteadystatehadbeenlowered.Rais-ingthetemperatureinasingledaygaveonlyatransienteectandsteadystatewasre-establishedwithineightdays.However,aslowincreaseintemperatureledtomoredele-teriouseectonthedigesterstability(Pecketal.,1985).Digestersworkingwithimposeddailycyclictempera-tureuctuationhavebeenthesubjectofonlyafewstudieswithdierentpurposes:Man-changetal.,2006simulatedtheheatingfailureofathermophilicanaerobicdigesterfedwithmunicipalorganicsolidwastesbyrapidlydecreasingthetemperatureandthenre-establishingtheoptimaltem-perature.Theeectoftemperatureuctuationsbetween10and20Conapsychrophilicanaerobicsequencingbatchreactortreatingswinemanurehasalsobeenevalu-ated.Resultssuggestthattheperformanceofanaerobicsequencingbatchreactors(ASBRs)willdeterioratesigni-cantlyiftheoperatingtemperatureisdecreasedfrom20to10C.However,theASBRswillremainstableanditwillrecoverat20C(Masseetal.,2003).Inanotherstudy,thegasproductiondynamicswasinvestigatedusingalab-oratoryscaledigesterfeddailywithdairymanurethatwasoperatedataconstanttemperature,aswellaswithapro-grammedtemperatureuctuationof3.3Caboutameanof35.8C.Thedatasuggestedthatitwouldbepossibletomanipulategasproductionbyheatingandcoolingthecontentofthedigester.Thusitmaybepossibletoreducegasstoragevolumebymatchingtheproductiontoavary-ingenergydemand(Chayovanetal.,1988).Inhighlands,suchastheBolivianAltiplano,wherethelivestockiscomposedmainlyofllamas,cowsandsheep(currentlymorethan1.8,0.6,and6.2millionanimals,respectively),theuseofthemanurefromtheseanimalsinanaerobicdigestioncouldsatisfytheenergydemandsforcookingandlighting(especiallyinremoteruralareas)andreducetheuseofrewoodandthedeforestationthatgoeswithit.However,theextremeenvironmentalcondi-tionsconstitutealimitingfactorforthebiomethanationprocess.TheBolivianAltiplanohasanaverageelevationofnearly4000m,itissweptbystrong,coldwinds,andhasanarid,chillyclimate,withlargedierencesintemper-

ature.Theaveragehighsduringthedayrangefrom15to20Candtheaveragelowsrangefrom15to3Cwithanatmosphericpressurearound460500mmHg.Thelargevariationinambienttemperatureislikelytoaecttosomeextenttheoperatingtemperatureoflow-costsim-pleanaerobicdigestors.Thiseectofdailytemperatureuctuationsontheanaerobicdigestionprocesshasrarelybeeninvestigated.Theaimofthecurrentworkwastoassesstheeectofdailytemperaturevariationsonthesemi-continuousanaer-obicdigestionofmanuresfromtheBolivianAltiplanousinganexperimentalmodelsystemwithforcedtempera-turevariation.Thedailytemperaturevariationsweresim-ulatedbyaforcedsquare-waveoscillationofthereactortemperatureofananaerobicdigester,whichusedamixtureofllama,cowandsheepmanureasfeedstock.Theperfor-manceofthedigesterwasanalysedwithrespecttovolu-metricbiogasproductionrate,methaneyieldandvolatilesolidreductioninthesubstrate.

2.Methods

2.1.Feedstockandpreparation

Llama,sheepandcowmanurewerecollectedfromfarmsintheBolivianAltiplano(19Slatitude,68Wlon-gitude).Themanureswereseparatelymincedandpulver-izedwithasemi-industrialcutter(CUT-3,Metvisa,Brazil).Thesampleswerepackedinto500gpolyethylenebagsandstoredat10Cinafreezer.ThecharacteristicsofthevariousmanuresaregiveninTable1.Batchesofequalvolumesofthethreemanures(33.3%(VS/VS)each)werepreparedwithllamamanure(5.7%byweight),cowmanure(13.4%byweight),andsheepman-ure(4.2%byweight).Eachbatchwasdilutedwithtapwater(76.6%)toobtainthedesiredsolidcontent(6%ofVSw/w).Theslurrywashomogenizedinadomesticelec-tricblender(HamiltonBeach908,HamiltonBeachCom-mercial,USA)andfractionated(withavolumedenedbythevalueofthedesiredhydraulicresidencetime,HRT=30days).Thesampleswerepackedintopolyethyl-enebagsandstoredinafreezer.Thesamplesforeachdaywerewithdrawnfromthefreezerandallowedtothawovernight.

Table1Characteristicsoffreshundilutedmanureusedinexperimentsat18,25,35CandtemperatureuctuationAnalysisLlamaCowSheepmanuremanuremanureTotalsolids(%w.w)49.5(3.2)19.8(1.1)77.6(2.2)Volatilesolids(%ofTS)70.3(2.2)74.9(1.6)61.3(5.6)Totalnitrogen(%ofTS)1.7(0.1)1.6(0.2)1.1(0.3)Totalorganiccarbon(%ofTS)29.5(2.3)26.5(5.7)18.9(4.1)Totalphosphorous(%ofTS)0.4(0.1)0.4(0.1)0.5(0.1)Totalpotassium(%ofTS)1.5(0.2)0.7(0.3)1.8(0.5)Standarddeviationfromvesamplesinparentheses.

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2.2.Apparatus

Abench-scalebioreactormadeofstainlesssteelwithatotalcapacityof15Landaliquidvolumeof9.3Lwasused(Fig.1).Thedigesterhadabuilt-incoolingandheat-ingsystemfortemperaturecontrol.Thejacketedcylindri-calvesselwasequippedwithaangedtop,towhichaangeplatewithagasoutletportwastted.Thisallowedgascollectionandmeasureofpressure.Animpeller(60rpm)wasusedforstirringandthestirrerdriveshaftswereinsertedthroughagas-tightbearing.Thereactorwasfedfromalateralportthrougha50mmIDballvalve.Theeuentwasdrawnfromthebottomofthereactorthrougha12.7mmballvalve.

2.3.Experimentalprocedure

Twokindsofexperimentswereconducted;anaerobicdigestionatconstanttemperatures(35,25and18C),andanaerobicdigestionduringforcedsquare-wavecyclingoftemperature.Thelowandhightemperaturesinthelatterkindofexperimentswere11and25,15and29,and19and32C,respectively.

2.3.1.ADexperimentsatconstanttemperaturesTheeectoftemperatureondigestionofllamacowsheepmanuremixturewastestedinthebench-scalereactorwith9.3-Lactivevolumedescribedabove.Forthestart-upperiod(60daysat35C),theinitialmediumwaspreparedwith5%llamamanure,11%cowmanure,4%sheepman-ure,64%tapwaterand16%activeslurryfromasemi-con-tinuousdigesterworkingwithllamacowsheepmanureat25CwithHRTof50days(7.3%TS,71%VS,andpH=7.6).Llama,cowandsheepmanure(33.3%VS/VSofeach)withaVScontentof60.1%w/wwasfeddailyinanamountgivingaHRTof30days.Afterthestart-upperiodthereactortemperaturewaskeptattheselectedtemperatures(60dayseachof35,25and18Cinthisorder)bywatercirculatinginthewater

jacketsurroundingthereactor.Thereactorwasfedonceadayat30daysHRTregimeand60.1%w/wVScon-tent.Thecontentofthereactorwasstirredfor15mineveryhourat60rpm.ThepHandthesolidcontentoftheslurrywereanalysedevery10days.Biogaswascollectedinaseparate30Lplas-ticbottleandthevolumewasmeasuredonceadaybydis-placementofacidiedwater(pH=2)atzerogaugepressureandambienttemperature.Thevolumeswererecalculatedtostandardtemperatureandpressure(0C,760mmHg).

2.3.2.Cyclicsquare-wavetemperatureexperimentsAfternishingtheexperimentdescribedaboveat18C,thebench-scaledigesterwassubjectedtoadailysquare-waveuctuationoftemperatureinordertostudytheeectontheanaerobicdigestionprocess.Thereactoroperatedattemperatureintervals1125,1529,and1932Csucces-sivelyfor60daysateachinterval.Thesquare-wavetem-peratureuctuationconsistedof12hathightemperaturefollowedby12hatlowtemperature.Thereactorwascon-trolledbyaPtsensorPLC-Softwareheater/coolersystemthatchangedthetemperatureoftheslurryinthereactorfromlowtohigh(orhightolow)inlessthan30min.Thereactorwasfedonceadayatthebeginningofthehightemperaturehalf-cycle.TheHRTwas30daysandtheload-ingratewas2kgVSm3d1with6(0.1)%w/w.Thefeed-stockwasamixtureofllamacowsheepwith33.3%VSofeach.Thecontentofthereactorwasstirredatintervalsof5min(on/o)allthroughthedayat60rpmtoimprovetheheattransferenceandthecontrolsystem.ThepH,solidcon-tent,biogasvolumeandmethanecontentweremeasuredaccordingtotheproceduredescribedabove.

2.4.Analyticalmethods

Methaneandcarbondioxideconcentrationsinthebiogasweredeterminedwithagaschromatograph(ShimadzuModelGC14B,Japan)equippedwithathermalconductivity

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Fig.1.Schematicdiagramofbench-scaledigester:1.vessel,2.feedinlet,3.euentvalve,4.motor,5.agitationimpeller,6.pHprobe,7.Pt100electrode,8.accumulationandmeasurementbiogassystem,9.electrovalves,10.pumps,11.thermostaticheater,12.cooler,13.pressostat.

Biogasproduction(LL-1d-1)VSineffluent%(VS/w.w)Methanecontent(%)R.Alvarez,G.Liden/BioresourceTechnology99(2008)727872847281

detector(TCD)andaCarboxen-1010plotCapillarycolumn30m0.53mmID(Supelco,USA).Theinjector,detectorandoventemperatureswere130,200,and100C,respectively.Heliumservedasthecarriergasatapressureof300kPa.Totalsolids(TS),volatilesolids(VS),pH,totalorganiccarbon(TOC),totalKjeldahlnitrogen(TKN),potassiumandphosphorusweredeterminedaccordingtostandardmethods(Clescerietal.,2000).Thetotalsolids(TS)con-tentwasdeterminedafterheating(105Cfor1h),cooling,desiccating,andweighingproceduresthatwererepeated

untiltheweightchangewaslessthan4%.VolatilesolidsweredeterminedbyignitionoftheresidueproducedinTSanalysistoconstantweightinamuefurnaceatatem-peratureof550C.TOCwasdeterminedbyhightempera-turecombustionmethod(Method5310B).TKNwasmeasuredusingthesemi-micro-Kjeldahlmethod(Method4500-NorgC),PotassiumandPhosphorusweremeasuredbyspectrophotometry(Method3500-Kand4500-P,respectively).

3.Results

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3.1.Anaerobicdigestionatconstanttemperature

Thefermentationofamixtureofllamacowsheepmanurewasfoundtobequitestablewithrespecttodailybiogasproduction,methanecontentandvolatilesolidineuent.Oncestabilized,thedailyvariationindigesterper-formance(Fig.2)at35,25,and18Cwaslessthan5%withrespecttothevariablesabove.Astobeexpected,thevolumetricbiogasproductionratedecreasedasthetemperaturewaslowered.Thereductionfrom35to25Cresultedinareductionof30%involumetricbiogasproductionrate,whereasthe7Creductionfrom25to18Ccausedareductionof51%(Table2).Clearly,theanaerobicdigestionisaprocessthatisstronglydependentontemperature.Ontheotherhand,themethanecontentinthebiogasincreasedatlowtemperature.Themethanecon-tentinthebiogasincreasedfrom49.9%to61.1%between35and18C,whichpartlycounteractedthedecreaseinvol-umetricgasproductionrate.Thevolumetricmethanepro-ductionratewasreducedfrom2094(at35C)to1676mlCH4d1(at25C)representingareductionof20%.Afurtherreductionof47%(from1676to894mlCH4d1)wasseenwhenthetemperaturewasreducedfrom25to18C.Themethaneyieldfollowedthesamepattern.

3.2.Anaerobicdigestionwithaforcedcyclictemperaturevariation

Fig.2.Dailybiogasproduction(a),methanecontent(b)andVSineuent(c)atdierenttemperatures:18C(),25C(h)and35C(N)fromanaerobicdigestionofllamacowsheepmanuremixtures.

Asquare-wavetemperaturevariationwasimposedonthebench-scaledigesterinordertoexperimentallyinvesti-

Table2Measuredandcalculatedparametersfrombench-scalereactordigestingmixtureofllamacowsheepmanurewithsquare-wavetemperatureuctuationanddierentconstanttemperatures

(kgVSm3d1)Digester(m3kg1VSadd)(mld1)BiogasatTemperaturerange(C)

OLR

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ReductionofVS(%)

Methanecontent(%)

VolumetricbiogasproductionrateMethaneyield(Ld1L1)DailybiogasBiogasatlowT(%)highT(%)

11252.03013.8(2.9)56(2)0.22226(155)3970.069(0.005)15292.03014.7(1.2)55(2)0.32738(98)3970.084(0.003)19322.03019.0(1.5)56(3)0.32890(32)6940.089(0.005)182.03019.1(2.8)61(1)0.21464(94)0.048(0.003)252.03025.7(3.3)56(2)0.32977(124)0.092(0.002)352.13029.4(1.0)49(1)0.54198(139)0.109(0.005)Standarddeviationfrom10consecutivedaysinparentheses.

Methanecontent(%)Methanecontent(%)Biogasproduction(ml)Biogasproduction(ml)Temperature(oC)Temperature(oC)7282R.Alvarez,G.Liden/BioresourceTechnology99(2008)72787284

gatetheeectofadailychangingtemperatureontheanaerobicdigestionprocess.Thereactorwasoperatedbetween11and25,15and29,and19and32Csucces-sivelyfor60daysateachinterval.Thevariationimposedwasasquare-wave,inwhichthetemperaturewasmain-tainedatthehighlevelfor12hfollowedby12hduringwhichthetemperaturewaskeptlow.Foreachtemperatureintervalthedigesterrequiredanadaptationperiodofbetween20and40daystoreachthepseudo-steady-state(i.e.whenthemeasuredresponseduringonecyclelooksthesameasthesubsequentones).Alongeradaptationper-iodwasrequiredforthecyclingexperimentsconductedatlowertemperatures.Thebiogasproductionwasmeasuredeverysecondhour,whichprovidedsomedynamicinformationontheresponsetochangesinvolumetricbiogasproductionrateandmeth-anecontent(Figs.3aand4a).Figs.3band4bshowgasproductionandmethanecontentinadigesterworkingiso-thermicallyat25C.Althoughthereisaresponsefromthedailyfeedingalsointheisothermalcase,thedierencesbetweenthegasproductioncurvesdemonstratethestrong

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inuencethatthetemperatureexertsontheprocess.Time-averagedsteady-stateresultsaresummarizedinTable2.Thevaluesofmethanecontent,productivityandyieldwerebasedonanaverageofmeasurementsovera10-dayperiodaftertworetentiontimes.Inthisstudy,themeasuredpH-valuewas7.20.5throughout.Onaday-averagedbasis,themethanecontentinthegaswasbetween55and56%.Whenthedigesterwasworkingwithforcedoscillationsbetween11and25C,mostofthedailybiogasproduction(2226mld1)wasobtainedduringthe12hof

Time(h)Fig.4.Methanecontentinbiogas(barsymbols)andtemperature(s)duringforcedperiodicvariationoftemperature(a)andataxedoperatingtemperatureof25C(b).

hightemperature,whichcorrespondedto97%ofthetotalproduction.Whenthetemperaturewasreducedto11C(about2030mincoolingtime)adrasticreductioninbio-gasgenerationwasobserved,andinthefollowing12hoursonly3%ofthedailybiogasproductionwasobtained.A

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similarpatternwasobservedinthedigesterworkingattemperaturesbetween15and29C.Thedigesterworkingbetween19and32Cshowedsomewhathigheractivityatthelowtemperature,and6%ofthedailybiogasproduc-tionwasobtainedat19C(173mld1),andtheremaining94%wasproducedat32C(2717mld1).Inalltheforcedcyclingexperimentsitwasclearthatthevolumesofgasobtainedintheperiodsoflowtemperature(12h)wereverylow.Interestingly,theproductivityduringthelowtemper-ature(19C)periodintheforcedcyclingexperimentwork-ingbetween19and32Cwasverymuchlowerthantheproductivityfortheexperimentrunningataconstanttem-peratureof18C(cfTable2).Volumetricbiogasproductionrateandmethaneyieldbothincreasedwithtemperature.Theaveragevolumetricbiogasproductionrateat1125Cwas0.22Ld1L1(withayieldof0.07m3CH4kg1VSadd).Increasingthetemperaturerangetoalowtemperatureof15Candahightemperatureof29Craisedthevolumetricbiogasproductionrateby25%,to0.27Ld1L1(0.08m3CH4kg1VSadd).However,increasingthetem-

Time(h)Fig.3.Gasproduction(barsymbols)andtemperature(s)duringforcedperiodicvariationoftemperature(a)andataxedoperatingtemperatureof25C(b).

peraturefurthertoalowandahightemperatureof19and32C,respectively,gaveonlyafurtherincreasedbio-gasproductionof7%(to0.29Ld1L1and0.09m3CH4kg1VSadd).

4.Discussion

R.Alvarez,G.Liden/BioresourceTechnology99(2008)727872847283

Theimposedsquaredailycyclictemperatureuctuationgaveastrongmagnicationofthecyclicvariations

Theresultsobtainedinthepresentstudyshowthatanaerobicdigestionofmanuresfrom,e.g.llamas,cowsandsheepinasemi-continuousprocesssubjecttoasquare-wavedailytemperatureuctuationmayoperateinastablefashion,althoughwithproductivitychangingoverthe24hcycle.BoththepH(7.2)intheeuentandthemethanecontentofthebiogas(5556%)werestable,whichisasignofawellbalancedbiomethanationprocess(MountfortandAsher,1978)withexpectablebiogasvol-umes(Alvarezetal.,2006).Thesquare-wavetemperatureuctuationthatwasimposedonthedigesterresultedinaninterestingbehav-iour.Ineect,thedailybiogasvolumesobtainedwhenthedigesterwasworkingat1125Candat1529Cwereobtainedduringthe12hofhightemperature(25and29C,respectively),astheproductionduringthe12hoflowtemperature(11and15C)merelyamountsto3%ofthetotaldailyyield.Despitethealmostnon-existentperformanceatlowtemperature,thesuddenraiseintemperatureresultedinaperiodwithunexpect-edlyhighanaerobicdigestionactivity.Thebiogasvol-umesproducedinthisperiodweremorethanexpectedofadigesterworkingfor12hatasimilarconstanttemperature.Thedigesterworkingatthe1125Ctemperatureuctuationhadavolumetricbiogasproductionrateof0.24Ld1L1andamethaneyieldof0.07m3CH4kg1VSadd,thismeansthatitsvolumetricbiogasproductionratewas30%higheranditsmethaneyield40%higherthanthatofthedigesterworkingattheaveragetemper-ature18Cinanisothermicprocess(Table2).Theseresultssuggestthattherearepositiveeectswhendigest-ersaresubjectedtoperiodiccyclicuctuationsintem-perature.Adigesterworkingat1932Contheotherhandhadavolumetricbiogasproductionrateof0.3Ld1L1andamethaneyieldof0.09m3CH4kg1VSadd,numberswhicharemoresimilartowhatwasobtainedwhenthedigesterwasworkingataconstant25C.Increasedgasproductionassociatedwithtemper-atureuctuationshasbeenpreviouslynoticedbyChayo-vanetal.(1988).Foradigesterworkingataconstanttemperature(Fig.3b),adailycyclicuctuationintherateofgaspro-ductionwasobservedasaresultofthefeeding.Duringthersthoursafterfeeding,therewasaclearincreaseinbiogasproduction,whichwasfollowedbyadecline.Asmallvariationinmethaneconcentrationinthegaswasobservedaswell(0.52.5%abovethedailyaverage).How-ever,themeasurementofthisvariationwasmaskedbydilutioninthegasspacevolumeinthedigesterandbythecollectingbottle.Thevariationsduetothedailyfeedingtothereactorsaremostlikelyaresultfrommetabolicvari-ations,ashaspreviouslybeenreportedbyMountfortandAsher(1978),andothers(HawkesandYoung,1980;Chayovanetal.,1988).

describedabove(Figs.3aand4a)withanimmediateresponsetosuddenincreasesoftemperatureandtoaddedsubstrate.Atlowtemperature,ontheotherhand,thebio-gasproductionalmostceased.Thesuppressionofacido-genicactivityduetorapidtemperaturedrops(tobelow20C)hasbeenpreviouslyreportedbyChaetal.(1997).Inthatstudy,thenumberofbacteriawasslowlyreduced,buttheacidogensbecametemporarilyinactiveduringtheperiodoflowtemperature.Theanaerobicfermentationprocessappearscapableofanimmediateresponsetosuddenincreasesintemperatureandtoaddedsubstrate,whichsuggeststhatthemetaboliccapacityofthemethanogenicbacteriaiswellmaintained.Itappearsthatsomecomponentsofthellamacowsheepmanurearerapidlyconvertedtobiogas(e.g.carbohy-drates),whereasthebulkofthematerialsisbrokendownmoreslowly(HawkesandYoung,1980).

5.Conclusion

Amixtureofmanuresofllama,cowandsheepwasshowntobeasuitablefeedstockforbiogasproductionontheBolivianAltiplano.Amethanecontentinthebiogasof55%andavolumetricbiogasproductionratebetween0.2and0.3Ld1L1wereobtainedataHRTof30daysfortheconditionsinvestigated.Dailysquare-wavecyclicuctuationsoftemperature,designedtomimicuctuationsofambienttemperaturesontheBolivianAltiplano,causedlargecyclicvariationsintherateofgasproduction.Thedigesterrequiredanadaptationperiodbetween20and40daystoreachthepseudo-steady-statewithalongerper-iodrequiredforthelowertemperatureregion.Interest-ingly,thebiomethanationprocessrespondedimmediatelytothesuddenincreaseintemperature.Thissuggeststhatactivityofthemethanogenicbacteriaarewellpreservedduringtheperiodatlowtemperature.

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