Effect of liquid surface area on hydrogen sulfide oxidation during micro-aeration in dairy manure

12 Pages·2017·7.55 MB·English

RESEARCHARTICLE
Effect of liquid surface area on hydrogen
sulfide oxidation during micro-aeration in
dairy manure digesters
WalterMulbry1*,KaitlynSelmer1,StephanieLansing2
1 AgriculturalResearchService,UnitedStatesDepartmentofAgriculture,Beltsville,Maryland,UnitedStates
ofAmerica,2 DepartmentofEnvironmentalScienceandTechnology,UniversityofMaryland,CollegePark,
Maryland,UnitedStatesofAmerica
*[email protected]
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a1111111111 Abstract
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Althoughthereareavarietyofcommerciallyavailablebiologicalandchemicaltreatments
forremovalofhydrogensulfide(H S)frombiogas,managingbiogasH Sremainsasignifi-
2 2
cantchallengeforagriculturaldigesterswherelaborandoperationalfundsareverylimited
comparedtomunicipalandindustrialdigesters.Theobjectivesofthisstudyweretoevaluate
OPENACCESS headspaceaerationforreducingH Slevelsinlowcostplugflowdigestersandtocharacter-
2
Citation:MulbryW,SelmerK,LansingS(2017) izetherelationshipbetweentheliquidsurfaceareaandH2Soxidationrates.Experiments
Effectofliquidsurfaceareaonhydrogensulfide withreplicatefieldscaleplugflowdigestersshowedthatH Slevelsdecreasedfrom
2
oxidationduringmicro-aerationindairymanure
3500ppmvto<100ppmvwhenheadspaceoxygenlevelswere0.5to1%.Methaneproduc-
digesters.PLoSONE12(10):e0185738.https://
tionwasnotaffectedbyaerationratesthatresultedinheadspaceoxygenlevelsofupto1%.
doi.org/10.1371/journal.pone.0185738
Pilotscaleexperimentsusing65to104LdesulfurizationunitsshowedthatH Soxidation
2
Editor:JohnM.Senko,TheUniversityofAkron,
ratesincreasedwithincreasesinliquidsurfacearea.Theseresultssupportthehypothesis
UNITEDSTATES
thatH Soxidationratesarelimited,inpart,bythesurfaceareaavailableforoxygentransfer,
2
Received:August2,2017
andcanbeincreasedbygrowthofbiofilmscontainingH Soxidizingbacteria.Maximum
2
Accepted:September18,2017 removalratescorrespondedto40to100gSm-2d-1ofliquidsurfaceareaatbiogasretention
Published:October4,2017 timesof30to40min.
Copyright:Thisisanopenaccessarticle,freeofall
copyright,andmaybefreelyreproduced,
distributed,transmitted,modified,builtupon,or
otherwiseusedbyanyoneforanylawfulpurpose.
TheworkismadeavailableundertheCreative Introduction
CommonsCC0publicdomaindedication.
Untreatedmanurefromanimaloperationscanresultinnoxiousodorsandtheunintentional
DataAvailabilityStatement:Allrelevantdataare
releaseofthegreenhousegasmethane(CH ).However,whenproperlytreatedinananaerobic
withinthepaperanditsSupportingInformation 4
digester,animalwastecanbetransformedintoenvironmentalandeconomicbenefits:1)the
files.
capturedmethanebecomesasourceofrenewableenergy,2)emissionsofmethaneandnox-
Funding:ThisworkwasfundedbytheUSDA
iousodorsaresharplyreduced,and3)thefertilizervalueofthemanureincreasesasaportion
AgriculturalResearchService.Thefunderhadno
oforganicNisconvertedtoplantavailableammonia-Nincreases[1].
roleinstudydesign,datacollectionandanalysis,
decisiontopublish,orpreparationofthe Biogasfrommanuredigestersusuallycontains55to65%CH4,35to45%CO2,and0.1to
manuscript. 0.4%ofhydrogensulfide(H S)[2].Inadditiontoitstoxicityandreactivitywithmetalsand
2
cement,H SisreadilyconvertedintoSO andH SO ,whicharealsohighlycorrosive.
Competinginterests:Theauthorshavedeclared 2 2 2 4
thatnocompetinginterestsexist. Althoughthereareavarietyofcommerciallyavailablebiologicalandchemicaltreatmentsfor
PLOSONE|https://doi.org/10.1371/journal.pone.0185738 October4,2017 1/12
Effectofliquidsurfaceareaonmicro-aeration
removalofH Sfrombiogas(recentlyreviewedby[2,3],allrequiresomelevelofchemicalor
2
waterinputsandmaintenance.Inpractice,managingbiogasH Sremainsasignificantchal-
2
lengeforagriculturaldigesterswherelaborandoperationalfundsareverylimitedcompared
tomunicipalandindustrialdigesters.Asanalternativetotreatbiogas,relativelylowvolumes
ofair(oroxygen)canbeinjectedintothedigesterheadspace.Thebasisofthismethodliesin
thepresenceofsulfide-oxidizingbacteriainthemanurefeedstock[4]thatconvertdissolved
sulfideintoSoandSO 2-[5].ThefeasibilityofH Soxidationundermicro-aerobicconditions
4 2
hasbeendemonstratedinpilot-scaleexperiments(recentlyreviewedby[2,3])andbymoni-
toringoflargescalesewagesludgedigesters[6].However,furtherresearchisneededtotrans-
ferthesefindingstopracticeonplug-flowagriculturaldigesterswherebiogascompositionand
aerationsystemsaremonitoredinfrequently,ifatall.
Thereweretwospecificobjectivesofthisstudy.Thefirstobjectivewastoevaluatehead-
spaceaerationforreducinghydrogensulfide(H S)levelsinlowcostplugflowmanuredigest-
2
ers.Sixfield-scaleTaiwanese-modeldigesterswereusedtodeterminetheeffectofaeration
rateonH Sconcentrationsandmethaneproductionunderfieldconditions.Thesecondobjec-
2
tivewastodeterminetherelationshipbetweentheliquidsurfaceareaindigestersandH Soxi-
2
dationratesduringheadspaceaeration.WehypothesizedthatH Soxidationrateswere
2
limited,inpart,bythesurfaceareaavailableforgrowthofbiofilmscontainingH Soxidizing
2
bacteria.Inthispartofthestudy,weusedreplicatepilot-scaledesulfurizationunits[7]that
wereoperatedatarangeofH Sloadingrates.
2
Materialsandmethods
Substrate
DairymanurewasobtainedfromtheUSDA’sDairyResearchUnitwithintheBeltsvilleAgri-
culturalResearchCenter(BARC)(Beltsville,Maryland,USA).Thedairy’sfreestallbarn
housesapproximately100dairycowsandusessawdustasbeddingontopofrubberpillows.
ManureismechanicallyscrapedintoholdingpitsandpumpeddailyfromthepitstoaFAN
screw-presssolidseparatorsystem.Thesolidsarecollectedandcomposted,andthesolids-sep-
aratedliquidmanure(containingroughly3%TS)ispumpedoverthecourseofadayintothe
fullscaleBARCdigester(540m3totalvolume,400m3workingvolume)fortreatment.During
thecourseofthisstudy,theBARCdigesteroperatedwithoutmixingattemperaturesranging
from30to35˚Cwithbiogaspressureof2000to2500Pa(20–26cmofwater)andtypicallypro-
duced65m3d-1ofbiogascontaining63to66%CH ,32to34%CO ,2000to4000partsper
4 2
millionbyvolume(ppmv)H S.
2
Field-scaleanaerobicdigesters
AnaerobicdigestionexperimentswerecarriedoutusingsixmodifiedTaiwanese-modelfield-
scale(FS)digesters(Fig1)attheBARCdairy[8,9].EachFSdigesteriscomposedofatapered
5.2mlong,0.9mdiameterPVC-baseddigesterbagwitha1.0mmthickmembrane.The
digesterbagswereplacedinside1.1mdiametercorrugatedhigh-density-polyethylene
(HDPE)drainagepipesforprotectionandinsulation.EachFSdigesterhasatotalcapacityof3
m3andwasoperatedataliquidcapacityof67%(2m3workingvolume),with33%headspace
forbiogascollection.TheFSdigestersareplug-flowreactorsandwereoperatedwithoutmix-
ing.Digestersweremaintainedat28±2˚Cbycirculatingaheated30%glycolsolutionthrough
heatexchangermateriallocatedbetweenthedigesterbagsandfoaminsulation.Pumps,valves
andheatignitionwerecontrolledelectronicallythroughaLabview™softwareprogram
(NationalInstrumentsCorp.,Austin,TX,USA).
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Effectofliquidsurfaceareaonmicro-aeration
Fig1.Imagesofthesixplug-flowfield-scale(FS)digestersusedinthisstudy.A,Digesterbagpriorto
installation.Thebiogasventtube(10cmdiameter)isnotshownbutislocatedinthetopcenterofthebag.B,
Fieldsiteoverview,withthedigesterbagsplacedinsideinsulatedHDPEcorrugateddrainagepipes.
https://doi.org/10.1371/journal.pone.0185738.g001
Priortothestudy,eachFSdigesterwasfilledwith1m3ofinoculumfromtheBARCfarm
digesterand1m3ofseparatedmanure.Eachdigesterwassubsequentlyloadedwith160Ld−1
separatedmanure,correspondingtoanorganicloadingrateof1.5kgVSm−3d−1)andhydrau-
licretentiontime(HRT)of17days.Correspondingly,eachdigesterproduced160Ld−1of
digestedeffluentthatwascollectedinacentralsumppriortotransferbacktothefarm’s
manurelagoon.Theamountofthemanuresolidswaschosentokeepthedigesterorganic
loadingrate(OLR)withinthesuggestedrangeof1to3kgVSm−3d−1[10].Steadystatecondi-
tions,definedasstablemethaneproductionandmethanecontent,wasreachedinthedigesters
aftertheywereoperatedfortwohydraulicretentiontimes(HRT)(datanotshown).
Afterachievingsteadystateconditions,randomlychosenpairsofdigesterswereassigned
foroneofthreetreatments(noaeration,lowrateaeration(1to5%ofairtobiogas(v/v),or
highrateaeration(5to10%ofairtobiogas(v/v),andoperatedfor18weeks(7.4HRTs).For
digesterschosenforheadspaceaeration,airwaspulsed20timesperdayintotheheadspaceof
eachdigesterusinganelectronictimer(model50015,Woods/ColemanCable,Waukegan,IL)
PLOSONE|https://doi.org/10.1371/journal.pone.0185738 October4,2017 3/12
Effectofliquidsurfaceareaonmicro-aeration
andanaquariumpump(model9902,Petco,SanDiego,CA)thatdeliveredairtotwooutlets
thatfloatedonthesurfaceofthedigestate.
Pilot-scaledesulfurizationexperiments
Polypropylenebarrels(Uline,PleasantPrairie,WI)containingdigestatefromthefarmscale
BARCdigesterwereusedaspilot-scaledesulfurizationunitstoremoveH SfromBARC
2
digesterbiogas.Inthefirstsetofexperiments,sealed65Lbarrels(35cmdiameter,60cm
height)containing6Lofdigestate(59Lheadspace)wereusedtodeterminetherelationship
betweenliquidsurfaceareaandH Soxidationrates.Pairsofbarrelswereoperatedvertically
2
orhorizontallywithliquidsurfaceareasof0.099and0.150m2,respectively.Inthesecondset
ofexperiments,pairsofopentop104Lbarrels(43cmdiameter,71cmheight,104L)with
removablegastightlidsandcontaining30Lofdigestate(74Lheadspace)wereusedinexperi-
mentsinwhichweattemptedtoincreaseH Soxidationcapacitybyincreasingthewettedsur-
2
faceareaforbiofilmgrowth.Inonepairofbarrels,averticalpieceofcottonterryclothfabric
(29X29cm)wassuspendedverticallyintoandabovethedigestatesurfaceineachbarrel.The
fabricwassaturatedwithdigestateatthebeginningoftheexperiment.Anotherpairofbarrels
wasoperatedwithouttheadditionoffabric.Inbothexperiments,biogasfromtheBARC
digesterwascontinuouslyintroducedintotheheadspaceofeachbarrel(approximately5cm
abovethedigestatesurface)atratesrangingfrom0.1to3Lmin-1.Ambientairwascontinu-
ouslypumpedintotheheadspaceofthebarrelsusingsmallaquariumpumps(described
above)atratescorrespondingto5%(v/v)ofthebiogasflowrate(rangingfrom5to150ml
min-1).
Barrelswereinsulatedusing2"closedfoaminsulationboard(R-10)underneaththebarrels
and2"fiberglassinsulation(R-13)wrappedaroundthesides.Barrelsweremaintainedattem-
peraturesbetween20to25˚Cusing50Wheatingpadstapedtothesidesofthebarrels(model
756–500,Sunbeam,BocaRaton,FL).Flowratesofbiogasandairweredetermineddailyusing
amassflowmeter(modelGFM17,AalborgInstruments)androtameter(DwyerInstruments),
respectively.
Analyticalmethods
BiogasproducedbytheFSdigesterswasmonitoredusinglow-pressurecumulativegasmeters
(modelPGM.75,EKMMetering,SantaCruz,CA,USA).Althoughbiogasreadingswere
recordeddaily,biogasproductionvalueswerecalculatedusingweeklyaverages.Gascomposi-
tion(CH ,CO ,O ,H S)ofbiogasfromFSdigesters,biogastheBARCdigester,andtreated
4 2 2 2
biogasfromeachdesulfurizationunitweredetermineddailyusingaportablegasanalyzer
(Biogas5000,LandtecInstruments,Dexter,MI).Theinstrumentwascalibratedusinggasstan-
dardsprovidedbythemanufacturer.
InfluentandeffluentsamplesfromtheFSdigesterswerecollectedweeklyandanalyzedfor
totalsolids(TS),volatilesolids(VS),pH,andalkalinityaccordingtoStandardMethods[11].
Digestatesamplesfromdesulfurizationunitswerecollectedonlyatthebeginningandendof
theexperiments.
Statisticalmethods
Analysisofvariance(ANOVA)wasperformedtotestthesignificanceamongtreatmentsfor
maximumH Soxidationrate(gSd-1m-2).Post-hocpairwisecomparisonsoftreatmentswere
2
conductedusingTukey’sHonestSignificantDifferencetest(R3.1.2;95%confidencelevel)
[12].
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Effectofliquidsurfaceareaonmicro-aeration
Resultsanddiscussion
Useoffield-scaledigesterstodeterminetheeffectofaerationrateon
biogasH SconcentrationsandCH production
2 4
Sixfield-scaledigesterswereusedtodeterminetheeffectofaerationrateonhydrogensulfide
concentrationsandmethaneproductionunderfieldconditions(Fig1).Aftersteadystatecon-
ditionswereachieved,theFSdigesterswereoperatedat28˚Cusingsolidsseparateddairy
manure.TheHRTandOLRwere17daysand1.5±0.1kgVSm3d-1,respectively,duringthe
studyforalldigesters.Thetwonon-aerateddigestersproduced0.9to1.7m3biogasperday,
correspondingtobiogasretentiontimesof14to26hours.Themeanmethaneproductionrate
fromthenon-aerateddigesterswas0.45±0.02m3d-1m-3andspecificmethaneproduction
valuewas0.32±0.01m3CH d-1kg-1VS.Biogasproducedfromthenon-aerateddigesters
4
containedCH concentrationsof62%to68%,CO concentrationsof32%to37%,andH S
4 2 2
concentrationsof2900to4500ppmvoverthe18weekstudyperiod(notshown).
Resultsshowedthatincreasedlevelsofaerationofdigesterheadspaceresultedindecreased
levelsofH S.Twopairsofdigesterswerechosentoreceiveaerationatdifferentratesoveran
2
18-weekperiod.Anotherpairofdigestersdidnotreceiveaeration.Airwaspulsedintothe
headspace(20pulsesperday)asitwasdifficulttoaccuratelymaintainthelowairflowrates
neededforcontinuousaeration.Ataerationratescorrespondingto2%to10%ofairpervol-
umeofbiogas,residualoxygenlevelsinthebiogasrangedfrom0.1to3%.Althoughwepre-
dictedthatanyamountofexcessoxygenwouldindicatecompleteoxidationofH S,H Slevels
2 2
inthebiogaswereconsistentlylessthan500ppmvonlywhenresidualoxygenlevelswithinthe
biogaswere(cid:21)0.3%(v/v)(Fig2,panelA).H Slevelsinthebiogaswereconsistentlylessthan
2
100ppmvonlywhenresidualoxygenlevelswithinthebiogaswere(cid:21)0.5%(v/v).Theseresults
areconsistentwiththosefromotherstudiesusingsewagesludgedigesters.Hydrogensulfide
removalefficienciesof<99%havebeenreportedfromdigesterswithbiogasretention
times>5hr(recentlyreviewedby[2,3]).
AerationofdigesterheadspacedidnotsignificantlyaffectCH production.Asexpected,
4
CH andCO concentrationsinthebiogasdecreasedwithincreasinglevelsofaerationbecause
4 2
ofdilutionwithair(primarilynitrogen).However,meanmethaneproductionvaluesfromaer-
ateddigesters(0.43±0.01m3CH d-1)werenotsignificantlydifferent(p=0.277)thanvalues
4
fromdigestersoperatedwithoutaeration(0.45±0.02m3CH d-1).Therewasnoapparent
4
trendwithregardtotheeffectofaerationonCH productionuptolevelsyieldingbiogasoxy-
4
genconcentrationsupto1%(Fig2,panelB).Withrespecttotheeffectofbiogasoxygencon-
centrationsabove1%,theresultsarelessclearbecauseofrelativelyfew(andvariable)results
undertheseconditions.TherewerenodifferencesinthepHoralkalinityvaluesofeffluents
fromdigestersreceivingaerationcomparedthosenotreceivingaeration(resultsnotshown).
Wedidnotmeasuresulfurconcentrationsindigestereffluentsbecauseweexpectedthatmuch
oftheelementalsulfurprecipitatewouldberetainedwithsolidsintheseunmixeddigesters
[9].Therearefewresultsfromotherstudieswithwhichtocompareourresultsbecausemost
studieshaveusedsingledigestersoperatedunderdifferentconditionsthroughtimerather
thanreplicatedigestersoperatedsimultaneously.However,effectsofmicro-aerationonCH
4
productionhavebeenconsideredtobeminimalorslightlypositive[3,6].
Useofpilot-scaledesulfurizationunitstodeterminetheeffectofwetted
surfaceareaonH Soxidationratesduringmicro-aeration
2
AlthoughH SlevelsintheFSdigesters(withrelativelylongbiogasretentiontimesof14to26
2
hours)weresignificantlyreducedbyaeration,wesoughttodeterminetheeffectivenessof
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Effectofliquidsurfaceareaonmicro-aeration
Fig2.Hydrogensulfideconcentrationandmethaneproductionasafunctionofresidualoxygen
concentrationsinmicro-aeratedFSdigesters.A,InfluenceofaerationonH Sconcentrationsinbiogas.
2
Valuesaremeansof2to12dailymeasurementsfromduplicateaeratedfieldscaledigestersoperatedfor18
weeks.Duringthisperiod,H Sconcentrationsfromduplicatenon-aerateddigestersrangedfrom3000to
2
4000ppmvwithameanvalueof3570ppmv.Verticalbarsshowstandarderrorvalues(threeerrorbarsare
clippedattheaxislimit).B,EffectofaerationonCH production.Valuesarecalculatedfromweeklymeans
4
fromduplicateaeratedfielddigestersoperatedwithandwithoutaeration.Verticalbarsshowstandarderror
values.
https://doi.org/10.1371/journal.pone.0185738.g002
aerationunderarangeofbiogasflowratesandmuchlowerretentiontimes.Inaddition,since
themajorityofH Soxidationwithindigestersislikelyduetomicrobialbiofilmslocatedonthe
2
digestatesurface,wealsosoughttodeterminetheeffectofvaryingtheavailableliquidorwet-
tedsurfaceareaonsulfuroxidationratesduringheadspaceaeration.Fortheseexperiments,
wemademinormodificationstoa10LdesulfurizationunitdescribedbyRamosandcowork-
ers[7].Inthefirstsetofexperiments,wedeterminedH SoxidationratesasafunctionofH S
2 2
loadingrateusingpolypropylenebarrelswithtwodifferentliquidsurfaceareas.Pairsof65L
barrelscontainingdigestatewereoperatedvertically(0.099m2liquidsurfacearea)orhorizon-
tally(0.150m2liquidsurfacearea)andsubjectedtomicro-aerationunderdifferentflowrates
ofbiogasfromtheBARCdigester.
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Effectofliquidsurfaceareaonmicro-aeration
Resultsshowthatthehorizontaltanks(containing50%moreliquidsurfaceareacompared
totheverticaltanks)oxidizedH Satmaximumratesthatwereapproximately3-foldhigher
2
thantheverticalbarrels(Fig3,Table1).Whennormalizedforsurfacearea,maximumH S
2
oxidationratesfromhorizontalunitsareapproximately2-foldhigherthanthosefromthever-
ticalunits(Table1).Sincethebarrelswereotherwiseidentical,webelievethatthelikelyexpla-
nationforthedifferentratesisthatincreasedliquid/gassurfacearealeadstomoresurface
biofilmcontainingH SoxidizingbacteriaandhigherratesofH Soxidation.Therearefew
2 2
resultsfromotherstudieswithwhichtocompareourresultsbecausemoststudieshavenot
beendesignedtodeterminemaximumH Soxidationratesandfewprovideinformation
2
neededtodeterminedigesterliquidsurfacearea.ThedesulfurizationunitdescribedbyRamos
andcoworkersreducedtheeffluentbiogasH Scontentfromapproximately3000ppmvto
2
between100and200ppmv[7].Undertheirconditions,thecalculatedH Soxidationratewas
2
25gSd-1m-2.Thisvalueiswithintherangeofvaluesdeterminedinourstudy(Table1).
Nghiemandcoworkersconductedastudyinwhichmicro-aerationwascontrolledbyoxida-
tionreductionvaluesinthedigestate[13].Undertheirconditions,effluentbiogasH Sconcen-
2
trationsdecreasedfromapproximately6000ppmvto30ppmvandthecalculatedH S
2
oxidationratewas2.6gSd-1m-2(Table1).Thisvalueislowerthanvaluesdeterminedinthis
studybutislikelyanunderestimateofthemaximumratebecausetheirstudywasnotdesigned
forthispurpose.
Inthesecondsetofexperiments,wedeterminedtheeffectofincreasingthewettedareafor
biofilmgrowthontheH Soxidationrate.Intheseexperiments,wedeterminedH Soxidation
2 2
ratesusing104Lbarrelscontainingdigestate(Fig4,panelA).Averticalpieceofcottonterry-
clothfabric(29X29cm)wassuspendedverticallyintoandabovethedigestatesurfaceineach
ofapairofbarrels(Fig4,panelC).Thefabricwassaturatedwithdigestateatthebeginningof
theexperiment.Asecondpairofbarrelswasoperatedwithoutfabric(Fig4,panelB).Barrels
weresubjectedtodifferentflowratesofbiogasandaeratedat5%v/vofbiogasflowas
describedabovefor30days.
Resultsshowthattheunitscontainingfabric(withapproximately50%morewettedsurface
areacomparedtotheunitswithoutfabric)oxidizedH Satmaximumratesthatwereapproxi-
2
mately50%higherthanunitswithoutfabric(Fig5,Table1).Whennormalizedforwettedsur-
facearea,maximumH Soxidationratesfromunitswithandwithoutfabricwerenearly
2
identical(Table1).Useoftallerpiecesofsaturatedfabric(29cmwidex59cmhigh)insubse-
quentexperimentsdidnotresultinincreasedratesofH Soxidation(resultsnotshown).Itis
2
possiblethatadditionofwettedfabricabove29cmdidnotleadtoincreasedoxidationrates
becausefabricabovethisheightdidnotmaintainadequatemoisturelevels.Indeed,we
observedthatthefabricremainedvisiblywetonlyabout12cmabovethedigestatesurface.
Althoughthemaximumoxidationratesmeasuredintheseexperimentsareusefulindeter-
mininglimitingfactorsinthemicro-aerationprocess,digesteroperatorsarefocusedonthe
maximumH SloadingratesthatachievespecificH Stargetconcentrations.Ourresultssug-
2 2
gestthatbiogasH Sconcentrationswerereducedfromapproximately3500ppmvtolessthan
2
500ppmvatH Sloadingratescorrespondingtoapproximately25to40%ofthemaximum
2
oxidationrates(Table1).BiogasH Sconcentrationswerereducedtobelow100ppmvatH S
2 2
loadingratescorrespondingto8to18%ofthemaximumoxidationrates.
Althoughdigestatesfromdesulfurizationunitsturnedyellowandcontainedvisibleflocsof
sulfur-containingsolids,wedidnotcharacterizesulfurrecoveryintheseunits.Alkalinityand
pHvalueswerelowerindigestatesfromunitsreceivingaerationcomparedtothosenotreceiv-
ingaeration.However,wedidnotcollectsamplesneededfordeterminingarelationship
betweenpH,alkalinity,andoxidizedsulfur(sampleswerecollectedonlyatthebeginningand
endofexperiments).Ramosandcoworkerscollectedsulfur-containingsolidsintheirstudyof
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Effectofliquidsurfaceareaonmicro-aeration
Fig3.HydrogensulfideoxidationratesasafunctionofH Sloadingrateinverticalandhorizontal65
2
Lbarrels.Valuesaremeansofdailymeasurementsfromduplicateaeratedbarrels.Redcircles,horizontal
barrels.Bluetriangles,verticalbarrels.Verticalbarsshowstandarderrorvalues.Dottedlinesshowestimated
maximumremovalratescorrespondingtovaluesshowninTable1.
https://doi.org/10.1371/journal.pone.0185738.g003
Table1. ComparisonofH Soxidationratesusingpilot-scaledesulfurizationunits.
2
65Lvertical 65Lhorizontal 104Lbarrelno 104Lbarrelwith 10Ldesulfur- 50Ldigester
barrel barrel fabric fabric izationunit[7]1 [13]2
Headspacevolume(L) 59 59 74 74 9 20
Liquid/wettedsurfacearea(m2) 0.099 0.150 0.146 0.224 0.049 0.126
BiogasH S(ppmv) 3500 3500 3500 3500 3000 6000
2
BiogasH S(gm-3) 4.88 4.88 4.88 4.88 4.18 8.38
2
BiogasflowrateatmaximumH S 1.5 1.5 2.0 2.5 0.15 0.027
2
oxidationrate2(LPM)
Biogasretentiontime(min) 39 39 37 30 60 740
MaximumH Soxidationrate3(gSd-1) 4.3±0.3 12.4±1.3 15.3±0.5 22.4±0.3 n.d.4 n.d.4
2
MaximumH Soxidationrate3(gSd-1m-2) 43±3a 83±7b 104±5c 101±2bc n.d.4 n.d.4
2
MaximumH Sinputrateforeffluent n.d. 24(29%)5 27(26%) 44(43%) 25 n.d.4
2
content<500ppmvH S(gSd-1m-2)
2
MaximumH Sinputrateforeffluent n.d. 6.6(8%)4 9.3(9%) 18(18%) n.d. 2.6
2
content<100ppmvH S(gSd-1m-2)
2
1Experimentsuseda10L(25cmdiameter)plasticcontainerandachievedeffluentbiogasH Svaluesof100to200ppmv.MaximumH Soxidationrates
2 2
werenotdetermined.
2Experimentsuseda50L(40cmdiameter)digesterandachievedeffluentbiogasH Svaluesof30ppmv.MaximumH Soxidationrateswerenot
2 2
determined.
3Valuesaremeans±SEof6or7dailymeasurementsfromduplicatebarrels.Treatmentmeanswithdifferentlettersarestatisticallysignificantatthe0.05
significancelevel.
4n.d.:notdetermined.
5ValuesinparenthesesindicatetheoreticalH Sinputratewitheffluentcontent<500ppmvor<100ppmvH Sasapercentageofthemaximumoxidation
2 2
rate.
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Effectofliquidsurfaceareaonmicro-aeration
Fig4.Imagesof104Lmicro-aerationunits.A,Viewofinsulatedbarrels.B,Viewofsulfurdepositionon
digestatesurfaceinbarrelreceivingaeration.C,Viewofsaturatedfabric(29x29cm)inbarrelsreceiving
aeration.
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Effectofliquidsurfaceareaonmicro-aeration
Fig5.Hydrogensulfideoxidationratesasafunctionofloadingratein104Lbarrelswithandwithout
verticalstripsofcottonfabric.Bluetriangles,barrelswithoutfabric.Redcircles,barrelswithsaturated
cottonfabricinheadspaceabovedigestate.Valuesaremeansofdailymeasurementsfromduplicateaerated
barrels.Verticalbarsshowstandarderrorvalues.Dottedlinesshowestimatedmaximumremovalrates
correspondingtovaluesshowninTable1.
https://doi.org/10.1371/journal.pone.0185738.g005
adesulfurizationunitandestimatedthattherecoveredsolidscontained60%ofinputH S-S
2
(assumingthatallH S-SwasconvertedtoS0)[7].
2
OurresultssuggestthatmaximumH Soxidationratesduringheadspaceaerationarelim-
2
ited,inpart,bythewettedareaavailableforbiofilmgrowthand/oractivity.Verticalsaturated
fabricabovethedigestatesurfaceprovidesasuitablehabitatforthebiofilmactivity.Although
itmaybepossibletomaximizeoxidationefficiencyfurtherbyreducingfabricheightbelow29
cm,resultsfromexperimentsusingshorterlengthsoffabricwereinconclusive(notshown).It
islikelythatH Soxidationratescouldbeincreasedbyadditionofmultiplestripsofadurable
2
absorbentmaterialpositionedverticallyinthedigesterheadspace(orinanexternaldesulfuri-
zationunit).
Althoughtheefficiencyofmicro-aerationisinfluencedbyotherfactors(suchasH Scon-
2
centration,biogasretentiontime,digestatetemperature,andadequatemixingofoxygenatthe
digestatesurface),thesulfuroxidationvaluesbasedonliquidsurfaceareamaybeusefulfor
estimatingH Soxidizationcapacitiesofdigestersanddesulfurizationunits.Using10gSm-2
2
d-1fortheH Soxidationrateneededtoachievebiogaseffluentcontaining<100ppmvH S,
2 2
thefield-scalehorizontalplug-flowdigesters(withasurfaceareaofapproximately3.3m2)
couldbeoperatedtoproduceupto33gH Sd-1.Thisratecorrespondstoadailybiogaspro-
2
ductionvalueofabout6m3d-1(assumingacontentof3500ppmvH S(5gSm-3))thatis
2
3-foldhigherthanourtypicalproductionratesof1–2m3d-1[9].Thesulfuroxidationvalues
canalsobeusedtoestimatethesizeofanexternaldesulfurizationunitneededtotreatdigester
biogas.Forthefarm-scaleBARCdigester,theestimatedH S-Soutputisapproximately325g
2
Sd-1(assuming65m3d-1biogascontaining3500ppmvH S).Usingvaluesof25or10gSm-2
2
d-1,externaldesulfurizationunitsofapproximately13m2or33m2wouldberequiredto
achievebiogasH Sconcentrationsbelow500ppmvorbelow100ppmv,respectively.
2
PLOSONE|https://doi.org/10.1371/journal.pone.0185738 October4,2017 10/12