Delft University of Technology
Nitrous oxide emission from full-scale municipal aerobic granular sludge
van Dijk, Edward J.H.; van Loosdrecht, Mark C.M.; Pronk, Mario
DOI
10.1016/j.watres.2021.117159
Publication date
2021
Document Version
Final published version
Published in
Water Research
Citation (APA)
van Dijk, E. J. H., van Loosdrecht, M. C. M., & Pronk, M. (2021). Nitrous oxide emission from full-scale
municipal aerobic granular sludge. Water Research, 198, [117159].
https://doi.org/10.1016/j.watres.2021.117159
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Nitrous
oxide
emission
from
full-scale
municipal
aerobic
granular
sludge
Edward
J.H.
van
Dijk
a,b,∗,
Mark
C.M.
van
Loosdrecht
a,
Mario
Pronk
a,ba Department of Biotechnology, Delft University of Technology, Van der Maasweg 9, Delft 2629 HZ, the Netherlands b Royal HaskoningDHV, Laan1914 35, Amersfoort 3800 AL, the Netherlands
a
r
t
i
c
l
e
i
n
f
o
Article history:
Received 15 January 2021 Revised 9 April 2021 Accepted 13 April 2021 Available online 19 April 2021
Keywords:
Nereda
Aerobic granular sludge Nitrous oxide
Greenhouse gas emissions
a
b
s
t
r
a
c
t
Thenitrousoxidesemissionwasmeasuredover7monthsinthefull-scaleaerobicgranularsludgeplant inDinxperlo,theNetherlands.Nitrousoxideconcentrationsweremeasuredinthe bulkliquidand the off-gasoftheNereda® reactor.Combinedwiththebatchwiseoperationofthereactor,thisgaveahigh informationdensityandabetterinsightintoN2Oemissioningeneral.Theaverageemissionfactorwas 0.33%basedonthetotalnitrogenconcentrationintheinfluent.Theyearlyaverageemissionfactorwas estimatedtobebetween0.25%and0.30%.Theaverageemissionfactoriscomparabletocontinuous acti-vatedsludgeplants,usingflocculentsludge,anditislowcomparedtoothersequencingbatchsystems. Thevariabilityintheemissionfactorincreasedwhenthereactortemperaturewasbelow14°C,showing higheremissionfactorsduringthewinterperiod.Achangeintheprocesscontrolinthewinterperiod reducedthevariability,reducingtheemissionfactorstoalevelcomparabletothesummerperiod. Dif-ferentprocess control mightbenecessaryathigh and lowtemperaturesto obtainaconsistentlylow nitrousoxideemission.Rainyweatherconditionsloweredtheemissionfactor,alsointhe dryweather flowbatchesfollowingtherainyweatherbatches.Thiswasattributedtothefirstflushfromthesewerat thestartofrainyweatherconditions,resultinginatemporarilyincreasedsludgeloading.
© 2021TheAuthors.PublishedbyElsevierLtd. ThisisanopenaccessarticleundertheCCBYlicense(http://creativecommons.org/licenses/by/4.0/)
1. Introduction
Nitrousoxideisagreenhousegascontributingtoglobal warm-ing.Amoleculeofnitrousoxidehasa298timesgreatereffecton global warming than a molecule of carbon dioxide (IPCC,2007). Nitrousoxidecanbeproducedasaby-productofnitrificationand denitrification processes (Kampschreur et al., 2009). Although in general only a smallfraction ofthe influent ammonium is emit-ted asnitrousoxide,the largegreenhouse warmingpotential can make nitrous oxide emission the dominant factor in the carbon footprint of a wastewater treatment plant(Daelman etal., 2015;
Desloover etal., 2012). Emission ofnitrous oxide has been stud-ied for many wastewatertreatment process configurations under many process conditions, showinga wide range of emission fac-tors. These emission factors, defined as the amount of nitrous oxide emitted relative to the nitrogen load to the plant, gener-ally fall between 0% and5%, but highervalues are also reported (Vasilakietal.,2019).
∗Corresponding author.
E-mail address: e.j.h.vandijk@tudelft.nl (E.J.H. van Dijk).
Several pathways are shown to be of importance for ni-trous oxide production in the wastewater treatment process (Kampschreur et al., 2008; Wunderlin et al., 2012). In the nitri-fication process, the intermediate product hydroxylamine can be oxidizedtonitrousoxide(bothbiologicallyandchemically).Under oxygen-limitedconditions,nitrifierscandenitrifynitritetonitrous oxide,theso-callednitrifier-denitrification pathway.Underanoxic conditionsnitrousoxidecanbeproducedbyheterotrophic denitri-fiers by imbalanced enzyme activity, nitrite accumulationor lack ofbiodegradable COD (Wunderlin etal., 2012). Atthe same time denitrificationcanbea sinkfornitrousoxide,whenthereducing capacityof nitrous oxide exceeds the production capacityduring denitrification (Conthe et al., 2019). Fluctuating influent concen-trationsandseasonalvariationsinfullscaleplantscombinedwith thevarietyofpathwaysleadingtonitrousoxideformationmakeit verycomplextofindtheunderlyingprocessesthatleadtoelevated nitrousoxideemissions(Daelmanetal.,2015,2013;Vasilakietal., 2019).
Research on nitrous oxide emissions has mainly focused on wastewater treatment processes with flocculent sludge and few laboratory studies have been performed with aerobic granular sludge(Jahnetal.,2019).Thisismainlybecausetheaerobic
gran-https://doi.org/10.1016/j.watres.2021.117159
ularsludge(AGS)processisarelativelynewwastewatertreatment process. The technology has been rapidly adopted over the last decade andcurrentlyalmost 70full-scale aerobic granularsludge installationsareoperationalorunderconstruction.Thesefull-scale aerobic granularsludgeinstallations areall Nereda® installations, which is a registered trademark of Royal HaskoningDHV.Forthe Nereda® process, only short-term (2 weeks) measurements were reported in a pilot reactor andthe full-scale reactor in Epe, the Netherlands. Bothshowedan averagenitrous oxide emission fac-torofabout0.7%ofthetotalnitrogenintheinfluent(Roestetal., 2012),whichiscomparabletoconventionalactivatedsludgeplants. However,otherstudieshaveshownthattogetreliabledataon ni-trous oxideemissions long-termmeasurement campaignsare re-quired (Daelman etal., 2015; Gruber etal., 2020; Vasilaki etal., 2019).
There are several laboratory studies reporting on nitrous ox-ide formation in AGS reactors. These studies reported a wide rangeofemissionfactorsfrom1%(Lochmatteretal.,2014)to22% (Zhangetal.,2015).Duetothestrongdeviationfromconditionsin full-scale reactors in theseexperiments,it isuncertain how rele-vantthereportedvaluesareregardingfull-scaleinstallations. Lab-oratory systems are very good at isolating a specific parameter, but translationtowards full-scale WWTP’sis challengingbecause ofdifferencesininfluentcomposition,process controlandreactor designandoperation.
Becauseofthepotentiallysignificantcontributiontothecarbon footprintoffull-scaleAGSprocesses,itisimportanttoquantifythe nitrous oxide emission factors.Hereto the nitrousoxide emission fromafullscaleNereda® planttreating domesticwastewaterwas monitoredfor7consecutivemonths.Twodifferentmethodswere usedtomeasurethenitrousoxideemissions,namelybymeasuring the nitrous oxideconcentrationscontinuously in thewaterphase andbymeasuringitintheoff-gasduringaeration.Theformerhas theadvantageofshowingnitrousoxidekineticsunderanoxic con-ditions, when the aeration is turned off.The latterhas the ben-efit of measuring the direct nitrous oxide emission without the need foraconversionalgorithm.Combined withthe dynamic be-haviouroftherepeatedbatch-wiseoperatedsystem,ahigh infor-mation densitycould beobtainedon thenitrous oxidebehaviour from the plant. The goalwas toget better insight inthe nitrous oxideemissionsofthefull-scaleAGSprocess,aswellasto under-standthemajorfactorspreventingandleadingtoelevatednitrous oxideemissionsinfullscaleAGSsystems.
2. Methodology 2.1. Plantdescription
All themeasurements took place atthe Dinxperlowastewater treatment plant which is located in the municipality of Aalten, the Netherlands(Fig. 1,geohash:u1hwgpzr). Thetreatment plant is operated bythe waterauthority Waterschap Rijn enIJssel and the plantwastakeninto operation in2013. The influent consists mainly ofdomesticwastewater(seeTable 1)andtheplantis de-signed for11,000p.e.,treatingonaverage3,100m3day−1,witha
peakflowof570m3 h−1.Itconsistsof3separatereactorsofeach
1,250 m3.Current effluentrequirementsare: CODof125 mgL−1,
totalnitrogenof15mgL−1,totalphosphorusof2mgL−1,and to-tal suspendedsolidsof 30mgL−1,allyearly averaged values.On topofthis,theeffluentrequirementforphosphorusis1mgL−1 in thesummerand3mgL−1 inthewinter.Theinfluentiscollected inaninfluentbufferandthentreatedinoneofthethreeNereda® reactors.Theeffluentispolishedbymeansofasandfilterwiththe possibilityofirondosingtoremoveremainingphosphorus.
Table 1
Average influent and effluent composition dur- ing the nitrous oxide measurement campaign at WWTP of Dinxperlo, the Netherlands (period Au- gust 2017 - March 2018).
Parameter Unit Influent Effluent COD mg L −1 531 28 BOD mg L −1 202 2.0 N tot -N mg L −1 54 6.0 NO 2 -N mg L −1 0.05 NO 3 333 -N mg L −1 3.3 P tot mg L −1 6.4 1.1 PO 4 -P mg L −1 0.9 SS mg L −1 198 5.0
2.2. Nitrousoxidemeasurements
Thenitrous oxideemission fromthereactorwasmeasured by determiningthe nitrous oxideconcentration in theoff-gas ofthe reactorduringaeration (Fig.2).Apolyethylenefloating hoodwith across-sectional area of0.55m2 wasused tocapturetheair
es-caping thesurfacearea ofthereactor duringaeration. The inside of the hood was partially filled with polyurethane foam to re-duce theheadspaceandlimitthe gasretentiontime tothe anal-ysers.Partoftheoff-gasthatpassedthroughthehoodwas trans-ported via a transparent hose and cooled to 4°C to remove the moisture.Thegasconcentrationsweremeasuredintwoonlinegas analysers(RosemountNGA2000MLTforoxygenandcarbon diox-ide;Servomex4900formethaneandnitrousoxide).Calibrationof theanalyserswasperformedusinggascylinderscontainingknown concentrationsofthestudied gases.Foraccurate calculations, the temperature,pressureandrelativehumidityoftheoutsideairwere alsomeasured,usingamicrosensor(BoschBME280).Thenitrous oxide concentrations were converted into mass fluxes using the methoddescribedbyBaeten(Baeten,2020;Baetenetal.,2021).
Additionaltothenitrousoxideconcentrationintheoff-gas,the nitrousoxideconcentrationinthebulkliquidwasmeasuredusing anitrousoxidesensorfromUnisenseEnvironment.Thissensorwas placedonemeterbelowthewatersurfaceofthereactor.
2.3. Sizedistribution
Thegranule sizedistribution oftheaerobic granularsludgein thereactorswasmeasuredbypouringasampleofthesludgeover a seriesofsieves withdifferentmesh sizes(212,425,630,1000, 1400 and2000 μm). A mixedsampleof 100 mL wasfilteredfor the determinationof thetotal dryweight. The obtainedgranular biomassofthedifferentsievefractionsandthemixedsamplewere driedat105°Cuntilnochangeinweightwasdetectedanymore.
2.4. Onlinemeasurements
The reactor was equipped with probes for dissolved oxygen andtemperature(Hach;LDO), redoxpotential (Hach;Redox), wa-ter level(Endress+Hauser,radar), suspended solids(Hach,Solitax TS),andnitrate(Hach;Nitratax).Ammoniumandphosphatewere continuously measured using a filterunit andauto sampling de-vice(Hach;FILTRAX, AMTAXandPHOSPHAX). The filterunit was situated0.5meterbelowthewatersurface.Samplingwasdoneat anintervalof5minutes.
2.5. Offlinesampling
Samples for analyses of influent and effluent were col-lected usingrefrigerated auto samplers,collecting 24-hours flow-proportional samplesforbothinfluent andeffluent.Thechemical
Fig. 1. Photograph of WWTP in Dinxperlo, the Netherlands. The Nereda® reactors are located on the right, attached to the building with the sloped roof. The sludge buffer and the sand filter are located to the right of the Nereda® reactors. The inlet works, including the influent buffer are located at the bottom. The old pre-existing aeration tank and clarifier on the left are now part of a public water garden (on the top).
Fig. 2. Schematic representation of the off-gas measurement set-up. Both reactor off-gas and outside air were cooled to 4 °C to remove the moisture before it passed through the analysers.
analysesofCOD,TN,NH4+-N,PO43−-P,NO3−-N,NO2−-Ninthe
re-actorwereperformedbyusingtheappropriateHachLangecuvette kits.
2.6. Emissionfactors
Offline samples were taken every 14 days. To calculate the emissionfactoratotalnitrogenconcentrationintheinfluent(CTN,i)
per batch wasneeded. Therefore,thetotalnitrogenconcentration intheinfluent wascalculatedforeverybatchusingthepeak am-monium concentration duringaeration measured by theanalyser (CNH4,max),theremainingeffluentammoniumconcentrationofthe
previous batch measured by the analyser (CNH4,e), and the
ex-changeratio(ER).
CTN,i=CNH4,max− CERnh4,e
(
1− ER)
fadsforg (1)Herethefads isthe factorcompensatingforadsorption of
am-moniumto the granules(Bassinetal., 2011) andforg is theratio
betweentotalnitrogen andammoniumin theinfluent. The com-binedeffectofthesetwofactorswasfoundbycorrelatingthe es-timated CTN withthe actual valuesfound by the 14 days offline
sampling.Anaveragevalueoffads xforgof1.79wasfoundwithan
r-squaredof0.75.
The emission factor wascalculated by dividing the total out-going loadof nitrous oxide withthe total incomingloadof total nitrogen,accordingtothefollowingequation:
EF= MN2O CTN,iVbatch
(2)
HereMN2O isthetotalmassofthenitrousoxideintheoff-gas
Time (min) 15 30 45 60 75 90 105 120 135 150 165 180 195 210 225 240 255 270 285 300 315 330 345 360 Dry weather Fill/draw Reaction/aeration Settling Wet weather Fill/draw Reaction/aeration Settling
Fig. 3. Batch scheduling for the Nereda® reactor in Dinxperlo.
2.7. Simultaneousnitrification/denitrification
During the aerated phase part of the nitrified nitrogen is di-rectly convertedto nitrogengasbecause oftheanoxicconditions in the granule. The average efficiency of simultaneous nitrifica-tion/denitrification(SND)duringaerationisexpressedas:
SND=1− CNO3,e− CNO3,min fadsforgCNH4,max− CNH4,e− CN,org
(3)
HereCNO3,ministheminimumnitrateconcentrationatthestart oftheaerationphase,CNO3,eisthenitrateconcentrationattheend
of theaeration phaseandCN,org istheestimatedvalue ofthe
or-ganicnitrogenintheeffluent.Forthelatteravalueof1.5mgL−1 isassumed.
2.8. Processcontrol
Theaerationwascontrolledusinganovelprocesscontrol devel-opedforaerobicgranularsludge(vanDijketal.,2018).Thiscontrol strategy targetsa nitrate productionrateto maximize simultane-ousnitrification/denitrification.Asaresult,dissolvedoxygenlevels inthereactorareminimizedasistheenergyconsumption.
During dryweather conditionsthe reactors havea fixed cycle time of 6 hours. When the flow increases due to rainy weather, the cycle willadapt to treat the increasedamount of water.The feedingtimewillincreasefrom60minutesto75minutesandthe feed flow fromthe buffer increasesfromabout180 m3 h−1 toa
maximumof600m3h−1.Asaresult,thecycletimewilldecrease
toaminimumof4hours(seeFig.3).
3. Results
3.1. Plantperformanceandoperation
The measurements were executed at theNereda® installation of Dinxperlo, the Netherlands. When the trial started the plant wasalreadyinoperationforfouryearsandthereactorscontained an aerobic granularsludgebedwitha MLSS concentration of8.0 g L−1. The three reactors showed normal operation during the wholetrialperiod.ThemeasurementsweredoneinReactor#1.In
Table 1theaverage influentandeffluentquality duringthe mea-surementcampaignisshown.
In Fig. 4 a typical batch from Dinxperlo is shown. The figure shows online measurements of the concentration of ammonium, nitrate,phosphateanddissolvedoxygenduringthreecycles.Since thesesensorswerepositionedatthetopofthereactor,andthe re-actorwasplug-flowfedfromthebottom,themeasurements dur-ingfeedingrepresenttheeffluentconcentrations.Thecyclestarted withafeedphase,whereinfluent wasaddedtothebottomof re-actorandeffluentwasdecantedfromthetopsimultaneously.After feeding,thereactionphasestarted,wherethereactorwasaerated. The reactor wasmixed by the aeration. At the start ofthe aera-tionphasetheconcentrationsofammoniumandphosphateappear
toincrease, whichwascausedby themixingofthebottom layer with influent water and top layer with the effluent water. After thereactionphasethebiomasswasallowedtosettleandthecycle restartedforthenextbatch.
3.2. Monthlyaveragenitrousoxideemission
Thenitrousoxideemissionthroughoff-gasfromReactor1was measuredfromthe9thAugust2017to18thMarch2018(thewater
phasesensor wasavailable fromthe4th ofOctober2017). Inthis
periodtheaveragenitrousoxideemissionfactorwas0.33%.
Fig.5showsthemonthlyaveragenitrousoxideemissionsover the whole measuring period. There was a distinct difference be-tween the summer andautumn period, compared to the winter period.In December the nitrous oxide emission factorstarted to risefromanaverageof0.22%inthefirst4monthstoamaximum of 0.64% in February. In March, the emissions dropped again to thepre-Decemberlevels. Theaveragewatertemperaturedeclined steadilyoverthesameperiod,from20.6°CinAugustdownto9.7 °CinMarch2018.
3.3. Batchaveragenitrousoxideemission
Fig.6showstheemissionfactorsaswellasthewater tempera-tureperbatch.ThegraphsshowthatintheperiodbetweenAugust 2017andmidDecember2017theemissionfactorformostbatches wasbetween0%and0.5%,withalimitednumberofbatchesrising above 0.5%. Inthis periodthe emission factoraveraged to 0.22%. StartingfromDecember2017thevariabilityoftheemissionfactors increased.Therewerestillmanybatchespresentwithanemission factorofalmostzero,butthemaximumvaluesincreasedupto2%. In the period between December 2017 andthe end of February 2018theemissionfactoraveraged to0.42%.Starting fromthelast weekofFebruary2018thisvariabilitywasagaincomparabletothe periodbeforeDecember2017.
3.4. Effectoforganicloadingrate/N-load
The variability ofthe nitrous oxideemission factor(and with thatthetotalemissionfactor)seemstobeinfluencedbythe max-imum ammonium concentration inthe batch (Fig. 7). The maxi-mum emissionfactor(upto 2%)wasreachedatammonium con-centrationsbetween5and10mgL−1,butinthisrangemany val-ues closeto zero were also measured. At ammonium concentra-tions below 5 mg L−1 and above 10 mgL−1, only few emission factorsabove1%weremeasuredwithmostvaluesbetween0%and 0.5%.
3.5. Effectofrainevents
Fig.8showstheeffectoftheflowtotheWWTPonthenitrous oxideemission factor. Duringdryweather flow conditions(DWF) the influent flow ranged between 0 and 150 m3/h averaging at
Fig. 4. Concentration profiles of ammonium (green), nitrate (orange), phosphate (black) and oxygen (blue dashed) for three typical consecutive batches. The grey diagonal striped area indicates the feeding phase, the grey diamond grid shows the reaction phase and the grey solid area shows the settling phase.
Fig. 5. Monthly averaged nitrous oxide emission factor and temperature profile.
Fig. 6. Emission factor (blue dots) and average temperature (black dashed line) per batch.
Fig. 7. The nitrous oxide emission factor related to the ammonium peak calculated per batch.
105 m3/h. Rain weather flow (RWF)conditions are characterized
withaninfluent flowupto600m3/h(RWF/DWFratioof6).
Dur-ing dryweather flowtheemissionfactor(0.40%)washigherthan duringrainweatherflow (0.13%).Althoughthetotalloadof nitro-gentransportedtotheWWTPwillnotdiffermuchbetweenDWF andRWFconditions,thecycletimewasshortenedduringRWFto handletheincreasedinfluentflow.ThecycletimeforDWFbatches
wastypically 6hours, whilethe cycletime forRWFbatcheswas shortenedtoaminimumof4hours.
3.6. Effectoftemperature
Temperature had some effect on the variability of the emis-sionfactorasshowninFig. 9.Temperaturesabove 14°Cresulted
Fig. 8. Difference in emission factor of nitrous oxide between dry and rain weather flow conditions.
Fig. 9. Effect of temperature on the nitrous oxide emission factor.
Fig. 10. Diurnal pattern of nitrous oxide emission. Average emission (blue) based on start time of the batch (standard deviation in black).
formostbatchesinan emissionfactorbetween0and0.5%while attemperaturesbelow14°Ctheemissionfactorvaried between0 and2%.Itisuncertainwhetherthiswassolelyrelatedto tempera-ture,becauseinMarchthetemperaturewaslow(averageat10°C) whiletheemissionfactordidnotshowthisincreasedvariability.
3.7. Diurnalpattern
The emission factor did not show a clear diurnal pattern. In
Fig. 10theaverageemission factorperbatch isshownasa func-tion of the time of the day. The samevariability was presentas inthepreviousgraphs,leadingtoarelativelyhighstandard devia-tion.Thevaluesofthebatchesstartingbetween3:00hand16:00h showed a lower emission than the batches between 17:00h and 02:00h. A problemin thisanalysis wasthe batch-wiseoperation ofthereactor.Theanalysiswasdonebasedonthestartingtimeof
thebatch, whichwasseveralhours beforethe actualemission of nitrousoxidewasmeasured.Duetothe6hourscycletimeduring DWFout of1043 batches analysed in thisstudy, only19 started between3 AM and 4 AM,while 71 batches started 6 PM and7 PM.
3.8. Dynamicnitrousoxidebehaviour
Thecomplexityanddynamicsofnitrous oxide emissionsis il-lustratedinFig.11.Netproductionofnitrousoxideaswell asnet consumption ofnitrous oxidewas visiblein theonline measure-ments.The firstbatch started[marker1]without anynitrous ox-ideinthebulk liquidandthereforealsonoemission throughthe off-gasatthestartoftheaerationphase.Atthestartofthe aera-tion,thedissolvedoxygenconcentrationincreasedandafirstpeak of nitrous oxide emission could be observed [marker 2]. Also, a
Fig. 11. Batch with high concentrations of nitrous oxide (start at marker [marker 1]): initial peak due to denitrification of residual NO 3 −[marker 2]; after a decline of nitrous oxide in the bulk liquid production of nitrous oxide [marker 3]; after a drop in the O 2 concentration an increase of the nitrous oxide concentration in the bulk liquid [marker 4]; no denitrification of nitrous oxide at the end of the cycle at the top of the reactor because sludge has settled [marker 5]; second reaction phase starts [marker 6]; drop of nitrous oxide in the bulk liquid by pre-denitrification and mixing [marker 7]; drop in the oxygen concentration [marker 8] led to increase in nitrous oxide production [marker 9]; and no nitrous oxide left at the end of the next cycle [marker 10]. The grey diagonal striped area indicates the feeding phase, the grey diamond grid shows the reaction phase and the grey solid area shows the settling phase.
simultaneous decreaseof thenitrate concentrationwasobserved, causedbymixingofthenitrateremainingfromthepreviousbatch with newly fed influent, low innitrate.When the dissolved oxy-gen concentration increased further,the nitrous oxide concentra-tion inthebulkliquidandoff-gasdecreased again(marker [2]to [3]). In this period, the nitrous oxide production ratewas lower thanthecombinedeffectofstrippingthroughtheoff-gasand den-itrification ofnitrous oxide.Furtheron intheaeration phase,the nitrous oxide concentration in both the bulk liquid andthe off-gas increased[marker 3]. Towards theend ofthe aeration phase thedissolvedoxygen concentrationwaslowered[marker4]anda suddenincreaseofthenitrousoxideconcentrationinthebulk liq-uidwasseen.Hereaftertheaeration,andthusmixing,wasstopped allowing the biomass tosettle [marker 5].The reactorwas ready to receive thenext influent batch [marker 5].During thefeeding (marker[5]to[6])thenitrousoxideconcentrationinthebulk liq-uidstayed constantbecausethesensorwassituatedatthetop of the reactor andthe sludgebed hadsettledto the bottom ofthe reactor. Therewasno sludgepresentinthetoplayerandno bio-logicalprocesses occuredinthe toppartofthereactor.After this feeding phase, the aeration phase started again, and the nitrous oxideconcentration droppedduetomixingbeforetheproduction startedagain[marker7].Halfwaythroughthereactionphase,the oxygen concentration inthe bulk liquid waslowered[marker 8]. This ledtoa periodwherethe productionofnitratewaslimited, but ammoniumwas still beingconverted,thus optimizingfor si-multaneous nitrification anddenitrification. After loweringofthe oxygen concentration,asimilar, althoughlower, initialincrease of the nitrous oxide could be seen [marker9] aswas visibleinthe previous batch[marker2].Attheendofthissecondbatchthe ni-trousoxidewasalmostcompletelyremoved[10].
The processcontrol usedhereautomaticallybalances nitrifica-tion anddenitrificationtooptimizesimultaneousnitrification and denitrificationtogetamaximumtotalnitrogenremoval.Thiswas done by dynamically altering thedissolved oxygen set-point and sometimes thisresultedina dropoftheoxygen concentration in the reactionphase aspreviously described. InFig.12 an example of thisbehaviour isshown. This seems totrigger anitrous oxide productionresponse.Whentheoxygen concentrationdropped [1] thenitrousoxideproductioninthebulkliquidandtheoff-gas in-creased[2].
On the 24th of February the process control was changed to
a fixed oxygen set-point (2.5 mgL−1) duringthe reactionphase, with the reaction phase being split-up in an aeration phase and anunaeratedpost-denitrificationphase.Thishadanimmediate ef-fectonthenitrousoxideproduction.Anexampleofabatchunder thisnew processcontrol isshown inFig.13. Duringthereaction
phase,almostnonitrousoxidewasproduced.Onlywhenthe aer-ation wasstopped inthe post-denitrificationphase, somenitrous oxidewasproduced,butthisdidnotleadtoanyemission,because theaeration was switchedoff and thenitrous oxide was denitri-fiedbeforethe endofthecycle. Thischangeledto adecreaseof the emission factorto 0.15% duringthe three weeksthis process controlwasused.
The nitrogen removal also changed slightly by the change in theprocesscontrol.Theaverageammoniumeffluentconcentration was10%higher(3.2mgL−1afterthechange,comparedto2.9mg L−1 beforethechange).The averagenitrateeffluentconcentration wassimilarunderbothprocesscontrols(5.3mgL−1).TheSND ef-ficiencyover thewholeperiodwas69%(+/-15%).Itwas58%(+ /-12%)afterthechangecomparedto70%(+/-15%)beforethechange. A typical RWFevent is shown in Fig. 14. When the flow to-wardstheWWTPincreasedbecauseofrainyweather,afirstflush arrivedattheWWTP,increasing theammonium loadinthe reac-tor.The loadinthisbatch wastoohighforthe aerationcapacity andthereducedaerationduration,leavingsomeelevatedlevelsof ammoniumintheeffluent.Theprocesscontrolfocussedmainlyon nitrification, aerating the system atmaximum capacity. Little ni-trous oxideis formedin thisbatch.In thetwo batcheshereafter, theloadreturnedtonormallevels, butstill thefocuswasmainly onnitrification. Inthese batches,littlenitrous oxide wasformed. WhentheRWFeventwasfinishedandtheflowtotheWWTP re-turned to normal,the cycletimes lengthened again, but still the emissionofnitrousoxidesremainedclosetozero.
4. Discussion
4.1. Longtermnitrousoxideemissions
This study is the first long-term campaign measuring nitrous oxide emissions of a full-scale AGS reactor treating sewage. A Nereda® reactor atthe wastewatertreatment plantofDinxperlo, theNetherlands wasmonitoredfor7consecutivemonths. Inthis period an average emission factor of 0.33% was measured. This means that 0.33%of the incoming nitrogen loadwas emitted as nitrous oxide with the off-gas. The daily averaged emission fac-tor ranged from 0.02% to 1.58%. These values are comparable to the values found in previous (short term) Nereda® research (Roestetal.,2012).Theaveragevaluefoundintheshort-term re-search (0.7%)arehigherthan theaveragevalue found inthe cur-rentresearch, butthe value of0.7% iswell within thevariability ofthislongtermstudy,stressingtheimportance oflong-term re-search. Since theemission was measured foronly 7months, the higherwintervaluescontributeproportionatelystronginthe
aver-Fig. 12. Increase of the nitrous oxide production [marker 2] when the oxygen concentration dropped to a value below 1 mg L −1 [marker 1]. The grey diagonal striped area indicates the feeding phase, the grey diamond grid shows the reaction phase and the grey solid area shows the settling phase.
Fig. 13. Dynamics within a Nereda® cycle; different process control. The nitrous oxide appeared only in the water phase when the aeration was turned off [marker 1], leading to a very low emission factor. The grey diagonal striped area indicates the feeding phase, the grey diamond grid shows the reaction phase and the grey solid area shows the settling phase.
Fig. 14. Typical RWF event showing a first flush [marker 1], showing little emission in the first batch [marker 2] and almost zero emission in the consecutive rainy weather batches [marker 2 to 3]; after the RWF event the emission factor stayed almost zero for a few batches [marker 4 to 6]. The grey diagonal striped area indicates the feeding phase, the grey diamond grid shows the reaction phase and the grey solid area shows the settling phase.
age value presented.A12 monthyearly averaged emission factor isestimatedtobeintherangeof0.25%to0.30%.
Compared toconventional activated sludge(CAS) systems,the values obtainedfall well withinthe reportedranges inliterature. For example, in a study investigating seven CAS plants in Aus-tralia, theemission factorranged between0.6% and25.3%,based on the kilograms of nitrogen denitrified (Foley et al., 2010). An-other studyperforming shorttermmeasurements in 12plants in the United States showedan emission factorranging from 0.01% to1.5%(Ahnetal.,2010).Morerecentlyalong-termmeasurement campaign in Switzerland showed an emission factor for the CAS systemof1.6%– 2.0%whileaflocculentsludgeSBRsystemshowed an emission factor of 2.4% (Gruber etal., 2020). An overview of the emission factors for different wastewater treatment systems, adapted from (Vasilaki etal., 2019) is shownFig. 15.This under-lines that thevalues found inthisstudyare comparableto most other wastewater treatment systems but are considerably lower than the valuesgenerally reported forsequencing batch reactors
(SBR) systems.This is remarkable, since the AGS system used in thisstudyisoperatedasanSBR.Thisshowshighemissionfactors arenotintrinsictoSBRsystemsandthatthecorrectprocess con-ditionscanalsoleadtolowemissionfactors.
4.2. Seasonalanddiurnalvariations
ForCASsystemsastrongseasonaleffecthasbeenreportedfor the emission factor (Daelman et al., 2015; Gruber et al., 2020), showing higher emissions at lower temperatures or increasing temperatures in early spring. In this study, a seasonal effect is also visible, as can be seen in Figs. 5 and 9. The variability of the nitrous oxide emission increases when the water tempera-turedropsbelow14°CinDecember.Attemperaturesabove14°C the emissionfactor per batch variesbetween0% and0.5%, while below 14 °C the emission factor ranges between 0% and 2.5%. Contradictorily,March shows thelowest monthly emission factor (0.15%), at the lowest average temperature of 9.7 °C. This differ-ence may be caused by a change in process control in March.
Fig. 15. Emission factor of nitrous oxide for different wastewater treatment systems, adapted from ( Vasilaki et al., 2019 ). Process groups: AGS: Aerobic Granular Sludge, A/O: Anoxic/oxic reactor, A 2 /O: anaerobic-anoxic-oxic reactor, CAS: conventional activated sludge, MLE: Modified Ludzack-Ettinger reactor, OD: oxidation ditch, SBR: sequencing batch reactor, PN and PN/A: partial-nitritation and partial-nitritation-anammox. process.
Daelmanetal.(2015)suggestedthisseasonaleffectwasprimarily caused by an increase of NO2− concentrations inearly spring.In
thecurrentstudyeffluentnitrite concentrationswereconsistently low,withan averagevalueof0.05mgL−1 (Table1).Although ni-triteconcentrationswerenot measuredduringthecycle,elevated levelsofnitriteduringthecyclewouldalso,atleastpartially,have endedupintheeffluent.Sincethiswasnotthecasehere,no ma-joreffectofnitriteonthenitrousoxideemissionsisexpected.
The drop in the temperaturein December was relatedto the inflow of melting snow,andin 4 days the watertemperature in thereactordroppedfrom13°Cto8°C.Attheselowtemperatures of 8 °C the nitrification rates dropped considerably, butthe am-monium effluentconcentrationwasstill belowtheconsentvalue. Atthesametime,thenitrousoxideconcentrationsinbothoff-gas andbulkliquidwerealmost zeroorbelowthedetectionlimitfor severaldays,untilthetemperatureincreasedagainabove 9°C.In the week before thisevent the variability ofthe emission factor was alreadyincreasing, butafter thisevent thevariability ofthe emission factorofnitrous oxide furtherincreasedafterthe water temperaturewasrecoveredtotemperaturesabove10°C.
Adiurnalpatternisobservedinthedata(Fig.10).Thebatches startingbetween4h and14h show alower averageemission fac-torthan thebatchesstartingbetween15h and3h.The relationis not asclear asfor CAS systems (Daelman et al., 2015), which is mainly causedbythefactthat theAGSreactorisa batchsystem, runningabout4batchesperday,whichdoesnotgivea high res-olutionoverthedayasinCAS systems.Thelowestaverage emis-sionisfoundbetween3hand15h.Itisuncertainwhatcausesthis, butvariationsinbatchloadingmayplayarole.Sincethereactors are operatedwithafixedcycletime of6hoursduringDWF con-ditions,thevolumetricexchangeratioislowerifthetotalflowto thewastewatertreatmentplantislower.
4.3. Nitrousoxideinthecycle
Nitrous oxide can be produced by both nitrification and by denitrification(Kampschreur etal.,2008;Wunderlin etal., 2012). Ammonia-oxidising bacteria can produce nitrous oxide from ox-idation of hydroxylamine and from denitrification of nitrite un-der oxygen deprived circumstances. Denitrificationcan be both a source anda sink fornitrous oxide (Conthe etal., 2019). Nitrous oxideisalsoanintermediateintheheterotrophicdenitrificationof nitrate.At thesametime,nitrous oxidecanbe removedby deni-trification(Contheetal.,2019).Inaerobicgranularsludge nitrifica-tionanddenitrificationhappensimultaneouslyduringtheaeration phase,whichmakeitdifficulttodistinguishnitrousoxide produc-tionfromnitrificationanddenitrificationduringtheaerationphase (DeKreuketal.,2005).Nevertheless,thereseemstobe clear
evi-dencethatboth processescontributeto theproductionofnitrous oxides.Inmostbatchesthereisnodissolvednitrousoxidepresent atthe startof thecycle. Inthesecycles nitrous oxideproduction coincides with the conversion ofammonium andthe production ofnitrate(Fig.11at[marker3]).Often,apeakofnitrousoxideat thestartoftheaerationphaseisobserved(Fig.11at[marker2]). Thispeakseemstobecausedbydenitrificationofnitrateleftover fromthe previous cycle,because nitrification hasnot startedyet. Itisnotclearifnitrousoxideisproducedbypartialdenitrification causedbylackofCODattheendofthepreviouscycleorbyrapid denitrificationonreadilybiodegradableCOD fromthe fresh influ-ent.Inboth casestheincrease ofthenitrous oxideconcentration isprimarilycausedbymixingofthereactor(aeration)andthe de-creaseseems to be primarily causedby denitrification ofnitrous oxides on readily biodegradable COD, although in the lattercase strippingofnitrousoxidealsoplaysarole.
It is likely that denitrificationacts more asa sink for nitrous oxideatthestartofthecycleafterfeedingandthatdenitrification acts more asa source fornitrous oxide at the end of the cycle, whenmostCODfromthefeedingphase(bothstoragepolymersin thesludge andCOD inthe bulk liquid)is consumed.Thatwould implicatethenitrousoxideproductionobservedwhennitrification startsasa net productionrateresulting fromproduction by am-monia oxidizing bacteria and denitrification by heterotrophic or-ganisms. Understandingthe mechanisms andwhen nitrous oxide production exceeds consumption could be important forthe de-velopmentofnitrousoxideemissioncontrolstrategies.
Thedissolvedoxygenconcentrationisconsideredanimportant parameter to control nitrous oxide emissions, andconcentrations below1mgL−1duringnitrificationwouldstimulatenitrifier den-itrification due to oxygen limitation (Kampschreur et al., 2009). Oxygen limitation inbiofilms is a well-known factor even under higheroxygenconcentrations(Ødegaardetal.,1994).Itistherefore notsurprisingthat loweroxygenconcentrationsseemtoresultin highernitrous oxide emissions.In Fig.11andFig. 12 an increase ofthenitrousoxideemission canbeseen ifthedissolved oxygen concentration dropsbelow1mgL−1.Adecreasingdissolved oxy-genconcentrationwillalsoshifttheprocess todenitrification be-causethesizeoftheanoxiczonewithintheaerobicgranuleswill increase (De Kreuk et al., 2005). Since this decreasing dissolved oxygenconcentrationmostlyhappenstowardstheendofthe reac-tionphase,denitrificationmightactmoreasa sourcethanasink ofnitrousoxideascarbonavailabilityislow.Thiswouldresultin a double effect on the nitrous oxide emission: both the nitrifier pathwayandthedenitrifierpathwaycouldincrease nitrous oxide productioninthissituation.
A clear effect of rain events is shownin Fig. 14. Rain events causethebatchsizetoincrease,becausemorewaterarrivesatthe
WWTP.Intotal173batches(18%)were classifiedasRWFbatches. These batches hadan average emission factorof 0.09%, whichis lessthanone-third ofthe0.33%foundforallbatches.The reason fortheseloweremissionsduringRWFconditionsisuncertain,but there areseveralprocessesinfluencingtheemission factorduring RWF.UnderRWFconditionstheflowtotheWWTPincreasesfrom 0-175m3/hduringDWFconditionstoupto600m3/hunderRWF conditions.Sincethereactorisoperatedasasequencingbatch re-actor,theschedulingneedstobeadaptedtohandletheincreased inflowofwastewater(Chenetal.,2020).Thisisdonebydecreasing thetotalcycletimeandrunningmorebatchesperday.Thisleads toshorter,moreintenseaerationphases, withhigheroxygen con-centrations.Onaverage,theaerationphaseduringRWFis35 min-utes shorterthanduringDWF.SecondlyRWFbatchescanbesplit intwogroups.Atthestartofaraineventafirstflusharrivesatthe WWTP,duetothepresenceofpressurepipelinesinthesewer.The loadexceedstheaerationcapacity,leadingtoincomplete nitrifica-tionandthuslesspotentialfornitrousoxideproduction.Afterthis initialpeakload,theloadreturnstoamoreaveragevalue,butthe aeration remains relativelyshortandintense,focussingon nitrifi-cation. Thelackofcycletime duringRWFresultsinan increased nitrate effluent concentration (1.7 mgL−1 under DWFconditions and 4.4mg L−1 under RWFconditions). The total SND efficiency on averagewas69% (+/-15%), butwasslightlylower duringRWF (65%(+/-13%))comparedtoDWF(70%(+/-15%)),butduringDWF batches,SNDhappensatlowerdissolvedoxygenconcentrations.It also appearsthat theRWFeventinfluencesthe DWFbatches fol-lowingtheRWFevent(Fig.14,[5]and[6]).IntheseDWFbatches nitrousoxideemissionsareclosetozero.Thismightbetheresult ofthe firstflushatthe startoftherainevent, whichresultsina highsludgeloading.Ahighsludgeloadwillresultinhigher stor-agepolymerconcentrationinthegranularbiomassinthisspecific batch,whichmightstretchouttothefollowingbatches.Thisleads to moredenitrificationcapabilityoftheplant. Itappears thatthe denitrification process in thesebatches acts mainly asa sink for nitrous oxide,denitrifyingnitrous oxideatahigherratethan itis produced.Eventually,thepositiveeffectofthehighersludge load-ingwilldissipate,andnormalnitrousoxideemissionswillreturn.
The nitrousoxideconcentrationsinthebulkliquidareseldom higher than 0.3 mg L−1 and in most cases the concentration at theendofthecycleisclosetozero.Thismeans thatinmost cy-clesthedenitrificationcapacityofnitrousoxideisalsopresent to-wardstheendofthecycle.Thisstudyobservednitrousoxide con-version ratesup to 1mg N2O-N L−1 h−1. Thisrateis a netrate,
because it happenssimultaneously withdenitrificationof nitrate, whichalsocan producenitrousoxideasanintermediateproduct. Inmostcasesinlessthan 30minutesofanoxicconditionsall ni-trous oxidein the bulk liquid isdenitrified. This suggestsa high nitrousoxideconversionpotentialispresentiftherightconditions aremet.
4.4. Effectofprocesscontrol
The nitrous oxide emission factors varied between 0.02% and 1.58%perday,mostofthebatchesbeingbelow0.5%.Theemission of nitrous oxide might be lowered by changing the process con-trol.Thedecreaseoftheemissionfactorfrom0.57%inFebruaryto 0.15%inMarchbychangingtheprocesscontroltoafixedaeration strategyisanexampleofthis.Amorestabledissolveoxygen con-centrationduringtheaerationphaseledtoaremarkabledecrease in the emission ofnitrous oxide. Onthe other hand,the process control focusing on simultaneous nitrification and denitrification didnotleadtoelevatednitrousoxideemissioninthesummer pe-riod. Differentprocess controlstrategiesmaybe necessaryduring summerandwinterconditionstolimitnitrousoxideemission un-derallconditions.
It also seems that the production ofnitrous oxideduring the cycleisincreasing,whentheDOdropsbelow1mgL−1.Thiscould beeasily preventedby adjustingtheprocess control asto not al-low forthe oxygen to dip belowthe required setpoint. Another potentialimprovementrelatestotheinitialpeakatthestartofthe aeration (Fig. 11).This initial peak could be preventedby adding a pre-denitrification phase to the cycle, aiming to remove this residualnitrate,simultaneouslyremovingthenitrousoxideformed during settling and feeding. Another option is to focus on post-denitrification to prevent highamounts of residual nitrate to be presentinthenext cycle,therebylimitingthenitrousoxide emis-sionpeak.
Compared with a continuously fed activated sludge system, an SBR system gives a much higher information density on the changes innitrous oxide productionand consumption. Thisgives the possibility to develop effective process control strategies to minimize the nitrous oxideemissions. Amaximum nitrous oxide concentrationof0.3mgL−1 andanetdenitrificationrateupto1 mgL−1 h−1wasmeasured.Thiswouldmeanthatadenitrification phase of 20 minutes should be enough to remove nitrous oxide fromthe water phase in most cases.Splitting the main aeration phase andadding one or more intermediate denitrificationsteps couldbeaneffectivemeasuretominimizenitrousoxideemission.
4.5. DifferencewithconventionalSBRsystems
The AGS reactor wasoperated asa sequencing batch reactor. A comparison with flocculent SBR systems is therefore of inter-est.AsshowninFig.15,regularSBRsystemshaveshowntohave higheremissionfactorsthancontinuouslyfedactivatedsludge sys-tems.Highnitrous oxideemissionsinSBRsareattributedto sud-denchanges inthe concentrationsof ammonium,nitrate and ni-trite within the cycle or to accumulated dissolved nitrous oxide during anoxic settling and decanting in the subsequent aerobic phase(Vasilakietal.,2019).TheseconventionalSBRsystemsshow emissionfactors up to5.6%which ismuch higherthanthevalue of0.33%found inthisstudy.Thismightbe causedbydifferences in process conditions. The feeding in the AGS process is strictly anaerobic, which is achievedby plug flow feeding fromthe bot-tomofthereactor.Bythisplug flow,thenitrateremaininginthe reactorfrom theprevious cycle ispushed upwards whilethe re-actorisfilledwithfreshinfluent fromthebottom.Thislimitsthe contactbetweensludge,CODandnitrate,preventingproductionof highlevelsofnitrousoxideduringthefeedingphase.While anaer-obicplug flow feedingisa requirementinAGS systems,itis un-commonin SBRsystems. Forexample,in a studyby ( Rodriguez-Caballeroetal.,2015)theSBRreactorswerealternatinglyfed anox-icallyandaerobicallyresultinginanemissionfactorof6.4%.
4.6. Comparisonwaterphaseandgasphasemeasurement
The nitrous oxide concentration was measured by two differ-entmethods: firstly,bymeasuringthenitrousoxideconcentration inthe bulkliquid bymeans ofan onlinesensor andsecondly by measuringthenitrousoxideconcentrationsintheoff-gasviaagas analyser.Thelatterhasthebenefitofmeasuringtheemission dur-ingaeration directlywithouttheneed forconvertingwaterphase concentration intoemissions tothe air.The downsideof the off-gasmethodisthelackofinformationaboutwhathappensduring theanaerobic andtheanoxic phase,when theaeration is turned off.The water phase sensor givesdirect insight intothe produc-tion of nitrous oxide in the anoxic phase andthe denitrification ofnitrous oxideintheanoxicphase.Thewaterphasesensorthus providesinformationthatotherwisewouldbelostorobscuredby othernitrousoxideformingprocessesduringaeration.Fortheuse inprocesscontrolbothmethodscanbeused,butthewaterphase
suringcampaignspanningsummerandwinter.
• The yearly average emission factor was estimated between 0.25%and0.30%
• Theemissionfactorwascomparablewithcontinuouslyfed ac-tivatedsludgeplantswithlowemissionsandlowerthanvalues foundforconventionalSBRsystems.
• Bothnitrification anddenitrificationappeared tocontribute to the nitrous oxide production, denitrification acting both as a sourceandasinkfornitrousoxide.
• Post-denitrificationsignificantlyreducedthenitrousoxide con-centrationinthereactor.
• Anincreasedvariabilityoftheemissionfactorwasobservedat lowtemperatures.
• Different process control between summer and winter could limittheemissionfactor.
• In the winter period, aeration on a fixed oxygen setpoint re-ducedtheemissionfactorcomparedtoaeration usingvariable oxygensetpoint.
• Atemporaryincreaseofthesludgeloadingdecreasedthe emis-sionfactorforseveralbatches.
DeclarationofCompetingInterest
Theauthorsdeclarethattheyhavenoknowncompeting finan-cialinterestsorpersonalrelationshipsthatcouldhaveappearedto influencetheworkreportedinthispaper.
Acknowledgments
This research wassupportedby RoyalHaskoningDHV.The au-thorswish tothankWaterschapRijn enIJsselforthecooperation inthisprojectandSuellenEspindola,PascalleVermeulenandJelle Langedijkfortheirdedicationtothisproject.
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