Treatment Wetland Aeration without Electricity?

12 Pages·2016·2.09 MB·English

water
Article
Treatment Wetland Aeration without Electricity?
Lessons Learned from the First Experiment Using a
Wind-Driven Air Pump
JohannesBoog1,*,JaimeNivala1,ThomasAubron1,ScottWallace2,ChristopherSullivan3,
ManfredvanAfferden1andRolandA.Müller1
1 HelmholtzCenterforEnvironmentalResearch(UFZ),EnvironmentalandBiotechnologyCenter(UBZ),
Permoserstrasse15,04318Leipzig,Germany;[email protected](J.N.);[email protected](T.A.);
[email protected](M.v.A.);[email protected](R.A.M.)
2 NaturallyWallaceConsultingLLC,7801VauxhillDrive,P.O.Box99587,Raleigh,NC27624,USA;
[email protected]
3 RegionalDevelopmentVictoria,121ExhibitionStreet,Melbourne3000,Australia;
[email protected]
* Correspondence:[email protected];Tel.:[phone]
AcademicEditors:HansBrix,CarlosA.AriasandPedroN.Carvalho
Received:30August2016;Accepted:24October2016;Published:2November2016
Abstract:Aeratedtreatmentwetlandshavebecomeanincreasinglyrecognizedtechnologyfortreating
wastewatersfromdomesticandvariousindustrialorigins. Todate,treatmentwetlandaerationis
provided by air pumps which require access to the energy grid. The requirement for electricity
increasestheecologicalfootprintofanaeratedwetlandandlimitstheapplicationofthistechnology
toareaswithcentralizedelectricalinfrastructure. Windpoweroffersanotherpossibilityasadriver
forwetlandaeration,butitsuseforthispurposehasnotyetbeeninvestigated. Thispaperreportsthe
firstexperimentaltrialusingasimplewind-drivenairpumptoreplacetheconventionalelectricair
blowersofanaeratedhorizontalsubsurfaceflowwetland. Thewind-drivenairpumpwasconnected
to a two-year old horizontal flow aerated wetland which had been in continuous (24 h) aeration
sincestartup. Thewind-drivenaerationsystemfunctioned,howeveritwasnotspecificallyadapted
towetlandaeration. Asaresult,treatmentperformancedecreasedcomparedtopriorcontinuous
aeration. Inconsistentwindspeedatthesitemayhaveresultedininsufficientpressurewithinthe
aerationmanifold,resultingininsufficientairsupplytothewetland. Thispaperdiscussesthelessons
learnedduringtheexperiment.
Keywords: aeratedwetland;treatmentwetland;domesticwastewater;horizontalflow;energydemand
1. Introduction
Aerated treatment wetlands have become an increasingly recognized technology for treating
wastewatersfromdomesticandvariousindustrialoriginsunderdifferentclimateconditions[1–5].
Themainadvantageofthistechnologyisitshighoxygensupplytothemicrobialcommunitypresent,
whichenablesincreasedratesofaerobicmicrobialdegradationofpollutants. Aswastewaterdischarge
standardsbecomeincreasinglystringent,aeratedtreatmentwetlandsoffereffectiveremovalofkey
pollutantssuchasorganiccarbon,ammoniumnitrogen,andpathogens[6–9]andalsohaveareduced
land requirement compared to conventional treatment wetland designs [10]. Aerated treatment
wetlands have higher operation costs compared to passive treatment wetland designs. However,
comparedtoaconventionalactivatedsludgewastewatertreatmenttechnology,aeratedwetlandshave
loweroperationcosts[11].
Water2016,8,502;doi:10.3390/w8110502 www.mdpi.com/journal/water
Water2016,8,502 2of12
Electricalenergyistheconventionalsourceofpowerforairblowersinaeratedtreatmentwetlands.
Thesourceoftheelectricalenergy,whichisoftenfossilfuels,affectstheecologicalfootprintofthe
treatmentsystem. Severalattemptshavebeenmadetodecreasetheenergydemandforairsupplyby
usingintermittent[12–14]andspatial[15]aerationpatterns. Aeratedwetlandsrequiretheexistenceof
centralizedelectricalinfrastructureinordertooperate. However,accesstotheenergygridmaynotbe
availableinremoteareassuchascampgrounds,ruralcommunities,orinlessdevelopedcountries.
Whenconsideredinconjunctionwiththeconceptofdecentralizedwastewatertreatment,effective
wastewatertreatmenttechnologiesthatcanbeoperatedwithoutaconnectiontotheenergygridare
advantageous. Locallyavailablerenewableenergyshouldbeexploitedwhereverpossible.
Windpoweroffersapromisingpossibilityforwetlandaerationbutitsuseinthiscontexthas
notyetbeeninvestigated. Electricityproductionfromwindpowergenerallyoccursonlyonalarge
scale. Thus, an aerated wetland driven by electrical energy from wind power would still require
somesortofinfrastructuretobeconnectedtothegrid. Onthecontrary,small-scalemechanicalwind
power stations are readily available on the market and are often, for example, used to aerate fish
farmponds. Qinetal.[16]reportstheuseofsuchadevicetodriveareverseosmosissystemtreating
aquaculturewastewater. Thisdemonstratesthattheconceptofdecentralizedenergyproductionto
operatedecentralizedwastewatertreatmentsystemsisplausible,andthuscouldalsobeappliedto
aeratedtreatmentwetlands.
Fromaneconomicperspective,theconceptofwind-poweredaerationsimplyseesthetransferof
thecostassociatedwiththepurchaseoftheelectricpumpandthecostofelectricityandmaintenance
overthelifeofthesystemreplacedwiththecapitalcostofthewind-poweredairpump. Depending
on the life span of an aerated treatment wetland, wind-powered aeration could be economically
favorableoverthelongterm. Intheory,small-scalewindpowerstationscouldbeconstructedwith
localmaterialsandshouldbespecificallyadaptedtowetlandaeration,furtherimprovingtheecological
andeconomicalsustainabilityofthetreatmentsystem. Windspeedfluctuationsarelikelytoresult
inintermittentaeration,whichhasbeenshowntoimprovetotalnitrogenremoval[12–14],andthus
perhapsnotbeadetrimenttotreatment.
Aerated treatment wetlands are generally operated with saturation and configured as either
horizontalorverticalflow. Theinternalhydraulicsofthesetwodesignsaredifferent. Aeratedvertical
down-flow systems have been reported to be extremely well mixed along the vertical direction,
as a result of the counter-movement of water and air, and exhibit hydraulics similar to that of
one continuously stirred tank reactor [12]. Horizontal flow wetlands, on the other hand, exhibit
hydraulicssimilartothatofthreetofivetanks-in-series,whichisduetothedifferentdimensionsof
thewetlandandthefactthatthedirectionofflowandthemovementofairbubblesareperpendicular
to one another [17]. In the end, the variation in the distribution of retention times in horizontal
systemsissmaller,whichreducesthepossibilityofinflowtobetransferredtotheoutlettooquickly.
Therefore,thehydraulicbehaviorofahorizontal-flowaeratedwetlandwilleffectivelyreducetherisk
ofshort-circuiting. Thismaybeimportanttoconsiderforatreatmentwetlandwithawind-driven
aerationsystemwithrespecttovariablewindspeeds. Forthisreason,theinvestigationwasperformed
onahorizontal-flowaeratedwetland.
In order to retain the benefits of wetland aeration while eliminating the need for ongoing
electricitycosts,thecurrentstudyinvestigatedawind-poweredmechanicalairpumpthatreplaces
the electrical-driven aeration device of an aerated horizontal subsurface flow wetland. The main
objectives of this study were to assess the suitability of a wind-driven air pump for a horizontal
aeratedwetlandtreatingprimarilysettleddomesticsewage,andtocomparetreatmentperformanceof
awind-drivenaeratedwetlandtothatofawetlandaeratedcontinuouslywithanelectric-poweredair
pumpregardingtheremovaloforganiccarbon,nitrogenandpathogens. Thispaperwilldiscussthe
lessonslearnedduringtheexperimentandthechallangesofwind-drivenwetlandaeration.
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esxcpheermimateinc toafl tsheetu epx.perimental setup. 
The  wind‐driven  air  pump  was  a  conventional  six‐blade  wind  rotor  (d  =  1.5  m,  
Thewind-drivenairpumpwasaconventionalsix-bladewindrotor(d=1.5m,collarheight=3m)
collar height = 3 m) with a piston pump that was mechanically connected through a bevel drive. 
withapistonpumpthatwasmechanicallyconnectedthroughabeveldrive. Polyethylene(PE)pipes
Polyethylene (PE) pipes (d = 32 mm) were used to couple the piston pump to the aeration system. 
(d=32mm)wereusedtocouplethepistonpumptotheaerationsystem.
Monitoring of the experimental wetland was conducted from August 2010 to August 2014. In 
Monitoring of the experimental wetland was conducted from August 2010 to August 2014.
July 2012, the aeration device of the test system was changed from the electrical blower to the  
In July 2012, the aeration device of the test system was changed from the electrical blower to the
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2.2. Experimental Procedure 
2.2. ExperimentalProcedure
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breetlwateioennsthhiep abiretflwoeweng ethnee raaitre fdlobwy tgheenweriantded-d briyv ethnea wiripnudm‐dpriavtean caeirrt apiunmlepv ealt oaf cwerintadins pleeveedl wofa swninodt 
sstpraeiegdh twfoarsw naordt .stSruacighhatfsomrwalalrwdi.n Sdupcohw ae srmstaaltli owninisde apsoiwlyearf fsetcatteidonb yisq euaisciklya nafdf/ecotreidn tbeyn sqeucihckan agneds/ionr 
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in the wind. The direct mechanical coupling to the piston pump transferred this fluctuation to the 
generated air flow, which contributes to the existing variability in air flow rate that results from the
Water2016,8,502 4of12
mechanicalcouplingtothepistonpumptransferredthisfluctuationtothegeneratedairflow,which
contributestotheexistingvariabilityinairflowratethatresultsfromtheup-and-downmotionofthe
piston. Athermalmassflowmeter(TSI4043,TSIGmbH,Aachen,Northrhine-Westfalia,Germany)
combinedwithahigh-resolutiondataloggerwithasamplingfrequencyof32Hz(HandyLogV/C
-rugged-,Driesen+KernGmbH,BadBramstedt,Schleswig-Holstein,Germany)wasusedtoassessair
flowrates. Forthemeasurements,theairpumpwasconnectedtoapipewithanopenend,inorder
tomakeameasurementwithoutthebackpressureofthewetlandaerationsystem. Severaltechnical
andlogisticaldifficultiesdidnotallowthequantificationoftheairflowwhilethewind-drivenpump
wasconnectedtothewetlandaerationsystem. Ameasurementcampaignconsistedofseveraldays
ofmonitoringairflowandwindspeedsimultaneously,whichsometimesincludedacoupleofdays
waitingforwind.
2.2.2. SamplingandWaterQualityAnalysis
Forlogisticalreasons,aquasi-weeklysamplingroutinewasestablished. Grabsamplesweretaken
fromtheinfluentandeffluentoftheexperimentaltreatmentwetlandandanalyzedforredoxpotential
(ORP,Multi350i,SenTix® ORP,WTW,Weilheim,Bavaria,Germany),dissolvedoxygen(DO,Multi
350i,CellOx325,WTW,Weilheim,Bavaria,Germany),five-daycarbonaceousbiochemicaloxygen
demand(CBOD ,DIN38409H52,OxiTOP®,WTW,Weilheim,Bavaria,Germany),totalorganiccarbon
5
(TOC,DINEN1484,TOC-VCSN,ShimadzuDeutschlandGmbH,Duisburg,Northrhine-Westfalia,
Germany), total nitrogen (TN, DINEN 12660, TNM-1, Shimadzu Deutschland GmbH, Duisburg,
Northrhine-Westfalia,Germany),ammonianitrogen(NH -N,DIN38406E5,EPOSANALYZER5060,
4
EppendorfAG,Hamburg,Germany),nitratenitrogen(NO -N,DIN38405D9,EPOSANALYZER5060,
3
EppendorfAG,Hamburg,Germany),andE.coli(Colilert-18Quanti-Tray™,IDEXXLaboratoriesInc.,
Westbrook, ME, USA). Inflow and outflow were measured with a magnet inductive flow meter
(Promag10, Endress+Hauser Messtechnik GmbH+Co. KG, Weilam Rhein, Baden-Württemberg,
Germany)andatippingcounter,respectively.
2.3. DataProcessing
Influentandeffluentconcentrationdatawasscreenedforoutliers. Outliersweredefinedasdata
relatedtoinfrastructuremalfunctionsand/orsamplingerrors,andwereidentifiedusingtimeseries
plotsofthecorrespondingwaterqualityparameters. DetectionlimitsforCBOD ,NH -NandNO -N
5 4 3
were0.3,0.03,and0.02mg·L−1respectively. Fordataprocessing,resultsreportedasbelowdetection
limitwerereplacedwiththevalueofthecorrespondingdetectionlimit. Thisapproach,ifanything,
slightlyunderestimatestreatmentperformanceofthesystem. Missingvalueswhichwerearesultof
thequasi-weeklysamplingscheduleand/orduetoinfrastructuremalfunctionswerenotconsideredas
problematicasonlysimpledescriptivestatisticsandtimeseriesplotswereappliedforthedataanalysis.
Information about data below detection limit and number of outliers can be reviewed in TableS1.
Theregression analysis of the air flow measurement data was done in the statistic environment R
version3.1.2[20]usingthepackagesegmented[21].
3. Results
3.1. WindSpeedDistributionatExperimentalSite
On a yearly basis, the average wind speed varied from 0 to 7 m·s−1 with an overall mean of
around3m·s−1andmaximumvaluesintherangeof8–15m·s−1. Thewindspeedtimeseriesexhibits
slightlyloweraveragevaluesduringsummercomparedtowinterseasons(Figure2a). Figure2bshows
thewindspeeddistributionduringSeptember2013. Thismonthlypatternshowsaconstantwind
speedfluctuationwithameanof4m·s−1andfluctuationsbetween0and10m·s−1. Adivergencefrom
thispatterncanbeidentifiedinthefirstandlastweekofthepresentedmonth. Figure2bshowsthat
thespeedanddurationofthewinddiffersgreatlyfromonedaytoanother. Thisisalsoobservedinthe
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hourlywinddatashowninFigure2d. Overall,mostwindoccursduringthedaytime. Ingeneral,the
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distributionofwindspeedatthesitecanbetermedashighlyvariable,containingmaximumvaluesof
2c0onmt·asi−n1in;gh omwaexviemr,utmhe vloanluge-tse romf 2a0v emra∙sg−e1; whoinwdesvpeere, dthies qlounitge‐tleorwm. Tahveersatrgoen wgeisntdfl supcetuedat iiosn qsuiintew lionwd. 
sTpheee dstwroenrgeeosbt sfelurvcteudaotinonasd iani lwyianndd shpoeeudrl ywberaes ios.bserved on a daily and hourly basis. 

Figure 2. Wind speed time series at the research facility in Langenreichenbach, Germany at different 
Figure2.WindspeedtimeseriesattheresearchfacilityinLangenreichenbach,Germanyatdifferent
time scales: (a) Period of record; (b) monthly (September 2013); (c) weekly (1–7 September 2013); and 
time scales: (a) Period of record; (b) monthly (September 2013); (c) weekly (1–7 September 2013);
(d) daily (5 September 2013). 
and(d)daily(5September2013).
3.2. Air Flow Measurement 
3.2. AirFlowMeasurement
The air flow generated by the wind‐driven piston pump and the wind speed are positively 
The air flow generated by the wind-driven piston pump and the wind speed are positively
correlated. This correlation shows a breakpoint at a wind speed of around 5 m∙s−1. The rate of 
correlated. Thiscorrelationshowsabreakpointatawindspeedofaround5m·s−1. Therateofincrease
increase of air flow over wind speed declines with wind speeds greater than 5 m∙s−1, which might be 
ofairflowoverwindspeeddeclineswithwindspeedsgreaterthan5m·s−1,whichmightbecaused
caused by a stall effect on the rotor blades. A stall effect is a common phenomenon in fluid dynamics 
by a stall effect on the rotor blades. A stall effect is a common phenomenon in fluid dynamics of
of wind rotor blades [22]; it describes the demolition of an air flow on a wing above a certain 
windrotorblades[22];itdescribesthedemolitionofanairflowonawingaboveacertainthreshold
threshold wind speed. A segmented regression with two linear parts was fit to the data; the 
windspeed. Asegmentedregressionwithtwolinearpartswasfittothedata; thebreakpointwas
breakpoint was identified at a wind speed of approximately 5.4 m∙s−1 as indicated in Figure 3. The 
identifiedatawindspeedofapproximately5.4m·s−1asindicatedinFigure3. Theregressionlinein
regression line in Figure 3 also shows that a minimum wind speed is required in order to produce 
Figure3alsoshowsthataminimumwindspeedisrequiredinordertoproduceanysignificantair
any significant air flow. The maximum measured air flow was approximately 2200 NL∙h−1 at a wind 
flow. Themaximummeasuredairflowwasapproximately2200NL·h−1atawindspeedof9.5m·s−1.
speed of 9.5 m∙s−1. Remembering that the wind speed at the site fluctuated from 0 to 7 m∙s−1 on 
Remembering that the wind speed at the site fluctuated from 0 to 7 m·s−1 on average, a range of
average, a range of airflows from 1000 to 1800 NL∙h−1 can be expected if the back pressure of the 
airflows from 1000 to 1800 NL·h−1 can be expected if the back pressure of the aeration system is
aeration system is neglected. Unfortunately, the aeration system does exert a back pressure, which is 
neglected. Unfortunately,theaerationsystemdoesexertabackpressure,whichisaresultoffrictional
a result of frictional losses and the hydrostatic pressure of the water level. This implies a reduction in 
losses and the hydrostatic pressure of the water level. This implies a reduction in actual air flow
actual air flow delivered to the wetland. It also means an effective increase of the threshold wind 
deliveredtothewetland. Italsomeansaneffectiveincreaseofthethresholdwindspeedrequiredto
speed required to initiate a significant air flow into the wetland. Boog [17] estimated that after 
considering these losses, the air volume reaching a small‐scale aerated wetland is approximately 
50%  of  the  maximum  value  (listed,  for  example,  on  the  manufacturer  specification  sheet). 
Considering the pump specifications used in Boog [17], the reported flow loss would correspond to a
Water2016,8,502 6of12
initiateasignificantairflowintothewetland. Boog[17]estimatedthatafterconsideringtheselosses,
theairvolumereachingasmall-scaleaeratedwetlandisapproximately50%ofthemaximumvalue
(listed,forexample,onthemanufacturerspecificationsheet). Consideringthepumpspecifications
Water 2016, 8, 502  6 of 11 
usedinBoog[17],thereportedflowlosswouldcorrespondtoabackpressureoftheaerationsystem
(binaccklu dpirnegsstuhree hoyfd trhoes taaetircaptiroens ssuyrseteomft h(ienwcluatdeirncgo ltuhme nh)yodfroapstpartoicx ipmreastseulyre1 8o0f mthbea rw. Tatheers ecollousmsens)a roef 
oanplpyrofuxrimthaetreelyxa 1c8e0r bmatbeadr.f Torheaswe ilnodss-edsr iavreen oanilryp fuumrthpe.r exacerbated for a wind‐driven air pump. 

Figure 3. Relationship of wind speed and air flow generated by the wind‐driven pump during the 
Figure3. Relationshipofwindspeedandairflowgeneratedbythewind-drivenpumpduringthe
open end measurement (the pressure loss within the aeration system itself was neglected). The black 
open end measurement (the pressure loss within the aeration system itself was neglected). The
line indicates the two‐step segmented linear regression with the breakpoint at a wind speed of 
blacklineindicatesthetwo-stepsegmentedlinearregressionwiththebreakpointatawindspeedof
aapppprrooxxiimmaatteellyy 55..44 mm·∙ss−−11 (R(R2 2= =0.08.88)8. ).
Air flow by the electric aeration system during phase one was not measured directly but can be 
Air flowby theelectric aerationsystem duringphase one wasnot measured directlybut can
inferred from Boog [17], who measured the air flow rate of two electric air pumps of the same type as 
be inferred from Boog [17], who measured the air flow rate of two electric air pumps of the same
in this study, in a planted replicate wetland at the same site as in this study to approximately  
typeasinthisstudy,inaplantedreplicatewetlandatthesamesiteasinthisstudytoapproximately
1000 NL∙h−1 each. This translates to an estimate of 2000 NL∙h−1 of air flow generated by electric 
1000NL·h−1 each. This translates to an estimate of 2000 NL·h−1 of air flow generated by electric
aeration during phase one.  
aerationduringphaseone.
3.3. Treatment Performance of the Experimental System 
3.3. TreatmentPerformanceoftheExperimentalSystem
The influent wastewater exhibits typical characteristics of primary settled domestic wastewater, 
Theinfluentwastewaterexhibitstypicalcharacteristicsofprimarysettleddomesticwastewater,
including a low redox potential as well as low dissolved oxygen and nitrate nitrogen content, and 
includingalowredoxpotentialaswellaslowdissolvedoxygenandnitratenitrogencontent,and
high concentrations of organic carbon, ammonia nitrogen, and the pathogen indicator organism  
high concentrations of organic carbon, ammonia nitrogen, and the pathogen indicator organism
E. coli (Table 1). Referring to the time series plots in Figure 4, it can be seen that the water quality of 
E.coli(Table1).ReferringtothetimeseriesplotsinFigure4,itcanbeseenthatthewaterqualityof
tthhee iinnflfulueennttfl fuluctcutuataetseso voevretri mtiem.eC.B COBDOD,T5,O TCO,EC., cEol.i ,caonli,d aDnOd cDoOnc ecnotnrcaetinotnrastoioscnisll aotsecailrloauten daraoustnadb lea 
5
stable central tendency over the period of record. A slight increase in total nitrogen and ammonia 
centraltendencyovertheperiodofrecord. Aslightincreaseintotalnitrogenandammonianitrogenis
nitrogen is visible between 2010 and 2013. In 2013, an additional chamber was installed into the 
visiblebetween2010and2013. In2013,anadditionalchamberwasinstalledintotheseptictankin
septic tank in order to provide a longer retention time during primary treatment. 
ordertoprovidealongerretentiontimeduringprimarytreatment.
During the period from August 2010 to August 2012, the wetland system was equipped with 
DuringtheperiodfromAugust2010toAugust2012,thewetlandsystemwasequippedwith
eelleeccttrriicc aaeerraattiioonn ppuummppss tthhaatt rraann 2244h h·d∙d−−11.. DDuurriinngg tthhiiss ttiimmee,, tthhee wweettllaanndd ccoonnssiisstteennttllyy pprroodduucceedd aa  
high‐quality effluent (low carbon, complete nitrification). This is reflected in mean outflow levels of 
high-quality effluent (low carbon, complete nitrification). This is reflected in mean outflow levels
o3f.13 .1mmg∙gL·−L1 −o1f oCf BCOBOD5D,  1,21.28. 8mmgg∙L·L−1 −T1OTCO,C  4,34.32. 2mmgg∙L·L−1− 1TNTN, ,anandd  00.1.1  mmgg∙·LL−−1 1NNHH4‐N-N, ,aass  wweellll aass  
5 4
88..00 mmgg·∙LL−−11 DDOO, ,3388..77 mmgg∙·LL−−1 1NNOO3‐N-N, a,nadn da arerdedoox xppootetenntitaial loof f221111.7.7 mmVV.. AAddddiittiioonnaallllyy,, tthhee wweettllaanndd 
3
eliminated  approximately  four  orders  of  magnitude  of  E.  coli,  down  to  an  average  effluent 
eliminatedapproximatelyfourordersofmagnitudeofE.coli,downtoanaverageeffluentconcentration
ocofn2c.8enlotrgatio(Mn PoNf 21.80 0lomg1L0(M−1P).NT 1o0ta0l mniLtr−1o)g. eTnotcaol nncietrnotgraetni ocnonrceemnotrvaatliown arsemapopvraolx wimasa taeplypr5o3x%im—attheliys 
10
53%—this means that denitrification may have taken place despite the fact of continuous aeration in 
means that denitrification may have taken place despite the fact of continuous aeration in space
space and time. Boog [12] and Foladori [8] report similar findings. It is quite likely that simultaneous 
and time. Boog [12] and Foladori [8] report similar findings. It is quite likely that simultaneous
nitrification–denitrification induced by non‐uniform oxygen distribution, non‐aerated micro zones, 
nitrification–denitrificationinducedbynon-uniformoxygendistribution,non-aeratedmicrozones,
and/or biofilm layering took place. The probability of a potential volatilization of ammonia as a 
and/or biofilm layering took place. The probability of a potential volatilization of ammonia as a
contribution to TN removal is quite low owing to constant effluent pH levels of approximately 7. 
contributiontoTNremovalisquitelowowingtoconstanteffluentpHlevelsofapproximately7. These
These  results  are  in  agreement  with  other  studies  such  as  Boog  [17],  Headley  et  al.  [3],  
Labella et al. [1], and Nivala et al. [23], which report a similar treatment performance for carbon, 
nitrogen, and pathogen removal in aerated treatment wetlands.
Water2016,8,502 7of12
resultsareinagreementwithotherstudiessuchasBoog[17],Headleyetal.[3],Labellaetal.[1],and
Nivalaetal.[23],whichreportasimilartreatmentperformanceforcarbon,nitrogen,andpathogen
removalinaeratedtreatmentwetlands.
Table1.Waterqualityresultsoverthecourseofthestudy.
Parameter Influent Effluent
Period 2010–2014 2010–2012 2012–2013 2013–2014
Aeration - 24helectric wind-driven wind-driven
Hydraulicloadingrate(L·m−2·d−1) 130 130 65
Organicloadingrate(gCBOD5m−2·d−1) 36.5 36.5 18.2
Median −179.9 211.7 −97.3 −267.0
Mean −193.1 206.1 −15.5 −265.9
RedoxPotential(mV)
StdDev 80.8 61.7 204.5 21.0
N 164 48 39 39
Median 0.5 8.0 3.1 3.1
DissolvedOxygen(mg·L−1) Mean 0.6 8.0 3.2 2.8
StdDev 0.5 2.2 1.7 1.2
N 157 46 38 36
Median 270.0 3.1 52.2 49.7
CBOD5(mg·L−1) MStdeaDnev 28980..05 44..97 3522..26 2510..51
N 163 31 38 39
Median 152.8 12.8 30.2 21.8
TOC(mg·L−1) Mean 161.1 20.1 33.1 25.5
StdDev 67.2 18.1 12.4 13.4
N 163 45 39 38
Median 81.7 43.2 73.3 76.3
TN(mg·L−1) Mean 80.9 43.0 74.7 74.9
StdDev 17.7 10.9 13.7 10.9
N 165 46 40 38
Median 62.2 0.1 56.2 65.3
NH4-N(mg·L−1) MStdeaDnev 6117..44 02..85 6107..32 6181..16
N 158 41 39 38
Median 0.2 38.7 0.4 0.1
NO3-N(mg·L−1) MStdeaDnev 00..33 3182..42 38..31 15..34
N 97 48 21 19
Median 6.8 2.6 5.6 5.6
E.colilog10(MPN×100mL−1) MStdeaDnev 60..73 20..88 50..58 50..46
N 163 47 41 37
In August 2012, the electrical air pump was replaced by the wind-driven air pump. Figure 4
showsthechangeineffluentwaterqualityafterinstallationofthewind-poweredpump. Nitrification
ceased,andCBOD ,TOC,andE.colieffluentconcentrationsincreased. Effluentdissolvedoxygen
5
concentrationsdroppedtoapproximately3mg·L−1;theremainingDOcouldbearesultoftheeffluent
samplecollection. Effluentsamplesweretakenfrominsidethecontrolbuilding,ratherthandirectly
fromthewetland. Apossiblereaerationwhilepassingfromthewetlandeffluentpipetothesampling
locationmayhaveinfluencedthedissolvedoxygenconcentrationintheeffluent.
ThechangeintreatmentperformancewasratherrapidforTN,NH -N,NO -N,andE.coli.Onthe
4 3
contrary,thedecreasedperformanceforCBOD andTOC,aswellasthedropinredoxpotentialand
5
dissolvedoxygenconcentrations, occurredmoreslowly. Itisinterestingthatthetransitionperiod
of CBOD , ORP, DO, and redox potential is characterized by high fluctuations. A possible reason
5
for this may be that the wind-driven air pump generated an unsteady supply of air during the
first two months of operation. This could have served at least for a temporal partial removal of
organiccarbon(nitrificationwasprobablyinhibitedduetofastergrowthofheterotrophicorganisms
inoxygen-limitedenvironments). Astimepassed,theefficiencyofthewind-drivenairpumpmay
havechanged,resultinginafurtherdeclineintreatmentperformance. Theaerationlinesmayhave
experiencedaccumulationofsludgewhichinturncouldhaveincreasedthecounterpressure,further
hinderingaeration.
Water2016,8,502 8of12
Water 2016, 8, 502  8 of 11 

FFigiguurere 44.. TTiimmee sseerriieesso offi niflnufleunetn(tr e(dreddo tdteodttelidn el,incier,c ucilracrumlaarr kmerasr)kaenrds) eafflnude enftf(lgureenetn (sgorleidenli nsoe,lisdq ulianree,d 
smquaarrkeedr sm)caornkceerns)t rcaotinocnesn.tTrahteiovnesr. tiTchale svoelirdticlianle sionldidic alitnese tihnedsicwaittecsh tfhroe mswelietcchtr ifcraolmto ewleinctdri-cdarli vteon 
waienrdat‐idornivaennd aethraetivoenr taincadl tdhoet vteedrtilcinael droefteterds tlionteh reefreerdsu toct tiohne roefdtuhcetihoynd orfa tuhleic hlyodardaiunlgicr laotaedfirnogm ra1t3e0 
frtoom65 1L3·0m to− 26·5d L−∙1m. −2∙d−1. 
The treatment performance did not recover during the rest of the time the wind‐driven air 
Thetreatmentperformancedidnotrecoverduringtherestofthetimethewind-drivenairpump
pump was in operation. In August 2013, after a year of less than optimal results, the hydraulic 
wasinoperation. InAugust2013,afterayearoflessthanoptimalresults,thehydraulicloadingrate
loading rate was reduced by 50%. Unfortunately, treatment performance did not improve. The water 
wasreducedby50%. Unfortunately,treatmentperformancedidnotimprove. Thewaterqualitydata
quality data indicates that the decrease in treatment performance was caused by a lack of oxygen, 
pointing to problems with implementation and/or operation of the wind‐powered air pump.
Water2016,8,502 9of12
indicates that the decrease in treatment performance was caused by a lack of oxygen, pointing to
problemswithimplementationand/oroperationofthewind-poweredairpump.
4. Discussion
By switching from electrical to wind-driven aeration, treatment performance of the aerated
wetlanddecreasedquickly. Treatmentperformancebecameoxygen-limited. Itispossiblethatthe
wind-drivenairpumptemporarilyprovidedaerationtothewetlandforashortperiodoftime,butit
isclearthatoverthelongertermtheefficiencyoftheaerationsystemwaspoor. Theaerationsystem
ofwetlandsoperatingwithanelectricalairpumpcanbevisuallycheckedbydiggingaholeintothe
gravel,exposingthewaterlevel,andwaitingtoobserveairbubblesrisingtothesurface. Duringthe
twoyearsofwind-poweredaeration,veryfew(ifany)airbubblescouldbeobservedcomingfromthe
wind-poweredairpump,evenonawindyday.
The counter-pressure in the aerated wetland is already quite high and reduced the air flow
byapproximately50%undercontinuouselectricaeration,whichcorrespondstoabackpressureof
approximately180mbar. Theorificesoftheaerationlinesdonotpreventwaterentrancefromoutside
if the counter pressure is not compensated. In the end, the wind-powered air pump, which was
unsteady as a result of fluctuating wind speeds, may have not provided enough pressure to clear
thiswateroutoftheaerationlines. Additionally,thedecreasedtreatmentperformancemighthave
resultedinanaccumulationofsludgeintheaerationlineswhichcouldhaveincreasedthenecessaryair
pressuretocleanthelinesandinturnfurtherinhibitedaeration,resultinginanunfavorablefeedback
loop. This may especially be a problem during long periods without wind. Under the absence of
anycounter-pressure,asduringtheairflowmeasurement,thewindpowersystemwascapableof
producingasignificantairflow. Unfortunately,itprovednottobepowerfulenoughwithrespectto
thefactofthecounter-pressurefromtheaerationsystem. Italsohastobetakenintoaccountthat
thelocationoftheexperimentalsiteLangenreichenbachinGermanyisnotalocationknowntobe
especiallywindy.
Whenaimingtouseawindpowerstationforwetlandaeration,thedistributionofwindspeedat
thesiteofinterestmustbeexaminedtodeterminewhetherornotthesiteissuitableforsuchadevice.
Specialattentionshouldfocusontheminimumwindspeedduringatimeframewhichwouldallow
sufficientaerationforremovaloforganiccarbonandammonianitrogen. Siteshavingperiodswithout
anysignificantwindonadailybasisarenotparticularlysuitableforwind-drivenaeration.
Thewind-poweredairpumptechnologyitselfisofgreatimportance. Thepumpcapacitycurve
ofacertainwind-drivenairpumpwithrespecttocounter-pressureandwindspeedshouldbeknown
orexaminedinadvancetoapotentialapplication. Ifanairpumpcannotovercometheapproximately
180mbarofcombinedpressurelossesintheaerationlinesincludingthehydrostaticpressureofthe
watercolumn,airwillnotbedeliveredtothewetland. Thefurtherincreaseofthispressurelossdue
towaterentranceduringphaseswithoutairflowasaresultofwindspeedbelowaminimumlevel
mightbepreventedbyusinganon-returnvalveintheaerationlines. Apistonpumpisprobablya
decentchoicefromtheperspectiveofairflowgeneration. Inthisstudy,thewindpowerstationwhich
drovethepistonpumphadaconventionalsix-bladerotorwithahorizontalcollar. Whenconducting
an experiment under similar wind speed conditions, it might by more suitable to use a Savonius
rotor,which(duetoitsverticalcollarconstruction)maybemoreeffectiveatlowwindspeedsand
will function regardless of any changes in wind direction. Further adaptations of wind-powered
equipmenttowetlandaerationshouldresultinperformanceimprovementscomparedtothecurrent
study. Itmightalsobefavorabletouseawindpowersystemwithahighercollarheight,suchas15m,
tocatchthewindwell-aboveobstaclessuchasbuildingsandtrees.
Thepotentialaccumulationofsludgeintheaerationlinesshouldalsobeconsidered. Inthisstudy,
thehydraulicloadingrateofthewetlandwasnotchangedwhenmovingfromthe24-helectricalair
pumptotheintermittentlyoperatingwind-poweredairpump. Takingthisintoaccount,itwouldbe
advisabletostartwithalowerloadingratetoreducetheinflowoxygendemand. Atthesametime,
Water2016,8,502 10of12
thismightbefavorabletoincreasethehydraulicretentiontimetospanitbeyondthemeandurationof
non-windyconditions. Finally,suchtechnologymaybebettersuitedasapolishing(tertiary)treatment
stepasopposedtosecondarytreatment.
Usingsmall-scalewindpowerstationstogenerateandstoreelectricalenergyforthesupplyof
electricairblowersmightbeanotherinterestingconceptforwetlandaeration. Thisconcept,including
abatterysystemforenergystorage,couldbeabletoovercometheproblemoflongerperiodswithout
aminimumwindspeednecessarytooperatethewindpowerstation. Here,thechallengeistofinda
windpowerstationwhich,atthesametime,isabletoprovideacertainamountofelectricalenergyto
supplytheelectricairpumpsandtostoreexcessenergyinthebattery. Thebatterywillhavetohavea
storagecapacitywhichallowsthesupplyofacertainamountofelectricalenergyduringamaximum
timeperiodwithouttheminimumwindspeed. Thebatteryshouldalsobeabletorechargeduringa
minimumtimeperiodwiththeminimumwindspeed. Thetechnicalrealizationofthisconceptmay
havetobeextendedtoavoltagetransformer,ascommerciallyavailableequipmentofawindpower
station,battery,andairpumphavedifferentrequirementsofelectricalparameterssuchasin-and
outputvoltageandcurrent. Sofar,theuseofawindpowerstationtogenerateelectricalenergyfor
drivingconventionalairpumpsisaninteresting,butmorecomplex,aerationconceptcomparedtoa
mechanicalwind-drivenairpump.
5. Conclusions
Thisstudyreportsthefirstapplicationofawind-drivenairpumpfortheaerationofatreatment
wetlandtreatingdomesticwastewater. Afterswitchingfromelectricaltowind-drivenaeration,the
treatment performance of the pilot system deteriorated quickly to a level of a passive horizontal
subsurfaceflowwetland. Reducingthehydraulicloadingrateby50%didnotimprovetreatment
performance. Waterqualityresultsindicatethatthewind-drivenairpumpdidnothavethecapacity
toprovidetherequiredamountofoxygenequivalenttocontinuousaerationundertheexperimental
conditions. Yetunderdifferentconditionsandwithadaptationsofthewind-poweredequipmentto
wetlandaerationsystems,theprinciplemaysucceed. Ingeneral,apotentialwind-drivenairpump
wouldhavetocompensateapressurelossofapproximately180mbarataminimumwindspeedand
shouldbeequippedwithanon-returnvalve.
Anotherpotentialconceptcouldmakeuseofsmall-scalewind-powerdevicestogenerateelectrical
energytosupplyconventionalelectricalairpumps. Thisoptionallowsenergystorageusingbatteries
andmayovercometheproblemofnoaerationduringlongperiodswithoutwind.
Furtherresearchinthiscontextshouldfocuson(1)priorcharacterizationoftheonsitewindspeed
distribution;(2)priorassessmentofthepotentialforwind-drivenairpumpstoprovideanairflow
at a given wind speed and counter-pressure or the potential for a wind-power station to generate
electricityatacertainlevelofwindspeedandabatterywiththecapacitytostoreenergyforacertain
timeperiod;and(3)consideringtrialswithalowerhydraulicloadingrateorfortreatingwastewater
withalowerinfluentoxygendemand.
SupplementaryMaterials:Thesupplementarymaterialsareavailableonlineatwww.mdpi.com/2073-4441/8/
11/502/s1.
Acknowledgments: Johannes Boog acknowledges the Helmholtz Center for Environmental Research
(HelmholtzZentrumfürUmweltforschung—UFZ)andtheHelmholtzInterdisciplinaryGraduateSchoolfor
EnvironmentalResearch(HIGRADE)foradditionalfundingandsupport.Theauthorsgratefullyexpresstheir
gratitudetoKatyBernhardandGritWeichertfortheirsupportandassistanceinsamplecollectionandanalysis;
andtoSybilleMothes,CarolaBönischandKarstenMarienforanalyticalsupport.Thisworkwassupportedinpart
byfundingfromtheGermanMinistryofEducation&Research(BMBF)withinthecontextoftheSMART-MOVE
Project(Ref.02WM1355).
AuthorContributions: ChristopherSullivan,ScottWallace,ThomasAubronandJaimeNivalaconceivedthe
idea. JohannesBoog,ChristopherSullivan,ThomasAubronandJaimeNivaladesignedtheexperimentsand
conductedtheexperiments.JohannesBoogandJaimeNivalaanalyzedandsynthesizedthedata.JohannesBoog,
JaimeNivala,ManfredvanAfferdenandRolandA.Müllerwrotethepaper.