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University of Wollongong University of Wollongong
Research Online Research Online
Faculty of Engineering and Information Sciences - Papers: Part A
Faculty of Engineering and Information Sciences
1-1-2015
The role of forward osmosis and microfiltration in an integrated osmotic-The role of forward osmosis and microfiltration in an integrated osmotic-
microfiltration membrane bioreactor system microfiltration membrane bioreactor system
Wenhai Luo University of Wollongong, wl344@uowmail.edu.au
Faisal I. Hai University of Wollongong, faisal@uow.edu.au
Jinguo Kang University of Wollongong, jkang@uow.edu.au
William E. Price University of Wollongong, wprice@uow.edu.au
Long D. Nghiem University of Wollongong, longn@uow.edu.au
See next page for additional authors
Follow this and additional works at: https://ro.uow.edu.au/eispapers
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Recommended Citation Recommended Citation Luo, Wenhai; Hai, Faisal I.; Kang, Jinguo; Price, William E.; Nghiem, Long D.; and Elimelech, Menachem, "The role of forward osmosis and microfiltration in an integrated osmotic-microfiltration membrane bioreactor system" (2015). Faculty of Engineering and Information Sciences - Papers: Part A. 3957. https://ro.uow.edu.au/eispapers/3957
Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: research-pubs@uow.edu.au
The role of forward osmosis and microfiltration in an integrated osmotic-The role of forward osmosis and microfiltration in an integrated osmotic-microfiltration membrane bioreactor system microfiltration membrane bioreactor system
Abstract Abstract This study investigates the performance of an integrated osmotic and microfiltration membrane bioreactor (O/MF-MBR) system for wastewater treatment and reclamation. The O/MF-MBR system simultaneously used microfiltration (MF) and forward osmosis (FO) membranes to extract water from the mixed liquor of an aerobic bioreactor. The MF membrane facilitated the bleeding of dissolved inorganic salts and thus prevented the build-up of salinity in the bioreactor. As a result, sludge production and microbial activity were relatively stable over 60 days of operation. Compared to MF, the FO process produced a better permeate quality in terms of nutrients, total organic carbon, as well as hydrophilic and biologically persistent trace organic chemicals (TrOCs). The high rejection by the FO membrane also led to accumulation of hydrophilic and biologically persistent TrOCs in the bioreactor, consequently increasing their concentration in the MF permeate. On the other hand, hydrophobic and readily biodegradable TrOCs were minimally detected in both MF and FO permeates, with no clear difference in the removal efficiencies between two processes.
Disciplines Disciplines Engineering | Science and Technology Studies
Publication Details Publication Details Luo, W., Hai, F. I.., Kang, J., Price, W. E., Nghiem, L. D. & Elimelech, M. (2015). The role of forward osmosis and microfiltration in an integrated osmotic-microfiltration membrane bioreactor system. Chemosphere, 136 125-132.
Authors Authors Wenhai Luo, Faisal I. Hai, Jinguo Kang, William E. Price, Long D. Nghiem, and Menachem Elimelech
This journal article is available at Research Online: https://ro.uow.edu.au/eispapers/3957
1
The role of forward osmosisand microfiltration in an integrated1
osmotic-microfiltration membranebioreactor system2
Revisedmanuscript submitted to Chemosphere3
April 20154
WenhaiLuo a, FaisalI. Hai a, JinguoKangb, William E. Priceb, LongD. Nghiema*,5
MenachemElimelechc6
a StrategicWaterInfrastructureLaboratory, Schoolof Civil Mining andEnvironmental7
Engineering,Universityof Wollongong,Wollongong,NSW2522,Australia8
b StrategicWaterInfrastructureLaboratory,Schoolof Chemistry,Universityof Wollongong,9
Wollongong,NSW2522,Australia10
c Departmentof ChemicalandEnvironmentalEngineering,YaleUniversity,NewHaven,CT11
06520-8286,UnitedStates12
* Correspondingauthor:longn@uow.edu.au; Ph:+61(2) 42214590.
2
Abstract13
This study investigates the performanceof an integrated osmotic and microfiltration14
membranebioreactor(O/MF-MBR) systemfor wastewatertreatmentand reclamation. The15
O/MF-MBR systemsimultaneouslyusedmicrofiltration (MF) and forward osmosis(FO)16
membranesto extract water from the mixed liquor of an aerobic bioreactor.The MF17
membranefacilitatedthe bleedingof dissolvedinorganicsaltsandthuspreventedthe build-18
up of salinity in the bioreactor.As a result, sludgeproductionand microbial activity were19
relatively stableover 60 daysof operation. Comparedto MF, the FO processproduceda20
betterpermeatequality in termsof nutrients,total organiccontent,aswell ashydrophilicand21
biologically persistentTrOCs. The high rejection of the FO membranealso led to the22
transportof severalhydrophilic and biologically persistent TrOCs to the MF permeate. On23
the other hand,hydrophobicand readily biodegradableTrOCs were minimally detectedin24
bothMF andFO permeates,with no cleardifferencein theremovalefficienciesbetweentwo25
processes.26
Key words: Osmoticmembranebioreactor(OMBR); Forwardosmosis(FO); Microfiltration27
(MF); Traceorganicchemicals(TrOCs);Salinity build-up;28
3
1. Introduction29
Water reuseis an importantmeasureto tackle water scarcityand environmentalpollution,30
which arekey factorshamperingeconomicdevelopmentandthreating the nautralecosystem31
(Wintgens et al., 2008; Hochstrat et al., 2010). Safe and reliable water reuse requires32
adequateremovalof salts,nutrients,pathogenicagents,andtraceorganicchemicals(TrOCs)33
from the reclaimed effluent. TrOCs are a diverserangeof emergingorganic chemicals of34
eitheranthropogenicor natural origin. They occurubiqituouslyin munucipalwastewaterat35
concentrations in the rangeof a few nanogramsper lit re (ng/L) to severalmicrogramsper36
lit re (µg/L) (Luo et al., 2014). TheseTrOCspresentarguably the mostvexing challengeto37
practicalpotablewater reuse(Wintgenset al., 2008; Lampardet al., 2010; Dreweset al.,38
2013; Luo etal., 2014).39
Adequate removal of TrOCs is also essentialto facilitate water reuse for agriculture40
production. It has been demonstratedthat the occurrenceof pharmaceuticals, such as41
carbamazepineandtriclocarban, in reclaimedwastewater(Tanoueet al., 2012) andbiosolids42
(Wu et al., 2012) usedto grow fruits and vegetablescan bio-accumulatein ediblepartsof43
theseproduces.Therefore, a major technicalchallengefor the water industry is to develop44
new treatmentprocessesthat can reliably and cost-effectively removethese TrOCs during45
waterreuse.46
Recentefforts in wastewatertreatmentand reusehave led to the emergenceof a novel47
osmoticmembranebioreactor(OMBR) process(Achilli et al., 2009; Cornelissenet al., 2011;48
Nawazet al., 2013), which integratesforward osmosis(FO) with the conventionalactivated49
sludgetreatmenttechnology.In the OMBR system,the osmoticpressuredifferencebetween50
the mixed liquor and draw solution (e.g. NaCl) induceswater diffusion through a semi-51
permeable FO membrane. The FO membrane can effectively retain small organic52
contaminantsin the bioreactor, therebyfacilitating their subsequentbiodegradation(Alturki53
et al., 2013; Codayetal., 2014). Indeed,recentstudieshaveshowntheexcellentperformance54
of OMBR for TrOC removal, particularly the compoundswith relatively large molecule55
weightand/or featuredwith negativecharge(Alturki et al., 2012; Lay et al., 2012; Holloway56
et al., 2014). Thus,OMBR canpotentiallyproducehigh quality reclaimedwaterfor potable57
reuse,irrigation,or directdischargein environmentallysensitiveareas.58
Despitethepotentialof OMBR, salinitybuild-up in thebioreactorcausedby high rejectionof59
theFOmembraneandreversetransportof thedrawsolutionremainsa technicalchallengefor60
4
its further development(Van der BruggenandPatricia,2015). The high bioreactorsalinity61
canreducethedriving force for watertransport(Lay et al., 2010). Sludgecharacteristics and62
microbial community can also be altered with the elevated bioreactor salinity and63
subsequentlyworsenthe biological treatmentand membraneperformance(Qiu and Ting,64
2013). A shortsludgeretentiontime (SRT) is expectedto control the build-up of salinity in65
the bioreactor. However, in an OMBR systemwith an operatingSRT of 10 days, the66
bioreactorsalinity still increasedsubstantially, exertinginhibition on the microbial activity67
(Wanget al., 2014a). The shortSRT could alsoadverselyaffect the biological performance68
(Grelier et al., 2006) and increasethe cost for wastesludgedisposal.Severalstudieshave69
recentlyproposedthe integrationof an microfiltration (MF) or ultrafiltration (UF) process70
with OMBR to bleedout inorganicsaltsfrom the bioreactor(Holloway et al., 2014, 2015;71
Wanget al., 2014b). By applying theapproach,Holloway et al. (2014,2015)showeda stable72
operationof a pilot UFO-MBR treating raw domesticwastewaterover a period of four73
months. Removal to below the detection limit was reported for 15 out of 20 TrOCs74
investigatedin their study in 2014usinga pilot reverseosmosisprocessfor drawsolutionand75
cleanwaterrecoveries(Hollowayet al.,2014).76
Building uponthe existingliteratureon this topic, we aimedto evaluatethe performanceof77
an integratedosmoticandmicrofiltration membranebioreactor(O/MF-MBR) by specifically78
comparingpermeatequalities betweenthe FO and MF processes and examining sludge79
stability in the bioreactor.Thesystemperformancewasalsoassessedin termsof waterflux,80
bioreactorsalinity,andmembranefouling. TrOC removalwasrelatedto their hydrophobicity81
andmolecularstructures to mechanisticallyelucidatetheir fate within the integratedO/MF-82
MBR system.The interactionbetweenFO andMF in the integratedsystemwith regardsto83
thefateandremovalof TrOCswasalsodiscussed.84
2. Materials and methods85
2.1 Representativetraceorganicchemicals86
A stocksolutioncontaining30 representativeTrOCs(TableS1, SupplementaryData)were87
preparedin pure methanoland storedat -18 °C in the dark. The stock solution was used88
within less than a month. These TrOCs were selectedto representfour major groupsof89
chemicalsof emergingconcern– pharmaceuticaland personalcare products,endocrine90
disruptingcompounds, pesticides,andindustrialchemicals– thatareubiquitousin municipal91
wastewater. They have a diverserangeof properties,including hydrophobicity,molecular92
5
weight,andfunctionalgroups(TableS1,SupplementaryData). Hydrophobicityof anorganic93
compoundcan be measuredby Log D, which is the effective octanol-water partition94
coefficientata givensolutionpH (NghiemandColeman,2008). Basedon their Log D values95
at pH of 7, the selectedTrOCs can be classifiedas hydrophilic (i.e. Log D pH 7 < 3) or96
hydrophobic(i.e.Log D pH 7 > 3).97
2.2 FO andMF membranes98
A flat-sheet,cellulosebasedmembranesuppliedby HydrationTechnologyInnovations(HTI,99
Albany, USA) was usedin the FO process.The FO membraneis composedof a cellulose100
triacetate active (CTA) layer reinforced by a polyester mesh for mechanicalsupport101
(McCutcheonand Elimelech,2008). It is noteworthythat thin film composite(TFC) FO102
membraneswith embeddedpolyesterscreensupporthavealso beenreleasedby HTI and103
severalothermanufacturesin recentyears. Both CTA andTFC membraneshavetheir own104
positive attributes.Findingsfrom this study are specific to the OMBR processratherthan105
specificmembranepropertiesandthusapplicableto all typesof FOmembranes.106
A hollow fibre, polyvinylidene fluoride MF membranemodule from Mitsubishi Rayon107
Engineering(Tokyo, Japan) wassubmergedin thebioreactor.Theeffectivesurfaceareaand108
nominalporesizeof theMF membranewere740cm2 and0.4µm, respectively.109
2.3 Experimentalsystem110
The integratedO/MF-MBR systemusedin this study was composedof a cross-flow FO111
configuration,a submergedMF membranemodule,anda 10 L aerobicbioreactor(Fig. 1). An112
electricalair pump(Heilea,Ningbo,China)wasusedto continuouslyaeratethe reactorvia a113
coarsediffuser.A Masterflexperistalticpump(Cole-Parmer,VernonHills, USA) wasusedto114
draw permeatethrough the MF membranewith an operationon/off time of 14/1 min.115
Transmembranepressure(TMP) of theMF membranewascontinuouslymonitoredby a high116
resolution(±0.1kPa)pressuresensor(ExtechInstruments,Nashua,USA).117
A detaileddescriptionof the cross-flow FO configurationis availableelsewhere(Alturki et118
al., 2012). Briefly, theFOconfigurationcomprisedtwo semi-cellsmadeof acrylicplasticand119
a drawsolutiondeliveryandcontrol equipment.TheFO membranewasplacedbetweentwo120
semi-cellsto sealthefeedanddrawsolutionchannelswith a length,width, anddepthof 145,121
95, and 2 mm, respectively.The effective membranesurfaceareawas 138 cm2, with the122
active layer facing the feedchannel(i.e. FO mode).The mixed liquor in the bioreactorwas123
6
circulatedto the feedchannelby a Masterflexperistalticpump(Cole-Parmer,VernonHills,124
USA). On theotherside, a gearpump(Micropump,Vancouver,USA) wasusedto circulatea125
draw solution to the draw solution channel.The circulation flow rateof both the feed and126
draw solutionswas 1 L/min (i.e. a cross-flow velocity of 9 cm/s)monitoredby rotameters127
(Cole-Parmer, Vernon Hills, USA). The draw solution reservoirwas placedon a digital128
balance connectedto a computer. During the experimentalperiod, the draw solution129
concentrationwas kept constantby a conductivity controller equippedwith a conductivity130
probeanda Masterflexperistalticpumpto automaticallydosea concentrateddrawsolutionto131
the draw solution reservoir.The controller accuracywas 0.1 mS/cm (i.e. 0.05 g/L NaCl).132
Both theconcentratedandworking drawsolutionreservoirswereplacedon the samedigital133
balanceto avoidexperimentalerrorsby theconcentrationcontrolequipment.134
[FIGURE 1]135
2.4 Experimentalprotocol136
A submergedMF-MBR systemwasfirst initiatedto seedthebioreactorwith activatedsludge137
from the Wollongong WastewaterTreatmentPlant (Wollongong, Australia). The initial138
mixed liquor suspendedsolid (MLSS) concentrationin the bioreactorwasapproximately5139
g/L. Syntheticwastewaterwas used to simulate medium strengthmunicipal sewageand140
consistedof 100 mg/L glucose,100 mg/L peptone,17.5mg/L KH2PO4, 17.5mg/L MgSO4,141
10 mg/L FeSO4, 225 mg/L CH3COONa and 35 mg/L urea. The MF-MBR systemwas142
stabilized in a temperature-controlled room (22 ± 1 °C) at a working volume of 6 L, a143
hydraulicretentiontime (HRT) of 24 h, anda dissolvedoxygenconcentration(DO) of 5 ± 1144
mg/L. Comparedto a typical MBR system,a longerHRT wasusedin thisstudyto maintaina145
relativelylow waterflux to minimizethemembranefouling. Therelativelyhigh aerationrate146
of 8 L/min usedhere to preventsludgesettlementand scour the membranesurfacealso147
resultedin a higher DO concentrationthan that in a typical MBR system.No sludgewas148
wasted(exceptfor weeklysamplingof 90 mL mixed li quor) to systematicallyinvestigatethe149
build-up of salinity in the bioreactor.Stability of the bioreactorwas determinedby sludge150
production,biomassactivity, andremovalof organicmatterandnutrients.In practice,regular151
sludgewithdrawalcanalleviatesalinity build-up to someextent.152
Once stabilized, the cross-flow FO processwas connectedto the bioreactorto form an153
integratedO/MF-MBR system.At thesametime, theTrOC stocksolutionwasspikedto the154
influent to obtain5 µg/L of eachof the 30 compounds.The integratedsystemwasoperated155
7
continuouslyfor 60 daysundertheconditionsasmentionedabove.To minimizethebiosolids156
blockage in the narrow feed channel of the cross-flow FO system, the initial MLSS157
concentrationin thebioreactorwasadjustedto 2 g/L. Giventheunstablewaterflux of theFO158
process,thepermeateflux of MF wasadjusteddaily to maintaina constantHRT of 24 h. The159
drawsolutionandconcentrateddrawsolutionwere58.5and351g/L NaCl, respectively. The160
drawsolutionwasreplacedeveryday to avoidoverflow andcontaminantaccumulation.The161
concentrateddraw solution was also addedmanuallyon a daily basis.Membranecleaning162
wasnot conductedduringthis study.163
2.5 Analyticalmethods164
Total organiccarbon (TOC) andtotal nitrogen(TN) of theinfluent,mixed liquor supernatant,165
MF and FO permeateswere analysedusing a TOC/TN-VCSH analyser(Shimadzu,Kyoto,166
Japan). Orthophosphate(PO43-) was measuredby a Flow Injection Analysis system167
(QuichChem8500,Lachat,USA). MLSS andmixedliquor volatile suspendedsolid (MLVSS)168
concentrationswere determinedfollowing the StandardMethods for the Examinationof169
WaterandWastewater.Specific oxygenuptakerate(SOUR)of the sludgewastested based170
on the techniquedescribedby Choi et al. (2007). Mixed liquor pH and conductivity were171
measuredusing an Orion 4-Star Plus pH/conductivitymeter(ThermoScientific, Waltham,172
USA).173
TrOC concentrationsin the feed,mixed liquor supernatant,MF permeate,anddrawsolution174
were determinedweekly using an analytical methoddescribedby Hai et al. (2011). The175
methodinvolved solid phaseextractionand derivation, followed by gas chromatography-176
massspectrometry(GC-MS) analysisusinga ShimadzuGC-MS system(Kyoto, Japan).177
In this study,the MF andFO processeswereoperated simultaneously to extractwaterfrom178
the bioreactor.Permeatesamples could thusbe obtainedseparatelyfrom the MF-MBR and179
OMBR channels(i.e. bioreactor-MF and -FO streams, respectively). Against the feed180
contaminantconcentration,the removal efficiency through the MF-MBR channel was181
definedas:182
%100)1(e
���edF
MF
CC
R (1)183
where, CFeed and CMF were contaminantconcentrationsin the feed and MF permeate,184
respectively.Unlike the MF process,contaminants permeatedthrough the FO membrane185
8
were diluted by the draw solution. The dilution factor (DF) was calculatedusing a mass186
balance:187
FO
DS
VV
DF � (2)188
where,VDS and VFO were draw solution and FO permeatevolumesuntil samplingtime. As189
notedabove,to avoid solution overflow and contaminantaccumulation,the draw solution190
wasreplacedeveryday. Thus,the overall removalthroughthe OMBR channelwasdefined191
as:192
%100)1(eed
��� DFCC
RF
DS (3)193
whereCDS was contaminantconcentrations in thedrawsolutionreservoir.194
In this study, TrOC accumulationin biosolidswas not consideredfor removalassessment195
becauseonly compoundsin the aqueousphasecould transportthrough the MF and FO196
membranes.It is alsonoteworthythatTrOC removalhereonly indicatesthedisappearanceof197
parent moleculesbut not necessarilycompletemineralization.Indeed, biodegradationof198
certain TrOCs would produce stable intermediates/metabolites in the bioreactor and199
permeates.However,detaileddiscussionof theseaspectsis beyondthescopeof this study.200
3. Resultsand discussion201
3.1 Processperformance202
3.1.1 Salinity build-up, waterflux, andmembranefouling203
The integrationof the MF membraneinto OMBR preventedthe build-up of salinity in the204
bioreactor, becausedissolvedinorganic salts were readily permeablethrough the micro-205
porous membrane(Fig. 2). After a small increasein the first week, the mixed liquor206
conductivity stabilizedat approximately700 µS/cm (i.e. a salinity of 0.4 g/L NaCl). The207
result comparesfavourably with our previousstudy where a rapid increasein the mixed208
liquor conductivity from 268 to 8270 µS/cm was observedwithin sevendays using the209
similar experimentalconfiguration and conditions without housing the submergedMF210
membranein thebioreactor(Alturki etal., 2012).211
[FIGURE 2]212
Two distinctstagesof waterflux declinecouldbeobservedin theFO processwith time (Fig.213
2). The water flux decreasedrapidly from 6.5 to 3.4 L/m2h within the first week mainly214
9
becauseof salinity build-up in thebioreactorandmembranefouling. With thedecreasein the215
bioreactorsalinity, the waterflux of the FO processdecreasedslightly andthenstabilizedat216
approximately1.7 L/m2h from day 45 onward. The elevatedsalinity could increasethe217
osmotic pressurein the mixed liquor side and thus reduce the driving force for water218
transport.On theotherhand,high salinity could leadto doublelayercompressionandreduce219
electrostaticinteractionamong the macromoleculefunctional groups,resulting in a thicker220
and more compactfouling layer (Nghiem et al., 2005). Indeed,a thick cake layer was221
observedon themembranesurfaceat a feedcross-flow velocity of 9 cm/sin this study(Fig.222
S1,SupplementaryData).The fouling layer could increasethe hydraulicresistanceto water223
permeationand cause severe concentrationpolarization adjacent to membranesurface,224
therebyreducingthewaterflux (HoekandElimelech,2003; Boo et al.,2012).225
It is noteworthythat the stablewaterflux of approximately1.7 L/m2h wasmuchlower than226
that observedby Holloway et al. (2015). The different flux behavioursbetweenthe two227
studiescouldbeattributedto thedifferencein hydrodynamicsadjacentto membranesurface228
betweenthesubmergedandcross-flow FOsystems.In ourcross-flow FOsystem,particulates229
in mixed liquor wereproneto adhereto the membranesurfacein the narrowfeedchannel,230
particularlyata low feedcross-flow velocityof 9 cm/s.231
The TMP value of the MF membraneonly increasedto 5 kPa (0.05 bar) by the endof the232
experiment(Fig. S2, SupplementaryData), indicating a negligible membranefouling. The233
low membranefouling could be attributedto the small water flux and high aerationrate234
appliedin this study.Over 60 daysof experiment,the water flux of MF wasadjustedfrom235
1.6 to 2.6 L/m2h. By consideringthe gradual flux decline in the FO process,this flow236
adjustmentwas necessaryto keepa constantHRT of 24 h during the entire experimental237
period.On theotherhand,the low MLSS concentrationin thebioreactor(2 – 3.3 g/L) could238
alsominimizethemembranefouling.239
3.1.2 Biological performance240
Biological performanceof the integratedO/MF-MBR systemwas assessedwith regardsto241
the removal of basic contaminants(i.e. TOC, TN, and PO43--P), sludgeproduction,and242
biological activity. The removalof basiccontaminantswasstableafter a short-term salinity243
build-up in the bioreactor(Fig. 3). The stableremovalcanalsobe determinedby the small244
standarddeviation of thesecontaminantconcentrationsin different units of O/MF-MBR245
during thecourseof theexperiment(TableS2,SupplementaryData).246
10
Dueto thehigh rejectionof theFOmembrane,permeatequality of FOwassuperiorto thatof247
MF, particularly in termsof TN andPO43--P concentrations(Fig. 3). The removalof TOC248
from theOMBR channelwasover98%during the entireexperimentalperiod(Fig. 3a).The249
result is consistentwith that reportedby Hancocket al. (2013). Given the excellentremoval250
of TOC from the bioreactor (indicated by low TOC concentrationin the mixed liquor251
supernatant),the benefitsof FO over MF werenot significant.However,the removalof TN252
throughthe MF-MBR channelonly varied in the rangeof 20 – 65%, with relatively high253
concentrationin the permeate(Fig. 3b). Since the removal of TN in aerobicbioreactors254
occursmainly via assimilationto the biomass(Hai et al., 2014), it was not surprisedto255
observethe relatively low andunstableremoval.By contrast,TN removalfrom the OMBR256
channelrangedfrom 60 to 90%, althoughtherewasa smalldeclinefrom day40 onward.This257
declinewaslikely dueto theincompleterejectionof NH4+-N andaccumulatedNOx
--N by the258
FO membrane(Irvine et al., 2013; Luo et al., 2015). A small andvariableremovalthrough259
the MF-MBR channelwas also observedfor PO43- -P (Fig. 3c), possibly due to the low260
biomassassimilationand/or phosphorusprecipitationunderthe nearlyneutralpH condition261
in thebioreactor(Qiu andTing, 2014). Nevertheless,PO43--P couldnot bedetectedin theFO262
permeate.Indeed,the FO membranecanalmostcompletelyretainPO43- -P dueto the large263
hydratedradiusandnegativechargeof theorthophosphateions(Holloway et al.,2007).264
[FIGURE 3]265
The MLSS concentrationgradually increasedwith time after a slight decreasein the first266
week(Fig. 4). Thesmall decreasein the MLSS concentrationat the beginningwaspossibly267
due to the inhibitory effects of the elevatedbioreactorsalinity on microbial mass. This268
inhibition wasalsoevidencedby a reductionin biomassactivity asindicatedby theSOURof269
the sludge (Fig. S3, SupplementaryData). With the bioreactorsalinity stabilizing at a270
relatively low level (0.4 g/L NaCl), the sludgeconcentrationin the bioreactorincreased271
graduallywith theMLVSS/MLSSratio of 0.75± 0.05from day7 onward.At thesametime,272
theSOURof thesludgealsoincreasedandsubsequentlylevelledoff at 4.5mg O2/g MLVSS273
h. This stableSOURvalueis in goodagreementwith thatreportedpreviouslyin conventional274
MBRs(Hanetal., 2005; Choi et al.,2007).275
[FIGURE 4]276
3.2 Removalof traceorganicchemicals277
11
The removal of most TrOCs selectedhere was stable during the entire courseof the278
experiment (Fig. 5). There are only six exceptions,namely, clofibric acid, atrazine,279
carbamazepine,propoxur,diclofenacand fenoprop. The removalof thesesix compoundsis280
shownas a function of time in Fig. 6. During biological treatment,TrOC removalcan be281
evaluatedusinga qualitativepredictiveframeworkdevelopedby Tadkaewet al. (2011)based282
on their molecularproperties,suchashydrophobicityandfunctionalgroups.Accordingto the283
scheme,TrOCs investigatedin this studycould be generallyclassifiedas hydrophobic(i.e.284
Log D pH 7 > 3) or hydrophilic(i.e.Log D pH 7 < 3) (section2.1).285
[FIGURE 5]286
[FGIURE 6]287
3.2.1HydrophobicTrOCs288
Of 30 TrOCsselectedin this study,all elevenhydrophobiccompoundscould be effectively289
removed(> 85%) from both OMBR andMF-MBR channels(Fig. 5). Previousstudieshave290
demonstratedtheexcellentremovalof thesehydrophobicTrOCs duringbiological treatment291
(Radjenovi� et al., 2009; Nguyen et al., 2012). Due to the high hydrophobicityof these292
compounds,they can easily absorb on the activatedsludge and thereby facilitate their293
biodegradation(transformation)in the bioreactor(Tadkaewet al., 2011). As a result,apart294
from bisphenolA andoctocrylene,therewasno cleardifferencein theconcentrationof these295
hydrophobicTrOCs betweenthe MF and FO permeates(Fig. 5). It is noteworthy that296
bisphenolA andoctocryleneconcentrationsin theFO permeatewerehigherthanthosein the297
MF permeate.Their high concentrationsin the FO permeatewere possibly due to cake-298
enhancedconcentrationpolarizationcausedby the foulant layer on the membranesurface299
(Vogel et al., 2010). These two compoundsare hydrophobic.Thus, their accumulation300
adjacentto the membranesurfacedue to cake-enhancedconcentrationpolarizationcould301
enhancetheir transportacrosstheFO membranevia hydrophobicinteractions(Nghiemet al.,302
2004). Furtherstudiesarenecessaryto ascertainthe effectsof the sludgecakelayer on the303
rejectionof TrOCs,particularlythehydrophobiccompounds,in theFOprocess.304
3.2.2HydrophilicTrOCs305
Significantvariationin theremovalof hydrophilicTrOCswasobservedfrom bothMF-MBR306
andOMBR channels. By accountingfor therelativelylargeporesof theMF membrane, their307
removalthroughtheMF-MBR channelwasmainly governedby theactivatedsludge.Indeed,308
previous studieshave shown a large variation in the removal of hydrophilic TrOCs in309
12
conventionalMBRs, which was determinedby their intrinsic biodegradabilitydue to their310
weakadsorptionontobiosolids(Tadkaewet al., 2011). In this study,the removalof six very311
hydrophilic TrOCs(i.e. Log D pH 7 < 1), including salicylic acid,metronidazole,ketoprofen,312
naproxen,primidone, and ibuprofen,was higher than 85% throughthe MF-MBR channel.313
The excellent removal of thesecompoundscould be attributedto the presenceof strong314
electrondonatingfunctionalgroups,suchasamineandhydroxyl groups,in their molecular315
structures(Table S1). Containingthesefunctional groupsallowed compoundseasily to be316
electrophilically attacked by oxygenasesfrom the aerobicbacteria.The oxygenasesarekey317
reactantsresponsiblefor biodegradationof organic compounds(Kanazawaet al., 2003;318
Tadkaewet al., 2011). Sincethesehydrophilic TrOCscould be effectively removedin the319
bioreactor,the benefitsof FO over MF were not significant (Fig. 5). It is noted that the320
removalof salicylic acid from the OMBR channelwasslightly lower than that throughthe321
MF-MBR channel. Theexactreasonis still unclearbut it couldbeattributedto theeffectsof322
cake-enhancedconcentrationpolarizationin theFOprocessasnotedabove.323
Dueto thehigh rejectionof the FO membrane,the removalthroughtheOMBR channelwas324
moreeffectivethanthat from the MF-MBR channelfor the six hydrophilicTrOCsshownin325
Fig. 6. The removalof thesecompoundswaslow andhighly variablethrough theMF-MBR326
channelbecauseof their resistanceto biodegradation.Tadkaewet al. (2011)haveattributed327
their low biodegradationto the presenceof one or more strong electron withdrawing328
functionalgroup(e.g.chlorine,amideandnitro groups)and/ortheabsenceof strongelectron329
donatingfunctional groupsin their molecularstructures. Despitethe low removalof these330
compoundsin the bioreactor,their high rejection by the FO membraneensuredexcellent331
removalfrom theOMBR channel.Thebenefitsof theFOmembranefor TrOC rejectionhave332
alreadybeenhighlightedin severalrecentstudies(Alturki et al.,2013; Codayet al., 2014).333
With the exceptionof clofibric acid, the rejection of thesehydrophilic and biologically334
persistentTrOCsby theFO membraneincreasedtheir permeationthroughtheMF membrane335
andthusreducedtheremovalby theMF-MBR channel(Fig. 6). Theremovalof clofibric acid336
via the MF-MBR channel gradually increasedwith time, althoughsomefluctuationswere337
observed. The reasonfor this phenomenonis not clear, possibly due to an enhanced338
biodegradationwith the increasedMLSS concentrationin the bioreactor(Cirja et al., 2008).339
Of six biologically persistentcompoundsnotedabove,theremovalof atrazineby the OMBR340
channel was also observed to decreasegradually with time. Atrazine has moderate341
hydrophobicity(Log DpH 7 = 2.6), andthusthe observedlow andreducedremovalcould be342
13
attributed to its adsorptionand partitioning into the membranesurface followed by a343
diffusion throughthemembrane(Nghiemetal., 2004).344
4. Conclusion345
This studycomparedthe waterquality of the FO andMF permeatesin an integratedO/MF-346
MBR systemregardingtheconcentrationof TOC,TN, PO43--P andTrOCs.TheFO permeate347
had a higher water quality than the MF permeatedue to the effective rejectionof the FO348
membrane.Theconcentrationof hydrophobicTrOCsandhydrophiliccompoundscontaining349
strongelectrondonatingfunctional groupswas low in both MF and FO permeatesas they350
could be well removedby the activatedsludge.However,the concentrationof hydrophilic351
and biologically persistentTrOCs which containedstrongelectronwithdrawing functional352
groupsin theFO permeatewasmuchlower thanthat in theMF permeate. In addition,dueto353
the high rejectionof the FO membrane,thesehydrophilic andbiologically persistentTrOCs354
couldaccumulatein the bioreactorandbe transferredinto theMF permeate.Thus,thewater355
flux ratio betweenMF andFO canbeoptimisedto reducesalinity build-up in thebioreactor356
while ensuringadequateMF permeatequality.357
5. Acknowledgement358
This researchwassupportedunderAustralianResearchCouncil’sDiscoveryProjectfunding359
scheme(project DP140103864).The authorswould like to thank the ChineseScholarship360
CouncilandtheUniversityof Wollongongfor thePhDscholarshipsupportto WenhaiLuo.361
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18
LIST OF CAPTIONS
Fig. 1: Schematicdiagramof a laboratory-scaleintegratedO/MF-MBR hybrid system.Draw
solution was replaceddaily to avoid overflow and contaminantaccumulationin the draw
solution reservoir.High concentrateddraw solution was addedmanuallyon a daily basis.
Sampleswere taken from feed, bioreactor,MF permeate,and draw solution reservoirfor
analysis.
Fig. 2: Variation of mixed liquor conductivity and FO water flux with time. Experimental
conditions:HRT = 24 h; DO concentration= 5 ± 1 mg/L; draw solution = 58.5 g/L NaCl;
cross-flow rate= 1 L/min (i.e. cross-flow velocity = 9 cm/s);FO mode;temperature= 22 ± 1
°C. Waterflux of MF wasadjustedfrom 1.6 to 2.6 L/m2h to compensatethe flux declineof
FOto keepaconstantbioreactorworkingvolumeandHRT.
Fig. 3: Removalof (a) TOC, (b) TN, and(c) PO43--P by OMBR andMF-MBR channelsof
theintegratedO/MF-MBR system.
Fig. 4: Variationof biomassconcentrationin thebioreactorwith time.
Fig. 5: MeasuredTrOC concentrationsin the feed,MF andFO permeates,andtheir removal
by MF-MBR andOMBR channelsof an integratedO/MF-MBR system.Error barsrepresent
thestandarddeviationof eightmeasurements(onceaweek).
Fig. 6: Time-dependentremovalof six hydrophilic and biologically persistentTrOCs (i.e.
diclofenac,atrazine,carbamazepine,propoxur,fenopropandclofibric acid) via MF-MBR and
OMBR channelsof theintegratedO/MF-MBR system.
19
Peristaltic pump
Computer
High Conc.tank
Biological reactor
Circulating pump
Flowmeter
Flowmeter
Balance
FO membrane cell
Feed Drawsolution
MF
MF permeate
Mixed liquor
Cond. controller
Peristaltic pump
Peristaltic pump
Figure 1
20
0 10 20 30 40 50 600
200
400
600
800
1000
1200
Mixedliquorconductivity(�S/cm)
Time (d)
Mixed liquor conductivity
0
1
2
3
4
5
6
7
FO water flux
FOwaterflux(L/m
2 h)
Figure 2
21
0
2
4
6
20
40
60
80
100
0
5
10
15
20
25
30
35
40
0 10 20 30 40 50 600
2
4
6
8
10
12
14
(a)
(b)
(c)
Concentration in: Feed Mixed liquor supernatantMF permeate FO permeate
Removal by: MF-MBR channel OMBR channel
TOC(mg/L)
0
20
40
60
80
100
Removal(%)
TN(mg/L)
0
20
40
60
80
100
Removal(%)
PO
43--Pconcentration(mg/L)
Time (d)
0
20
40
60
80
100
Removal(%)
Figure 3
22
0 10 20 30 40 50 600.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Biomassconcentration(g/L)
Time (d)
MLSSMLVSS
0.0
0.2
0.4
0.6
0.8
1.0
MLVSS/MLSS ratioMLVSS/MLSSratio
Figure 4
23
Salicy
licac
id
Clofibr
icac
id
Metro
nidaz
ole
Fenop
rop
Ketopr
ofen
Napro
xen
Primido
ne
Ibupr
ofen
Propo
xur
Diclof
enac
Enter
olacto
ne
Carba
mazep
ine
Gemfib
rozil
Amitri
ptyli
ne
DEET
Estriol
Atrazin
e
Pentac
hloro
phen
ol
Ametryn
Benzo
phen
one
4-ter
t-But
ylphe
nol
Estron
e
Bisphe
nolA
Oxybe
nzon
e
17-e
thyny
lestra
diol
17-e
strad
iol
-Estr
adiol
-17-
acet
ate
4-ter
t-Octy
lphen
ol
Triclos
an
Octocr
ylene
0
1000
2000
3000
4000
5000Hydrophobic (Log D > 3)Hydrophilic (Log D < 3)
�
Concentration(ng/L)
Concentration in: Feed MF permeate FO permeate
0
20
40
60
80
100
�
�
Removal by: MF-MBR channel OMBR channel
Removal(%)
-1
0
1
2
3
4
5
6
7Log D
LogDatpH=7
Figure 5
24
0
20
40
60
80
100
0
20
40
60
80
100
0 10 20 30 40 50 600
20
40
60
80
100
0 10 20 30 40 50 60
Diclofenac MF-MBR channel OMBR channel
Removal(%)
Atrazine
Carbamazepine
Removal(%)
Propoxur
Fenoprop
Removal(%)
Time (d)
Clofibric acid
Time (d)
Figure 6
The role of forward osmosisand microfiltration in an integrated
osmotic-microfiltration membranebioreactor system
SupplementaryData
Revisedmanuscript submitted to Chemosphere
April 2015
WenhaiLuo a, FaisalI. Hai a, JinguoKangb, William E. Priceb, LongD. Nghiema*,
MenachemElimelechc
a StrategicWaterInfrastructureLaboratory, Schoolof Civil Mining andEnvironmental
Engineering,Universityof Wollongong,Wollongong,NSW2522,Australia
b StrategicWaterInfrastructureLaboratory,Schoolof Chemistry,Universityof Wollongong,
Wollongong,NSW2522,Australia
c Departmentof ChemicalandEnvironmentalEngineering,YaleUniversity,NewHaven,CT06520-8286,UnitedStates
* Correspondingauthor:longn@uow.edu.au; Ph:+61(2) 42214590.
TableS1:Physicochemicalpropertiesof theselectedtraceorganicchemicals.
CompoundsChemicalformula
Log Dat pH = 7
MW(g/mol)
Chemicalstructure
Salicylicacid C7H6O3 -1.13 138.1
Clofibric acid C10H11ClO3 -1.06 214.6
Metronidazole C6H9N3O3 -0.14 171.2
Fenoprop C9H7Cl3O3 -0.13 269.5
Ketoprofen C16H14O3 0.19 254.3
Naproxen C14H14O3 0.73 230.3
Primidone C12H14N2O2 0.83 218.3
Ibuprofen C13H18O2 0.94 206.3
Propoxur C11H15NO3 1.54 209.2
Diclofenac C14H11Cl2NO2 1.77 296.2
Enterolactone C18H18O4 1.89 298.33
Carbamazepine C15H12N2O 1.89 236.3
Gemfibrozil C15H22O3 2.07 250.3
Amitriptyline C20H23N 2.28 277.4
DEET C12H17NO 2.42 191.3
Estriol C18H24O3 2.53 288.4
Atrazine C8H14ClN5 2.64 215.7
PentachlorophenolC6HCl5O 2.85 266.4
Ametryn C9H17N5S 2.97 227.3
Benzophenone C13H10O 3.21 182.2
4-tert-Butylphenol C10H14O 3.4 150.2
Estrone C18H22O2 3.62 270.4
BisphenolA C15H16O2 3.64 228.3
Oxybenzone C13H10O 3.89 228.2
17� -ethynylestradiol
C20H24O2 4.11 296.4
17� -estradiol C18H24O2 4.15 272.4
� -Estradiol 17-acetate C20H26O3 5.11 314.4
4-tert-Octylphenol C14H22O 5.18 206.3
Triclosan C12H7Cl3O2 5.28 289.5
Octocrylene C24H27N 6.89 361.5
Source:SciFinderScholar(ACS)database.
TableS2: Basicwaterquality in differentunitsof O/MF-MBR (average± standard
deviation*)
WaterparametersContaminantconcentration(mg/L)
Feed Mixed liquor supernatant MF permeate FOpermeateTOC 71.4± 9.6 2.7± 1.2 2.2± 1.0 1.7± 0.8TN 18.3± 4.9 14.3± 4.3 12.2± 4.1 5.3± 3.5
NH4+-N 10.5± 1.7 2.5± 1.4 1.0± 0.4 0.6± 0.3
PO43--P 10.9± 1.1 9.1± 1.5 8.9± 1.6 0.0± 0.0
*Standarddeviationwascalculatedfrom 20 measurements(onceevery3 days).
Fig. S1: Photographof the FO membranesurfaceat the conclusionof the experiment.
Membranecleaningwas not conducted.Experimentalcondition: CTA-FO membrane;FO
mode;draw solution = 1 M NaCl; cross-flow rate = 1 L/min (i.e. cross-flow velocity = 9
cm/s);HRT = 24 h; DO concentration = 5 ± 1 mg/L; temperature= 22 ± 1 °C; MF waterflux
= 1.6– 2.6mL/min.
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