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Biochemical
Engineering
Journal
j ou rn a l h o m ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / b e j
Regular
article
The
role
of
EPS
in
the
foaming
and
fouling
for
a
MBR
operated
in
intermittent
aeration
conditions
Riccardo
Campo
a,
Marco
Capodici
b,
Gaetano
Di
Bella
a,∗,
Michele
Torregrossa
b aFacoltàdiIngegneriaeArchitettura,UniversitàdiEnna“Kore”,CittadellaUniversitaria,94100Enna,ItalybDipartimentodiIngegneriaCivile,UniversitàdegliStudidiPalermo,Ambientale,Aerospaziale,deiMateriali,ScuolaPolitecnica,VialedelleScienzeed8,
90128Palermo,Italy
a
r
t
i
c
l
e
i
n
f
o
Articlehistory: Received18April2016
Receivedinrevisedform29August2016 Accepted16November2016
Availableonline18November2016 Keywords: IA-MBR Filamentousbacteria Foaming Fouling EPS
a
b
s
t
r
a
c
t
ThisworkinvestigatesthecausesoffoamingandfoulinginanIntermittentAerated–Membrane BioRe-actor(IA-MBR)usedforwastewatertreatment.Theexperimentwasdividedintothreeperiodswith differentaeratedregimesexpressedwithdifferenttaeration/tcycleratio(PeriodI:60min/180min,PeriodII: 80min/180min,PeriodIII:30min/90min).
Theadvancedfoamingtestsusedallowthestudyoftheroleofextracellularpolymericsubstances (EPSs)onfoamingandfouling.Ingeneral,inthePeriodsIandII,goodcorrelationsbetweentheEPSs andtheModifiedScumIndexwithoutpurification(MSI0)andtheFoamPowerwereobserved.Theresults shownthatthefilamentousmicroorganisms,whichmainlygrewinthePeriodIII,playalsoakeyfactorin thefoamingandfouling.Inparticular,whenanetproliferationoffilamentousbacteriaoccurred(during periodIIIwithahighernumberofcyclesperday)bothEPSsconcentrationandfilamentousabundance influencethemodifiedscumindexobtainedaftertwopurificationsteps(MSI2).Finally,theprocessing datashowthattheEPSshamperedalsothemembranefiltrationbutimprovethepre-filtrationactionof cakelayer.Ontheotherhand,theco-presenceofEPSsandfilamentousbacteriareducedtheeffectocake layeraspre-filter.
©2016ElsevierB.V.Allrightsreserved.
1. Introduction
Membrane Bioreactors (MBRs) represent nowadays a
well-consolidatedadvanced available technologyin orderto comply
withstringentmandatoryrequirementsintermsofquality
stan-dardsof treated wastewater.Indeed, MBRsystems guarantee a
highereffluentqualityalsointermsofpathogenicbacteriaremoval,
alowerecologicalfootprintandalowersludgeproductionif
com-paredtoaconventionalactivatedsludge(CAS)system.However,
thegreatestdownsideofMBRsisrepresentedbytheoccurrence
offouling andfoaming phenomena[1].Membrane fouling
con-sistsof thedepositionof particlesonthemembrane surfaceor
colloidalparticleswithintheporesofthemembrane.Thetangible
consequence of fouling is representedby an increase of
trans-membranepressure(TMP)yieldingareductionofpermeabilityand
thusadecreasingeffluentflow[1].Fourgroupsoffactorsaffect
membranefouling:membrane materials,mixedliquor features,
influentwastewatercompositionandoperatingconditionssuchas
∗ Correspondingauthor.
E-mailaddress:gaetano.dibella@unikore.it(G.DiBella).
sludgeretentiontime (SRT),hydraulicretentiontime(HRT)and
foodtomicroorganismratio(F/M)[2–4].Particularly,the
mem-branefoulingisstronglyinfluencedbymixedliquorfeatures.More
indetail,amongothersparameters,sludgeviscosity,mixedliquor
suspendedsolids(MLSS)concentration,presenceandabundanceof
filamentousbacteria,extracellularpolymericsubstances(EPSs)and
solublemicrobialproducts(SMPs)concentrationsarerecognized
tobethekeyfactorsaffectingthemembranefouling[5].Despite
manyauthorsoutlinedthatmembranefoulingispromotedbyhigh
EPSconcentrations[6,7],thereisstilluncertaintywhetherfouling
resultsmainlyinfluencedbyproteinsorpolysaccharidesfractionof
EPSandSMP.Suchuncertaintyislikelyduetodifferencesin
exper-imentalmethodsavailableforEPScontentassessmentandalsoto
thedifferentconfigurationofMBRtreatmentplants[3,8].
Ontheotherhand,thefoamingrepresents,aswellasfouling,
anotherdownsideofMBRsthatcanleadtoasignificantdecreasing
ofbiologicalperformancesofMBRsandalsotoadecreasein
mem-branefiltrationefficiency[9].Foamingarisesasaviscouslayeron
thesurfaceofwastewatertreatmentplantreactors;itresultsin
afloatingbiomassthatcancauseseriousoperationalproblemsto
thetreatmentprocess.In particular,itispossibletodistinguish
two differentkindof foaming:biologicalfoamingand chemical
http://dx.doi.org/10.1016/j.bej.2016.11.012
foaming.Thebiologicalfoamingtriggerfactorisrecognizedtobe
theunbalancedF/Mratio;whilethechemicalfoamingisattributed
toa loadingshock caused by thepresence of synthetic
surfac-tants[10].Amongthevariouskindofmicroorganismstraceable
intheactivatedsludge,a significantpresenceof Microthrix
par-vicellaandNocardioformshasbeenobservedusingconventional
microscopyduringbiological foamingoccurrence.For such
rea-son,theaforementionedmicroorganismshavebeendefined“foam
formingbacteria”[11].Furthermore,otherstudiesoutlinedalsothe
presenceofotherbacteriaspeciesduringfoamingoccurrence.For
thisreason,bacterialstrainssuchastheEikelboomType0092have
beendefined“notfamousfoam-formingbacteria”[9,11].
DespitethefoaminginCASisattributedtotheabundanceof
theaforementionedstrains,ithastobestressedthatfoamingin
MBRsystemscanalsooccurintheabsenceof“foam-forming”or
“notfamousfoam-forming” microorganisms. In these casesthe
foamabundancewasattributedtotheconcentrationoftheprotein
fractionofEPS[12].However,bothEPSsandfilamentousbacteria
influencethesludgehydrophobicityandthereforeimpacton
foam-ingoccurrence. Although thefoaming occurrence canseriously
affectthecorrectworkingofaMBR,studiesdealingwithfoamingin
MBRsarescarce[9,12,13].Furthermore,foamingoccurrencecanbe
encouragedbytheimplementationofnewmanagementstrategies
suchastheIntermittentAeration(IA).Althoughsuchprocesscan
offerseveraladvantages[14,15],theintermittentaerationimpact
onmicroorganismscanresultinaproliferationoffoam-forming
microorganismssuchasfilamentousbacteria.Indetails,theIA
pro-cessconsistsofalternatingaerobicandanoxicphaseswithinthe
samestirred reactor.The implementationofsuchmanagement
strategyallowsachievingtheammonia oxidationaswellasthe
nitratereductionsavingenergy.Indeed,theairsupplynecessary
tothebioreactorresultdecreasedandalsotheabsenceofreturn
activatedsludgerichinnitratesfromnitrificationtodenitrification
yieldtheIAacosteffectivesolution[15].However,thepresence
oftheanoxicphasecanseriouslyimpactinthestrains
develop-mentfavoringtheproliferationoffoam-formingmicroorganisms
likefilamentousbacteria[14].Bearinginmindtheprevious
obser-vations,and considering that extensivestudies onIntermittent
Aerated-MembraneBioReactor(IA-MBR)arelacking,thepresent
studyaimstoinvestigatethefoulingandfoamingmechanismsinan
IA-MBRsystemtreatingdomesticwastewater.Specificfoamtests
andadetailedfoulingtendencyassessmenthavebeencarriedout
inordertooutlinetherootsofbothphenomenainIA-MBRsystems.
Furthermore,adetailedinvestigationfocusedtohighlightthe
influ-enceexertedbyotherparameters,suchasEPSandSMPfractions
andconcentration,sludgeviscosity,TSSconcentrationand
filamen-tousmicroorganisms’abundancehasbeenreported,improvingthe
knowledgeaboutthesetwodrawbacksofthisnewandinnovative
MBRconfiguration.
2. Materialandmethods
2.1. Experimentalsetup
ThepilotplantwasbuiltatPalermomunicipalwastewater
treat-mentPlantof“AcquadeiCorsari”anditwasthesameasthatused
byCapodicietal.[15].Inparticular,theIAreactor(400L),theMBR
(50L)tankandtheOxygenDepletionReactor(ODR)(28L),mainly
composedthepilotplant.Thefirstonewasprovidedwith
aera-tionandstirringdevices;theMBRreactorwasdownstreamtheIA
reactoranditwascontinuouslyaeratedinordertomitigatethe
membranefouling;theODRreactorwascontinuouslystirredin
ordertoavoidTSSsettling.AlogicalschemeofthePilotplantis
showninFig.1.
Fig.1. PilotPlantLay-out.
Throughoutalltheexperimentalperiod,thefollowingmost
rel-evantoperationalparametersweremeasured:DissolvedOxygen
(D.O.),pH,OxidationReductionPotential (O.R.P.).Theywereall
monitoredon-linebymeansofprobesappropriatelylocatedinthe
differentsectionsofthepilotplant[15].
Thetemperaturewasnot regulatedandwasaffectedbythe
environmentalconditions(roomtemperature):however,the
vari-ationwasrelativelysmall,changingfrom18to24◦Cduringthe
experimentalperiod.
TheMBRcompartmentwasequippedwithtwoultrafiltration
(UF)hollowfiberGEZenonZW10®modules(nominalsurfaceof
singlemembrane unit:0.93m2; pore dimension:0.04m)in a
submergedconfiguration.
Thedurationsofsuctionandbackwashweresetto9and1min,
respectively,withatotalfiltrationcycleof10min.Theplantwas
fedcontinuouslyandthepermeatefluxfromeachmembranewas
equalto20–23Lm−2h−1;whilethenetpermeatefluxwasequal
to17–19Lm−2h−1.
Thetransmembranepressure(TMP)wasmeasuredbymeans
of analogical vacuum-meters connected to the upper heads of
the membrane modules. TMP data were acquired with two
data-loggers connected tothevacuum-meters,stored and then
post-processed.
Theexperimentalcampaign,whichlasted120days,wasdivided
intothreeperiodswithdifferentaerationregimes[15].Indetails,
Period I (lasted 57days) had a 180min cycle length (tcycle)
with60minofaeration (taeration)and120minwithoutaeration
(taeration/tcycle=60/180).InperiodII(lasted47days)thecyclelength
wasthesameasthatofperiodIandthetaeration/tcycleratiowas
equalto80/180.Finally,periodIII(lasted16days)was
character-izedwithacyclelengthof90minwithtaeration/tcycle=30/90.During
thenot-aeratedphase,themixedliquorwascontinuouslymixedto
avoidSuspendedsolidssettling.Themainoperationalparameters
arereportedinTable1.
Biochemicaloxygendemand(BOD5),chemicaloxygendemand
(COD), soluble COD, ammonium (NH4-N), nitrate ammonium
(NO3-N),nitriteammonium(NO2-N),totalsuspendedsolids(TSS)
andvolatilesuspendedsolids(VSS)weremeasuredthreetimesper
weekinordertoevaluatethepilotplantperformance,accordingto
StandardMethods[16].
2.2. Foamingtestandfilamentousinvestigations
Proposedfoamtestsandcharacterizationscanbecategorized
as:
1)Qualitytest:Foamingrateandfoampower;
Table1
Mainoperationalparameters.
Period n◦cycles/day Ox/Cycle[min/min] Anox/Cycle[min/min] F/M[kgBOD5kg−1VSSd−1] COD[gm−3] NH4-N[gm−3] MLSS[kgm−3]
I 8 60/180 120/180 0,08±0.02 380±48 29±5 ∼2.7
II 8 80/180 100/180 0,06±0.01 483±25 23±3 ∼4.3
III 16 30/90 60/90 0,05±0.01 414±34 19±5 ∼3.7
3)Microscopic observation: Filamentous bacteria identification andabundance.
Generally, foam quality is measured on site by meansof a specificapparatus.Todefinethefoam stabilityand “geometric” characteristics(thicknessandfeatures),theFoamingRate (FMR) is used according to Blackall’s method [17]. The FMR value is
measuredbypouring50mLofactivatedsludgeintoa40mm
diam-eter×500mmhighglasscylinderandbubblinggasthroughitvia
a40–90mporesizediffuserat200mLmin−1.
Foamcharacteristicsarerecordedintermsofitsvolume,bubble
size,speedofformationandtimetakenforthefoamtocollapseafter
aerationhasceased(Table2).
FoamPower(FP)evaluatesthefoampotential/stabilityofMBR
activatedsludgeinaccordancewiththeconceptsusedinfood
sci-ence.FPisdefinedas“theconsumedsamplevolume(mL)from
foamingby1Lofaeration”[12].100mLofactivatedsludgesample
isplacedinatransparent7cm2cross-section(A)verticalcylinder
andaeratedfor30s(T)atanairflow(QAir)of5Lmin−1.TheFPis
thendeterminedfromthedifferenceintheinterfacelevelsofthe
sampleliquidandfoambeforeandafteraeration(H0):
FP= A·Ho
T·QAir
(1)
Whilesomewhatrudimentary,FMRmeasurementisusefulfor
quicklyassessing the extentof foaming risk (nonefor FMR=0;
overt/onerousforFMR=6–7).Bycontrast,FPisscientifically
rigor-ous,andessentialforanalysisoffoamingpotentialandpredictionof
foamingrisk.Furthermore,FPhasbeencloselycorrelatedwithEPS
concentration[9],andsocanbeindirectlyassociatedtothe
physi-ologyoftheactivatedsludgeandwithotherphenomenadependent
onthem(suchasmembranefoulingrate).However,bothtestsare
simpleandcanbeconducteddirectlyinsituwithsimilarsimple
equipment.
MoreusefulforfoamingassessmentinanMBRplantisthe
Mod-ifiedScumIndex(MSI)[10].TheMSIallowsthequantificationof
thefoamfractionfromEPS(generallymoreabundantbutless
sta-blefoam)comparedwiththatduetothepresenceoffilamentous
bacteria(usuallylowerinmassbutmorehighlystable).
MSIisdeterminedusinga2-laliquotofactivatedsludge,taken
fromtheaerationtank,which isplacedintheflotationcelland
aeratedatanintensityof10LairL−1h−1for15mintoformathick,
stablelayerofscum.Onceasingaerationthescumlayerisseparated
Table3
MSIclassification.
Effectofscumflotationstep Effectofscumpurificationstep
MSI0[%] MSI1[%] MSI2[%] MSI3[%]
Low 0–10 0–10 0–7.5 0–5
Moderate 10–15 10–15 7.5–12.5 5–10
Serious 15–25 15–20 12.5–17.5 10–25
Excessive >25 >20 >17.5 >15
fromthenon-floatingsludgeandthetestrepeatedseveraltimes (dependingonthefilamentousbacteriacontent)ontheresidual unfloatedmaterialuntilallofthefoam-formingmicro-organisms aretransferredintothescum.Witheachsteptheextractedscumis stored.Thisfirstphaseiscalled“flotationstep”.Theportionofscum separatedinthisphasecontainsthefoamproducedbyboththeEPS contributionandthegrowingbiomass.Thedrymassofrecovered scumcanbedeterminedandtheMSI0calculatedas:
MSI= MassofMassSuspendedofscumSolidsrecoveredinthesample×100 (2) Followingtheflotationstep,thefloatingscumisplacedbackin theflotationcellandre-suspendedwithwatertoatotalvolume of 2L.Subsequently, the“purification step”is performedunder controlledaerationconditionsfor15minandtheprocess of re-suspensionanddilution(withpurewater)isrepeated,withsteps of15min,untiltheamountoffoamformedisconstant.These por-tionscontainonlythefoam-formingbiomass.Theequationabove providestheScumIndexforeachpurificationstepinthe step-by-stepprocess(MSI1,MSI2,MSI3,etc)togaugefoamingpropensity (Table3)basedondeterminationsusingsimpleequipment(Fig.2).
Ingeneral,ithasbeendemonstratedthatthedilutionofthe
sep-aratedscumsamplescouldhaveanegativeinfluenceontheoriginal
impactof EPSconcentration.For this reason,it isbelieved that
theMSI0representsmainlytheeffectoffoamingduetoEPS.
Con-versely,purifyingthefoambydilutionandre-suspensionisolates
thecontributionfromfilamentousbacteria.
Concerningmicroscopicinvestigation,theabundanceand
dom-inanceof filamentous micro-organismsby microscopic analysis
can beestimated using the criteria suggested by Jenkinset al.
[18].Thismethodisbasedonobservingmorphologicaland
phys-iologicalcharacteristicsofbacteria,includingbranching,motility,
filamentshapelengthanddiameter,location,growthofattached
Table2
FoamingRateclassification.
FR Effects
0 Reactiontoaerationasforpurewater.Bubblesbreaksurfacebutareunabletofoamorhavenostability.
1 1–3cmoffoamwithfragile,ill-formedbubbles.Insufficientstabilityoffoamfilms.Immediatecollapseonaerationbeinghalted.
2 Intermittentfilmssufficientlystabletolastfor>5–10s.Usuallygeneratedfromafragilefoamstructureoflimitedheight.Filmsunstableonaerationbeing halted.
3 Foaming(bubblesabout1cmdia.)to3–8cmheight.Infrequentregularfilmformation,withbothfilmandfoamsemi-stableonaerationbeinghalted.Films have10–30sstability.
4 Initially8–15cmoffoam(about1cmdia.bubbles)withstablefilmsbeingformedatregularintervals.Bodyofthefoamandfilmsstablefor3–5minonce aerationceases.
5 Formationofstablefoam5–10cminheightwithin2min;afterwhichcollapseto3–5cmheight,whichisstablewhenaerationishalted.Nofilms. 6 Stablefoam15–30cminheightwithnofilm.Bubblesizeof∼0.5cmduringproduction,increasingto2–3cmdia.within3–5minfromtimethataerationis
halted.
7 Densestablefoam>30cmover2minaeration.Bubblesizeabout0.3cmduringproductionoffoamandmax.1cmdia.within3–5minafteraerationishalted. Foamissufficientlystabletoshownochangeinheight10–15minafteraerationishalted.
Fig.2.MSIprotocol.
epiphyticbacteria,sheath,cellsepta,cellshape,flocsize,sulphur deposits,andNeisserand Gramstaining.Theabundanceof fila-mentousmicro-organisms(Ab)isevaluatedusingthesubjective “scoring”ofthefilamentsaccordingtotheindexproposedby Jenk-insetal.,assigninganumericalvaluetothepresenceoffilaments onascalefrom0to6[18].
Morespecifically,thequantitativeestimationoffoam-forming
micro-organismscanbecalculatedusinga“fastmethod”.A50mL
sampleisplacedontheslideandthesamplestained(Gramand
Neisser)toidentifythenumberofGramandNeisserpositive
fila-ments.Thisisbasedonthenumberofintersectionsofacertaintype
ofpositivefilamentdetectedalongwiththetotalnumberofsuch
positivefilamentsperunitareaofthemicroscopeslide.Themean
numberofintersectionsacrossfivesamples(XAVG)isnormalised
againstthetotal mass of volatilesuspended solids(VSS)in the
microscopicsample(VinL)andcorrectedforthemagnification
MtoproducetheNormalizedIntersectionsNumber(NIN):
NIN= XAVG·M
VSS·V ·10
6 (3)
wherethe106factorconvertsltoL.
2.3. Foulinganalysis
Themembranefoulinghasbeeninvestigatedbymeanofthe
“ResistancesinSeriesModel”[7].Morespecifically,theprotocol
basedonthephysicalcleaningapproachhasbeenusedtodefinethe
characteristicresistancesderivingfromthemainfouling
mecha-nisms.Indetail,theadoptedcleaningprotocolwascharacterizedby
aseriesofdifferentstepsandactionsreproducedalmostidentically
asbetteroutlinedinthefollowing.
1First,permeatefluxandTMParemeasuredduringnormalplant
operations,beforethecleaningaction.Onthebasisofthese
val-ues, thetotal resistancetofiltrationcan beevaluated(Rtot,1),
accordingtoEq.(4):
Rtot,1=
TMP1
J1·
(4)
whereJ1isthepermeatefluxofthefouledmembrane[m3m−2s−1],
TMP1isthetransmembranepressureinthesameconditions[Pa],
isthepermeateviscosity[Pas]thatisalmostequaltothatof
wateratworkingtemperature.
2Afterwards,themembraneisextractedfromthebioreactorand
physicallycleanedthrough3operationalsteps.Firstly,the
mem-braneisrinsedwithtapwaterat0.4–0.5barfor15min,gently
rubbingthemembranesurface(orthemembranefibersinthe
case of hollow fiber modules). Thereafter, the membrane is
cleanedinpurewater bymeansofmembraneshaking,either
mechanical(forsmallmembranemodulesusedinbenchscale
plants)ormanual(forbiggermodulesusedinpilotplant
appli-cations). Finally, 5min rinsing with ultrapure water (at low
pressure,<0.2bar)iscarriedout.
3Subsequently,thecleanedmembraneisimmersedincleanwater
andsubjectedtoanormalfiltrationcycle(withthesame
inordertomeasuretheresistancetofiltrationincleanwater
(Rtot,cw),accordingtoEq.(5):
Rtot,cw=TMPcw
Jcw· (5)
whereJcw[m3m−2s−1]andTMPcw[Pa]arethepermeatefluxand
thetransmembranepressureincleanwaterafterphysicalcleaning;
isthewaterviscosity[Pas].
4Thecleanedmembraneisthereafterimmersedinthe
bioreac-torandthensubjectedtonormalfiltrationcycles(withthesame
fluxesandeventuallyordinarybackwashingwithanaverageflow
ratenextto900mLmin−1).Itis worthnoting thattheentire
processofphysicalcleaninghadashortduration(about½–1h)
duringwhichtheMLSSconcentrationwasassumedtobealmost
constant.Thus,itispossibletoevaluatethefinaltotalresistance
tofiltration(Rtot,2),accordingtoEq.(6):
Rtot,2=
TMP2
J2·
(6)
where J2 is the permeate flux during filtration of the same
mixedliquorasthatofstep1,aftermembranephysicalcleaning
[m3m−2s−1],TMP2isthetransmembranepressureunderthesame
conditions[Pa],whereasisthepermeateviscosity[Pas].
Thecakecontribution,duetoeitherreversible(RC,rev)or
irre-versible (RC,irr) phenomena, can be computed, after cleaning,
accordingtothesteps2–3oftheaforementionedprotocol.Inthis
context,thetotalresistancecanbeexpressedbasingonthespecific
depositionmechanisms:
Rtot,1=Rm+RPB+RC,rev+RC,irr (7)
whereRPBistheresistancedueto“PoreBloking”.
TheresistancesRtot,cwandRtot,2canbeexpressedaccordingto
Eqs.(8)and(9):
Rtot,cw=Rm+RPB (8)
Rtot,2=Rm+RPB+RC,rev (9)
Finally,theRISmodelisappliedinthefollowingway,through
Eqs.(10)–(12),inordertocomputethedifferentfouling
mecha-nisms:
RPB=Rtot,cw−Rm (10)
RC,irr=Rtot,1−Rtot,2 (11)
RC,rev=Rtot,2−Rtot,cw (12)
2.4. Viscosity,hydrophobicity,flocdimensionsandextracellular
content
Finally,thefollowingmainparametersthatdefinetheactivated
sludgefeatureshavebeenmeasured:
• Theviscosityofthemixedliquorwasinvestigatedtoevaluatethe
sludgerheologyandtheirroleinfoamingandfouling.Arotational
rheometer(Brookfielddigitalviscometer,modelDV-E)equipped
withconcentriccylindersandanadapterforlowviscositywas
usedtoestimatethemixerliquorviscosityunderconstant
tem-perature(20◦C).
• ForthehydrophobicityofmixedliquortheRosenberg’smethod
wasapplied[19].
• Theflocsizedistributionofthesludgesuspensionwas
deter-minedbyusingaMalvernMastersizer2000instrument(Malvern
InstrumentsLtd., Worcestershire, UK)with a detection range
of0.02–2000m.Thecharacteristicdiametersd10,d90anddm
(average)weredetermined.
• The total EPSs (EPST) were evaluated by using the “heating
method”[1,4,20],whichallowstheachievementofthesoluble
fractionofEPST(SMPs)aswellastheboundfractionEPSbound.
Inordertomeasuretheproteinandcarbohydratecontents,the
extractedboundEPSsandSMPswereanalyzedusingLowry’sand
Anthrone’smethods,respectively[21,22].In particular,inthis
workthesumofproteinandcarbohydratecontentwas
consid-eredas:
EPST=[EPSbound,P+EPSbound,C]+[SMPP+SMPC] (13)
wherethesubscriptsymbol“P”or“C”indicatestherelativecontent
ofproteinsorcarbohydratesintheEPSTandSMPs,respectively.
3. Resultsanddiscussion
3.1. Mixedliquorcharacterization(EPS,viscosity,floc
dimensions)
Concerningtheremovalefficiencies,thepilotplantreducedthe
averageinfluentCODof326mg/LtoamedianeffluentCOD
con-centrationof27mg/L(92%removal)inthePeriodI,from421mg/L
to18mg/LinthePeriodII(96%removal),from401mg/Lto14mg/L
inthePeriodIII(97%removal).Ingeneral,eachphase(anoxicand
aerobic)contributedtotheCODremoval,withasimilarspecific
CODremovalinbothphases.
Regardingthe TNremoval, differentbiological performances
wereobservedduringeachperiod.
• InthePeriodI,theaveragenitrificationanddenitrification
effi-ciencieswereabout71%and85%,respectively:theunsatisfying
nitrificationefficiencywaslikelyduetothelowabundanceof
autotrophicbiomassduetolowF/Mratioapplied,thataffected
significantlythenitrificationefficiency.
• InthePeriodII,theaveragenitrificationanddenitrification
effi-ciencieswereabout95%and72%,respectively:inthiscase,the
nitrificationwascompletebecausethephase ofaeration were
higher thanPeriodI (taeration/tcycle=0.44 inPeriod2 and 0.33
inPeriod1);contrarytheefficiencyofnitrificationwasslightly
lower(thanperiod II) becausenowthere aremore nitrateto
denitrified.
• InthePeriodIII,theaveragenitrificationanddenitrification
effi-ciencieswereabout98%and55%,respectively:theunsatisfying
denitrificationefficiencywasstrictlydependentonthenumber
ofdailycycle(withatcycleof90minthereare16cycleperday,
significantlyhigherthan8cycleperdaysofperiod1and2);such
acircumstance(displacementbetweenaeratedandno-aerated
phase)significantlyaffectedthetotalnitrogenremovalefficiency,
thatinperiod3waslowerthan60%,asaveragevalue
Adetaileddiscussionconcerningtheremovalperformanceof
removaloftheplantwasreportedin[15].
On the other hand, someimportant observation concerning
Mixedliquorcharacteristicscanbedonebeforetodiscussfoaming
andfoulingphenomenainIA-MBR.
Duringtheinvestigatedperiodtheaveragemixedliquor
sus-pendedsolidMLSSconcentrationsforthepilot-scaleMBRranged
from5to7gTSS/landtheaveragepercentageofvolatilefraction
wasalmostequalto75%.Theobservedyieldcoefficient(Yobs)was
nearto0.07gVSSgCOD−1.Actually,It’simportanttounderlinethat
themixedliquorsuspendedsolidMLSSconcentrationwaslower
duringtheinitialdays(<7gTSS/l),duetoanexcessivesludge
0
100
200
300
400
500
600
700
0
20
40
60
80
100
120
EPS bound [mg · gVSS -1] Time [days] EPSbound,c EPSbound,p I II III0
100
200
300
400
500
0
20
40
60
80
100
120
SMP [mg · gVSS -1] Time [days] SMPc SMPp I II III0
200
400
600
800
1000
1200
5 12 19 26 29 36 39 41 43 48 50 55 57 60 64 67 71 74 78 85 90 92 95 99 102 105 109 113 116 119 EPS [mg · gVSS -1] Time [days]EPSt Protein Carbohydrate
I II III
a)
b)
c)
Fig.3.SpecificEPSbound(a)andSMP(b)distribution,inproteinandcarbohydratefraction,andrelationshipbetweenproteinsandcarbohydrateswithtotalEPSs(c).
reduced,theMLSSvaluesrosereachingafinalvalueof7gTSS/land
thereafternosignificantbiomassgrowthhasbeennoticed[15].
TheEPSsinthemixedliquor,thatwereknownaskeyfactorsin
theformationoffoamingandfoulinginotherplants[12,13,23–27],
areshowninFig.3,intermsofEPSboundandSMPsbothinprotein
andcarbohydratefractions
Thepredominanceofprotein,especiallyintheformofEPSbound,
could be due to the presence of large quantities of
endoen-zymes,whichwereliberatedbylysis,andextracellularproducts
intheflocs,dependingonmicroorganismtype,substrate
proper-ties[28,29]andoperatingconditions.Furthermore,due totheir
hydrophobicity,proteinsgenerallyhavehigheraffinityforsludge
flocsthandopolysaccharides[30].Infact,sincethepolysaccharides
aremore readilybiodegradable than protein, theyare
metabo-lized;contrary,proteins,thataremolecularlymorecomplex,tend
toadheretothesludgeflocsbecomingthemaincomponentsof
EPSbound.Ingeneral,theEPSsformationisfavoredinIAconditions
becausethealternationofaeratedno-aeratedrepresentastress
factorformicroorganisms,whichcouldleadtocellularautolysis
[5].
In particular, until day 36 in the Period I
(taeration/tcycle=60min/180min) high values of EPST were
observed due to acclimation of inoculated biomass (collected
from a CASP) to the new IA condition. Subsequently theEPST
decreased,denotinganacclimatizationofbiomasstotheregime
ofalternationofanoxic/oxicphases.AtthebeginningofPeriodII
(taeration/tcycle=80min/180min),there wasa furtherdecreaseof
EPSTthatwaslikelyduetoadifferentmetabolicbehaviorofthe
biomass[31].However,inthefollowingdaysofPeriodII,anew
biomassadaptationoccurredwithagradualincreaseofEPST.
Finally,thePeriodIII(taeration/tcycle=30min/90min)was
char-acterizedbyafurtherdecreaseofEPSTduetothereductionofthe
anoxictimepercycle,withasubsequentmetabolicinhibition.
Finally,Fig.4ashowsthetrendsofviscosityand
hydrophobic-ityduringthethree periods;thesevariables arestrictly related
tothecontents ofpolysaccharidesandproteins, respectively.In
thiscontext, theconcentrationofpolysaccharides results
corre-latedwelltotheviscosityof themixedliquor(Fig.4b).Onthe
otherhand,inFig.4cthecorrelationbetweenproteincontentand
Fig.4.Distributionofviscosityandhydrophobicityofmixedliquor(a);correlations betweenpolysaccharidesandviscosity(b)andbetweenproteinsand hydrophobic-ity(c).
0 200 400 600 800 0 20 40 60 EPS T [mg · gVSS -1] MSI [%] R² = 0,69 R² = 0,63 R² = 0,97 0 200 400 600 800 1000 0 100 200 300 EPS T [mg · gVSS -1] FP [ml · L-1]
e)
R² = 0,84 0,00 10,00 20,00 30,00 40,00 50,00 60,00 0 20 40 60 80 100 120 MSI [%] Time [days]MSI_0 MSI_1 MSI_2
I II III 0 1 2 3 4 0 20 40 60 80 100 120 FMR [ -] Time [days] 0 50 100 150 200 250 300 0 20 40 60 80 100 120 FP [ml · L -1] Time [days] FP
a)
I II III I II IIIc)
b)
d)
Fig.5.ModifiedScumIndex(MSI)(a),FoamRate(FMR)(b),FoamPower(FP)(c),andcorrelationsMSI-EPST(d)andFP-EPST(e).
hydrophobicityarereported.Ingeneral,thehydrophobicityis
cou-pledtohighproteinconcentrationinthemixedliquor,andthis
circumstancereducesthesludgeaggregation[32].
3.2. Foamingphenomenonanalysis
TheresultsoftheFoamTests,showninFig.5,arerepresentative
forsamplestakenattheendoftheaerobicphase.
DuringPeriodI,theMSI0(Fig.5a)increaseduntil50%(day39th)
accordingtotheincreaseofEPST.Subsequently,itdecreasedto
val-uesintherangeof10–20%attheendofPeriodI.Similarly,inthe
PeriodII,thevariationofMSI0iscorrelatedtoEPSsvariation:
ini-tiallyMSI0decreased(untilday90th)andthenincreasedtoabout
35%.InperiodIII,theMSI0decreasedtoabout20%.Thus,in
agree-mentwithpreviousstudies[9,12],alsoinanIA-MBRthegeneral
trendofMSI0iscorrelatedwiththevariationofEPSSinthe
biore-actor.Contrarily,afterthecompleteprocessof“purification”the
effectofEPSSwas“masked”.Morespecifically,theMSI2wasless
affectedbytheEPST,andshowedatrendsimilartothatof
filamen-tousmicroorganisms.ThecorrelationbetweenMSI2andthelevel
offilamentousabundanceisdiscussedinthenextparagraph.
Thequalityandstabilityofthefoamlayer,intermsofFMRand
FPtestsrespectively,areshowninFig.5bandc.Bothparameters
increasedcontinuouslyduringthewholemonitoringperiod:this
wasprobablyduetothestabilizingeffectofthefilamentousthat
wereselectedduringtheexperiment(seenextsections).
Ingeneral,usingthepreviousFMRclassification,inthePeriod
IandIIthefoamappearedas“fragileanddecreasingiftheaeration
ceased”.Ontheotherhand,inPeriodIIIthefoamlayerwas3–8cm
inheightwith“infrequentregularfilmformation,having10–30s
sta-bility,andwithbothfilmandfoambeingsemi-stableuponaeration
beinghalted”.
ConcerningFig.5c,theresultsshownthatthetrendofFPstarted
froma value nexttoabout 50mlL−1,then it rapidlyincreased
toover250mlL−1(day36),incorrespondenceofasimultaneous
increaseofEPST(mainlyproteinfraction).Subsequently,thevalue
decreasedtoabout50mlL−1byday92.Finally,attheendofperiod
IIandduringperiodIII,therewasanincreaseto150mlL−1andthis
lattertrendismainlyduetothefilamentousmicroorganisms,as
furtherdiscussed.
Fig.5d,esummarizethecorrelationbetweenEPST,MSIandFP
data,confirmingthattheEPSareakeyfactorinthefoaming.In
detail,thecorrelationbetweenMSIandEPSTbecameprogressively
weakergoingfromMSI0toMSI2(Fig.5d).Ontheotherhand,the
absenceofsignificantcorrelationsbetweenMSI2andEPSSconfirms
thatthepurificationstepreducestheeffectofEPSsonfoaming,
infavorofthecontributionoffilamentousbacteria[9,13].In
par-ticular,whenthemixedliquorwaspurifiedfromscumduringthe
foamingtests,themajoramountofEPSwasremovedwiththescum
andsothecorrelationbetweentheextracellularpolymeric
sub-stancesandthesubsequentscumindexesbecamegraduallyless
important.
Finally, Fig.5eshows that alsothe EPSand FP are strongly
correlated. The polymeric and jellynature of EPS, involves the
productionofevermorepersistentbiologicalfoamunderaerobic
conditions,thegreateristheconcentrationofextracellular
poly-mericsubstances.So,FPisaparameterthatsatisfactorilydescribes
the“probability”ofthemixedliquorfoaminginalloperative
condi-tions(suchascausedbyonlyEPS,orcoupledEPSwithfilamentous
bacteria).
3.3. FilamentousbacteriainIA-MBR
Filamentousbacteriaplayanimportantroleinthedevelopment
of sludge features andmembrane permeability. Thus, the
anal-ysisoffilamentous bacteriainthesludge suspension,shownin
Fig.6a,providesusefulinformation.Inthiscontext,themicroscopic
observationsandinvestigationsaccordingtoJenkinsetal.[18]
car-riedoutsinceday44,highlightedthepresenceofthreedominant
speciesof filamentousbacteria:EikelboomType0092, Microthrix
parvicellaand,insmallerquantities,Nocardioforms.
TheEikelboomType0092(“notfamous”bacteria)asoneofthe
dominantspeciesisinagreementwithotherstudies[9,29,33,34].
Fig.6.Trendoffilamentousbacteria(a)(witha1-a3microscopicobservations)andtheircorrelationswithMSI2(b)andFP(c).
favoredbytheoperatingconditionsimposedintheIA-MBRandby
thelackofdissolvedoxygenduringtheanoxicphases,especiallyin
thePeriodIII.
Correlationsbetweenthedevelopmentoftotalfilamentous
bac-terialspeciesandtheScumIndexandthefoampowerinPeriod
IIIarealsoshowninFig.6(bandc).Itisimportanttounderline
that,converselytothecorrelationbetweenMSIandEPSs,thebest
correlationbetweenMSIandNINwasobservedinPeriodIIIwith
MSI2.Thisresultconfirmstheresultsofpreviousexperiments[9]
andpreviousobservations:ingeneral,thedatashowasignificant
correlationafterpurificationsteps.Furthermore,inthePeriodIII,
thepotentialoffoamingisalsocoupledtofilamentous
microorgan-isms,asconfirmedbythecorrelationbetweenFPandNIN(Fig.6c).
In general, in an IA-MBR, the main foam-forming bacteria
growthstronglydependsonthealternationoftheaerobic/anoxic
phases.
• Fewercyclesperdayandhigherdurationoftheanoxicphasefor
singlecycle,determinedagreaterproductionofEPS,whichwas
themaincauseoffoaminginPeriodsIandII.
• Ontheotherhand,ahighernumberofcyclesperday,asinPeriod
III,stimulateaproliferationofdifferentfilamentous
microorgan-ismsthatbecomeakeyfoam-formingfactorintermofstability
andfoampotential.
3.4. Membranefoulinganalysisandrelationwithfoaming
Thevariationofthetotalresistancestofiltrationisreportedin
Fig.7a.Thetotalhydraulicresistancetrendwasstronglyinfluenced
byphysical cleanings,which alloweda partialrecoveryof
per-meabilityandtheestimationofspecificresistances.Noprevailing
foulingmechanism,betweenthereversibleandirreversible
foul-ing,wasobservedfromtheanalysisandcomparisonofthespecific
foulingresistances(Fig.7b).OnlyinthePeriodIII,theirreversible
foulingmechanisms,intermsofRCirrandRPB,weredominant.
On theother hand, Fig.7c underlines theimportant role of
boundand solubleEPSonthemembrane fouling development.
Morespecifically,thehistogramreportsthesingularratiobetween
eachfactor(EPSs,SMPs,RPB,RCake)andthetotalfractionofEPSTor
RT.Inthiscontext,Fig.7cshowsanoppositetrendforthethree
peri-odswithregardtoEPSboundandSMP.FromPeriodItoperiodIII,the
microorganismsmodifytheirmetabolism,decreasingtheircellular
autolysis(soimplyinglowerreleaseofSMP)[5,31]andincreasing
theproductionofexopolymersasstoragesubstrateintheformof
EPSbound.
Particularly, in Period I, which hasa cycle time of 180min
with120minwithoutaeration,biomassissubjectedtoahigher
metabolicstressthanintheremainingtwoperiods.Successfully,
therewasaslightincreaseofboundEPSboundfraction(from77%in
PeriodIto96%inPeriodIII),whichcorrespondstoanequallysmall
decreaseofSMP(from23%inperiodIto4%inperiodIII).
Contem-porarily,anincreaseofirreversiblefouling,bothasirreversiblecake
(Rcake,irr)andasporeblocking(RPB)wasobservedattheexpenseof
adecreaseinthereversiblefouling(Rcake,rev).
Thefoulingfromirreversiblecakeisattributabletothe
produc-tionofEPSbound,whichgivesagelatinousconsistencytothecake
thatcanberemovedwithaphysicalcleaning.
Theconcomitantincreaseofporeblockingisprobablydueto
de-flocculationwhichtypicallyoccursinaMBRsystem,asdiscussedin
thefollowing,andwhichinvolvesflocdisintegration,witha
possi-bleocclusionofthemembraneporesbydispersedmicroorganisms
andsolubleproducts.
Regardingthefoulingrate(Fig.8a)expressedasincreaseinthe
totalresistanceovertime,itisgreaterinthefirstperiodbecause
thelowconcentrationofTSSinmixedliquoravoidedthe
Fig.7.Totalresistancefouling(a),specificresistances(b)andinfluenceofSMPsandEPSboundonfoulingmechanism(c).
developmentof a cakelayer,mainlymade ofirreversiblecake,
impliedaconsiderabledecreaseoffoulingratefortheremaining
daysofexperimentation.Furthermore,thefoulingrateisclosely
relatedtotheproteincontentofthemixedliquorexpressedbythe
ratioProteins/Polysaccharides(PN/PS),asshowninFig.8b.Dueto
theirhydrophobicnature(Fig.4c),proteinstendtoadheretothe
surfaceoftheflocs,effectivelyconstitutingEPSbound,and
contribut-ingtotheirreversiblecakedeposition.
3.5. Analysisoftheflocdimensions
The resultsof themorphology observations are reported in
Fig.9a,wheredmtrendsareshownforeachphaseaswellasthe
d10andthed90data.
Ingeneral,thedataconfirmthatintheMBRthebiological
aggre-gatesare smallerthanflocs ina ConventionalActivated Sludge
Plant.Thisisduetotheloworganicloadingrateconditions,and
tothehighturbulence inmembrane bioreactors [35–37].More
Fig.9. Flocdimensionsvariations(a)andcorrelationbetweenspecificcakeresistancetofiltrationandthemeanflocdimensions(b).
specifically,throughoutthethreeperiods,therewasaslightsludge
de-flocculation.Inparticular,inPeriodI,de-flocculationoccurred
untilday36thwithapartialre-flocculationoccurredattheendof
thisperiodduetotheincreaseofEPSsinthebioreactor(seeFig.4a).
ThePeriodIIwascharacterizedbyaslightde-flocculation,whilein
periodIIIanewre-flocculationwasnoted,partiallydueto
filamen-tousmicroorganisms,whichactasbridgesfortheformationofnew
sludgeflocs.
Consequently,thespecificcakelayerresistance(␣)was
influ-enced by de-flocculation (Fig.9b). In particular, thesuperficial
cakedepositionincreasedwhen theflocsize decreased. In this
circumstance, the permeability of the resulting cake layer was
stronglyaffectedbythepresenceoffilamentousmicroorganisms
orexcessiveflochydrophobicityandsludgeviscosity.
3.6. Considerationontherelationshipbetweenfoamingand
fouling
InFig.10,allcorrelationbetweenthemainoperational
param-eter(EPS,andNIN)andspecificfoulingandfoamingmeasurement
(MSI,Resistances),areshown.
In general, the high production of extracellular polymeric
substances (Period I and II) involves the formation of a very
hydrophobicsludgethatimpliestheformationofcompactcake
layer,improvingitspre-filtrationaction(Fig.10a).On theother
hand,the filamentous bacteria (Fig. 10b)promote thebridging
betweenthecellularaggregatesand theformationofaswollen
cakewithareductionofthepre-filtereffect(PeriodIII).
Finally,therewassimilarcorrelationbetweenMSI0andFPwith
theRt(Fig.10candd).Thisindicatesthatthephenomenaof
foam-ingandfoulingarenotindependentbutinterconnectedbymeans
ofEPSs.Indeed,anincreaseoffoamingquantity(intermsofMSI0)
andpotential(intermsofFP)inevitablyinvolvesaworseningof
thepermeabilityofthemembrane.Thisresultisapparentlyin
dis-agreementwiththeviewtakenbytheauthorsCosenzaetal.[13],
thataffirmedthatmembranefoulingtendtodecreaseiffoaming
increase.Likely,thedifferenceisduetothehigherpresenceof
over-allEPSboundintheIA-MBR,whichcontributetotheproduction
ofahigherquantityoffoam.Furthermore,inthecurrentwork,the
foamingintheIA-MBRsystemwasmainlyduetotwodifferent
causes:firsttoEPSs(PeriodIandII)andsubsequently(PeriodIII)
tofilamentousbacteriaandEPSstogether.Contrarily,inthe
experi-enceofCosenzaandcoauthors,thefoamingwasmainlycorrelated
toEPSscontribution(withaverylowfilamentousabundance).In
anycase,itisimportanttorememberthatthefoam“persistence”is
relatedtothegeometricfeaturesofBioreactor,whicharedifferent
inbothexperimentations.
4. Conclusions
ThefoamformationoccurredspontaneouslyintheIA-MBR,with
highvaluesof MSI(>30%)and FP(>150mLL−1).In general,the
resultsshowthatextracellularpolymericsubstancesplaykeyroles
infoamandfouling.Furthermore,ifthehighconcentrationofEPS
iscoupledtothepresenceoffilamentousbacteria(asitoccurswith
shortdurationoftheaeration-noaerationcycles)thefoaming
phe-nomenonworsens.Fortheanalyzedcasestudy,inPeriodIandII
(bothwithacycledurationof180min)thefoamwasduetoEPSs
production,asshownbytheMSI0trend.Intheseperiods,thereare
feweraerationcyclesperdaybuttheanoxicphaseforeachcycleis
longer.Ontheotherhand,thehighernumberofcyclesperdayin
PeriodIII(durationofthecycleof90min),despitetheshorter
dura-tionoftheanoxicphaseforeachcycle,promotedtheproliferation
offilamentousbacteriathatbecameakeyfoam-formingfactor,as
confirmedbyFP-NINcorrelation.
However,thisstudyprovidesanewoutlookabouttheroleofEPS
inthefoamingforMBRsandalsoanewinsightintothemembrane
foulingmechanism.
Acknowledgements
ThisresearchwasfundedbytheNationalResearchProjectPRIN
2009(ProjectTitle:ExperimentalanalysisonMBRsystemstostudy
andtorealizeamodeloffoulingincludingthephysicaland
micro-biologicalfeaturesofsludgeandtheoperationalconditionsthat
influencethisphenomenonCUPB71J11000610001);Italian
Min-istryofEducation,UniversityandResearch.
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