85 6 Selection Pressure Theory for Aerobic Granulation in Sequencing Batch Reactors Yu Liu and Zhi-Wu Wang CONTENTS 6.1 Introduction 85 6.2 Is Aerobic Granulation Inducible? 86 6.3 Earlier Understanding of Aerobic Granulation 86 6.4 Brief Review of Parameters Contributing to Aerobic Granulation 87 6.4.1 Substrate Composition and Loading 88 6.4.2 Hydrodynamic Shear Force 88 6.4.3 Feast-Famine Regimen 89 6.4.4 Feeding Strategy 89 6.4.5 Dissolved Oxygen 89 6.4.6 Reactor Conguration 90 6.4.7 Solids Retention Time 90 6.4.8 Cycle Time 90 6.4.9 Settling Time 91 6.4.10 Exchange Ratio 92 6.4.11 Discharge Time 92 6.5 Main Selection Pressures of Aerobic Granulation 92 6.6 A Selection Pressure Theory for Aerobic Granulation in SBRs 93 6.7 Failure of Aerobic Granulation in Continuous Microbial Culture 97 6.8 Upscaling Aerobic Granular Sludge SBRs 100 6.9 Prediction of Settling Velocity of Bioparticles 102 6.10 Conclusion 107 References 107 6.1 INTRODUCTION It appears from the preceding chapters that almost all research on aerobic granula- tionhasbeenconductedinsequencingbatchreactors(SBRs),whilenosuccessful aerobic granulation has been observed in continuous microbial culture. As shown in chapter5,itisbelievedthataerobicgranulesformthroughself-immobilizationof 53671_C006.indd 85 10/29/07 7:16:14 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 86 Wastewater Purification bacteria when suitable selection pressure is provided in the SBR. Compared to con- tinuousmicrobialculture,theuniquefeatureofanSBRisitsabilitytobeoperated inacyclicmode,andacycleofSBRforaerobicgranulationmaycompriselling, aeration, settling, and sludge discharge. Anumberofparametershavebeenknowntoinuencethepropertiesofaerobic granules formed in SBRs. Basically, contributing parameters include substrate com - position,organicloading,hydrodynamicshearforce,feast-famineregimen,feeding strategy, dissolved oxygen, reactor conguration, solids retention time, cycle time, settling time, and volume exchange ratio, while only the parameters associated with selectionpressureonsludgeparticlescancontributetotheformationofaerobicgran - ules, as shown in chapters 4 and 5. In an SBR, three major selection pressures have been identied, and they are settling time, volume exchange ratio, and discharge time (see chapters 4 and 5). In fact, selection pressure in terms of upow velocity has been shown to serve as an effective driving force towards successful anaerobic granulation in upow anaerobic sludge blanket (UASB) reactors (Hulshoff Pol, Heijnekamp, and Lettinga1988;Alphenaar,Visser,andLettinga1993).Thekeytosuccessfulaerobic granulationistoidentifyandmodelselectionpressures;thus,thischapterattempts toofferanoverviewofthemajorselectionpressuresforaerobicgranulationinSBRs, and subsequently a unied selection pressure theory is described as well. 6.2 IS AEROBIC GRANULATION INDUCIBLE? Inviewofmodernmolecularbiology,theinformationonaerobicgranulationmay reside in the genetic makeup of the microbial species involved. According to Calleja (1984),“thedepositionofstructuralandregulatorygenesmaydeterminewhether the aggregation function of cells is constitutive or inducible.” If the capability of microorganismsforaerobicgranulationisconstitutive,thatis,whateverstagethe cellisinwithregardtoitscellcycleoritslifecycle,aerobicgranulationwillbe present, provided the environmental conditions allow it to occur. On the contrary, if it is inducible, it will be present only when the cells are physiologically competent under given conditions. Aerobicgranulationhasbeenobservedintheculturesofdifferentmicrobial species. Such experimental evidence indeed supports the view that aerobic granula - tion is a microbial self-aggregation induced by environmental conditions through changing microbial surface properties and metabolic behaviors, as shown in the preceding chapters. Therefore, it is reasonable to consider that aerobic granulation would be species-independent, and represents an inducible rather than constitutive microbiological phenomenon (Y. Liu et al. 2005b). 6.3 EARLIER UNDERSTANDING OF AEROBIC GRANULATION Basedonmicroscopicobservations,Beunetal.(1999)proposedaschematicmech- anismofaerobicgranulationinSBR(gure6.1).Thismechanismshowsthatthe growthoflamentousfungiisaprerequisiteofaerobicgranulation.Afterreactor seeding,thesefungicaneasilyformpelletswithacompactstructureunderhydro- dynamicshearconditions,andthenthefungalpelletcansettlequickly,whileother 53671_C006.indd 86 10/29/07 7:16:14 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Selection Pressure Theory for Aerobic Granulation 87 bacteriawithoutthispropertyarewashedoutoftheSBR.Obviously,thesepellets may provide a protective matrix in which bacteria can further grow into colonies up toadiameterof5to6mm.Subsequently,thelargepelletsbreakupduetomicrobial lysis in their inner parts, probably caused by oxygen and nutrient limitations. Because of their good settleability, bacterial colonies produced from the breakup of the fungal pelletsareeasilyretainedintheSBR,andfurthergrowtoaerobicgranules. Development of aerobic granules requires aggregation of microorganisms. For bacteria in a culture to aggregate, a number of conditions have to be met. So far, it has been believed that intercellular communication and multicellular coordina - tionarecrucialforbacteriatoachieveanorganizedspatialstructure.Accordingto research on cell-to-cell communication in biolms (Davies et al. 1998; Pratt and Kolter 1998), it is a reasonable consideration that a cell-to-cell signaling mechanism wouldalsobeinvolvedintheformationofaerobicgranules,aswellasintheorgani - zation of the spatial structure of granule-associated bacteria. In the study of aerobic granulation by two coaggregating bacterial strains, it was found that the coaggregat - ingbacterialstrainscouldproduceautoinducer-likesignalsduringaerobicgranula - tion (Jiang et al. 2006). The benets of an organized microbial structure include more efcient proliferation, access to resources and niches that cannot be utilized by isolated cells, collective defense against antagonists that eliminate isolated cells, andoptimizationofpopulationsurvivalbydifferentiationintodistinctcelltypes (Shapiro 1998). Obviously, a sound understanding of the cell-to-cell communication in aerobic granulation is essential. Y.LiuandTay(2002)proposedagenericfour-stepmodelforaerobicgranulation. Step 1: Physical movement to initiate bacterium-to-bacterium contact or bacterial attachment onto nuclei Step 2: Initial attractive forces to keep stable multicellular contacts Step3: Microbialforcestomakecellaggregationmature Step 4: Steady-state three-dimensional structure of microbial aggregate shaped by hydrodynamic shear forces The microbial aggregates would be nally shaped by hydrodynamic shear force to formacertainstructuredcommunity.Theoutershapeandsizeofmicrobialaggre - gates are determined by the interactive strength/pattern between aggregates and of hydrodynamic shear force, microbial species, and substrate loading rate. This four-step modelforaerobicgranulation,aswellasthatshowninFigure10.1,stillcannotexplain what is the key driving force of aerobic granulation. In this regard, a more profound understanding of the mechanisms responsible for aerobic granulation is needed. 6.4 BRIEF REVIEW OF PARAMETERS CONTRIBUTING TO AEROBIC GRANULATION Aerobic granulation is the gathering together of cells through cell-to-cell immobiliza- tion to form a fairly stable and multicellular association. Evidence shows that aerobic granulationisagradualprocessfromseedsludgetocompactaggregates,furtherto granularsludgeandnallytomaturegranules(seechapter1).Obviously,forcellsin 53671_C006.indd 87 10/29/07 7:16:15 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 88 Wastewater Purification a culture to aggregate, a number of conditions have to be fullled. The focus of this sectionisthustoidentifythemaindrivingforcesofaerobicgranulationinSBR. 6.4.1 SUBSTRATE COMPOSITION AND LOADING Asshowninchapter1,aerobicgranuleshavebeencultivatedsuccessfullyfortreat- ingawidevarietyofwastewaters.Itisevidentthattheformationofaerobicgranules is independent of or insensitive to the characteristics of feed wastewater, while the microbial structure and diversity of mature aerobic granules are closely related to the type of wastewater (chapter 1). Theessentialroleoforganicloadingintheformationofaerobicgranuleswas discussed in chapter 1. It has been found that relatively high organic loading facili - tates the formation of anaerobic granules in upow anaerobic sludge blanket (UASB) reactors (Hulshoff Pol, Heijnekamp, and Lettinga 1988; Kosaric et al. 1990). This isduemainlytothefactthatthehighorganicloading-enhancedbiogasproduction resultsinanincreasedupowliquidvelocityknownasthemajorselectionpres - sureforanaerobicgranulationintheUASBreactor(HulshoffPol,Heijnekamp,and Lettinga 1988). In contrast to anaerobic granulation, it appears from chapter 1 that aerobicgranulescanformacrossaverywiderangeoforganicloadingratesfrom2.5 to 15 kg COD m –3 day –1 ,whilenitrifyingandP-accumulatinggranulescanalsobe developed at a very wide range of ammonia-nitrogen and phosphate loadings. These indicate that the substrate loading in the range studied so far is not a determinant of aerobic granulation in SBRs. As concluded in chapter 1, aerobic granulation in SBRs wouldbesubstrateconcentration-independent,butthekineticbehaviorofaerobic granulesisrelatedtotheappliedsubstrateloading(seechapter7). 6.4.2 HYDRODYNAMIC SHEAR FORCE InabubblecolumnorairliftSBR,hydrodynamicshearforceiscreatedmainlyby aerationthatcanbedescribedroughlybytheupowairvelocity(seechapter2). Colonization of bacteria Granules of bacterial colonies Inoculation Pellet formation Shear Oxygen limitation Lysis FIGURE 6.1 Illustration of aerobic granulation proposed by Beun et al., (1999). 53671_C006.indd 88 10/29/07 7:16:17 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Selection Pressure Theory for Aerobic Granulation 89 Ahighershearforcefavorstheformationofmorecompactanddenseraerobicgranules (chapter2).Similartotheformationofbiolms,aerobicgranulescanformatdifferent levels of hydrodynamic shear forces. It is believed that the structure of mature aerobic granulesisdeterminedbyhydrodynamicshearforce,butthereisnoconcreteevidence toshowthatshearforceisaprimaryinducerofaerobicgranulationinSBRs. 6.4.3 FEAST-FAMINE REGIMEN AnSBRisoperatedinasequencingcycleoffeeding,aeration,settling,anddis- charge of supernatant. In SBR, the aeration period consists of two phases: a degrada- tionphaseinwhichthesubstrateisdepletedtoaminimum,followedbyanaerobic starvationphaseinwhichtheexternalsubstrateisnolongeravailable.Itislikely thatmicroorganismsintheSBRaresubjecttoaperiodicfeastandfamineregimen, called periodic starvation (Tay, Liu, and Liu 2001). There is evidence showing that bacteria become more hydrophobic under the periodic feast-famine conditions, andhighcellhydrophobicityinturnfacilitatesmicrobialaggregation(Bossierand Verstraete1996).Infact,theperiodicfeast-famineregimeninSBRscanberegarded asakindofmicrobialselectionpressurethatmayalterthesurfacepropertiesofthe cell. However, it has been revealed in the preceding chapters that aerobic granules cannotsuccessfullybedevelopedifthesettlingtimeintheSBRisnotproperly controlled even though the periodic feast-famine regimen was present. As shown inchapter14,short-termC,N,P,andKstarvationsreducegranuleextracellular polysaccharide content, inhibit microbial activity, weaken structural integrity, and subsequently worsen settleability of aerobic granules. So far, no solid experimental evidenceshowsthatstarvationcanactasatriggerofaerobicgranulationinSBRs. 6.4.4 FEEDING STRATEGY McSwain, Irvine, and Wilderer (2004) reported that intermittent feeding is an effec- tiveoperatingstrategyforenhancingaerobicgranulationinSBRs.Forthispurpose, differentllingtimeswereappliedtoSBRs,resultingindifferentdegreesoffeast- famine to microorganisms. A high feast-famine ratio or pulse feeding to the SBR was found to be favorable for the formation of compact and dense aerobic granules. This seems to indicate that the feeding strategy may inuence the characteristics of aerobicgranulesformedinanSBR,butitisunlikelytoplaytheroleofatriggerof aerobic granulation. 6.4.5 DISSOLVED OXYGEN Dissolvedoxygen(DO)concentrationisanimportantparameterintheoperation of aerobic wastewater treatment processes. Evidence shows that aerobic granules canformatDOconcentrationsaslowas0.7to1.0mgL –1 in an SBR (Peng et al. 1999;Tokutomi2004),whiletheycanalsobesuccessfullydevelopedatrelatively highDOconcentrationsof2to6mgL –1 (Tsuneda et al. 2003; Yang, Tay, and Liu 2003;Qin,Liu,andTay2004a).Obviously,ifanaerobicconditionismaintained bysufcientaeration,theDOconcentrationwouldnotbeadecisiveparameterof aerobic granulation. 53671_C006.indd 89 10/29/07 7:16:17 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 90 Wastewater Purification 6.4.6 REACTOR CONFIGURATION InacolumnSBRahigherratioofreactorheight(H)todiameter(D)canensurea circularowtrajectory,whichinturncreatesamoreeffectivehydraulicattrition to microbial aggregates. On the other hand, a high H/D ratioalsoimprovesoxygen transfer.Q.S.Liu(2003)lookedintoaerobicgranulationinacolumn-typecontinu - ous activated sludge reactor, and found that aerobic granulation failed, while Pan (2003)showedthataerobicgranulescouldbedevelopedinSBRswithvarious H/D ratios.Thesestudiesindicatethataerobicgranulationmaynotbeassociatedwith the H/D ratio. 6.4.7 SOLIDS RETENTION TIME Y. Li (2007) systematically investigated the role of solids retention time (SRT) in aerobicgranulationinSBR,andfoundthatSRTupto40dayshadnosignicant inuence on aerobic granulation (gure 6.2). It is apparent that a complete aerobic granularsludgeblanketwasnotdevelopedovertheSRTrangeof3to40daysif selection pressures were too weak in the SBR (Y. Li 2007). In fact, in the past 100 years of research and application history of the conventional activated sludge process, aerobic granulation has never been reported in the processes operated in an extremelywiderangeofSRT.Thus,thereisnoreasontobelievethatSRTwouldbe aninducerofaerobicgranulationinSBR. 6.4.8 CYCLE TIME If an SBR is run at an extremely short cycle time, microbial growth should be suppressedbyinsufcientreactiontimeforbacteriatobreakdownsubstrates.As a result, the sludge loss due to hydraulic washout cannot be compensated for by 0 20 40 60 80 100 3691240 SRT (days) Fraction of Aerobic Granules (%) FIGURE 6.2 Fractionofaerobicgranulesversussolidsretentiontime(SRT)inSBRs operated at extremely low selection pressures. (Data from Li, Y. 2007. Ph.D. thesis, Nanyang Technological University, Singapore.) 53671_C006.indd 90 10/29/07 7:16:18 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Selection Pressure Theory for Aerobic Granulation 91 bacterial growth. For example, a complete washout of the sludge blanket and subse- quentfailureofnitrifyinggranulationwasobservedinanSBRrunataveryshort cycletime(seechapter3).Onthecontrary,ifthecycletimeiskeptmuchlongerthan thatrequiredforcompletedegradationofsubstrates,hydrolysisordecayofbiomass occursandeventuallycausesanegativeeffectonmicrobialaggregation(chapter3). Panetal.(2004)reportedthatattheshortestHRTof1hour,thestrong hydraulic pressure triggered biomass washout and led to reactor failure, while at the longestHRTof24hours,aerobicgranulesweregraduallysubstitutedbybioocs. Therefore,itseemsreasonabletoconsiderthatthecycletimeofSBRsshouldbe shorttosuppressbiomasshydrolysis,butlongenoughforbiomassgrowthandaccu - mulationinthesystem.However,evenforSBRsoperatedattheoptimumcycletime, aerobic granulationstillfailedifthesettlingtimewaskeptlongerthan15minutes (seechapter3).Consequently,cycletimeisnotadecisivefactorinaerobicgranula - tion in SBR. 6.4.9 SETTLING TIME InacolumnSBR,wastewateristreatedinsuccessivecyclesofafewhours.Atthe endofacycle,settlingofthebiomasstakesplacebeforetheefuentiswithdrawn. Sludge that cannot settle down within the preset settling time is washed out of the reactor through a xed discharge port, as illustrated in gure 6.3. Basically, a shortsettlingtimepreferentiallyselectsforthegrowthoffast-settlingbioparticles. Thus,thesettlingtimeactsasamajorhydraulicselectionpressureexertedonthe microbial community. As discussed in chapter 4, aerobic granules were success - fullycultivatedandbecamedominantonlyintheSBRsoperateatasettlingtimeof less than 5 minutes, while a mixture of aerobic granules and suspended sludge was developedintheSBRsrunatthelongersettlingtimes.Sofar,ashortsettlingtime hasbeencommonlypracticedasaneffectivemeansofcontroltoenhanceaerobic Discharge port L H Aeration FIGURE 6.3 Schematicofacolumn-typeSBRforaerobicgranulation. 53671_C006.indd 91 10/29/07 7:16:19 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 92 Wastewater Purification granulationinSBRs(Jiang,Tay,andTay2002;Lin,Liu,andTay2003;Q.S.Liu, TayandLiu2003;Y.Liu,Yang,andTay2003;Yang,Tay.andLiu2003;Wang, Du,andChen2004;Huetal.2005).Atalongsettlingtime,poorlysettlingbioocs cannot be withdrawn effectively, and they may in turn out compete granule-forming bioparticles(seechapter7).Thispointstothefactthatsettlingtimecanberegarded asadecisivefactorinaerobicgranulationinSBRs. 6.4.10 EXCHANGE RATIO TheexchangeratioinanSBRisdenedastheliquidvolumewithdrawnattheend ofthegivensettlingtimeoverthetotalreactorworkingvolume(seechapter5).For column SBRs with the same diameter, the exchange ratio is proportionally related to the height(L)ofthedischargeportfromthewatersurface(gure6.3).Alargerexchange ratioisassociatedwithahigherL.Thefractionofaerobicgranulesinthetotalbiomass wasfoundtobeproportionallyrelatedtotheexchangeratio,forexample,onlyinthe SBRs run at the higher exchange ratios of 60% and 80% were aerobic granules domi- nant,andamixtureofaerobicgranulesandbioocsinsteadofpureaerobicgranules developed at smaller exchange ratios of 40% and 20% (see chapter 5). It appears that aerobic granulation is highly dependent on the exchange ratio of the SBR. 6.4.11 DISCHARGE TIME TheessentialroleofdischargetimeinaerobicgranulationinSBRshasbeendem- onstratedinchapter5.Aprolongeddischargetimeresultsinafailureofaerobic granulationeventhoughbothsettlingtimeandvolumeexchangeratiowereproperly controlled, that is, the discharge time of efuent from the SBR is one of the key parametersthatdetermineaerobicgranulationinanSBR.Todevelopaunied theoryforaerobicgranulationinSBRs,theroleofdischargetimeinaerobicgranu - lation should be taken into account seriously. 6.5 MAIN SELECTION PRESSURES OF AEROBIC GRANULATION Aerobicgranulationisamicrobialphenomenonthatisinducedbyselectionpressure through changing microbial surface properties and metabolic behavior, as documented intheprecedingchapters.Comparedtocontinuousmicrobialculture,SBRisall- and-draw process that is fully mixed during the batch reaction step. The sequential stepsofaerationandclaricationinanSBRoccurinthesametank (MetcalfandEddy 2003). The operation of nearly all SBRs employed for aerobic granulation comprises four steps: feeding, aeration, settling, and discharge (gure 6.4). Feeding Aeration Settling Discharge FIGURE 6.4 Cycle operation of an SBR for aerobic granulation. 53671_C006.indd 92 10/29/07 7:16:20 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Selection Pressure Theory for Aerobic Granulation 93 Itappearsfromthediscussioninsection6.4,thatsettlingtime,volumeexchange ratio, and discharge constitute the main selection pressures on aerobic granulation inSBR,thatis,nomatterhowothervariablesaremanipulated,aerobicgranulation would not be successful without proper control of these three main selection pres - suresintheSBR.Thismeansthatoptimizationandscaleupofanaerobicgranular sludgeSBRmustobviouslytakeaccountoftheseselectionpressures. 6.6 A SELECTION PRESSURE THEORY FOR AEROBIC GRANULATION IN SBRS Itisnowclearthatthesettlingtime,exchangeratio,anddischargetimeinSBRsare the most effective selection pressures for aerobic granulation. Successful and stable aerobic granulation in SBRs closely depends on those applied selection pressures. Y.Liu,Wang,andTay(2005)proposedaselectionpressuretheorybywhichthe threeidentiedkeyparameterscanbeuniedintoaneasyconceptofminimum settlingvelocityofbioparticles.Followingisadiscussionofthisapproach. InpresentoperationofacolumnSBRforaerobicgranulation,theefuentisdis - chargedatadischargeoutlet(gure6.3),thatis,thevolumeofmixedliquorabove the discharge port is withdrawn immediately at the end of the preset settling time. Asshowningure6.3,foranSBRwithagivendiameter,thevolumeexchangeratio translates to the suspension discharge depth. According to the well-known Stokes formula,thesettlingvelocityofaparticlecanbecalculatedasfollows: V gd s pp ()RR M 2 18 (6.1) in which V s isthesettlingvelocityoftheparticle,d p is the diameter of the particle, S p is the density of the particle, S is the density of the solution, and µ is the viscosity of the solution. Equation 6.1 shows that the settling velocity of the particle is deter - minedmainlybythedensityanddiameterofaggregatesinanSBR. ForacolumnSBR(gure6.5a)withtheefuentdischargedatanoutletlocated at depth L, that is, at the end of the designed settling time (t s ), the volume of suspen- sionabovethedischargeportwillbewithdrawnduringthepresetdischargetime( t d ). Ifthedistanceforbioparticlestotraveltothedischargeportis L, the corresponding traveltimeofthebioparticlesisgivenby: Traveling time to the discharge port L V s (6.2) in which V s isthesettlingvelocityofthebioparticles.Ascanbeseeningure6.5a, L is proportionally related to the volume exchange ratio. Equation 6.2 shows that a high V s resultsinshorttraveltimeofbioparticlestothe dischargeport.Thisimpliesthatbioparticleswithatraveltimethatislongerthanthe designedsettlingtimewillbedischargedoutoftheSBR.Thus,aminimumsettling 53671_C006.indd 93 10/29/07 7:16:22 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 94 Wastewater Purification velocity, (V s ) min exists for the bioparticles to be retained in the reactor. According to Y. Liu, Wang, and Tay(2005), ( V s ) min canbedenedas: V L s min effective settling time (6.3) As discussed in chapter 9, there appears to exist a minimum discharge time t d , min atwhichthefractionofaerobicgranulesintheSBRiscloseto100%,thatis,afull granular sludge blanket can be developed at t d , min .Ifthedischargetime(t d )issetto be longer than t d , min , a portion of the liquor above the discharge port will continue to settle during discharge time t d , and this will eventually lower the effective selection pressure on microorganisms (gure 6.5b). Therefore, for t d > t d , min , the settling time should be calibrated in order to account for the effect of the longer discharge time. AccordingtoY.Liu,Wang,andTay(2005),theeffectivesettlingtimeinvolvedin equation 6.3 can be expressed as follows: Effective settling time settling time prese tt relaxation of settling time due to t s tt d (6.4) Y. Liu, Wang, and Tay (2005) further thought that if the discharge ow rates at t d and t d,min are Q d and Q d,max ,respectively,theycanbecalculatedinawaysuchthat: Q V t d e d ,max ,min (6.5) Q V t d e d (6.6) L H Discharge port Q d,max Q d Aeration t d = t d, min t d > t d, min (a) (b) FIGURE 6.5 (a)SchematicofacolumnSBR;(b)hypotheticalowsduringdischarge. 53671_C006.indd 94 10/29/07 7:16:26 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC [...]... Relaxation of setting time due to td 1 td ,miin td td td ,min td - td ,min td 2 (6. 8) In this case, equation 6. 4 becomes: td - td ,min ts Effective settling time td 2 (6. 9) Combining equation 6. 9 with equation 6. 3 leads to: Vs L min ts td td ,min 2 (6. 10) td Equation 6. 10 integrates the three major selection pressures (i.e., ts, td, and L) in SBRs into an easy concept of the minimum settling velocity required... driving force of aerobic granulation in sequencing batch reactors Process Biochem 39: 579–584 Qin, L., Liu, Y., and Tay, J H 2004b Effect of settling time on aerobic granulation in sequencing batch reactor Biochem Eng J 21: 47–52 Renko, E K 1998 Modelling hindered batch settling II A model for computing solids profile of calcium carbonate slurry Water SA 24: 331–3 36 Schmidt, J E and Ahring, B K 19 96. .. analytically defined in a way such that: M ps M pw (6. 16) p Vps Vpw in which Mpw is the mass of water contained in the bioparticles Combining equations 6. 15 and 6. 16 yields the following relationship: M ps p w Vps Vpw Vpw Vps Vpw 1 (6. 17) © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor 103 5 367 1_C0 06. indd & Francis Group, LLC 10/29/07 7: 16: 46 AM 104 Wastewater Purification It appears from equations 6. 15... and characteristics of phosphorusaccumulating microbial granules in sequencing batch reactors Appl Microbiol Biotechnol 62 : 430–435 Liu, Q S 2003 Aerobic granulation in sequencing batch reactor Ph.D thesis, Nanyang Technological University, Singapore Liu, Q S., Tay, J H., and Liu, Y 2003 Substrate concentration-independent aerobic granulation in a sequential aerobic sludge blanket reactor Environ Technol... Prog 21: 62 1 62 6 With permission.) In order to determine the constant involved, equation 6. 23 is linearized as follows: Y X ln (6. 24) in which: Y ln Vs SVI 2 dp (6. 25) Thus, plotting Y versus X gives a straight line with a slope of – and an intercept of ln Figure 6. 14 and figure 6. 15 show changes in size, biomass concentration, SVI, and corresponding settling velocity of microbial aggregates in the... guidelines for selecting and adjusting the settling time according to the parameters describing settleability in terms of particle size and SVI In fact, this strategy has been applied successfully to accelerate aerobic granulation in SBRs by adjusting the settling time according to changes in the settleability of bioparticles (Qin, Liu, and Tay 2004b, 2004a) 6. 10 CONCLUSION This chapter shows that aerobic. .. mechanism of aerobic granulation in a sequencing batch reactor Ph.D thesis, Nanyang Technological University, Singapore © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor 109 5 367 1_C0 06. indd & Francis Group, LLC 10/29/07 7: 16: 58 AM 110 Wastewater Purification Wang, Z.-W., Liu, Y., and Tay, J H 20 06 The role of SBR mixed liquor volume exchange ratio in aerobic granulation Chemosphere 62 : 767 –771 Yang,... Singapore With permission.) 6. 7 FAILURE OF AEROBIC GRANULATION IN CONTINUOUS MICROBIAL CULTURE The selection pressure theory for aerobic granulation is further supported by experimental observations in a continuous activated sludge reactor in which selection pressure in terms of the minimum settling velocity (equation 6. 10) is absent or extremely weak Q S Liu (2003) reported the failure of aerobic granulation. .. were conducted with aerobic granules having defined mean size in the range of 0.23 to 2.4 mm, while the biomass concentration was kept constant at 1.2 kg m–3 In this case, equation 6. 23 reduces to: Vs in which: 2 dp SVI e X (6. 26) (6. 27) 2 Figure 6. 16 shows a plot of d p/SVI against Vs A good agreement between the equation 6. 26 prediction and the experimental data is obtained, indicated by a © 2008... for Aerobic Granulation 95 Ve in equation 6. 5 is the exchange volume above the discharge port, as shown in figure 6. 5b, and the hypothetical flow that can settle is Qd,max − Qd (figure 6. 5b) Thus, the relaxation of settling time due to td can be given as: Qd ,max x Relaxation of settling time due to td Qd td Qd ,max td ,min (6. 7) Substitution of equations 6. 5 and 6. 6 in equation 6. 7 yields the following . 85 6 Selection Pressure Theory for Aerobic Granulation in Sequencing Batch Reactors Yu Liu and Zhi-Wu Wang CONTENTS 6. 1 Introduction 85 6. 2 Is Aerobic Granulation Inducible? 86 6.3 Earlier. 92 6. 4.11 Discharge Time 92 6. 5 Main Selection Pressures of Aerobic Granulation 92 6. 6 A Selection Pressure Theory for Aerobic Granulation in SBRs 93 6. 7 Failure of Aerobic Granulation in Continuous. loading in the range studied so far is not a determinant of aerobic granulation in SBRs. As concluded in chapter 1, aerobic granulation in SBRs wouldbesubstrateconcentration-independent,butthekineticbehaviorofaerobic granulesisrelatedtotheappliedsubstrateloading(seechapter7). 6. 4.2