51 4 Aerobic Granulation at Different Settling Times Lei Qin and Yu Liu CONTENTS 4.1 Introduction 51 4.2 Effect of Settling Time on the Formation of Aerobic Granules 52 4.3 Effect of Settling Time on the Settleability of Sludge 54 4.4 Effect of Settling Time on Cell Surface Hydrophobicity 55 4.5 Effect of Settling Time on Production of Extracellular Polysaccharides 56 4.6 Effect of Settling Time on Microbial Activity of Aerobic Granules 57 4.7 Accumulation of Polyvalent Cations in Aerobic Granules 58 4.8 Effect of Shift of Settling Time on Aerobic Granulation 60 4.9 Effect of Settling Time on Microbial Population 62 4.10 Rationale Behind Settling Time-Initiated Aerobic Granulation 62 4.11 Conclusions 65 References 65 4.1 INTRODUCTION The selection pressure in terms of upow liquid velocity has been demonstrated tobeadrivingforceofanaerobicgranulationinupowanaerobicsludgeblanket (UASB) reactors (Hulshoff Pol, Heijnekamp, and Lettinga 1988; Alphenaar, Visser, andLettinga1993).Althoughaerobicgranulationisnowstudiedextensivelyin SBRs,itisnotyetclearhowtheaerobicallygrowngranulesareformedinSBR.The mainfeatureofacolumnSBRisitssuccessivecycleoperation,andeachcyclecon- sistsoflling,aeration,settling,anddischarging.Attheendofeachcycle,settling of biomass takes place before efuent is withdrawn and sludge that cannot settle down within a given settling time is washed out of the reactor together with efuent through a xed discharge port. As aerobic granules are much denser than suspended ocs,theyrequirelesstimetosettlethanocsdo. ItappearsthatinSBRthesettlingtimeislikelytoexertaselectionpressureon the sludge particles, that is, only particles that can settle down below the discharge pointwithinthegivensettlingtimeareretainedinthereactor;otherwise,theyare discharged. This chapter aims to offer in-depth insights into the role of settling time in aerobic granulation in SBR. Such information would be useful for further setting upapracticalguidelineforsuccessfulaerobicgranulationinSBR. 53671_C004.indd 51 10/29/07 7:14:29 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 52 Wastewater Purification 4.2 EFFECT OF SETTLING TIME ON THE FORMATION OF AEROBIC GRANULES Qin,Liu,andTay(2004a)investigatedtheeffectofsettlingtimeonaerobicgranu- lationinfourcolumnreactors,namelyR1,R2,R3,andR4,eachwithaworking volume of 2.5 liters, which were operated in sequencing batch mode (gure 4.1). R1 toR4wererunatsettlingtimesof20,15,10,and5minutes,respectively,whilethe other operation parameters were kept the same. The duration of different operation stages and operation conditions applied for different reactors are shown in table 4.1. Efuent was discharged at the middle point of each SBR, which gives a volume exchangeratioof50%.Thesequentialoperationofthereactorswasautomatically controlled by timers, while two peristaltic pumps were employed for inuent feeding andefuentwithdrawal.Inordertolookintotheeffectofsettlingtimeonaerobic Settling Height Reactor Timer Effluent Influent Air Peristaltic Pump Peristaltic Pump Air Pump FIGURE 4.1 Schematic diagram of experimental system. (From Qin, L. 2006. Ph.D. thesis, Nanyang Technological University, Singapore. With permission.) TABLE 4.1 Operation Strategies of R1 to R4 Operation Time (min) R1 R2 R3 R4 Feed 5555 Aeration 210 215 220 230 Settling 20 5 15 2 10 1 5 — Discharge 5555 Total cycle time 240 240 240 240 Minimal settling velocity (m h –1 ) 1.89 2.52 3.78 7.56 Source: Qin, L. (2006) Ph.D. thesis, Nanyang Technological University, Singapore. With permission. 53671_C004.indd 52 10/29/07 7:14:30 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Aerobic Granulation at Different Settling Times 53 granulation,intherstphaseofthestudy,R1toR4wererunatrespectivesettling timesof20,15,10,and5minutes. Theseedsludgehadameanocsizeof0.11mm,andasludgevolumeindex (SVI)valueof230mLg –1 .After7daysofoperation,aerobicgranuleswererst observed in R4 operated at the settling time of 5 minutes. On day 10, tiny aggregates appearedinR1toR3runatrespectivesettlingtimesof20,15,and10minutes.After 3 weeks of operation, the four reactors reached steady state. The respective biomass concentrations in R1 to R4 at steady state were 5.3, 4.9, 5.5, and 5.4 g L –1 .Figure4.2 shows that aerobic granules had a very regular and spherical outer shape, and the size of mature aerobic granules seems to increase gradually with the decrease of the settlingtime.Kimetal.(2004)alsoreportedthatgranulescultivatedwithamini - mumsettlingvelocityof0.7mh –1 hadameansizeof1to1.35mm,whereasgranule sizevariedfrom0.1to0.5mmandrarelyexceeding1mmwhencultivatedwitha lower minimum settling velocity of 0.6 m h –1 . Other studies also showed that settling time employed would have an impact on the formation, size, and structure of aerobic granulesatsteadystate(Beun,vanLoosdrecht,andHeijnen2002;McSwain,Irvine, andWilderer2004). One of the prominent differences between aerobic granules and suspended ocs is the magnitude of the micropellets. It is observed that in R1, R2, and R3 aerobic granulescoexistedwithsuspendedocs,whereasinR4largeaerobicgranules became dominant over suspended ocs. The fractions of aerobic granules in steady- stateR1toR4areshowningure4.3.ItisobviousthatonlyinR4runattheshortest AB CD FIGURE 4.2 Morphology of aerobic granules developed in R1 (a), R2 (b), R3 (c), and R4 (d). Bar: 2mm.(FromQin,L.,Liu,Y.,andTay,J.H.2004a.Biochem Eng J 21:47–52.Withpermission.) 53671_C004.indd 53 10/29/07 7:14:32 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 54 Wastewater Purification settling time of 5 minutes were aerobic granules the dominant form of growth; whereasthefractionofaerobicgranuleswasonlyabout10%inR1,15%inR2, and 35% in R3. These results clearly indicate that a mixture of aerobic granules and suspendedsludgedevelopedinR1toR3insteadofapureaerobicgranularsludge blanketasobservedinR4.Thefractionsofaerobicgranulesinthereactorsseemto berelatedtothesettlingtimes.McSwain,Irvine,andWilderer(2004)alsoobserved a similar phenomenon in two SBRs operated at different settling times of 2 and 10minutes,respectively.Atalongersettlingtime,poorlysettlingocscannotbe effectively withdrawn, and they may outcompete granule-forming bioparticles. As a result, the longer settling time would lead to failure of aerobic granulation due to the absence of strong selection pressure. 4.3 EFFECT OF SETTLING TIME ON THE SETTLEABILITY OF SLUDGE SVI has been commonly used to describe the settleability and compactness of acti- vated sludge in the eld of environmental engineering. Figure 4.4 shows the rela - tionshipbetweenthesettlingtimeandSVIobservedinsteady-stateR1toR4.Itwas foundthattheSVIwascloselyrelatedtothesettlingtime,thatis,amorecompact microbialstructureoftheaerobicgranulescouldbeexpectedatashortersettling time. The SVI decreased from 230 mL g –1 in seed sludge to 49 mL g –1 in R4 after the formation of aerobic granules. However, in SBRs with partial aerobic granulation (R1toR3),theSVIwasmuchhigherthanthatinR4.Inconsiderationofthefraction ofaerobicgranulesineachreactor(gure4.3),itisreasonabletoconsiderthat theSVIisdeterminedbythedegreeofaerobicgranulationaswellasthesizeand densityofaerobicgranules.McSwain,Irvine,andWilderer(2004)reportedthat aerobicgranulesdevelopedintheSBRoperatedatasettlingtimeof2minuteshad anSVIof47mLg –1 , while an SVI of 115 mL g –1 wasfoundfortheocculentSBR Settling Time (min) 3 6 9 12 15 18 21 Fraction of Aerobic Granules (%) 0 20 40 60 80 100 120 FIGURE 4.3 Fraction of aerobic granules developed at different settling times. (Data from Qin,L.,Liu,Y.,andTay,J.H.2004a.Biochem Eng J 21: 47–52.) 53671_C004.indd 54 10/29/07 7:14:33 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Aerobic Granulation at Different Settling Times 55 operatedatasettlingtimeof10minutes.Theimprovementofsettlingabilitywith decrease of settling time can be attributed to the increase of size and number or so-calledfractionofaerobicgranulesinthereactorsasocsareeffectivelywashed out at short settling time. 4.4 EFFECT OF SETTLING TIME ON CELL SURFACE HYDROPHOBICITY Figure4.4showstheeffectofsettlingtimeoncellsurfacehydrophobicity.Alowcell surfacehydrophobicitywasfoundtobeassociatedwithalongsettlingtime.Thecell surfacehydrophobicitytendedtoincreasefrom20%fortheseedsludgetoastable valueof48%inR1,58%inR2,63%inR3,and72%inR4.Likely,thecellsurface hydrophobicity is inversely related to the settling time, that is, the microbial com - munity developed at short settling time exhibits a high cell surface hydrophobicity. Asshowningure4.3,thepartialaerobicgranulationwasobservedinR1,R2,and SVI (mL g –1 ) 40 60 80 100 120 140 160 Settling Time (min) 0 5 10 15 20 25 Cell Surface Hydrophobicity (%) 45 50 55 60 65 70 FIGURE 4.4 Effect of settling time on SVI (D) and cell surface hydrophobicity ($). (Data fromQin,L.,Liu,Y.,andTay,J.H.2004a.Biochem Eng J 21: 47–52.) 53671_C004.indd 55 10/29/07 7:14:34 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 56 Wastewater Purification R3,whereasaerobicgranulesweredominantinR4.Itappearsthattheselection pressure-induced change in cell surface hydrophobicity contributes to cell-to-cell aggregation.Infact,ithasbeenwellknownthatcellsurfacehydrophobicityhighly contributestotheformationofbiolmandanaerobicgranules(seechapter9). Evidence shows that bacteria can change their surface hydrophobicity under somestressfulconditions(seechapter9).Thecellsurfacehydrophobicityofthe seed sludge was about 20%; however, after the appearance of aerobic granules in R1toR4,thecellsurfacehydrophobicitywasgreatlyimproved(gure4.4).InR4 dominatedbyaerobicgranules,thecellsurfacehydrophobicitywasmuchhigher thanthoseinR1toR3.Thesettlingtimeseemstoinducechangesincellsurface hydrophobicity,andashortersettlingtimeorastrongerhydraulicselectionpressure resultsinamorehydrophobiccellsurface.Researchonanaerobicgranulationalso showed that anaerobic granular sludge in UASB reactors was more hydrophobic than the nongranular sludge washed out (Mahoney et al. 1987). It seems that microbial associationhastoadaptitssurfacepropertiestoresistbeingwashedoutfromthe reactors through microbial self-aggregation at short settling time. 4.5 EFFECT OF SETTLING TIME ON PRODUCTION OF EXTRACELLULAR POLYSACCHARIDES Extracellularpolysaccharides(PS)areproducedbymostbacteriaoutofcellwall with the purpose of providing cells with the ability to compete in a variety of environments,providingamodeforadhesiontosurfaceorself-immobilization(see chapter 10). Figure 4.5 shows that a shortened settling time would stimulate the pro - ductionofPS,forexample,anincreasefrom60.0to166.2mgg –1 volatile solids (VS) was observed in the mature granules with the decrease of settling time in R1 to R4, whereas the production of extracellular proteins (PN) was not signicantly inuencedbythesettlingtime,rangingfrom16.5to25.0mgg –1 VS. It appears Settling Time (min) 0 5 10 15 20 25 PS/PN (mg mg –1 ) 3 4 5 6 7 8 FIGURE 4.5 Effect of settling time on PS/PN ratio. (Data from Qin, L., Liu, Y., and Tay, J. H. 2004b. Process Biochem 39: 579–584.) 53671_C004.indd 56 10/29/07 7:14:36 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Aerobic Granulation at Different Settling Times 57 from gure 4.5 that the PS/PN ratio was inversely correlated to the settling time, that is, a shorter settling time would stimulate cells to produce more polysaccharide. Together with gure 4.3, these seem to suggest that extracellular polysaccharides playanessentialroleintheformationandfurthermaintainingthestructureand stabilityofaerobicgranules. ThePS/PNratiosintheaerobicgranulescultivatedinR2toR4aremuchhigher than that in the seed sludge (about 0.5 mg mg –1 ). This is consistent with the earlier nding by Vandevivere and Kirchman (1993) that the content of extracellular poly - saccharides for attached cells was ve times higher than for free-living cells. The failureofaerobicgranulationinSBRwasalsoobservedduetotheinhibitionof the production of extracellular polysaccharides (Yang, Tay, and Liu 2004), while thedisappearanceofaerobicgranulesinSBRwasfoundtobetightlycoupledtoa drop of extracellular polysaccharides (Tay, Liu, and Liu 2001). It has been reported thathighshearforcecaninducebothaerobicbiolmsandgranulestosecretemore extracellular polysaccharides, leading to a balanced structure of biolm or granules under given hydrodynamic conditions (Ohashi and Harada 1994; Tay, Liu, and Liu 2001;LiuandTay2002).Infact,thereiscontroversialreportwithregardtothe essential role of extracellular polysaccharides in aerobic granulation (chapter 10). 4.6 EFFECT OF SETTLING TIME ON MICROBIAL ACTIVITY OF AEROBIC GRANULES Microbialactivitycanbequantiedbythespecicoxygenutilizationrate(SOUR)in termsofmilligramsofoxygenconsumedpermilligramofvolatilebiomassperhour. To reect the microbial activity of aerobic granules, aerobic granules were sampled justduringthehalfhourofreactionperiod,andSOURwasmeasuredimmediately after sampling (Qin, Liu, and Tay 2004a). The correlation between the SOUR and settlingtimeispresentedingure4.6.TheSOURwasfoundtobeinverselyrelated Settling Time (min) 0 5 10 15 20 25 SOUR (mg O 2 g –1 VSS h –1 ) 230 240 250 260 270 280 290 FIGURE 4.6 Effect of settling time on microbial activity in terms of SOUR. (Data from Qin,L.,Liu,Y.,andTay,J.H.2004b.Process Biochem 39: 579–584.) 53671_C004.indd 57 10/29/07 7:14:37 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 58 Wastewater Purification to the settling time, that is, a shorter settling time would signicantly stimulate the respirometricactivityofmicroorganisms.Theseresultsmayimplythatbacteriamay regulatetheirenergymetabolisminresponsetothechangesinhydraulicselection pressure exerted on them. The catabolic activity of microorganisms is directly correlated to the electron transportsystemactivity,whichcanbedescribedbySOUR.Asshowningure4.6, the SOUR was closely related with the hydraulic selection pressure in terms of settling time, for example a shorter settling time results in a remarkable increase of SOUR. This may indicate that the microbial community responds metabolically to changes in hydraulic selection pressure. As pointed out earlier, shorter settling time may trigger theproductionofextracellularpolysaccharides.ThecorrelationbetweenthePS/PN ratio and SOUR is further shown in gure 4.7. More extracellular polysaccharides weresecretedathigherSOUR.Itismostlikelythatwhenthemicrobialcommunityis exposed to an increased hydraulic selection pressure, much energy produced through the catabolism would go for the synthesis of extracellular polysaccharides rather than forgrowth,thatis,underahighselectionpressure,themicrobialcommunitywould havetoregulateitsmetabolicpathwayinordertomaintainabalancewiththeexter - nal forces through consuming nongrowth-associated energy for the production of polysaccharides and the improvement of cell surface hydrophobicity. 4.7 ACCUMULATION OF POLYVALENT CATIONS IN AEROBIC GRANULES Thecontentsofpolyvalentcations(Ca,Mg,Fe,andAl)inaerobicgranulescultivated in R1 to R4 are shown in table 4.2. The calcium content increased signicantly at the shortersettlingtimes,whilethetotalcontentofMg,Fe,andAlinaerobicgranules didnotshowmuchdifferenceatvarioussettlingtimes(gure4.8).Theincreased SOUR (mg O 2 g –1 VSS h –1 ) 220 240 260 280 PS/PN (mg mg –1 ) 2 3 4 5 6 7 8 FIGURE 4.7 RelationshipsbetweenPS/PNandSOUR.(DatafromQin,L.,Liu,Y.,and Tay, J. H. 2004b. Process Biochem 39: 579–584.) 53671_C004.indd 58 10/29/07 7:14:38 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Aerobic Granulation at Different Settling Times 59 calcium content of aerobic granules would result in a decrease of the ratio of volatile solids(VS)tototalsolids(TS)from0.88to0.53.Itappearsthataerobicgranules tendtoselectivelyaccumulatecalciumthatcouldplayapartintheinitiationand development of aerobic granules. In fact, it has been generally believed that multi- valentpositiveions,especiallycalcium,canfavorbothanaerobicandaerobicgranu - lation(SchmidtandAhring1996;Teo,Xu,andTay2000;Yu,Tay,andFang2001; Jiang et al. 2003). Accumulation of calcium content in aerobic granules has been observedinaerobicgranulescultivatedundershortsettlingtimesof1to3minutes and an organic loading rate of 4.8 kg chemical oxygen demand (COD) m –3 day –1 (Wang, Du, and Chen 2004). Figure4.8clearlyshowsthatthecalciumcontentofaerobicgranulesinR4oper - atedattheshortestsettlingtimeof5minutesisabout18%ofdryweight,whichis much higher than those in the granule-suspended sludge mixtures cultivated in R1 toR3.However,thetotalcontentsofiron,magnesium,andaluminuminaerobic granules are minor and independent of the selection pressure as compared to the calcium, that is, the microbial community prefers to accumulate calcium instead of iron,magnesium,andaluminum.Infact,itwasobservedthattheaccumulationof calciumwasaccompaniedbyarapidincreaseingranulesize,whileanucleuswas observed in the aerobic granule with high calcium content. The selective accumulation of calcium would be a defensive strategy of the microbial community to selection pressure to increase its settleability to resist washout from the reactor. According to the proton translocation-dehydration theory developedforanaerobicgranulation,Teo,Xu,andTay(2000)proposedabiological explanation for the selective calcium accumulation in anaerobic granulation, and they considered that the positive effect of calcium on anaerobic granulation was probablyduetothecalcium-induceddehydrationofbacterialcellsurfaces,which wasobservedbyXu,Jiao,andLiu(1993),thatis,thecalcium-inducedcellfusion mightinitiatetheformationofacellcluster,whichactsasamicrobialnucleusfor further granulation. Ithasbeenreportedthatthecalciumcontentinanaerobicgranuleswasabout 14.6%bydryweight(Fukuzaketal.1991).Infact,calciumisaconstituentof TABLE 4.2 Metal Content in Aerobic Granules in Percent by Dry Weight R1 R2 R3 R4 AS a AG b Ca 2.039 2.130 7.287 18.757 0.975 14.556 Mg 0.256 0.264 0.238 0.262 0.187 1.904 Fe 0.3813 0.3261 0.3533 0.4826 1.4420 0.087 Al 0.0041 0.0053 0.0049 NA c 0.4440 NA c Microelement d 0.0446 0.0481 0.0408 0.0439 0.9038 NA c a Activated sludge. b Anaerobic granules (data from Fukuzak et al. 1991). c Not available. d Microelements including Co, Cu, Mn, Ni, and Zn. Source: Data from Qin, L. (2006) Ph.D. thesis, Nanyang Technological University, Singapore. 53671_C004.indd 59 10/29/07 7:14:39 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 60 Wastewater Purification extracellular polysaccharides and/or proteins, which are used as adsorbing and link- ing materials in the anaerobic granulation process (Morgan, Evison, and Forster 1991).However,differentviewsexistregardingtheroleofcalciuminbiogranulation, for example calcium has been thought not to induce granulation, and the contribution of calcium to anaerobic granulation was overestimated (Guiot et al. 1988; Thiele et al. 1990). As presented in chapter 13, the accumulation of calcium in aerobic granules maynotbeaprerequisiteofmicrobialgranulation.InR1toR4,theVS/TSratioof aerobicgranulesdeclinedfrom88%to53%whenthecalciumcontentinaerobic granules increased from 20.4 to 187.6 mg g –1 TS.Itisobviousthatcalciumand calcium-relatedcompoundswouldbemainlyresponsibleforthereducedVScontent inaerobicgranules.Asaresult,aerobicgranulesaresubstantiallymineralizedat high calcium contents. 4.8 EFFECT OF SHIFT OF SETTLING TIME ON AEROBIC GRANULATION After the stabilization of the four reactors, the settling times in R1 to R3 were further shortenedfrom20to5,15to2,and10to1minutes,respectively,withoutchanging the other operation parameters. As shown in gure 4.3, the fraction of aerobic gran - ules is in the range of 10% to 35% in R1 to R3 operated at respective settling times of20,15,and10minutes.Inordertoconrmtheeffectofsettlingtimeorhydraulic selection pressure on aerobic granulation, the settling times in steady-state R1, R2, andR3wereshiftedfrom20to5,15to2,and10to1minutesonday60accord - ingly.Thisledtoimmediatewashoutofthelightanddispersedsludgefromthe reactors, while only heavier granules remained. Two weeks after the shift of settling time, R1 to R3 gradually restabilized, and aerobic granules completely replaced suspendedsludgeandbecamedominantinR1toR3.Figure4.9showsacomparison Settling Time (min) 5101520 Metal Content (mg g –1 ) 0 50 100 150 200 FIGURE 4.8 The accumulation of polyvalent cations in aerobic granules developed at varioussettlingtimes,Ca(gray)andtotalMg,Fe,andAl(white).(DatafromQin,L.,Liu,Y., andTay,J.H.2004a.Biochem Eng J 21: 47–52.) 53671_C004.indd 60 10/29/07 7:14:40 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC [...]... (Vs)min (m h–1) Sludge size (mm) SVI (ml g–1) References 2 12a NAb 47 McSwain et al 20 04 4–5 10 2.5 NAb Beun et al 2002 5 49 Qin 2006 NAb 115 McSwain et al 20 04 1.89 0 .4 144 Qin 2006 75 0.7 1–1.3 50 Kim et al 20 04 75 b 2.15 2.4a 20 a 7.56 10 0.6 0. 1-0 .5 85 Kim et al 20 04 The minimum settling velocities were obtained from McSwain et al (2005) Data of sludge size are not available 10 minutes As shown in. .. identified in the steady-state sludge harvested from longer-settlingtime SBRs than sludge from shorter-settling-time reactors (figure 4. 10) This observation might suggest settling time could cause species selection during the cultivation of aerobic granules It seems that further investigation into the species selection with the effect of settling time is still needed in order to obtain more detailed information... settling velocity and hence a traveling time longer than the designed settling time are discharged from the reactor, that is, a minimum settling velocity, (Vs)min, for bioparticles to be retained in the reactor, is only defined with a given L and settling time: (Vs )min L settling time (4. 3) Only those bioparticles with a settling velocity greater than (Vs)min can be retained in the system Equation 4. 3... Inoc D3 D7 D 14 D28 D160 D220 D3 Reactor 2 D7 D 14 D28 D160 D220 FIGURE 4. 10 DGGE gel showing samples from R1 (10 minutes of settling time) and R2 (2 minutes of settling time) taken over time (for example, D3 means on day 3) Arrows indicate differences in dominated bands during start-up; boxes indicate dominant bands at steady state (From McSwain, B S., Irvine, R L., and Wilderer, P A 20 04 Water Sci Technol... biofilm developed in an attachedgrowth reactor Water Sci Technol 29: 281–288 Qin, L 2006 Development of microbial granules under alternating aerobic- anaerobic conditions for carbon and nitrogen removal Ph.D thesis, Nanyang Technological University, Singapore Qin, L., Liu, Y., and Tay, J H 2004a Effect of settling time on aerobic granulation in sequencing batch reactor Biochem Eng J 21: 47 –52 Qin, L., Liu,... obtain more detailed information 4. 10 RATIONALE BEHIND SETTLING TIME-INITIATED AEROBIC GRANULATION Figure 4. 3 shows that aerobic granules are dominant only when the settling time is as short as 5 minutes, and a mixture of aerobic granules and suspended sludge is developed at longer settling times In SBR, a short settling time preferentially selects for the growth of good settling bacteria, and the sludge... 53671_C0 04. indd & Francis Group, LLC 10/29/07 7: 14: 44 AM Wastewater Purification 64 R1 R2 FIGURE 4. 11 Steady-state sludge (day 200) from reactors 1 and 2 (scale = 1 mm) with different settling times of 10 and 2 minutes, respectively (From McSwain, B S et al 2005 Appl Environ Microbiol 71: 1051–1057 With permission.) TABLE 4. 3 Comparison of Sludge Characteristics with the Settling Time Settling time (min)... and Tay, J H 2004b Selection pressure is a driving force of aerobic granulation in sequencing batch reactors Process Biochem 39: 579–5 84 Schmidt, J E and Ahring, B K 1996 Granular sludge formation in upflow anaerobic sludge blanket (UASB) reactors Biotechnol Bioeng 49 : 229– 246 Tay, J H., Liu, Q S., and Liu, Y 2001 The effects of shear force on the formation, structure and metabolism of aerobic granules... aggregation during upflow anaerobic sludge bed-filter (UBF) reactor start-up In Granular anaerobic sludge: Microbiology and technology, eds Lettinga, G., Zehnder, A J B., Grotenhuis, J T C., and Hulshoff Pol, L W Wageningen, 187–1 94 The Netherlands: Purdoc Hulshoff Pol, L W., Heijnekamp, K., and Lettinga, G 1988 The selection pressure as a driving force behind the granulation of anaerobic sludge In Lettinga,... Obviously, the selection of good settling sludge is crucial for © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor 62 53671_C0 04. indd & Francis Group, LLC 10/29/07 7: 14: 42 AM Aerobic Granulation at Different Settling Times 63 aerobic granulation In fact, absence of anaerobic granulation in the UASB reactors was observed at very weak hydraulic selection pressure in terms of liquid upflow velocity (OFlaherty . discharge pointwithinthegivensettlingtimeareretainedinthereactor;otherwise,theyare discharged. This chapter aims to offer in- depth insights into the role of settling time in aerobic granulation in SBR 0.262 0.187 1.9 04 Fe 0.3813 0.3261 0.3533 0 .48 26 1 .44 20 0.087 Al 0.0 041 0.0053 0.0 049 NA c 0 .44 40 NA c Microelement d 0. 044 6 0. 048 1 0. 040 8 0. 043 9 0.9038 NA c a Activated sludge. b Anaerobic granules. 2.15 49 Qin 2006 10 2 .4 a NA b 115 McSwain et al. 20 04 20 1.89 0 .4 144 Qin 2006 75 0.7 1–1.3 50 Kim et al. 20 04 75 0.6 0. 1-0 .5 85 Kim et al. 20 04 a The minimum settling velocities were obtained