Wastewater Purification: Aerobic Granulation in Sequencing Batch Reactors - Chapter 5 potx

69 5 Roles of SBR Volume Exchange Ratio and Discharge Time in Aerobic Granulation Zhi-Wu Wang and Yu Liu CONTENTS 5.1 Introduction 69 5.2 The Role of SBR Volume Exchange Ratio in Aerobic Granulation 70 5.3 Effect of Volume Exchange Ratio on Aerobic Granulation 71 5.4 Effect of Volume Exchange Ratio on Sludge Settleability 73 5.5 Effect of Volume Exchange Ratio on Production of Extracellular Polysaccharides 75 5.6 Effect of Volume Exchange Ratio on Calcium Accumulation in Aerobic Granules 75 5.7 VolumeExchangeRatioIsaSelectionPressureforAerobic Granulation 76 5.8 Effect of Discharge Time on Formation of Aerobic Granules 78 5.9 Effect of Discharge Time on Settleability of Bioparticles 79 5.10 Effect of Discharge Time on Cell Surface Hydrophobicity 82 5.11 Effect of Discharge Time on Production of Extracellular Polysaccharides 82 5.12 Conclusions 83 References 84 5.1 INTRODUCTION It appears from the preceding chapters, among all the operation parameters that have beendiscussedsofar,onlysettlingtimecanserveasaneffectiveselectionpressure for aerobic granulation. However, a basic question to be addressed is if there are still other parameters that can also play the roles of selection pressure in aerobic granulation other than the identied settling time. The answer to such a question is essential for developing the design and operation strategy for rapid and stable aerobic granulation in both small- and large-scale sequencing batch reactors (SBRs). This 53671_C005.indd 69 10/29/07 7:15:18 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 70 Wastewater Purification chapter looks into two other potential candidate parameters that may act as selection pressuresinaerobicgranulationinSBR,namelySBRvolumeexchangeratioand discharge time. 5.2 THE ROLE OF SBR VOLUME EXCHANGE RATIO IN AEROBIC GRANULATION According to gure 5.1, the mixed liquor volume exchange ratio, or volume exchange ratio for short, is the volume of efuent that is withdrawn after a preset settling time dividedbythetotalworkingvolumeofacolumnSBR: Volume exchange ratio    R, R( , ( 2 2 (5.1) in which r istheradiusofacolumnSBR,andH is the working height of the column SBR. This equation clearly shows that the volume exchange ratio is proportionally related to L.Tolookintothepotentialroleofvolumeexchangeratioinaerobic granulation,Wang,Liu,andTay(2006)designedandranfouridenticalcolumn SBRs at different volume exchange ratios of 20% to 80% (gure 5.1), while the other operatingconditionswereallmaintainedatthesamelevels. P Feeding pump 80% Air 60% 40% 20% Discharging pump P P P Substrate 4°C P FIGURE 5.1 Schematics of four SBRs operated at the respective volume exchange ratios of 80%,60%,40%,and20%.(FromWang,Z W.2007.Ph.D.thesis,NanyangTechnological University,Singapore.Withpermission.) 53671_C005.indd 70 10/29/07 7:15:22 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Roles of SBR Volume Exchange Ratio and Discharge Time 71 5.3 EFFECT OF VOLUME EXCHANGE RATIO ON AEROBIC GRANULATION Wang,Liu,andTay(2006)investigatedtheeffectofvolumeexchangeratioonthe formationofacetate-fedaerobicgranules.Theevolutionofsludgemorphologyinthe course of SBR operation at different volume exchange ratios is shown in gure 5.2. Morphologiesofaerobicgranulesformedinfourreactorsappearedtobecloselycorre - latedwiththeappliedvolumeexchangeratio;thatis,only8daysafterreactorstartup, 80% 60% 40% 20% 0d 8d 16d 24d 30d FIGURE 5.2 Morphologies of sludge cultivated at different volume exchange ratios in the course of aerobic granulation in SBRs; scale bar: 6 mm. 53671_C005.indd 71 10/29/07 7:15:26 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 72 Wastewater Purification aerobicgranulesrstappearedintheSBRoperatedatthehighestvolumeexchange ratioof80%,whileaerobicgranulesweresubsequentlyobservedatthevolume exchangeratiosof60%,40%,and20%,respectively,6,12,and20dayslater.Itcan be seen in gure 5.2 that the larger and more spherical aerobic granules were formed atthehighervolumeexchangeratioof80%,whereasbioocswerecultivatedand becamepredominantintheSBRoperatedatthelowervolumeexchangeratioof20%. It is apparent from gure 5.2 that only a mixture of bioocs and aerobic granules wascultivatedatsmallvolumeexchangeratios.Analysisofthefractionofaerobic granules formed in each SBR reveals that nearly a pure aerobic granular sludge blanket wasindeeddevelopedatthevolumeexchangeratioof80%(gure5.3).Incontrast, almostnoaerobicgranulationwasfoundatthevolumeexchangeof20%,indicating afailedgranulation(gure5.3).ItisthusreasonabletoconsiderthattheSBRvolume exchange ratio can play an essential role in aerobic granulation, and a high SBR volume exchange ratio facilitates rapid and successful aerobic granulation in SBR. As presented in the preceding chapters, aerobic granules can be simply distin - guished from bioocs by their large particle size. The mean size of aerobic gran - ules cultivated at different volume exchange ratios are presented in gure 5.4. The size of the aerobic granules tended to increase with the increase in the SBR volume exchange ratio, for example, the size of aerobic granules developed at the volume exchangeratioof20%wassmallerthan1mm,whereasaerobicgranulesaslargeas about3.8mmwereobtainedatthevolumeexchangeratioof80%. In the operation of nitrogen-removal SBRs, Kim et al. (2004) also manipulated theSBRdischargeheightsoastoimposeonmicroorganismstwoslightlydifferent selectionpressuresintermsofminimumsettlingvelocityof0.6and0.7mh –1 .Even such a marginal difference in the minimum settling velocity could also result in distinctmorphologiesofcultivatedsludge.Forexample,largebioparticlesof1.0to 2.0 mm were harvested at the ( V s ) min of 0.7 m h –1 , while only small bioparticles of 0.1 to0.5mmwerecultivatedatthe( V s ) min of 0.6 m h –1 (Kim et al. 2004). Microscopic observation further revealed that the high volume exchange ratio SBR favored the Volume Exchange Ratio (%) 0 20 40 60 80 100 Fraction of Aerobic Granule (%) 20 40 60 80 100 FIGURE 5.3 Fraction of aerobic granules in four SBRs run at volume exchange ratios of 20% to80%.(DatafromWang,Z W.,Liu,Y.,andTay,J H.2006.Chemosphere 62: 767–771.) 53671_C005.indd 72 10/29/07 7:15:28 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Roles of SBR Volume Exchange Ratio and Discharge Time 73 cultivation of spherical granular sludge (gure 5.5b), while only bioocs instead ofgranularsludgeweredevelopedintheSBRrunatthelowvolumeexchange (gure5.5a).SimilartothendingsbyWang,Liu,andTay(2006),thetimefor formation of aerobic granules and to reach steady state was signicantly shortened atahighvolumeexchangeratio(Kimetal.2004). 5.4 EFFECT OF VOLUME EXCHANGE RATIO ON SLUDGE SETTLEABILITY In the eld of biological wastewater treatment, sludge volume index (SVI) has been used commonly as a good indicator of microbial sludge settleability. Figure 5.6 Volume Exchange Ratio (%) 0 20406080100 Mean Size (mm) 0 1 2 3 4 FIGURE 5.4 Comparison of mean size of aerobic granules developed at volume exchange ratiosof20%to80%.(DatafromWang,Z W.,Liu,Y.,andTay,J H.2006. Chemosphere 62: 767–771.) 115 µm 1.2 mm A B 2 µm FIGURE 5.5 Morphology of steady-state granules obtained at different minimum settling velocities,(a):0.6mh –1 and (b): 0.7 m h –1 ,respectively.(FromKim,S.M.etal.2004.Water Sci Technol 50:157–162.Withpermission.) 53671_C005.indd 73 10/29/07 7:15:30 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 74 Wastewater Purification shows comparison of the settleability of sludge cultivated at different volume exchange ratios in SBRs. It can be seen that the sludge SVI was inversely correlated to the volume exchange ratio, that is, a sludge with excellent settleability would be developed at a high volume exchange ratio. For example, the settleability of sludge cultivatedatthevolumeexchangeratioof80%isalmostthreetimessuperiorto thatharvestedatthevolumeexchangeratioof20%.Kimetal.(2004)alsoreported similar results showing that high volume exchange ratio SBR corresponding to a high ( V s ) min could promote the development of sludge with excellent settleability, indicatedbyalowSVIof50mLg –1 (gure 5.7). FIGURE 5.6 Sludge volume index (SVI) versus volume exchange ratios in SBRs. (Data fromWang,Z W.,Liu,Y.,andTay,J H.2006.Chemosphere 62: 767–771.) FIGURE 5.7 Sludge volume index (SVI) versus minimum settling velocities (V s ) min deter- mined from the volume exchange ratios. (Data from Kim, S. M. et al. 2004. Water Sci Technol 50:157–162.) 53671_C005.indd 74 10/29/07 7:15:31 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Volume Exchange Ratio (%) 0 20 40 60 80 100 SVI (mL g –1 ) 20 40 60 80 100 (V s ) min (m h –1 ) 0.6 0.7 SVI (ml g –1 ) 40 50 60 70 80 90 Roles of SBR Volume Exchange Ratio and Discharge Time 75 5.5 EFFECT OF VOLUME EXCHANGE RATIO ON PRODUCTION OF EXTRACELLULAR POLYSACCHARIDES Extracellular polysaccharides (PS) are a kind of bioglue that interconnects individ- ual cells into the three-dimensional structure of attached-growth microorganisms (seechapter10).Ahighappliedvolumeexchangeratiowasfoundtostimulatecells toproducemorePS(gure5.8).Asdiscussedinchapter10,PSindeedisnotan essential cell component under normal living conditions, and its production is only necessarywhenmicrobialcellsaresubjectedtostressfulconditions.Figure5.8 seemstoindicatethatthehighSBRvolumeexchangeratiocanimposeapressureon microbialsludge,leadingtoanenhancedproductionofPS. 5.6 EFFECT OF VOLUME EXCHANGE RATIO ON CALCIUM ACCUMULATION IN AEROBIC GRANULES Calcium ion was accumulated signicantly in aerobic granules developed at high volumeexchangeratio,forexample,thecalciumcontentingranulescultivatedat thevolumeexchangeratioof80%wasalmostthreetimeshigherthanthatobtained atthevolumeexchangeratioof20%(gure5.9).Figure5.10showsfurtherthat themeansizeoftheaerobicgranulestendedtoincreasewiththecalciumcontent, whileaninversetrendwasfoundforSVI.AccordingtoStokeslaw,theincreasein particle size will improve the settling ability of particles, and this in turn results in a lowered SVI (gure 5.10). The improved settleability of bioparticles can effectively preventthemfrombeingwashedoutoftheSBRatahighvolumeexchangeratio (gure 5.9). Thus, it is most likely that the selective accumulation of calcium would be a defensive strategy of microbial aggregates to resist the hydraulic discharge from the reactor through the calcium-promoted increases in their size and settleability in termsofSVI(gure5.10).Infact,itisgenerallybelievedthatcalciummayfacilitate Volume Exchange Ratio (%) 0 20 40 60 80 100 EPS Content (g g –1 SS) 0.0 0.1 0.2 0.3 0.4 0.5 FIGURE 5.8 Extracellular polysaccharide production at different volume exchange ratios. (DatafromWang,Z W.,Liu,Y.,andTay,J H.2006.Chemosphere 62: 767–771.) 53671_C005.indd 75 10/29/07 7:15:32 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 76 Wastewater Purification anaerobic granulation (Schmidt and Ahring 1996; Yu, Tay, and Fang 2001), while evidence also shows that the removal of calcium from the anaerobic granule matrix results in lowered strength of upow anaerobic sludge blanket (UASB) granules (Pereboom 1997). Consequently, a certain amount of calcium in biogranules would improve their long-term stability. 5.7 VOLUME EXCHANGE RATIO IS A SELECTION PRESSURE FOR AEROBIC GRANULATION It appears from chapter 4 that the settling time of SBR can serve as a selection pres- sureforaerobicgranulation,forexample,atashortsettlingtime,bioparticleswith poorsettleabilitywouldbewashedoutaccordingtotheminimumsettlingvelocity: ()6 , T S S min  (5.2) in which t s is settling time and L is the traveling distance of the bioparticles above the discharge port, which is proportionally correlated to the volume exchange ratio of SBR (gure 5.11). Atadesignedsettlingtimeanddischargeheight,bioparticleswithasettling velocity less than ( V s ) min arewashedoutofthereactor,whilethosewithasettling velocity greater than ( V s ) min areretained(gure5.11).Itisobviousthattheselection pressure in terms of minimum settling velocity ( V s ) min is not only a function of settling time ( t), but also depends on the discharge height (L), which can be translated to the volume exchange ratio as given in equation 5.2. This means that the volume exchange ratio can be another essential selection pressure for successful aerobic granulation. Volume Exchange Ratio (%) 0 20 40 60 80 100 Calcium Content (g g –1 SS) 0.00 0.05 0.10 0.15 0.20 0.25 FIGURE 5.9 Calciumcontentofsludgecultivatedatdifferentvolumeexchangeratiosin SBRs.(DatafromWang,Z W.,Liu,Y.,andTay,J H.2006.Chemosphere 62: 767–771.) 53671_C005.indd 76 10/29/07 7:15:34 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Roles of SBR Volume Exchange Ratio and Discharge Time 77 Mean Bioparticle Size (mm) 15 30 45 60 75 90 105 Calcium Content (g g –1 SS) 0.06 0.09 0.12 0.15 0.18 0.21 0.24 SVI (mL g –1 ) 0.5 1.0 1.5 2.0 2.5 3.0 3.5 FIGURE 5.10 Correlations among size of bioparticles (O), SVI (/), and calcium content. (DatafromWang,Z W.,Liu,Y.,andTay,J H.2006.Chemosphere 62: 767–771.) L FIGURE 5.11 SchematicdiagramofacolumnSBR. 53671_C005.indd 77 10/29/07 7:15:36 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 78 Wastewater Purification Accordingtoequation5.2,theminimumsettlingvelocityisthefunctionofsettling timeanddischargeheightorvolumeexchangeratioforanSBRwithagivendiam - eter.Bycontrolling( V s ) min , bioparticles can be effectively selected according to their respective settleability. This means that selection of bioparticles indeed can be realized by manipulating settling time and volume exchange ratio. To examine the collective effects of SBR volume exchange ratio and settling velocity on aerobic granulation, gure5.12showsthecorrelationofthefractionsofaerobicgranulesinSBRsto( V s ) min calculatedfromvarioussettlingtimes(seechapter6)andvolumeexchangeratios.As expected,thedegreeofaerobicgranulationinanSBRisdeterminedby( V s ) min . Itappearsfromgure5.12thatata( V s ) min lessthan4mh –1 ,onlyapartialaerobic granulation can be achieved in SBR, and the growth of suspended sludge seems to be promotedinthiscase.Thetypicalsettlingvelocityofconventionalactivatedsludgeis generallylessthan5mh –1 (Giokasetal.2003).ThisimpliesthatforanSBRoperated ata( V s ) min below the settling velocity of conventional activated sludge, suspended sludge cannot be effectively withdrawn. In this case, suspended sludge will easily outcompeteaerobicgranules,whichwillleadtotheinstabilityandevenfailureof aerobicgranularsludgeSBRs.Nowitisclearthatsuspendedsludgewilltakeoverthe entire reactor at low ( V s ) min ,asshowningure5.12.Toachieverapidandenhanced aerobicgranulationinSBRs,theminimumsettlingvelocity( V s ) min must be controlled atalevelhigherthanthesettlingvelocityofsuspendedsludge(seechapter6). 5.8 EFFECT OF DISCHARGE TIME ON FORMATION OF AEROBIC GRANULES Asillustratedingure5.13,dischargetimeofSBR(t d )isdenedasthetimepreset to withdraw the volume of the mixed liquor above the discharge port of the SBR, and (V s ) min (m h –1 ) 0246810 Fraction of Granules (%) 0 20 40 60 80 100 FIGURE 5.12 Fractionofaerobicgranulesversus(V s ) min , obtained from studies of volume exchange ratio (D) and settling time ($).(DataonvolumeexchangeratiofromWang,Z W., Liu, Y., and Tay, J H. 2006. Chemosphere 62:767–771;dataonsettlingtimefromQin,L., Liu, Y., and Tay, J. H. 2004. Biochem. Eng. J. 21: 47–52.) 53671_C005.indd 78 10/29/07 7:15:37 AM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC [...]... exchange ratio in aerobic granulation in SBRs can be reasonably interpreted by the concept of minimum settling velocity, while the mechanism by which discharge time can influence aerobic granulation will be further discussed in chapter 6 REFERENCES Arrojo, B., Mosquera-Corral, A., Garrido, J M., and Mendez, R 2004 Aerobic granulation with industrial wastewater in sequencing batch reactors Water Res 38:... 2007 Insights into the mechanism of aerobic granulation in sequencing batch reactor Ph.D thesis Nanyang Technological University, Singapore Wang, Z.-W., Liu, Y., and Tay, J.-H 2006 The role of SBR mixed liquor volume exchange ratio in aerobic granulation Chemosphere 62: 767–771 Yu, H Q., Tay, J H., and Fang, H H P 2001 The roles of calcium in sludge granulation during UASB reactor start-up Water Res 35: ... 15 min 20 min FIGURE 5. 14 Morphology of the steady-state sludge cultivated at different discharge times; scale bar: 3 mm (From Wang, Z.-W 2007 Ph.D thesis, Nanyang Technological University, Singapore With permission.) 2 .5 Mean Size (mm) 2.0 1 .5 1.0 0 .5 0.0 0 5 10 15 Discharge Time (min) 20 25 FIGURE 5. 15 Mean size of bioparticles versus different discharge times observed in SBRs (Data from Wang, Z.-W... aerobic granulation 5. 9 EFFECT OF DISCHARGE TIME ON SETTLEABILITY OF BIOPARTICLES As presented in the preceding chapters, settleability of bioparticles can be evaluated by a simple parameter, namely the SVI Figure 5. 17 shows a comparison of the © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor 79 53 671_C0 05. indd & Francis Group, LLC 10/29/07 7: 15: 39 AM 80 Wastewater Purification 5 min 10 min 15. .. Technol 50 : 157 162 Pereboom, J H F 1997 Strength characterisation of microbial granules Water Sci Technol 36: 141–148 Qin, L., Liu, Y., and Tay, J H 2004 Effect of settling time on aerobic granulation in sequencing batch reactor Biochem Eng J 21: 47 52 Schmidt, J E and Ahring, B K 1996 Granular sludge formation in upflow anaerobic sludge blanket (UASB) reactors Biotechnol Bioeng 49: 229–246 Wang, Z.-W 2007... for aerobic granulation in an SBR A high volume exchange ratio favors aerobic granulation, and a short discharge time has the same function © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor 83 53 671_C0 05. indd & Francis Group, LLC 10/29/07 7: 15: 45 AM 84 FIGURE 5. 21 Wastewater Purification Filamentous PS observed in aerobic granules, scale bar = 3 µm The essential role of volume exchange ratio in aerobic. .. time of 3 minutes was four times higher than that at 0 .5 minutes of discharge time, indicating that the sludge with poor settleability © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor 81 53 671_C0 05. indd & Francis Group, LLC 10/29/07 7: 15: 42 AM 82 Wastewater Purification 0 .5 Effluent TSS (gL–1) 0.4 0.3 0.2 0.1 0.0 0 .5 1.0 3.0 Discharge Time (min) FIGURE 5. 18 Total suspended solids (TSS) in the effluents... hydrophobicity was increased from 26% for the seed sludge to a stable value of 71%, 65% , 52 %, and 39% in SBRs run at discharge times of 5 to 20 minutes This implies that a microbial community developed at short discharge time would exhibit a high cell surface hydrophobicity As shown in figure 5. 16, incomplete aerobic granulation was observed in SBRs run at the discharge times of 10, 15, and 20 minutes, while... (%) 70 60 50 40 0 5 10 15 20 25 Discharge Time (min) FIGURE 5. 19 Cell surface hydrophobicity at different discharge times (Data from Wang, Z.-W 2007 Ph.D thesis, Nanyang Technological University, Singapore.) PS Content (g g–1SS) 0.4 0.3 0.2 0.1 0.0 0 5 10 15 20 25 Discharge Time (min) FIGURE 5. 20 Polysaccharide content of sludge cultivated at different discharge times (Data from Wang, Z.-W 2007 Ph.D... Technological University, Singapore.) © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor 80 53 671_C0 05. indd & Francis Group, LLC 10/29/07 7: 15: 41 AM 81 Roles of SBR Volume Exchange Ratio and Discharge Time Fraction of Aerobic Granules (%) 100 80 60 40 20 0 0 5 10 15 20 25 Discharge Time (min) FIGURE 5. 16 Fraction of aerobic granules cultivated at different discharge times (Data from Wang, Z.-W 2007 Ph.D thesis, . in gure 5. 16, incomplete aerobicgranulationwasobservedinSBRsrunatthedischargetimesof10, 15, and 20 minutes, while successful aerobic granulation was only achieved at the discharge time of 5 minutes be further discussed in chapter 6. REFERENCES Arrojo,B.,Mosquera-Corral,A.,Garrido,J.M.,andMendez,R.2004.Aerobicgranulation with industrial wastewater in sequencing batch reactors. Water Res 38:. 767–771. Yu,H.Q.,Tay,J.H.,andFang,H.H.P.2001.Therolesofcalciuminsludgegranulation during UASB reactor start-up. Water Res 35: 1 052 –1060. FIGURE 5. 21 FilamentousPSobservedinaerobicgranules,scalebar=3µm. 53 671_C0 05. indd 84 10/29/07 7: 15: 46 AM © 2008