287 16 Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria Yu Liu and Zhi-Wu Wang CONTENTS 16.1 Introduction 287 16.2 Improved Stability of Aerobic Granules by Selecting Slow-Growing Nitrifying Bacteria 288 16.3 Improved Stability of Aerobic Granules by Selecting Slow-Growing P- or Glycogen-Accumulating Organisms 294 16.4 Improved Stability of Aerobic Granules by Selecting Aged Aerobic Granules 296 16.5 Conclusions 296 References 298 16.1 INTRODUCTION Thereisevidenceshowingthatthestabilityofaerobicgranulesispoorerthanthatof anaerobic granules developed in upow anaerobic sludge blanket (UASB) reactors (Morgenrothetal.1997;Pengetal.1999;ZhuandLiu1999).Experimentalresults fromtwopilotplantsoperatedassequencingbatchbubblecolumnsdemonstrated the feasibility of the aerobic granulation technology in treating real industrial waste- water;however,abigconcernremainsgranulestability,aswellastheeconomic competitiveness (Inizan et al. 2005). Obviously, the poor stability of aerobic granules wouldlimititsapplicationinwastewatertreatmentpractice. The instability of aerobic granules is probably due to the fact that aerobic bacte- ria can grow much faster than anaerobic bacteria do. In fact, the stability of biolm is closelyrelatedtothegrowthrateofbacteria,thatis,thehighergrowthrateofbacteria resulted in a weaker structure of biolm (Tijhuis, van Loosdrecht, and Heijnen 1995; Y.Liu1997;Kwoketal.1998).Todate,thequestionofhowtoimprovethestability ofaerobicgranulesremainsunanswered.Therefore,thischapterexploresamicro- bial selection-based strategy for improving the stability of aerobic granules. This wouldbeveryusefulforthedevelopmentofafull-scaleaerobicgranularsludge sequencing batch reactor (SBR) for wastewater treatment. 53671_C016.indd 287 10/2/07 3:07:05 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 288 Wastewater Purification 16.2 IMPROVED STABILITY OF AEROBIC GRANULES BY SELECTING SLOW-GROWING NITRIFYING BACTERIA Under hydrodynamic conditions, the growth of aerobic granules after the initial cell- to-cellattachmentisthenetresultofinteractionbetweenbacterialgrowthanddetach- ment, while the balance between growth and detachment processes in turn leads to anequilibriumorstablegranulesize(Y.LiuandTay2002).Thus,sizeevolution of the microbial aggregates can be used to describe the growth of granular sludge. Figure16.1showstheevolutionofmicrobialaggregatesintermsofsizeobservedat different substrate N/COD ratios. It can be seen that the size of microbial aggregates increases gradually and nally stabilizes. According to the granular growth curves shown in gure 16.1, the aerobic granulation process can be categorized in three phases, that is, the acclimation or lag phase, granulation, and maturation, indicated byastablegranulesizeinthefourreactors. The specic growth rate (µ d )bysizeofmicrobialaggregatescanbedenedas: M D D$ DT $ (16.1) in which D is the mean size of the microbial aggregates, and t is operation time. In thegranulationphase,asshowningure16.1,integratingequation16.1gives: ln $T D M constant (16.2) Hence, the observed size-dependent specic growth rate of microbial aggregate can be determined from the slope of the straight line described by equation 16.2. It should bepointedoutthatthisapproachhasbeensuccessfullyemployedtoestimatethe Time (days) 0 102030405060 Bioparticle Mean Size (mm) 0.0 0.7 1.4 2.1 Acclimation Granulation Maturation FIGURE 16.1 Changesinsizeofmicrobialaggregates.D: substrate N/COD ratio of 5/100; $:10/100; d :20/100; c :30/100.(DatafromLiu,Y.,Yang,S.F.,andTay,J.H.2004. J Biotechnol 108: 161–169.) 53671_C016.indd 288 10/2/07 3:07:09 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria 289 growth rates of biolms and anaerobic granules (Y. Liu 1997; Yan and Tay 1997). Figure 16.2 shows the effect of substrate N/COD ratio on µ d .Itisobviousthatahigher substrate N/COD ratio results in a lower specic growth rate of aerobic granules. AccordingtoY.Liu,Yang,andTay(2004),theoverallactivityofthehetero- trophicpopulationinstableaerobicgranulescanbequantiedbyitsspecicoxygen utilization rate (SOUR) H , while the overall nitrifying activity is represented by the sumoftheactivitiesofammoniaoxidizerandnitriteoxidizer,namely(SOUR) N . The relative activity of the nitrifying population over the heterotrophic population in aerobic granules developed at different substrate N/COD ratios is shown in gure 16.2. The (SOUR) N /(SOUR) H ratio exhibits an increasing trend with the increase of sub- strate N/COD ratio. It has been reported that the activity distribution of the nitri - fying population over the heterotrophic population in biolms was proportionally related to the relative abundance of two populations under given conditions (Moreau et al. 1994). Figure 16.3 further indicates that the increased (SOUR) N /(SOUR) H ratio would result in a lower observed growth rate of aerobic granules and an improved cell surface hydrophobicity; gure 16.4 and gure 16.5 reveal that aerobic granules withlowgrowthratehavesmallersizeandmorecompactstructure.Ascanbeseen in gure 16.6, both specic gravity and the sludge volume index (SVI) of aerobic granules are closely correlated to the cell surface hydrophobicity, that is, high cell surfacehydrophobicityleadstoacompactstructureoftheaerobicgranule. Itappearsfromgure16.1thataerobicgranulationisagradualratherthaninstant processfromdispersedsludgetomatureaerobicgranuleswithastablesize.The acclimation phase observed in gure 16.1 implies that a newly inoculated culture does notbegingrowingimmediately,andaperiodofabout10daysisrequiredforbacteria toadopttoanewenvironmentinsteadofgrowth.Theobservedgrowthratebysize andmeansizeatequilibriumofaerobicgranulesarecloselyrelatedtothesubstrate N/COD ratio, that is, higher substrate N/COD ratio results in smaller granules with FIGURE 16.2 Effect of substrate N/COD ratio on µ d (D)and(SOUR) N /(SOUR) H ($) ofaerobicgranules.(DatafromLiu,Y.,Yang,S.F.,andTay,J.H.2004.JBiotechnol 108: 161–169.) 53671_C016.indd 289 10/2/07 3:07:10 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 290 Wastewater Purification lower growth rate (gure 16.2). Figure 16.2 also reveals that the nitrifying population inaerobicgranulesisenrichedwiththeincreaseofthesubstrateN/CODratio.Asa result, the heterotrophs in aerobic granules become less and less dominant at high substrate N/COD ratio. It seems that the high substrate N/COD ratio is an important factor that selects nitrifying population. Since the growth of nitrifying bacteria is much slower than heterotrophs (Sharma and Ahlert 1977), aerobic granules may offer aprotectivematrixforthenitrifyingpopulationtogrowonwithouttheriskofbeing washedoutofthesystem. (SOUR) N /(SOUR) H 0.1 0.2 0.3 0.4 0.5 0.6 0.7 μ d (d –1 ) 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 Cell Hydrophobicity (%) 68 72 76 80 84 88 92 FIGURE 16.4 Effect of µ d onstablegranulesize.(DatafromLiu,Y.,Yang,S.F.,and Tay, J. H. 2004. JBiotechnol108: 161–169.) (SOUR) N /(SOUR) H 0.1 0.2 0.3 0.4 0.5 0.6 0.7 + d (d -1 ) 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 Cell hydrophobicity (% ) 68 72 76 80 84 88 92 + d ( ) and cell hydrophobicity ( ) of aerobic granules FIGURE 16.3 Effect of (SOUR) N /(SOUR) H on µ d (F) and cell hydrophobicity (&)ofaerobic granules. (Data from Liu, Y., Yang, S. F., and Tay, J. H. 2004. JBiotechnol108: 161–169.) 53671_C016.indd 290 10/2/07 3:07:40 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria 291 It appears from gure 16.3 that the specic growth rate of aerobic granules is closely related to the distribution of the nitrifying population over the hetero - trophic population in aerobic granules. This suggests that the enriched nitrifying populationinaerobicgranulesismainlyresponsiblefortheloweredgrowthrateof aerobic granules developed at high substrate N/COD ratios. In a study of anaerobic granulation, Yan and Tay (1997) thought that if granulation is purely the result of bacterial aggregation and growth and the granule formed is ideal, a relationship betweenspecicgrowthratebysizeandthatbybiomasscanbederivedasfollows: ! FIGURE 16.5 Effect of µ d on specic gravity (D)andSVI($)ofaerobicgranules.(Data from Liu, Y., Yang, S. F., and Tay, J. H. 2004. JBiotechnol108: 161–169.) " ! FIGURE 16.6 Relationships of specic gravity (D)andSVI($)tocellhydrophobicity ofaerobicgranules.(DatafromLiu,Y.,Yang,S.F.,andTay,J.H.2004.JBiotechnol 108: 161–169.) 53671_C016.indd 291 10/2/07 3:07:43 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 292 Wastewater Purification M P R P R M GD 8 D8 DT $ D$ DT $ D$ DT ¥ § ¦ ´ ¶ µ 11 6 6 3 1 3 3 3 (16.3) in which µ g isspecicgrowthratebybiomass(gbiomassg –1 biomass d –1 ), X is bio- mass concentration of granules, and S isdensityofgranules.Accordingtoequation 16.3,thespecicgrowthratebysizecanbeconvertedtothespecicgrowthrateby biomass. The respective µ g value of aerobic granules developed at substrate N/COD ratios of 5/100, 10/100, 20/100, and 30/100 is 0.3, 0.21, 0.15, and 0.12 d –1 (Y. Liu, Yang,andTay2004).Theµ g values of nitrifying population-enriched aerobic granulesarecomparablewiththosefoundinnitrifyingbiolms(Oga,Suthersan, and Ganczarczyk 1991). Aerobicgranuleshavebeenconsideredtohaverelativelylowstability(Morgenroth et al. 1997; Zhu and Liu 1999). Obviously, the poor stability of aerobic granules will limit their application in wastewater treatment. The cause behind the poor stability ofaerobicgranuleswouldbeduetothefastgrowthofheterotrophicbacteriathat dominate aerobic granules. The nitrifying population grows much more slowly than heterotrophs, while the physical structure of nitrifying biolms is much stronger than heterotrophic biolms (Oga, Suthersan, and Ganczarczyk 1991). Figure 16.3 reveals thattheobservedgrowthrateofaerobicgranulescanbesignicantlyloweredby enrichment of the nitrifying population, and this can be realized through properly controllingthesubstrateN/CODratio.Ascanbeseeningures16.4and16.5,the loweredgrowthrateinturnresultsinasmallersizeofaerobicgranules,butwitha higherspecicgravity,indicatingacompact,strongmicrobialstructure.Itfurther appearsfromgure16.7thatlargegranuleshavealoosestructure.Thisobservationis consistentwiththosefoundinbiolms,thatis,thecompactnessofbiolmisreduced withtheincreaseinbiolmthickness(Kwoketal.1998;Y.LiuandTay2002).These allpointtothefactthatthestructuralstabilityofaerobicgranulescanbesignicantly improved by selecting slow-growing nitrifying bacteria. Aerobicgranulationisknownasamicrobialself-immobilizationprocessthat shouldbesimilartothegrowthofbiolm(Y.LiuandTay2002).Inastudyof biolms,thereisevidencethatthestrengthofbiolmsisnegativelyrelatedtothe growth rate of microorganisms (Tijhuis, van Loosdrecht, and Heijnen 1995). Kwok etal.(1998)reportedthatthebiolmdensitydecreasedasthegrowthrateincreased, whilethedensityofnitrifyingbiolmwasfoundtobehigherthanthatofhetero - trophic biolm (Oga, Suthersan, and Ganczarczyk 1991). This is consistent with the results reported in gures 16.4 and 16.5. Similarly, in the anaerobic granulation pro- cess,itwasalsoobservedthatahighbiomassgrowthrateledtoareducedstrength of anaerobic granules, that is, partial loss of structural integrity and disintegration occurs at high biomass growth rates (Morvai, Mihaltz, and Czako 1992; Quarmby andForster1995).Itbecomesclearthatthehighobservedgrowthratewouldencour- agetheoutgrowthofaerobicgranules,leadingtoarapidincreaseinthesizeofthe granules,aswellasaloosestructurewithlowbiomassdensity. As discussed earlier, a high substrate N/COD ratio appears to favor the selection of nitrifying bacteria in aerobic granules, thereby one possible operation strategy 53671_C016.indd 292 10/2/07 3:07:45 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria 293 thatcanhelptoimprovethestabilityofaerobicgranulesistoselectslow-growing nitrifying bacteria in aerobic granules by controlling the feed N/COD ratio. Amushroom-likestructurewasobservedinaerobicgranulescultivatedatthesub - strateN/CODratioof20/100(gure16.8a),andasimilarstructurewasalsoobserved ingranulesdevelopedatthesubstrateN/CODratioof30/100.However,theaerobic granules developed at the substrate N/COD ratio of 5/100 displayed a nonclustered structure. CLSM (confocal laser scanning microscope) images of FISH (uorescent in situ hybridization) further revealed that the nitrifying population was dominant in the clusters (gure 16.9). Figure 16.8b shows that the top layer mainly consists of cocci-shaped bacteria, while rod-shaped bacteria are dominant subsequently. Tay et al. (2002) also reported that the nitrifying population was mainly located at a depthof70to100µmfromthesurfaceofthegranule.Infact,previousresearch showed that biolm of mixed bacterial communities formed thick layers consisting of differentiated mushroom-like structures (Costerton et al. 1994), which are very similar to that observed in gure 16.8a. Figure 16.2 shows that the relative abundance ofthenitrifyingpopulationovertheheterotrophicpopulationintheaerobicgranules grownatthesubstrateN/CODratioof5/100isverylowascomparedtothegranules developedathighsubstrateN/CODratios.AthighsubstrateN/CODratio,competi - tion between nitrifying and heterotrophic populations on nutrients is signicant. It has been demonstrated that biolm can form the mushroom-like structure by simply changing the diffusion rate, that is, the biolm structure is largely deter - mined by nutrient concentration (Wimpenny and Colasanti 1997). In fact, bacteria may sense and move towards nutrients (Prescott, Harley, and Klein 1999). Because of their slow growth rate, the mushroom-like structure would result from the demand ofthenitrifyingpopulationonnutrients,anditinturnensuresthatthenitrifying population in aerobic granules can maximize access to nutrients. As Watnick and Kolter(2000)noted,inmixedbiolms,bacteriadistributethemselvesaccordingto whocansurvivebestintheparticularmicroenvironment,andthehighcomplexity Stable Bioparticle Mean Size (mm) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 Specific Gravity 1.01 1.02 1.03 1.04 1.05 1.06 1.07 FIGURE 16.7 Relationshipbetweenstablemeansizeandspecicgravityofaerobic granules.(DatafromLiu,Y.,Yang,S.F.,andTay,J.H.2004.JBiotechnol108: 161–169.) 53671_C016.indd 293 10/2/07 3:07:46 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 294 Wastewater Purification ofamicrobialcommunitywouldbebenecialtoitsstability.Thesendingsseem to indicate that the mushroom-like structure of densely slow-growing nitrifying bacteriawouldcontributetothestabilityofaerobicgranulesdevelopedathighsub - strateN/CODratios.Inastudyofactivatedsludgeocstability,asimilarremark wasalsomadebyWilen,Jin,andLant(2003).Consequently,theorganizationofdif - ferentmicrobialpopulationsmayhaveaneffectonthestabilityofaerobicgranules. 16.3 IMPROVED STABILITY OF AEROBIC GRANULES BY SELECTING SLOW-GROWING P- OR GLYCOGEN-ACCUMULATING ORGANISMS It is clear that selection of slow growing organisms can improve the density and stabilityofaerobicgranules.deKreukandvanLoosdrecht(2004)thoughtthat to lower the growth rate of organisms in aerobic granules, easily biodegradable EHT = 30.00 kV WD = 19 mm Mag = 5.00 K X Photo No. = 2198 Detector = SE11 µm EHT = 30.00 kV WD = 19 mm Mag = 130 X Photo No. = 2177 Detector = SE120 µm B A FIGURE 16.8 Mushroom-likestructureofanaerobicgranuledevelopedatasubstrate N/CODratioof20/100.(FromLiu,Y.,Yang,S.F.,andTay,J.H.2004.JBiotechnol 108: 161–169. With permission) 53671_C016.indd 294 10/2/07 3:07:49 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria 295 substrate needs to be converted to slowly degradable organics, namely microbial storage polymers. It has been known that phosphate- or glycogen-accumulating organismscanperformsuchaconversionofexternalorganiccarbontostorage polymers. The experimental work by de Kreuk and van Loosdrecht (2004) showed that the selection or enrichment of P-accumulating or glycogen-accumulating organ - isms in aerobic granules indeed would lead to stable aerobic granules. Heterotrophicbacteriagrowingontheslowlybiodegradablestoragepolymers, such as poly- C-hydroxybutyrate(PHB)orglycogen,mayhavesmallergrowthrates ascomparedtothosegrowingoneasilybiodegradableorganicsubstrates(Cartaetal. 2001).Forpromotingtheconversionofanexternalcarbonsourcetothestoragepoly - mers,alonganaerobic feedingperiodhasbeenoftenpracticedfollowedbyanaerobic reactionphase.ByimplementingsuchanoperationstrategyinanSBR,selectionof slow-growing P- or glycogen-accumulating organisms would be expected (de Kreuk andvanLoosdrecht2004).Onthecontrary,Li,Kuba,andKusuda(2006)foundthat whentheaerobicllingtimewasextendedfrom5to30minutes,thedenseandcom - pactaerobicgranulesweregraduallyshiftedintoalightandlooselamentousgranular structure,thatis,theextensionoftheaerobicllingtimeeventuallyledtoinstability andthefailureoftheaerobicgranularsludgeSBR.Ithasbeenreportedthatwhen dosage of external phosphate was no longer available, P-accumulating organisms tendedtograduallydisappearandbereplacedbyglycogen-accumulating organisms in aerobic granules. Even in this case, the characteristics of aerobic granules seemed nottochangesignicantly,andsmooth,denseandstableaerobicgranulescouldbe maintained in the SBR (de Kreuk and van Loosdrecht 2004). Sofar,evidenceshowsthatahighdissolvedoxygen(DO)concentrationis necessary for stable aerobic granulation in SBRs (see chapter 8). However, low oxygen FIGURE 16.9 Distribution of ammonium-oxidizing bacteria (AOB) in an aerobic granule. WhitecolorrepresentsAOB.(CourtesyofDr.V.Ivanov,NanyangTechnologicalUniversity, Singapore.) 53671_C016.indd 295 10/2/07 3:07:50 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC 296 Wastewater Purification concentration is desirable in order to make aerobic granulation technology economi- cally competitive over the conventional activated sludge processes. According to the substrate availability, the operation of an SBR can be roughly divided into two dis - tinctphasesorperiods,thatis,feastandfamineperiods(Tay,Liu,andLiu2001;Q S. Liu2003;deKreukandvanLoosdrecht2004).Theoretically,thefeastperiodisthe periodinwhichtheexternalenergysource(e.g.substrate)isavailableformicrobial growth, while after depletion of the external substrate, the culture comes to the famine phase in which only internally stored polymers are available for microbial use. Y.Q.LiuandTay(2006)lookedintothepossibilityofvariableaerationinan aerobicgranularsludgeSBR,andtheyfoundthataftertheaerationratewasreduced from 1.66 to 0.55 cm s –1 in the famine period, the settleability of aerobic granules in the SBR with reduced aeration was the same as that of aerobic granules in the SBR with constant aeration rate of 1.66 cm s –1 .Itisapparentfromgure16.10thatreduc- ingtheaerationrateduringthefamineperiodwouldnothaveasignicanteffect on the stable operation of the aerobic granular sludge reactor, whereas the aeration rate in the feast period is crucial for the stable operation of the aerobic granular sludge.Obviously,byimplementinganoperationstrategywithreducedaerationin thefaminephase,asignicantreductioninenergyconsumptionwouldbeexpected inaerobicgranularsludgeSBRs. 16.4 IMPROVED STABILITY OF AEROBIC GRANULES BY SELECTING AGED AEROBIC GRANULES Itcanbeseenintheabovediscussionthatselectionofslow-growingbacteriacansig- nicantlyimprovethestabilityofaerobicgranulesdevelopedinSBRs.Intermsofthe processoperation,alongsolidsretentiontime(SRT)meansalowspecicmicrobial growthrate.Basedonthisbasicidea,Li,Kuba,andKusuda(2006)triedtocontrol thegrowthrateofaerobicgranulesbyspecicallyselectingyoungoragedgranules. Whenyoungaerobicgranuleswereregularlyremoved,moreandmoreagedgranules wouldaccumulateinthesystem,leadingtoareducedbiodiversityofthoseremaining aerobicgranules.Ithasbeenthoughtthatthereducedbiodiversityduetoenriched aged aerobic granules would help to select slow-growing bacteria and thus increase the stabilityofaerobicgranules(Li,Kuba,andKusuda2006).Alongwiththetakeoutof youngaerobicgranules,granulesremainingintheSBRwouldbecomemoreaged,and subsequentlyaremarkableincreaseinthegranuleashcontentwasobserved(Li,Kuba, andKusuda2006).Iftheagedaerobicgranules wereremovedfromSBR,Li,Kuba, and Kusuda (2006) found that large, loose aerobic granules appeared and dominated the system. This may be due to the fact that lamentous microorganisms grew exces - sively in the system, eventually leading to instability of aerobic granules. 16.5 CONCLUSIONS Thestabilityofaerobicgranulesiskeytolong-termandstableoperationofaerobic granular sludge bioreactors. In this respect, the selection and enrichment of slow-growing organisms, such as nitrifying bacteria, P-accumulation and glycogen- accumulating organisms, appears to be the most feasible engineering strategy. 53671_C016.indd 296 10/2/07 3:07:50 PM © 2008 by Taylor & Francis Group, LLC © 2008 by Taylor & Francis Group, LLC [...]... Selection of slow growing organisms as a means for improving aerobic granular sludge stability Water Sci Technol 49: 9–17 Inizan, M., Freval, A., Cigana, J., and Meinhold, J 2005 Aerobic granulation in a sequencing batch reactor (SBR) for industrial wastewater treatment Water Sci Technol 52: 335–343 Kwok, W K., Picioreanu, C., Ong, S L., van Loosdrecht, M C M., Ng, W J., and Heijnen, J J 1998 Influence of biomass... 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Prescott, L., Harley, J., and Klein, D 1999 Microbiology Boston: McGraw-Hill Quarmby, J and Forster, C F 1995 An examination of the structure of UASB granules Water Res 29: 2449–2454 Sharma, B and Ahlert, R C 1977 Nitrification and nitrogen removal Water Res 11: 897–925 Tay, J H., Liu, Q.-S., and Liu, Y 2001 The effects of shear force on the formation, structure and metabolism of aerobic granules Appl Microbiol . Selecting Slow-Growing Nitrifying Bacteria 288 16. 3 Improved Stability of Aerobic Granules by Selecting Slow-Growing P- or Glycogen-Accumulating Organisms 294 16. 4 Improved Stability of Aerobic Granules. 287 16 Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria Yu Liu and Zhi-Wu Wang CONTENTS 16. 1 Introduction 287 16. 2 Improved Stability of Aerobic Granules by Selecting. Stability of Aerobic Granules by Selecting Slow-Growing Bacteria 293 thatcanhelptoimprovethestabilityofaerobicgranulesistoselectslow-growing nitrifying bacteria in aerobic granules by controlling the