Wastewater Purification: Aerobic Granulation in Sequencing Batch Reactors - Chapter 16 ppt

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 upow 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 biolm is
closelyrelatedtothegrowthrateofbacteria,thatis,thehighergrowthrateofbacteria
resulted in a weaker structure of biolm (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.
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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 specic growth rate (µ
d
)bysizeofmicrobialaggregatescanbedenedas:
M
D
D$ DT
$
 (16.1)
in which D is the mean size of the microbial aggregates, and t is operation time. In

thegranulationphase,asshowningure16.1,integratingequation16.1gives:
ln $T
D
M constant (16.2)
Hence, the observed size-dependent specic 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.)
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Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria 289
growth rates of biolms 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 specic growth rate of aerobic granules.
AccordingtoY.Liu,Yang,andTay(2004),theoverallactivityofthehetero-
trophicpopulationinstableaerobicgranulescanbequantiedbyitsspecicoxygen
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 biolms 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 specic gravity and the sludge volume index (SVI) of aerobic
granules are closely correlated to the cell surface hydrophobicity, that is, high cell

surfacehydrophobicityleadstoacompactstructureoftheaerobicgranule.
Itappearsfromgure16.1thataerobicgranulationisagradualratherthaninstant
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.)
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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.)
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Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria 291
It appears from gure 16.3 that the specic 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
betweenspecicgrowthratebysizeandthatbybiomasscanbederivedasfollows:





        
  !








FIGURE 16.5 Effect of µ
d
on specic gravity (D)andSVI($)ofaerobicgranules.(Data
from Liu, Y., Yang, S. F., and Tay, J. H. 2004. JBiotechnol108: 161–169.)





        
" !



















FIGURE 16.6 Relationships of specic gravity (D)andSVI($)tocellhydrophobicity
ofaerobicgranules.(DatafromLiu,Y.,Yang,S.F.,andTay,J.H.2004.JBiotechnol
108: 161–169.)
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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
isspecicgrowthratebybiomass(gbiomassg
–1
biomass d
–1
), X is bio-
mass concentration of granules, and S isdensityofgranules.Accordingtoequation
16.3,thespecicgrowthratebysizecanbeconvertedtothespecicgrowthrateby
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
granulesarecomparablewiththosefoundinnitrifyingbiolms(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 biolms is much stronger than
heterotrophic biolms (Oga, Suthersan, and Ganczarczyk 1991). Figure 16.3 reveals
thattheobservedgrowthrateofaerobicgranulescanbesignicantlyloweredby
enrichment of the nitrifying population, and this can be realized through properly
controllingthesubstrateN/CODratio.Ascanbeseeningures16.4and16.5,the
loweredgrowthrateinturnresultsinasmallersizeofaerobicgranules,butwitha
higherspecicgravity,indicatingacompact,strongmicrobialstructure.Itfurther
appearsfromgure16.7thatlargegranuleshavealoosestructure.Thisobservationis
consistentwiththosefoundinbiolms,thatis,thecompactnessofbiolmisreduced
withtheincreaseinbiolmthickness(Kwoketal.1998;Y.LiuandTay2002).These
allpointtothefactthatthestructuralstabilityofaerobicgranulescanbesignicantly
improved by selecting slow-growing nitrifying bacteria.
Aerobicgranulationisknownasamicrobialself-immobilizationprocessthat
shouldbesimilartothegrowthofbiolm(Y.LiuandTay2002).Inastudyof
biolms,thereisevidencethatthestrengthofbiolmsisnegativelyrelatedtothe
growth rate of microorganisms (Tijhuis, van Loosdrecht, and Heijnen 1995). Kwok
etal.(1998)reportedthatthebiolmdensitydecreasedasthegrowthrateincreased,
whilethedensityofnitrifyingbiolmwasfoundtobehigherthanthatofhetero
-
trophic biolm (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
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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 biolm 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 signicant.
It has been demonstrated that biolm can form the mushroom-like structure
by simply changing the diffusion rate, that is, the biolm 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,inmixedbiolms,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 Relationshipbetweenstablemeansizeandspecicgravityofaerobic
granules.(DatafromLiu,Y.,Yang,S.F.,andTay,J.H.2004.JBiotechnol108: 161–169.)
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294 Wastewater Purification

ofamicrobialcommunitywouldbebenecialtoitsstability.Thesendingsseem
to indicate that the mushroom-like structure of densely slow-growing nitrifying
bacteriawouldcontributetothestabilityofaerobicgranulesdevelopedathighsub
-
strateN/CODratios.Inastudyofactivatedsludgeocstability,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)
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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
whentheaerobicllingtimewasextendedfrom5to30minutes,thedenseandcom
-
pactaerobicgranulesweregraduallyshiftedintoalightandlooselamentousgranular
structure,thatis,theextensionoftheaerobicllingtimeeventuallyledtoinstability
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
nottochangesignicantly,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.)
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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
.Itisapparentfromgure16.10thatreduc-
ingtheaerationrateduringthefamineperiodwouldnothaveasignicanteffect
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,asignicantreductioninenergyconsumptionwouldbeexpected
inaerobicgranularsludgeSBRs.
16.4 IMPROVED STABILITY OF AEROBIC GRANULES
BY SELECTING AGED AEROBIC GRANULES
Itcanbeseenintheabovediscussionthatselectionofslow-growingbacteriacansig-

nicantlyimprovethestabilityofaerobicgranulesdevelopedinSBRs.Intermsofthe
processoperation,alongsolidsretentiontime(SRT)meansalowspecicmicrobial
growthrate.Basedonthisbasicidea,Li,Kuba,andKusuda(2006)triedtocontrol
thegrowthrateofaerobicgranulesbyspecicallyselectingyoungoragedgranules.
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.
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Improved Stability of Aerobic Granules by Selecting Slow-Growing Bacteria 297
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© 2008 by Taylor & Francis Group, LLC
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SVI (mL g
–1

)
0
20
40
60
80
100
120
140
Biomass Concentration (g L
–1
)
0
2
4
6
8
"
 
! #$





FIGURE 16.10 SVI, biomass conc e ntration, and average particle size of aer obic granular
sludge i n two SBRs operated at (
c
) reduced aeration rate of 0.55 cm s
–1

and ( )constant
aeration rate of 1.66 cm s
–1
.(DatafromLiu,Y.Q.andTay,J.H.2006.JBiotechnol
124 : 338– 346. )
298 Wastewater Purification
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