Available online at www.sciencedirect.com
Bioresource Technology 99 (2008) 7672 7677
Is sludge retention time a decisive factor for aerobic granulation in SBR?
a a,* b
Yong Li , Yu Liu , Hailou Xu
a
Division of Environmental and Water Resources Engineering, School of Civil and Environmental Engineering, Nanyang Technological
University, 50 Nanyang Avenue, Singapore 639798, Singapore
b
School of Chemical Engineering and Life Sciences, Singapore Polytechnic, 500 Dover Road, Singapore 139651, Singapore
Received 30 October 2007; received in revised form 28 January 2008; accepted 30 January 2008
Available online 10 March 2008
Abstract
This study investigated the role of sludge retention time (SRT) in aerobic granulation under negligible hydraulic selection pressure.
Results showed that no successful aerobic granulation was observed at the studied SRTs in the range of 3 40 days. A comparison anal-
ysis revealed that hydraulic selection pressure in terms of the minimum settling velocity would be much more effective than SRT for
enhancing heterotrophic aerobic granulation in sequencing batch reactor (SBR). It was shown that SRT would not be a decisive factor
for aerobic granulation in SBR.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Sludge retention time; Aerobic granulation; SBR; Selection pressure
1. Introduction of aerobic granules in SBR. So far, there is no research
available in the literature with regard to the essential role
Sludge retention time (SRT) is one of the most impor- of SRT in the formation of aerobic granules in SBR, i.e.,
tant design and operation parameters in the activated the effect of SRT on aerobic granulation remains unknown.
sludge process. It has been known that SRT may have It has been shown that aerobic granulation in a SBR is dri-
remarkable effect on bioflocculation of activated sludge. ven by hydraulic selection pressure in terms of minimum
Basically a SRT of 2 days is often required for the forma- settling velocity of bioparticles (Liu et al., 2005a). Thus,
tion of flocculated activated sludge with good settling abil- to investigate the effect of SRT on aerobic granulation in
ity (Ng, 2002), while the optimum SRT for good SBR, the interference of hydraulic selection pressure needs
bioflocculation and low effluent COD was found to be in to be avoided. For such a purpose, this study aimed to
the range of 2 and 8 days (Rittmann, 1987). It has seen show if SRT is essential for aerobic granulation in case
believed that a SRT shorter than 2 days favors the growth where hydraulic selection pressure is absent, and it is
of dispersed bacteria that in turn would result in increased expected to offer in-depth insights into the mechanism of
SVI and effluent COD concentration. aerobic granulation as well as operation strategy for suc-
In aerobic granular sludge sequencing batch reactor cessful aerobic granulation in SBR.
(SBR) without intentional control of SRT, it was found
that SRT would vary in a very large range of one to forty
2. Methods
days along with granulation (Pan, 2003), while Beun et al.
(2000) reported that the SRT increased from 2 days to 30
2.1. Experimental set-up and operation
days, and then dropped to 17 days, finally the SRT was sta-
bilized at 9 days along with the formation and maturation
Five columns (127 cm in height and 5 cm in diameter),
each with a working volume of 1.96 L, were operated as
*
sequencing batch reactors, namely R1, R2, R3, R4 and
Corresponding author.
E-mail address: [e-mail] (Y. Liu). R5, which were seeded with the activated sludge taken
0960-8524/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.biortech.2008.01.073
Y. Li et al. / Bioresource Technology 99 (2008) 7672 7677 7673
from a local municipal wastewater treatment plant. R1 R5 were determined using standard methods (APHA, 1998).
were run at a respective SRT of 3, 6, 9, 12 and 40 days, The size of sludge was measured by a laser particle size ana-
while the other operation conditions were kept the same, lyser (Malvern Mastersizer Series 2600, Malvern), or an
i.e. 4 h of total cycle time, 5 min of filling, 30 min of settling image analyser (IA) (Image-Pro Plus, v 4.0, Media Cyber-
and 5 min of effluent withdrawal. The remaining time in netics). Cell surface hydrophobicity was determined using
each cycle was the aeration period. In the last 2 min of aer- the method developed by Rosenberg et al. (1980). In this
ation, a certain volume of the mixed liquor was discharged method, 2.5 mL hexadecane was used as the hydrophobic
out of the reactor in order to maintain the desired SRT. phase, and cell surface hydrophobicity was expressed as
Fine air bubbles were introduced at a flow rate of 3.0 L the percentage of cells adhering to the hexadecane after
min 1 through a dispenser located at the bottom of each 15 min of partitioning.
reactor. At the end of the settling phase, supernatant was
discharged from an outlet located at 0.6 m height from 2.4. Bacterial tests
the reactor bottom. A hydraulic retention time of 8 h was
maintained in the reactors. The sequential operation of 2.4.1. Pretreatment of biosample
the reactors was automatically controlled by timers, while 45 ml of mixed liquor was collected from each SBR, and
two peristaltic pumps were employed for influent feeding was then transferred into a sterile 50 ml centrifuge tube.
and supernatant withdrawal. The sample was centrifuged for 15 min at 4500 rpm. The
Synthetic wastewater used for granule cultivation con- supernatant was removed, and the sludge harvested was
sisted of sodium acetate 732.5 mg L 1, NH4Cl 22.0 mg L 1, resuspended in 45 mL of 0.85% saline. The sample sus-
K2HPO4 7.5 mg L 1, CaCl2 2H2O 9.5 mg L 1, pended in saline was mounted to a mortar, and was grin-
MgSO4 7H2O 6.25 mg L 1, FeSO4 7H2O 5 mg L 1 and ded by the pestle till it was completely disintegrated.
microelement solution 1.0 mL L 1. This composition gave a These were all done in a laminar flow hood in order to pre-
COD concentration of 500 mg L 1, while microelements vent potential contamination, while aseptic technique was
solution contained: H3BO3 0.05 g L 1, ZnCl2 0.05 g L 1, practiced at every step. In addition, the centrifuge tube, sal-
CuCl2 0.03 g L 1, MnSO4 H2O 0.05 g L 1, (NH4)6 ine, the mortar and pestle were all autoclaved at 121 °C for
MO7O24 4H2O0.05 g L 1, AlCl3 0.05 g L 1, CoCl2 6H2O 20 min for sterilization before use.
0.05 g L 1, NiCl2 0.05 g L 1.
2.4.2. Nutrient agar preparation
Nutrient agar was used as the growth medium for
2.2. Control of SRT
microbial isolation. For this purpose, 28 g of nutrient agar
was dissolved in 1 l of RO water, and was then autoclaved
In order to control SRT, the mixed liquor was discharged
at 121 °C for 20 min. After autoclaving, the agar was left to
from SBR during aeration, i.e., discharge of the mixed liquor
cool at room temperature for 15 min, and it was then
was done in the last 2 min of each aeration period. For a
poured out into SterilinÓ disposable Petri dishes.
desired SRT, the corresponding volume of mixed liquor to
be discharged can be calculated from the following formula:
2.4.3. Spread plate method
VX
SRTź ð1Þ Nine hundred microlitre of saline was pipetted into a
6ðV X þV XÞ
e e s
number of Eppendorf tubes for serial dilution. After the
sample had been dispersed, the suspension was stirred,
in which V is the volume of the reactor, 6 is the number of
and 100 lL of the sample was taken out. This volume of
cycles per day, Xe is the suspended solid concentration in
sample was added to a sterile 1.5 mL Eppendorf tube con-
the supernatant after settling, X is biomass concentration
taining 900 lL of sterile 0.85% saline, and it was further
in the complete-mix reactor equal to biomass concentration
in the discharged mixed liquor, while Ve and Vs are the vol- diluted by serial dilution into Eppendorf tubes, each con-
taining 900 lL of sterile saline. Serial dilution was carried
ume of discharged supernatant after sludge settling, and
out from 10 1 to 10 8 for each sample. From tubes with
the volume of discharged mixed-liquor in each cycle,
dilution factors of 10 3 to 10 8, 100 lL of the sample from
respectively. Eq. (1) can be rearranged to
each dilution was inoculated into a plate of nutrient agar
V V X
e e
V ź ð2Þ respectively. The sample was then spread around the plate
s
6SRT X
uniformly with the aid of a spreader, which was then cov-
To achieve a desirable SRT, Eq. (2) can be used to deter- ered and placed in a 30 °C incubator. Duplicates were done
mine corresponding volume of mixed liquor to be
for each dilution.
discharged.
2.4.4. Isolation of bacteria
2.3. Analytical methods After 5 days of the incubation at 30 °C, the total bacte-
rial number per plate was counted. Plates that did not have
Biomass concentrations in terms of total solids (TS) and a total count of between 25 and 250 colonies were dis-
volatile solids (VS) as well as sludge volume index (SVI) carded. From the plates that had 25 250 colonies, colonies
7674 Y. Li et al. / Bioresource Technology 99 (2008) 7672 7677
with similar morphologies were grouped together and
SRT= 3 days
numbered. Within each group, three colonies were picked
SRT= 12 days
SRT= 6 days
out and each was subcultured into a plate containing nutri-
SRT= 40 days
SRT= 9 days
ent agar, respectively. Triplicates were done in order to
0.35
minimize variation within each group.
0.3
2.4.5. Identification by biochemical kits 0.25
API 20 NE kit (bioMérrieux) was used for microbial
0.2
identification. Fresh subcultures of microorganisms from
0.15
each plate of isolate were inoculated as per the manufactur-
ers instructions. Each plate was tested twice with the kit to
0.1
ensure valid results. Numerical profiles obtained from the
0.05
test were determined by the API software, using API data-
base version 6.
0
40
10 20 30
0
Time (days)
3. Results
Fig. 1. Changes in aggregate size in the course of operation of SBRs at
different SRTs.
3.1. General observation by image analysis
On day 3 after the start-up of SBRs, some microbial
found to be less than 20% in all the reactors, indicating that
aggregates with a regular shape appeared in R1 run at
bioflocs would be dominant form of biomass.
the SRT of 3 days, while very few regular-shape aggregates
were observed on day 4 and day 5 in the SBRs operated at
3.3. Settleability of sludge
the SRTs of 6 40 days. After the first a few days, the evo-
lution of sludge morphology became insignificant in R1
Changes in the sludge volume index (SVI) at different
R5 until the reactors were stabilized in terms of constant
SRTs were determined in the course of SBR operation
biomass and effluent concentrations after the 30-day oper-
(Fig. 2). The SVI observed in all the reactors tended to
ation. At the steady state, it was found that aerobic gran-
decrease rapidly in the first 10-days of operation, and grad-
ules with a size bigger than 0.35 mm only accounted for a
ually approached a stable level of around 50 ml g 1 in all
very small fraction of total biomass in SBRs, i.e., bioflocs
the cases. In addition, a horizontal comparison across the
were absolutely the dominant form of biomass in all five
SRTs also shows that the SVI of sludge cultivated at the
SBRs operated at the SRT of 3 40 days.
SRT of 40 days decreased more slowly than those devel-
oped at the relatively short SRTs.
3.2. Evolution of sludge size
3.4. Biomass concentration
Fig. 1 shows the size evolution of microbial aggregates
in R1 R5 operated at different SRTs. The seed sludge
The biomass concentration in terms of MLSS was mea-
had a mean size of about 75 lm. A significant increase in
sured along with the reactor operation (Fig. 3a). The bio-
the aggregate size was observed in the first four days of
operation in all the SBRs. From day 4 onwards, the aver-
SRT= 3 days
age size of aggregates gradually stabilized in the SBRs run
SRT= 12 days
SRT= 6 days
at different SRTs of 3 40 days. It appears that no aerobic
SRT= 40 days
SRT= 9 days
granular sludge blanket was developed in the SBRs oper-
400
ated at the large SRT range of 3 40 days. Only a few aer-
350
obic granules with round shape were found after 30-days of
300
operation, while relatively a large quantity of tiny aggre-
gates seemed dominant in the sludge community cultivated 250
at the different SRTs.
200
The size distribution of aggregates was determined on
150
day 30. The peak values of the size distributions fell into
100
a narrow range of 150 350 lm in R1 R5. These seem to
50
indicate that the SRT in the range studied would not have
0
remarkable effect on the formation of aerobic granules.
10 20 30 40
0
Based on the size distribution, the fraction of aerobic gran-
Time (days)
ules defined as microbial aggregates with a mean size bigger
than 350 lm and a round shape (Qin et al., 2004) was Fig. 2. Changes in SVI in course of SBR operation at different SRTs.
Average floc size (mm)
-1
SVI (mL g )
Y. Li et al. / Bioresource Technology 99 (2008) 7672 7677 7675
3.5. Substrate removal kinetics
SRT= 3 days
SRT= 12 days
SRT= 6 days
SRT= 40 days
SRT= 9 days The TOC profiles within one cycle were determined after
21 days of operation in R1 R5. A fast TOC degradation
a 9
was observed in all five SBRs, i.e., nearly all input TOC
8
was removed during the first 20 min. These eventually lead
7
to a long famine period which has been believed to favor
6
aerobic granulation in SBR (Tay et al., 2001; Li et al.,
5
2006). Fig. 3b further revealed that the calculated TOC
4
removal rate was proportionally related to the SRT
3
applied, i.e., a higher TOC removal rate is observed at a
2
longer SRT. However, the lower specific TOC removal rate
1
was observed at higher SRT. This can be reasonably
0
explained by the differences in biomass concentrations as
010 20 40
30
shown in Fig. 3a.
Time (days)
3.6. Cell surface hydrophobicity
TOC removal rate Specific TOC removal rate
b
50 15
The cell surface hydrophobicities of sludges cultivated at
different SRTs were found to fall into a narrow range of
40 12
25 40%, while the seed sludge had a cell surface hydropho-
30 9
bicity of 22%. Only the cell surface hydrophobicity of
sludge developed at the SRT of 3 days seems slightly higher
20 6
than that of the seed sludge, whereas the cell surface hydro-
phobicities of sludges cultivated at the SRTs longer than 3
10 3
days are pretty comparable with that of the seed sludge.
These mean that the SRT in the range studied would not
0 0
have remarkable effect on the cell surface hydrophobicity.
3 6 9 12 40
SRT (d)
3.7. Shift in microbial population
Fig. 3. (a) Biomass concentration versus operation time; (b) TOC removal
rate versus SRT.
The sludges cultivated in R2 and R3 were sampled on
day 3, 10, 17, 24 for microbial analysis. Basically, total
12 species, namely No. 1 12, were isolated (Table 1). It
mass concentrations in R1 R5 gradually increased up to a
was found that the isolates Nos. 1, 5 and 8 were very close
stable level. It was found that the biomass concentration at
to the strain Brevundimonas vesicularis, while the isolates
steady state was proportionally related to the SRT applied,
Nos. 4, 7 and 9 could belong to the strain Comamonas tes-
i.e., a longer SRT would lead to a higher biomass
tosterone. Because of limitation of biokits (API 20 NE), the
accumulation.
isolates Nos. 10, 11 and 12 could not be identified and
Table 1
Distribution of microbial isolates identified in the course of operation of R2 and R3
Isolate no. Closest relative No. of species in R2 (108 CFU g 1 dry biomass) No. of species in R3 (108 CFU g 1 dry biomass)
Day 3 Day 10 Day 17 Day 24 Day 3 Day 10 Day 17 Day 24
1 Brevundimonas vesicularis 66.5 14.5 9.3 15.3 17.2 11.7 13.3 6.4
2 Ochrobactrum anthropi 29.6 6.2 26.7 11.1 27.1 6.7 13.3 7.1
3 Chryseobacterium indologenes 0.0 0.0 62.7 25.0 39.4 11.7 0.0 0.0
4 Comamonas testosterone 22.2 10.4 0.0 0.0 39.4 11.7 0.0 0.0
5 Brevundimonas vesicularis 0.0 0.0 16.0 0.0 0.0 16.8 16.8 9.5
6 Sphigobacterium spiritivorum 32.0 14.5 29.3 34.7 29.6 16.8 0.0 0.0
7 Comamonas testosterone 0.0 35.3 37.3 26.4 0.0 8.4 88.7 13.5
8 Brevundimonas vesicularis 0.0 0.0 80.0 40.3 14.8 23.5 10.6 12.7
9 Comamonas testosterone 17.2 8.3 0.0 0.0 0.0 0.3 7.1 0.8
10 Not identified 96.1 104.0 17.3 0.0 71.4 72.1 10.6 0.0
11 Not identified 0.0 0.0 82.7 61.1 0.0 0.0 35.5 20.6
12 Not identified 0.0 0.0 86.7 61.1 0.0 0.0 42.6 21.4
-1
MLSS (g L )
-1
-1
min )
-1
-1
-1
(mg L min g
MLSS)
TOC removal rate (mg L
Specific TOC removal rate
7676 Y. Li et al. / Bioresource Technology 99 (2008) 7672 7677
further study is needed in this regard. Table 1 further In fact, the observed growth yield (Yobs) determined at dif-
shows the population shifts in the course of operation of ferent SRTs decreased from 0.23 g MLSS g g 1 COD at
R2 and R3. It can be seen that the dominant species varied the SRT of 3 days to 0.05 g MLSS g 1 COD at the SRT
along with the reactor operation, e.g. isolate No. 10 was the of 40 days.
most dominant species on day 3 onwards in R2 and R3, Liu et al. (2005b) also reported a growth yield of
but this species completely disappeared from R2 and R3 0.29 MLSS g 1 COD and a decay rate of 0.023 0.075 d 1
on day 24. Isolates 11 and 12 were found to be undetectable for glucose-fed aerobic granules. In activated sludge model
on day 3 and day 10, while they became dominant starting No. 3 (Gujer et al., 1999), the decay rate for heterotrophic
from day 17 in both R2 and R3. It should be realized that bacteria has been reported in the range of 0.1 and 0.2 d 1
the shifting patterns of microbial species in R2 and R3 are at the 10 and 20 °C, respectively. Basically, a cycle of
similar, nevertheless the density of the isolates in terms col- SBR consists of feast and famine phases (Liu and Tay,
ony forming units (CFU) g 1 dry biomass is much higher 2004; McSwain et al., 2004). In this study, almost all exter-
in R2 than in R3. nal organics could be removed within the first half an hour
of each cycle, i.e., more than 75% of each SBR cycle would
be subject to famine condition, which would trigger a sig-
4. Discussion
nificant microbial decay eventually leading to the low
observed growth yields.
Existing evidence shows that the formation and struc-
It appears from Table 1 that in R2 and R3 operated
ture of aerobic granules are associated very closely with cell
at the respective SRT of 6 and 12 days, the shift pattern
surface hydrophobicity which can initiate cell-to-cell aggre-
and distribution of microbial species isolated did not
gation that is a crucial step towards aerobic granulation
show significant difference. For instance, on day 24, 10
(Liu et al., 2004). It is observed that the cell surface hydro-
isolates were found in the sludges cultivated in R2 and
phibicities of the sludges cultivated at the SRT of 3 40 days
R3, out of which 6 were the same. These seem to imply
are pretty comparable with that of the seed sludge. These
that in the present operation mode of SBRs, the selection
seem to imply that that the SRT in the range studied would
of microbial species by the applied SRT would be weak,
not induce significant changes in cell surface hydrophobic-
and such a weak selection on species may in turn, at
ity, and the low cell surface hydrophobicity observed in
least partially explain the fact that the properties of
turn may partially explain unsuccessful aerobic granulation
sludges developed in all five SBRs only showed some
in SBR. In addition, Liao et al. (2001) reported that hydro-
marginal differences as discussed earlier. As no successful
phobicities of sludges in terms of contact angle only
aerobic granulation was observed in R2 and R3, it is
increased from 25 to 35 degrees as the SRT was prolonged
hard to draw a solid conclusion with regard to the pos-
from 4 to 20 days.
sible correlation between aerobic granulation and the
In the field of environmental engineering, the SRT is
observed changes in microbial species. In fact, it has
correlated to the specific substrate utilization rate by the
been thought that aerobic granulation would not be clo-
following expression:
sely related to a particular microbial species because aer-
1
obic granules grown on a very wide spectrum of organic
źY qs Kd ð3Þ
t
SRT
carbons have been developed, including acetate, glucose,
in which qs is the specific substrate utilization rate in a cy- phenol, p-nitrophenol, nitrilotriacetic acid (NTA) and
cle, and Kd is the specific decay rate. According to Eq. (3), ferric-NTA complex synthetic and real wastewaters
Yt and Kd can be estimated from the plot of 1/SRT versus (Beun et al., 2000; Tay et al., 2001; McSwain et al.,
qs, i.e., 0.29 g MLSS g 1 COD for Yt and 0.12 d 1 for Kd. 2004; Schwarzenbeck et al., 2004; Nancharaiah et al.,
2006; Yi et al., 2006).
Qin et al. (2004)
Wang et al. (2006) This study
As discussed earlier, SRT in the range studied would not
100%
have a significant effect on the formation of aerobic gran-
ules in SBR. For a column SBR, the travel distance of bio-
80%
particles above the discharge port is L (distance between
water surface and discharging port). For a designed settling
60%
time (ts), bioparticles with a settling velocity less than L/ts
would be washed out of the reactor, while only those with a
40%
settling velocity greater than L/ts will be retained. Accord-
ing to Liu et al. (2005a), a minimum settling velocity
20%
(Vs)min exists in SBR, and it can be defined as follows:
L
0%
ðV Þminź ð4Þ
s
2
04 6 8 10
ts
(Vs)min (m h-1)
Eq. (4) shows that a long L or a short settling time would
result in a larger (Vs)min, and vice versa.
Fig. 4. Fraction of aerobic granules versus (Vs)min.
Fraction of aerobic granules
Y. Li et al. / Bioresource Technology 99 (2008) 7672 7677 7677
Giokas, D.L., Daigger, G.T., Sperling, M., Kim, Y., Paraskevas, P.A.,
It has been believed that aerobic granulation in a SBR is
2003. Comparison and evaluation of empirical zone settling velocity
driven by hydraulic selection pressure in terms of minimum
parameters based on sludge volume index used a unified settling
settling velocity of bioparticles (Liu et al., 2005a). This
characteristics database. Water Res. 37, 3821 3836.
means that to study the effect of SRT on aerobic granula-
Gujer, W., Henze, M., Mino, T., van Loosdrecht, M., 1999. Activated
tion in SBR, the interference of hydraulic selection pressure
sludge no. 3. Water Sci. Technol. 39, 183 193.
Liao, B.Q., Allen, D.G., Droppo, I.G., Leppard, G.G., Liss, S.N., 2001.
needs to be avoided. In this study, in order to look into the
Surface properties of sludge and their role in bioflocculation and
effect of SRT on aerobic granulation without interference
settleability. Water Res. 35, 339 350.
of hydraulic selection pressure, the selection pressure in
Li, Z.H., Kuba, T., Kusuda, T., 2006. The influence of starvation phase on
terms of (Vs)min was minimized to an extremely low level
the properties and the development of aerobic granules. Enzyme
of 0.76 0.78 m h 1. Qin et al. (2004) studied aerobic gran- Microb. Technol. 38, 670 674.
Liu, Y., Tay, J.H., 2004. State of the art of biogranulation technology for
ulation at different settling times with a fixed L, while
wastewater treatment. Biotechnol. Adv. 22, 533 563.
Wang et al. (2006) investigated aerobic granulation at dif-
Liu, Y., Yang, S.F., Tay, J.H., Liu, Q.S., Qin, L., Li, Y., 2004. Cell
ferent L at the constant settling time. Using those as well as
hydrophobicity is a triggering force of biogranulation. Enzyme
the data obtained in this study, a correlation of the fraction
Microb. Technol. 34, 371 379.
of aerobic granules and (Vs)min is shown in Fig. 4. It can be
Liu, Y., Wang, Z.W., Qin, L., Liu, Y.Q., Tay, J.H., 2005a. Selection
pressure-driven aerobic granulation in a sequencing batch reactor.
seen that the fraction of aerobic granules is proportionally
Appl. Microbiol. Biotechnol. 67, 26 32.
correlated to (Vs)min. Moreover, at a (Vs)min less than
Liu, L.L., Wang, Z.P., Yao, J., Sun, X.J., Cai, W.M., 2005b. Investigation
4mh 1, aerobic granulation is not favored in SBR, instead
on the formation and kinetics of glucose-fed aerobic granular sludge.
the growth of suspended sludge would be greatly encour-
Enzyme Microb. Technol. 36, 712 716.
aged. It should be realized that the typical settling velocity
McSwain, B.S., Wilderer, P.A., Irvine, R.L., 2004. The effect of intermit-
tent feeding on aerobic granule structure. Water Sci. Technol. 49, 19
of conventional activated sludge is generally less than
25.
5mh 1 (Giokas et al., 2003). These imply that for a SBR
Nancharaiah, Y.V., Schwarzenbeck, N., Mohan, T.V.K., Narasimhan,
operated at a (Vs)min lower than the settling velocity of con-
S.V., Wilderer, P.A., Venugopalan, V.P., 2006. Biodegradation of
ventional sludge, suspended sludge could not be effectively
nitrilotriacetic acid (NTA) and ferric-NTA complex by aerobic
withdrawn. As the result, suspended sludge will take over
microbial granules. Water Res. 40, 1539 1546.
Ng, H.Y., 2002. Performance of a membrane reactor and a completely
the entire reactor at low (Vs)min just as observed in this
mixed activated sludge system at short solid retention times. Ph.D.
study no matter how SRT was controlled. These results
Thesis, University of California, Berkley.
indicate that SRT would not be a primary factor governing
Pan, S., 2003. Inoculation of microbial granular sludge under aerobic
aerobic granulation in SBR.
conditions. Ph.D. Thesis, Nanyang Technological University,
Singapore.
Qin, L., Liu, Y., Tay, J.H., 2004. Effect of settling time on aerobic
5. Conclusion
granulation in sequencing batch reactor. Biochem. Eng. J. 21,
47 52.
This study for the first time systematically investigated
Rittmann, B.E., 1987. A critical evaluation of soluble microbial formation
the role of SRT in aerobic granulation in SBR. No success-
in biological processes. Water Sci. Technol. 19, 517 528.
ful aerobic granulation was observed at all studied SRTs,
Rosenberg, M., Gutnick, D., Rosenber, E., 1980. Adherence of bacteria to
hydrocarbons: a simple method for measuring cell-surface hydropho-
i.e., bioflocs were the dominant form of biomass at the
bicity. FEMS Microbiol. Lett. 9, 29 33.
SRTs studied. Different from the conventional activated
Schwarzenbeck, N., Erley, R., McSwain, B.S., Wilderer, P.A., Irvine,
sludge process, aerobic granulation in SBR is unlikely
R.L., 2004. Treatment of malting wastewater in a granular sludge
dependent on SRT, and this may have great engineering
sequencing batch reactor (SBR). Acta Hydrochim. Hydrobiol. 32, 16
implication in the design, optimization and operation of
24.
Tay, J.H., Liu, Q.S., Liu, Y., 2001. Microscopic observation of aerobic
a full scale aerobic granular sludge SBR.
granulation in sequential aerobic sludge reactor. J. Appl. Microbiol.
91, 168 175.
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