Biotechnology Advances 23 (2005) 335  344
www.elsevier.com/locate/biotechadv
Research review paper
A unified theory for upscaling aerobic granular
sludge sequencing batch reactors
Yu Liu*, Zhi-Wu Wang, Joo-Hwa Tay
Division of Environmental and Water Resources Engineering, School of Civil and Environmental Engineering,
Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore
Received 18 March 2005; received in revised form 1 April 2005; accepted 1 April 2005
Available online 28 April 2005
Abstract
Aerobic granular sludge sequencing batch reactors (SBR) are a promising technology for treating
wastewater. Increasing evidence suggests that aerobic granulation in SBRs is driven by selection
pressures exerted on microorganisms. Three major selection pressures have been identified as
follows: settling time, volume exchange ratio and discharge time. This review demonstrates that
these three major selection pressures can all be unified to one, the minimal settling velocity of
bioparticles, that determines aerobic granulation in SBRs. The unified selection pressure theory is a
useful guide for manipulating and optimizing the formation and characteristics of aerobic granules in
SBRs. Furthermore, the unified theory provides a single engineering basis for scale up of aerobic
granular sludge SBRs.
© 2005 Elsevier Inc. All rights reserved.
Keywords: Aerobic granulation; Sequencing batch reactor; Selection pressure; Scale up
Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 336
2. Major selection pressures in SBR . . . . . . . . . . . . . . . . . . . . . . . . . . 336
3. A unified selection pressure theory for aerobic granulation . . . . . . . . . . . . . 338
* Corresponding author.
E-mail address: [e-mail] (Y. Liu).
0734-9750/$ - see front matter © 2005 Elsevier Inc. All rights reserved.
doi:10.1016/j.biotechadv.2005.04.001
336 Y. Liu et al. / Biotechnology Advances 23 (2005) 335 344
4. Guidelines for upscaling aerobic granular sludge sequencing batch reactors. . . . . . 341
5. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343
1. Introduction
Aerobic granulation is a recently developed environmental biotechnology for waste-
water treatment and has been commonly reported in sequencing batch reactors (SBR)
(Morgenroth et al., 1997; Beun et al., 1999; Peng et al., 1999; Tay et al., 2001, 2004; Yang et
al., 2003; Arrojo et al., 2004; Schwarzenbeck et al., 2004; Zheng et al., 2005). Accumulated
evidence shows that aerobic granules form through self-immobilization of bacteria when
suitable selection pressure is provided in the SBR (Kim et al., 2004; McSwain et al., 2004;
Qin et al., 2004a,b; Hu et al., 2005; Wang et al., 2004; Liu et al., 2005). Compared to
continuous microbial culture, the unique feature of a SBR is its ability to be used in a cyclic
operation. A cycle may comprise filling, aeration, settling and sludge discharge.
Many factors influence the properties of aerobic granules formed in SBRs (Liu and Tay,
2004). Contributing factors include substrate composition, organic loading, hydrodynamic
shear force, feast famine regime, feeding strategy, dissolved oxygen, reactor config-
uration, solids retention time, cycle time, settling time and volume exchange ratio. While
all these factors influence properties of granules, only the factors associated with selection
pressure on sludge particles contribute to the formation mechanism of granules (Liu et al.,
2005). In SBR, two major selection pressures have been identified as the settling time and
the volume exchange ratio (Hu et al., 2005; McSwain et al., 2004; Qin et al., 2004a,b;
Wang et al., 2004; Liu et al., 2005). A third important selection pressure for aerobic
granulation has been shown to be the discharge time of SBR (Arrojo et al., 2004; Wang,
2005). Similarly, selection pressure is a key driving force for successful anaerobic
granulation in upflow anaerobic sludge blanket reactors (Hulshoff Pol et al., 1988;
Alphenaar et al., 1993).
Although nearly all research on aerobic granulation has been carried out in laboratory
scale SBRs, aerobic granulation technology is moving towards industrial use. A key factor
in successful scale up of aerobic granular sludge SBRs is the identification and modeling
of selection pressures that determine aerobic granulation. This review shows that the major
selection pressures for aerobic granulation in SBRs can be unified into a single
contributing influence, for scale up of SBRs.
2. Major selection pressures in SBR
Aerobic granulation is a microbial phenomenon that is induced by selection pressure
through changing microbial surface properties and metabolic behavior (Qin et al., 2004a,b;
Liu et al., 2005). So far, aerobic granulation has been reported in SBRs only. Compared to
continuous microbial culture, SBR is a fill-and-draw process that is fully mixed during the
batch reaction step. The sequential steps of aeration and clarification in a SBR occur in the
same tank (Metcalf and Eddy, 2003). The operation of nearly all SBR systems used in
Y. Liu et al. / Biotechnology Advances 23 (2005) 335 344 337
100
80
60
40
20
0
5 10 15 20
Settling time (min)
Fig. 1. Ratio of aerobic granules biomass to total biomass at different settling times. The discharge time and the
volume exchange ratio were fixed at 5 min and 50%, respectively (Qin et al., 2004a).
aerobic granulation comprises four steps: 1. feeding, 2. aeration, 3. settling, and 4.
discharge. The variables that impact on aerobic granulation in SBRs have been reviewed
elsewhere (Liu and Tay, 2004).
Available evidence shows that of the various possibilities, settling time and volume
exchange ratio constitute the major selection pressures on aerobic granulation. Thus, no
matter how other variables are manipulated, aerobic granulation in SBR fails without
proper control of settling time and volume exchange ratio (Liu et al., 2005). Figs. 1 and 2
clearly show that the fractions of aerobic granules in SBRs are highly correlated with
settling time and volume exchange ratio. For a reactor of a given diameter, the volume
exchange ratio translates to suspension discharge depth.
Arrojo et al. (2004) suggested discharge time as an additional parameter that influenced
selection pressures for aerobic granulation in SBR. Wang (2005) conducted a series of
systematic studies on the effect of discharge time on aerobic granulation and concluded
that aerobic granulation was closely related to discharge time at fixed values of settling
time and volume exchange ratio (Fig. 3). In summary, all evidence points to settling time,
100
80
60
40
20
0
80 60 40 20
Exchange ratio (%)
Fig. 2. Ratio of aerobic granules biomass to total biomass at different exchange ratios. The settling time and the
discharge time were fixed at 6 min and 1 min, respectively (Wang, 2005).
Fraction of aerobic granules (%)
Fraction of aerobic granules (%)
338 Y. Liu et al. / Biotechnology Advances 23 (2005) 335 344
100
80
60
40
20
0
510 15 20
Discharge time (min)
Fig. 3. Ratio of aerobic granules biomass to total biomass at different discharge times. The settling time and the
volume exchange ratio were fixed at 5 min and 50%, respectively (Wang et al., 2004).
volume exchange ratio and discharge time, as the major selection pressures for aerobic
granulation in SBRs. Any scale up of aerobic granular sludge SBR must obviously take
account of these selection pressures. Unfortunately, in the environmental engineering
literature, no method exists for unifying these selection pressures into a single entity for
use in scale up.
3. A unified selection pressure theory for aerobic granulation
Successful and stable aerobic granulation in SBRs is closely dependent on the applied
selection pressures in the forms of settling time, volume exchange ratio or discharge depth,
and discharge time (Arrojo et al., 2004; Hu et al., 2005; Kim et al., 2004; McSwain et al.,
2004; Qin et al., 2004a,b; Liu et al., 2005; Wang, 2005). Here we show that the three major
selection pressures can in fact be unified into one.
Consider a column SBR (Fig. 4a) with the effluent discharged at outlet located at depth
L, i.e. at the end of the designed settling time (ts), the volume of suspension above the
discharge port will be withdrawn during the preset discharge time (td). If the distance for
bioparticles to travel to the discharge port is L, the corresponding traveling time of
bioparticles can be calculated as follows:
L
Travelling time to distance port ź ð1Þ
Vs
where Vs is settling velocity of bioparticles. L is proportional to the volume exchange ratio
(Fig. 4a).
Eq. (1) shows that a high Vs results in short traveling time to discharge port. This
implies that bioparticles with a traveling time that is longer than the designed settling time,
will be discharged out of the SBR, i.e. there is a minimum settling velocity, (Vs)min for the
bioparticles to be retained in the reactor. (Vs)min can be defined as follows:
L
ðVsÞmin ź ð2Þ
effective settling time
Fraction of aerobic granules (%)
Y. Liu et al. / Biotechnology Advances 23 (2005) 335 344 339
L
H Discharge
Qd, max Qd
port
Aeration
td = td, min td > td, min
(a) (b)
Fig. 4. a) Schematic of a column SBR; b) hypothetical flows during discharge.
(Vs)min depends on the depth of the discharge port (L). From Fig. 3, there appears to exist a
minimum discharge time td,min at which the fraction of aerobic granules in the SBR is
close to 100%, i.e. a full granular sludge blanket is developed at td,min. If the discharge
time (td) is set to be longer than td,min, Fig. 4b illustrates that a portion of the liquor above
the discharge port would continue to settle during discharge time td, and this would
eventually lower the effective selection pressure on microorganisms. For td > td,min, the
settling time needs to be calibrated to take account of the discharge time. Therefore, the
effective settling time for use in Eq. (2) is defined as:
Effective settling time ź settling time preset ðtsÞ
þ relaxation of settling time due to td: ð3Þ
If the discharge flow rates at td and td,min are Qd and Qd,max, respectively, they can be
defined as follows:
Ve Ve
Qd;max ź and Qd ź : ð4Þ
td;min td
Here Ve is the exchange volume above the discharge port as shown in Fig. 4b. The
hypothetical flow that can settle is Qd,max Qd (Fig. 4b). Thus, the relaxation of settling
time due to td can be expressed as follows
Qd;max Qd
Relaxation of settling time due to td ź td td;min : ð5Þ
Qd;max
340 Y. Liu et al. / Biotechnology Advances 23 (2005) 335 344
Substitution of Eq. (4) in Eq. (5) leads to the following equation:
2
td;min td td;min
Relaxation of settling time due to td ź 1 td td;min ź :
td td
ð6Þ
Thus, Eq. (3) becomes
td td;min 2
Effective settling time ź ts þ : ð7Þ
td
Combining Eq. (7) with Eq. (2) gives
L
ðVsÞmin ź : ð8Þ
2
td td;min
ts þ
td
Eq. (8) combines the three major selection pressures (i.e. ts, td and L) in SBR into that
of the minimum settling velocity required for successful aerobic granulation. Fast settling
bioparticles tend to be heavy spherical aggregates while the slow settling particles tend to
be small, light and of irregular shapes. Clearly, bioparticles can be selected according to
their settling velocity. This has been confirmed in laboratory scale aerobic granular sludge
SBR (Liu et al., 2005). Eq. (8) provides a plausible explanation for why ts, L and td in
SBRs can serve as the effective selection pressures that allow for selecting particles that
settle easily.
The relationship between (Vs)min and the ratio of biomass of aerobic granules to the
total biomass, in stable SBRs operated at different selection pressures is shown in Fig. 5.
It can be seen that the fraction of aerobic granules in the reactor increases with the
increase of (Vs)min. Fig. 5 shows that at (Vs)min values smaller than 1.0 m· h 1, only
suspended bioflocs are cultivated and no aerobic granules are developed. As (Vs)min
100
80
60
40
20
0
0 2 4 6 8
(Vs)min (m h-1)
Fig. 5. Relationship between the mass fraction of aerobic granules and (Vs)min. .) at different settling times (Qin
et al., 2004a); o) at various volume exchange ratios (Wang, 2005); n) at different discharge times (Wang, 2005).
Fraction of aerobic granules (%)
Y. Liu et al. / Biotechnology Advances 23 (2005) 335 344 341
increases above 1.0 m· h 1, aerobic granular sludge blanket starts to form. At (Vs)min
value of 4.0 m· h 1, aerobic granules prevail over suspended flocs. As the typical settling
velocity of suspended activated sludge is generally less than 3 to 5 m· h 1 (Giokas et al.,
2003), these results seem to indicate that if the SBR is operated at a (Vs)min value below
that of suspended flocs, suspended sludge would not be effectively washed out of the
reactor. The specific growth rates and growth yield of aerobic granules are much lower
than that of suspended activated sludge (Tay et al., 2004; Yang et al., 2004), i.e.
suspended sludge can easily outcompete aerobic granules. Such outcompetition represses
the formation and growth of aerobic granules and eventually leads to disappearance of
aerobic granular sludge blanket in the SBR if suspended sludge is not effectively
withdrawn (Qin et al., 2004a,b; Liu and Tay, 2004). Therefore, (Vs)min must be controlled
at a level higher than the settling velocity of suspended sludge, or successful aerobic
granulation would not be achieved and maintained stably. Eq. (8) indicates that enhanced
selection of bioparticles for rapid aerobic granulation can be realized through proper
control of settling time, discharge time and the volume exchange ratio (or depth of
discharge port) in SBRs.
4. Guidelines for upscaling aerobic granular sludge sequencing batch reactors
Aerobic granules have been successfully developed in laboratory scale SBR with aspect
ratios of 1.9 to 20 (Morgenroth et al., 1997; Beun et al., 1999; Tay et al., 2001, 2004; Yang
et al., 2003; Qin et al., 2004a; McSwain et al., 2004; Schwarzenbeck et al., 2004; Zheng et
al., 2005). It had been proposed that SBRs should have a high aspect ratio to improve
selection of granules that settle (Beun et al., 2002). However, from Eq. (8), aspect ratio
does not appear to influence selection pressure in a SBR. Nevertheless, a large aspect ratio
is desirable because it allows more leeway to operators in manipulating L and, therefore,
(Vs)min (Kim et al., 2004; Liu et al., 2005).
In upscaling aerobic granular sludge SBR, settling time, discharge time and volume
exchange ratio (or depth of outlet port) must be properly controlled and manipulated (Eq.
(8)). Compared to the volume exchange ratio and discharge time, control of the settling
time is more flexible in manipulating the operation of a full-scale SBR. To avoid initial
washout of biomass, settling time should be gradually shortened from 20 min to as short as
2 min (Lin et al., 2003; Hu et al., 2005; Qin et al., 2004a,b; Tay et al., 2004). According to
Fig. 5, (Vs)min for enhanced aerobic granulation should not be less than 8 m· h 1.
Successful aerobic granulation has been obtained settling velocities of 10 and 16.2 m· h 1
(Beun et al., 2000, 2002).
As essential aspect of design of aerobic granular sludge SBR is the estimation of the
discharge time. Discharge time greatly influences the formation of aerobic granules (Eq.
(8)) and determines the discharge pumping rate (Eq. (4)) that relates to energy
consumption. In practice engineers and operators have limited scope for manipulating
the volume exchange ratio or depth of the outlet port of the reactor. Indeed, most
laboratory scale aerobic granular sludge SBRs are actually operated at a fixed volume
exchange ratio (Morgenroth et al., 1997; Tay et al., 2001; Lin et al., 2003; McSwain et al.,
2004; Wang et al., 2004). In practice it may be preferable to control the settling time and
342 Y. Liu et al. / Biotechnology Advances 23 (2005) 335 344
discharge time in order to achieve the minimum settling velocity required for aerobic
granulation; hence, Eq. (8) can be rewritten as follows:
td td;min 2
L
ts ź : ð9Þ
ðVsÞmin td
The following example considers a full scale column-type SBR with a diameter of 4 m
and reactor height of 8 m. A volume exchange ratio of 50% is assumed. This corresponds
to a discharge depth (L) of 4 m and a minimum discharge time of 5 min. The latter value is
based on laboratory studies, e.g. Fig. 3. If the minimum settling velocity is increased in
steps of 2 m· h 1 from an initial value of 8 m· h 1, the corresponding settling time and
discharge time can be determined by Eq. (9); thus,
ðtd 5Þ2
At ðVsÞmin ź 8 m· h 1; ts ź 30
td
ðtd 5Þ2
At ðVsÞmin ź 10 m· h 1; ts ź 24
td
ðtd 5Þ2
At ðVsÞmin ź 12 m· h 1; ts ź 20
td
ðtd 5Þ2
At ðVsÞmin ź 16 m· h 1; ts ź 15
td
ðtd 5Þ2
At ðVsÞmin ź 20 m· h 1; ts ź 12 :
td
The above equations show relationships of the settling time to the discharge time at
different desired minimum settling velocities (Fig. 6). The salient points from Fig. 6 are as
follows: 1. any pair of ts and td that satisfies the above ts td relationship would result in
30
(Vs)min (m/h)
25
8
10
20
12
16
20
15
10
5
0
5 10 15 20 25 30 35 40
td (min)
Fig. 6. Relationship between settling time and discharge time for a desired (Vs)min.
s
t (min)
Y. Liu et al. / Biotechnology Advances 23 (2005) 335 344 343
the desired (Vs)min for aerobic granulation, i.e. a longer td would be compensated by a
shorter ts, and vice versa; 2. for any given settling time, we can compute the discharge time
required; 3. the longest settling time for the desired (Vs)min can be estimated; 4. the longest
discharge time allowed for achieving desired (Vs)min can be calculated. Such information
is essential for rational design and operation of aerobic granular sludge SBRs.
It should be realized that the discharge time directly determines the effluent pumping
rate that relates to the energy cost, i.e. a short discharge time results in a higher effluent
pumping rate. From the point of view of the SBR operation, we prefer a long discharge
time in order to reduce pumping power. However, a long discharge time can be obtained
only by shortening the settling time as shown in Fig. 6. Studies in laboratory scale SBRs
suggest that optimal settling time for successful aerobic granulation is less than 5 min (Tay
et al., 2001; McSwain et al., 2004; Tay et al., 2004; Qin et al., 2004a,b). If a similar settling
time applies to the above full scale SBR, we can compute the discharge time needed for
achieving a desired (Vs)min for aerobic granulation, e.g. 34.4 min for a (Vs)min of 8 m· h 1.
Consequently, Eq. (8) offers a guide for design and operation engineers to manipulate
aerobic granulation process through adjustments of the settling time, discharge time and
the volume exchange ratio.
5. Summary
Current understanding of aerobic granulation in SBRs identifies three major selection
pressures that select for formation of bioparticles and their characteristics. These selection
pressures are settling time, volume exchange ratio and discharge time. Analysis presented
in this review shows that the three major selection pressures can be unified into that of
minimal settling velocity alone. Design guidelines are provided for successfully scaling up
SBRs to obtain stable aerobic granulation.
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