A unified theory for upscaling aerobic granular

Research review paper

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,50Nanyang Avenue,Singapore 639798,Singapore

Received 18March 2005;received in revised form 1April 2005;accepted 1April 2005

Available online 28April 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.

Keywords:Aerobic granulation;Sequencing batch reactor;Selection pressure;Scale up

Contents

Introduction......................................3362.

Major selection pressures in SBR ..........................3363.A unified selection pressure theory for aerobic granulation .............3380734-9750/$-see front matter D 2005Elsevier Inc.All rights reserved.doi:10.1016/j.biotechadv.2005.04.001

*Corresponding author.

E-mail address:cyliu@https://www.360docs.net/doc/e913386188.html,.sg (Y .Liu).

Biotechnology Advances 23(2005)335

–344

https://www.360docs.net/doc/e913386188.html,/locate/biotechadv

4.Guidelines for upscaling aerobic granular sludge sequencing batch reactors......

3415.Summary........................................

343References (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 https://www.360docs.net/doc/e913386188.html,pared 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

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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.1and 2clearly 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,

20406080100510Settling time (min)

F r a c t i o n o f a e r o b i c g r a n u l e s (%)1520Fig.1.Ratio of aerobic granules biomass to total biomass at different settling times.The discharge time and the volume exchange ratio were fixed at 5min and 50%,respectively (Qin et al.,2004a

2040608010080604020

F r a c t i o n o f a e r o b i c g r a n u l e s (%)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 6min and 1min,respectively (Wang,2005).

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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 (t s ),the volume of suspension above the discharge port will be withdrawn during the preset discharge time (t d ).If the distance for bioparticles to travel to the discharge port is L ,the corresponding traveling time of bioparticles can be calculated as follows:

Travelling time to distance port ?L V s

e1Twhere V s is settling velocity of bioparticles.L is proportional to the volume exchange ratio (Fig.4a).

Eq.(1)shows that a high V s 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,(V s )min for the bioparticles to be retained in the reactor.(V s )min can be defined as follows:

V s eTmin ?L

effective settling time e2

5101520

F r a c t i o n o f a e r o b i c g r a n u l e s (%)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 5min and 50%,respectively (Wang et al.,2004).

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(V s )min depends on the depth of the discharge port (L ).From Fig.3,there appears to exist a minimum discharge time t d ,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 t d ,min .If the discharge time (t d )is set to be longer than t d ,min ,Fig.4b illustrates that a portion of the liquor above the discharge port would continue to settle during discharge time t d ,and this would eventually lower the effective selection pressure on microorganisms.For t d N t d ,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 ?settling time preset t s eT

trelaxation of settling time due to t d :e3T

If the discharge flow rates at t d and t d,min are Q d and Q d,max ,respectively,they can be defined as follows:

Q d ;max ?V e

t d ;min and Q d ?V e t d :e4T

Here V e is the exchange volume above the discharge port as shown in Fig.4b.The hypothetical flow that can settle is Q d,max àQ d (Fig.4b).Thus,the relaxation of settling time due to t d can be expressed as follows

Relaxation of settling time due to t d ?Q d ;max àQ d Q d ;max ?t d àt d ;min àá:e5T

t d = t d , min t d > t d , min

(a)Aeration (b)

Fig.4.a)Schematic of a column SBR;b)hypothetical flows during discharge.

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Substitution of Eq.(4)in Eq.(5)leads to the following equation:

Relaxation of settling time due to t d ? 1àt d ;min t d t d àt d ;min àá?t d àt d ;min àá2t d

:e6T

Thus,Eq.(3)becomes

Effective settling time ?t s tt d àt d ;min àá2t d

:e7T

Combining Eq.(7)with Eq.(2)gives

V s eTmin ?

L t s tt d àt d ;min

àá2t d :e8TEq.(8)combines the three major selection pressures (i.e.t s ,t d 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 t s ,L and t d in SBRs can serve as the effective selection pressures that allow for selecting particles that settle easily.

The relationship between (V s )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 (V s )min .Fig.5shows that at (V s )min values smaller than 1.0m d h à1,only suspended bioflocs are cultivated and no aerobic granules are developed.As (V s )

min

F r a c t i o n o f a e r o b i c g r a n u l e s (%)(V s )min (m h -1)

Fig.5.Relationship between the mass fraction of aerobic granules and (V s )min ..)at different settling times (Qin et al.,2004a );o )at various volume exchange ratios (Wang,2005);n )at different discharge times (Wang,2005).

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Y.Liu et al./Biotechnology Advances23(2005)335–344341 increases above1.0m d hà1,aerobic granular sludge blanket starts to form.At(V s)min value of4.0m d hà1,aerobic granules prevail over suspended flocs.As the typical settling velocity of suspended activated sludge is generally less than3to5m d hà1(Giokas et al., 2003),these results seem to indicate that if the SBR is operated at a(V s)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,(V s)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 of1.9to20(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, (V s)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 from20min to as short as 2min(Lin et al.,2003;Hu et al.,2005;Qin et al.,2004a,b;Tay et al.,2004).According to Fig.5,(V s)min for enhanced aerobic granulation should not be less than8m d hà1. Successful aerobic granulation has been obtained settling velocities of10and16.2m d hà1 (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

discharge time in order to achieve the minimum settling velocity required for aerobic granulation;hence,Eq.(8)can be rewritten as follows:

t s ?L V s eTmin àt d àt d ;min àá2t d

:e9TThe following example considers a full scale column-type SBR with a diameter of 4m and reactor height of 8m.A volume exchange ratio of 50%is assumed.This corresponds to a discharge depth (L )of 4m and a minimum discharge time of 5min.The latter value is based on laboratory studies,e.g.Fig.3.If the minimum settling velocity is increased in steps of 2m d h à1from an initial value of 8m d h à1,the corresponding settling time and discharge time can be determined by Eq.(9);thus,

At V s eTmin ?8m d h à1;

t s ?30àt d à5eT2t d At V s eTmin ?10m d h à1;t s ?24àt d à5eT2

t d

At V s eTmin ?12m d h à1;t s ?20àt d à5eT2

t d

At V s eTmin ?16m d h à1;t s ?15àt d à5eT2

t d

At V s eTmin ?20m d h à1;

t s ?12àt d à5eT2

d :Th

e above equations show relationships o

f the settlin

g time to the discharge time at different desired minimum settling velocities (Fig.6).The salient points from Fig.6are as follows:1.any pair of t s and t d that satisfies the above t s àt d relationship would result in

152025

t s (m i n )t d (min)

Fig.6.Relationship between settling time and discharge time for a desired (V s )min .

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Y.Liu et al./Biotechnology Advances23(2005)335–344343 the desired(V s)min for aerobic granulation,i.e.a longer t d would be compensated by a shorter t s,and vice versa;2.for any given settling time,we can compute the discharge time required;3.the longest settling time for the desired(V s)min can be estimated;4.the longest discharge time allowed for achieving desired(V s)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 than5min(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(V s)min for aerobic granulation,e.g.34.4min for a(V s)min of8m d hà1. 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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