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Review

Recent advances in membrane bioreactors(MBRs): Membrane fouling and membrane material

Fangang Menga,c,*, So-Ryong Chaeb, Anja Drewsc, Matthias Kraumec, Hang-Sik Shind,Fenglin Yanga

aKey Laboratory of Industrial Ecology and Environmental Engineering, MOE, School of Environmental and Biological Science and Technology,

Dalian University of Technology, Dalian 116024, PR ChinabDepartment of Civil and Environmental Engineering, School of Engineering, Duke University, Durham, NC 27708, USAcChair of Chemical Engineering, Technische Universitat Berlin, Str. des 17. Juni 135, MA 5-7, 10623 Berlin, GermanydDepartment of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, South Korea

a r t i c l e i n f o

Article history:

Received 7 April 2008

Received in revised form

19 December 2008

Accepted 22 December 2008

Published online 3 January 2009

Keywords:

Membrane bioreactor (MBR)

Membrane fouling

Extracellular polymeric substances

(EPS)

Soluble microbial products (SMP)

Membrane modification

Abbreviations: AFM, atomic force microscBAC, biologically activated carbon; BAP, bdynamics; CLSM, confocal laser scanning mgradient gel electrophoresis; DO, dissolvedobservation through membrane; EPS, extrachybridization; FTIR, Fourier transform infrarretention time; MBR, membrane bioreactor;mixed liquid suspended solid; NF, nanofiltracarbon; PAN, polyacrylonitrile; PCR, polymePOEM, polyoxyethylene methacrylated; PPHRO, reverse osmosis; SEM, scanning electronretention time; TFC, thin film composite; Ttration; VSS, volatile suspended solid.

* Corresponding author. Key Laboratory ofBiological Science and Technology, Dalian U

E-mail address: fgmeng80@126.com (F. M0043-1354/$ – see front matter ª 2009 Elsevidoi:10.1016/j.watres.2008.12.044

a b s t r a c t

Membrane bioreactors (MBRs) have been actively employed for municipal and industrial

wastewater treatments. So far, membrane fouling and the high cost of membranes are

main obstacles for wider application of MBRs. Over the past few years, considerable

investigations have been performed to understand MBR fouling in detail and to develop

high-flux or low-cost membranes. This review attempted to address the recent and current

developments in MBRs on the basis of reported literature in order to provide more detailed

information about MBRs. In this paper, the fouling behaviour, fouling factors and fouling

control strategies were discussed. Recent developments in membrane materials including

low-cost filters, membrane modification and dynamic membranes were also reviewed.

Lastly, the future trends in membrane fouling research and membrane material develop-

ment in the coming years were addressed.

ª 2009 Elsevier Ltd. All rights reserved.

opy; AGMBR, aerobic granular sludge membrane bioreactor; ANN, artificial neural network;iomass-associated products; BOD, biological oxygen demand; CFD, computational fluidicroscopy; COD, chemical oxygen demand; CST, capillary suction time; DGGE, denaturingoxygen; DOC, dissolved organic carbon; DON, dissolved organic nitrogen; DOTM, directellular polymeric substance; F/M, food to microorganism ratio; FISH, fluorescence in situ

ed spectroscopy; HP-SEC, high performance size exclusion chromatography; HRT, hydraulicMF, microfiltration; MFE, membrane flux enhancer; MFR, membrane fouling reducer; MLSS,tion; NMR, nuclear magnetic resonance; OLR, organic loading rate; PAC, powdered activatedrase chain reaction; PE, polyethylene; PES, polyethersulfone; PFS, polymeric ferric sulfate;FMM, polypropylene hollow fiber microporous membrane; PVDF, polyvinylidene fluoride;microscopy; SMP, soluble microbial products; SRF, sludge resistance to filtration; SRT, solid

MP, transmembrane pressure; UAP, substrate-utilisation-associated products; UF, ultrafil-

Industrial Ecology and Environmental Engineering, MOE, School of Environmental andniversity of Technology, Dalian 116024, PR China. Tel.: þ86 411 84706172.eng).er Ltd. All rights reserved.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21490

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14902. Fundamentals of membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1491

2.1. Characteristics of membrane fouling and its importance in MBRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14912.2. Classification of membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1492

2.2.1. Removable and irremovable fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14922.2.1.1. Definition of removable and irremovable fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14922.2.1.2. Formation of the cake layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14922.2.1.3. Irremovable fouling in MBRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1493

2.2.2. Biofouling, organic fouling, and inorganic fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14942.2.2.1. Biofouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14942.2.2.2. Organic fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14952.2.2.3. Inorganic fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1495

3. Fouling factors and control strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14963.1. Bound EPS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1496

3.1.1. Definition of bound EPS and SMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14963.1.2. Effect of bound EPS on membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14963.1.3. Behaviour and control of bound EPS in MBRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1498

3.2. SMP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14993.2.1. Effect of SMP on membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14993.2.2. Behaviour and control of SMP in MBRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1499

3.2.2.1. Control of SMP via adjustment of operation conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14993.2.2.2. Control of SMP via addition of adsorbents/coagulants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1501

3.3. Hydrodynamic conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15013.3.1. Effect of hydrodynamic conditions on membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15013.3.2. Favorable hydrodynamic conditions mitigating membrane fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1502

3.4. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15024. Developments of membranes/filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1503

4.1. Influence of membrane characteristics on MBR performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15034.2. Application of low-cost filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15044.3. Membrane modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15044.4. Dynamic membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1505

5. Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1506

1. Introduction activated sludge is the main cause that puzzles us. Further-

Annual publications on MBR fouling(Google scholar)

1

10

100

1000

1994 1996 1998 2000 2002 2004 2006 2008

Year

Num

ber

of p

ublic

atio

ns

Fig. 1 – The diagram showing the annual publication on

MBR fouling.

MBRs are being increasingly used for wastewater treatment

that requires excellent effluent quality, e.g., water reuse or

water recycling (Judd, 2006, 2008; Liao et al., 2006; Yang et al.,

2006; Wang et al., 2008a). MBRs allow high concentrations of

mixed liquor suspended solids (MLSS) and low production of

excess sludge, enable high removal efficiency of biological

oxygen demand (BOD) and chemical oxygen demand (COD),

and water reclamation. However, membrane fouling is

a major obstacle to the wide application of MBRs. Additionally,

large-scale use of MBRs in wastewater treatment will require

a significant decrease in price of the membranes.

During the last few years, Chang et al. (2002a) and Le-Clech

et al. (2006) reviewed MBR fouling by focusing on almost all the

fouling factors; namely, they provided a very comprehensive

review on sludge characteristics, operational parameters,

membrane materials and feedwater characteristics. In recent

years, a considerable number of papers were published, e.g.,

the annual publication reached nearly 400 in 2006 and 2007

(see Fig. 1). To date we are still confused with MBR fouling,

even though numerous investigations have been performed.

In fact, the complex nature of membrane foulants and

more, these investigations were of different focus and there-

fore, it is necessary to summarize and compare the results

obtained in recent years.

To complement the current knowledge on MBR fouling,

this review paper was mainly focused on two issues:

MBR fouling(Foulants)

Activatedsludge

SRT, HRT, F/M,DO, OLR

Aeration,Cleaning

Biomass-related aspects

Operation conditions

Controlfouling

Modifysludge

Determinefouling

Fig. 2 – Schematic illustration showing the fouling affecting

factors and controlling approaches.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 2 1491

fundamentals of membrane fouling (see Section 2) and sludge

characteristics (see Section 3). The operating parameters such

as SRT, HRT, dissolved oxygen (DO) and food to microor-

ganism ratio (F/M) have no direct effect on membrane fouling;

but they determine the sludge characteristics and, the opti-

misation of them can modify activated sludge (see Fig. 2).

Therefore, to find out the effective approaches (e.g., suitable

operating parameters) for the modification of activated

sludge, the influence of operating parameters on sludge

characteristics was analysed in this review paper. In addition,

this paper also updated recent challenges for characterisation

and control of membrane fouling by incorporating some new

findings such as the influence of filamentous bulking on MBR

fouling and the use of computational fluid dynamics (CFD).

Lastly, the development of new membrane/filter materials to

overcome current problems related to membrane cost and

membrane fouling was discussed.

2. Fundamentals of membrane fouling

2.1. Characteristics of membrane fouling and itsimportance in MBRs

Membrane fouling is a major obstacle that hinders faster

commercialisation of MBRs. As shown in Fig. 3, membrane

fouling in MBRs can be attributed to both membrane pore

a b

Fig. 3 – Membrane fouling process in MBR

clogging and sludge cake deposition on membranes which is

usually the predominant fouling component (Lee et al., 2001).

Membrane fouling results in a reduction of permeate flux or

an increase of transmembrane pressure (TMP) depending on

the operation mode.

With respect to MBRs, membrane fouling occurs due to the

following mechanisms: (1) adsorption of solutes or colloids

within/on membranes; (2) deposition of sludge flocs onto the

membrane surface; (3) formation of a cake layer on the

membrane surface; (4) detachment of foulants attributed

mainly to shear forces; (5) the spatial and temporal changes of

the foulant composition during the long-term operation (e.g.,

thechange of bacteriacommunity and biopolymer components

in the cake layer). In other words, the membrane fouling can be

defined as the undesirable deposition and accumulation of

microorganisms, colloids, solutes, and cell debris within/on

membranes. Given the complex nature of the activated sludge,

it is not surprising that the fouling behaviour in MBRs is more

complicated than that in most membrane applications.

Generally, as shown in Fig. 4, a three stage fouling history might

be proposed (Cho and Fane, 2002; Zhang et al., 2006a):

- Stage 1: an initial short-term rapid rise in TMP;

- Stage 2: a long-term weak rise in TMP;

- Stage 3: a sharp increase in dTMP/dt, also known as TMP

jump (Cho and Fane, 2002).

Fig. 4 shows the schematic illustration of the occurrence of

TMP jump. The TMP jump is believed to be the consequence of

severe membrane fouling. Cho and Fane (2002) attributed the

TMP jump to the changes in the local flux due to fouling

eventually causing local fluxes to be higher than the critical

flux. Latterly, Zhang et al. (2006a) reported that the sudden

jump was possibly not only due to the local flux effect, but also

caused by sudden changes of the biofilm or cake layer struc-

ture. Due to oxygen transfer limitation, the bacteria in the

inner biofilms tend to die and release more extracellular

polymeric substances (EPS). A more recent investigation also

confirmed that the sudden jump of TMP was closely related to

the sudden increase in the concentration of EPS at the bottom

of cake layer, which might be attributed to the death of

bacteria in the inner of cake layer (Hwang et al., 2008).

The occurrence of the TMP jump also depends on operating

conditions. Zhang et al. (2006a) observed that an abrupt TMP

jump of over 10 kPa was observed at 24 and 48 h for the fluxes

of 30 and 20 L/(m2 h), respectively, in a lab-scale MBR which

was used to treat synthetic wastewater. However, there was

no TMP jump during the 280 h operation at 10 L/(m2 h). Pollice

et al. (2005) reported that the TMP jump was more frequently

observed in small-scale experiments. It should be borne in

mind that fouling rates measured in lab-scale are

Sludge particles

Colloids

Solutes

s: (a) pore blocking and (b) cake layer.

Filtration time

TM

P

TMP Jump

Fig. 4 – Schematic illustration of the occurrence of TMP

jump.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21492

inappropriate to describe long-term full-scale operation due

to distinct and inherent differences between them (Kraume

et al., 2009). From these investigations it can be concluded that

the interactions between TMP jump and these operating

parameters are very complex, and TMP jump occurs inevitably

during long-term operation of MBRs. Thus, the overall goal of

fouling control is to retard the occurrence of the TMP jump via

modifying sludge characteristics or decreasing membrane

flux (e.g., operation below critical flux).

In MBRs, the prediction or measurement of fouling resis-

tance is significant for the understanding of fouling extent, or

for the optimisation of operating conditions. Furthermore, the

knowledge of the filtration mechanisms can help to find an

appropriate method to avoid fouling. Some empirical models

have also been proposed, which aim at expressing the relation

between sludge characteristics or operating conditions and

membrane fouling. Table 1 shows the expressions developed

to describe membrane flux or membrane fouling resistance in

MBRs. These empirical models are helpful for the under-

standing and mitigation of membrane fouling in MBRs.

2.2. Classification of membrane fouling

2.2.1. Removable and irremovable fouling

2.2.1.1. Definition of removable and irremovable fouling.Membrane fouling is a very complicated phenomenon and

results from multiple causes. Particle sizes of sludge flocs,

colloids and solutes in mixed liquor may strongly affect

fouling mechanisms in a membrane filtration system. If fou-

lants are comparable with the membrane pores (i.e., colloids),

or smaller than the membrane pores (i.e., solutes), adsorption

on pore wall and pore blocking may occur. However, if fou-

lants (i.e., sludge flocs and colloids) are much larger than the

membrane pores, they tend to form a cake layer on the

membrane surface.

By now, the concepts of reversible fouling and irreversible

fouling are confusing because of different definitions

proposed in publications. Generally, the irreversible fouling

should be defined as the fouling that cannot be removed by

any methods including chemical cleaning. But, some previous

studies defined the irreversible fouling as the fouling that can

be removed by chemical cleaning but cannot be removed by

physical cleaning. Here, we define three types of fouling:

removable fouling, irremovable fouling and irreversible

fouling. As shown in Fig. 5, the removable fouling can be easily

eliminated by implementation of physical cleaning (e.g.,

backwashing) while the irremovable fouling needs chemical

cleaning to be eliminated. The removable fouling and revers-

ible fouling are the same. The removable fouling is caused by

loosely attached foulants; however, irremovable fouling is

caused by pore blocking and strongly attached foulants during

filtration. The irreversible fouling is a permanent fouling

which cannot be removed by any approaches. In general,

removable fouling is attributed to the formation of cake layer,

and the irremovable fouling is attributed to pore blocking.

2.2.1.2. Formation of the cake layer. In many cases, cake layer

formation linked with removable fouling was considered as

the major contributor to membrane fouling in MBRs. Lee et al.

(2001) reported that the filtration resistances included

membrane resistance (12%), cake resistance (80%), blocking

and irremovable fouling resistance (8%), indicating that the

formation of cake layer is the main cause leading to

membrane fouling. Table 2 shows the relevant reports about

the importance of cake layer formation on membrane fouling.

Recently, a large number of scientific investigations have

been performed in order to gain a better understanding of

cake layer formation and cake layer morphology. Chu and Li

(2005) reported that the cake layer was not uniformly distrib-

uted on the entire surface of all of the membrane fibers. The

membranes were covered partially by a static sludge cake that

could not be removed by the shear force due to aeration, and

partially by a thin sludge film that was frequently washed

away by aeration turbulence. The filtration resistances of the

sludge cake and thin sludge film were 308� 1011 and

32.5� 1011 m�1, respectively. They also pointed out that the

deposited biopolymers allow easier and faster bacterial

adhesion. In addition, the EPS holds the flocs more tightly on

the membrane and increases the difficulty of cake removal by

aeration turbulence. Jeison and van Lier (2007) performed

a study on a lab-scale anaerobic submerged membrane

bioreactor (AnMBR) for over 200 days, and observed that cake

formation was removable on a short-term basis, however,

cake consolidation was observed when a long-term operation

was performed at a flux close to the critical flux. The consol-

idated cake could not be removed by the back-flush cycles,

and required an external physical cleaning procedure. At the

same time, Di Bella et al. (2007) found that the cake in an

aerobic MBR had a mainly removal nature. These investiga-

tions suggest that the cake layer formed with aerobic sludge

and anaerobic sludge might have different removability.

A cake layer can be described as a porous media with

a complex system of interconnected inter-particle voids. Yang

et al. (2007) simulated the intra-layer flow field by using the

three-dimensional volumetric grid model and confocal laser

scanning microscopy (CLSM) analysis, and observed that there

was a very complex flow pattern in the fouling layer. Because

of the inter-connectivity of the neighboring pores, the flow

direction may even be the reverse of that of the pressure

gradient (dP/dx). Recently, multiphoton microscopy, which

provides in situ 3D characterisation, was employed to char-

acterise protein or yeast fouling, and a combination of 3D

images and resistance data could be used to identify the

Table 1 – Expressions developed to describe membrane flux or membrane fouling resistance (modified after Judd andJefferson, 2003).

Application Expressiona Remarks Ref.

Classical cake

filtration

J ¼ DPmðRmþaCMLSSÞ

J is membrane flux (L/m2 h), DP is TMP, CMLSS is the

biomass concentration (mg/L), m is sludge viscosity

(mPa s), a is specific cake resistance (m/kg).

(Shimizu et al., 1993;

Chang et al., 2001; Chang

and Kim, 2005)

Concentration

polarisation

J ¼ aþ b logðDCDOCÞ DCDOC is the differential DOC concentration between the

activated sludge and the permeate (mg/L).

(Ishiguro et al., 1994)

Cross-flow

MBR

J ¼ DP

m

�Rm þ 843DPC0:926

MLSSC1:37CODm0:326

� CMLSS is sludge concentration (mg/L), CCOD is COD value

(mg/L), m is sludge viscosity (mPa s).

(Sato and Ishii, 1991)

Cross-flow

MBR J ¼ J0exp

�kReðCMLSS � CMLVSSÞ

CMLVSS

� MLSS is sludge concentration (kg/m3), MLVSS is volatile

sludge concentration (kg/m3), k is a constant related with

TMP, Re is Reynolds number. J0 is the initial membrane

flux (L/m2 h).

(Krauth and Staab, 1993)

Submerged

MBR

Rt ¼ Rm þ am The accumulation, detachment and consolidation of EPS

on the membranes were considered. m is EPS density on

the membrane surface (kg/m2).

(Nagaoka et al., 1998)

Submerged

MBR

Rt ¼ Rm þ am m ¼ kmVpXTSS

A The activated sludge model No. 1 (ASM1) is used to

describe membrane fouling. Where a is specific resistance

of accumulated mass (m/kg), m is accumulated mass on

the membranes (kg/m2), A is membrane area (m2), Vp is

permeate volume (m3), XTSS is total suspended solids, km is

efficiency of cross-flow velocity, ranging from 0 to 1.

(Lee et al., 2002)

Submerged

MBR

K ¼ ð8:93� 107Þ � C0:532MLSS � J0:376 � U�3:05

a K is the increasing rate of filtration resistance (1/(m h)), X is

sludge concentration (mg/L), J is membrane flux (L/m2 h),

ULr is observed cross-flow velocity of the tap water in the

membrane zone (m/s).

(Liu et al., 2003)

Submerged

MBR

Rf ¼ 2:25 expðMLSS� 9� 10�5Þ þ 0:111EPS�1:99� 10�2PSD� 3:20

Rf is the fouling resistance after 4 hours’ filtration with

a constant TMP of 3.97 kPa, EPS is the bound extracellular

polymeric substances (mg/g-MLSS), PSD is mean particle

size (mm).

(Meng et al., 2006b)

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 2 1493

dominant fouling mechanism (Hughes et al., 2006, 2007). The

multiphoton microscopy might provide a potential method to

study MBR fouling, especially for the study of soluble micro-

bial product (SMP) fouling.

2.2.1.3. Irremovable fouling in MBRs. Although most of the

recent research activities are focused on the fundamental

understanding of the cake layer, the investigation and control

of irremovable membrane fouling is of great importance for

long-term and sustainable operation of MBRs. During initial

filtration, colloids, solutes and microbial cells pass through

and precipitate inside the membrane pores. But, during the

long-term operation of MBRs, the deposited cells multiply and

yield EPS, which clog the pores and form a strongly attached

fouling layer. At the same time, some inorganic substances

might progressively precipitate onto the membranes or into

the membrane pores (see Section 2.2.2). The occurrence of

MBR fouling is a very complex process. Thus, how to predict

and control fouling is of great significance for MBR operation.

Operation below the critical flux is an effective approach to

avoid severe fouling including removal and irremovable

fouling within a given filtration system. Field et al. (1995)

introduced critical flux concept, operation below the critical

flux concept is called sub-critical flux or non-fouling operation

and is expected to lead to little irremovable fouling. For

a short-term membrane filtration, when the permeate flux is

set below the critical flux, the TMP remains stable and fouling

was removable. In contrast, when it exceeds the critical flux,

the TMP increases and might lead to a TMP jump. As a matter

of fact, for a long-term operation of MBRs, irremovable fouling

can occur even if they are operated below the critical flux.

Ognier et al. (2004) reported that despite the initial choice of

sub-critical flux filtration conditions, gradual fouling was seen

to develop which, after long periods of operation without

intermediary membrane regeneration, proved to be hydrau-

lically irremovable. The critical flux value depends on

membrane characteristics, operating conditions (i.e., aeration

intensity, temperature), and sludge characteristics. Further

discussion of critical flux can be found in recent review arti-

cles (Pollice et al., 2005; Bacchin et al., 2006). The concept of

critical flux has been popularly used in the study of MBR

fouling (Guglielmi et al., 2007b; Lebegue et al., 2008; Wang

et al., 2008b). However, most of the investigations on the

determination of critical flux are based on ex-situ measuring,

which cannot offer the real fouling propensity. Recently, an in

situ method was developed by de la Torre et al. (2008), which

can provide more reliable information about critical flux than

ex-situ methods. Huyskens et al. (2008) developed an on-line

measuring method, which was used to evaluate the remov-

able and irremovable fouling propensity of MBR mixed liquor

in a reproducible way. These studies imply that it is possible to

develop on-line or in situ method to determine critical flux or

removable/irremovable fouling. It is also of high interest to

develop a unified measuring method or apparatus.

Since irremovable fouling plays an important role in long-

term operation of MBRs, sometimes chemical cleaning is

required to maintain MBR operation. But, chemical cleaning

for the elimination of irremovable fouling should be limited to

a minimum frequency because repeated chemical cleaning

may shorten the membrane lifetime and disposal of spent

Removable fouling and irremovable fouling Irremovable fouling

New membraneIrreversible fouling

Initial filtration

Long-term filtration

Physical cleaning

Chem

ical cleaning

Sludge flocs Colloids Solutes

Fig. 5 – Schematic illustration of the formation and removal

of removable and irremovable fouling in MBRs.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21494

chemical agents causes environmental problem (Yamamura

et al., 2007).

2.2.2. Biofouling, organic fouling, and inorganic fouling

2.2.2.1. Biofouling. From the viewpoint of fouling compo-

nents, the fouling in MBRs can be classified into three major

categories: biofouling, organic fouling, and inorganic fouling.

A fundamental understanding of the formation of membrane

Table 2 – Role of cake formation in membrane fouling.

Feedwater Effect of cake layeron membrane fouling

Ref.

Synthetic

wastewater

- Cake formation was entirely

governing the applicable flux

(Jeison and

van Lier,

2007)

Municipal

wastewater

- The resistance of cake layer

accounts for 95–98% of the total

filtration resistances

(sludge filtration)

(Ramesh

et al., 2007)

- The specific filtration resistance of

cake sludge was about 258 times

higher than that of bulk sludge

(Wang et al.,

2007)

- The cake resistance was the

dominant resistance and the

bulking sludge could cause a severe

cake fouling

(Meng and

Yang, 2007c)

- Cake layer resistance was

the major resistance

(Chu and Li,

2006)

Agricultural

wastewater

- At high permeate flux, cake

resistance (Rc) prevailed internal

fouling resistance (Rf). At low

permeate flux, Rf affect more

greatly than Rc

(Shin et al.,

2005)

foulants will help us to propose more effective approaches for

fouling control. Biofouling refers to the deposition, growth and

metabolism of bacteria cells or flocs on the membranes, which

has aroused a significant concern in membrane filtration

processes (Pang et al., 2005; Wang et al., 2005). For a low

pressure membrane such as microfiltration and ultrafiltration

for treating wastewater, biofouling is a major problem

because most foulants (microbial flocs) in MBRs are much

larger than the membrane pore size. Biofouling may start with

the deposition of individual cell or cell cluster on the

membrane surface, after which the cells multiply and form

a biocake. Many researchers suggest that SMP and EPS

secreted by bacteria also play important roles in the formation

of biological foulants and cake layer on membrane surfaces

(Flemming et al., 1997; Liao et al., 2004; Ramesh et al., 2007).

The deposition of bacteria cells can be visualised by tech-

niques such as scanning electron microscopy (SEM), CLSM,

atomic force microscopy (AFM), and direct observation

through the membrane (DOTM). DOTM and CLSM have been

extensively used to characterise membrane biofouling (Li

et al., 2003; Jin et al., 2006; Yun et al., 2006; Zhang et al., 2006a;

Hwang et al., 2007; Lee et al., 2007). The DOTM approach was

originally developed by Fane’s group at the University of New

South Wales to record the deposition behaviour in simple

cases of latex particles and flocs (Li et al., 2003; Zhang et al.,

2006a). Zhang et al. (2006a) used a DOTM to observe the

interactions between the bioflocs and the membrane surface.

The images showed that the bioflocs could move across the

membrane surface by rolling and sliding. More recently, CLSM

has become a powerful approach for characterisation of

membrane biofouling, which can not only identify the

deposited cell, but also present the 3D structure of the fouling

layer. Ng et al. (2006b) applied CLSM to visualise the bacterial

distribution on the membrane surface, and found that

bacteria were widely present on the fouled membrane. The

combination of CLSM and image analysis can visualise or

quantify the architecture of bio-cake layer (Lee et al., 2008).

Yun et al. (2006) characterised the biofilm structure and ana-

lysed its effect on membrane permeability in MBR for dye

wastewater treatment. They found that membrane filter-

ability was closely associated with the structural parameters

of the biofilms (i.e., porosity, biovolume). The visualisation of

biofouling using these techniques is helpful for understanding

of the floc/cell deposition process and the microstructure or

architecture of the cake layer.

In addition, a few investigations have been performed to

study the microbial community structures and microbial colo-

nisation on the membranes in MBRs (Chen et al., 2004; Jinhua

et al., 2006; Zhang et al., 2006c; Miura et al., 2007). The microbial

community structures can be investigated using microbiology

methods such as polymerase chain reaction denaturing

gradient gel electrophoresis (PCR–DGGE) and Fluorescence In

Situ Hybridization (FISH). Zhang et al. (2006c) reported that the

microbial communities on membrane surfaces could be very

different from the ones in the suspended biomass. They

provided a list of bacteria that might be the pioneers of surface

colonisation on membranes. Miura et al. (2007) studied the

microbial communities in a full-scale submerged MBR used to

treat real municipal wastewater delivered from the primary

sedimentation basin of a municipal wastewater treatment

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 2 1495

facility over 3 months. They also reported that the microbial

communities on membrane surfaces were quite different from

those in the suspended biomass. In this study, their FISH and

16S rRNA gene sequence analyses revealed that a specific

phylogenetic group of bacteria, the Betaproteobacteria, probably

played a major role in development of the mature biofilms,

which led to severe irremovable membrane fouling. Jinhua et al.

(2006) reported that g-Proteobacteria more selectively adhered

and grew on membranes than other microorganisms, and the

deposited cells have higher surface hydrophobicity than the

suspended sludge. The high shear stress induced by aeration

can select the deposition of cells. Some cells can be detached

easily by the shear stress, but other ones still adhere to

membrane surface tightly. The selective deposition of the cell

relies on the affinity of cells to membranes. And, due to the

anoxic condition in the cake layer, the temporal change of

microbial community structure would take place. We can see

that some of the bacteria in the sludge should play an important

role in membrane biofouling. The fundamental understanding

of deposition behaviour of bioflocs/cells and mechanisms of

cell attachment in MBRs will be crucial for the development of

appropriate biofouling control strategies in the future.

2.2.2.2. Organic fouling. Organic fouling in MBRs refers to the

deposition of biopolymers (i.e., proteins and polysaccharides)

on the membranes. Due to the small size, the biopolymers can

be deposited onto the membranes more readily due to the

permeate flow, but they have lower back transport velocity

due to lift forces in comparison to large particles (e.g., colloids

and sludge flocs). Recently, in order to provide a unique

insight into the composition (protein and carbohydrate),

Metzger et al. (2007) have performed a more detailed study to

characterise deposited biopolymers in MBRs. After membrane

filtration, the fouling layers were fractionated into upper

layer, intermediate layer and lower layer by using rinsing,

backwashing and chemical cleaning. The results showed that

the upper fouling layer was composed of a porous, loosely

bound cake layer with a similar composition to the sludge

flocs. The intermediate fouling layer was contributed equally

by SMP and bacteria aggregates, and had a high concentration

of polysaccharides. The lower layer, representing the irre-

movable fouling fraction and predominated by SMP, had

a relative higher concentration of bound proteins. This study

revealed the spatial distribution of biopolymers on the

membrane surface.

In order to figure out the detailed information on the

deposited biopolymers, identification of these matters is

indispensable. Fourier transform infrared (FTIR) spectroscopy,

solid state 13C-nuclear magnetic resonance (NMR) spectros-

copy and high performance size exclusion chromatography

(HP-SEC) are powerful analytical tools for investigation of the

organic fouling. They have been proven as effective

approaches for identification and characterisation of organic

fouling in MBRs. Through the FTIR spectra, the major

components of the biopolymers were identified as proteins

and polysaccharides (Zhou et al., 2007). 13C-NMR analysis by

Kimura et al. (2005) also suggested that the foulants were rich

in proteins and polysaccharides, however, different F/M ratios

would change the nature of the foulants. The HP-SEC and

fluorescence analyses can help to conclude that membrane

fouling layer appeared to be made of protein-like substances,

organic colloids and humic-like substances. A study by Tey-

chene et al. (2008) showed that the fouling layer was mostly

governed by the deposition of soluble compounds whereas the

impact of the colloidal fraction (poorly present in the super-

natant) was less; and the results of HP-SEC and fluorescence

analyses revealed the important role of protein-like

substances (polypeptides) in MBR fouling. But, an early study

by Rosenberger et al. (2006) demonstrated that poly-

saccharides and other non-settleable organic matter with

a molecular weight larger than 120,000 Da were found to

impact on membrane fouling. Additionally, high poly-

saccharide concentrations in sludge supernatant corre-

sponded to high fouling rates. These studies confirm that SMP

or EPS is the origin of organic fouling, and it plays significant

roles in the development of MBR fouling. In addition to the

molecular size, the deposition of SMP or EPS on membranes

strongly depends on its affinity with membranes.

2.2.2.3. Inorganic fouling. In general, membrane fouling in

MBRs is mainly governed by biofouling and organic fouling

rather than by inorganic fouling, although all of them take

place simultaneously during membrane filtration of activated

sludge. Up to now, thereby, most of the researchers attributed

membrane fouling to the deposition of bacteria cells and

biopolymers; the inorganic fouling in MBRs has been

mentioned by only a few papers. Kang et al. (2002) investigated

the filtration characteristics of organic and inorganic

membranes in a membrane-coupled anaerobic bioreactor, in

which a thick cake layer composed of biomass and struvite

(MgNH4PO4$H2O) formed on the membranes, especially on the

inorganic membrane. Ognier et al. (2002) pointed out there

was severe CaCO3 fouling in a pilot MBR with a ceramic

ultrafiltration membrane module. In this study, the synthetic

wastewater was prepared with hard tap water (concentrations

of Ca2þ and Mg2þ are 120 mg/L and 8 mg/L, respectively).

They found that the high alkalinity of the activated sludge

(pH¼ 8–9) could cause the precipitation of CaCO3. The inves-

tigations by Kang et al. (2002) and Ognier et al. (2002) suggested

that on inorganic membranes inorganic fouling may occur

more easily. In general, a cake of inorganic matter can be

irremovable due to the cohesive properties. More recently,

Wang et al. (2008b) observed that the cake layer was formed by

organic substances and inorganic elements such as Mg, Al, Fe,

Ca, Si, etc. The organic foulants coupled with the inorganic

precipitation enhance the formation of a cake layer. Lyko et al.

(2007) also found that metal substance was a more significant

contributor to membrane fouling than biopolymers. Some-

times, the fouling caused by inorganic scaling is not easy to be

eliminated even by chemical cleaning (You et al., 2006). These

findings indicate that inorganic fouling has become more and

more important in MBRs. But, the understanding of inorganic

fouling is still not clear. The investigation on the limiting

concentration of metal ions in the feed wastewater that can

lead to inorganic fouling will be of great interest, since the

chemical composition of the wastewater is in close relation

with the formation of precipitation.

The inorganic fouling can form through two ways (see

Fig. 6): chemical precipitation and biological precipitation. A

great number of cations and anions such as Ca2þ, Mg2þ, Al3þ,

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21496

Fe3þ, CO32�, SO4

2�, PO43�, OH� and others are present in MBRs.

Concentration polarisation will lead to higher concentration

of retained salts on the membrane surface. Chemical precip-

itation occurs when the concentration of chemical species

exceeds the saturation concentrations due to concentration

polarisation. Additionally, the fouling layer on membranes

can protect the surface layer from shear stress as biofilm or

biocake is elastic in nature leading to greater concentration

polarisation and precipitation of inorganics (Sheikholaslami,

1999). Carbonates are one kind of the predominant salts in

inorganic fouling. The aeration and the CO2 produced by

microorganisms can affect the super-saturation of carbonates

and the pH of the sludge suspension. The carbonates of metals

such as Ca, Mg, and Fe can increase the potential of

membrane scaling (You et al., 2005).

Biological precipitation is another contribution to inor-

ganic fouling. The biopolymers contain ionisable groups such

as COO�, CO32�, SO4

2�, PO43�, OH�. Metal ions can be easily

captured by these negative ions. In some cases, calcium and

acidic functional groups (R–COOH) can form complexes and

build a dense bio-cake layer or gel layer that may exacerbate

flux decline (Costa et al., 2006). When the metal ions in treated

water pass through the membranes, they could be caught by

the bio-cake layer via complexing and charge neutralisation

and then accelerate membrane fouling. Metal ions play

a significant role in the formation of fouling layers, which can

bridge the deposited cells and biopolymers and then form

a dense cake layer. There exists a synergistic interaction

among biofouling, organic fouling and inorganic fouling.

Despite the fact that inorganic fouling is a troublesome

phenomenon in MBRs, it is possible to avoid or limit inorganic

fouling by pretreatment of feedwater and/or implementation

of chemical cleaning. But the presence of a small quantity of

metal ions such as calcium can be beneficial for the

membrane permeation in MBRs due to its positive effect on

sludge flocculation ability (Kim and Jang, 2006). As inorganic

fouling can result in severe irremovable fouling, chemical

cleaning is more effective than physical cleaning in the

removal of inorganic precipitation. Chemical cleaning agents

such as EDTA might efficiently remove inorganics on the

COO-

COO-

COO-

Mn+

Mn++nOH- M(OH)nMn++CO3

2- MCO3

Mn++OH-+CO2 MCO3

Mn++SO42- MSO4

------------

Chemical precipitation

Biological precipitation

Sludge flocs Colloids Solutes Crystal

Fig. 6 – Schematic illustration of the formation of inorganic

fouling in MBRs.

membrane surface. EDTA can form a strong complex with

Ca2þ, biopolymers associated with Ca2þ ions are replaced by

EDTA via a ligand exchange reaction (Al-Amoudi and Lovitt,

2007).

3. Fouling factors and control strategies

The factors affecting membrane fouling can be classified into

four groups (Le-Clech et al., 2006): membrane materials,

biomass characteristics, feedwater characteristics, and oper-

ating conditions. The complex interactions between these

aspects complicate the understanding of membrane fouling.

For a given MBR process, the fouling behaviour is directly

determined by sludge characteristics and hydrodynamic

conditions. But, operating conditions (i.e., SRT, HRT and F/M)

and feedwater have indirect actions on membrane fouling by

modifying sludge characteristics. Table 3 gives the relation-

ship between various fouling factors and membrane fouling

on the basis of recent literature. In this review paper, the

major fouling-causing factors including bound EPS, SMP, and

hydrodynamic conditions are discussed. The fouling control

strategies based on operating conditions and feedwater are

proposed and summarized.

3.1. Bound EPS

3.1.1. Definition of bound EPS and SMPEPS in either bound or soluble form are currently considered

as the predominant cause of membrane fouling in MBRs.

Bound EPS consist of proteins, polysaccharides, nucleic acids,

lipids, humic acids, etc. which are located at or outside the cell

surface. Soluble EPS and SMP are the same. SMP can be

defined as the pool of organic compounds that are released

into solution from substrate metabolism (usually with

biomass growth) and biomass decay (Barker and Stuckey,

1999). Thus, SMP can be subdivided into two categories (Las-

pidou and Rittmann, 2002): substrate-utilisation-associated

products (UAP), which are produced directly during substrate

metabolism, and biomass-associated products (BAP), which are

formed from biomass, presumably as part of decay.

The interrelations between bound EPS and SMP are very

complex. A unified theory for EPS and SMP was proposed by

Laspidou and Rittmann (2002), who pointed out that cells use

electrons from the electron-donor substrate to build active

biomass, and they produce bound EPS and UAP in the process.

Part of the bound EPS can be hydrolysed to BAP. Some SMP can

be utilised by active biomass as recycled electron donors; and

some can be adsorbed by the biomass flocs and then, become

bound EPS. In addition, the generation of bound EPS and UAP

is in proportion to substrate utilisation.

3.1.2. Effect of bound EPS on membrane foulingBound EPS have been reported not only as major sludge floc

components keeping the floc in a three-dimensional matrix,

but also as key membrane foulants in MBR systems. Cho et al.

(2005b) found a close relationship between the bound EPS and

the specific cake resistance and established a functional

equation in which the specific cake resistance was propor-

tional to the EPS concentration. Ahmed et al. (2007) also

Table 3 – Relationship between various fouling factors and membrane fouling.

Sludge condition Effect on membrane fouling Ref.

Sludge condition

MLSS - MLSS[ / normalized permeabilityY (Trussell et al., 2007)

- MLSS[ / fouling potential[ (Psoch and Schiewer, 2006a)

- MLSS[ / cake resistance[, specific cake resistanceY (Chang and Kim, 2005)

Viscosity - Viscosity[ / membrane permeabilityY (Li et al., 2007a)

- MLSS/Viscosity[ / membrane permeabilityY (Trussell et al., 2007)

- Viscosity[ / membrane resistance[ (Chae et al., 2006)

F/M - F/M[ / fouling rates[ (Trussell et al., 2006)

- MLSS (2–3 g/L): F/M[ / irremovable fouling[

MLSS (8–12 g/L): F/M[ / removable fouling[

(Watanabe et al., 2006)

- F/M[ / Protein in foulants[ (Kimura et al., 2005)

EPS - polysaccharide[ / fouling rate[ (Drews et al., 2006)

- bound EPS influences on specific cake resistance (Cho et al., 2005c)

- polysaccharide[ / fouling rate[ (Lesjean et al., 2005)

- bound EPS[ / membrane resistance[ (Chae et al., 2006)

- The loosely bound EPS contributes to most of the

filtration resistance of the whole sludge

(Ramesh et al., 2007)

SMP - SMP is more important than MLSS (Zhang et al., 2006b)

- colloidal TOC relates with permeate flux (Fan et al., 2006)

- filtration resistance is determined by SMP (Jeong et al., 2007)

- SMP is probably responsible for fouling (Sperandio et al., 2005)

- polysaccharide is a possible indicator of fouling (Le-Clech et al., 2005)

- SMPY / fouling indexY (Jang et al., 2006)

- fouling rates correlate with SMP (Trussell et al., 2006)

Filamentous bacteria - filamentous bacteria[ / sludge viscosity[ (Meng et al., 2007a)

- bulking sludge could cause a severe fouling (Sun et al., 2007)

- filamentous bacteriaY / cake resistanceY (Kim and Jang, 2006)

Operating condition

SRT - SRT decrease from 100 to 20 d / TMP[ (Ahmed et al., 2007)

- SRT decrease from 30 to 10 d / fouling[ (Zhang et al., 2006b)

- SRTs[ / fouling potentials of SMP[ (Liang et al., 2007)

- SRT decrease from 5 to 3 d / fouling[ (Ng et al., 2006c)

HRT - HRTY / membrane fouling[ (Meng et al., 2007a)

- HRTY / membrane fouling[ (Chae et al., 2006)

- HRTY / membrane fouling[ (Cho et al., 2005a)

Aeration - aeration intensity[ / permeability[ (Trussell et al., 2007)

- air-sparging improves membrane flux (Psoch and Schiewer, 2006a)

- larger bubbles for fouling control are preferable (Phattaranawik et al., 2007)

- air backwashing for fouling control is preferable (Chae et al., 2006)

- bubble-induced shear reduces fouling significantly (Wicaksana et al., 2006)

- air scouring can prolong membrane operation (Sofia et al., 2004)

Permeate flux - sub-critical flux mitigates irremovable fouling (Lebegue et al., 2008)

- sub-critical flux mitigates fouling (Guo et al., 2007)

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 2 1497

observed that as bound EPS concentration rose, the specific

cake resistance increased, and this consequently resulted in

the rise of TMP. A recent study by Ji and Zhou (2006) indicated

that both composition and quantity of attached EPS on the

membrane surface influenced membrane fouling, and the

total biopolymers in sludge suspension played a more

important role than bound EPS in reflecting the extent of

membrane fouling. Ramesh et al. (2006) fractionated bound

EPS into loosely bound EPS and tightly bound EPS, and proved

that the fouling resistance was primarily caused by the loosely

bound EPS, but not by the tightly bound EPS. The loosely

bound EPS correlates with the performance of flocculation and

sedimentation processes (Li and Yang, 2007b).

Several studies, however, reported that bound EPS had

little correlation with membrane fouling. Rosenberger and

Kraume (2003) found that contrary to some literature, no

impact of bound EPS on the filterability could be observed.

Instead, the soluble EPS or SMP was found to have great

impact on the filterability of sludge. This was confirmed by

a more recent work reporting no clear relation between bound

EPS and membrane fouling as its concentration was smaller

than 10 mg/g SS (Yamato et al., 2006). In order to have a better

understanding of sludge characteristics and their effects on

membrane fouling, several investigations have been carried

out (Germain et al., 2005; Fan et al., 2006). These investigations

showed that activated sludge has very complex impacts on

3 5 10 20 30 50 60 70 100

Optimum SRT

Lee et al. (2003)

Ng et al. (2006b)

Zhang et al. (2006b) Han et al. (2005)

Fouling tendency

Ahmed et al. (2007)

Fig. 7 – Comparison of recent literature about the effects of

SRT on fouling rate.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21498

membrane fouling process. Bound EPS cannot be considered

as the sole cause for membrane fouling, even though it has

great effects on sludge characteristics and membrane fouling.

Despite the fact that the research results on bound EPS are

different from each other, it must be addressed that bound

EPS concentrations are closely connected to sludge charac-

teristics such as sludge volume index, flocculation ability,

hydrophobicity, surface charge, sludge viscosity. Therefore,

considering the important roles of bound EPS in sludge char-

acteristics and membrane fouling, bound EPS should be

controlled in order to mitigate membrane fouling more

efficiently.

3.1.3. Behaviour and control of bound EPS in MBRsAccording to literature, there is no efficient approach to

control the bound EPS directly since the MBRs include living

microorganisms and their metabolites. Therefore, most of the

recent reported literature is focused on finding suitable oper-

ating parameters in order to modify the sludge suspension.

SRT is one of the most important operating parameters

affecting MBR performance, in particular membrane fouling

(Grelier et al., 2006). Cho et al. (2005b) reported that as SRT

decreased, the amount of bound EPS in sludge flocs increased

at MLSS condition of 5000 mg/L. A recent investigation

reported by Ng et al. (2006b) showed that a longer SRT may

improve membrane permeation (10-day and 20-day SRTs

were better than 3-day and 5-day SRTs). They also observed

that membrane fouling rate increased with rising SMP and

bound EPS concentrations, both of which increased with

decreasing SRT. Masse et al. (2006) found that bound EPS

content decreased from 45–70 to 20–40 mg/gVSS when SRT

increased from 10 to 53 d. The above-mentioned results

suggest that too short SRT might do harm to membrane

performance. A too long SRT, however, was also found to

result in excessive membrane fouling. Lee et al. (2003) repor-

ted that as SRT increased from 20 days to 40 and 60 d, the

overall fouling resistance increased. Han et al. (2005) also

found that membrane fouling increased with increasing SRT

(30, 50, 70, and 100 d) due to large amount of foulants and high

sludge viscosity. Pollice et al. (2008) observed that the capillary

suction time (CST) and sludge resistance to filtration (SRF)

values, which are used to characterise the sludge filterability,

were minimized for SRT in the range of 40–80 d. These

reported results indicate that in order to control bound EPS

concentration and membrane fouling, the optimum SRT of

MBRs should be controlled at 20–50 d depending on HRT and

feedwater (see Fig. 7). But some investigators observed that

a long SRT will benefit membrane permeation. Ahmed et al.

(2007) reported that the membrane fouling became less when

SRT increased from 20 days to 40, 60, and 100 d. The study was

performed in an MBR equipped with a sequential anoxic/

anaerobic reactor for synthetic wastewater treatment. The

contrary result probably results from this special MBR process.

Sludge loading rate and correspondingly HRT and organic

loading rate (OLR) are main operating parameters affecting the

production of bound EPS since they govern biomass growth

and decay. In addition, HRT can govern both the F/M of the

bioreactor and the MLSS concentration. Meng et al. (2007a)

reported that there were high bound EPS concentrations and

high sludge viscosity as F/M ratio increased. The formation of

bound EPS is growth-related and is produced in direct

proportion to substrate utilisation (Laspidou and Rittmann,

2002). Thus, the increase of organic loading rate or F/M ratio

will induce the generation of more bound EPS. In addition,

aeration intensity, dissolved oxygen and feed substrates have

been proven as important parameters affecting bound EPS.

With increased aeration rates, protein/carbohydrate ratios of

sludge flocs decreased (Ji and Zhou, 2006). Li and Yang (2007b)

used six lab-scale bioreactors to grow activated sludge with

different carbon sources including glucose and sodium

acetate, and different SRTs of 5, 10 and 20 d. The sludge that

was fed on glucose had more EPS than the sludge that was fed

on acetate. For any of the feeding substrates, the sludge had

a nearly constant tightly bound EPS value regardless of the

SRT, but the loosely bound EPS content decreased with the

SRT, indicating that SRT is more important than feed

substrates on the control of bound EPS. A more recent inves-

tigation also showed that the protein/carbohydrate (P/C¼ 2, 4,

and 8) ratios of feedwater correlated strongly with bound EPS

composition (Arabi and Nakhla, 2008). It was found that with

increasing P/C ratio of feedwater, the P/C ratio of bound EPS

also increased slightly, but both protein and carbohydrate

concentrations decreased. It can be concluded from these

studies that there are several factors either alone or combined

with each other that play an important role in the formation of

bound EPS. It is of interest to know in what way the factor

(SRT, HRT, F/M, DO, etc.) impacts on the formation of bound

EPS.

In recent years, filamentous bulking has been found to

have a strong influence on MBR fouling (Meng and Yang,

2007c; Su et al., 2007; Sun et al., 2007). The overgrowth of

filamentous bacteria leads to a sharp increase of bound EPS

concentration and then induces the increase of sludge

viscosity and sludge hydrophobicity. In addition, the fila-

mentous bacteria can enlace and fix the foulants on the

membrane surface (see Fig. 8). Sun et al. (2007) observed that

with increasing sludge volume index (SVI), which results from

filamentous bulking, the average increasing rate of TMP

increased and the stable filtration period was shortened. Until

now, there are only a few studies about the cause and control

of filamentous bacteria in MBR processes, even though it

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 2 1499

exhibits significant impacts on sludge characteristics and

membrane fouling. Filamentous bulking can be controlled by

selectors, optimisation of operating conditions, addition of

coagulants and chlorine (Chudoba et al., 1973a,b; Caravelli

et al., 2003). Another important approach for the control of

filamentous bulking is to provide sufficient DO and alkalinity

for the sludge, because in many cases the filamentous bulking

is caused by the low DO of sludge suspension or low pH of

feedwater (Liu and Liu, 2006).

Another interesting approach for fouling control is the use

of aerobic granular sludge membrane bioreactor (AGMBR)

Because of the large size and dense structure, aerobic granular

sludge has little chance to deposit on membranes, even

though the granular sludge might have high bound EPS

concentrations. Li et al. (2005b) reported that the membrane

permeability of AGMBR was more than 50% higher than that of

conventional MBR, but the AGMBR had more severe irremov-

able fouling, which resulted from the deposition of SMP (i.e.,

colloids and solutes) on membranes.

3.2. SMP

3.2.1. Effect of SMP on membrane foulingIn fact, fouling behaviour cannot be attributed solely to bound

EPS due to the complex nature of sludge suspension. Recently,

the influence of SMP on MBR fouling has attracted much

attention (Rosenberger et al., 2005, 2006; Jeong et al., 2007;

Drews et al., 2008; Paul and Hartung, 2008). Due to the

membrane rejection, the SMP is more easily accumulated in

MBRs, which results in the poor filterability of the sludge

suspension. Geng and Hall (2007) observed that the floc size

distribution and the amount of soluble EPS or SMP in the

mixed liquor were the most important properties that signif-

icantly influenced the fouling propensity of sludge, but the

content of bound EPS was not found to be directly associated

with membrane fouling. Furthermore, several attempts have

shown that polysaccharide-like substances in SMP contribute

to fouling more than protein-like substances (Rosenberger

et al., 2006; Yigit et al., 2008). From Table 4 it also can be seen

that the impacts of SMP on membrane fouling depend on SMP

concentration, membrane materials and operation modes.

Since SMP has been recognized as significant membrane

foulant, scientific research on SMP or sludge supernatant

became one of thehot topics in membranefouling. Rosenberger

et al. (2006) reported that the SMP of the sludges (soluble and

Fig. 8 – SEM images showing fouling cake laye

colloidal materials) was found to impact on fouling and to cause

the difference in membrane performance between two iden-

tical MBRs. Iritani et al. (2007) reported that the relative

contribution of the supernatant to the membrane fouling of an

anaerobic activated sludge is nearly 100%, indicating that SMP

is the controlling factor in microfiltration of activated sludge.

But, the concentration of colloids and solutes in the superna-

tant was not mentioned in the paper. Lyko et al. (2007) analysed

the SMP in supernatant and permeate as well as bound EPS

extracted from fouled membranes in the full-scale MBR, and

found an important influence of soluble humic substances and

carbohydrates in complexes with metal cations on membrane

fouling. They also suggested that dissolved organic carbon

(DOC) was an alternative to complex and costly measurements

of SMP components (Lyko et al., 2008). These investigations

suggest that the occurrence of SMP in MBRs impacts on

membrane fouling significantly, and SMP concentration and

SMP composition would determine its fouling propensity.

Furthermore, the occurrence of SMP in MBR effluent concerns

the implementation of post-treatment for water recycling (e.g.,

the RO fouling in MBRþ RO process), and the discharge of SMP-

rich water brings additional troubles to local environment (e.g.,

the occurrence of dissolved organic nitrogen (DON)).

3.2.2. Behaviour and control of SMP in MBRsSMP can accumulate on the membranes or penetrate into

membrane pores. Accumulation and detachment of

membrane foulants are determined by particle convection

towards the membrane surface and the back transport rate of

the deposited particles from membrane surface into the bulk.

The back transport mechanisms in membrane filtration

include inertial lift, shear-induced diffusion and Brownian

diffusion. It is difficult to control the back transport of colloids

and solutes only by enhancing aeration intensity due to the

small size of these substances. The control of SMP concentra-

tion in MBRs is crucial. In general, the control of SMP can be

achieved by two approaches: adjustment of operation param-

eters (i.e., SRT, HRT, DO concentration, temperature, aeration)

and addition of adsorbents or coagulants to reduce SMP

concentration.

3.2.2.1. Control of SMP via adjustment of operation conditions.The effect of various process parameters on the production,

accumulation and elimination of SMP is of considerable

concern for researchers and engineers. Barker and Stuckey

r formed with filamentous bulking sludge.

Ta

ble

4–

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w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21500

(1999) summarized the process parameters (i.e., feed

strength, HRT, OLR, SRT, substrate type, temperature,

biomass concentration and reactor type) affecting the

production of SMP in conventional activated sludge process.

In MBRs, the cause and control of SMP formation were also

investigated, especially in the recent few years.

Shin and Kang (2003) reported that at an SRT of 20 d, an

influent DOC of 112 mg/L and an HRT of 6 h, the produced

SMP was 4.7 mg DOC/L of which 57% was removed or

retained by the membrane. At a long SRT, SMP concentra-

tion in the MBR reactor and effluent increased to some

extent and then became stable, and finally decreased. Lee

et al. (2003) found a decreasing contribution of the SMP to

overall membrane fouling with increasing SRT (20–60 d).

Zhang et al. (2006b) operated a submerged MBR equipped

with Kubota flat-sheet membranes at a short SRT of 10 d

and a moderate SRT of 30 d. During steady operation the

total amount of EPS extracted from the flocs and the

supernatant was approximately the same for the two SRTs

under the same organic loading rate. However, the soluble

polysaccharide concentration in the sludge suspension was

about 100% higher for the SRT of 10 d than that for 30 d.

More recently, Liang et al. (2007) presented an experimental

study on SMP in MBR operation at different SRTs of 10, 20,

and 40 d for the treatment of readily biodegradable

synthetic wastewater. They observed that accumulation of

SMP in the MBR became more pronounced at short SRTs.

Similarly, Rosenberger et al. (2006) found that at an SRT of

8 d, the polysaccharide concentration varied in the range of

3–15 mg/L; while at an SRT of 15 d, it varied in the range of

3–8 mg/L. It can be seen that most of the reported results

mentioned above showed that the SMP concentration

decreased with increasing SRT. Therefore, it is feasible to

control SMP concentration in MBRs by selecting suitable

operation parameters.

In MBRs, SMP are actually eliminated to a large extent via

biodegradation, adsorption or other mechanisms (Drews

et al., 2006). Drews et al. (2007) performed a comprehensive

study to elucidate and quantify the effects of varying envi-

ronmental conditions on SMP elimination. It was observed

that DO and nitrate concentrations appeared to have an

impact on SMP elimination and thereby on SMP concentra-

tion, with SMP elimination being lower at low DO concen-

tration. At the same time, Min et al. (2007) observed that

under DO limited conditions the sludge suspension con-

tained a larger amount of high molecular weight compounds

which lead to higher cake resistance. The low DO concen-

tration could lead to poor flocculation of individual activated

sludge cells, so that the number of small particles in the

sludge supernatant and soluble COD would increase as DO

concentration decreased (Kang et al., 2003; Jin et al., 2006). As

a result, a higher DO gave rise to a better filterability of sludge

suspension (Kang et al., 2003). Sudden temperature changes

led to spontaneous SMP release and increase in fouling rates

(Drews et al., 2007). Morgan-Sagastume and Grant Allen

(2005) found that deflocculation of sludge flocs occurred

under a temperature shift from 30 to 45 �C, which caused an

increase in turbidity and in SMP concentration. To achieve

low SMP concentrations, a sufficient supply of oxygen is

required in the bioreactor and sudden temperature change

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 2 1501

should be avoided (Drews et al., 2007). In addition, substrate

type or feedwater composition affects the formation and

elimination of SMP. McAdam et al. (2007) observed that carbon

substrate had a great influence on floc stability. Acetic acid

resulted in the production of high concentrations of small

particles (i.e., colloids and solutes) due to the weakly formed

flocs. Ethanol, on the other hand, encouraged the growth of

strong flocs that were capable of withstanding shear.

3.2.2.2. Control of SMP via addition of adsorbents/coagulants.Addition of adsorbents or coagulants into sludge suspension

can decrease the level of solutes and colloids or enhance the

flocculation ability. The addition of powered activated carbon

(PAC) is a simple and convenient method for fouling control.

The PAC can not only incorporate into the bioflocs forming

biologically activated carbon (BAC) (Ng et al., 2006a), but also

adsorb biopolymers in the sludge suspension. The addition of

PAC to MBRs provides a solid support for biomass growth, and

hence reduces floc breakage (Hu and Stuckey, 2007). Moreover,

the BAC flocs in MBRs are very strong and dense, which can

help to prevent particle accumulation on the membranes.

Akram and Stuckey (2008) studied the impacts of PAC

addition on the performance of a submerged anaerobic MBR,

and found that in the presence of 1.67 g-PAC/L, the combined

effects of adsorption of fine colloids and solutes, and the

formation of a thin cake layer resulted in significant flux

improvement from 2 to 9 L/(m2 h). However, the addition of

3.4 g-PAC/L reduced the flux to 5 L/(m2 h). As discussed by the

authors, this evidence might be due to the increased sludge

viscosity at high PAC addition. It suggests that PAC addition

can improve membrane flux significantly; but, if the addition

of PAC is beyond the optimal value, it will do harm to

membrane permeation. However, in some cases when the

MBRs were operated without wastage the performance of

the MBR(BAC) was worse than the conventional MBR. Thus,

the improved performance of the MBR(BAC) requires regular

replacement of aged BAC with fresh PAC (Ng et al., 2006a).

Coagulants can remove SMP by charge neutralisation and

bridging (Wu et al., 2006). Addition of an optimum calcium

concentration could induce lower SMP concentration, lower

hydrophobicity, lower concentration of filamentous bacteria

and better flocculation, which resulted in the reduction in

cake layer resistance and pore blocking resistance (Kim and

Jang, 2006). Attempts have been also made to use alum, ferric

chloride, and chitosan as coagulants or filter aids (Iversen

et al., 2008; Ji et al., 2008; Koseoglu et al., 2008; Song et al., 2008;

Tian et al., 2008; Zhang et al., 2008c). Zhang et al. (2008c)

reported that the addition of ferric chloride at the optimal

concentration could reduce both SMP with MW> 10 kDa in

the supernatant and the fraction of small particles (sludge

flocs) in the range of 1–10 mm. The improvement of membrane

flux or sludge filterability also depends on the coagulant used.

Ji et al. (2008) found that the membrane fouling rate of the

MBRs was in the order of Control MBR without coagu-

lant>Al2(SO4)3 added MBR>Chitosan added MBR> Poly-

meric ferric sulfate (PFS) added MBR. In addition, cationic

polymeric chemicals were found to be favorable due to their

steady and successful performance in fouling control

(Koseoglu et al., 2008). It has been reported that polymeric

coagulants could supply more positive charges and longer

chain molecules, so that they had a better effect on filterability

enhancement of sludge suspension than monomeric coagu-

lants, while excess addition of polymeric coagulant led to

‘‘colloidal re-stabilization’’ (Wu et al., 2006).

Recently, a so-called membrane fouling reducer (MFR) or

membrane flux enhancer (MFE), which is a modified cationic

polymer, has been developed to reduce membrane fouling in

MBRs (Guo et al., 2008; Koseoglu et al., 2008). The addition of

MFR can lead to the flocculation of activated sludge. SMP is

also entrapped by the microbial flocs during the course of the

flocculation, leading to an increase in the concentration of

bound EPS (Hwang et al., 2007). With the addition of MFRs, the

cost of aeration can decrease 40–55% to achieve the same flux

(Yoon et al., 2007). Lee et al. (2007) determined the optimum

dosage as 0.025 mg/mg MLSS. But if the addition of MFR is

beyond the optimum MFR dosage, soluble matters can be

released from the microbial flocs to the bulk solution. Most

MFRs and other additives have no or slight negative effect on

the biomass activity (Iversen et al., 2008). These investigations

further indicate that the addition of coagulants into sludge

suspension is an effective and convenient method to control

or eliminate SMP.

But there are still many problems which need to be

answered, such as the occurrence and fate of SMP in MBR, the

change of SMP components after coagulants being added,

the dynamic process of SMP production and elimination, and

the accumulation and detachment of SMP on membranes.

Moreover, the potential impacts of coagulants or adsorbents

on biomass community or biomass metabolism need to be

taken into account (Iversen et al., 2009), and the discharge of

some chemicals that are used as coagulants or adsorbents

might be a potential environmental risk. At this point,

biomass-friendly coagulant or adsorbent (e.g., powdered

activated carbon, chitosan) should be preferred unless the

above-mentioned problems are made clear.

3.3. Hydrodynamic conditions

3.3.1. Effect of hydrodynamic conditions onmembrane foulingSince submerged MBRs are more popular than cross-flow

MBRs in scientific research and real applications, most of the

recent studies on hydrodynamic conditions are focused on the

reduction of aeration demand by enhancement of aeration

efficiency. In a cross-flow MBR, the membrane fouling can be

limited by increasing cross-flow velocity. In a submerged MBR,

shear stress is created by aeration, which not only provides

oxygen to the biomass, but also maintains the solids in

suspension and scours the membrane surface to alleviate

membrane fouling. The aeration can be used to generate

a shear stress on the membrane surface without requiring

a recirculation pump. Hong et al. (2002) examined the effect of

aeration on cake removal and suction pressure using a pilot-

scale submerged MBR and concluded that aeration was

a significant factor governing the filtration conditions.

Previous investigation (Han et al., 2005) showed that the cake-

removing efficiency of aeration did not increase proportion-

ally with the increase in the airflow rate and that the airflow

rate had an optimum value from the cake-removing point of

view.

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21502

Aeration is an important parameter determining both the

size of the sludge flocs and the membrane filtration or fouling

control. A high aeration rate certainly can reduce sludge

attachment to the membranes, but it also has a significant

influence on the biomass characteristics. Too high aeration

intensity will lead to breakage of sludge flocs and production

of SMP. Under high aeration intensity, the colloids and solutes

would become the major membrane foulants (Fan and Zhou,

2007), because the resistance of colloids and solutes cannot be

reduced effectively by increasing shear stress. The back

transport of the colloids and solutes from the membrane

surface is determined by Brownian diffusion, especially for

solutes. The increase of aeration can improve shear-induced

diffusion and inertial lift. Due to this the concentration

gradients are steeper causing higher diffusion rates. The

aeration intensity is expected to have a very complex influ-

ence on MBR performance.

3.3.2. Favorable hydrodynamic conditions mitigatingmembrane foulingIn a low pressure membrane process, such as MBRs, the bubble

size and bubble flow rate play significant roles in hydrody-

namic conditions and energy demand. Fane et al. (2005)

compared the effect of two nozzle sizes, 0.5 and 1.0 mm

diameter, on bubble size and membrane fouling. The larger

nozzle could produce higher bubble sizes than the

smaller nozzle. However, the fouling control, characterised by

dTMP/dt, was noticeably improved using the smaller nozzle

with the smaller bubbles.

A more recent study by Prieske et al. (2008), however,

suggests that the smaller bubble size (1 mm) could induce

a slower circulation velocity than large bubbles (2 and 3 mm)

due to a higher gas holdup in the downcomer, and concluded

that larger bubbles seem to be more efficient for air scour of

the membrane surface because the resulting drag and lift

forces on the membranes are much higher due to higher

circulation velocities. It also has been reported that fouling

reduction increased with the airflow rates up to a given value

and beyond this flow rate no further enhancement was ach-

ieved (Ndinisa et al., 2006a). In addition, under certain condi-

tions intermittent airflow can achieve better fouling control

than continuous filtration and it also reduces energy

requirements.

For submerged hollow fibers in bubble-enhanced systems,

such as the membrane bioreactor, the preferred fiber orien-

tation should be vertical rather than horizontal though the

overall effect of fiber orientation on filtration is smaller than

the turbulence caused by the two-phase flow (Chang et al.,

2002b). The size of the gap between the submerged flat-sheet

membranes is also important for two-phase flow and fouling

control (Ndinisa et al., 2006a). As the gap was increased from

7 mm to 14 mm, the fouling became worse and the degree of

fouling reduction by two-phase flow decreased by at least 40%

based on suction pressure rise (dTMP/dt). Moreover, fiber

movement and fouling control are influenced by fiber tight-

ness with significantly improved performance for slightly

loose fibers (Wicaksana et al., 2006). In recent years, to look for

a more efficient membrane fouling control, air-sparging in

MBRs has been paid more and more attention to (Delgado

et al., 2008; Psoch and Schiewer, 2008). The benefits of air-

sparging are the enhancement of hydrodynamic conditions

and the efficient use of aeration.

The interaction of two-phase flow with membranes in

MBRs is a complex issue. During the last few years, the anal-

ysis of the hydrodynamics and the fluid flow pattern adjacent

to the membrane have been studied and visualised by CFD

mathematical modeling and simulation (e.g., Ahmad et al.,

2005). The multi-phase flow simulation by CFD technique can

provide microscopic understanding of the fouling mecha-

nism, and it has been proven to be a powerful tool to aid

membrane module design (Li et al., 2006). Ndinisa et al. (2006b)

studied the fouling in a submerged flat sheet MBR using two-

phase flow characterisation and CFD simulation. It was found

that the flux enhancement by the increasing bubble size was

primarily due to an increase in the overall shear stress on the

membrane and to more turbulence generated by introduction

of the gas phase.

In addition to aeration intensity, the rheological properties

of sludge suspension not only have major impacts on oxygen

transfer and sludge conditioning in the next step, but also

have a strong influence on transport phenomena near the

membrane surface. For a given aeration intensity, the increase

of sludge viscosity will weaken the hydrodynamic conditions

close to the membranes. An example is the sharp decrease of

the shear stress at the membrane surface with increasing

sludge viscosity (Meng et al., 2007b). According to the rheo-

logical properties of activated sludge, Van Kaam et al. (2008)

proposed an intermittent aeration mode, which allows acti-

vated sludge to restructure and can effectively prevent MBR

fouling. Of particular interest is the energy saving of the

intermittent aeration.

In brief, enhancement of hydrodynamic conditions is one

of the effective approaches to mitigate membrane fouling in

MBRs. But, the hydrodynamic conditions has close relation

with aeration intensity, bubble size, membrane module

configuration, MLSS concentration and sludge viscosity, etc.

Therefore, the hydrodynamic conditions in MBRs is very

complex; and optimisation of membrane module and aeration

combined with CFD modeling and simulation might be helpful

for the improvement of the hydrodynamic conditions.

Besides bound EPS, SMP and hydrodynamic conditions we

mentioned above, attempts have been made to control fouling

or modify sludge by using ultrasound, ozone and electric field

(Chen et al., 2007; Huang and Wu, 2008; Sui et al., 2008; Wen

et al., 2008). Experimental results showed that ultrasound

could control membrane fouling effectively although

membrane damage may occur under some operation condi-

tions (Wen et al., 2008). One interesting method is the use of

an electric field, which could prevent the sludge flocs and

colloids depositing onto the membrane surface. In addition,

attempts also have been made to control MBR fouling by

developing novel filtration modes and/or backwashing

conditions (Wu et al., 2008a,b).

3.4. Summary

So far, there is a great number of investigations focused on

membrane fouling, most of which still consider bound EPS,

SMP and hydrodynamic conditions as the main factors

affecting membrane fouling. At least, these investigations

Table 5 – Summarization of fouling control strategiesbased on various fouling factors.

Controlstrategy

Control item and its effect on membranefouling factor

Hydraulic

control

- HRTY / sludge viscosity[ (Meng et al., 2007a),

EPS[ (Chae et al., 2006)

- aeration[ / permeability[ (Psoch and Schiewer,

2006a; Trussell et al., 2007), fiber movement[

(Wicaksana et al., 2006), cake-removing efficiency[

(Chang and Judd, 2003), and cake resistanceY (Psoch

and Schiewer, 2006b; Fan and Zhou, 2007)

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 2 1503

indicate that membrane fouling has more or less relation with

the above-mentioned factors. The adjustment of operating

conditions or feedwater can be used to modify sludge char-

acteristics (e.g., SMP) and then control membrane fouling.

Table 5 presents the fouling control strategies based on

various fouling factors. A schematic illustration about favor-

able conditions mitigating membrane fouling is also given in

Fig. 9. Regarding the discussion about other fouling factors

such as MLSS concentration, sludge viscosity and sludge

particle size, it can be found in recent review papers (Chang

et al., 2002a; Le-Clech et al., 2006).

- periodical backwashing / flux[ (Psoch and

Schiewer, 2006a) and operation period[

(Chae et al., 2006), total resistanceY (Psoch and

Schiewer, 2006b)

- sub-critical/low flux operation / sustainable

operation (Bacchin et al., 2006; Guglielmi et al.,

2007a)

Chemical

control

- powdered activated carbon / EPSY (Ying and Ping,

2006), irremovable foulingY (Ng et al., 2006a; Ying

and Ping, 2006)

- membrane fouling reducer / cake porosity[,

soluble EPSY (Hwang et al., 2007; Lee et al., 2007)

- flocculation/coagulation / organic matterY

(Zhang et al., 2008c)

- chemically enhanced backwashing / remove

fouling (Kim et al., 2007)

Biological

control

- SRT[ / bound EPSY (Ahmed et al., 2007), SMPY

(Liang et al., 2007)

- MLSS/viscosityY / permeate flux[ (Li et al., 2007a;

Trussell et al., 2007), cake foulingY (Chang and Kim,

2005; Chae et al., 2006)

- F/M ratioY / fouling resistanceY (Trussell et al.,

2006)

- filamentous bacteriaY / bound EPSY (Meng et al.,

2006a)

Fig. 9 – Favorable conditions mitigating membrane fouling.

4. Developments of membranes/filters

4.1. Influence of membrane characteristics on MBRperformance

Membrane characteristics such as pore size, porosity, surface

charge, roughness, and hydrophilicity/hydrophobicity, etc.,

have been proven to impact on MBR performance, especially on

membrane fouling. The determination of suitable membrane

pore sizes has been extensively investigated in the 1990s. Pore

size distribution is likely to be one of the parameters affecting

membrane performance. A narrow pore size distribution is

preferred to control membrane fouling both in MBR process

and in conventional membrane separation process.

The membrane materials always show different fouling

propensity due to their different pore size, morphology and

hydrophobicity. Polyvinylidene fluoride (PVDF) membrane is

superior to polyethylene (PE) membrane in terms of preven-

tion of irremovable fouling in MBRs used for the treatment of

municipal wastewater (Yamato et al., 2006). Regarding MBR

processes, the fouling behaviour of the membrane used is

determined by the affinity between foulants (e.g., EPS/SMP)

and membrane. Zhang et al. (2008b) studied the affinity

between EPS and three polymeric ultrafiltration membranes,

and observed that the affinity capability of the three

membrane was of the order: Polyacrylonitrile (PAN)

< PVDF< Polyethersulfone (PES). It suggests that among these

membranes the PAN membrane is more fouling-resistant.

Inorganic membranes, such as aluminum, zirconium, and

titanium oxide, show superior hydraulic, thermal, and

chemical resistance. A stainless steel membrane was used for

MBR, and the result showed that the stainless steel membrane

could obtain a higher permeate flux (Zhang et al., 2005b), and

it is a potential alternative for the treatment of high temper-

ature wastewater (Zhang et al., 2006d). In the stainless steel

membrane bioreactor, thermophilic bacteria could be culti-

vated when the MBR was operated at higher temperature. But,

these inorganic membranes are not the preferred option for

large-scale MBR plants because of their high costs. In addition,

inorganic membranes can induce severe inorganic fouling

(i.e., struvite formation). So, the inorganic membranes might

be used only in some special applications such as high

temperature wastewater treatment.

In general, membrane fouling occurs more readily on

hydrophobic membranes than on hydrophilic ones because of

the hydrophobic interaction between foulants and membranes

As a result, much attention has been given to reduce membrane

fouling by modifying hydrophobic membranes to relatively

hydrophilic (Yu et al., 2005a,b). The objective of this paper is to

review the recent and current developments of membranes in

MBRs, so the main content of Section 4 is focused on the novel

and significant findings in membranes or filters used for MBRs.

The detailed discussion on the impacts of membrane material

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21504

on membrane fouling can be found in recent reviews (Chang

et al., 2002a; Le-Clech et al., 2006).

4.2. Application of low-cost filters

Both capital and operation costs of the MBRs must be reduced

in order to increase their competitiveness with respect to

conventional activated sludge processes. In MBRs, the

membranes are just used as filters, which have small pores.

Since an improved effluent quality might not always be

required and only standard criteria are stipulated, it should be

feasible to substitute the membranes by cheaper filters with

large pore size. The low-cost filters investigated include (Ye

et al., 2006; Iversen et al., 2007; Seo et al., 2007; Satyawali and

Balakrishnan, 2008): non-wovens, meshes and filter cloths.

Non-wovens are composed of a random network of over-

lapping fibers having multiple connected pores through which

the fluid can flow (Chang et al., 2006). Due to its large pore size

or porosity, the non-woven could obtain a high-flux even at

very low pressure. There are little differences in the effluent

water quality between the non-woven bioreactor and the

membrane bioreactor (Meng et al., 2005). In 1998, non-woven

and coarse pore filter modules were applied in the pilot-scale

and full-scale wastewater treatment plant in Tokyo (Asou

et al., 1998). The performance of both pilot-scale and full-scale

plants was acceptable for real municipal wastewater. Then,

Seo et al. (2003) in Korea developed an anaerobic/aerobic

bioreactor in which a non-woven module was submerged into

the aerobic compartment. The COD removal efficiency was

91.6% producing an effluent COD concentration around

13 mg/L. These results confirmed that non-wovens can be

used in wastewater treatment processes to substitute the

membranes, though it cannot be compared with conventional

membranes in some cases. But, as a new filter used in

wastewater treatment process, the non-woven still has its

own shortcomings. Compared with polymeric membranes,

some non-woven filters have a lower tensile strength and tear

strength, and a lower resistance to microbiological corrosion.

It is, therefore, expected that the lifetime of non-woven filters,

which depends on the material, might be small.

Mesh bioreactors are potential processes used for waste-

water treatment, especially when a small area requirement is

a high priority (Kiso et al., 2005). Fuchs et al. (2005) obtained

a much higher flux of 50–150 L/(m2 h) at very low pressure

depending on operating conditions (i.e., MLSS concentration,

aeration, HRT). Due to the much larger pore size of the mesh,

the effluent of a mesh bioreactor is not of the same excellent

quality as that of a membrane bioreactor (Fuchs et al., 2005;

Wang et al., 2006). But, a self-forming dynamic membrane

(SFDM) forms on the coarse mesh during the filtration of

activated sludge (Fan and Huang, 2002). As soon as the SFDM is

formed, it can improve effluent quality significantly. Besides

the low cost, it can be concluded that the high permeation at

very low suction pressure is another attractive character of

these filters.

The major problem limiting the application of low-cost

filters is the severe fouling due to their rough surface and the

too large pore size. The sludge flocs can entrap in the void

among the fiber matrix and it is difficult to be removed by

shear stress. Due to the large pore size, pore blocking by

sludge flocs is one of the main reasons leading to filter fouling

(Moghaddam et al., 2006). In order to avoid the rapid decline of

permeate flux of filter cloth, Ye et al. (2006) precoated the filter

cloth with powdered activated carbon (PAC) before

submerging it into a bioreactor. It was found that the pre-

coated filter could not only mitigate membrane fouling, but

improve the effluent quality significantly as well. It indicates

that the severe fouling of low-cost filters can be resolved by

modifying the filters to improve the surface roughness,

hydrophilicity, surface charge and so on (see Section 4.3).

4.3. Membrane modification

The main objective of new membrane material development is

to reduce the high cost of investment for the membrane

modules or to enhance and maintain membrane flux. To

improve the anti-fouling property of polypropylene hollow

fiber microporous membranes (PPHFMMs) in an MBR for

wastewater treatment, the PPHFMMs were subjected to surface

modification by NH3 and CO2 plasma treatment by Yu et al.

(2005a,b, 2008). The water contact angle reduced significantly

after NH3 and CO2 plasma treatment because –NH2 groups and

–COOH groups were grafted on the membrane surface. Fouling

indices of the NH3 and CO2 plasma treated PPHFMMs were

lower than those of the unmodified PPHFMMs. Although the

plasma treatment processes have many advantages, such as

a very shallow modification depth compared to other surface

modification techniques, it still has drawbacks. For example,

the chemical reactions of the plasma treatment are rather

complex, so the surface chemistry of the modified surface is

difficult to understand in detail and thus, currently it is not

possible to extend plasma treatment on large-scale (Yu et al.,

2007).

To overcome these disadvantages of plasma treatment, Yu

et al. (2007) applied the surface graft polymerisation method

to improve the membrane permeation in MBRs. In the study,

the surface modification of polypropylene microporous

membranes was accomplished by UV irradiation in aqueous

acrylamide solutions. The contact angle data showed that the

hydrophilicity of the surface modified membranes increased

strongly with the increase of the grafting degree. Even though

the modified membrane showed better filtration ability in the

MBR than the unmodified membrane, this method has the

disadvantage of employing high-energy methods, such as UV

irradiation, plasma treatment, gamma irradiation, and

chemical reaction, resulting in an increase in membrane

production cost (Asatekin et al., 2006).

Recently, a self-assembly technique, which is one of the

simplest and most effective methods to prepare a thin film on

the membrane surface, was employed for fabricating a fouling

resistance membrane in MBR (Asatekin et al., 2006). In this

study, commercial polyvinylidene fluoride ultrafiltration

membranes (PVDF UF) were coated with the amphiphilic graft

copolymer polyvinylidene fluoride-graft-polyoxyethylene

methacrylated (PVDF-g-POEM), to create thin film composite

nanofiltration membranes (TFC NF). The new TFC NF

membranes exhibited no irremovable fouling in 10-day dead-

end filtration of model EPS (bovine serum albumin, sodium

alginate and humic acid) at concentrations of 1000 mg/L. The

anti-fouling properties of the TFC NF membranes were

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 2 1505

attributed to both the nanoscale dimensions of the hydro-

philic channels through the coating, which greatly restrict the

size of permeate species, and the unique properties of poly-

ethylene oxide (PEO), which can resist the adsorption of EPS

on the membrane surface. Meanwhile, TiO2 embedded poly-

meric membranes have been prepared by a self-assembly

process and applied to the filtration of MBR sludge (Bae and

Tak, 2005a,c). The surface of a TiO2 embedded membrane can

was more hydrophilic than that of neat polymeric membrane

due to the higher affinity of metal oxides to water. Therefore,

hydrophobic adsorption between sludge suspension and TiO2

embedded membrane can be reduced, and deposited foulants

are readily removed by cross-flow (Bae and Tak, 2005a,c). They

confirmed that self-assembly technique can be successfully

used to modify the membranes for membrane fouling control

in MBRs. Recently, Zhang et al. (2008a) attempted to modify

non-woven filter by dip-coating PVA (polyvinyl alcohol). The

results obtained from two parallel MBRs indicated that the

flux decline of modified non-woven filter was only 12%, in

comparison of the original non-woven filter of 40%.

In addition to membrane modification, the development of

economical, high-flux, non-fouling membranes is still needed

before viable MBR processes can be achieved (Shannon et al.,

2008). The non-fouling or low-fouling membrane should have

much narrower pore size distributions, stronger hydrophi-

licity, and larger porosity than the currently used membranes.

At this point, the microsieve membrane, which has very

uniform pore size (Brans et al., 2006; Ning Koh et al., 2008), can

provide a useful alternative for the development of narrow

pore size distribution membranes. On the other hand, nano-

technology might be of interest for the development of strong

hydrophilic membranes.

4.4. Dynamic membranes

In recent years, dynamic membranes have been investigated

in MBRs in order to improve membrane performance.

Dynamic membranes were firstly reported in 1965 by inves-

tigators at the Oak Ridge Laboratories engaged in desalination

research (Marcinko et al., 1966). Dynamic membranes can be

prepared by filtering a solution containing either inorganic or

organic materials through a porous support (Fan and Huang,

2002). There are two basic types of dynamic membranes:

precoated and self-forming. The precoated membrane is

produced by passing a solution of one or more specific

components over the surface of a porous support. The self-

forming membrane is formed by the components in the

solution to be filtered.

Many hydrated oxides, natural polyelectrolytes, and

synthetic organic polymers can be used for the preparation of

precoated membranes. It has been also reported that modified

membranes coated with an even PVA hydrogel layer show

dramatically high anti-fouling characteristics and good flux

recovery compared to inadequately modified membranes and

unmodified membranes (Na et al., 2000). Although these

investigations were based on conventional membrane sepa-

ration, they provide valuable information for the application

of dynamic membrane in MBRs.

The precoated dynamic membrane bioreactor was firstly

reported by Li et al. (2005a), who prepared the dynamic

membrane by circulating kaolinite suspension through the

ceramic membrane module to improve MBR performance.

The precoated dynamic membrane bioreactor had satisfying

performance on organic substances and nitrogen removal.

But, the influence of precoated membranes on fouling or

rejection was not discussed. Ye et al. (2006) proposed a pre-

coated dynamic membrane bioreactor used for municipal

wastewater treatment. The filter cloth with a pore size of

56 mm and powdered activated carbon were used as support

filter and precoating reagent. It was found that during the

long-term operation the removal efficiencies of organic

carbon and ammonia were as good as traditional hollow fiber

membrane bioreactors. The precoated filter cloth had a more

excellent performance with respect to fouling control than

uncoated filter cloth and hollow fiber membranes (Zhang

et al., 2005a). In addition, the precoated membrane had a low

irremovable fouling during the operation of MBRs. This finding

coincides with a previous study (de Amorim and Ramos, 2006).

It suggests that precoated dynamic membranes can help to

improve the filterability of filter cloths, mesh filters and non-

woven filters, so it provides these low-cost filters with a larger

potential in MBRs.

Self-forming dynamic membranes were firstly used in

conventional membrane separation process. The self-forming

dynamic membrane implies that the rate of particle convec-

tion towards the membrane surface is balanced by the rate of

back transport. The self-forming dynamic membrane can

improve both the permeate flux and rejection of solutions.

The performance of self-forming dynamic membranes is

determined by concentration, type, shape, molecular weight

of filtering solution and cross-flow velocity.

Currently, self-forming dynamic membranes have been

introduced into MBRs in order to explain the formation and

action of bio-cake layer on the membrane surface. A study by

Lee et al. (2001) gave an explanation about self-forming

dynamic membranes in MBRs. As the membrane filtration

reaches a steady state, a dynamic membrane will have been

formed, which mainly consists of large sludge flocs. It was

reported that the fouling layers forming on the membrane

surface act as barriers which protect membrane surfaces and

pores from being fouled (Lee et al., 2001) because EPS, soluble

organics and colloidal particles could be rejected or bio-

degraded by the dynamic membrane composed of living

microorganisms. Thus, the foulants have fewer chances to

deposit on the membrane surface. Fan and Huang (2002)

reported self-forming dynamic membranes formed on

a 100 mm coarse mesh instead of MF or UF membranes. The gel

layer formed on the mesh surface had a structure like

conventional membranes and played a key role in the self-

forming dynamic membrane. From these reported findings, it

can be expected that that self-forming dynamic membranes

can not only mitigate membrane fouling in MBR, but also

provide an alternative to improve the performance of some

cheaper filters (i.e., filter cloth, non-woven, coarse mesh). It

should be pointed out that the performance of the dynamic

membrane is dependent upon many factors such as cake

density, cake structure and cake components. For example,

when the cake layer formed on the membrane surface

becomes thick, the oxygen in the bulk cannot be transported

into the inner regions of the cake (Zhou et al., 2008) and then

w a t e r r e s e a r c h 4 3 ( 2 0 0 9 ) 1 4 8 9 – 1 5 1 21506

the bacteria will lose their activity and release a great deal of

biopolymers. In such case, the so-called dynamic membrane

will lead to severe membrane fouling.

5. Conclusions and perspectives

In this paper, recent advances of research on membrane

fouling and membrane material in MBRs were reviewed. From

the viewpoint of fouling reversibility with physical and

chemical cleanings, membrane fouling includes removable

and irremovable fouling, in which the latter will be paid more

and more attention to in MBRs, especially in long-term oper-

ation. From the viewpoint of fouling components, the fouling

in MBRs can be classified into three major categories:

biofouling, organic fouling and inorganic fouling. The results

obtained from recent investigations on bound EPS, SMP, fila-

mentous bacteria and hydrodynamic conditions are updated.

In the coming few years, membrane fouling is still a hot

issue in research and application of MBRs. According to recent

literature and our own experience, the future study on

membrane fouling should include:

(1) Studies on membrane fouling mechanisms should focus

on identification and characterisation of membrane fou-

lants (i.e., chemical and biological components of foulants,

bacteria community of the foulants). Cake formation, pore

blocking, and EPS/SMP adsorption on/within the

membranes could all be important. Of particular impor-

tance could be the interaction and interrelation between

these mechanisms and sludge characteristics.

(2) Development of procedures for the visualisation and

characterisation of membrane fouling in MBRs. Direct

monitoring and in situ techniques will offer more useful

information about the formation of membrane foulants.

(3) Development of more effective and easy methods to

control and minimize membrane fouling. Generally,

removable fouling is controlled by creating shear stress on

the membrane surface. Although air bubbles are used to

promote shear stress and to enhance the membrane flux,

they also have strong impact on biomass characteristics.

Moreover, enforced aeration will need more energy.

Research should be directed to optimisation of the current

coarse aeration methods for submerged membrane

modules. Lastly, alternative filtration concepts to limit the

deposition of foulants onto the membrane surface should

be developed.

(4) Study the fouling behaviour in full-scale MBR plants in

order to reflect the real fouling behaviour.

(5) Development of novel membrane modules for MBRs to

reduce their capital costs and enhance their hydrody-

namic conditions.

(6) Modeling of mass transfer and membrane fouling by

mathematical approaches such as CFD, Monte Carlo

simulation, fractal theory, artificial neural network (ANN).

In other words, a comprehensive investigation should be

performed to understand, control and reduce membrane

fouling, especially avoiding severe fouling; it is just like

a systematic physical examination on a person to understand

his/her health condition and to avoid the occurrence of

illness, especially fatal diseases.

In recent years, there are considerable investigations about

the impacts of membrane materials, pore size, hydrophilicity/

hydrophobicity, etc., on membrane fouling; however, most of

the recent investigations are focused on the application

of low-cost filters to substitute the membranes, modification

of membranes to enhance their hydrophilicity and use of

dynamic membranes to improve the performance of

membranes or low-cost filters. In the future, to our knowl-

edge, the study on membrane materials in MBRs should still

focus on:

(1) Development of anti-fouling membranes or modification

of current membranes.

(2) Enhancement of the performance of low-cost filters by

modifying their surface properties.

Acknowledgements

The first author is a research fellow of the Alexander von

Humboldt Foundation. This work is partially supported by

Alexander von Humboldt Foundation (07.2007–12.2008).

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