Fouling in membrane bioreactors used in wastewater treatment

Pierre Le-Clech ∗, Vicki Chen, Tony A.G. FaneUNESCO Centre for Membrane Science and Technology, School of Chemical Engineering, The University of New South Wales,

Sydney 2052, NSW, Australia

Abstract

The membrane bioreactor (MBR) can no longer be considered as a novel process. This reliable and efficient technology has become a legitimatealternative to conventional activated sludge processes and an option of choice for many domestic and industrial applications. However, membranefouling and its consequences in terms of plant maintenance and operating costs limit the widespread application of MBRs. To provide a betterunderstanding of the complex fouling mechanisms and propensities occurring in MBR processes, this review compiles and analyses more than300 publications. This paper also proposes updated definitions of key parameters such as critical and sustainable flux, along with standard methodsto determine and measure the different fractions of the biomass. Although there is no clear consensus on the exact phenomena occurring on themembrane interface during activated sludge filtration, many publications indicate that the extracellular polymeric substances (EPS) play a majorrole during fouling formation. More precisely, the carbohydrate fraction from the soluble microbial product (also called soluble EPS or biomasssupernatant) has been often cited as the main factor affecting MBR fouling, although the role of the protein compounds in the fouling formationis still to be clarified. Strategies to limit fouling include manipulating bioreactor conditions, adjusting hydrodynamics and flux and optimizing

module design.

Keywords: Membrane bioreactors; Fouling; Activated sludge; Operating conditions; Cleaning

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181.1. MBR history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2. Fouling mechanisms for complex fluids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.1. Concepts of critical and sustainable flux in mixed species environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192.2. Effect of operating modes on performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 202.3. Cake structure and the effect of mixed species on cake morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212.4. Effect of membrane morphology and surface chemistry on fouling mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 232.5. Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3. Roadmap for MBR fouling parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243.1. Membrane characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.1.1. Physical parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 253.1.2. Chemical parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

3.2. Feed–biomass characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2.1. Nature of feed and concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.2.2. Biomass fractionation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283.2.3. Biomass (bulk) parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.2.4. Floc characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

∗ Corresponding author. Tel.: +61 2 93855762; fax: +61 2 93855966.E-mail address: p.le-clech@unsw.edu.au (P. Le-Clech).

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45. .

owh(dhlsraspoadmsmtlaa1ap(u1potarange of MBR systems commercially available, most of which

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction

Membrane bioreactor (MBR) technology combines the bio-logical degradation process by activated sludge with a directsolid–liquid separation by membrane filtration. By using microor ultrafiltration membrane technology (with pore sizes rangingfrom 0.05 to 0.4 �m), MBR systems allow the complete physi-cal retention of bacterial flocs and virtually all suspended solidswithin the bioreactor. As a result, the MBR has many advan-tages over conventional wastewater treatment processes. Theseinclude small footprint and reactor requirements, high effluentquality, good disinfection capability, higher volumetric loadingand less sludge production [1]. As a result, the MBR process hasnow become an attractive option for the treatment and reuse ofindustrial and municipal wastewaters, as evidenced by their con-stantly rising numbers and capacity. The current MBR markethas been estimated to value around US$ 216 million and to rise toUS$ 363 million by 2010 [2]. However, the MBR filtration per-formance inevitably decreases with filtration time. This is due tothe deposition of soluble and particulate materials onto and intothe membrane, attributed to the interactions between activatedsludge components and the membrane. This major drawback andprocess limitation has been under investigation since the earlyMBRs, and remains one of the most challenging issues facingfurther MBR development [3].

1.1. MBR history

The MBR process was introduced by the late 1960s, assoon as commercial scale ultrafiltration (UF) and microfiltra-tion (MF) membranes were available. The original process wasintroduced by Dorr-Olivier Inc. and combined the use of anactivated sludge bioreactor with a crossflow membrane filtra-

tion loop [4]. The flat sheet membranes used in this processwere polymeric and featured pore size ranging from 0.003 to0.01 �m [5]. Although the idea of replacing the settling tank

uaf

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

f the conventional activated sludge process was attractive, itas difficult to justify the use of such a process because of theigh cost of membranes, low economic value of the producttertiary effluent) and the potential rapid loss of performanceue to fouling. As a result, the focus was on the attainment ofigh fluxes, and it was therefore necessary to pump the mixediquor suspended solids (MLSS) at high crossflow velocity atignificant energy penalty (of the order 10 kWh/m3 product) toeduce fouling. Due to the poor economics of the first gener-tion MBRs, they only found applications in niche areas withpecial needs like isolated trailer parks or ski resorts for exam-le. The breakthrough for the MBR came in 1989 with the ideaf Yamamoto et al. to submerge the membranes in the biore-ctor [6]. Until then, MBRs were designed with the separationevice located external to the reactor and relied on high trans-embrane pressure (TMP) to maintain filtration. The other key

teps in the recent MBR development were the acceptance ofodest fluxes (25% or less of those in the first generation), and

he idea to use two-phase bubbly flow to control fouling. Theower operating cost obtained with the submerged configurationlong with the steady decrease in the membrane cost encouragedn exponential increase in MBR plant installations from the mid990s. Since then, further improvements in the MBR designnd operation have been introduced and incorporated into largerlants. While early MBRs were operated at solid retention timesSRT) as high as 100 days with mixed liquor suspended solidsp to 30 g/l, the recent trend is to apply a lower SRT (around0–20 days), resulting in more manageable mixed liquor sus-ended solids (MLSS) levels (10–15 g/l). Thanks to these newperating conditions, the fouling propensity in the MBR hasended to decrease and overall maintenance has been simplifieds less frequent membrane cleaning is necessary. There is now a

3.2.5. Extracellular polymeric substances (EPS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323.2.6. Soluble microbial products (SMP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

3.3. Operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.3.1. Aeration, crossflow velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.3.2. Solid retention time (SRT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363.3.3. Unsteady state operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

3.4. Fouling mechanisms in MBRs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.4.1. Constant TMP operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 383.4.2. Constant flux operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

4. Mitigation of MBR fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.1. Removal of fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.1.1. Physical cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 414.1.2. Chemical cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

4.2. Limitation of fouling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.2.1. Optimization of membrane characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.2.2. Optimization of operating conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 424.2.3. Modification of biomass characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

se submerged membranes although some external modules arevailable; these external systems also use two-phase flow forouling control. In terms of membrane configurations, mainly

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ollow fiber and flat sheet membranes are applied for MBRpplications [7].

The economic viability of the current generation of MBRsepends on the achievable permeate flux, mainly controlledy effective fouling control with modest energy input (typi-ally ≤ 1 kWh/m3 product). More efficient fouling mitigationethods can be implemented only when the phenomena occur-

ing at the membrane surface are fully understood. The plethoraf publications dealing with MBR fouling and published withinhe last 5 years tends to dilute the accessibility of informa-ion and may lead to some confusion. This review presents

state-of-the-art assessment of MBR fouling based on theost recent and relevant papers on the subject. After discus-

ion of fouling mechanisms for complex fluids, a comprehensiveoadmap for MBR foulants and fouling parameters will be pro-osed. Finally, a review of the current methods for foulingitigation in MBR systems will detail design options to opti-ize MBR operation. This review aims to open doors to new

deas and directions for optimized and more sustainable MBRrocesses.

. Fouling mechanisms for complex fluids

Significant advances in understanding fouling of individualomponents such as bacteria, yeast, proteins, and colloids haveccurred in microfiltration and ultrafiltration literature [8–11].uch of this literature has focused on the effect of charge (via pH

ariation or salt concentration), crossflow, concentration, mem-rane hydrophilicity, membrane pore size and flux (constantressure or constant flux). While some broad trends for simpleolloids are valid for macromolecules (the most commonly stud-ed of which are proteins), the labile nature of proteins and rangef polydispersity of naturally occurring macromolecules suchs polysaccharides and humic substances add a particular com-lexity to the fouling mechanisms. In addition, the interactionetween the suspended colloids or those in the deposited “cake”n a mixed species environment has the potential to significantlyhange the nature of the foulant layer in terms of resistance andeversibility, even for simple model systems. In this section, theeuristics providing insights to fouling in such mixed speciesnvironment are considered in the context of broad observationsnd commonly used tools to decipher them.

.1. Concepts of critical and sustainable flux in mixedpecies environment

Optimizing flux to control fouling has been pursued sincehe mid-1980s. Moderating TMP differences within modulesad been utilized to reduce excessive localized fouling. Whileuch of the existing literature has been performed under con-

tant pressure conditions, the use of constant flux and monitoringf resultant TMP rise have proved to be particularly useful inhe context of monitoring fouling in complex fluids and is cur-

ently the mode of choice in many MBR applications (Fig. 1).ypically, increasing flux steps are imposed and the TMP mon-

tored for its stability at each step. When the TMP is no longertable at each flux step and increases rapidly to indicate rapid

tctf

esulting data obtained during the study of the effect of membrane state (BW:ackwashed, chemically cleaned or new) on the fouling rate (dP/dt) [14].

ccumulation of foulants, this is usually referred to as the crit-cal flux. The original critical flux hypothesis for MF assumeshat a critical flux exists below which a decline of permeabilityith time does not occur, and above which fouling is observed

12]. Since this first definition, the critical flux concept has beenefined with numerous different meanings, definitions and meth-ds of determination, reviewed in [13,14]. Two distinct forms ofhe critical flux concept have been defined, with, respectively,o fouling and little fouling occurring at sub-critical operationor the strong and weak forms. In practice, the flux obtaineduring sub-critical flux (strong form) equates to the clean waterux obtained under the same conditions. In the alternative weakorm, the sub-critical flux is the flux rapidly established andaintained during the start-up of the filtration, but does not nec-

ssarily equate to the clean water flux.The critical flux depends on the back transport provided by the

rossflow or turbulence generated by imposed liquid flow and/orubbling as well as the specific solute–membrane interactions,hich are affected by charge and hydrophobicity. Solute size

lso plays a significant role in determining the regime of backransport whether it is diffusive or inertial lift [8]. High local

oncentrations that promote local aggregation due to concen-ration polarization will also determine the cohesiveness of theoulant layer.

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For single particles, the force balance between convectiverag to the membrane and back transport due to crossflow can benalyzed with various shear enhanced diffusivity models [15] toredict the critical flux at which deposition occurs at a particularydrodynamic condition. The reversibility of the deposition haseen less well documented but can be assessed by examining theysteresis of the TMP versus flux profile [16,17]. The hysteresisechnique is the most recently formalized technique to deter-

ine critical flux. By using this method, Chen et al. [16] studiedhe transition from concentration polarization to cake forma-ion during the membrane filtration of colloidal silica. Recently,spinasse et al. [18] published a proposed standard method for

he determination of critical flux using hysteresis effects. In thisethod, critical flux is defined as the lowest flux that creates

n irreversible deposit on the membrane. Models proposed byacchin and co-workers [19–21] to incorporate intermolecu-

ar forces with convective transport have been used to predictegimes where particle aggregation begins to dominate on theembrane surface.For macromolecules, the “apparent” critical flux can also be

etermined however in these systems, background adsorptionccurs even with no convection and a slow increase in mem-rane resistance is always detected even at low fluxes. Theinetics of this adsorption, particularly for proteins, can be cru-ial as conformational changes of the initial adsorbed layer canhange the surface chemistry of the membrane surface withime. This continual adsorption may close off smaller poreshat cause redistribution of flow throughout membrane structure.

hile concentration polarization may not be initially presentor these small macromolecules in microfiltration, the adsorp-ion followed by retention of an increasing fraction of the feedan result in rapid loss in transmission and resultant polariza-ion. High initial flux (typical of constant pressure experiments)an also generate aggregation of some macromolecules such asroteins [22,23].

For complex fluid systems, one common practice to exper-mentally determine the critical flux value is to incrementallyncrease the flux for a fixed duration. This leads to relativelytable TMP at low fluxes (indicating little fouling), and an ever-ncreasing rate of TMP rises at fluxes beyond the critical fluxalues [16,24–28]. Since zero rate of TMP increase is generallyot attained in filtration of complex fluids, no critical flux, in itstrictest (or strong) definition, can be defined (Fig. 1). In fluidsith both macromolecules and particulates, membrane fouling

akes place even at low flux rates, but changes dramatically whenhe so-called critical flux (in its weak form) is reached. Althoughpparently straightforward in principle, the precise identifica-ion of the critical flux value from flux-stepping experimentstrongly depends upon the conditions used (step duration, stepeight, initial state of the membrane) [14]. The most importantarameter remains the step height, which needs to be kept asmall as possible for higher accuracy in the determination ofhe critical flux value [13]. Unfortunately, no standard protocol

xists, such that the comparison of critical flux values reported inhe literature is difficult. The determination of the exact value ofhe critical flux (i.e. the passage from little to severe fouling) ishen left to the judgement of the researchers, although rigorous

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athematical expressions have been reported [14]. In spite of therbitrary aspect of this method, critical flux determination by thishort-term experiment remains an efficient approach to assesshe fouling behavior of a given filtration system and to compareifferent operating conditions. Interestingly, this method wasecently used as a standard test to assess the fouling propen-ity of an MBR on a daily basis [29]. This approach allows thelotting of fouling intensity against an MBR parameter such asiomass characteristics.

It is also now generally accepted that the short-term determi-ation methods for the critical flux (especially the flux-steppingpproach) does not yield predictive absolute permeability dataor extended operation of complex fluids. For example, the foul-ng rate (dTMP/dt) values measured for long-term experimentsre always significantly lower than the equivalent values mea-ured for the short-term flux-step experiments [13,14,30–32].n addition, a second phase of TMP increase has been observedhen long-term filtration was carried out at sub-critical flux

experimentally determined in short-term experiments) even forimple model feeds, such as alginate solutions. A number ofodels have been proposed to account for the development ofsecond phase of TMP increase which may occur after hun-

reds of hours of operation [17,31,33,34]. Most of the modelsave focused on the potential for slow pore closure or blockagehat results in high local fluxes due to redistribution of flow andubsequent rapid fouling.

The challenge remains to use short-term experimental data toroject long-term fouling characteristics in such mixed systemshere foulant inventory and fractionation may play important

oles. Thus the focus may be shifted to considering a sustain-ble flux (see Section 4.2.2) where reversibility of the foulanteposition and global operational constraints for productivitynd costs are taken into account.

.2. Effect of operating modes on performance

Constant pressure filtration behavior is typified by a rapidux decline at the start of filtration followed by a more grad-al decrease until a steady-state or a pseudo-steady-state flux iseached. Four filtration models (Table 1), originally developedor dead-end filtration [35], have been proposed to describe thenitial flux decline.

Comparison between operating modes (constant pressure andonstant flux) have been limited [27,36,37]. Constant flux oper-tion avoids excessive fouling of membranes as well as beingost effective for submerged membrane operations [27]. Vyas etl. [38] investigated the performance of different combinationsf constant pressure and constant flux crossflow microfiltrationf lactalbumin suspensions, since in their case the critical fluxs too low to be an economic operation. It was found that oper-ting under constant flux just above the critical flux followedy constant TMP operation causes severe membrane fouling.t appears that during the constant TMP period, small parti-

les continued to permeate through the relatively thinner cake,ormed during the low constant flux filtration, into the mem-rane pores. In contrast, constant TMP operation followed byery low constant flux operation can offer scope to reduce

Table 1Empirical dead-end filtration equations

Law Physical cause Description Equation

Cake filtration Boundary layer resistance Deposit of particles larger than the membrane pore size onto themembrane surface

t/V = AV + B

Complete blocking Pore blocking Occlusion of pores by particles with no particle superimposition −ln(J/J0) = At + BIntermediate blocking Long-term adsorption Occlusion of pores by particles with particle superimposition 1/J = At + BStandard blocking Direct adsorption Deposit of particles smaller than the membrane pore size onto the

pore walls, reducing the pore sizet/V = At + B

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urface fouling by reducing the convective force towards theembrane.While fouling is generally observed as being slower in con-

tant flux operation, there is some evidence that the depositionnder these low fouling conditions may be more irreversible ashe resultant mechanism tends to be predominately internal foul-ng by macromolecular species. Constant flux operation mayenerate a substantial initial deposit, but its effect on subse-uent deposition of macromolecules may be beneficial in someircumstances by serving as a prefilter for species which maytherwise infiltrate more deeply into the membrane pores. Inddition, the constraints of productivity in terms of flux and per-eation of targeted species for applications such as fermentation

edefine the optimal flux operational mode.Intermittent filtration combined with continuous crossflow

ay allow deposits to relax as long as the particles are still labile.n appropriate regions suggested by Bacchin et al. [19] whereoagulation or aggregation has not occurred, this approach mayllow removal of foulant cakes. However, for many biologicalolids, the cohesive strength of the cake may be significant androteins adsorption and gel formation result in strong attractiveonds to the membrane materials.

.3. Cake structure and the effect of mixed species on cakeorphology

Once the cake is formed on the membrane surface, the cakeayer offers an additional resistance for filtration. The perme-bility of the cake layer can be affected by flux, electrostaticnteractions, and particle size. General observations by Petsevt al. [39] include:

When salts do not cause aggregation in the feed, the perme-ability of the cake layer sharply decreases with the increasein electrolyte concentration.The permeability of the cake layer sharply decreases with theincrease in permeate flux because the increased flux results ina more compressed cake layer.The permeability of the cake layer increases with the surface

potential of the particles due to the increase in the inter-particle repulsion. However, above a certain value of surfacepotential, a plateau value for the permeability is reached.The permeability of the cake layer passes through a minimumwith the increase in the particle size.

fi0Xof

al flux.

The reason for such behavior is that for very small parti-les, inter-particle repulsion (electrostatic repulsion) exerts aominant effect on the voidage of the cake layer. In this rangef particle size, the inter-particle repulsion decreases with thencrease in particle size resulting in the decrease of permeability40]. After a certain value of particle size, the effect of the elec-rostatic force becomes negligible and the permeability increasesith the increase in particle size. Fane et al. [41] observed a

imilar dependency of the permeability of the cake layer onarticle size. They proposed a different explanation based onhe counter balance of the Brownian diffusion (dominating formaller particles) and particle migration due to hydrodynamicorces (dominating for larger particles).

The development of the cake layer during microfiltrationas also studied [42–47]. Cakes formed in the crossflow modeay have higher specific cake resistances than cakes formed in

ead-end filtration and may even increase with the increase inembrane resistance [48]. Many of these observations can be

xplained by size dependent particle deposition and the depen-ence of the specific resistance on the particle size. Based on theass transfer mechanisms, there is a maximum diameter of par-

icle that can deposit on the membrane surface. As the crossflowelocity increases, the cut-off diameter decreases, allowing thatmaller particles to deposit on the membrane surface. Thus spe-ific resistance may increase. In addition, cake formation duringrossflow tends to eliminate larger particles from the deposits,eading to cake containing a finer fraction of the particle sizeistribution. Plugging and catastrophic reduction in permeabil-ty of the retained cake is another potential cause of the twotage TMP increase during sub-critical flux operation indicatedarlier [34]. However, Keskinler et al. [44] reported that the spe-ific resistance for lower crossflow velocities was greater thanhe one obtained in higher crossflow velocities for all yeast celloncentrations tested. In contrast, during the membrane filtra-ion of monodisperse latex particles, no effect of stirring speedas found on the specific resistance values [43].The compressibility indexes of the cake have been found to

e different in crossflow and dead-end filtration. Keskinler et al.44] found that non-living yeast cakes formed in the crossflowode are more compressible than cakes formed in dead-endltration. The compressibility index was found to be 1 and

.39 for the crossflow and dead-end filtration, respectively.ujiang et al. [49] found similar trends during microfiltrationf talc suspensions. By contrast, Mota et al. [50] reported that,or spherical particles, the compressibility index (n) both in

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ead-end and crossflow filtration were similar basing on thetudies at relatively low crossflow velocity. Tanaka et al. [45]ound lower compressibility index in crossflow filtration thanhat in dead-end filtration during microfiltration of rod-shaped. subtilis, which are 0.6 and 0.8, respectively. They explained

hese differences as follows. During the crossflow filtrationt lower TMP, the cells were arranged by the shear from theeginning of cell deposition on the membrane surface, thus theake showed higher specific cake resistance than that in theead-end filtration. While at higher TMP, the cells depositiont the initial stage of crossflow filtration in particular tended toeposit in a manner similar to that in dead-end filtration due tohe high permeate flux; therefore the specific cake resistanceecame close to that in the dead-end filtration. This may providehe reason that the compressibility is lower in crossflow filtrationhan dead-end filtration. Hughes and Field [51] recently showedhat increasing shear stress reduced the amount of reversibleouling in yeast filtration but the irreversible componentemained constant. The potential for size segregation and lateralransport for yeast cells near the membrane wall has also beenbserved [52]. Foley [53] recently reviewed factors affectinglter cake properties of microbial suspensions.

For complex fluids such as membrane bioreactors effluent,ermentation broths, and natural organic matter, the foulingnteractions of the colloidal component are affected by the poten-ial for small macromolecules to penetrate and adsorb into the

embrane structure and foulant cake structure. On the otherand, colloids or particles can affect the initial deposition of theacromolecules by adsorbing them on their surfaces or provid-

ng a secondary layer that entraps aggregates of these macro-olecules. Studies to elucidate this phenomenon have been

arried with yeast and protein mixtures. Davis and colleagues54–56] showed that the presence of yeast actually preventedouling of bovin serum albumin (BSA) in microfiltration as theeast layer on the membrane surface captured the BSA aggre-ates and prevented them from fouling the internal structure ofhe membrane. In this case, the cake layer formed by the yeastarticles can be considered as a prefilter (Fig. 2). They observedigher protein transmission and higher flux in the presence ofhe yeast cake than in its absence. Recent studies by Ye andhen [57] showed that the critical flux of the mixtures of yeastnd BSA showed little change from critical flux measured for

east alone; however, the reversibility of the deposited formedy these mixed layer is substantially reduced. Thus the macro-olecules can serve to bind the particulates together. Results

Fig. 2. Cake layer as prefilter.

metbiopbfntcTto

ith alginate, a microbial polysaccharide, showed increasingpecific resistance with time, indicating a consolidation of theake layer formed which may be due to infiltration of smallractions of the alginates among the alginate aggregates initiallyrapped by the microfilter. When both alginate and protein areresent, the transmission of both components was reduced whilehe compressibility of the mixed deposit was increased. Thus theigidity and compressibility may vary substantially dependingn the chemical nature of the extracellular components boundr soluble in MBR or fermentation broths.

Some researchers indicated that particles in the mixed feedolution determine the flux behavior during the membrane fil-ration. Timmer et al. [58] found that the small quantities ofilicates completely determined the flux behavior in the cross-ow microfiltration of �-lactoglobulin solutions. Causserand etl. [59] studied the permeability changes in clay cake due torotein adsorption. A minimum limiting flux was found at thesoelectric point of the clay–protein complex. Interestingly, theyound that at higher pH values, the mixture behavior was simi-ar to the protein, whereas below pH 4.5, the limiting flux wasimilar to those observed for the filtration of clay suspensionslone. By optimizing the electrostatic interactions between pro-eins and an adsorptive surface like clay, Causserand et al. [60]mproved protein fractionation and decreased membrane foul-ng by the protein, which was attributed to the formation of aecondary membrane by clay particles on top of the original par-icles. Hwang et al. also showed that capture of BSA in bed ofatex particles can be related by standard capture equation foreep-bed filtration. Interesting studies by van Oers et al. showedow the presence of silica sols can reduce rejection of polyethy-ene glycol (PEG) and dextran by providing a high polarizationayer (unstirred) zone near the membrane [61]. In contrast, theejection of PEG and dextran increased in the presence of BSA.he compressibility of the BSA layer leads to highest rejectionccurring at the highest pressure of filtration.

The impact of large particles on the fouling process is not easyo gauge. Researchers have indicated that fouling can be reducedy adding suspended solids during UF of organic moleculesuch as polysaccharides and proteins. Panpanit and Visvanathan62] investigated the role of bentonite addition in the UF foril/water emulsions. It was found that the addition of ben-onite up to a certain concentration dramatically decreased the

embrane fouling. This was because the reduction of oil/watermulsions concentration by bentonite adsorption and the forma-ion of larger particles when oil/water emulsion contacted withentonite. However, beyond the limiting concentration, the fluxmprovement gradually declined, possibly due to the formationf packed cake of particles on the membrane surface. This com-osite cake structure is illustrated in Fig. 3. Recent studies withentonite and alginate mixtures during constant flux MF showedormation of a bentonite cake near the membrane while the algi-ate formed a viscous layer above the cake. Particle velocitieshrough this viscous layer dropped steadily as filtration time pro-

eeded, indicating densification of the viscous gel layer (Fig. 4).his may provide insight into the cohesive and transport charac-

eristics of such composite layers. In contrast, compact cellularr particulate cakes which form with swollen macromolecular

us

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Fig. 3. Composite cake structure.

nderlayer may disengage spontaneously if the cake build-up isufficient to create high shear stress due to crossflow [63].

In systems with the microorganisms, the likelihood of dif-erent cell wall properties precluded researchers from makingefinitive statements regarding the effects of cell size and shapen filtration characteristics. In this context, Foley and his co-orkers used polymorphic microorganisms to conduct a detailed

nvestigation of the effect of cell size and shape on filtrationehavior [48,64–67]. The shape of this microorganism, rangingrom yeast-like to filamentous, could be varied in a controlleday by altering its growth conditions. The structure of the cellall was reasonably constant and independent of the cell shape.he results clearly showed that the specific cake resistancend compressibility of the microbial filter cakes was stronglyelated to cell morphology, in particular the mean aspect ratiof the cells. The potential errors in calculating specific resis-ance and tortuosity is significant in mixed species cakes andhe effect of cell shapes and structure in a compressible media

ay require better understanding of the associated extracellularaterial [50,68].Recent work by Ohmori and Glatz [69] and earlier work by

odgson et al. [68] have shown that the filtration properties oficrobial suspensions were dependent not only on the cell shape

ut also on the physical characteristic of the cell and associatedxtracellular matrix. Ohmori and Glatz [69] found that changes

ig. 4. Velocity profile during filtration of binary model solution with directbservation apparatus (500 mg/l alginate–50 mg/l bentonite solution; apparentulk velocity = 2 mm/s; constant flux of 56 l/m2 h). The background picturehows the fouling layer after 2 h of filtration.

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n carbon source for the fermentation of C. glutamicum affectedhe microfiltration performance. The specific cake resistance ofells cultivated with sucrose was half as much as those culti-ated with glucose at neutral pH, and were almost the sameelow pH 4.0. The authors attributed these differences in spe-ific cake resistance, as well as their pH dependencies, to theigher hydrophobicity and lower surface charge of cells grownn sucrose. By performing extracellular matrix modification ofarine bacteria SW8 with a proteolytic enzyme and a chelating

gent, the important role of matrix in resistance was confirmedy the changes of flux and specific cake resistance [68].

.4. Effect of membrane morphology and surface chemistryn fouling mechanisms

Conventional wisdom generally attributes lower fouling tomooth hydrophilic membranes with high porosity and narrowore size distribution. This has been supported by extensive workith various biological fluids, particularly proteins solutions

10,11,70,71]. Reduction in the macromolecular adsorption withydrophilic surfaces or by mitigating charge interactions willeduce the rate of pore closure due to this mechanism. Met-amuuronen et al. [72] reported that much lower critical fluxesere observed for the ultrafiltration of baker’s yeast when aydrophobic polysulfone membrane was used as opposed to aydrophilic regenerated cellulose membrane. This phenomenons more obvious at pH 6 where both membranes have a zetaotential of zero. By using matrix-assisted laser desorption ion-zation mass spectrometry (MALDI-MS) for quantitative analy-is, Chan et al. [73] studied the membrane fouling by proteinixtures on hydrophilic and hydrophobic 30 kDa moleculareight cut-off (MWCO) UF membranes. It was found that, for

he hydrophobic membrane, the deposition exceeded quantitiesorresponding to a monolayer above and below the apparentritical flux. When a hydrophilic membrane was employed,overage in excess of a monolayer was only found above thepparent critical flux. Interestingly, while high molecular weightroteins appear to dominate the apparent critical flux, the pre-ominant proteins observed on the membrane by this techniqueended to be the lower molecular weight species [74] that pene-rate the pores. In mixed species feeds, the surface chemistry ofhe membrane may be masked by adsorption of the multitudesf macromolecular species thus the benefits of hydrophilicityay be obscured during the long-term fouling.At a given fixed flux, one would initially expect the pore size

f the membrane to be irrelevant to the convective force exertedn the particles and to any back diffusion or shear inducediffusion effect. However, as pore size decreases, hindered trans-ort of macromolecules exacerbates local polarization and theotential for aggregation and fouling. The local porosity andssociated local convective velocities as opposed to average fluxcross the whole membrane surface also need to be consideredhen comparing membranes with widely varying porosities.

he influences of pore size on the fouling were found to dif-

er in various studies. In the microfiltration of 0.4 wt.% BSAolutions, Chen [75] found that the critical flux increased withore size when track-etched membranes of pore size 0.1, 0.2 and

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.4 �m were used. In comparison, Wu et al. [76] investigated theffect of membrane pore size (50 kDa, 100 kDa and 0.2 �m) onritical flux for three types of feed fluids. For all feed fluidsested: 0.5% silica, 0.15% BSA and 5% yeast cell suspension,he critical flux decreased with increasing membrane pore size.

Narrow pore size distributions reduce the inhomogeneousow distribution between pores that lead to preferential deposi-

ion and blockage of large pores [77,78]. Similarly, high porosityeans that local flux at the pore entrance will be reduced. Mem-

ranes with interconnecting pore structures also have the advan-age that surface blockage can be mitigated [79]. Membrane

orphology will determine initial macromolecular transmissionnd fouling mechanisms, particularly at low flux operation. Theransition between pore closure and cake formation is critical inhe fouling progression in mixed species feed. As the effectiveore size is reduced, the local flux increases, increasing the con-ective forces to the pore. Larger particles are then pulled in andccelerate the foulant build-up.

Typically, membrane blocking laws (constant flux and con-tant pressure mode) have been used to establish when thisransition between pore blockage and cake formation takes place80]. Ho and Zydney [81] have developed a combined porelockage and cake formation model, with the cake layer onlyorming over the regions of the membrane that have already beenlocked by the initial deposit in the membrane pores. Unlikeost prior pore blockage models, it was assumed that someuid is still allowed to flow through the pores blocked by largeggregates. The model was successfully used to analyze the pro-ein fouling [81], alginate fouling [33] and humic acid fouling82] during microfiltration.

.5. Summary

In complex fluids, the interactions between the macromolec-lar and particulate components of the feed can result in unex-ected and rapid changes in fouling. The kinetics and inventoryf macromolecules adsorbing will dictate the initial foulinghase. Progressive closure of pores or membrane surface resultsn a change in transmission and species convected to the surfacend the foulant cake. While the initial low fouling phase at lowux (or “sub-critical” flux) features slow progressive adsorptionf macromolecules on the membrane surface, a more rapid foul-ng phase then occurs. During that period, pore closure results innhanced rejection of macromolecules and deposition of largerarticles (Fig. 5). Evolution of this foulant cake and its irre-ersibility depend on both its composition and the hydrodynamicnvironment under which it was established. The interactionetween particulate and macromolecular fouling needs to beonsidered with many of the same complexities observed inouling studies of natural organic matter. Macromolecular foul-ng can increase particulate adhesion, but particles can affecthe transmission and infiltration of macromolecules into the

embrane pores. Greater understanding of the foulant struc-

ure in mixed specie systems will allow better control measureso prevent foulant build-up or to disengage the foulant layer. Theessons learnt from such studies are important for understandingouling in MBRs.

Fig. 5. Progressive pore blockage leading to rapid TMP increase.

. Roadmap for MBR fouling parameters

All the parameters involved in the design and operation ofBR processes have an influence on membrane fouling. For the

urpose of this review, three categories of factors are defined, i.e.embrane and module characteristics, feed and biomass param-

ters and operating conditions (Fig. 6). While some of thesearameters have a direct influence on MBR fouling, many othersesult in subsequent effects on phenomena exacerbating foulingropensity. The complex interactions between these parametersomplicate the perception of MBR fouling and it is thereforerucial to fully understand the biological, chemical and physicalhenomena occurring in MBRs to assess fouling propensity and

Fig. 6. Factors affecting fouling in submerged MBRs.

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.1. Membrane characteristics

.1.1. Physical parameters

.1.1.1. Pore size and distribution. The effects of pore size (andistribution of pore size) on membrane fouling are stronglyelated to the feed solution characteristics and in particularhe particle size distribution. Depending of the pore size andhe type of biomass filtered, results reported in the literatureave shown opposite trends. If particle size is smaller thanore size, pore blocking and/or restriction is expected. It isherefore expected that large pore membranes like MF wouldresent higher fouling propensity compared to UF membranes.able 2 reports results obtained in 11 studies during which

he pore size effects have been assessed by different foulingarameters. It is quite clear from this table that the pore sizelone cannot predict hydraulic performances as no general trendas observed between these two parameters. The complex and

hanging nature of the biological suspension present in MBRystems and the large pore size distribution of the membraneenerally used in MBR are the main reasons for the unde-ned general dependency of the flux propensity on pore size83,84]. Additionally, the duration of the experiment and otherperating parameters like crossflow velocity (CFV) and con-tant pressure or constant flux operation have a direct influ-nce on the determination of the optimization of the membraneore size (Table 2). For example, when MF and UF mem-ranes were compared in a similar environment (with a CFVf 0.1 m/s), the MF membrane produced a hydraulic resistanceround twice that of the UF membrane. In that same study, theouling behaviors of the MF and UF membranes were differ-nt when operated at higher CFV. This was due to the effectf CFV on critical flux of particulates (Section 3.3.1). Inter-stingly, the dissolved organic carbon (DOC) rejection of both

embranes were similar after 2 h of operation, indicating the

reation of a dynamic membrane layer on the MF membrane85].

bft

able 2ffect of pore size on MBR hydraulic performances

embranes tested Optimum Test duration

.1,0.22, 0.45 �m 0.22 �m 20 h

0, 30, 50, 70 kDa70 kDa 110 min50 kDa 110 days

0 kDa, 0.3 �m 70 kDa 8 h

0 kDa, 0.3 �m30 kDa 2 h0.3 �m Merge

.1, 0.2, 0.4, 0.8 �m 0.8 �m n/a00 kDa, 0.1, 1 �m 1 �m 3 h

.3, 1.5, 3, 5 �m5 �m 25 min0.3 �m 45 days

.4, 5 �m0.4 �m 1 dayNo effect From 50 days

.01, 0.2, 1 �m No effect Few hours00 kDa, 0.1, 1 �m 0.1 �m n/a.05, 0.4 �m 0.05 �m n/a

a Constant flux operation, non-marked references are constant TMP operation.

The long-term effect of UF membrane pore size on hydraulicerformances has been assessed by He et al. for anaerobic MBRperated under constant TMP [87]. The smallest MWCO tested20 kDa) featured the largest permeability lost within the first5 min of filtration when compared to 30, 50 and 70 kDa mem-ranes. However, when operated for extended time (over 100ays) with regular hydraulic and chemical cleaning, the largestWCO membrane (70 kDa) experienced the greater fouling

ate, as 94% of its original permeability was lost, compared tonly 70% performance decrease for the other three membranes.s a result, the 30 and 50 kDa membranes provided the bestverall hydraulic performances, indicating the possibility of anptimum membrane pore size for a given application and for aiven filtration time. These results also revealed that the experi-ent duration is crucial to fully assess the fouling propensity ofmembrane. Similar trends showing the time dependency for

arge pore MF with the highest initial fouling for the smallerore and the greater long-term fouling for the larger pore wereeported for pore size ranging from 1.5 to 5 �m operated at con-tant TMP [90]. While the quest for the highest “steady-state”ermeability is probably desirable, it is important to be awarehat conclusions derived from flux decline data could be some-imes deceptive, as an intrinsically high flux membrane mayppear to foul more for the same increment of resistance.

It is expected that smaller pore membranes would reject aider range of materials, and the resulting cake layer features aigher resistance compared to large pore membranes. However,his type of fouling is more reversible and is easily removed dur-ng the maintenance cleaning than fouling due to internal porelogging obtained in larger pore membrane systems. The irre-ersible fouling, due to the deposition of organic and inorganicaterials onto and into the membrane pores is the main cause

f the poor long-term performances of larger pore size mem-

ranes. However, when testing membranes with pores rangingrom 0.4 to 5 �m (at constant TMP), Gander et al. observedhe opposite results, i.e. higher initial fouling for large pore and

Other References

– [86]a

High concentrated[87]

Feed, anaerobic– [88]

CFV = 0.1 m/s[85]

CFV = 3.5 m/s– [89]a

Based on critical flux test [84]a

– [90]

– [91]

Based on critical flux test [92]a

Anaerobic [93]– [94]

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ignificant flux decline when the small pore membrane was usedver an extended period of time [91] (Table 2). Characterizationf the molecular weight (MW) distribution of the compoundsresent in the supernatant of MBRs operated with four pore sizesranging from 0.1 to 0.8 �m) has also been presented [89]. Withpparent lower fouling rate, the 0.8 �m pore size MBR featuredslightly higher concentration of most of the macromoleculesresent in the bioreactor supernatant. However, it seems unlikelyhat the small differences in MW distribution are the main causef the various fouling rates observed between the four MBRystems.

In another study based on short-term experiments, sub-ritical fouling resistances and fouling rates increased linearlyor membrane resistances ranging from 0.4 to 3.5 × 10−11 m−1,orresponding to membrane pore size from 1 down to 0.01 �m,espectively [84]. These results indicated the creation of aynamic layer of greater overall resistance for more selectiveembranes under sub-critical conditions. However, it was also

ostulated that increasing the pore size may decrease the depo-ition onto the membrane at the expense of internal adsorption.ong-term trials confirmed this theory as progressive internaleposition eventually leads to catastrophic increase in resistance14,95]. This again emphasizes the importance of test durationn fouling studies.

.1.1.2. Porosity/roughness. Membrane roughness and poros-ty were suggested as potential reasons for the different foulingehaviors observed when four MF membranes with nominalore sizes narrowly ranged between 0.20 and 0.22 �m wereested in parallel [96]. The four membranes were operated underhe same constant pressure, and therefore produced differentnitial fluxes. The track-etched membrane, with its dense struc-ure and small but uniform cylindrical pores, featured the lowestesistance due to pore fouling. In contrast, the other three mem-ranes presented interwoven sponge-like microstructures andere more prone to pore fouling due to their highly porous net-ork. Although all membranes featured similar nominal pore

ize, polyvinylidene fluoride (PVDF), mixed cellulose estersMCE) and polyethersulfone (PES) membranes presented dif-erent fouling behaviors. While fouling was mainly due to cakeormation for the PVDF and MCE membranes, pore block-ng was responsible for 86% of the total hydraulic resistancehen the PES membrane was used. Overall, the PES membrane

howed a 50% higher fouling resistance than the PVDF andCE membranes. It was suspected that membrane microstruc-

ure, material and pore size distribution were all affecting MBRouling significantly [96]. Comparison between two microp-rous membranes prepared by the stretching method revealed thenfluence of the pore aspect ratio (mean major axis length/mean

inor axis length) on fouling in an MBR. With both membraneshe average pore size and pure water flux were identical, but lessouling was observed with the membrane having the higher porespect ratio (elliptical pore) rather than with the circular pore

embrane [97]. With roughness values (measured by AFM)

anging from 2.4 to 33.2 nm, for 20 and 70 kDa MWCO mem-ranes, respectively, initial fouling was observed to decreasehile irreversible resistance increased [87]. However, in this

slfis

tudy based on an anaerobic MBR, membrane morphology andore size were changing simultaneously, so it was not possibleo clearly determine the effect of roughness on MBR fouling.owever, an assumption was made that the large “filling-inoints” present on rougher membranes are more prone to thereation of fouling layers, compared to the fewer and smallercrevices” observed on smoother membranes [87]. Detailed dis-ussion about the effect of membrane surface properties on cellttachment could be found in [98], while Ho and Zydney gaveore details about membrane morphology and MBR fouling

99].

.1.1.3. Membrane configuration. The current trend in MBResign tends to favor submerged over sidestream configurationsn the majority of the studies dealing with domestic wastewa-er treatment. As a result, comparison between these two MBRonfigurations will be discussed only briefly in this review, butore details can be found in [100–103]. Based on short-term

ritical flux tests, a direct comparison between submerged andidestream MBRs showed that similar fouling behavior wasbtained when the two configurations operated at superficial gaselocity (UG) of 0.07–0.11 m/s and superficial liquid velocityUL) of 0.25–0.55 m/s for submerged and sidestream, respec-ively [102]. An increase of UG in the submerged MBR waslso found to have more effect in fouling removal than a similaraise of CFV (or UL) in the sidestream configuration (also seeection 3.3.1). This may be due to the benefit of unsteady stateow achieved by bubbling.

In submerged MBR processes, the membrane can be con-gured as vertical flat plates, vertical or horizontal hollow finebers (filtration from out-to-in) or, more rarely as tubes (filtrationrom in-to-out). Although the tubular configuration is generallyreferred for sidestream processes, the effect of the lumen sizen submerged MBR fouling has been investigated [104,105].hile hollow fiber modules are generally cheaper to manufac-

ure, allow high membrane density and can tolerate vigorousackwashing, fluid dynamics and distributions may be probablyasier to control for flat plate and tubular membranes, wherehe membrane channel width is well defined [106]. As a result,ollow fibers may be more prone to fouling and require morerequent washing and cleaning. An interesting discussion of theelative performances of hollow fibers and flat plate membranesas initiated by Gunder and Krauth [107] and revealed the bet-

er hydraulic performance of the flat plate in their studies. Twoypes of submerged MBR of comparable size, operated for theame length of time for sewage treatment have also been com-ared [108]. The differences observed were mostly due to theifferent operating and maintenance conditions (see Section 4.1)ather than the module designs per se. Although the price of theat plate MBR is estimated to be 20–25% higher than hollow-ber-based-systems, fouling rate and maintenance operation areenerally less for the former configuration. This observation maye due to the design flux at which MBR were operated in this

tudy, i.e. 20–27 l/m2 h for flat plate and 23–33 l/m2 h for hol-ow fiber [108]. The backwashing requirement of the hollowber MBR (up to 25% of the permeate volume [108]) may alsolightly complicate the process. In another study, the effect of

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embrane configuration was assessed when hollow fiber andat plate MBRs (featuring similar pore size of 0.4 �m) weresed for high-strength wastewater treatment [109]. Once bothystems were operated at similar flux, it was found that theat sheet MBR fouled slightly more and could not recover itsriginal performance after water cleaning. However, chemicalleaning managed to remove most of the fouling (probably dueo pore blocking in this specific case). Finally, each configu-ation has specific footprint, airflow requirement, and integrityesting which may favor one process over another one for a givenpplication. More details about these two configurations (with aocus given on aeration intensity) and the effect of their physicalarameters are available from Cui et al. [106].

Amongst the numerous membrane manufacturers, Kubotaflat plate configuration), Zenon, Mitsubishi and US Filter (hol-ow fiber configuration) are the main membrane suppliers for

BR systems. Few large-scale studies based on comparisonf these commercially available MBR systems have been con-ucted. The city of San Diego, California, and the researchonsultant, Montgomery Watson Harza, have been evaluatinghe MBR process through various projects since 1997, includingeasibility of using MBR to produce reclaimed water [110,111],ptimization of MBR operation, and parallel comparison andost estimations of the four leading MBR suppliers [112]. MBRsere evaluated for their ability to produce high quality effluent

nd to operate with minimum fouling. In terms of hydraulicerformances, it was shown that all four processes were ableo cope with flux rates exceeding 33 l/m2 h and HRTs as lows 2 h. A 6-year-development programme has also been initi-ted for the introduction of MBR technology in the Netherlandsarket. Started in 2000, a comparative study of four 750 m3/day-BRs carried out by DHV water has been reported [113,114].

inally, three MBR plants, treating a design flow of 300 m3/dayach, have been operated in parallel during 2003 and 2004 iningapore. This most recent study reported MBR power con-umption of less than 1 kWh/m3 of treated water [115], whilenergy consumption around 1.9 kWh/m3 was reported for 2001116] and up to 2.5 kWh/m3 in 1999 [117]. Although these threetudies have been conducted with the MBR systems running inarallel (with the same influent water), the MBR maximum flux,perating conditions and general design applied were those rec-mmended by the suppliers, and therefore somewhat differentor each system. This makes it difficult to make a fair compar-son, so it is not possible to classify the MBRs as a function ofheir relative hydraulic performances, which need to be consid-red along with the cleaning protocols applied to each systemsee Section 4.1).

An important parameter for submerged hollow fibers is likelyo be packing density. The distance between adjacent membraness suspected to directly impact on mass transfer and thereforehe shear and aeration demands. Moreover, increasing the pack-ng density could lead to severe clogging by gross solids ando the slower rise of bubbles, limiting their effect on fouling

imitation. Experiments carried out with a model bundle of ninebers revealed the overall module performance to be much worse

han that of an individual fiber [118,119]. It was also clearlyhown that the surrounded fibers are less productive than the

goti

uter fibers. At high feed concentration and low cross-velocity,urrounded fibers become completely blocked and eventuallyroduce negligible flux. Finally, it was advised that the pack-ng density should be lower than 30% in order for the bundle toerform similarly to single fibers. In low packing density config-rations, cake layers from adjacent fibers do not interfere withach other and the effect of CFV may be more evenly distributed,imiting overall fouling [119]. A mathematical model based onubstrate and biomass mass balance also revealed the significantole played by packing density in the overall MBR performance,nd the hydrodynamics of the biomass in particular [120].

The effects of other membrane characteristics including hol-ow fiber orientation, size and flexibility are discussed in theeview of Cui et al. [106]. For hollow fiber membranes usedor yeast filtration, higher critical fluxes were measured forlightly loose membranes (95%), with small diameter (0.65 mm)nd greater length (80 cm) [121]. Contradictory results showinglightly higher specific flux for shorter membranes (30 cm cf.00 cm) has been reported [122]. The pressure drop due to theermeate flow in the lumen of the hollow fiber could be the mainause behind the effect of membrane length on rapid and severeouling. Significant pressure loss (up to 53 kPa) was measuredor long fibers (60 cm). Below the critical length of 15 cm, pres-ure loss was minimal at less than 11 kPa [97]. Further discussionf fouling distribution in hollow fibers can be found elsewhere31,123–127].

.1.2. Chemical parameters

.1.2.1. Hydrophobicity. Because of the hydrophobic interac-ions occurring between solutes, microbial cells and membrane

aterial, membrane fouling is expected to be more severe withydrophobic rather than hydrophilic membranes [92,128–130].n many reported studies, change in membrane hydrophobic-ty often occurs with other membrane modifications such asore size and morphology, which make the correlation betweenembrane hydrophobicity and fouling more difficult to assess.

n a recent study for example, the contact angle measurementhowed that the apparent hydrophobicity of polyethersulfonePES) membranes decreased (from 55◦ to 47◦) with the increasen MWCO (from 20 to 70 kDa membranes, respectively) [87].he effect of membrane hydrophobicity was studied in detailuring comparison of two UF membranes of similar character-stics [131]. Based on the greater solute rejection and foulingnd cake resistances reported for the hydrophobic membrane,he authors were able to postulate on the effects of membraneouling on the removal performances of the MBR process. Itas concluded that the greater solute rejection was mainly due

o the dynamic layer formed by adsorption and/or sieving in theake deposited on the membrane, and, to a lesser extent, due toirect adsorption into membrane pores and on the surface.

Numerous anti-fouling studies have been based on membraneurface modification, and will be reviewed in Section 4.2.1.urprisingly, Fang and Shi [96] indicated that membranes of

reater hydrophilicity tend to be more vulnerable to depositionf foulants of hydrophilic nature. In MBRs, activated sludge con-ains substantial amounts of hydrophilic EPS, which has beendentified as an important foulant (Section 3.2.5). However, in

to

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3

3

b[trueicpfttf

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aTtmtifmtthutntbf[apimirst

teboperating conditions are numerous and include the type of feedwater used [144], permeability of the membrane, particle sizeand hydrodynamics conditions [143]. Examples of interactionsbetween suspended and dissolved solids and membrane fouling

his study, the most hydrophilic membrane also featured morepen pores, which could be another reason for severe fouling.

Notwithstanding the significance of the membrane hydropho-icity on the early stage of the fouling formation, this parameters expected to play only a minor role during extended filtrationeriods. Once initially fouled (i.e. conditioned), the membrane’shemical characteristics would become secondary to those of theludge materials covering the membrane surface.

.1.2.2. Materials. Although featuring superior chemical, ther-al and hydraulic resistances, ceramic membranes are not the

referred option for MBR applications due to their high cost.owever, ceramic membranes have been successfully used for

everal MBR applications, such as treatment of high-strengthndustrial waste [132,133] and anaerobic biodegradation [134].eramic membranes, in modules which require higher pressurend turbulence, are generally used in sidestream configura-ions. The benefit of turbulence promoters in such MBR sys-ems has been reported [135]. The potential advantage of usingeramic membrane was demonstrated in a test comparing 0.1 �meramic with 0.03 �m polymeric multi-channel membrane mod-les operated in sidestream air-lift mode. The ceramic MBR didot substantially foul for short-term experiments with fluxes upo 60 l/m2 h, while the polymeric membrane critically fouled atround 36 l/m2 h [136]. However, in the same study, the over-ll cost of the ceramic membrane was reported to be around anrder of magnitude more expensive than the polymeric materials.inally, novel stainless steel membrane modules have recentlyhown good hydraulic performance and fouling recovery whensed in an anaerobic MBR for wastewater treatment [137]. How-ver, the large majority of the membranes used in MBRs areolymeric-based. A direct comparison between polyethylenePE) and PVDF membranes clearly indicated that the later leadso a better prevention of irreversible fouling and that PE mem-rane fouled more quickly [138]. In that same study, the authorslso mentioned that the composition of the irreversible foulingas dependant of the membrane material, as some fractions of

he organic matter present in the biomass presented a higherffinity with certain polymeric materials.

.2. Feed–biomass characteristics

.2.1. Nature of feed and concentrationAlthough the effects of wastewater properties on mem-

rane fouling are undeniable for direct wastewater filtration139–141], fouling in the MBR is mostly affected by the interac-ions between the MBR membrane and the biological suspensionather than wastewater per se [88]. However, in the rare cases ofsing saline sewage as feed, the resulting higher fouling rate gen-rally leads to a more frequent cleaning [142]. The most strik-ng effect of the wastewater nature is on the physico-chemicalhanges in the biological suspensions [13,14]. For example, therotein fraction measured in the extracted EPS (eEPSp) has been

ound to be significantly lower when biomass was fed with syn-hetic feed (chemical oxygen demand: COD of 460 mg/l) ratherhan with real sewage (COD of 140 mg/l). Simultaneously, theouling rate was higher using synthetically fed MBR [14]. For

Fp

hese reasons, the fouling propensity of the wastewater is indi-ectly taken into consideration during the characterization of theiomass (Section 3.2.3).

.2.2. Biomass fractionationActivated sludge biomass can be fractionated into three ide-

lized components, i.e. suspended solids, colloids and solutes.his approach has often been applied to account for the rela-

ive contribution of each biomass fraction on MBR fouling. Theethodology applied to appropriately separate the biomass frac-

ions varies from one study to another but remains a crucial stepn the definitions of the different biomass fractions and there-ore, the interpretation of the results. Unfortunately, no standardethod exists. However, Fig. 7 shows a typical protocol where

he biomass sample is centrifuged, the resulting supernatant ishen filtered with a dead-end membrane cell, with the calculatedydraulic resistance (Rsup) being attributed to colloidal and sol-ble species (Rcol and Rsol, respectively). Another portion ofhe biomass suspension is then filtered by a microfilter (withominal pore size of around 0.5 �m). The fouling properties ofhis coarse-filtered supernatant are attributed solely to the solu-le matter with resistance Rsol. Calculations assess the relativeouling contributions of the suspended solids and the colloids143]. In another approach, the concentration of colloids waslso characterized by the difference between the levels of TOCresent in the filtrate passing through 1.5 �m filtration paper andn the permeate collected from the MBR membrane (0.04 �m

embrane) [29]. Although fractionation methods may signif-cantly vary for different studies (see references from Fig. 8),esults are often reported in terms of hydraulic resistances foruspended solids (Rss), colloids (Rcol) and soluble species (Rsol),he sum of which being the total resistance (Rt).

Although an interesting approach for studying MBR fouling,he fractionation experiment neglects any coupling or synergisticffects which may occur among the different components of theiomass. The interactions between each biomass fraction and the

ig. 7. Experimental method for the determination of the relative foulingropensity for the three biomass fractions.

Fig. 8. Relative contributions (in %) of the different biomass fractions to MBRfbe

wishrsb

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esrsignificant impact on the determination of the effect of the MLSSconcentration. Similarly, the test duration can be a factor. WhileMBR performances are expected to decrease for higher MLSS(at applied flux superior to Jc), the MLSS concentration may not

Table 3Influence of shift in MLSS concentration (g/l) on MBR fouling

MLSS shift Fouling parameters References

Fouling increase0.09–3.7 Rc: 21 to 54 × 1011 m−1 and αc: 18.5 to

0.7 × 108 m/kg[146]

2.4–9.6 Rp: 9 to 22 × 1011 m−1 [96]7–18 Jc: 47–36 l/m2 h (for SRT: 30–100 days) [155]

2.1–9.6 Jc: 13–8 l/m2 h [154]1–10 Jc: 75–35 l/m2 h [92]a

2–15 Limiting flux: 105–50 l/m2 h [156]a

1.6–22 Stabilized flux: 65–25 l/m2 h [157]a

Fouling decrease3.5–10 Jc: <60 to >80 l/m2 h [26]a

No (or little) effect9–14 No impact on fouling rate [158]

4.4–11.6 No impact between 4 and 8 g/l, slightlyless fouling for 12 g/l

[84]

6–18 Similar fouling rates for J < 10 l/m2 h,and slightly lower fouling rates for

[148]

ouling. For SRT increase from 8 days (1) to 40 days (2); F/M ratio of 0.5, resultsased on modified fouling index (3); based on flux reduction after 600 min ofach fraction filtration (4); for SRT increase from 20 days (5) to 60 days (6).

ere discussed in Section 2.3. The protocol illustrated in Fig. 7s limited because it relies on dead-end filtration tests with apecific membrane. However, studies on biomass fractionationave also been reported for crossflow and submerged configu-ations. An attempt to compare results obtained from differenttudies is reported in Fig. 8 where relative contributions haveeen calculated.

The relative contribution of the biomass supernatant (solublend colloids, generally defined as soluble microbial productsr SMP) to overall membrane fouling ranges from 17% [143]o 81% [145]. These wide discrepancies may surprise and arerobably explained by the different operating conditions andiological states of the suspension used in the reported stud-es. They also confirmed the relatively low fouling role playedy the suspended solids (biofloc and the attached EPS) com-ared to those of the SMP (Section 3.2.6). In terms of foulingechanisms, soluble and colloidal materials are assumed to be

esponsible for the pore blockage of the membrane, while sus-ended solids account mainly for the cake layer resistance [145]Section 3.4.2). However, because MBRs are typically operatedt modest flux, the formation of a biomass cake tends not toccur. The smaller species (like SMP) are much more likely toeposit.

.2.3. Biomass (bulk) parameters

.2.3.1. MLSS concentration. Often considered at first sight ashe main foulant parameter, MLSS concentration has indeed aomplex interaction with MBR fouling, and controversial find-ngs about the effect of this parameter on membrane filtrationave been reported. If the other biomass characteristics are notccounted for, the increase in MLSS concentration seems to havemostly negative impact (higher TMP or lower flux) on the MBRydraulic performances [146,147]. However, some authors haveeported positive impact [26,148], and some observed insignif-

cant impact [84,149,150]. The existence of a threshold abovehich the MLSS concentration has a negative influence was also

eported (at 30 g/l [151]). A more detailed fouling trend has beenescribed by Rosenberger et al. [152]. While a rise in MLSS

eems to decrease fouling at low MLSS concentration (<6 g/l),ore fouling is expected as the MLSS concentration increases

bove 15 g/l. The level of MLSS does not appear to have signif-cant effect on membrane fouling between 8 and 12 g/l. Anothertudy [153] reviewed the significant effect of MLSS for con-entrations lower than 5 g/l, and indicated that hydrodynamicsmore than MLSS concentration) control the critical flux (Jc)or greater MLSS levels [153]. This is only partially verifiedy the data reported in Table 3. More subtle studies showedpparent contradictory trends from data obtained in the sametudy. For example, the cake resistance (Rc) was observed toncrease and the specific cake resistance (αc) to decrease as

LSS increased. Although having similar meaning conceptu-lly Rc and αc seemed to behave inversely [146]. This can beeconciled by noting:

c = αcmc (1)

here mc is the cake load/area of membrane. The cake load mcould tend to rise with MLSS concentration. Bin et al. observed

he permeate flux to decrease (but at a lower fouling rate) whenLSS increased [154]. This was explained by the creation of a

apid fouling cake layer (potentially protecting the membrane) atigh concentration, while progressive pore blocking created byolloids and particles was thought to take place at lower MLSSoncentration.

Since the value of Jc is often determined during short-termxperiments, it is expected that Jc indicates the deposition ofuspended solids rather than colloidal and soluble materials. As aesult, the flux value at which the experiment is carried out, has a

higher J4–15.1 Jc decreased from 25 to 22 l/m2 h [24]

3.6–8.4 – [149]

a Sidestream MBR.

pocMrfl

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lay a significant role in fouling propensity when the MBR isperated at low fluxes. In that later case, EPS components andoncentrations have more effect on the MBR fouling than theLSS concentration (Sections 3.2.5 and 3.2.6). Contradictory

esults may also arise from the mode of filtration, i.e. constantux versus constant TMP (Section 3.4).

Empirically derived equations predicting flux performanceave been proposed in numerous papers [96,159–161]. How-ver, these equations have limited use as they are generallybtained under very specific conditions and take into accountome specific operating parameters and disregard some others.

mathematical expression linking MLSS concentration, EPSnd TMP with cake specific resistance has been proposed byho et al. [162]. In this study, specific resistance did not change

ignificantly for MLSS ranging from 4 to 10 g/l and when thePS and TMP were kept constant.

The experimental method used for changing MLSS con-entration can also significantly impact upon biomass charac-eristics since biomass acclimatization periods are not alwaysespected [147]. Although the removal performances are gener-lly high for MBR processes, MLSS concentration also plays aignificant role in this regard. For example, an optimal MLSSoncentration at 6 g/l was obtained based on the highest CODemoval [163] and on the highest virus removal [164].

The lack of a clear correlation between MLSS concentrationnd any other foulant characteristics indicates that the MLSSoncentration (alone) is a poor indicator of biomass foulingropensity [13,165]. These authors recommended the use of fun-amental operating parameters like HRT and SRT for predictionf foulant production. This has been supported by the relativelytable foulant characteristics obtained once true steady-state wasstablished in the bioreactor. Current studies tend to considerhe non-settleable organic substances (rather than the MLSSoncentration) as the main players in the fouling propensity inBRs (see Sections 3.2.5 and 3.2.6).

.2.3.2. Viscosity. In the MBR, like in conventional activatedludge processes, biomass viscosity is closely related to its con-entration, and has been cited as a foulant parameter [166].

critical MLSS concentration exists under which the viscos-ty remains low and rises only slowly with the concentration.bove this critical value, suspension viscosity tends to increase

xponentially with the solids concentration [145]. This criti-al value was observed to change from 10 to 17 g MLSS/l forifferent operating conditions (conventional and hybrid (pre-oagulation/sedimentation) MBRs, respectively). Similar obser-ations were reported for the behavior of the capillary suctionime (CST), another parameter closely related to viscosity [30].he importance of MLSS viscosity is that it modifies bubbleize and can dampen the movement of hollow fibers in sub-erged bundles [121]. The net result of this phenomenon would

e a greater rate of fouling. Increased viscosity also reduceshe efficiency of mass transfer of oxygen and can therefore

ffect dissolved oxygen (DO) [167]; fouling tends to be worset low DO (see below). The effect of MLSS concentration oniscosity at different shear rates obtained from a submergedBR is shown in Fig. 9. These results also indicate the pseudo-

tpiM

ig. 9. Viscosity obtained at different MLSS concentrations and shear rates105].

lastic (or “shear-thinning”) property of the sludge obtained inBR.

.2.3.3. Temperature. Temperature impacts on membrane fil-ration through its influence on the permeate fluid viscosity168]. The common approach to comparing hydraulic perfor-ance obtained at different temperatures is to normalize the

perating flux at a reference temperature (generally 25 ◦C). Thisould be done by applying a temperature correction factor [169].o avoid the interference of the temperature effects on MBRouling, non-linear regression between critical flux and temper-ture was obtained [29]:

c,t = Jc,20 × 1.025t−20 (2)

Interestingly, experiments carried out at two sets of temper-tures (17–18 and 13–14 ◦C) featured different hydraulic resis-ances even after the flux had been normalized [170]. The greateresistances observed at low temperature were explained by fourhenomena occurring in the system: (1) within that temperatureange, the sludge viscosity (rather than permeate viscosity) wasalculated to increase by 10%, reducing the shear stress gener-ted by coarse bubbles, (2) intensified defloculation tend to occurt low temperature, reducing biomass floc size and releasingPS to the solution, (3) particle back transport velocity, calcu-

ated with the Brownian diffusion coefficient (linearly related toemperature), is less at low temperature, and (4) biodegradationf COD was also reduced at decreased temperature, resulting inhigher concentration of solute and particle COD in the reactor

170]. This last phenomenon was also observed by Fawehinmit al. [171] with higher SMP levels measured in an anaerobicBR operated at 20 ◦C rather than at 30 ◦C. All of these fac-

ors are directly linked to membrane fouling, so it is expected tobserve greater deposition of materials on the membrane surfacet lower temperatures [158].

.2.3.4. Dissolved oxygen (DO). The average level of DO in

he bioreactor is controlled by the aeration rate, which not onlyrovides oxygen to the biomass but also tend to limit foul-ng formation on the membrane surface. The effects of DO on

BR fouling are therefore multiple and may include changes in

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iofilm structure, SMP levels, and floc size distribution [89]. Asgeneral trend, higher DO tends to lead to better filterability, and

ower fouling rate. This was explained by the lower specific cakeesistance of the fouling layer which featured larger particle sizesnd greater porosity [172,173]. As expected, significant differ-nces were observed in microbial communities and resultingiofouling when the MBR was operated under various DO lev-ls (from 6 to less than 0.1 mg/l in [172]). Surprisingly, the CODn biomass suspension (i.e. an indicator for SMP level) decreasedrom 37 to 27 mg/l for DO of 3.4 and 0.9 mgO2/l, respectively174] and therefore cannot explained the hydraulic performancesbtained in MBR operated at higher DO. Moreover, the con-ribution of SMP to membrane filterability was found to be ainimum compared to those of the physico-chemical proper-

ies of cake layer (i.e. particle size and porosity) [173]. In atudy obtained with anoxic and aerobic sludges [175], floc dete-ioration was observed and used as a possible explanation forhe higher fouling rates obtained for the denitrification assay.he effect of oxygen limitation causing a lowering of the cellurface hydrophobicity, and consecutive floc deterioration, wasoncluded to be the main reason for the worsen MBR foulingor anoxic conditions.

In the MBR, fouling may also be due to the creation of aiofilm layer on the membrane surface. As a general definition,acterial biofilm characterizes the population of microorgan-sms concentrating, depositing and/or growing at the solid/liquidnterface [176]. As described later in the review (Section 3.2.5),he formation of biofilms is possible through the active rolef EPS which surround the microorganisms. Biofilm proper-ies such as adhesion strength (interaction between microorgan-sms and membrane) and cohesion strength (interaction betweenicroorganisms themselves) can be determined and are directly

ependant of the nature of the EPS [176]. As the thickness ofhe biological fouling layer increases with extended MBR filtra-ion time, some biofilm regions have been observed to becomenaerobic [86]. Because of the poor oxygen transfer within theiofilm structure, the fouling sub-layers (on the membrane sur-ace) may become anaerobic, and therefore affect membraneouling differently. Endogenous decay, similar to that expectedithin the fouling layer, was simulated and revealed the levelf carbohydrate in the extracted EPS (eEPSc) to significantlyncrease. Since the transition between aerobic to anaerobic con-itions seems to produce a large amount of EPS, this phe-omenon could also be responsible for MBR fouling [86].ore details about fouling in anaerobic MBR can be found

n [1].The direct impact of air bubbles (as a foulant parameter) on

BR filtration was even investigated by Jang et al. However, itas concluded that the effect of air blocking on the surface cane ignored in MBR processes with high MLSS concentrationas it accounted for less than 1% of total resistance Rt) [177].owever, under some circumstances, air bubbles my be presentr be formed in the lumens of hollow fibers and this can be

etrimental [178]. Finally, it is important to keep in mind thathe aeration rate (discussed in Section 3.3.1) controls biologicalequirements and parameters such as DO, ammonium/nitrateatio [179].

otkh

.2.4. Floc characteristics

.2.4.1. Floc size. In MBR systems, aggregation of theicroorganisms, and the formation of large floc is a signifi-

ant element in the effective separation of suspended biomassrom the treated water, although it is more critical in CASP.n terms of floc size, biomass suspensions in MBRs feature aide distribution, which ranges significantly from one study to

nother. Comparison of the aggregate size distribution of CASPnd MBR sludges was carried out [180] and revealed a distinctifference in terms of mean particle sizes (160 and 240 �m,espectively). A bimodal distribution was even observed for

BR sludge (5–20 and 240 �m); the high concentration of smallolloids, particles and free bacteria was caused by their com-lete retention by the membrane. In this study, sludges wereollected from small-scale experimental rigs and measured byarticle size analyzer (MasterSizer 2000). In another study, par-ial characterization of the floc (up to 100 �m) reported the flocize to range from 10 to 40 �m, with a mean size of 25 �m143]. These authors also claimed that the floc size distributionbtained with the MBR sludge are lower than the results gener-lly obtained from CASP. In comparison, the particles presentn the supernatant, obtained after 4 h of gravitational sedimenta-ion, have a mean size of around 9 �m. The floc size distributionsbtained with three MBR operated at different SRTs were sim-lar, although the mean floc size increased slightly from 5.2 to.6 �m for SRT increasing from 20 to 60 days [181].

Given the large size of the floc particles, compared to the poreize of the membrane generally used in MBR, it is expectedhat floc cannot directly block pore entrances. Nor would theoc deposit on the membrane surface due to drag forces result-

ng from the low/modest fluxes and the shear induced backransport phenomenon experienced by large particles. However,ndependent of their size, biological floc play a major role inhe formation of the fouling cake on the membrane surface. Theffect of the EPS level on floc size will be discussed in Sec-ion 3.2.5. The addition of aerobic granules, activated carbon orolymer can significantly increase the floc size. Their effect onBR fouling is reported in Section 4.2.2.

.2.4.2. Hydrophobicity/surface charge. In the MBR process,ike in CASPs, hydrophobic flocs lead to high flocculationropensity and low interaction with the (generally) hydrophilicembrane. However, reports of highly hydrophobic flocs foul-

ng MBR membranes can be found in the literature. Relativeydrophobicity of floc can be directly measured by bacterialdhesion to hydrocarbons (hexane) [182], or estimated by con-act angle determination [128]. Although the direct effect of flocydrophobicity on MBR fouling is difficult to assess, hydropho-icity measurement of sludge and EPS solutions revealed thathe decrease of EPS relative hydrophobicity may cause floc dete-ioration (and consequent increase of Rc) [175,182]. EPS levelnd filamentous index (parameter related to the relative pres-nce of filamentous bacteria in sludge) have a direct influence

n the relative hydrophobicity and zeta potential measured inhe biomass floc. The excess growth of filamentous bacteria,nown to be responsible for severe MBR fouling, also resulted inigher EPS levels, lower zeta potential, more irregular floc shape

aith[

ft−[tfansfdflrfl

3

tsm

taem(bsm

ba[aoacicmtbo[wfttt(aiets

tedegothchscb[

Fig. 10. Simplified representation of EPS, eEPS and SMP.

nd higher hydrophobicity [183]. In another example, foam-ng sludge showed greater flux decline (more than 100 times)han the non-foaming sludge. The increase was attributed to theydrophobic and waxy nature of the foaming sludge surface184].

Due to the negative charges from ionization of the anionicunctional groups, flocs (and EPS) of most activated sludge fea-ure zeta potential and surface charges ranging from −0.2 to

0.6 mequiv./g VSS and from −20 to −30 mV, respectively185]. The surface charge of MBR microbial floc (obtained byhe titration method) confirms the general trend, as it rangedrom −0.7 to −0.4 mequiv./g VSS when the MBR was operatedt various SRT (from 20 to 60 days) [181]. This was accompa-ied by an increase in contact angle (from 34◦ to 44◦). In thistudy where the fouling resistance caused by microbial floc wasound to increase with the SRT, contact angle and surface chargeemonstrated a strong positive correlation with the microbial-oc-caused fouling propensity. For CASP, Liu and Fang alsoeviewed a positive effect of long SRT on hydrophobicity andocculation [185].

.2.5. Extracellular polymeric substances (EPS)Given the large number of recent publications dealing with

he fouling of MBRs by bio-polymeric substances, a majorection of this review is focused on their nature, method of deter-ination and influence on MBR fouling.Extracellular polymeric substances (EPS) are the construc-

ion materials for microbial aggregates such as biofilms, flocsnd activated sludge liquors. The term “EPS” is used as a gen-ral and comprehensive concept for different classes of macro-olecules such as polysaccharides, proteins, nucleic acids,

phosphor-)lipids and other polymeric compounds which haveeen found at or outside the cell surface and in the intercellularpace of microbial aggregates [186]. They consist of insolubleaterials (sheaths, capsular polymers, condensed gel, loosely

iwch

Fig. 11. Proposed method for EPS and S

ound polymers and attached organic material) produced byctive secretion, shedding of cell surface material or cell lysis175]. The functions of EPS matrix are multiple and includeggregation of bacterial cells in flocs and biofilms, formationf a protective barrier around the bacteria, retention of waternd adhesion to surfaces [187]. With its heterogeneous andhanging nature, EPS can form a highly hydrated gel matrixn which microbial cells are embedded [188]. Therefore theyan be responsible for the creation of a significant barrier to per-eate flow in membrane processes. Finally, bioflocs attached

o the membrane can play a major nutrient source during theiofilm formation on the membrane surface [189]. Their effectsn MBR filtration have been reported for more than a decade190] and have received considerable attention in recent yearsith many reports indicating the EPS to be the most significant

actor affecting fouling in MBRs [83]. In this review, distinc-ion will be made between the EPS extracted artificially fromhe biological cell floc (eEPS) and the soluble EPS present inhe activated sludge supernatant and unassociated with the cellsoluble microbial products or SMP). The term “EPS” is useds a general parameter to characterize bio-polymeric substancesn the reactor (Fig. 10). It is important to recognize that thexact definitions of eEPS, and SMP are directly dependant ofhe methods used to obtain and characterize chemically theseolutions.

Studies on the effects of EPS in MBR fouling rely on extrac-ion of EPS from the sludge floc. So far, no standard method ofxtraction exists, making comparison between research groupsifficult. Methods of extraction are numerous and include cationxchange resin [175,191,192], heating methods [193], centrifu-ation with formaldehyde [194]. These techniques, along withthers, have been compared under various conditions to assessheir efficacies [185], and results have revealed that formalde-yde extraction was the most effective to extract the largestoncentration of eEPS. However, because of its simplicity, theeating method is sometimes preferred (Fig. 11). Typically, theolution containing eEPS is then characterized by its relativeontent of protein (eEPSp) and carbohydrate (eEPSc), measuredy photometric methods (Lowry et al. [195] and Dubois et al.196] methods, respectively). Although the EPS characterization

s sometimes reported in terms of polysaccharides, this reviewill only use the term carbohydrate, which definition comprises

ompounds like polysaccharides. While eEPSp has generally aydrophobic tendency, eEPSc is more hydrophilic [185]. eEPS

MP extractions and measurements.

Table 4Concentration of the eEPS components in different MBR systems (units are in mg/gSS by default)

eEPSp eEPSc Other Details References

25–30 7–8 Humic: 12–13 R (10) [180]29 36 SUVA: 2.8–3.1 l/m mg S [199]a

120 40 S (∝) [200]31–116 6–15 TOC: 37–65 Four pilot-scale plants, municipal [165]20 14 – Pilot-scale plant, Industrial11–46 12–40 TOC: 44–47 Three full-scale plants, municipal25 9 TOC: 42 Full-scale plants, industrial

– – EPSp + EPSc = 8 – [175]30–36 33–28 – From 20 to 60 [181]73 30 – S (∝) [14]60 17 – R (∝)– – TOC: 250 mg/l S, MLSS: 14 g/l [198]– – TOC: 26–83 mg/gVSS From 8 to 80 [197]

116–101 22–24 – S (20) [174]

S , infi

s[i(iem(bwbnsaar

ira[bccblae

cromdMshb

irafiPeThis confirms previous findings obtained with CASP sludge.Size exclusion chromatography combined with infrared micro-spectroscopy techniques was used for CASP sludge [191].

, synthetic wastewater; R, real wastewater; SRT are given in days in bracket; ∝a Anaerobic upflow-sludge bed filter (UBF) and an aerobic MBR [199].

olution can also be characterized in terms of its TOC level197,198] and, less frequently, its aromaticity or hydrophobic-ty (by the measurement of the specific ultraviolet absorbanceSUVA) [199]). Table 4 reports eEPS concentrations from var-ous MBR set-ups and reveals a relatively narrow range of theEPSp and eEPSc levels measured. In most cases, eEPSp (with aaximum concentration of 120 mg/gSS) was greater than eEPSc

maximum concentration of 40 mg/gSS). Sludge flocs have alsoeen characterized in terms of protein and carbohydrate levels,ith colorimetric analysis carried out directly from the washediomass [174]. Low correlation was found between these twoew indicators and MBR fouling propensity. Finally, the mea-urement of humic substances, generally overlooked for proteinnd carbohydrate, have revealed their significant occurrence inctivated liquors [185], and may require more attention in futureesearch on MBR fouling.

As mentioned before, EPS has been identified as a major foul-ng parameter [25,184,201–203]. More recently, a functionalelationship between specific resistance, MLVSS, TMP, perme-te viscosity and eEPS was obtained by dimensional analysis197]. eEPS was found to have no effect on the specific resistanceelow 20 and above 80 mgEPS/gMLVSS, but played a signifi-ant role on MBR fouling between these two limits. This wasonfirmed by another study reporting no clear relation betweenound EPS (or eEPS) and membrane fouling for concentrationsower than 10 mg/gSS [138]. In another example obtained withn anaerobic MBR, specific resistance increased linearly withEPS rising from 20 to 130 mg/gSS [171].

In order to reach a better understanding of membrane foulingaused by EPS, further insights into eEPS identification wereecently obtained for MBR sludge [175,182]. In a study basedn an intermittently aerated MBR, eEPS solution featured threeain MW peaks at 100, 500 and 2000 kDa (fractionation con-

ucted by gel chromatography). In this example, eEPS with

W larger than 1000 kDa was assumed to be mainly respon-

ible for MBR fouling [198]. For drinking water treatment,igh performance size exclusion chromatography (HPSEC) haseen widely used for accurate characterization (or “fingerprint-

Fa

nite SRT (i.e. no wastage).

ng”) of the apparent molecular weight distributions of natu-al organic matters (NOM) [204]. The same technology waspplied to compare eEPS solutions obtained from different con-gurations, operating conditions or treatment plants (Fig. 12).reliminary results revealed, for example, the similarity of theEPS profiles between different treatment plants [13,30,165].

ig. 12. HPSEC profiles of eEPS (a) and SMP (b) from municipal plants oper-ted at different MLSS concentrations [13].

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esulting chromatographs of eEPS solution exhibited sevenistinct peaks. The analysis of the complex mixtures of eEPSolymers revealed the presence of 45–670 kDa MW proteins,nd 0.5–1 kDa MW carbohydrates. The presence of both pro-eins and carbohydrates around the biological cells was proposeds a key parameter in the floc formation, and therefore may alsoave a significant role in MBR fouling. From this point of view,he work reported for characterization of eEPS obtained fromASP and their influence of flocculation, settling and dewater-

ng may also be an interesting parallel for MBR fouling. Recenteviews [185,205] reporting these issues are valuable tools foruture EPS–MBR studies.

Since the EPS matrix plays a major role in the hydrophobicnteractions among microbial cells and thus in the floc forma-ion [185], it was proposed that a decrease in EPS levels mayause floc deterioration. Experimental results obtained duringhe comparative study of nitrification/denitrification in a MBRend to confirm this theory [175]. The repercussion of low EPSevel, and therefore floc deterioration on membrane fouling maye detrimental for the MBR performances. If verified more thor-ughly, this would indicate the existence of an optimum EPSevel for which floc structure is maintained without featuringigh fouling propensity.

Many parameters including gas sparging, substrate composi-ion [171], loading rate [206,207] affect EPS characteristics inhe MBR, but SRT probably remains the most significant of them208]. A clear decrease of EPS levels was observed for extendedRT, but this reduction became negligible for SRT greater than0 days [165]. However, Lee et al. [181] observed an increasen protein concentration (along with stable carbohydrate levels)hen SRT was increased. Further comments on the role of SRT

re given in Section 3.3.2.

.2.6. Soluble microbial products (SMP)In an attempt to protect the membrane from direct contact

ith MLSS, a dual compartment MBR (bioreactor coupled withettling tank in which membrane filters biomass supernatant)as been built in Singapore [207]. In this set-up, higher fil-ration resistance was observed from the membranes filtering

sduc

able 5oncentration of the SMP components (in mg/l)

MPp SMPc Other

8 25 Humic subs: 36– 3–14 –– 2–6.5 –

TOC: up to 8 mg/l0.5–9a n.d.–10a 4–37a (TOC)0.5–1a n.d. 11a

0.5a n.d. 1.5a

– – TOC: 30–70 mg/l3 7– – DOC: 5 mg/l– – TOC: 8–10 mg/l0–34 5–33 –4.5–6 4.5–3.7 –

.d., non-detected; S, synthetic wastewater; R, real wastewater; SRT are given in daya In mg/gSS.

upernatant rather than those filtering 4 g/l of biomass. Thisxample clearly indicates that the composition and concentrationf the organics present in the biomass supernatant (i.e. solubleicrobial products (SMP)) have a large impact on membrane

erformance. SMP are defined as soluble cellular componentshat are released during cell lysis, diffuse through the cell mem-rane, are lost during synthesis or are excreted for some purpose187,209]. In MBR systems, they can also be provided from theeed substrate (Fig. 10). It has been now widely accepted that theoncepts of soluble EPS and SMP are identical [152,175,187].uring filtration, SMP adsorb on the membrane surface, blockembrane pores and/or form a gel structure on the membrane

urface where they provide a possible nutrient source for biofilmormation and a hydraulic resistance to permeate flow [152].lthough the influence of dissolved matter has been studied fordecade, the concept of SMP fouling in the MBR is relativelyew as no report on SMP levels existed for MBRs prior to 200183]. In order to reveal the feasibility and relevance of liquidhase analyses on MBR filterability and potentially standardizehe method, Rosenberger et al. reported four MBR cases studiesased on SMP analysis for membrane fouling [152].

Three methods of separating the water phase from theiomass have been investigated, and the simple filtration throughlter paper was found to be the most effective technique overentrifugation and sedimentation [210]. Although these authorssed a large pore filter paper (12 �m), it is suggested in thiseview to filter the solution with, at least, a 1.2 �m filter in ordero remove colloids (Fig. 11). Similarly to eEPS, SMP solutions then characterized with its relative amount of protein andarbohydrate [210], with its TOC level [200] or more rarelyith SUVA measurement [211]. Examples of SMPp and SMPc

eported in the literature are given in Table 5.HPSEC analysis has also been conducted on SMP solutions

Fig. 12). Not surprisingly, the molecular weight distributionf the organics present in MBR supernatant was found to be

ignificantly different for reactors operated under various con-itions [158]. However, the SMP solution fingerprint was largelynchanged for weekly measurement from the same reactor, indi-ating no significant change in SMP characteristics when the

Operating conditions References

R (10) [180]R (8) [158]R (15)S (∞) [200]Four pilot-scale plants, municipalThree full-scale plants, municipal [165]Full-scale plant, industrialS (∞), MLSS 15 g/l [212]R (not available) [210]S (20) [211]R (21) [115]R (from 40 to 8) [213]S (20) [174]

s in bracket; ∞, infinite SRT (i.e. no wastage).

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iomass is acclimatized to given operating conditions. Whenompared to EPS molecular weight distribution, the SMP solu-ion featured generally larger macromolecules [165].

Comparison between acclimatized sludges obtained fromBR and CASP pilot plants revealed similar levels in terms

f eEPSp, eEPSc and eEPShumic [180]. The presence of theembrane in the MBR process does not seem to affect the

ontent of eEPS within the flocs. However, SMPp, SMPc andMPhumic levels were significantly greater for the MBR sludge,resumably due to the retention of large macromolecules by theembrane. Critical flux tests carried out under the same condi-

ions for both MBR and CASP sludges revealed the higher foul-ng propensity of MBR sludge over CASP (critical fluxes wereround 10–15 and 32–43 l/m2 h, respectively). Since the mea-ured levels of EPS was unchanged, only the SMP componentsan be accounted for the higher membrane fouling observed forBR sludge [180]. During this study, Cabassud and co-workers

bserved significant biological activity in the MBR supernatant,ndicating the presence of free bacteria (presumably submicronolloidal size), which could also be another cause of mem-rane fouling. Direct linear relationships between loss of MBRydraulic performances and SMP concentration has been alsoeported for an anaerobic MBR [171] and has been mathemat-cally modeled [214]. Different mechanisms for the interactionetween the macromolecules present in the supernatant and theembrane surface have also been proposed recently [158]. The

reation of the fouling layer on the membrane surface wouldct as a secondary membrane, increasing the retention and/orhe adsorption of macromolecules. The formation of a biofilmould also lead to the degradation of the macromolecules ashe permeate flows through the membrane. Finally, interactionetween the macromolecules and other solutes (humics, divalentations) within the membrane pores may be responsible for theeduction of the membrane pore size over time.

In an attempt to define the biomass fraction with the highestouling potential, Lesjean et al. [150] conducted a methodi-al comparison between permeate and supernatant solutions.ssuming that the materials observed in the biological super-atant and not in the permeate solution are responsible for MBRouling, this group clearly revealed the higher concentration ofarbohydrates, proteins and organic colloids in the MBR super-atant compared to those in the permeate. These findings con-rmed similar results previously reported [30,210]. Since, directelationships between the carbohydrate level in SMP solutionith fouling rate [150], filtration index and CST [213,215,216],

ritical flux tests [102], and specific flux [152] have been clearlyescribed, this reveals the SMPc as the major foulant indicatorn MBR systems. However, the nature and fouling propensityf SMPc were observed to change during the study of unsteadyBR operation [217]. In this specific study, it was not possi-

le to correlate SMPc to fouling. So far, the effect of the proteinraction contained in the SMP solution on MBR fouling has beenore rarely reported. Since a significant amount of proteins is

etained by the membrane (from 15% [210] to 90% [217]), its expected that this plays a role in MBR fouling. This wasecently confirmed by the value of specific resistance increasingy a factor of 10 as the SMPp increased from 30 to 100 mg/l

tspt

208]. However, in two separate studies, analyses of the foul-ng layer have revealed a higher concentration of carbohydratend lower concentration of proteins compared to their levels inhe activated sludge [86,218]. This further confirms the greateristribution of SMPc in the fouling layer compared to that ofMPp. With a smaller MW, humic substances contained in the

iquid phase are not retained by the membrane, and thereforeay not significantly participate to MBR fouling [217].As expected, many operating parameters affect SMP levels

n MBRs. As for eEPS, SMP levels decreased with increasingRT [165]. For SRT ranging from 4 to 22 days, SMPp and SMPc

evels were reduced by factors of 3 and 6, respectively [213]. Inheir long-term study, Rosenberger et al. identified temperaturend stress to the microorganisms (and at a lower degree, SRT)o be the main parameters affecting SMPc [158].

In order to obtain better control of the environmental con-itions, many research studies are based on the use of syn-hetic/analogue solutions, which attempt to model real wastew-ters. These solutions are sometimes very basic (mainly com-osed of glucose) and therefore are very easily biodegradable.s a result, it is expected that SMP levels in such systems are

ower than in real systems. Since it may be assumed that there arelmost no substrate residuals from glucose in the supernatant,he less biodegradable SMP induced by cell lysis or cell releaseould account for most of the SMP measured in synthetically fedBRs. This could explain the lower influence of SMP compared

o those of eEPS reported in some MBR studies. When using syn-hetic substrate, Cho et al. [197] concluded that the membraneouling was affected more by the bound EPS of activated sludgeoc rather than the dissolved organic matter. SUVA measure-ent carried out from supernatant of MBR fed with synthetic

olution confirmed the presence of a portion of larger, more aro-atic, more hydrophobic and double-bond-rich organics, which

riginated from the decayed biomass rather than the feed [211].nother important study [219], also based on synthetic wastew-

ter, revealed that soluble organics alone cannot predict MBRouling. By comparing filterabilities of attached and suspendedrowth microorganisms, Lee and co-workers observed the ratef membrane fouling of the attached growth system (MLSS.1 g/l and attached biomass of 2 g/l) to be about seven timesigher than that of suspended growth MBR (MLSS of 3 g/l).ith similar soluble fraction characteristics in both reactors,

he filtration discrepancy was explained by the formation of arotective dynamic membrane created by suspended solids. Theesults of Ng et al. [207] reported earlier, with more foulingrom supernatant that mixed liquor, can also be explained by thisechanism.Another group of organic materials has been recently

ntroduced by Wang et al. [220]. The biopolymer clustersBPC) have been defined as non-filterable material issued fromhe affinity clustering of the free EPS and SMP present in theludge cake deposited on the membrane surface. AlthoughPC is expected to accumulate on the pore of the sludge cake,

his material can be readily separated from the fouling cake byimple stirring. This work highlights the recent interest in theolymeric characterization of the fouling layer [172,221]. Forhis purpose, confocal laser scanning microscopy can be used

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o precisely locate different EPS compounds on the membraneurface [172,176,222,223].

.3. Operating conditions

.3.1. Aeration, crossflow velocityControlling fouling in submerged membrane systems

emains more challenging than in pumped crossflow or dead-endigs, in which the feed-liquid can be more accurately managed.ince the origin of the submerged MBR, bubbling has beenefined as the strategy of choice to induce flow circulation andhear stress on the membrane surface. Aeration used in MBRystems has three major roles: providing oxygen to the biomass,aintaining the activated sludge in suspension and mitigating

ouling by constant scouring of the membrane surface [224]. These of gas bubbling to enhance membrane processes, and MBRsn particular, has been thoroughly investigated and reviewed106]. While fundamental studies and mathematical models haveeen applied to well-defined membrane configurations (such asubular [225,226] and flat sheet [227–229]), the effect of bub-les on submerged hollow fiber modules is still being assessed.ore recently, the uneven distribution of the aeration turbulent

hear intensity has been taken into account in the developmentf a mathematical model [230]. Theoretical analysis becomesven more problematical when the complex nature of biomassixture (non-Newtonian fluid containing solutes, colloids and

articulates) present in MBRs is taken into account. However,eneral anti-fouling phenomena due to aeration occurring inBRs can be described.Basically, the bubbles flowing near to the membrane sur-

ace induce local shear transients and liquid flow fluctuations,ncreasing back transport phenomenon. The tangential shear athe membrane surface prevents large particle deposition on the

embrane surface. However, the effect of tangential shear is aunction of particle diameter, with lower shear induced diffu-ion and lateral migration velocity for smaller particles, leadingo more severe membrane fouling by fine materials [231]. Aer-tion also affects MBR performance by causing fiber lateralovement (or sway) in hollow fiber configurations [121]. The

ffect of bubbling can help to overcome issues related to highacking density in hollow fiber bundles. However, it is still ahallenging task for MBR designers to achieve effective aerationhroughout the population of fibers in a bundle [232]. All theseffects, described in more detail elsewhere [104,106,233–235],ontribute to a significant reduction in fouling propensity. Aovel explanation for the influence of aeration on MBR foul-ng has been proposed by Ji and Zhou [174]. According to theiresults, aeration rate directly controls the quantity and composi-ion of the polymeric compounds (EPS) in the biological flocs,nd ultimately the ratio of protein/carbohydrate deposited on theembrane surface. However, this mechanism cannot explain the

ffects of bubble-induced fouling control observed with modeleeds [121].

An optimum aeration rate, beyond which a further increaseas no significant effect on fouling suppression, was originallybserved by Ueda in 1997 [236], and has been verified in manyccasions since [84,237,238]. During MBR optimization stud-

g

tr

es, the limits of the aeration were demonstrated through its effectn the netflux-ratio (netflux over instantaneous flux calculatedor operation with membrane relaxation or backwashing). In thisase, aeration intensity could not further improve hydraulic per-ormances below the critical netflux-ratio of 0.85. Above thisalue, the aeration rate was able to limit, to a certain extend,he formation of severe fouling [239]. This study revealed that,or specific conditions, the filtration mode has more effect on

BR fouling than the changes in aeration rates. Intense aera-ion rate may also damage the floc structure reducing their size,nd releasing EPS in the bioreactor [174,240]. These phenomenaave been similarly described in the sidestream MBR configura-ion in which the circulation pump is responsible for the break upf bacterial flocs [241,242]. However, in a small crossflow cell,ouling was found to decrease linearly with increasing CFV (upo 4.5 m/s), and no CFV optimum was observed [243]. Detailedtudy of the various calculated hydraulic resistances revealedhat CFV values of 2 and 3 m/s were sufficient to prevent theormation of reversible fouling in UF (30 kDa) and MF (0.3 �m)ystems, respectively. Finally, it was shown that high CFV wasore effective in reducing fouling in the MF rather than in theF–MBR system. This was confirmed by analysis of the mass,

hickness and density of the fouling layer deposited in/on theembrane under different operating conditions. This may be

ue to the effect of CFV on critical flux of particulates. At aow CFV of 0.1 m/s, the UF membrane fouled less, being lessusceptible to the particle deposition, whereas at a high CFVf 3.5 m/s, the majority of particles would not deposit (raisedritical flux) on either membrane. However in current MBRseneration, the CFV is relatively low which could favor the usef tighter (UF) membranes.

Determination of the CFV induced by aeration of the mem-rane surface can be difficult to assess and techniques suchs electromagnetic flow velocity meter [244], particle imageelocimetry [232], constant temperature anemometry [245,246],ave be used for liquid velocity estimation in submerged MBReactors. Based on the observed CFV in tap water in the riserection (CFVtp) of a internal loop-airlift MBR system, Liu et al.et up a modified model for calculating the CFV of the aeratedctivated sludge (CFVas) over the membrane surface [237]:

FVas = 1.406 CFV1.226tp × µ−0.147 (3)

here µ is the viscosity of the MLSS, and CFVtp can be esti-ated from equation given in Liu et al.’s earlier publication

247].

.3.2. Solid retention time (SRT)SRT (and consequently the F/M ratio), which ultimately con-

rols biomass characteristics, is probably the most importantperating parameter impacting on fouling propensity in MBRs.perating an MBR at higher SRT leads inevitably to increase ofLSS concentration, but this in itself may not necessary lead to

reater fouling (Section 3.2.3) [248].Extremely low SRTs (down to 2 days) have been tested

o assess fouling propensity [249]. Not surprisingly, foulingate increased nearly 10 times when SRT was lowered from

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0 to 2 days (corresponding to F/M ratio from to 0.5 to.4 gCOD/gMLVSS/day and MLSS of 7.8–6.9 g/l). There is noeason (other than purely research-based work) to run MBRst such extreme conditions, and, as a general rule, F/M ratio isecommended to be maintained below 0.5. Other characteris-ics of performance at SRT as low as 0.25 day are discussed in250]. The reasons suggested for the increased fouling rate atery low SRT include the increased levels of production of EPS.t should be noted that an increased F/M ratio could occur duringransients in unsteady operation (Section 3.3.3).

At the other end of the spectrum, the temptation to run MBRst extended SRT is great, considering the advantages of this pro-ess over CASP. Indeed the early MBRs were typically run atery long SRTs to minimize excess sludge. Studies reportingperation at infinite SRT are generally bench scale, use syn-hetic feed (or very finely prefiltered sewage) [212,251–254] andherefore do not take into consideration the accumulation of inert

aterial in the tank. The progressive accumulation in the MBRank of non-biodegradable materials (like hair and lint), whichre not completely removed by the MBR pre-treatment pro-esses, undeniably leads to clogging of the membrane module255]. The increase in MLSS concentration related to extendedRT could also result in higher fouling propensity (see Section.2.3) even with the aeration raised significantly. Previous exper-ments revealed an increase of MLSS levels from 7 to 18 g/l anddecrease of F/M ratio from 0.15 to 0.05 kgCOD/kgMLSS/dayhen the SRT was increased from 30 to 100 days. Even after

ncreasing the aeration rate from 15 to 25 l/min, fouling wasearly twice as great for the longer SRT conditions [155]. Inhis scenario, the increased shear provided to control foulingould breakup biofloc as well as causing cell lysis. Moreover,he increase in aeration intensity to keep the high MLSS levelsn suspension and properly oxygenated may not be a sustainableption for the treatment process. During a 300-day operation ofpilot-scale MBR without wastage (infinite SRT), the MLVSS

ncreased steadily from 3 to 15 g/l and both removal efficiencynd membrane performances remained constant. At infinite SRT,ost of the substrate is consumed to ensure the maintenance

eeds and the synthesis of storage products. The very low appar-nt net biomass generation observed can also explain the lowouling propensity observed for high SRT operation in this study253]. These two studies show that extended-SRT-operation doesecessary offer lower fouling; other operating conditions suchs flux and aeration rates would also have a major influence onouling propensity. The other difficulty with very high SRT ishe raised viscosity that could attenuate the effect of bubbling.

The effects of SRT on biological parameters like MLSS, SMP,EPS concentrations, described in Section 3.2, also reveal theajor impact of this operating parameter on MBR fouling. Asresult, selection of the SRT must be considered very carefully

n order to optimize MBR operation (see Section 4.2.2). Sometudies reported the DOC of the supernatant to be independentf SRT [252]. The lower fouling generally observed at extended

RT is partially explained by the lower organic carbon concen-

ration in eEPS rather than in SMP. Overall, it is likely that theres an optimal SRT, between the high fouling tendency of veryow SRT operation and the high viscosity suspension prevalent

rtwA

or very long SRT. However, the difficulty related to properlycclimatize an MBR (pilot) plant to different SRTs and conductfair comparison does not allow the determination of an opti-um SRT value. This could also explain the discrepancies in theRT effects reported in the literature. Criteria such as designedate of sludge and MLSS concentration recommended by theembrane supplier is more prone to define the working SRT.his point is further discussed in Section 4.2.2.

.3.3. Unsteady state operationUnsteady state such as variations in operating conditions

flow input/HRT and organic load) and shifts in oxygen supplyave also been defined as additional factors leading to changesn MBR fouling propensity. In real-world applications, suchnsteady state conditions could occur regularly. In an experimentarried out with a large pilot-scale MBR, the effects of unstableow input and unintentional sludge wastage have been assessed217]. Although the level of polysaccharides in the filtrate variedn a chaotic manner, the concentration of this specific compoundncreased before and after each sludge withdrawal. While thencrease after wastage was due to the sudden stress experiencedy cells, increase before sludge withdrawal was explained by thencreasingly high MLSS concentration and the resulting low DOevel in the bioreactor. It was concluded that unsteady operationhanged the nature and/or structure (and fouling propensity) ofhe polysaccharide rather than the overall EPS formation, andherefore could worsen the fouling propensity. These findingsonfirmed results previously reported on the effects of transientonditions in feeding patterns. The addition of a spike of acetaten the feed water significantly decreased the filterability of theiomass in an MBR; this was due to the rise in SMP levelsesulting from the feed spike [256].

The effects of starvation conditions on the biological sus-ension have been assessed by incorporating different substratempulses in batch tests [257]. Exogenous phases were followedy starvation periods, both characterized by the So/Xo ratioEq. (4)). For high So/Xo, multiplication of bacteria cells wasbserved, while compound storage, characterized by decreasef MLVSS and the absence of SMPp production and bacteriaysis, were obtained at low So/Xo ratio. The low F/M conditionsenerally used in MBRs are theoretically close to what woulde considered as starvation conditions. Although the influencef these operating conditions on MBR fouling have not beeneported, the lower amount of SMPp produced may lead to lessevere fouling propensity:

So

Xo= HRT

(F

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ith So, the feed substrate concentrate and Xo, the MLVSSoncentration.

The start-up phase can also be considered as unsteady oper-tion and data collected before biomass stabilization (includinghe period necessary to reach acclimatization) may become

elevant in the design of MBRs. Cho et al. [197] reportedemporal changes of the bound EPS levels when the MBRas acclimated to three different SRTs (8, 20 and 80 days).s expected (considering the general trends described in

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dcacfhcisfsonsttofdtvt[i[86]. This conditioning period (from now called stage 1) hasnot been observed or described as such until recently, but maybe a key aspect of fouling creation in MBRs. The dynamics ofthe biomass detachment from the membrane in relation to the

ection 3.2.5), the concentration of eEPS was lower for theonger SRT (83–26 mgTOC/gSS for SRT of 8 up to 80 days,espectively). More interestingly, an initial latent phase wasbserved for which eEPS concentration did not vary signifi-antly. However, eEPS levels increased exponentially after 40ays of operation at 8-day-SRT, and after 70 days when theBR was operated with an SRT of 20 days. No change in eEPSas observed during the 80 days of operation at 80-day-SRT.

n another MBR set-up operated at infinite SRT, no significanthanges in SMP concentration during 100 days of operation werebserved, although the MLSS increased from 1.8 to 4.5 g/l in theeantime [258]. After a latent phase of 30 days, MLSS and SMP

evels started to significantly increase and stabilized by reachingplateau after 140 days of operation at infinite SRT (operated for10 days). In this example, eEPS increased continuously fromay 1 to also reach steady-state on day 140 [251]. Nagaoka andemoto [198] observed a rise of MLSS concentration from 4

o 14 g/l over 100 days (SRT value not given) along with a rel-tively constant increase in eEPS (from 50 to 250 mg TOC/l).rom these examples and the relationship between eEPS-SMPnd fouling propensity defined earlier, further research on thexact impact of unsteady states on MBR fouling is required tomplement more sustainable MBR operation. At this point inime, the evidence points to increased membrane fouling duringnsteady state (particularly when F/M is increasing).

.4. Fouling mechanisms in MBRs

.4.1. Constant TMP operationThe current trend in MBR design is to operate at constant flux,

nd as a result, very few recent studies report the operation ofBR at constant TMP. In the MBR, like other membrane filtra-

ion processes at constant TMP, a rapid flux decline is expected toccur during the initial stages of the filtration. The rate of foulinghen decreases before reaching a plateau. Bae and Tak recentlyummarized the hypothetical three-phase-process-mechanismsor initial cake layer formation occurring in MBR [143]. Theyltered MBR mixed liquor samples at 100 kPa over periods ofp to 5 h with a range of UF membranes. According to theseuthors, the main parameter affecting the initial fouling (phase 1)ould be the irreversible deposition of the soluble fraction of theiomass suspension (presumably SMP). During this phase, theludge particles and the colloids would not take part in foulingince they are supposed to be, respectively, removed by cross-ow (size effect) and to be in too low a concentration to havesignificant effect on fouling. Deposition of sludge particles

n the membrane surface and in the previously deposited lay-rs is the main phenomenon occurring during phase 2 when theux declines more slowly. Phase 3 is then defined when fluxppears to stabilize, indicating that permeation drag and backransport have reached equilibrium. Although reduced perme-tion drag limits further severe fouling, compaction of the cakeayer would play a significant role in the slight increase in filtra-

ion resistance observed during this last phase. As little foulingtill occurs during phase 3, this operation can be maintained dur-ng a certain filtration period, before cleaning of the membranes required. However, in this study, fluxes typically dropped to F

bout 10 l/m2 h which is less than normal constant flux MBRperation. It is important to note that in phase 3, two factorsre at work. Firstly, the lowered flow slows down the convec-ion of foulant; it becomes self-limiting. The other factor is thatnce the flux is low, the fouling resistance is high, relative tohe membrane resistance. To see significant further decline (sayy a further 50%) requires the fouling resistance to double. Itecomes increasingly difficult to detect fouling trends by simplynspecting the flux decline profile.

.4.2. Constant flux operationWith the constant flux approach, the convection of foulant

oes not diminish and fouling phenomena self-accelerates andan eventually create a sharp increase of TMP. With fouling rate,nd therefore cleaning frequency, increasing with flux, operationonditions favors the MBR to be run at modest fluxes to limitouling severity (Section 4.2.2). As a result, numerous studiesave reported the fouling behavior for long-term MBR filtrationarried out at sub-critical flux. However, these long-term exper-ments have revealed noticeable fouling for MBRs operated atub-critical flux. Since its first reference to MBRs in 2001 [28],ouling behavior over time is generally characterized by a two-tep pattern. During the first period, a very small TMP rise wasbserved. For trials carried out over extended time periods, aoticeable change in the rate of TMP increase then arises afterome critical time period (Fig. 13). Pollice et al. [32] reviewedhe phenomena and two parameters were introduced as indica-ors for operation under sub-critical conditions: the critical timever which the prolonged first step is maintained (tcrit) and theouling rate (dTMP/dt) during that step. Table 6 reports tcrit andTMP/dt for recent trials and reveals the long periods of filtra-ion (up to 1200 h) for which fouling rate can be maintained atery low values (down to 2 × 10−4 kPa/h). The fouling rates forhe high TMP-rise-period have also been reported previously148]. Prior to these two filtration-steps generally describedn the literature, a conditioning period has also been observed

ig. 13. Long-term filtration for constant flux operation (adapted from [14]).

Table 6Sub-critical long-term parameters (completed from [32])

Flux (l/m2 h) dTMP/dt (kPa/h) tcrit (h) References

17 0.005 >600 [259]22 0.011 1200 [259]25 0.024 300 [259]30 0.072 250 [259]n.a. 0.023 350 [260]20 – 600 [261]8 – 350 [262]30 0.036 360 [25]10 0.036 550 [263]8 0.03 72 [30]7 0.006 96 [14]9 0.004 240 [14]18 0.104 48 [14]12 0.0002 300 [264]4 0.013 192 [148]68

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ltration and aeration turbulence has also been rarely reported,ut was recently considered in the formation of a mathemati-al model [230]. Based on the recent work reported by Zhangt al. [86], a detailed analysis of the mechanisms and factorsnvolved in these three fouling stages follows and is summa-ized in Fig. 14.

.4.2.1. Stage 1—conditioning fouling. As in constant TMPperation, strong interactions between the membrane surfacend the EPS present in the mixed liquor are probably responsi-

tsto

Fig. 14. Fouling mechanisms for MBR operat

le for the initial stage of fouling during constant flux operation.gnier et al. [95] described the rapid fouling phenomena induc-

ng irreversible resistance and taking place in the early stagef MBR filtration (in frontal mode, i.e. dead-end operation).assive adsorption of colloids and organics has been observedven for zero-flux operation, and before any deposition mech-nism initiates [86]. Another detailed study based on passivedsorption revealed that the hydraulic resistance due to this pro-ess was almost independent of tangential shear. In terms ofelative hydraulic resistance contribution, the initial adsorptionas been reported to account for 20–2000% of the clean mem-rane resistance (mainly depending on the pore size) [263]. Inmore recent study, its contribution to the overall resistanceas found to become negligible once filtration was conducted

88]. The adsorption propensity (determined with the modifiedreundlich isothermal adsorption equation) was also studied inelation to the filtration modes employed in submerged MBRs265]. As a result, colloid adsorption and initial pore block-ng [170] of new or cleaned membranes by organics substancess expected in MBRs. The intensity of this effect depends on

embrane pore size distribution and surface chemistry (andspecially hydrophobicity) [95] (Sections 3.2.5 and 3.2.6). Intest cell equipped with direct observation through the mem-

rane (DOTM) technology, and with crossflow but zero flux,oc was visually observed to temporarily land on the membrane86]. This was defined as a random interaction process rather

han proper cake formation phenomenon. While some flocs wereeen to roll and slide across the membrane, biological aggregatesypically detached and left a residual footprint of smaller flocsr EPS material. Biomass approaching the membrane surface

ed at constant flux (adapted from [86]).

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as then able to attach more easily to the membrane, colonizehe separation surface and contribute to stage 2.

.4.2.2. Stage 2—slow (steady) fouling. Even though MBRsre operated below the critical flux for the biomass, biofloc mayandomly land (see above) and contribute to the second foul-ng stage. After stage 1, the membrane surface is expected toe mostly covered by SMP, leading to the higher attachmentropensity of biomass particles and colloids. Because of the lowritical flux determined for SMP species, further adsorption andeposition of organics on the membrane surface may also occururing stage 2. Since adsorption may take place not only at theembrane pores but also on the whole surface, biological flocsay initiate cake formation without directly affecting the perme-

bility in this stage. Over time, this phenomenon would worsen.he rate of EPS deposition, and resulting TMP rise, is expected

o increase when the operating flux is higher, leading to a shortertage 2 operation (Table 6). The fouling mechanisms describedbove would prevail even with a good hydrodynamic environ-ent that provides adequate surface shear over the membrane

urface. However as maldistributions of flow, shear or flux areenerally expected in MBRs, irregular fouling patterns can benticipated.

.4.2.3. Stage 3—TMP jump. With regions or pores of theembrane more fouled than others, flux is expected to sig-

ificantly decrease in those specific locations. As a result, theverall permeate productivity redistributes to the less fouledembrane areas or pores, for which local flux increases (seeection 2), exceeding a critical flux (defined as sustainable flux

n Section 4.2.2). These phenomena have a self-acceleratingature and severe fouling, characterized by an exponentialMP increase, is generally obtained if the filtration is main-

ained. The sudden rise in TMP or “jump” is a consequencef constant flux operation and several mechanisms can be pos-ulated for the rapid increase in TMP at a given condition86]:

(i) The inhomogeneous fouling (area loss) model. This modelwas proposed to explain the observed TMP profiles in nom-inally sub-critical filtration of upflow anaerobic sludge [25].The TMP jump appeared to coincide with a measured lossof local permeability at different positions along the mem-brane, due to slow fouling by EPS. It was argued that theflux redistribution (to maintain the constant average flux)resulted in regions of supra-critical flux and consequentlyin rapid fouling and TMP rise.

ii) The inhomogeneous fouling (pore loss) model. Similar TMPtransients have been observed for the crossflow MF of amodel biopolymer (alginate) [17]. These trends revealedthat the TMP transient can occur with relatively simplefeeds. The data obtained have been explained by a model thatinvolves flux redistribution among open pores, allowing for

the pore size distribution. Local pore velocities eventuallyexceed the critical flux of alginate aggregates that rapidlyblock the pores. This idea was also the basis of the modelproposed by Ognier et al. [31]. While the “area loss” model

atM

considers macroscopic redistribution of flux, the “pore loss”model focuses on microscopic scale. In MBR systems, it isexpected that both mechanisms occur simultaneously.

ii) The critical suction pressure model. Using a fine colloid,filtered in dead-end mode, onto an immersed hollow fiber,gradual TMP rise followed by a rapid increase in TMP wasobserved. Both autopsy and modeling suggested a criticalsuction pressure at which coagulation occurs at the baseof the cake [266]. The very thin dense layer observed nextto the membrane confirmed the rapid increase in resistanceleading to the TMP jump. Although this model was obtainedwith dead-end rather than crossflow operation, there is noreason why this mechanism could not apply to sidestreamor submerged MBRs. A requirement for that model is thatfouling continues to occur over time until the critical suctionpressure is reached, and that the deposit compound(s) havethe potential to coalesce or collapse. Biofilms and depositlayers in MBRs are likely to have this tendency.

iv) Percolation theory. According to percolation theory, theporosity of the fouling layer gradually reduces due to thecontinuous filtration and material deposition within thedeposit layer. At a critical condition, the fouling cake losesconnectivity and resistance, and TMP, increase rapidly. Thismodel has been proposed for MBRs [34], but the model indi-cates a very rapid change (within minutes), which has notbeen observed in practice. However, it is plausible that thepercolation theory approach, combined with the inhomo-geneous fouling (area loss) model, could satisfy the moregradual kinetics of the typical TMP transient. Similarly,fractal theory was successfully applied to describe cakemicrostructure and properties and to explain the cake com-pression observed during MBR operation [267].

v) The inhomogeneous fiber bundle model. Another manifes-tation of the TMP transient has been observed for modelfiber bundles where the flow from individual fibers wasmonitored [118]. The bundle was operated under suctionat constant permeate flow, giving constant average flux, andinitially this was evenly distributed amongst the fibers. How-ever over time, the flows became less evenly distributed sothat the standard deviation of the fluxes of individual fibersstarted to increase from the initial range of 0.1–0.15 up to0.4 l/m2 h. Consequently, the TMP rose to maintain the aver-age flux across the fiber bundle, mirroring the increase inthe fluxes standard deviation. At some point both TMP andstandard deviation showed a rapid rise. This is believed tobe due to flow maldistribution within the bundle leadingto local blockages between fibers and membrane fouling. Itwas possible to obtain more steadily TMP and standard devi-ation profiles when the flow regime around the fibers wasmore vigorous (higher liquid and/or air intensity). Althoughthis trend was observed for a small model bundle, the phe-nomena are likely to occur in larger bundles.

The mechanisms (i)–(v) listed above are all self-acceleratingnd this is a feature of stage 3 fouling. It is probable that morehan one of these mechanisms apply simultaneously when an

BR reaches the TMP jump condition.

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. Mitigation of MBR fouling

.1. Removal of fouling

.1.1. Physical cleaningPhysical cleaning techniques for MBRs include mainly mem-

rane relaxation (where filtration is paused) and membraneackwashing (where permeate is pumped in the reverse directionhrough the membrane). These techniques have been incorpo-ated in most MBR designs as standard operating strategies toimit fouling; although vigorous backwashing is not an optionor flat plate submerged membranes.

Backwashing (also called backflushing) has been found touccessfully remove most of the reversible fouling due to porelocking, transport it back into the bioreactor, and partially dis-odge loosely attached sludge cake from the membrane surface.n some cases, clogging near the membrane surface may also beartially loosened or removed by backwashing. The efficiency ofackwashing has been studied in detail [268–270]. Key parame-ers in the design of backwashing are its frequency, duration, theatio between those two parameters and its intensity. For exam-le, less frequent, but longer backwashing (600 s filtration/45 sackwashing) was found to be more efficient than more fre-uent backwashing (200 s filtration/15 s backwashing) [170]. Innother study based on factorial design, suction time (betweenand 16 min) was found to have more effect on fouling removal

han both the aeration intensity (0.3–0.9 m3/m2 h) and the back-ash time (25–45 s) [239]. Although more fouling is expected

o be removed when backwashing duration and frequency arencreased, optimization of backwashing is required in regard tonergy and permeate consumptions. This was achieved by theesign of a generic control system which automatically opti-ized the duration of the backwash according to the monitored

alue of TMP [271].This anti-fouling operation obviously affects operating costs

s energy is required to achieve a pressure suitable for flow rever-ion. Moreover, between 5 and 30% of the produced permeates used in the process. Comparison between submerged hollowber and flat sheet MBR revealed the slightly higher overallux obtained when operating the membrane constantly at lowux [108]. In this example, flat sheet membranes, which can-ot be backwashed, were operated constantly with flux rangingetween 20 and 27 l/m2 h. The hollow fiber MBR was operatedt higher flux (23–33 l/m2 h) but with 25% of the permeate prod-ct being recycled for backwashing (45 s of backwashing aftervery 600 s of operation).

Air can also be used as the backflushing medium [272].lthough improving the flux by nearly 400% (compared to con-

inuous operation), 15 min of air backwash was required every5 min of filtration to obtain this result [273]. However, air back-ashing is an efficient method for flux recovery, it may alsoresent potential issues of membrane breakage and rewetting.

Membrane relaxation (or non-continuous operation of the

embrane) significantly improves membrane productivity.nder relaxation, back transport of foulants is naturally

nhanced as non-irreversibly attached foulants can diffuse awayrom the membrane surface through the concentration gradi-

reNo

nt. The fouling removal efficiency of this method can be fur-her increased when air scouring is applied during relaxation149,274]. Detailed studies of the TMP behavior during thisype of operation revealed that although the fouling rate is gen-rally higher than for continuous filtration, membrane relaxationllows filtration to be maintained for longer period of timesefore the need for cleaning [207]. Although some have reportedhat this type of operation may not be economically feasible forarge-scale MBRs [149], further cost and productivity analysisre probably required to compare this method against backwash-ng. Recent studies assessing alternative strategies for foulingitigation tend to combine intermittent operation with frequent

ackwashing for optimum results [137,275].

.1.2. Chemical cleaningIt is expected that membrane relaxation and backwashing

ffectiveness tend to decrease with operation time as more irre-ersible fouling accumulates on the membrane surface. There-ore, in addition to the physical cleaning strategies, differ-nt types/intensities of chemical cleaning may also be recom-ended. They include:

Chemically enhanced backwash (on a daily basis),Maintenance cleaning with higher chemical concentration(weekly), andIntensive (or recovery) chemical cleaning (once or twice ayear).

Maintenance cleaning is used to maintain design permeabilitynd helps to reduce the frequency of intense cleaning. Intensiveleaning is generally carried out when further filtration is noonger sustainable because of an elevated TMP. Each of the four

ain MBR suppliers (Kubota, Memcor, Mitsubishi and Zenon)roposes their own chemical cleaning recipes, which differainly in terms of concentration and methods (Table 7). Under

ormal conditions, the prevalent cleaning agents remain sodiumypochlorite (for organic foulants) and citric acid (for inorgan-cs). Sodium hypochloride hydrolyzes the organic molecules,nd therefore loosen the particles and biofilm attached to theembrane. The effects of cleaning chemical agents like NaOCl

n microbial community have been also recently studied forodeled MBR processes [276]. It is also common for MBR

uppliers to adapt specific protocols for chemical cleanings (i.e.hemical concentrations and cleaning frequencies) for individ-al facilities [115,179,255]. It also has been mentioned that theevel of pollutants (measured as TOC) in the permeate rises justfter the chemical cleaning episodes [115]. This is important forBRs used in reclamation process trains (i.e. upstream of RO

or example). So far, no systematic studies on cleaning agentsr procedures have been published [277]. This is probably dueo the site-specific nature of the MBR fouling.

Maintenance cleaning, taking up to 30 min for a completeycle, is normally carried out every 3–7 days at a moderate

eagent concentration of 0.01 wt.% NaOCl. Recovery cleaningmploys rather higher reagent concentrations of 0.2–0.5 wt.%aOCl coupled with 0.2–0.3 wt.% citric acid or 0.5–1 wt.%xalic acid (Table 7).

Table 7Intensive chemical cleaning protocols for four MBR suppliersa (from [255])

Type Chemicals Concentration (%) Protocols

Mitsubishi CILNaOCl 0.3

Backflow through membrane (2 h) + soaking (2 h)Citric acid 0.2

Zenon CIPNaOCl 0.2

Backpulse and recirculateCitric acid 0.2–0.3

Memcor CIPNaOCl 0.01

Recirculate through lumens, mixed liquors and in-tank air manifoldsCitric acid 0.2

Kubota CILNaOCl 0.5

Backflow and soaking (2 h)Oxalic acid 1

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IL: cleaning in line where chemical solutions are generally backflow (under grnd drained; the module is rinsed before being soaked in the cleaning solution aa The exact protocol for chemical cleaning can vary from a plant to another.

Research on the efficiency of sonification for removing cakeayers in MBRs has also been carried out [96]. The sonifica-ion cleaning process is based on the breakdown of the foulingake into smaller fragments. Although sonification can success-ully remove the cake from the membrane surface, this cleaningethod was not effective on all types of fouling due to pore

locking and may even worsen this type of fouling. A combi-ation of sonification with backwashing and chemical cleaningppeared to achieve almost complete flux recovery [278]. Moreetails of the cleaning mechanisms are available in [278]. How-ver, sonification would be difficult to apply at a large-scale dueo the focused nature of the sonic energy.

.2. Limitation of fouling

It may also be possible to prevent fouling before its occur-ence by (1) improving the anti-fouling properties of the mem-rane, (2) operating the MBR under specific non-or-little-ouling conditions and/or (3) pre-treating the biomass suspen-ion to limit its fouling propensity.

.2.1. Optimization of membrane characteristicsIn MBRs, chemical modifications of the membrane surface

ave been shown to efficiently improve anti-fouling proper-ies. As mentioned above (Section 3.1.2), more severe foul-ng is expected when hydrophobic membranes are used in the

BR, and efforts have been focused on increasing membraneydrophilicity through membrane modification. Recent exam-les for MBRs comprise NH3 and CO2 plasma treatments ofolypropylene hollow fibers [128,129]. In both cases, X-rayhotoelectron spectroscopy (XPS) and SEM were used to char-cterize the structural and morphological nature of the modi-ed membrane surface. With the introduction of polar groupsfrom oxygen and nitrogen) on the membrane surface, mem-rane hydrophilicity significantly increased and new membranesresented better filtration performances and flux recovery thanhose of unmodified membranes. In another study, the addition

f TiO2 nanoparticles to the casting solution and a precoat ofiO2 allowed the preparation of two types of TiO2-immobilizedF membrane (entrapped and deposited, respectively), whichere also used in MBR systems [143,279]. As a result, lower

iaac

) inside the membrane. CIP: cleaning in place where membrane tank is isolatedsed to remove excess of chlorine.

ux decline was obtained with the TiO2-membranes comparedo that of unmodified membranes.

As it was possible to add a larger amount of TiO2 parti-les on as a precoat to a membrane, this filter showed greaterouling mitigation compared to that of the TiO2-entrapped-embrane. Similarly, when MBR membranes were precoatedith ferric hydroxide flocs and compared to an unmodifiedBR, both effluent quality and productivity were found to

ncrease [280]. This phenomenon was explained by the adsorp-ion of soluble organics on ferric hydroxide flocs, limiting theirect contact between the organics and the membrane. Finally,ouling phenomena have been used to investigate the creationf self-forming dynamic membrane coupled bioreactors [281].y using coarse pore-sized substrates and allowing cake andel layers to deposit on the surface, a self-forming membraneeveloped with a high flux and good removal efficiencies. How-ver, because of the nature of the filtration barrier, the effluentuality cannot be guaranteed, and this is of concern in manypplications.

.2.2. Optimization of operating conditions

.2.2.1. Aeration. Since the energy involved in providing aer-tion to the membrane remains a significant cost factor in MBResign, efforts have been focused on optimization of air flow-ate. The specific design of airflow patterns and location oferators have also been defined as crucial parameters in foul-ng mitigation. Recent developments in aeration design carriedut by MBR suppliers are often reported in patent format, andnvolve cyclic aeration systems [282], and improved aerator sys-ems [283,284] for example. A recent study reported a detailedomparison of various aeration devices used in tubular mem-ranes. The results indicated that complex aeration systems withultiple orifices injecting air homogeneously in the feed flow

eatured the highest performances [285]. As mentioned before,he effect of aeration varies from hollow fiber to flat plate mem-rane configurations. The presence of a bi-chamber (riser andown-comer) in a Kubota MBR plays a significant role in induc-

ng high CFV [244]. In the same study, lower uplift resistancend higher CFV were induced by uniformly distributed fineir bubbles (issued from a porous media with 0.5 mm holes)ompared to performances obtained with large bubbles (from

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mm hole diffuser) at similar aeration rates. The air/permeateatio (m3/m3) can also be a useful parameter to characterize thentensity of aeration required to obtain a given amount of treatedater. Values given by MBR suppliers may vary between 24 and0, depending of the membrane configuration (flat sheet versusollow fiber) and the MBR tank design (membrane and aerobicone combined into one tank or not) [115]. Preliminary workarried out in Singapore on large-scale MBRs revealed theseriginal ratios to be quite conservative, since it was possibleo decrease them (down to 56% of their original value) with-ut significant fouling increase [115]. Attempts to increase theritical flux in submerged MBR by varying the aeration ratesas been reported [286]. In order to minimize fouling duringigh throughput operation, aeration was increased and returnedo lower values for the low throughput period. Based on thesehort-term experiments, it was possible to use this technique toinimize energy consumption. However, a recent study fromhoi et al. [88] carried out with a crossflow MBR device, indi-ates the tangential shear to have no effect on flux decline whenseudo steady-state is reached. In other words, increasing CFVoes not decrease fouling intensity when the deposition layertarts to govern the permeate flux behavior. In the absence ofFV, flux decline was predominantly caused by reversible foul-

ng, while slightly higher irreversible fouling was detected whenFV was applied [88].

The intermittent operation of aeration has also been reportedor (de)nitrification MBR systems [198,287]. In this uncommoncenario of single tank MBR used for both anoxic and aerobiciological degradation, filtration is carried out during the aerobichase to take advantage of the anti-fouling properties of the aircouring. While some authors testing intermittent aeration do notecommend this type of operation as severe fouling was observeds soon air sparging ceases [170,238], others have reported thatfficient fouling control was achieved by intermittent bubbling288,289]. Pulsing air at a frequency of 1 s on/1 s off allowed anmprovement in operating flux ranging from 20 to 100% and wasound more efficient than lower frequencies (5–10 s on/5–10 sff) more conventionally applied in the industry [288]. However,uch system may require the operation of a robust activators andalves at these high frequencies and may not be economicallyractical.

.2.2.2. Other operating conditions. As mentioned beforeSection 3.3.2), SRT remains probably the main operatingarameter defining the characteristics of the biomass suspensionnd its fouling propensity. With the numerous reports defininghe relation between SRT and concentrations of both eEPS andMP, it appears that the overall performance of the MBR islosely related to the choice of SRT value. Further optimiza-ions of operating conditions through reactor design have beentudied and include the addition of a spiral flocculator [290],ibrating membranes [291], helical baffles [292], suction mode97] and high performance compact reactor [293], novel types

f air lift [104], porous and flexible suspended membrane car-iers [294] and the sequencing batch MBR [295] for example.inally, the membrane module design remains another impor-

ant parameter in the optimization of the MBR operation, and

ptpo

ore precisely, the use of air sparging. In a specially designedodule in which air bubbles were confined in close proximity

o the hollow fiber (rather than diffusing in the reactor), higherermeability was obtained [296].

.2.2.3. Sustainable flux. The energy demand for operation ispotential weakness for the future development of the MBR

rocess. It is recognized that the energy usage of MBRs is stilligher than conventional activated sludge systems due to theeed to control membrane fouling by different strategies. Athe end of the day, MBRs can be economically viable only if itelivers a reasonable flux rate without significant fouling. Sinceermeation rate and fouling decrease simultaneously, most MBRystems operate at low fluxes to limit rapid and severe membraneouling. The concept of sustainable flux in MBRs can be defineds the flux for which the TMP increases gradually at an accept-ble rate, such that chemical cleaning is not necessary [207]. Theate of TMP increase and the period of filtration before chemi-al cleaning is required are left to the operators discretion, andherefore a more detailed definition of sustainable flux cannote possible. While critical flux was mainly determined duringhort-term experiments, sustainable flux can only be assessedhrough longer filtration periods. However, sustainable flux canlso be defined as sub-critical flux by default. In such a system,ot only the flux value is of importance but also the strategiessed to maintain this given flux.

.2.3. Modification of biomass characteristics

.2.3.1. Coagulant/flocculent. Ferric chloride and aluminumulfate (alum) are two types of coagulant commonly used forater and wastewater treatments. Both have been added to

educe significantly membrane fouling in MBRs. Once dis-olved in water, alum forms hydroxide precipitates which adsorbaterials such as suspended particles, colloids and soluble

rganics. In MBR-based trials, the addition of alum led tosignificant decrease of the SMPc concentration, along with

n improvement in membrane hydraulic performances [297].ecause of back transport and shear induced fouling controlechanisms, large microbial flocs are expected to have a lower

mpact on membrane fouling. The permeability enhancementbserved for hybrid coagulant/MBR systems are therefore due tohe largest flocs formed. A recent MBR-based example reportedhat small biological colloids (from 0.1 to 2 �m) coagulated andormed larger aggregate when alum was added to MBR acti-ated sludge [298]. Although more expensive, ferric chlorideas found to have higher efficiency than that of alum. Zeolite has

lso been used in MBRs and allowed the creation of rigid flocshat have lower specific fouling resistance. Further details abouthe mechanisms of performance enhancement due to zeolite andlum can be obtained from [298]. The addition of ferric iron haslso been tested on an MBR for enhancing the production of iron-xidizing bacteria, responsible for the degradation of gaseous2S. In this study, specific ferric precipitate like ferric phos-

hate and K-jarosite (K-Fe3(SO4)2(OH)6) have been observedo foul the membrane [299]. Pre-treatment of the effluent is alsoossible and studies based on the pre-coagulation/sedimentationf effluent before its introduction in the bioreactor revealed the

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ouling limitation offered by this technique. Obviously, pre-reatment of the feed is a crucial step in the MBR process.undamental of sieving, current state-of-the-art mechanical pre-

reatment and results from the comparison between differentypes of sieves are given in [300]. In a recent example, the addi-ion of iron based coagulant controlled both irreversible foulingnd suspension viscosity [145]. Ferric hydroxide flocs have alsoeen used in the MBR process as a membrane pre-coating agent.ot only the specific flux of this set up was higher, but the efflu-

nt quality was also improved compared to the non-coated MBRystem [280]. In this study, additional ferric chloride was addedo successfully remove the non-biodegradable organics whichccumulated in the bioreactor. This operation also led to a rapidncrease in membrane specific flux.

.2.3.2. Adsorbent agents. Addition of adsorbents into biologi-al treatment systems decreases the level of pollutants, and morearticularly organic compounds. When PAC is mixed with theBR biological suspension, biologically activated carbon forms

nd is responsible for significant uptake of soluble organics. Dur-ng long-term runs, PAC gradually incorporates to the biofloco form some biologically activated carbon [301]. Adsorptionf EPS on PAC has been studied during the comparison ofidestream and submerged hybrid PAC-MBRs [302]. For thiseason, lower fouling propensity is expected in MBR processeshen biomass is mixed with adsorbents. Results conducted withnly MBR supernatant also clearly revealed lower fouling whenAC was added (up to 1 g/l) [303]. An optimum PAC concen-ration of 1.2 g/l was obtained for filtration of activated sludge262]. In this study, floc size distribution and apparent viscosityf the biomass were the main factors responsible for the lowerake resistance observed when PAC was added to the bioreactor.owever, no significant improvement of filtration was obtainedith the addition of 5 g/l of PAC and no sludge wastage [207]. Itas postulated that the originally introduced-PAC was quickly

aturated with organic pollutants. Only the regular addition ofAC into the bioreactor showed good fouling limitation, as theystem was operated at lower SRT. Results reported by Fang et al.304] confirmed this hypothesis as virgin PAC was responsibleor 22% reduction of the filtration resistance, while pre-sorbedAC only reduced the resistance by 14%.

Finally, a detailed mathematical model considering sub-rocesses like biological reactions in the bulk liquid solution,lm transfer from bulk liquid phase to the biofilm, diffusion withiological reaction inside biofilm, adsorption equilibrium at theiofilm–adsorbent interface, and diffusion within the PAC par-icles has been proposed for predicting performances for hybridAC/MBR systems [305]. Numerous other studies reported these of PAC for MBR fouling limitation, but generally failed tossess key issues such as extra operating cost and disposal ofhe elevated amount of sludge to be wasted. However, Ng et al.ecently assessed more clearly the long-term performances ofuch hybrid systems [301].

In order to obtain higher biological aggregates in the biore-ctor, aerobic granular sludge has also been used in MBR sys-ems [144]. With an average size around 1 mm, granular sludgencreased the membrane permeability by 50%, but lower clean-

mtir

ng recoveries were observed (88% of those obtained with aonventional MBR).

A novel membrane performance enhancer (MPE 50) has beenecently developed by Nalco and applied to MBRs. When 1 g/lf cationic polymer-based compound was added directly to theioreactor, SMPc was found to decrease from 41 to 21 mg/l306]. The interaction between the polymer and the solublerganics in general, and SMPc in particular, was named as theain mechanism responsible for the performance enhancementhen Nalco’s polymer was used. In another example, an MBRperated with MLSS as high as 45 g/l featured a lower foulingropensity when 2.2 g/l of polymer was mixed to the bioreactor.

Experiments conducted with different system configurationsf submerged hollow fiber membranes allowed direct compar-son of hydraulic performances for pre-flocculation and PACddition. Under the operating conditions used in this study, pre-occulation presented higher fouling mitigation than that of PACddition [290]. However, when both strategies were used simul-aneously, membrane performances were maximum [290,307].

. Conclusions

After more than 10 years of intensive research, consensusn the exact fouling phenomena in MBRs has not been reachedet. Originally, it was suspected that aeration rate and MLSSoncentration had the main impact on MBR fouling. Notwith-tanding their significant effects, new areas of research haveeen since developed around the more detailed characterizationf these parameters. Efforts now concentrate on optimizing airistribution along the membrane modules and on more precisedentification of the biological parameters, which have the mostnfluence on the membrane performances. With the significanthanges in biomass characteristics from one plant to another, its not surprising to observe different biomass parameters affect-ng MBR fouling with various propensities. These disparitiesre also partly due to the different analytical methods and instru-ents used in the reported studies. In other words, the quest for a

ingle fouling parameter in MBR seems in vain. A large numberf recent publications indicate the biomass supernatant (SMP)nd its carbohydrate fraction to be one of the main parametersffecting MBR fouling. However, the more detailed characteri-ation of the supernatant and the fouling layer currently carriedut also reveals the significant role played by the protein fraction.

The effect of pore size on membrane fouling is also crucial forBR design, but the assessment of an optimized membrane pore

ize is time-dependant. MF-based MBR systems seem to rely onnitial fouling and the resulting creation of a dynamic membraneo produce high product quality, while UF-based MBRs featureood rejection from the early stage of filtration. However, thiseview revealed no clear advantage of using tight membranesver more open pores (within a given flux range). Finally, theltration time (short-term versus long-term), the mode of opera-

ion (constant flux versus constant TMP), the initial stage of the

embrane (new versus cleaned), the operating conditions and

he cleaning protocol are also crucial elements when the foul-ng experiments are designed and should be carefully selected,eported and analyzed in view of the results. The critical flux

crgtwtart

airbitiMse

A

atY

mc cake load/membrane area (kg/m2)MALDI-MS matrix-assisted laser desorption ionization

mass spectrometryMBR membrane bioreactorMF microfiltrationMLSS mixed liquor suspended solids (g/l)MLVSS mixed liquor volatile suspended solids (g/l)MW molecular weightMWCO molecular weight cut-off (kDa)NOM natural organic matterPAC powdered activated carbonRc hydraulic resistance attributed to the cake layer

(m−1)Rcol hydraulic resistance attributed to colloid species

(m−1)Rm hydraulic resistance of the membrane (m−1)Rp hydraulic resistance attributed to pore blocking

(m−1)Rsol hydraulic resistance attributed to soluble species

(m−1)Rss hydraulic resistance attributed to the suspended

solids (m−1)Rsup hydraulic resistance attributed to the biomass

supernatant (m−1)Rt total hydraulic resistance (m−1)SMP soluble microbial products (mg/l)SMPc fraction of carbohydrate contained in the sludge

solution (mg/gSS)SMPp fraction of protein contained in the sludge solution

(mg/gSS)So substrate concentration (g/l)SRT solid retention time (day)SUVA specific ultra violet absorbance (m l mg/C)t temperature (◦C)tcrit critical time over which step one is maintained (h)TMP transmembrane pressure (mbar)TOC total organic carbon (mg/l)UF ultrafiltrationUG gas superficial velocity (m/s)UL liquid superficial velocity (m/s)V cumulative volume of permeateXo MLSS concentration (g/l)

Greek symbols

R

oncept and its determination with the flux-stepping experimentemains an interesting tool to assess fouling propensity for aiven operating condition, but cannot be used for long-term fil-ration predictions. Instead, the concept of sustainable flux, forhich filtration can be maintained over an extended period of

ime, is more appropriate for real MBR plants. Effectivenessnd strategies for physical and chemical cleanings are under-eported in the open literature, and there are still opportunitieso match cleaning protocols with the foulant species present.

At this stage in time, it is difficult to propose a short-listing ofll the parameters which could predict and/or model MBR foul-ng. The large number of studies published on the subject andeviewed in Section 3 reveals the complex interactions existingetween the different fouling parameters. Further understand-ng of the nature of MBR foulants and their interactions withhe membrane material may provide new directions for clean-ng agents and protocols, and fouling mitigation strategies for

BRs. In that effort, previous studies reported for flocculation,ettling and dewatering of activated sludge can be used as inter-sting parallels.

cknowledgments

The authors gratefully thank the Australian Research Councilnd the NewSouth Global Postdoctoral Fellowship Program forhe financial support of this study, and Prof Simon Judd, Dr. Yune and Ms. Yulita Marselina for their contribution to this review.

Nomenclature

BPC biopolymer clustersBSA bovin serum albuminCASP conventional activated sludge processCFV crossflow velocity (m/s)CFVas crossflow velocity of activated sludge (m/s)CFVtp crossflow velocity of tap water (m/s)COD chemical oxygen demand (mg/l)CST capillarity suction time (s)dTMP/dt rate of TMP increase (or fouling rate) (kPa/h)DO dissolved oxygen (mg/l)DOC dissolved organic carbon (mg/l)DOTM direct observation through membraneeEPSc fraction of carbohydrate contained in extracted

solution from sludge (mg/gSS)eEPSp fraction of protein contained in extracted solution

from sludge (mg/gSS)EPS extracellular polymeric substances (mg/gSS)F/M food to microorganisms ratioHPSEC high performance size exclusion chromatographyHRT hydraulic retention time (h)J flux (l/m2 h)Jc critical flux (l/m2 h)

Jc,t critical flux at t (l/m2 h)Jc,20 critical flux at 20 ◦C (l/m2 h)J0 initial flux (l/m2 h)

αc cake specific resistance (m/kg)µ dynamic viscosity of MLSS (mPa s)

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lossary

dhesion: Molecular attraction between materials. In membrane separationprocesses, mechanisms include mechanical (interlocking), chemical (ionic,covalent or hydrogen bonding) and dispersive (adsorption, van der Waalsforce) adhesions.

iofilm: Complex aggregation of microorganisms on a solid surface. This phe-nomenon is possible thanks to the excretion of the protective and adhesiveEPS.

iofouling: Combination of biofilm formation and bacterial adhesion and depo-sition on the membrane surface.

iological floc: Aggregation of bacteria thanks to the EPS action. The relativelylarge floc size (few hundred micrometers) allows the biomass settlement inCASP.

iomass: Microorganisms and other biological solid materials growing in thebioreactor. Biomass concentration can be characterized by the mixed liquorvolatile suspended solids (MLVSS) concentration.

iomass supernatant: Solution generally obtained after centrifugation (and/orfiltration) of the activated sludge biomass. The supernatant is composed by

colloidal and soluble species.

ioreactor: Tank designed to biologically treat wastewater by the use of biomass.arbohydrate: Chemical compounds made of hydrogen, carbon and oxygen,

such as sugars, starches, and cellulose. Polysaccharides are one type of car-bohydrate prevalent in MBRs.

C

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D

E

E

F

H

M

P

S

olloids: Very small, finely divided solids (particles that do not dissolve) thatremain dispersed in the aqueous phase due to their small size (from 1 nm to1 �m) and electrical charge.

onventional Activated Sludge Process (CASP): Process in which a clarifierfollows the aeration tank and is used for solids separation.

eposition: Settling of particles from a solution or suspension mixture on apre-existing surface.

xtracellular polymeric substances (EPS): Construction materials for microbialaggregates such as biofilms, flocs and activated sludge liquors (see Section3.2.5).

xtracted extracellular polymeric substances (eEPS): Solution obtained afterthe physical and/or chemical extraction of the EPS from the biological walls

of the microorganisms.

ouling: Undesirable accumulation of (particulate, colloidal, molecular) mate-rials on the internal or external structure of the membrane. If the foulinginvolves living things such as microorganisms, the term biofouling may beused.

S

ydraulic retention time (HRT): The HRT is equivalent to the theoretical deten-tion time for an ideal plug flow or completely mixed reactor and is calculatedas the volume of bioreactor divided by the influent flowrate.

embrane bioreactor (MBR): Technology combining biological degradationprocess by activated sludge with a direct solid–liquid separation by filtration.

roteins: Chemical substances based on the polymerisation of amino acids. Dur-ing biological degradation, proteins are hydrolysed to polypeptides, aminoacids, and then ammonia and simple organic compounds.

olids residence time (SRT): The SRT (also called the mean cell residence timeor sludge age) is equivalent to the average time that microorganisms spendin the MBR and is calculated as the mass of organisms in the reactor dividedby the mass of organisms generated/wasted from the reactor each day.

oluble microbial products (SMP): SMP (also called soluble EPS or biomass

supernatant) are defined as soluble cellular components that are releasedduring cell lysis, diffuse through the cell membrane, are lost during synthesisor are excreted. In MBR systems, they can also be provided from the feedsubstrate (see Section 3.2.6).