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Journal of Membrane Science 403– 404 (2012) 8– 14

Contents lists available at SciVerse ScienceDirect

Journal of Membrane Science

j ourna l ho me pag e: www.elsev ier .com/ locate /memsci

embrane biofouling and scaling in forward osmosis membrane bioreactor

insong Zhanga,b, Winson Lay Chee Loonga, Shuren Choua,b, Chuyang Tanga,b,ong Wanga,b, Anthony Gordon Fanea,b,∗

Singapore Membrane Technology Centre, Nanyang Technological University, Singapore 639798, SingaporeSchool of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singapore

r t i c l e i n f o

rticle history:eceived 12 September 2011eceived in revised form1 December 2011ccepted 28 January 2012vailable online 6 March 2012

eywords:embrane fouling

orward osmosisembrane bioreactor

a b s t r a c t

The forward osmosis membrane bioreactor (FOMBR) has received much attention recently. Due to thehigh rejection nature of FO membranes, the biomass, dissolved organic and inorganic compounds retainedin the bioreactor could cause membrane fouling by multiple mechanisms. In this study, a 45% perme-ate flux decrease was observed in a well controlled FOMBR equipped with a submerged hollow fiberFO membrane module with the active layer facing the draw solution (AL-DS) configuration. A series ofcharacterizations were performed to explore membrane fouling mechanisms in the FOMBR. It was foundthat a biofouling layer covered the substrate surface of the FO membrane with a combined structure ofbacterial clusters and extracellular polymeric substances (EPS), which contributed to 72% drop of themembrane mass transfer coefficient (Km) and around 10% increase in the hydraulic resistance. The inor-ganic fouling was caused by Ca, Mg, Al, Si, Fe and P that contributed 60% of the total hydraulic resistance

of the fouled membrane and decreased the Km by around 34%. These results suggest that in this appli-cation the FO fouling is governed by the coupled influences of biofilm formation and inorganic scaling.When the configuration was reversed with the active layer facing the feed solution (AL-FS), a negligibleflux decline was obtained by applying intermittent tap water flushing to the membrane surface, whichsuggests that the AL-FS orientation is favorable for FOMBR operation. An effective strategy for fouling

rnal s

control is to prevent inte

. Introduction

Membrane bioreactors (MBR) have been extensively applied forndustrial and municipal wastewater treatment, where particle-ree effluents are produced through the filtration of the activatedludge using microfiltration (MF) or ultrafiltration (UF) mem-ranes. However, the limited retention of these porous membraneso low molecular weight (MW) soluble microbial products (SMP)nd dissolved inorganic ions have restricted the reuse extent ofBR permeate. While reverse osmosis (RO) or nanofiltration (NF)

an be applied subsequently for the effluent post-treatment, theombined MBR-RO/NF treatment generally results in high capitalost and energy consumption [1–3]. As an alternative to conven-ional MBRs, forward osmosis membrane bioreactors (FOMBRs),ombining the biological and forward osmosis (FO) processes in oneystem, have been reported recently [4–6]. FO is a natural process

riven by the osmotic pressure difference across a semi-permeableembrane that retains solutes but allows water to transfer through

he membrane. Existing studies on FOMBR have demonstrated

∗ Corresponding author at: School of Civil and Environmental Engineering,anyang Technological University, Singapore 639798, Singapore.

E-mail address: AGFane@ntu.edu.sg (A.G. Fane).

376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2012.01.032

caling and the over-growth of biofilm on the membrane surface.© 2012 Elsevier B.V. All rights reserved.

acceptable permeate flux and remarkable removal efficiency fororganic compounds, nutrients, and other trace contaminants [7,8].If a suitable draw solution is readily available (e.g., seawater) orcan be regenerated cost-effectively, FOMBR may have the poten-tial to replace the conventional MBR-RO/NF process for wastewaterreclamation.

One of the major issues in FO operation is membrane fouling.Existing bench-scale studies on FO membrane fouling [5,9–12],using model foulants such as alginate, proteins, and humic acid,revealed that FO flux reduction can be strongly affected by mem-brane structure and orientation in addition to other operationalconditions. FO fouling is generally more pronounced when theactive rejection layer is facing the draw solution (AL-DS), sincethis orientation is prone to severe internal pore clogging in theporous membrane support layer [9,11]. The modification of thesupport layer structure (e.g., reduced porosity, increased tortuos-ity, or increased thickness in the case of a cake layer formed on thesupport surface) in the AL-DS orientation may further enhance theinternal concentration polarization (ICP) in the membrane support,leading to accelerated flux reduction [11]. Compared to the AL-DS

orientation, the active-layer-facing-feed-solution orientation (AL-FS) generally enjoys more stable flux performance, although itsmore severe ICP level means that this membrane orientation tendsto have relatively lower water flux [11]. Thus, one may operate

J. Zhang et al. / Journal of Membrane Science 403– 404 (2012) 8– 14 9

osis

aiAmmd

tsgoino[tdiat

tFsTis

2

2

iwvSmfiaMldh

30 laser at 405 nm, an Argon laser at 488 nm, a DPSS561-10

Fig. 1. Schematic diagram of the forward osm

n FO membrane either (1) in AL-DS to attain a significant highernitial flux (but with much higher fouling propensity), or (2) inL-FS to achieve an inherently more stable flux at the expense ofore severe ICP. Correspondingly, it is anticipated that differentembrane operation and fouling control strategies may be adopted

epending on the membrane orientation.Compared to FO fouling by model foulant compounds, fouling in

he FOMBR can be significantly more complicated as the activatedludge contains many different components (such as microor-anisms, extracellular polymeric substances, soluble macro/microrganics, and inorganics) which can interact with the membranen many different ways [13–15]. Cornelissen et al. [6] reportedegligible fouling by activated sludge for both AL-FS and AL-DSrientations in a short term experiment. In contrast, Achilli et al.4] observed ∼18% water flux drop in AL-FS orientation even withhe implementation of osmotic backwashing, and more severe fluxecline without backwashing. Up to now, the fouling mechanisms

n FOMBR have not been fully characterized. While both biofoulingnd scaling are anticipated, their exact roles and relative impor-ance are not well understood.

The objective of the current study is to systematically inves-igate the complex fouling phenomenon of FO membranes in aOMBR. The combined effect of membrane biofouling and inorganiccaling on the flux decline and the enhanced ICP were elaborated.he current study may provide important insights into FO foul-ng mechanisms during FOMBR operation and appropriate controltrategies, which could be helpful to guide process optimization.

. Materials and methods

.1. Operation of MBR

A schematic diagram of the submerged FOMBR used in this studys shown in Fig. 1. The bioreactor was fed with synthetic municipal

astewater [8] (see Supporting Information S1) and the effectiveolume of the bioreactor was 4 L. Seed sludge was collected fromingapore Ulu Pandan Demonstration MBR Plant. A submergedembrane module made of thin film composite (TFC) FO hollow

bers with an effective area of 0.025 m2 was installed in the biore-ctor. The TFC FO membrane, developed in house in the Singapore

embrane Technology Centre, has an ultra-thin polyamide skin

ayer (∼300 nm) on the inner surface of the hollow fiber (seeetails in Refs. [16,17]). The outer skin of the membrane, whichas ultrafiltration-like retention properties, was further modified

membrane bioreactor experimental system.

by plasma treatment (see Supporting Information S2) to have alow molecular weight cut-off (MWCO) of 6 kDa in order to reducethe tendency of membrane pore clogging by macromolecules.The water permeability A and solute permeability B of the mem-brane were determined by RO tests (A = 8.47 × 10−12 m/s Pa andB = 3.95 × 10−8 m/s). Compared to commercially available FO mem-branes (Hydration Technology Inc. (HTI)), the water flux of the TFChollow fibers developed in house was more than 3 times higherunder similar test conditions [16,17].

A 0.5 M sodium chloride (NaCl) solution was used as the drawsolution (osmotic pressure ∼ 23.5 bar as measured by a GonotecOSMONAT 030 cryoscopic osmometer). The DS crossflow velocitywas kept at 0.75 m/s. Its concentration was maintained constantby means of conductivity control. When the DS became dilutedby the FO permeate water, a concentrated NaCl solution (5 M)was dosed into the DS tank to keep the DS conductivity at thelevel of 47.3–47.5 ms/cm. The permeate flow rate was computedby a measuring column equipped with an overhead ultrasoundlevel transmitter and a controller (Cole-Parmer, 0.1 mm reso-lution, ±0.2% accuracy), where the volume of dosed 5 M NaClsolution was exclude from the flux calculation. Compressed air at5 L/min was introduced into the bottom of the membrane mod-ule to ensure a dissolved oxygen (DO) concentration of about6–8 mg/L and to generate shear stress for scouring the membranesurface. The reactor temperature was controlled at 23 ◦C. Dur-ing the entire FOMBR operation, the sludge retention time (SRT)was fixed at 10 days. The hydraulic retention time (HRT) was7 h initially but as membrane fouling occurred, the HRT eventu-ally decreased to 44 h due to permeate flux decline. Operationalparameters, such as pH, DO, conductivity, flux, temperature, anddraw solution flow rate, were measured by transducers and werelogged by Scada software (OMRON) into a computer data loggingsystem.

2.2. Confocal laser scanning microscopy

The morphologies of fouled FO membranes samples were char-acterized by a ZEISS LSM710 Confocal Laser Scanning Microscope(CLSM, ZEISS, Germany). The CLSM is equipped with a Diode405-

laser at 561 nm and a HeNe633 laser at 633 nm. The commercialLIVE/DEAD BacLightTM Bacterial Viability Kits (Molecular Probes,product #L7012) were used to stain the microorganisms on themembrane samples.

10 J. Zhang et al. / Journal of Membrane Science 403– 404 (2012) 8– 14

ductiv

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2

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2

ett

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Fig. 2. Feature of membrane permeate flux and con

.3. Field emission scanning electron microscopy (FESEM) andnorganic foulant analysis

Field-emission scanning electron microscopy (FESEM) was per-ormed using a JEOL JSM-6340F FESEM at an acceleration voltagef 5.0 kV. Prior to SEM observation, the membrane samples wereehydrated with ethanol, dried for 24 h in a Freeze Dryer (Christlphr 1-4 LD, Germany), and coated with a uniform layer of goldsing an Emitech SC7620 Sputter (45 s coating time). Elementalnalysis was performed using an energy diffusive X-ray analyzerEDX) coupled to the FESEM. In addition, the elemental composi-ions (Ca, Mg, P, Fe, Al) were determined by an inductively coupledlasma emission spectrometer (PerkinElmer, Wellesley, MA, USA;ptima 2000) for the supernatant of the mixed liquor and the HNO3

olution at pH 2 for dissolving the inorganic scaling of the fouledembranes.

.4. EPS and humic substance analysis

The biomass-bound EPS extraction was performed followinghe “formaldehyde plus NaOH extraction method”. The polysac-haride content in the EPS was measured by the phenol–sulphuriccid method using glucose as the calibration standard, and therotein content was determined by the Bradford-bovine serumlbumin (BSA) method, as reported in our previous papers [13,18].

Fourier Transform Infrared Spectrophotometer with Microscopyystem (FTIR-8400S-AIM-8800, Shimadzu) was applied to charac-erize the major functional groups of the EPS in the membraneouling layer. A 3D FTIR mapping of the EPS distribution in the

embrane biofouling layer was obtained using mapping softwareIM-MAP. Supernatant samples were prepared by pre-filtration ofctivated sludge with a 0.45 �m filter, and the dissolved organicsoncentration and molecular weight distribution were determinedy size-exclusion chromatography equipped with organic car-on detection and organic nitrogen detection (LC-OCD-OND). TheC-OCD-OND analysis was performed by DOC-LABOR Dr. Huber,ermany [19].

.5. Modeling FO performance

An analytical model of FO performance, which incorporates theffect of ICP, has been developed by Loeb et al. [20]. According tohis model, the FO permeate flux Jv can be determined by Eq. (1) for

he AL-DS orientation and Eq. (2) for the AL-FS orientation:

v = Km ln

(A�draw − Jv + B

A�feed + B

)for AL-DS orientation (1)

ity of the mixed liquor during long-term operation.

Jv = Km ln

(A�draw + B

A�feed + Jv + B

)for AL-FS orientation (2)

where �draw and �feed are the osmotic pressure of the draw solutionand feed solution, respectively; A and B are the water permeabil-ity and solute permeability of the rejection layer, respectively. Themass transfer coefficient Km of the porous support layer is relatedto the solute diffusion coefficient D and the membrane structureparameter S,

Km = D

S= D · ε

� · l(3)

In Eq. (3), the structural parameter S is a length scale for the ICPin the porous support layer, and its value is given by membranesupport layer thickness (l) times its tortuosity (�) over the porosity(ε).

3. Results and discussion

3.1. Membrane fouling behavior

Fig. 2 shows the FO permeate flux and the mixed liquor conduc-tivity at AL-DS orientation in the bioreactor during a 55-day run.The conductivity increased from 1.8 to 12 ms/cm within the first 12days, due to the retention of feed solutes by the FO membrane aswell as the diffusion of solutes from the high-concentration drawsolution [21,22]. A stabilized conductivity value was achieved after12 days as a result of the routine daily discharge of 10% mixed liquor.This equilibration time was on the same order to the reactor SRT (10days), which agrees well with a previous analytical model on soluteaccumulation in a FOMBR [21]. Corresponding to the increase in thefeed conductivity (and the dissolved solute concentration), the FOpermeate flux also experienced a significant decline in the first 12days, and became stable after that initial period. The flux reduc-tion in Fig. 2 may be attributed to (1) FO membrane fouling and (2)the increased solute concentration in the reactor [21,22]. The latterled to an increased mixed liquor osmotic pressure (as confirmedby independent osmotic pressure measurement, see SupportingInformation S3). In order to decouple the solute accumulation effectfrom that of the membrane fouling, Eq. (1) was used to estimate thebaseline FO permeate flux assuming no fouling conditions (con-stant A, B, and Km values), where the actual mixed liquor osmoticpressure was used in the calculation. Interestingly, the experimen-tal results (the filled diamond symbols in Fig. 2) agreed closely

with the non-fouling baseline (the dotted line) during the firsttwo days of operation, which suggests that the initial flux declinewas likely dominated by the solute accumulation in the FOMBR.The subsequent deviation from the baseline can be attributed to

J. Zhang et al. / Journal of Membrane Science 403– 404 (2012) 8– 14 11

Table 1Mass transfer coefficient, water permeability and hydraulic resistance of fouled, cleaned and virgin FO membranes.

Membrane sample Km (LMH) A (10−12 m/s Pa) Jva (at 11.5 ms/cm)

Virgin membrane 5.5 ± 0.17 8.47 ± 0.21 8Fouled membrane 2.31 ± 0.11 4.48 ± 0.29 3.37

0.1

0.22

Ffl(i

cflseIostfraTtsssoda

gitpthwclbwpompaoisttcbtF

3

o∼

Hydraulically cleaned membrane (biofilm removed) 3.98 ±Acid cleaned membrane 5.33 ±a Jv was evaluated for a feed water with a conductivity of 11.5 ms/cm.

O membrane fouling. In the current study, the measured stableux (∼3.9 ± 0.5 LMH) was 45% lower than the computed stable flux∼7.1 ± 0.4 LMH), indicating that fouling plays a significantly rolen the long term FO flux performance.

While fouling of conventional MF/UF based MBRs is mainlyaused by increased hydraulic resistance due to biofouling, FOMBRouling can involve additional mechanisms. The solute accumu-ation phenomenon in FOMBR may further promote membranecaling by inorganic minerals [23,24], especially in the AL-DS ori-ntation where feed solutes also experience a severe concentrativeCP [25]. Moreover, the foulants trapped in the porous support layerr on the surface of the support due to biofouling and inorganiccaling may in turn enhance the degree of ICP by reducing its massransfer coefficient [11]. To quantitatively identify the effects of bio-ouling and inorganic scaling on the mass transfer and hydraulicesistance of the membrane, a series of cleaning protocols waspplied to the fouled membrane to remove the different foulants.o remove the attached biofilm, the first cleaning step was to flushhe fouled membrane surface with Milli-Q water for 48 h. In theecond cleaning step, the membrane with the biofilm removed wasoaked in a HNO3 solution at pH 2 for 1 h to dissolve the inorganiccaling. The water permeability A and mass transfer coefficient Km

f the fouled membrane before and after each cleaning step wereetermined using a bench-scale RO/FO cross-flow setups following

previously established method [16].Table 1 compares the measured FO permeate flux Jv of the vir-

in, fouled, hydraulically cleaned, and acid-cleaned membranes. Andentical feed conductivity of 11.5 ms/cm was used for the evalua-ion to eliminate any difference caused by the different feed osmoticressure. The clean membrane had a permeate flux ∼8 LMH, andhis value was reduced to 3.37 LMH for the fouled membrane. Theydraulic flushing only partially recovered the flux to 4.68 LMH,hile the acid cleaning nearly fully restored the value to that of the

lean membrane. This result suggests that inorganic scaling wasikely playing a critical role in the current study, although the role ofiofouling was not negligible. The mass transfer coefficient Km andater permeability A of the different membrane samples are alsoresented in Table 1 to provide better mechanistic understandingf the underlying fouling mechanisms. A comparison of the fouledembrane with the virgin membrane reveals that both water

ermeability and mass transfer coefficient were severely reducedfter fouling. Upon hydraulic cleaning (the removal of the biofilm),nly 10% of the water permeability was recovered while the Km

ncreased by about 72% (2.31 ± 0.11 LMH vs. 3.98 ± 0.1 LMH). Thisuggests that the FO biofouling affected mainly the substrate struc-ure, which resulted in a flux decline by enhanced ICP. In contrast,he comparison of acid-cleaned membrane with the hydraulicallyleaned one indicates that inorganic scaling had adversely affectedoth the water permeability and mass transfer coefficient, althoughhe former was more severely affected. Further characterization ofO biofouling and scaling is presented in the following sections.

.2. Characterization of FO biofouling

CLSM and SEM were applied to characterize the morphologyf the foulants. The CLSM micrograph (Fig. 3A) shows a biofilm18 �m thick that covered uniformly the FO support surface.

4.94 ± 0.23 4.68 7.94 ± 0.9 7.78

FTIR was used to characterize the composition of biopoly-mers (e.g., proteins and polysaccharides) in the cake layer. TheFTIR spectrum (Fig. 3D) shows two sharp adsorption peaks at1635 cm−1 and 1543 cm−1 that are characteristics to protein sec-ondary structures [23]. The broad peak at ∼1037 cm−1 is assignedto polysaccharides or polysaccharide-like substances [26]. The FTIRresults confirmed the presence of proteins and polysaccharide sub-stances as major foulants. Their distribution in the biofilm wasfurther characterized by the 3D mapping of FTIR spectra (Fig. 3Band C). From Fig. 3, the bacteria clusters of biofilm, polysaccha-rides and proteins were found to coexist or overlap on the regionsof the biofilm. There may be synergistic or antagonistic interac-tions between different kinds of biopolymers that cause changesin their membrane fouling processes. For instance, the presence ofEPS and their reaction with solute ions reduced the mass trans-fer coefficient, which contributed significantly to permeate fluxdecline in RO and NF membrane processes [15,27]. The proteinand polysaccharide in the biofilm was extracted to determinetheir content quantitively. The protein-like and polysaccharide-like to biomass ratio in the biofilm were 24.7 ± 4.1 mg g MLSS−1

and 82.5 ± 3.4 mg g SS−1, respectively. In comparison, the corre-sponding values in the mixed liquor were 15.6 ± 6.1 mg g MLSS−1

and 21.0 ± 7.4 mg g SS−1, indicating the preferential accumulationof EPS in the biofilm.

SEM results (Fig. 4A) confirmed the presence of a surface foul-ing layer on the FO support. The presence of microorganism cellsin the foulant cake layer was also apparent. The cross-sectionalSEM image (Fig. 4B) showed mild internal pore clogging in this FOmembrane, which implies that the fouling occurred mainly on themembrane surface. As discussed earlier (Table 1), the biofilm onlydecreased the overall water permeability marginally but severelyreduced the mass transfer coefficient. In this FO process, the pres-ence of combined structures of bacteria and EPS covered themembrane substrate surface and plugged the porous membranesupport layer. As an additional part of the porous membrane sub-strate of the FO membrane, the fouling layer would hinder theback diffusion of retained solutes in the porous support due to thetortuous paths within the biofilm layer [28], which explains thesignificant contribution of the biofouling layer to the reduction inKm (Table 1).

Fig. 5 illustrates molecular weight distributions of the solubleorganics in the supernatant of the FOMBR. The mixed liquor wascentrifuge twice and pre-filtered with a 0.4 �m filter to removeall the particulates and most of the macromolecular EPS. The totalorganic carbon (TOC) concentration in the supernatant was around20.04 mg/L, while the EPS passed through the filter accountedfor 0.9 mg/L TOC. The low molecular weight humic substance atMW 1000 Da, 350–500 Da and below MW 350 Da were 9.1 mg/L,5.08 mg/L and 4.96 mg/L, respectively. The humic concentrationwas around 95% of total dissolved organics. In our previous studyof a conventional MBR, the humics was only 1.6 mg/L (7.1% ofdissolved TOC) [18]. This difference may be explained by thehigh retention of FO membrane of the humic substances. Mi

and co-workers have investigated flat sheet FO membrane foul-ing by model foulants such as protein, humic acid, and alginate[3,10,19,24]. Tang et al. also reported the AHA fouling effect onICP and flux decline [11]. These studies revealed that the organic

12 J. Zhang et al. / Journal of Membrane Science 403– 404 (2012) 8– 14

Fig. 3. (a) CLSM image showing the surfaces of fouled membrane. (b,c) 3D FTIR spectra distribution map of protein like EPS and polysaccharide like EPS. (d) FTIR spectra ofbiofilm surface.

Fig. 4. SEM image of fouling surface and cross section of FO membrane: (a) cake layer of the fouled FO membrane and (b) cross section image and EDX scan region of fouledFO membrane.

J. Zhang et al. / Journal of Membrane Science 403– 404 (2012) 8– 14 13

Table 2Analysis of inorganic components of reactor supernatant and acid washed solution.

Element

Ca Mg Al Si Fe P

Acid washed solution (mg/L) 144.36 ± 1.72 9.31 ± 0.38 0.96 ± 0.14 0.25 ± 0.01 4.8 ± 0.01 12.61 ± 0.01Reactor supernatanta (mg/L) 75.23 ± 0.29 103.31 ± 0.92 0.14 ± 0.02 10.19 ± 0.1 0.09 ± 0.02 5.0 ± 0.03Membrane inorganic scaling (g/m2) 24.44 9.31

a The reactor supernatant conductivity was 7 ms/cm, pH was 7–8.

Fl

fa

3s

sttbosAambTnasscc

ig. 5. LC-OCD chromatogram of dissolved organics in the supernatant of the mixediquor. MW: molecular weight.

ouling by protein, alginate and humic acid would significantlyffect the FO membrane performance, as confirmed in this study.

.3. SEM–EDX and inductively coupled plasma emissionpectrometer analysis

SEM–EDX was performed over the fouled FO membrane cross-ection (Fig. 4). The EDX results show that Ca, Mg, Al, Si, Fe werehe main elements in addition to carbon, and they were found onhe membrane surface and in the internal pores of the FO mem-rane (see Supporting Information S4). The high rejection propertyf the selective layer together with the high ICP level in the porousupport may be responsible for the formation of inorganic scaling.s reported in Table 1, this inorganic scaling can be removed bycid cleaning. The inorganic constituents in the supernatant of theixed liquor and the acid wash solution were further characterized

y inductively coupled plasma emission spectrometer (Table 2).he supernatant contained high concentrations of calcium, mag-esium, silica, and phosphorus, together with trace amounts ofluminum and iron. Many of these were also found in the acid wash

olution. Calcium was abundant in the acid wash solution, whichuggests that calcium based scaling (e.g., calcium carbonate and cal-ium phosphate) may have dominated the internal scaling in theurrent study. The relative concentrations of iron and aluminum

0

2

4

6

8

10

12

14

0 8 16 24 32 40 48 56 64 72

Flux

(LM

H)

Time(h

computed flux Fl

Tap water flush po int

Fig. 6. Evolution of FO membrane flux at AL-facing-FW configuration (clean w

0.96 0.25 4.8 12.61

were enriched in the inorganic scaling likely due to the low solu-bility of these elements. Interestingly, silicon did not seem to playa major role in the internal scaling, even though its concentrationwas relatively high in the supernatant.

3.4. FO membrane flux in AL-FS orientation

To test the effect of membrane orientation on FO fouling, a FOcross-flow membrane module was tested with a bench scale FOsetup [17]. The hollow fiber membrane was made of the samemembrane material as applied in the previous submerged module,but with a bigger inner diameter (1.31 mm). The water permeabil-ity A and solute permeability B of this big diameter FO membranewere 5.61 × 10−12 m/s Pa and B = 4.05 × 10−8 m/s, respectively. Thefeed of biomass in this test was taken from the FOMBR. The drawsolution remained the same at 0.5 M NaCl. The feed solution waspumped through the lumen side (active layer side) in the AL-FSconfiguration. The cross-flow velocities of the feed and draw solu-tion were set at 1.2 and 0.75 m/s, respectively. The experimentalflux, computed flux baseline by Eq. (2), and feed conductivity areillustrated in Fig. 6. To achieve a sustainable operation, tap waterflushing was applied to the lumen of membrane module for 15 minat each flush point to remove the attached biomass. The arrows inFig. 6 indicated the different times when the tap water flushing wasperformed.

In the AL-FS orientation, superior flux stability was observed andthe average water flux during the experiment was approximately8 LMH. However, flux decline can still be observed between eachtap water flushing operation. The fouling effect can be correlatedwith the time interval of the intermittent tap water flushing. Theshorter the interval (8 h) of cleaning operation applied, the betterflux recovery achieved. From the 100th hour, without tap watercleaning, the flux dropped from 8 LMH to 6 LMH within 24 h. Themembrane fouling behavior in this study is in good agreement withprior studies of the AL-FS configuration [4–6]. It demonstrated that

the permeate flux in the AL-FS configuration was less sensitive tothe feed conductivity and membrane fouling. In this orientation,even with the buildup of the biofilm layer on the membrane surface,which may give rise to cake enhanced concentration polarization

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Technology 37 (2003) 5581–5588.[30] T.H. Chong, F.S. Wong, A.G. Fane, Implications of critical flux and cake

4 J. Zhang et al. / Journal of Mem

henomenon [28–30], the FO permeate did not seem to be severelyffected. The inherent stability in this membrane orientation maye explained by its relatively lower initial operating flux and thevoidance of internal plugging and concentrative ICP (which pro-ote internal scaling). In addition, due to the more severe ICP level

n this orientation, any small flux decline will significantly lowerCP and thus increases the effective osmotic driving force, a phe-omenon reported as the ICP self-compensation effect by Tang et al.11].

cknowledgments

We would like to thank the Environment and Water Industryrogramme Office (EWI) of Singapore for funding support under theroject # EWI 0801-IRIS-05. We are also grateful to Singapore Eco-omic Development Board for funding the Singapore Membraneechnology Centre.

ppendix A. Supplementary data

Supplementary data associated with this article can be found, inhe online version, at doi:10.1016/j.memsci.2012.01.032.

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