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Journal of Membrane Science 349 (2010) 75–82

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

ouling of RO membranes by effluent organic matter (EfOM): Relating majoromponents of EfOM to their characteristic fouling behaviors

an Zhaoa, Lianfa Songb, Say Leong Onga,∗

Division of Environmental Science & Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260, SingaporeDepartment of Civil and Environmental Engineering, Texas Tech University, 10th and Akron, Lubbock, TX 79409, USA

r t i c l e i n f o

rticle history:eceived 10 August 2009eceived in revised form1 November 2009ccepted 12 November 2009vailable online 3 December 2009

eywords:embrane foulingrganic foulingO membraneffluent organic matterouling potentialxtracellular polymeric substancesarbohydrate

a b s t r a c t

Effluent organic matter (EfOM) has been considered by many to play an important role in fouling ofRO membranes used for wastewater reclamation. However, due to their heterogeneous composition,which is a mixture of structurally complex aquatic humic substances (AHS), soluble microbial products(SMP) or extracellular polymeric substances (EPS) and other poorly defined organic compounds, the frac-tional component(s) or physical–chemical properties responsible for the fouling phenomenon are stillnot well understood. This study aims to obtain a better understanding of interactions between fractionalcomponents of EfOM and RO membranes and attempts to identify the most influential fraction(s) orphysical–chemical properties governing the fouling process. Four EfOM fractions were isolated and frac-tionated from UF prefiltrated treated effluent based on hydrophobicity and charge characteristics. EPS wasextracted from the biological treatment stage to assess their fouling potential on RO membranes via well-controlled laboratory-scale experiments. The individual organic fractions were rigorously characterizedin terms of physico-chemical properties. A clear correlation was observed between the physico-chemicalproperties of EfOM fractions and their fouling potential. Under hydrodynamic and chemical conditionstypical of commercial applications, the hydrophilic neutral fraction, mainly composed of small size car-bohydrates, resulted in the highest flux decline and exhibited highest affinity towards the membrane. EPS

biopolymers, to which great importance has been associated with regard to causing RO organic fouling,resulted in less fouling than hydrophilic carbohydrates. Although EPS biopolymers tended to accumu-late on the membrane in much higher quantities, the cake layer formed was found to constitute a muchlower resistance towards filtration and has a much lower membrane affinity, probably due to their largemolecular sizes. The contribution of AHS and other hydrophilic fractions to membrane fouling was found

pared

to be much lower as com

. Introduction

Reverse osmosis (RO) process has been increasingly imple-ented to reclaim high quality water from treated wastewater

ffluent in many parts of the world. Considering the superiorejection capability of RO membranes and the high contaminantoadings in treated wastewater effluent as feed water to RO,ffective fouling control is critical to the technical viability andost-efficiency of this technology. In the past few years, progressas been made in containing fouling problems associated with

uspended solids and colloids, microbial contaminants and scal-ng through implementing microfiltration (MF) or ultrafiltrationUF) pretreatment, high-performance polymeric antiscalants andoutine disinfection procedures. However, membrane fouling by

∗ Corresponding author. Tel.: +65 6516 2890; fax: +65 6774 4202.E-mail address: eseongsl@nus.edu.sg (S.L. Ong).

376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2009.11.024

to hydrophilic carbohydrates and EPS biopolymers.© 2009 Elsevier B.V. All rights reserved.

effluent organic matter (EfOM) still remains as a major obsta-cle to the long-term operational sustainability of this process[1–5]. Recently, fouling problems associated with EfOM and algalorganic matter (AOM) of similar allochthonous origin have alsobeen reported for RO or nanofiltration (NF) processes treating sur-face water with increasing frequency. This phenomenon could beattributed to the fast-rising discharge volume of treated wastew-ater effluent into surface water bodies and/or widespread algalgrowth caused by eutrophication [6–9].

The challenges encountered could partly be attributed to thelack of knowledge on the composition and characteristics of EfOM.EfOM represents a large group of structurally complex, heteroge-neous and poorly defined organic compounds derived from raw

wastewater and microbial metabolic activities in biological treat-ment systems. Although a few major components of EfOM havebeen identified such as humic substances (AHS), soluble microbialproducts (SMP) or extracellular polymeric substances (EPS), lipids,nucleic acids and organic acids, the total amount of EfOM compo-

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ition identified only accounted for 15–20% of the effluent organicarbon content [10–14]. Secondly, these organic compounds aremall in size, mainly present in colloidal and soluble form, and areefractory towards extended biodegradation, which makes theirontrol difficult in commercial applications.

Most previous studies on membrane fouling associated withfOM have been focused on MF/UF membranes used for tertiaryltration of treated effluent. Some studies on EfOM fouling onO membranes have also been reported in the past few yearsue to increasing interest in treated effluent reclamation. How-ver, a high percentage of these studies were conducted withynthetic wastewater rather than real wastewater [2–5,15–18].n these studies, organic compounds such as commercial humiccids (HA), fulvic acids (FA), alginic acids or alginate salts, bovineerum albumin (BSA) and octanoic acids have been employed toodel specific EfOM components such as NOM, polysaccharides,

roteins and fatty acids. One key shortcoming of employing syn-hetic compounds is that these organic compounds might not beble to adequately simulate the composition of EfOM. For exam-le, the sodium alginate employed has been reported to have aolecular weight ranging from 12 to 80 kDa and the bovine serum

lbumin (BSA) has a molecular weight around 66 kDa. However,he preliminary characterization results of MF/UF prefiltered sec-ndary effluent obtained in this study and results reported by otheresearchers have unanimously confirmed that EfOM has an aver-ge molecular weight around 1 kDa. Thus, the group of syntheticMP or EPS macromolecules which have been considered as a majorause of fouling and extensively studied might not be present in realreated effluent in large quantities. Therefore, it would be more rel-vant to conduct the fouling experiments with real treated effluentamples.

Studies on organic fouling of high-pressure NF/RO membranesave been initiated focusing on NOM as these membranes haveeen widely adopted for seawater or surface water treatmentefore being extended to wastewater reclamation applications.xtensive studies with commercial or extracted humic acids asOM surrogates indicated that surface adsorption and gel orake layer formation might be the dominant fouling mechanismlthough much is still poorly understood. It involves initial depo-ition of organic foulants on the membrane governed by thenteractions between foulants and the membrane surface andubsequent development of a gel or cake layer governed byhe interactions between the foulants deposited and foulants inhe bulk solution. These molecular-level interactions have beeneported to be further influenced by the physico-chemical proper-ies of organic foulant and the membrane, solution chemistry andydrodynamic operation conditions, which makes the feed water

ouling potential dependent on source composition and operatingonditions and understanding of the fouling phenomenon moreifficult [19–21]. Fundamentally, membrane fouling behavior byfOM would also be deduced to be dependent on their compositionnd characteristics. Although there is a good understanding withespect to the effects of NOM physical–chemical characteristicse.g., MWD, hydrophobicity and charge, metal chelating proper-ies and biodegradability) on membrane fouling behaviors, theres a lack of knowledge concerning relationships among physico-hemical characteristics of EfOM, represented by its fractionalomponents, and their fouling potential and membrane foulingehaviors [10,22–24].

If the “critical” EfOM fraction(s) contributing most to RO foulingi.e., the structures and characteristics associated with the highest

ouling potential) could be identified, effective cleaning strate-ies specifically targeting at these organic components could beeveloped rather than using generic methods. With a better under-tanding of the relationship between the EfOM physico-chemicalroperties and fouling potential, it might also be possible to reduce

e Science 349 (2010) 75–82

fouling potential of feed water by modifying the concentrationand composition of EfOM through biological treatment or pre-treatment upstream of RO membranes. The concentration andphysico-chemical characteristics of EfOM have been found to behighly dependent on wastewater sources and the operating con-ditions of the preceding biodegradation process such as substratetype and strength, temperature, oxygen loading rate (OLD) andsludge retention time (SRT). It has been reported that the produc-tion of SMP can be minimized when an optimal SRT was adopted[13,23,25,26].

When membranes get fouled, the mass and structure of the foul-ing layer developed (e.g., thickness, porosity and compressibility)will determine: (i) the additional resistance to permeation flow andthus the flux decline rate and extent, and (ii) fouling reversibility bysubsequent cleaning cycles. Therefore, when investigating foulingbehaviors of individual EfOM fractions, it is also important to assessthe fouling layer mass accumulated on the membrane and relatethem to the flux decline profile to get insight into the fouling layerproperties, which might possibly be influenced again by physico-chemical characteristics of EfOM. The affinity of fouling layer withthe membrane is also worth studying as it might determine mem-brane permeability recovery during periodic cleaning cycles andthe long-term fouling development rate.

The objective of this study was therefore to investigate theimpacts of different fractional components of EfOM or physico-chemical characteristics on RO fouling behaviors. Specifically,well-controlled laboratory-scale fouling experiments were carriedout with EfOM fractions isolated from ultrafiltration prefilteredtreated effluent based on hydrophobicity and charge character-istics and EPS extracted from the biological treatment stage.Physico-chemical properties of individual fractions were rigor-ously characterized and their relationships with fouling behaviorsobserved were analyzed. As an operationally significant solutionchemistry parameter, the effect of calcium ions was also studied.The extent of flux decline was associated with the mass of foulinglayer deposited and the affinity of fouling layer with the membranewas also assessed after hydraulic cleaning was carried out on thefouled membrane.

2. Materials and methods

2.1. Treated wastewater effluent

UF (Zeeweed®500, Zenon, GE Infrastructure Water Process andTechnologies, USA) prefiltered secondary effluent was collectedfrom a local water reclamation plant employing a dual-membranesystem (UF-RO) and UV disinfection technology to produce highquality reclaimed water. The water samples were collected in 30 Ljerry cans and transported back to the laboratory where it wasstored at 4 ◦C in the darkness until use. The UF filtrate with a com-bined chloramine residual of 2–3 mg/L was subject to isolation andfractionation in the laboratory for subsequent analysis.

2.2. EfOM isolation and fractionation

Four major fractions based on hydrophobicity and charge char-acteristics, namely, aquatic humic substances (AHS), hydrophilicacids (HPIA), hydrophilic bases (HPIB) and hydrophilic neutrals(HPIN) were isolated and fractionated based on the method adaptedby Namour and Muller [23] for the preparation of synthetic solu-

tions. A three-column array of nonionic resin (XAD-8, Supelco,USA), strong cation exchange resin (AGMP-50, Bio-Rad, USA) andweak anion exchange resin (IRA-96, Rohm and Haas, USA) was usedin the fractionation process. AHS fraction was first eluted from theXAD-8 column in reverse direction with 0.1 M NaOH. HPIB and HPIA

brane Science 349 (2010) 75–82 77

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ractions were subsequently eluted from the AGMP-50 and IRA-6 column, respectively, with 1 M NaOH. Finally, the non-absorbedluate in the IRA-96 column was collected as the HPIN fraction. Tonsure the complete adsorption of EfOM components onto resindsorbents, all fractionation procedures were repeated once afterlution of fractionates and resin regeneration.

.3. EPS extraction

EPS was extracted from conventional activated sludge samplessing the steaming treatment. This method was recommended ashe most effective extraction method for activated sludge among

any chemical and/or mechanical methods due to its high extrac-ion efficiency and minimal cellular disruption and has been usedy many researchers [27,28]. Settled sludge flocs were first sepa-ated from supernatant by centrifuging at 2000 × g for 10 min. Afteriscarding the supernatant, the tubes containing the pellets wereopped up with DI water. The contents were blended in a vortexlender at high speed for 1 min to recover the capsule-bound mate-ial. Samples were steamed in an autoclave at 80 ◦C under 1 barressure for 10 min and then centrifuged while still hot at 8000 × g.uring centrifugation, the temperature was reduced to 15 ◦C. The

teaming treatment was used to reduce the disruptive effects onhe cells of boiling or autoclaving under normal autoclave condi-ions (120 ◦C, 16 bar). The supernatant obtained on centrifugationas filtered through 0.22 �m sterile disposable cellulose acetatelters to ensure that samples were free of cells. The EPS yields inhe extracts were represented by DOC, carbohydrate and proteinoncentrations.

.4. Synthetic solution preparation

Synthetic solutions spiked with organic fractionates and EPSxtract were used as feed water to membrane fouling experiments.ackground organic concentrations and solution chemistry wereelected to simulate conditions typical of commercial applications..0 L feed water with a DOC concentration of 5.3 ± 0.2 mg/L wassed to represent EfOM foulants in all fouling experiments, whichas confirmed by measuring the non-purgable organic carbon

NPOC) concentration of feed solution using a total organic car-on (TOC) analyzer (5050A, Shimadzu, Japan). That translated into.3 ± 0.2 g organic foulant (in terms of TOC) available per squareeter of membrane surface area. The total ionic strength was set

t 40 mM by adjusting the amount of NaCl addition, confirmed byeasuring TDS of feed solution at around 1600–1800 mg/L usingconductivity meter. CaCl2 was added at a concentration of 1 mMithout changing the total ionic strength to investigate the effect

f calcium ions on fouling behaviors in some experiments. Feedater pH was maintained constant at 8.0 ± 0.2 in all experiments

y adding 1 mM NaHCO3. All the feed solutions were prepared withilli-Q reagent water.

.5. Bench-scale membrane setup and fouling experiments

Commercially available polyamide thin film composite (PA-TFC)O membranes (AG, GE Infrastructure Water Process and Tech-ologies, USA) were used in this study. This membrane has beenell-characterized and widely used for brackish water applications

29]. Fouling experiments were carried out using a bench-scaleross-flow RO membrane test unit as shown in Fig. 1(a). Thetainless-steel rectangular membrane cell has the channel dimen-

ions of 10 cm long and 5 cm wide with a channel height of 0.2 cm.agnetically stirred feed water was delivered to the membrane cell

sing a high-pressure pump (Hydracell, Wanner Engineering, USA).emperature was maintained at 24 ± 1 ◦C by circulating coolingater through a stainless-steel coil immersed in the feed tank (Nal-

Fig. 1. Schematic diagrams of (a) laboratory-scale cross-flow RO membrane testunit and (b) experiment procedure and assessment of fouling reversibility.

gene, USA). The trans-membrane pressure monitored by a pressuregauge (USG, USA) and cross-flow velocity monitored by a floating-disk flowmeter (Blue-white, USA) were adjusted by using a by-passflow valve and a back-pressure regulator (GO, UK) located at thechannel outlet. Permeate flux was continuously monitored by acomputer-interfaced digital flow meter (Optiflow 1000, Agilent,USA).

The protocol developed for RO fouling experiments is summa-rized in Fig. 1(b). A new membrane specimen was used for eachfouling experiment and the membrane setup was initially equili-brated with deionized (DI) water for 24 h to allow for sufficientmembrane compaction and ensure reproducible flux decline curvesduring subsequent fouling experiments. When a stable flux wasachieved, the pure water flux was measured. Recirculation modefouling experiments (both the permeate and concentrate fully recy-cled) were carried out with synthetic solutions prepared from EfOMfractionate for a duration of 42 h. Hydrodynamic conditions suchas permeation flux and cross-flow velocity have been found to playimportant roles in controlling membrane fouling rate and extent.Therefore, the same initial flux and cross-flow velocity typicalof commercial applications were employed for all fouling exper-iments. Trans-membrane pressure in the range of 170–200 psiachieving an initial permeate flux of 1.0 × 10−5 m/s and a cross-flowvelocity of 0.1 m/s were maintained during the fouling experimentsto observe the resulting flux decline rate and extent. Samples of thefeed and permeate were taken at the start, the end and several pre-set time intervals of the fouling experiment and analyzed for TDSand NPOC to monitor salt and organic rejection efficiencies.

In order to assess the affinity of fouling layer towards the mem-brane or the reversibility of fouling occurred, hydraulic cleaningwith the purpose of removing all loosely or reversibly absorbedfoulants was conducted after each fouling experiment. The mem-

7 mbrane Science 349 (2010) 75–82

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rane unit was flushed with DI water at a higher cross-flowelocity of 0.4 m/s for 10 min without pressurization. The differ-nce between pure water fluxes measured at the initial and end ofhe fouling experiment was first calculated to determine the over-ll flux decline. The ratio of pure water flux recovered from the endux by hydraulic cleaning to the overall flux decline was then cal-ulated to determine the extent of reversible fouling as depicted inq. (1). The ratio of flux difference from the initial pure water fluxfter hydraulic cleaning to the overall flux decline was then calcu-ated to determine the extent of irreversible fouling as depicted inq. (2).

eversible fouling = JH − JEJ0 − JE

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rreversible fouling = J0 − JHJ0 − JE

(2)

here J0 and JE are pure water fluxes measured at the initial and endf the fouling experiment, and JH is the clean water flux measuredfter hydraulic cleaning as shown in Fig. 1(b).

.6. Analytical methods

.6.1. Non-purgable organic carbon and specific ultravioletbsorbance

Secondary effluent prefiltered through 0.035 �m UF mem-ranes was mainly used as feed water samples; the organic contentccentually measured is dissolved organic carbon (DOC) values.on-purgable organic carbon (NPOC) is measured using a TOC ana-

yzer (5050A, Shimadzu, Japan) equipped with an automatic samplenjector. It uses the US EPA approved persulfate oxidation methodor analyzing samples containing 2 �g/L to 10,000 mg/L of organicarbon. Replicate measurements of NPOC samples confirmed aoefficient of variance within 5%. Specific ultraviolet absorbanceSUVA) is often used to indicate the aromaticity or humic contentf an aquatic organic sample. A UV/Visible spectrophotometer (UV-60A, Shimadzu, Japan) was used to measure the UV absorbancet 254 nm (i.e., UVA254). The SUVA value of a water sample wasalculated as the ratio of UVA254 to the NPOC concentration.

.6.2. Apparent molecular weight distributionThe apparent molecular weight distribution of EfOM was deter-

ined by ultrafiltration fractionation according to the methodeveloped by Aiken [30]. EfOM samples were fractionated in paral-

el mode at a constant pressure of 25 psi provided with compresseditrogen with a stirred cell (8010, Amicon, USA). Regenerated cel-

ulose UF membranes (YM series, Millipore, USA) with nominalolecular weight cut-off of 1, 10 and 100 kDa were used.

.6.3. Carbohydrate and protein concentrationsPhenol sulfuric acid method is simple, rapid, and sensitive when

sed for measuring carbohydrate concentration. It is based on therinciple that simple sugars, oligosaccharides, carbohydrates andheir derivatives undergo hot acid hydrolysis prior to a reaction toive the sample a stable orange color that can be detected under90 nm absorbance. The reagents used are inexpensive and stable,iving a permanent color when reacted with the samples [28,31].owry method for protein concentration measurement is based onhe principle that under alkaline conditions the divalent copper ionCu2+) forms a complex with peptide bonds in which it is reducedo a monovalent ion (Cu+), known as the Biuret reaction. Monova-

ent copper ion is then reacted with Folin reagent to produce annstable product that becomes reduced to molybdenum/tungstenlue. This is detectable in the range of 500–750 nm. This method

s only sensitive to low concentrations of protein in the range of–100 mg/L and accurate for narrow pH ranges. Only small volume

Fig. 2. DOC distribution of six EfOM fractions with different hydrophobicity andcharge characteristics (average ± standard deviations determined from triplicatemeasurements).

of sample (0.2 mL) is required which will have little or no effect onpH of the reaction mixture. Absorbance at 650 nm was measuredin this study [28,32].

3. Results and discussions

3.1. Distribution, composition and characteristics of EfOMfractions

Before studying fouling behaviors of the individual EfOM frac-tions and EPS extract, a rigorous characterization of the organiccompounds obtained is essential to ensure the reliability and repro-ducibility of the preparative fractionation or extraction processand obtain information on the composition and physico-chemicalproperties of individual organic fractions. Fig. 2 shows EfOM frac-tionation results based on hydrophobicity and charge properties.In this study, the EfOM present in UF prefiltered secondary effluentwas primarily hydrophilic in nature with the hydrophilic fractionscollectively accounting for 62.9–69.9% of EfOM measured as DOC.The result obtained with the same secondary effluent without UFfiltration was not significantly different (data not shown). Shonet al. [33] reported similar hydrophobic/hydrophilic distributionresult of secondary effluents during summer seasons, while thehydrophobic components were found to dominate during winterseasons. Considering this study was undertaken in a tropical region,it appeared that high hydrophilic ratios of EfOM were related tosome extent to temperate temperature. As organic hydrophobic-ity has been observed as an important factor in determining thestrength of membrane–foulant interactions, this variation in thehydrophobic/hydrophilic distribution of EfOM might have impacton its fouling potential during subsequent RO filtration. Namourand Muller [23] also reported that extended biological treatmentcould result in significantly increased percentage of AHS fraction intreated effluent. Imai et al. [24] observed ozonation nearly doubledthe percentage of HPIA fraction and significantly decreased bothAHS and HPO-N fractions. These results suggested that it might bepossible to modify the fouling potential of treated effluent by alter-ing the hydrophobic/hydrophilic distribution ratios of differentEfOM fractions via changing operating conditions of the preced-ing biological treatment and/or adopting pertinent pretreatmentschemes.

To understand the composition of individual EfOM fractionsand EPS extract at the compound-class level, the concentrationsof carbohydrate and protein, currently identified as two majorgeneric components of EfOM, on unit DOC mass basis and SUVAvalue representing the AHS content or organic aromaticity, weremeasured for each organic fraction. As shown in Fig. 3, EfOM wasfractionated into more homogeneous organic groups with different

characteristics. The HPIN fraction was found to possess the highestconcentration of carbohydrate while exhibiting the lowest SUVAvalue which implied having a rather low percentage of aromaticorganic compounds. The EPS extract exhibited the highest pro-teinaceous content and medium carbohydrate content owing to the

Y. Zhao et al. / Journal of Membran

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ig. 3. Carbohydrate, protein and SUVA distribution of EfOM fractions and EPSxtract.

resence of macromolecular compounds such as proteins, polysac-harides, amino-sugars, nucleic acids, and other cell componentsriginated from microbial metabolic activities. In comparison, thePIA fraction was characterized by a rather low carbohydrate androtein contents but with a high SUVA value. This observationould be reasonably expected as HPIA fraction has been reported toainly comprise of various hydroxy acids with strong hydroxyl and

arboxyl characteristics. However, it was noteworthy that SUVAalue of individual EfOM fractions observed in this study appearedo correlate better with the proteinaceous content rather than theHS content. SUVA has been frequently viewed to mainly associateith AHS indicating the aromaticity of NOM. However, protein-like

ompounds such as aromatic amino acids actually have a muchtronger absorbance at 254 nm owing to the presence of phenolicunctional groups [34]. Therefore, for EfOM which is made up of aariety of structurally complex organic components, SUVA mightot necessarily be a good index for hydrophobic acids such as AHS.

Fig. 4 shows molecular weight distributions of the individualfOM fractions and EPS extract. It can be seen that despite exhibit-ng high polydispersivity, EfOM as a whole represented a groupf small organic molecules with around 80% of the DOC falling inhe <1 kDa molecular weight range. Other researchers also reportedimilar small molecular weight of EfOM remaining in treatedecondary effluent [24]. This phenomenon suggested higher chal-enges to devise an effective strategy for controlling fouling of

astewater reclamation membranes as conventional pretreatmentchemes could hardly deal with such small organic compounds effi-

iently. Except for AHS (which were mostly medium size organicomponents with molecular weight falling in the 1–10 kDa range),lmost all other EfOM fractions were primarily made up of smallrganic molecules. Jucker and Clark [35] reported similar molecular

ig. 4. Molecular weight distributions of EfOM (after UF filtration), effluent fractionsnd EPS extract.

e Science 349 (2010) 75–82 79

size results for hydrophobic humic and fulvic acids, which are themajor constituents of AHS fraction. In contrast, the EPS extract pos-sessed a significantly larger proportion of large macromolecules.The 10–100 and >100 kDa molecular weight groups accounted fora total of 39% of its DOC, whereas for the other organic fractions,not more than 18% of DOC fell in the two molecular weight groupscombined. Therefore, it should be noted that although this categoryof compounds have often been associated with organic fouling ofwastewater reclamation RO membranes, they were not present inlarge quantities in the UF prefiltered secondary effluent used as ROfeed water.

3.2. Membrane fouling by various EfOM fractions

3.2.1. Flux decline rate and extentFouling experiments were performed with the individual EfOM

fractions and EPS extract to evaluate their fouling potential duringRO membrane filtration. Typical flux decline curves as a functionof filtration time are depicted in Fig. 5. Except for the HPIA frac-tion, the other three EfOM fractions and EPS extract resulted indifferent extent of flux decline for the duration of the experiments.The almost unnoticeable flux decline caused by the HPIA fractioncould be reasonably expected considering the physico-chemicalcharacteristics of this fraction. The hydrophilic nature and promi-nently negatively charged acidic groups comprising this fractioncould be more easily repulsed from the similarly charged mem-brane. In comparison, the positively charged HPIB and neutrallycharged HPIN fractions would be less strongly rejected by thenegatively charged membrane and actually were found to resultin higher extent of flux decline. The strongest membrane foulingwas observed to result from the HPIN fraction followed by theEPS extract. Interestingly, significant difference was observed intheir fouling patterns. The HPIN fraction resulted in a nearly con-sistent flux decline rate over the filtration duration, whereas theEPS extract led to a rapid initial flux decline which gradually less-ened with increasing filtration time. This observation indicated EPSmacromolecules were more easily deposited and attached ontothe membrane surface under permeation drag, while the thick-ness of resulting fouling layer might reach a “pseudo-steady” stateearlier than the HPIN fraction. This difference observed in fluxdecline rates might be attributed to the different organics–organicsand organics–membrane interactions of these two fractions dur-ing membrane filtration and the resulting different fouling layerproperties, which would be fundamentally influenced by theirdifferent physico-chemical characteristics. This phenomenon willbe further investigated in the following sections. This “pseudo-steady” state could also be attributed to the depletion of more easilydepositing foulants in the bulk concentrate along with filtrationtime. Depletion of organic foulants, especially the “problematic”foulants which interact more with the membrane surface, is a lim-itation often observed with laboratory-scale recirculation modefouling experiments. However, this phenomenon would not appearin commercial applications when there is a continuous deliveryof foulants towards the membrane surface. Earlier characteriza-tion results showed that the HPIN fraction possessed the highestcontent of carbohydrates among all four major EfOM fractions,whereas the EPS extract contained significantly higher concentra-tions of proteinaceous matter, which was around two times of theother fractions. Therefore, this study suggested that carbohydratespresent in treated effluent affected RO membranes more signifi-cantly than protein-like matters on unit mass basis. Many studies

have reported the important role played by organic colloids ormacromolecules of microbial origin in organic fouling wastewa-ter reclamation membranes [2,9]. This might be due to their easierdeposition onto the membrane surface discussed above. However,considering their relatively lower concentrations in feed water,

80 Y. Zhao et al. / Journal of Membrane Science 349 (2010) 75–82

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ig. 5. Permeate flux as a function of time in membrane fouling experiments fed wnitial flux of 1.0 × 10−5 m/s; cross-flow velocity = 0.1 m/s; temp = 24 ± 1 ◦C. [NaHCOruns repeated for the total of 10 runs, reproducibility is satisfactory.

he actual contribution of these biopolymers to membrane organicouling (i.e., the specific resistance of the fouling layer formed andts reversibility during membrane cleaning) needs to be furthernvestigated. The negatively charged AHS fraction was found toesult in only minor flux decline despite the hydrophobic inter-ctions possibly due to the strong electrostatic repulsions. Hence,he above results suggested a close correlation between the compo-ition and physico-chemical characteristics of EfOM fractions andheir membrane fouling potential.

.2.2. The influence of calcium ionsAs an important water quality parameter, the influence of cal-

ium ions on organic fouling of NF/RO membranes has been studiedy several researchers mainly using model compounds [3–5,19]. Itsresence was observed to aggravate fouling phenomenon caused

y organic acids of both hydrophobic and hydrophilic nature (i.e.,umic acids and alginic acids) and to a lesser extent hydrophilicases (i.e., BSA) supposedly via charge neutralization, complexa-ion and forming calcium bridges. However, opposite effects werelso observed with fatty acids (i.e., octanoic acids) owing to the

ividual EfOM fractions and EPS extract. Operating conditions: �p = 170–200 psi forM, [CaCl2] = 1 mM if added, IS = 40 mM, pH = 8.0 ± 0.2, NPOC = 5.3 ± 0.2 ppm, V = 5 L.

decreasing organic hydrophobicity after calcium addition [5]. Theinfluence of calcium ions on the fouling potential of EfOM com-ponents at typical concentrations of commercial applications wasstudied while keeping pH and total ionic strength of the feed solu-tion constant. As shown in Fig. 5, the individual EfOM fractionsand EPS extract responded differently towards the presence of cal-cium ions. The resulting increase in flux decline rate and extentwas found to follow the order: HPIB∼AHS < EPS∼HPIN < HPIA. Thesignificant increase in the fouling potential of HPIA fraction couldbe reasonably expected, which is due to charge neutralizationand similar to the results obtained with model hydrophilic acidfoulants [3]. However, the lessened fouling caused by AHS and HPIBfractions after calcium addition was beyond expectation. Previousstudies with model compounds have reported calcium addi-tion would aggravate the fouling problem resulting in enhanced

organic deposition on the membrane surface [4,19]. The abnormalobservations obtained in this study might possibly be explainedby the different composition (e.g., functional group and chargedensity) of the organic surrogates and real organic components inreal treated effluent samples as revealed by earlier characteriza-

Y. Zhao et al. / Journal of Membrane Science 349 (2010) 75–82 81

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ig. 6. Relationship between deposited organic carbon mass and extent of fluxecline for EfOM fractions and EPS extract.

ion results. Other possibilities might be the formation of organicggregates which would be easily carried away by shear or changesn hydrophobicity of organic foulants [36]. More advanced foulingayer characterization techniques would be required to unravel thenderlying mechanisms. The presence of calcium ions did not sig-ificantly impact the fouling behavior of EPS extract either, possiblyue to the weak response by predominantly proteinaceous matters observed with the HPIB fraction.

.3. Mass of fouling layer deposited

When RO membranes become fouled, the resulting additionalydraulic resistance to permeate flow would be determined byhe mass (e.g., thickness) and properties of the fouling layer (e.g.,orosity and compressibility) deposited on the membrane sur-ace. Fundamentally, the initial deposition of organic molecules isostulated to be controlled by the interplay of permeation dragowards the membrane surface, shear force carrying the foulantsway from the membrane surface, and the membrane–organicnteractions (e.g., hydrophobic and electrostatic interactions). Thensuing fouling layer growth and the fouling layer properties wouldossibly be governed by the interplay of permeation drag force,hear force and organic–organic interactions. In commercial appli-ations where hydrodynamic conditions are most of the time fixed,hese membrane–organic and organic–organic interactions woulde inferred to be influenced by physico-chemical characteristicsf organic foulants together with solution chemistry. Therefore,hysico-chemical characteristics of EfOM would be expected tolso have an impact on the fouling layer mass and properties.ig. 6 shows the accumulated organic masses (in terms of TOC)uring filtration of individual EfOM fractions and EPS extract andheir correlation with flux decline extent. Interestingly, the organicarbon masses accumulated correlated well with flux declines forost fouling experiments except for the EPS extract. Although the

esulting flux decline extents differed by not more than 20%, thisraction revealed significantly higher (around three times) accumu-ated mass than the other EfOM factions. This finding suggested EPS

acromolecules have greater tendency to attach and deposit ontohe membrane; while for a given mass deposited, they impartedess additional resistance to permeation flow compared with thether EfOM fractions. The lower resistance of the EPS fouling layeright be attributed to their high molecular weight distributions

s shown in Fig. 4, which resulted in a fouling layer of signifi-antly higher porosity. For all EfOM fractions, the impact of theresence of calcium ions on flux decline extent was also reflected

n the accumulated mass. However, nearly unchanged flux declineas observed along with increased EPS accumulation after calcium

Fig. 7. Ratio of reversible and irreversible fouling after hydraulic cleaning.

addition. EPS molecules might interact with calcium ions more eas-ily compared with small EfOM molecules, while more in-depthcharacterization of the fouling layer is needed to corroborate thisfinding.

3.4. Affinity of fouling layer with the membrane

When studying membrane fouling and devising pertinentfouling control strategies, the affinity of fouling layer with themembrane is also an important aspect to consider as it will deter-mine fouling reversibility during cleaning cycles and thus thelong-term sustainability of membrane operations. Previous stud-ies on organic fouling of NF/RO membranes by NOM showed thatthe inner layer of organic molecules adsorbed onto the membranemore tightly via chemical adsorption and could only be removedvia chemical cleaning—the so-called “irreversible fouling”. In con-trast, the outer layer of organic molecules physically attached tothe membrane surface loosely and could be displaced via hydrauliccleaning—the so-called “reversible fouling”. Reversible fouling, theextent of membrane permeability restorable after hydraulic clean-ing, was measured to assess the affinity of fouling layer developedafter filtration of the individual EfOM fractions and EPS extract.As shown in Fig. 7, a much stronger affinity with the membranewas observed with hydrophilic carbohydrates which resulted inthe highest flux decline also. Interestingly, although EPS macro-molecules tended to attach and deposited on the membrane surfacein large quantities, their cake layer exhibited a much lower affinitytowards the membrane and also constituted a lower specific resis-tance. The porous fouling layer formed by EPS macromolecules andthe organic aggregates might be more susceptible to high shearforce during hydraulic cleaning. The application of biological filtra-tion as pretreatment to RO membranes has been cautioned becauseof potential carrying over of EPS macromolecules to downstreamRO processes. However, the findings of this study suggested thatthese biopolymers possibly constituted a much lesser long-termfouling threat than small hydrophilic carbohydrates if the poten-tial biofouling problem associated could be safely controlled. Alongwith flux decline results, this study suggested that it might be nec-essary to develop fouling control strategies specifically targeting athydrophilic neutral organic compounds in order to more efficientlycontrol the fouling of wastewater reclamation RO membranes.

4. Conclusions

Well-controlled laboratory-scale fouling experiments wereconducted to investigate the correlation between the physico-chemical properties of EfOM, represented by its fractional andfunctional components and their membrane fouling behaviors in

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erms of flux decline and fouling reversibility. Four EfOM frac-ions were isolated and fractionated from UF prefiltered treatedffluent in a tropical area wastewater reclamation plant and EPSere extracted from the biological treatment stage. Individual

fOM fractions and EPS extract exhibited distinct physico-chemicalroperties in terms of carbohydrate and protein concentrations,romaticity and molecular weight distribution. Organic foul-ng behavior of EfOM was observed to closely correlate to theomposition and physico-chemical properties of its components.oth hydrophilic in nature and strongly negatively charged, theydrophilic acid fraction resulted in minimal flux decline. Regard-

ess of the presence of calcium ions, the highest flux decline extentas observed with the hydrophilic neutral fraction mainly com-osed of small carbohydrates followed by the EPS extract. Therganic mass accumulated on the membrane surface was foundo correlate well with the flux decline except for the EPS extract.lthough EPS macromolecules showed a prominently higher depo-ition tendency onto the membrane surface, its fouling layerisplayed a much lesser specific resistance and affinity to the mem-rane as compared to the other EfOM fractions. Therefore, it woulde expected to impose a weaker fouling threat for long-term ROperation. The results obtained in this study suggested that foul-ng phenomenon of RO membranes treating wastewater mighte mitigated by reducing the concentrations of small hydrophiliceutral organics. The required changes in the composition orhysico-chemical properties of EfOM might be accomplished by

mplementing pertinent pretreatment or modifying operationalonditions of the preceding biological treatment.

cknowledgement

The authors would like to acknowledge the financial support byhe National University of Singapore through project R-264-000-10-112.

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