The effects of natural organic matter (NOM) fractions on fouling characteristics and flux recovery...

$: The effects of natural organic matter (NOM) fractions on fouling characteristics and flux recovery of ultrafiltration membranes$
*Corresponding author.

The effects of natural organic matter (NOM) fractions onfouling characteristics and flux recovery of ultrafiltration

membranes

A.W. Zularisama,b,c, A.F. Ismaila*, M.R. Salimc, Mimi Sakinaha,b, H. Ozakid

aMembrane Research Unit, Universiti Teknologi Malaysia, 81310 Skudai, Johor, MalaysiaTel. +60 (7) 553-5592; Fax +60 (7) 558-1463; email: [email protected]

bCentre for Advanced Water & Wastewater Scientific Research, Faculty of Civil and Environmental Engineering, KolejUniversiti Kejuruteraan and Teknologi Malaysia, KUKTEM, Gambang, Pahang, Malaysia

cFaculty of Civil Engineering, Universiti Teknologi Malaysia, Skudai, Johor, MalaysiadDepartment of Civil Engineering, Faculty of Engineering Osaka Sangyo University, Japan

Received 21 April 2006; accepted 16 October 2006

Abstract

Natural organic matter (NOM) which is a complex mix of particulate and soluble materials in surface water hasbeen identified and reported by previous studies as responsible for membrane fouling. However the component ofNOM which primarily causes the fouling problem is still not well understood, especially relating to the specificfraction that is mainly responsible for flux decline. Therefore, the main objective of this study is to gain a betterunderstanding on the ultrafiltration membrane fouling characteristics due to fractionated NOMs. The 68 kDa MWCOpolysulfone (PSF) and 50 kDa MWCO cellulose acetate (CA) membranes formed by a simple phase inversiontechnique were employed, and the experiments were carried out in a submerged membrane configuration. The UluPontian River water was fractionated into several components which were hydrophobic (HPO), transphilic (TPI)and hydrophilic (HPI) fractions using DAX-8 and XAD-4 ion exchange resins. The experimental results showedthat the HPI component of NOM exhibited the worst flux decline despite lesser DOC and UV254 rejections comparedto other fractions (HPO and TPI). On the other hand, the HPO fraction showed the highest DOC and UV254 removalsdespite possessing lower organics concentration than HPI fraction. Therefore the degree of flux decline is notnecessarily proportional with the DOC and UV254 removals and in fact the degree of DOC and UV254 rejections arerelatively independent of the amount of organics in the source water. Thus, it is worth to note that rejection andfouling are not greatly dependent on the SUVA and DOC concentrations but mostly on the rejection mechanismresulted from the membrane-feed interactions. The hydraulic resistance in series assessment elucidated that theconcentration polarization, adsorptive fouling and cake layer deposition were the dominant fouling mechanismsfor HPO, HPI and TPI fractions, respectively. Furthermore the effect of cleaning solutions on flux recovery was

Desalination 212 (2007) 191–208

doi:10.1016/j.desal.2006.10.0100011-9164/07/$– See front matter © 2007 Elsevier B.V. All rights reserved.

192 A.W. Zularisam et al. / Desalination 212 (2007) 191–208

found significant in nearly restoring the initial membrane permeability. Surprisingly, the HPI fraction was found toexhibit better flux recovery than HPO fraction despite its higher adsorption resistance (Ra). Therefore, in futureapplications, the type of fouling resistance and the dominant fouling mechanism are useful tools which can beemployed as quantitative measurements for flux recovery and cleaning techniques effectiveness.

Keywords: NOM; Fouling; Ultrafiltration; Flux recovery; Drinking water

1. Introduction

Excellent water quality produced by mem-brane filtration has made this advanced technol-ogy a promising process in providing better drink-ing water for water supply. Membrane filtrationprocesses involving microfiltration (MF), ultra-filtration (UF), nanofiltration (NF) and reverseosmosis (RO) in potable water production haveincreased rapidly for the past decade and wouldpotentially replace the conventional treatmentprocess trains which consist of ozonation–precipi-tation–coagulation–flocculation–chlorination–gravel filtration [1]. Besides that, membrane fil-tration process does offer extra advantages overconventional treatment such as small footprint,compact module and capable of handling widefluctuations in feed quality. MF and UF are em-ployed to remove microparticles and macromol-ecules, which generally include inorganic par-ticles, organic colloidal (i.e. microorganism) anddissolved organic matters (DOM). DOM is ubiq-uitous in natural surface water and often reclaimed[2] as an important factor for both the reversibleand irreversible fouling in water filtration. Un-fortunately, the presence of free chlorine contentthat is used as a disinfectant in the conventionaltreatment is found to react with this residual DOMconsisting in the natural organic matter (NOM).This reaction process has been found to have atendency to form disinfection by-products orDBPs [3–7] such as trihalomethanes (THMs),haloacetics (HAAs) and other halogenated organ-ics. Direct exposure to DBPs can cause cancersand nervous system complications as it is carci-nogenic [8].

Progressive reduction of membrane flux dur-ing surface water membrane treatment imposes aserious constraint for employing this technologyas a substitute for conventional treatment. This isdue to the impact of membrane fouling that leadsto an increase of hydraulic resistance and a re-duction in filtration efficiency. In the case of drink-ing water filtration, the colloidal material mayreduce the membrane permeability (reversiblefouling) by accumulating solutes on the adjacentmembrane surface (concentration polarization),precipitating and forming a cake or gel layer onthe membrane surface as they becomes supersatu-rated [9]. While the dissolved organic materialsmay cause irreversible fouling by altering the ef-fective membrane pores through precipitating andadsorbing onto the membrane matrix and withinthe pores space. Membrane fouling is dependenton several parameters such as membrane charac-teristics, source water properties, operational con-ditions and solution chemistry. Among these pa-rameters, a complex natural organic matter(NOM) contained in source water has beenclaimed by many researchers [10,11] to play a vitalrole as the primary membrane foulant. NOM is amix of particulate and soluble components of bothinorganic and organic origin that vary from onesource to another [12]. Besides that NOM is alsoa heterogeneous mixture with wide ranges inmolecular weight (MW) and functional groups(phenolic, hydroxyl, carbonyl groups and car-boxylic acid) and is formed by allochthonous in-put such as terrestrial, vegetative debris and au-tochthonous such as algae. Natural organic mat-ters that occur in natural brown water are polyphe-nolic molecules with MW ranging from 5000 to

A.W. Zularisam et al. / Desalination 212 (2007) 191–208 193

50000 Da [13]. In particular, NOM can be frac-tionated into three segments: the hydrophobicfraction (HPO), hydrophilic (HPI) and transphilic(TPI). The hydrophobic fraction represents almost50% of dissolved organic carbon (Fig. 1) withlarger MW. The hydrophilic fraction composes25–40% of dissolved organic carbon (DOC) withlower MW (polysaccharides, amino acids, pro-tein and etc.) and operationally defined as non-humic fraction. The transphilic fraction comprisesapproximately 25% of DOC in natural water butwith MW in between hydrophobic and hydro-philic fractions. A major fraction of NOM arisesfrom humic substances and is reported to repre-sent up to 60–70% of TOC in soils and 60–90%

Hydrophobic 49%

Hydrophilic 30%

Hydrocarbon 1%Amino Acids 3%Carbohydrates

10%Carboxylic Acids 7%

of DOC in most natural waters. Fig. 2 shows amodel of humic acid molecular structures.

The hydrophilic components were thought toimpact water quality less than the humic fraction,however recent studies done by Lin et al. [16] andCaroll et al. [9] have revealed that the non-humicfraction of NOM (hydrophilic and neutrals) ma-terials were responsible in determining the rateand extent of flux decline. Carroll et al. [9] per-formed MF of a hydrophobic hollow-fiber mem-brane with a single water source and concludedthat the major cause of fouling was due to thehydrophilic neutral, and not the humic substances.Fan et al. [17] reported the order of fouling po-tential of NOM fraction as hydrophilic neutral >

Fig. 1. Fraction of NOM in surface water based on DOC [14].

Fig. 2. Schematic of humic acid model structure [15].


hydrophobic acids > transphilic acids > hydro-philic charged. This finding was well supportedby a study carried out by Lee et al. [18] who foundthat polysaccharides and protein that have lowerUV to HPSEC-DOC/UV response, to significantlyfoul their low pressure (MF/UF) membrane.Polysaccharides are hydrophilic and possess neu-tral character that can cause adsorption on acharged hydrophobic UF membrane [19]. Theauthors suggested that the large neutral NOM frac-tions were the prime foulants rather than humicsubstances. Speth et al. [20] in their study alsofound hydrophilic neutrals fouled more than hy-drophobic acids. It can be reasoned due to bulkymacromolecular shape and neutral character ofpolysaccharides that make it prone to foul andadsorb on membrane surface. Lin et al. [16] per-formed a study on the effect of fractionated NOMonto a negatively charged UF membrane and ob-served that both large-sized molecules of hydro-phobic and hydrophilic NOM components causedworse flux decline. Besides that, Jarusutthirak etal. [21] in their study on the effect of effluent or-ganic matter for UF membranes also found thehydrophilic fraction to induce the highest foul-ing. They found that the high molecular weightof the hydrophilic component was responsible asthe prime contributor to NOM fouling. It can beclaimed that NOM fouling was a result of lowUV absorbing compounds and high molecularweight hydrophilic components that occurredthrough adsorption mechanisms. In this study theinfluence of NOM components characteristics onthe fouling of a submerged ultrafiltration mem-brane and fouling mechanisms were investigated.The potential foulants of NOM isolates whichwere detrimental to the employed submerged UFmembrane were studied and identified by isolat-ing the chosen surface water using non-ionicmacro-porous ion-exchange resins. The perfor-mance of the membrane and the associated foul-ing mechanisms were examined based on NOMisolates rejection, flux decline and hydraulic se-ries resistance. The effects of hydraulic and chemi-

cal cleaning were also investigated in order toevaluate their effectiveness on both the revers-ible and irreversible fouling as well as the mem-brane flux recovery.

2. Materials and methods

2.1. Fabrication of a UF membrane

The polysulfone polymer was supplied byAmoco Performance Product Inc, the additivepolymer of polyvinyl-pyrolidone PVP3000 wasbought from Fluka Milwaukee and the N-dimethylacetamide (DMAC), which was used assolvent, was purchased from Merck Darmstadt,Germany. On the other hand, the materials forfabricating the cellulose acetate membrane werecellulose acetate polymer (Aldrich Chemical),formamide (Merck, Germany) and acetone (FisonScientific Equipment). The polymers were firstdried in an oven to remove moisture contents. Theadditive, solvent and polymer were sequentiallymixed in a container until a homogenous mixturewas achieved at a constant operational tempera-ture of 60ºC. The formulated dope was spun byusing a forced convective dry/wet spinning tech-nique under pressurized nitrogen gas in the drygap as shown in Fig. 3 [22].

The dope was smoothly pumped to the spin-neret by a 30 Wt gear pump motor at 0.3 cm3/revand with dope extrusion rates (DERs) of3.5 cm3/min. Bore fluid of deionized water washydraulically injected into the spinneret at a con-stant flowrate (1.17 cm3/min) by an ISCO 500Dsyringe pump. Upon extrusion from the spinneret,the pre-nascent membrane passed through cylin-drical hollow perspex (forced convective tube)before entering the coagulation bath. The hollow-fiber filament was mechanically collected by awind-up drum with an applied jet stretch ratioconstantly maintained at one. The as-spun hol-low (HF) fibers were then rinsed thoroughly withwater to remove residual solvent and dried at roomtemperature prior to usage. The hollow-fiber


Fig. 3. Schematic diagram of a hollow-fiber spinning rig : 1) nitrogen cylinder; 2) dope reservoir 3) gear pump; 4) on-linefilter, 7 mm; 5) syringe pump; 6) spinneret; 7) forced convective tube; 8) roller; 9) wind-up drum; 10) refrigeration; 11)coagulation bath; 12) washing/treatment bath; 13) wind-up bath; 14) schematic spinneret.

membrane characteristics are shown in Table 1.The zeta potential of the membrane surface hasbeen precisely determined by using the stream-ing potential technique (Fig. 4). The HF mem-branes surface area was 0.11 m2 (potted bundle of200 filaments) and filtration occurred from theoutside to the inside of the fiber with permeatebeing collected through the membrane’s lumen.

Table 1Characteristics of the experimental membranes

Parameter MRUTM55 MRUTM66 Membrane type Hollow fiber Hollow fiber Membrane material Polysulfone Cellulose acetate External diameter, µm 600 600 Internal diameter, µm 300 300 Contact angle 56° 28° Zeta potential (mV @ pH 7) –27 –15.4 MWCO, kDa 68 50 Pure water flux; Jpwf at TMP of 250 mm Hg, L. m–2 h–1 13.7 25.3 Pure water permeability, L m–2.h–1.bar–1 43 ± 5 85 ± 7

2.2. Surface water quality

The surface water used in this study was takenfrom Ulu Pontian River at Kampung MelayuRaya, Pontian Kecil, Johor, Malaysia. The sourcewater was found to be soft and rich in NOM. Thesurface water characteristics are summarized asin Table 2. The raw water was prefiltered with a


Fig. 4. Zeta potential curves of PSF and CA membranes by streaming potential.

0.45 µm filter (Whatman GFC filter paper) to re-move particulate material and then fractionatedaccording to the procedure used by Caroll et al.[9] and Gray et al. [23]. The ion-exchange frac-tionation procedure was used to isolate the sur-face water into different components of NOM. Thesurface water was fractionated into a hydropho-bic (HPO) fraction which is Supelite DAX-8(Sigma-Aldrich) adsorbable, transphilic (TPI)which is Amberlite XAD-4 adsorbable and hy-drophilic (HPI) components which pass throughboth the DAX-8 and XAD-4 resins without anyadsorption.

Both the DAX-8 and XAD-4 were initiallywashed with methanol and deionized water prior

-35

-30

-25

-20

-15

-10

-5

0

5

0 1 2 3 4 5 6 7 8 9 10 11

pH

zeta

-po

tential (m

V)

CA

PSF

Table 2Surface water characteristics after prefiltration process

Source Ulu Pontian River Coordinate N1º 35.541′, E103º 30.704′ DOC, mg/L 6.8 UV254, cm–1 0.088 SUVA, L/mg.m 1.29 pH 7.03 Conductivity, µS/cm 37 Ca2+, mg/L 4.2 Turbidity, NTU 5.9

to fractionation works. The surface water pH wasadjusted to 2 before fed onto the DAX-8 columnat the rate of 5 ml/min [16] and 15 ml/min ontoXAD-4 resin. The DAX-8 is a non-functionalisedresin that retains strongly hydrophobic organicmatter attributed to humic substances while theXAD-4 retains weakly hydrophobic known as TPI[24]. The result of fractionation is shown inTable 3. The HPO and TPI components were col-lected after eluting the resins with NaOH. Thisfractionation technique was carried out based onDOC and mass balances measurements. The pH,Ca2+ content and ionic strength of fractionatedNOMs were adjusted to the approximate valuesof raw water since the fractionation procedureresulted in variations of these parameters. Thismodification procedure was undertaken in orderto minimize their effect during the comparison ofthe membrane performance. Solution pH, ionicstrength and Ca2+ concentrations were adjusted

Table 3DOC concentration of Ulu Pontian NOM fractions

Fraction DOC (mg/L) DOC (%) Hydrophobic (HPO) 2.38 35 Transphilic (TPI) 1.7 25 Hydrophilic (HPI) 2.6 38


using 1 mol/L NaOH, 1 mol/L NaCl, 1 mol/LCaCl2 and 1 mol/L HCL of analytical grade.

2.3. Membrane filtration operation and resistancein series

An ultrafiltration (UF) submerged reactor wasused throughout this experiment and the experi-mental set-up is illustrated in Fig. 5. The feedwater was pumped out through the HF lumen un-der negative pressure and was collected in a per-meate vessel. The feed pressure was monitoredby a vacuum gauge meter (Wika, Germany). Ul-trafiltration experiments were carried out on thethree fractions of Ulu Pontian River. Prior to fil-tration, the clean hollow-fiber (HF) membranewas initially soaked in deionized water (DI) for24 h before exposing to filtration solutions. Atthe preliminary filtration stage, the membrane wasfirst compacted with DI water for 30 min in orderto ensure total solvent removal from the mem-

Fig. 5. Schematic diagram of the submerged UF membrane reactor.

Pump

Measuring cylinder

Membrane

Backwashing

Reactor tank

Vaccum gauge

Back-wash pump

Feed tank

Caustic acidic solutions

Valve Flow meter

Drain

Permeate

brane. The solvent was assumed to be present inthe membrane due to inefficient drying work dur-ing the membrane manufacturing process. Themembrane resistance (Rm) was measured usingpure water flux (Jpwf) at several transmembranepressures (TMP; from 70 mm Hg to 480 mm Hg)according to Darcy’s law [Eq. (1)], where J is thepermeate flux (m3.m–2 s–1), ∆P is the transmem-brane pressure (Pa), µ is the dynamic viscosity(Pa.s) and Rm is the intrinsic membrane resistance(m–1). The pure water flux (Jpwf) was carried outwith membrane being filtered with deionizedwater (DI) until permeation reached a steady value(conducted under steady-state flow) and was de-termined as in Eq. (2) [25], where the Jpwf is thepure water flux, V is the permeate volume (L), Ais the membrane area (m2) and the “T is the sam-pling time. The Rm was then determined from theslope of linear relationship of water flux (J) vs.∆P [26].


m

PJR

∆=µ

(1)

pwfVJ

A T=

∆(2)

After the compaction and Rm determinationprocesses, the first pure water flux (Jpwf1) for re-sistance in series measurement was introduced atoperational transmembrane pressure of 250 mmHg until the pure water permeability was stabi-lized. Then the DI water permeation was replacedwith a NOM fractional source at the same TMPof Jpwf1. In particular the operational TMP for Jpwf1,NOM fraction filtrations, Jpwf2, Jpwf3 and Jpwf4 wereconstantly maintained at 250 mm Hg except forthe TMP of backflushed process which was higherthan this value. A 13.1 L feed solution was em-ployed to provide a continuous supply in eachNOM fraction filtration experiment. The opera-tional flux and flux decline were regularly mea-sured throughout the filtration experiment. Sam-plings for the UV254 and DOC rejection were pe-riodically taken until permeation reached a steady-state condition (permeate rate reached a constantvalue). The system was then replaced with DIwater to remove the concentration polarization(CP) layer and a second Jpwf2 was measured. Thesystem was then backflushed thoroughly with DIwater at a flowrate 2.5 times higher than the ini-tial NOM permeate flowrate. This procedure wasdone in order to ensure total removal of revers-ible cake deposition and gel layers from the mem-brane surface and pores. The rinsed membranewas then used for a third Jpwf3 measurement and adifference between Jpwf3 and Jpwf2 was used to de-termine the hydraulic resistance of cake or gellayer (Rc). The membrane was then taken out ofthe reactor and its irreversible foulants were ex-tracted with 0.1 N NaOH in ultrasonic bath at 45°C(Branson 3510) for 3 h and 0.1 N HCl (3 h) solu-tions, respectively [27]. The latter procedure wasto remove the adsorption layer which caused the

weak adsorption resistance (Rwa). After chemicalcleaning process, the next fourth Jpwf4 was takento complete and quantify the measurement param-eters in the resistance-in-series model [28–30].The above flux measurement results were used tocalculate parameters in the resistances-in-seriesmodel [Eq. (3)] where J is the permeate flux(m3.m–2.s–1), ∆P is the transmembrane pressure(Pa), µ is the dynamic viscosity (Pa.s), Rm is theintrinsic membrane resistance (m–1), Rcp is the con-centration polarization resistance (m–1), Rc is thecake layer resistance (m–1), Rwa is the weak ad-sorption resistance (m–1) and Rsa (m

–1) is the strongadsorption resistance which could not be revers-ible by both the hydraulic and chemical cleaning.The osmotic pressure differential across the mem-brane was assumed to be zero as ultrafilters rejecta negligible fraction of inorganic ions in solution[31].

( )m cp c wa sa

PJR R R R R

∆=µ + + + +

(3)

Parameters used to quantify the efficiency ofmembrane processes are flux (J) and solute rejec-tion (R) where the flux is defined as

QJA

= (4)

where Q is the permeate flowrate (L h–1) and A isthe membrane area (m2) and the rejection (R) as

1 p

f

CR

C= − (5)

where Cp is the permeate concentration in mg/Land Cf is the feed concentration in mg/L.

2.4. Analytical methods

UV254 absorbance spectra were recorded withPerkin Elmer Lamda 25 UV-visible spectropho-


tometer at 254 nm. UV254 absorbance is specifi-cally sensitive to the aromatic components and isan indicator of the humic substances presence[23]. Dissolved organic carbon (DOC) was deter-mined through HACH direct method 10129 us-ing DR 4000 spectrophotometer. The SUVA valuewas calculated by the ratio of UV254 to DOC andis a measurement of relative aromaticity of hu-mic substances in surface water. Conductivity(µs/cm) and pH values were measured usingWTW LF330 and Orion ATI pH meter respec-tively. Calcium and sodium concentrations weremeasured by inductively-coupled plasma massspectroscopy Elan 6000 (ICP-MS). Determinationof apparent molecular weight distribution (AMWD)was carried out using the UF fractionation tech-nique filtration protocol by employing a series ofUF membranes (Ultracel, Milipore). The NOMfractions were isolated into various size compo-nents by an ultrafiltration membrane with theMWCOs of 1 kDa, 5 kDa, 10 kDa and 50 kDa.

3. Results and discussion

3.1. Effects of NOM fractions on fouling charac-teristics

The Ulu Pontian River exhibited a relativelylow hydrophobic fraction concentration and there-fore can be classified as a hydrophilic surfacewater type (Table 3). This was consistent with itslow SUVA value (1.29 L/mg.m) and commonlysurface water with SUVA less than 3 Lm–1mg–1 isclassified as low-humic water [16]. The normal-ized permeate fluxes for the three fractions areshown in Fig. 6. All the three fractions showedobvious flux decline the moment they were fil-tered and the fouling that happened in these caseswas mostly attributed to the dissolved NOM asthey were all pre-filtered with a 0.45 µm filter.The HPI fraction was found to have the most sig-nificance fouling potential than other fractions andthis finding could be taken as the most outstand-ing phenomena observed in this study. The detri-

mental effect of the HPI fraction to membranepermeability was even more pronounced as itshowed greater flux decline and higher foulingrate than HPO and TPI components. However, thisfinding was found to contrast with the results ob-tained by Nilson and DiGiano [10], where in theirstudies the HPI components showed less foulingeffect than the humic fraction. Nevertheless theresult presented here showed that the HPI exhib-ited the highest flux decline by approximately52% of the initial flux after 120 min of operation.

On the other hand, both HPO and TPI fluxdeclines were only 35% and 20% in spite of hav-ing the same filtration duration. Thus, it is worthnoting that the HPI fraction was the prime foulantin NOM and was mainly responsible for foulingand major flux decline in the surface water filtra-tion. This could be reasoned from the low UVabsorbing compound and high molecular weightcomponents such as polysaccharides or polysac-charides types contained in the HPI fraction [18].Polysaccharides are aldehyde derivates of highpolyhydric alcohols which have neutral charac-ters that can cause adsorption on a hydrophobicUF membrane. Moreover, as the polysaccharides(hydrophilic neutral) have a linear macromolecu-lar shape and neutral charge characteristic, theyeasily foul and adsorb onto the membrane sur-face. This finding was well supported by the stud-ies carried out in [9,18], where they also foundthe hydrophilic component to cause more foulingpotential than humic substances. Although the fluxresults showed the HPI fraction as the most detri-mental foulant and exerted the highest flux de-cline but yet its DOC removal was the lowestamong other fractions (Fig. 7).

In this case (HPI fraction), the common sce-nario, whereby high flux decline would corre-spond to better rejection, did not occur. In fact, itseems that the flux decline is not proportionallylinear with the DOC removal and this case waseven worse when the UV254 removal was also theleast. So apparently, the HPI fraction has not onlythe most significant fouling potential but also


Fig. 6. Flux profile of Ulu Pontian River fractions by MRUTM55 membrane.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 15 30 45 60 75 90 105 120 135 150 165 180Tim e (m in)

Nor

mal

ized

Flu

x

HPO

HPITPI

Fig. 7. UV254 and DOC removal (%) of hydrophobic, transphilic and hydrophilic fractions by MRUTM55 membrane.

would result in the worst membrane fouling (fluxdecline) and the lowest NOM removal (DOC andUV254 rejections). On the other hand, the HPOfraction was found to be better rejected than otherfractions in terms of DOC and UV254 although ithas a lower organic concentration than the HPIfraction. Hence, this study also shows that thedegree of DOC and UV254 removal is not associ-ated with the amount of organics possessed by

the source water or necessarily proportional to theflux decline. This phenomenon is presumed tooccur due to the electrostatic interaction betweenthe HPO fraction and the negative surface chargeof the PSF membrane (–27 mV). The HPO frac-tion contains mainly humic acid and fulvic acidthat are anionic polyelectrolytes with negativelycharged carboxylic (COOH–), methoxyl carbon-yls (C=O) and phenolic (OH–) groups [32]. There-

05

1015202530354045505560657075808590

HPO HPI TPIUlu Pontian river fractions

UV25

4 re

mov

al %

DOC

rem

oval

%

UV Removal%

DOC Removal %


Fig. 8. Comparison of DOC removal (%) between MRUTM55 and MRUTM66 membrane on Ulu Pontian River frac-tions.

fore lower flux decline or higher HPO fractionrejection by the PSF membrane was apparentlydue to the charge repulsion between negative func-tional groups of HPO and negative surface chargeof the PSF membrane. In addition, the results ofFig. 7 also show that the UV254 removal was al-ways higher than DOC rejection for all the threefractions. This could be again due to the tendencyor preference of the polysulfone membrane to re-pulse the high aromatic compounds possessed bythe HPO fraction compared to other fractions andthat it was shown high by the percentage of UVremoval, meanwhile the DOC is not a specificmeasurement of certain compound but a total car-bon concentration comprised by the three frac-tions and therefore resulted in lower percentageremoval than the UV254. The effect of membranecharacteristics on NOM rejection was further stud-ied with MRUTM66 (CA) membrane. This cel-lulose acetate membrane has lower MWCO (50kDa), more hydrophilic and less negative charge(Table 1) compared to MRUTM55 (PSF) mem-brane. The synthesized CA membrane has beencharacterized and proven to possess zeta poten-tial of –15.4 mV at pH 7, which was distinctivelyless in negative charge than the PSF membrane(–27 mV at pH 7). In fact, the contact angle (in-

49

14

31

34

22

27

0 10 20 30 40 50 60 70 80 90

HPO

HPI

TPI

Frac

tiona

l of

Ulu

Pon

tian

river

DOC rem oval (%)

MRUTM55 (PSF)MRUTM66 (CA)

dex of hydrophobicity) of the CA membrane wasfound to be relatively lower (28°) than that of thePSF membrane (56°). From Fig. 8 it is clearlyseen that MRUTM66 membrane resulted in lesserDOC (%) rejection despite its lower MWCO thanthat of MRUTM55 membrane. This scenario isagain presumably related to the shape, charge andsize effects of NOM components towards the re-jection mechanism of MRUTM66 membrane. Ingeneral, MRUTM66 increased the DOC rejection(%) of the HPI fraction but reduced both the HPOand TPI fractions when compared with MRUTM55.The HPI fraction removal (in terms of % DOCrejection) by MRUTM55 was 14% but increasedto 22% when introduced to MRUTM66. How-ever the overall DOC removal was still dominatedby the HPO fraction followed by the TPI and HPIfractions. Better removal in the HPI fraction couldbe explained by the lower MWCO (50 kDa) pos-sessed by MRUTM66 membrane, whereas higherrejection of HPO and TPI compared to the HPIcomponent was due to a higher intrinsic molecu-lar weight (MW) possessed by the fractions (HPOand TPI). Both the HPO and TPI fractions hadbeen widely reported to have greater MW [33] orhigher relative molecular mass [34] than the HPIfraction by previous researchers. Furthermore, the


HPO and TPI fractions have more complex mo-lecular structures (aromatic shape) than the HPIfraction (aliphatic structure) that made them easierto be excluded by MRUTM66 membrane. Dur-ing filtration with MRUTM66 most of the HPOfraction was successfully passing through thepores as the membrane was negatively less incharge and the effect of electrostatic interactionbetween the functional groups of HPO andMRUTM66 (cellulose acetate) was no longer thedominant mechanism. Thus it is worth to note herethat the rejection mechanism imposed byMRUTM66 membrane was a pore size (steric)exclusion, while MRUTM55 was an electrostaticrepulsion mechanism. Based on the DOC removal(%) imposed by the three Ulu Pontian fractions,

Table 4Dominant rejection mechanism on NOM fraction with different membrane (from most dominant to less dominant)

Membrane characteristics Rejection mechanism i) Negatively charged and hydrophobic membrane such

as PSF membrane Electrostatic interaction >Hydrophobicity > Steric exclusion

ii) Less negatively charged and hydrophilic membrane such as CA membrane

Steric exclusion > Electrostatic interaction; Hydrophobicity

Fig. 9. Apparent molecular weight distribution (AMWD) of Ulu Pontian NOM fractions

the order of dominant rejection mechanisms isshown in Table 4.

The explanation on the effect of NOM frac-tion on MRUTM55 membrane fouling was fur-ther studied from the molecular weight distribu-tion of the three NOM fractions (Fig. 9). In gen-eral, all fractions showed molecular weight domi-nation of less than 1 kDa. However, there was arelatively higher HPI distribution compared toother fractions at the molecular weight lower than1 kDa whereas the HPO fraction exhibited highercontent of high molecular weight above 30 kDa.Thus, the substantial flux decline caused by theHPI fraction (Fig. 6) was shown to be respon-sible for the low molecular weight (<1 kDa) ofthe HPI component. This result was well sup-


ported by the study carried by Carrol et al. [9]who reported that the smaller size hydrophilicneutral was responsible in the rate and extent offouling the microfiltration membrane.

3.2. Resistance in series of NOM fractions

The flux decline by NOM fractions was fur-ther quantitatively studied through resistance inseries model [Eq. (3)]. The explanation of the fluxdecline caused by Ulu Pontian fractions was madeby correlating their flux decline with the incurredfouling mechanisms and hydraulic resistances(Table 5). There seems to appear a relationshipbetween the high fouling potential of the HPI frac-tion and its high irreversible resistance (Ra). It isworth to note that the HPI fraction exhibited thehighest Ra value and subsequently its flux declinewas more pronounced than that of the other NOMfractions. Thus this severe flux decline was mostlyattributed to the adsorption fouling mechanismof the non-humic fraction (HPI) which occurredduring membrane filtration. As a matter of fact,the HPI fraction was found to possess muchgreater Rsa than HPO and TPI. However the Rsa ofall NOM fractions was relatively lower than otherresistances thus suggesting that the flux declinewas mostly related to NOM components whichwere reversible to chemical cleaning. The HPIfraction was also found to show the highest weakadsorption resistance (Rwa) and this phenomenoncould be the answer to the highest flux declinecaused by the adsorption fouling mechanism. This

Table 5Percentage of fouling resistance to total resistance (RT) by NOM fractional components of Ulu Pontian River usingMRUTM55 membrane

*Ra is the adsorption resistance Ra (%) = Rwa (%) + Rsa (%)

Fraction Rm (%) Rcp (%) Rc (%) Ra (%)* Rwa (%) Rsa (%) HPO 59 22 3 16 15.6 0.4 HPI 55 1.6 12.5 30.9 27.32 3.58 TPI 65 9 19 7 7 0

adsorption fouling mechanism can be describedby saturation and precipitation processes of dis-solved organics in the membrane pores; in thiscase it can be compounds derived from polysac-charide (neutral) and amino sugar [35]. This HPIfraction was expected to have the capability ofpassing through the membrane pores (HPI has analiphatic structure) and not being able to be ex-cluded by the steric exclusion mechanism as itwas relatively smaller than the membrane MWCO(68 kDa).

The normal molecular mass of the HPI frac-tion found in river waters was reported [14,34] tobe lower than 10 kDa. In addition, as the polysac-charide does not have any ionizable functionalgroup, this would definitely help it to easily over-come the electrostatic repulsion of the membrane.The precipitated HPI compounds would then ad-sorb onto the pores, constrict, reduce the pore sizesto the extent of blocking them. This initial ad-sorption enhanced further mass accumulation ofHPI that cover the pores and later lead to a cakelayer formation. This statement is well supportedby the results of Table 5 that show that the HPIfraction has high cake resistance (12.5%). Mean-while, the HPO fraction, which has an aromaticstructure, showed the concentration polarization(CP) as the primary fouling mechanism to its fluxdecline (Table 6). This is supported by the high-est Rcp (22%) possessed by the HPO componentcompared to other fractions. Moreover thereseems to be an underlying rejection mechanismthat governs the preferential rejection of the HPO


fraction by the PSF membrane that consequentlyleads to high Rcp.

This result also indicates that the hydropho-bicity interaction between the HPO fraction andthe PSF membrane was found to be less signifi-cant in affecting the adsorptive fouling althoughprevious studies [36,37] claimed that intrinsicallyhydrophobic membranes tend to adsorb NOM andfoul more than hydrophilic membranes. This phe-nomenon could be well explained from the highhumic content and negative charge (due to car-boxylic and phenolic moieties) of the HPO frac-tion which resulted in a significant rejection bythe ionizable functional group of the polysulfonemembrane. Therefore, we can hypothesize thatthere are two dominant factors which are respon-sible for determining the HPO adsorption and thetype of fouling mechanism. The factors are ad-sorptive behavior (due to hydrophobicity prop-erty) and the electrostatic repulsion interaction(due to similarity in charge with the membrane).The HPO fraction possesses negative chargesfunctional groups that prevent it from adhering toor adsorbing onto the negatively charged mem-brane surface and therefore resulting in high Rcp.However, in this experiment the HPO fraction didexhibit a significant adsorption fouling mecha-nism with adsorption resistance (Ra) 9% higherthan that of the TPI fraction. The Ra of the HPOfraction showed a higher value than the TPI frac-tion despite its electrostatic interaction with themembrane. This scenario was believed to happenwhen the hydrophobic interactions were able toovercome the electrostatic repulsion incurred bythe pressure gradient of the suction process. Thus,

Table 6Dominant fouling mechanism in NOM fractions as a func-tion of hydraulic fouling resistance

Fraction Primary fouling mechanism HPO Concentration polarization HPI Adsorption TPI Cake formation

for the HPO fraction, the adsorption mechanismcould be taken as a secondary influential foulingmechanism after the concentration polarization.In general, it could be summarized that foulingpotential would be higher for the negativelycharged membrane when it is exposed to hydro-philic components compared to highly negativelycharged materials (HPO fraction) although theypossess high adsorptive tendency. The foulingmechanisms of the NOM fractions based on thehydraulic resistance can be disclosed as follows:HPO Concentration polarization > Adsorption

> Cake formationHPI Adsorption > Cake formation >

Concentration polarizationTPI Cake formation > Adsorption >

Concentration polarization

The responsible foulant that affected the mem-brane performance was the HPI fraction whichwas found to cause the highest flux decline dur-ing the membrane filtration. Furthermore, the ad-sorption fouling can be taken as the primary foul-ing mechanism that governs the membrane fluxdecline as the HPI fraction has the highest Ra.Therefore, in future applications, NOM hydropho-bicity or aromaticity (feed source characteristic)and charge interaction (membrane property) canbe employed as a quantitative predictor for NOMrejection and flux decline of membrane filtrationof surface water.

3.3. Flux recovery through membrane cleaning

In this study both the acidic and alkaline clean-ing solutions were employed, as they were re-ported to be effective in removing organic mat-ter, while the acidic solutions were effective ineliminating inorganic foulants. Membrane fluxrecovery was performed after each cleaning pro-cedure through filtration with DI water. The mem-brane flux recovery is a ratio of Jpwfn to Jpwf1 aftereither hydraulic or chemical washing. Based onFig. 10, the flux recovery of the HPO fraction on


the fouled membrane showed a higher value thanthe TPI fraction and this result was found to con-tradict the flux decline profile shown in Fig. 6 asthe TPI fraction exhibited less flux decline thanthe HPO fraction. This scenario happened due tothe concentration polarization fouling which wasthe dominant fouling mechanism of the HPO frac-tion, and the effect of Rcp was eliminated the mo-ment suction pressure was turned off. In otherwords, the flux recovery of the HPO fraction forthe fouled membrane in Fig. 10 was measuredwithout the presence of Rcp and this consequentlyresulted in higher flux recovery than it was ex-pected. Other NOM fractions showed lower fluxrecovery of fouled membranes due to their differ-ent dominant fouling mechanisms. In general, thehydraulic cleaning (backwash) did impart someeffect on removing the cake layer from the mem-brane surface fouled with NOM fractions. Thisfinding was proven from the escalation of fluxrecovery of each NOM fraction with the TPI frac-tion experiencing the highest percentage (17.2%)followed by the HPI fraction (8.7%) and the HPOfraction (2.6%). This phenomenon was again re-lated to the governing fouling mechanism attrib-uted to the NOM fraction; in this case it was the

Fig. 10. Flux recovery after physical and chemical cleaning procedures for MRUTM55 membrane.

cake layer formation. The TPI fraction had thehighest flux recovery after backwashing as thecake adsorption was its dominant fouling mecha-nism. In the case of the HPI fraction, its Rcp andRc were relatively small compared to Ra. The hy-draulic washing could only restore about 64% ofthe clean membrane’s flux, and thus this scenarioclearly alarmed us that it required chemical clean-ing in order to combat its high irreversible foul-ing.

Nevertheless, chemical cleanings showed moreobvious recovery than hydraulic washing by givenadditional substantial flux recovery to the HPIfraction by about 30%, the TPI fraction about 13%and the HPO fraction (18%). High recoverygained by the HPI fraction represented the pref-erence of this fraction to chemical solution thatwas mainly due to the influence of its primaryfouling mechanism (adsorption). The HPI frac-tion exhibited better flux recovery than the HPOfraction despite its higher adsorption resistance(Ra). This can be reasoned from the behaviors ofneutral properties such as polysaccharides oramino sugar which are present in the HPI frac-tion. These substrates were widely reported byprevious researchers [17,37] to cause the intrin-


sic neutral properties of the HPI fraction whichwere more fragile to chemical solution. On theother hand, the chemical cleaning effectivenessof the HPO fraction was primarily governed bythe hydrophobicity effect. This interaction resultedin adsorptive fouling of chemical adsorption thatled to relatively high strength bond from electronsharing between membrane charged moleculesand the HPO fraction. As a matter of fact, the HPOfraction was also found to exhibit less chemicalcleaning efficiency compared to the HPI fractionalthough it showed lower Ra, less flux decline andhigher flux recovery (96%) than the HPI fraction.This again could be related to the adsorptive foul-ing behavior which was influenced by the HPOhydrophobicity.

Overall, the flux recovery of the membranesfouled with NOM fractions could be restored bymore than 94% of the clean membrane. The irre-versible loss percentage ranged from 4% to 6%and was believed to be a result of the strong ad-sorption of dissolved NOM fractions occurringwithin the membrane matrix. The HPI fractionshowed the highest irreversible loss with 6% ofthat of the new membrane. Moreover this resultwas not found surprising as the HPI fraction hadthe highest Ra with the adsorption fouling as itsdominant mechanism. In summary, the hydraulicwashing and chemical cleaning could help in in-creasing the flux recovery of the fouled mem-brane. The cleaning technique with DI water wasfound capable of eliminating the loose cake layerwhereas the chemical solutions helped in moresignificant removal of NOM foulants. Types offouling resistance and dominant fouling mecha-nism would determine the preference of cleaningsolution and the percentage of flux recovery ob-tained. Therefore in future applications, the typesof fouling resistance and the dominant foulingmechanism can be employed as a quantitativepredictor in flux recovery effectiveness and thesuitability of cleaning solutions to be applied.

4. Conclusions

Based on the results of this study several con-clusions can be drawn as follows:

1) Each of the Ulu Pontian NOM fractioncaused a substantial flux decline and the order offouling potential based on flux decline was HPI> HPO > TPI.

2) The hydrophilic fraction (HPI) was foundto appear as the prime foulant responsible for thehighest flux decline during membrane filtration.

3) Each fraction of NOM exhibited differentprimary fouling mechanisms where the adsorp-tion fouling was found to be the major foulingmechanism that controlled the membrane perfor-mance.

4) The HPI fraction exerted the least DOC andUV254 removals despite its highest flux decline.Thus, the degree of flux decline did not propor-tionally correlate with the DOC and UV254 remov-als. In fact, this observation significantly denotesthat the DOC fractions, which are most effectivelyexcluded by the membrane, are not the same frac-tions that contribute to increasing the flux decline.

5) The HPO fraction showed the highest DOCand UV removal despite its lower organics amountcompared to the HPI fraction. Thus, the degree ofDOC and UV rejections did not necessarily de-pend on the amount of organics concentration inthe source water.

6) The HPI and HPO fractions produced strongirreversible fouling due to their neutral propertyand hydrophobicity interaction with the mem-brane.

7) Charge interaction is more influential fac-tor than hydrophobicity in determining the foul-ing mechanism and the extent of flux decline.

8) The electrostatic repulsion is more domi-nant than size exclusion as the rejection mecha-nism when filtering NOM with a charged mem-brane.

9) The HPI fraction exhibited better chemicalcleaning effectiveness than the HPO fraction de-spite its higher adsorption resistance.


10) The types of fouling resistance and primaryfouling mechanism would determine the prefer-ence of cleaning solution and the percentage offlux recovery.

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