Wat. Res. Vol. 34, No. 11, pp. 2861±2868, 2000 7 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S0043-1354(00)00051-8 0043-1354/00/$ - see front matter www.elsevier.com/locate/watres THE FOULING OF MICROFILTRATION MEMBRANES BY NOM AFTER COAGULATION TREATMENT

T. CARROLL*, S. KING, S. R. GRAY, B. A. BOLTOM and N. A. BOOKER CSIRO Molecular Science, Bag 10, Clayton South, Victoria, 3169, Australia

(First received 1 July 1999; accepted 1 September 1999)

AbstractÐMicro®ltration membranes used in drinking-water treatment are fouled by both colloidal material and natural organic matter (NOM) present in the raw water. The relative importance of these contributions to fouling may depend on whether or not the water is pretreated before micro®ltration, and on the type and extent of any pretreatment. In this study, the causes of fouling were determined for micro®ltration of a surface water through a polypropylene hollow-®bre membrane. Fouling was caused by colloidal material when the raw water was ®ltered untreated, and by NOM when the raw water was coagulated before ®ltration. The components of NOM which cause fouling of micro®ltration membranes are not yet well-established, and were also investigated in this study. NOM from the raw water was fractionated into four speci®c classes of compounds on the basis of hydrophobicity and charge. The rates of fouling by each NOM fraction were measured separately. The major contribution to fouling was attributed to the NOM fraction comprising small, neutral, hydrophilic compounds. The NOM fractions comprising humic and fulvic acids made only a minor contribution to fouling. 7 2000 Elsevier Science Ltd. All rights reserved

Key wordsÐfouling, micro®ltration, natural organic matter

INTRODUCTION micro®ltration, colloidal material may cause fouling Membrane micro®ltration is a drinking-water treat- by forming a cake on the membrane surface, while ment process which is particularly suitable for the dissolved material may cause fouling by precipitat- removal of suspended solids, especially bacteria, ing at the membrane surface or adsorbing within algae, and protozoa, such as Giardia and Cryptos- the membrane pore space. In the former two cases poridium (MacCormick, 1995). However micro®ltra- the resistance to ®ltration is related to the per- tion is less successful for the removal of dissolved meability of the surface cake, while in the latter contaminants such as natural organic matter case the ®ltration resistance is related to the e€ec- (NOM) (MacCormick, 1995). NOM is a well-estab- tive size of the membrane pores (i.e. the e€ective lished cause of colour in water, and is also linked membrane permeability). These ®ltration resistances to the formation of disinfection by-products with are generally incorporated into a Darcy's Law potential health implications (Rook, 1974). The model, in which the membrane ¯ux per unit trans- conventional approach to removing NOM from membrane pressure is inversely related to a time- drinking water is coagulation with aluminium-based dependent resistance, which is calculated as the sum or iron-based coagulants (Crozes et al., 1995; Pon- of separate ®ltration resistances assumed to be in tius, 1993). Coagulation upstream of a micro®ltra- series (Weisner and Aptel, 1996). The e€ect of col- tion process can also be used as a pretreatment to loidal fouling on micro®ltration performance is well meet water quality requirements for NOM removal described, and common models range from the where micro®ltration alone is inadequate (Vickers simple particle-size-based Kozeny±Carman model et al., 1995). (McCabe et al., 1993), to more sophisticated inter- Membrane ®ltration processes are prone to foul- particle and membrane-particle interaction models ing, with a progressive decline in membrane ¯ux (Faibish et al., 1998; Meagher et al., 1996). The with time on stream because of the accumulation of e€ect of adsorptive fouling on micro®ltration per- retained material. In the case of drinking-water formance is also well-described (Weisner and Aptel, 1996), although the causes of fouling by dissolved NOM are not well understood. This is because the extent of interactions of NOM with a membrane *Author to whom all correspondence should be addressed. Tel.: +61-3-95452387; fax: +61-3-95452386; e-mail: can depend on features such as membrane hydro- [email protected] phobicity (Laine et al., 1989), the pH, salinity and

2861 2862 T. Carroll et al. hardness of the water (Nystrom et al., 1996), and into ¯ocs (Wiesner and Laine, 1996). One or more the characteristics of the NOM (Jucker and Clark, of these processes could alter the respective contri- 1994). The latter is a particularly challenging area, butions of colloidal material and NOM to mem- since NOM represents a broad range of structurally brane fouling by untreated and pretreated waters. complex compounds with a wide size distribution, In this study the contributions of colloidal material and heterogeneous functional chemistry (Stevenson, and NOM to the fouling of a polypropylene hol- 1994). Historically NOM is divided into hydro- low-®bre micro®ltration membrane, and the rate- phobic and hydrophilic (humic and non-humic) controlling fouling mechanisms, were investigated components. The hydrophilic components were for both untreated and alum-treated surface water. thought to impact water quality less than the The components of NOM which foul a polypropy- hydrophobic components, although they are also lene membrane were studied by fractionating the less amenable to physicochemical treatment (Owen surface water NOM on non-functionalised resins et al., 1995). into four speci®c classes of compounds based on The contributions of colloidal material and NOM hydrophobicity and charge, and measuring the rates to the fouling of micro®ltration membranes may of fouling by these compounds. The NOM fouling depend on the characteristics of the raw water, the mechanism was inferred from the types of com- membrane material, and on the type and extent of pounds responsible for fouling. any water pretreatment. Coagulation pretreatment is known to reduce the rate of fouling of micro®l- tration membranes, possibly by aggregating ®ne MATERIALS AND METHODS particles to increase cake permeability or prevent Raw water fractionation pore blockage, conditioning of the cake by incor- The surface water source was the Moorabool River near poration of ®ne particles into highly-porous ¯ocs, Anakie, Victoria, Australia. This water had a turbidity of or precipitation or adsorption of dissolved material 3.9 NTU, a salinity of 62 ppm (as Na+), a hardness of 4.3

Fig. 1. Outline of raw water fractionation procedure. Fouling of a micro®ltration membrane by NOM 2863

Fig. 2. Single-®bre micro®ltration apparatus.

ppm (as Ca2+), and a dissolved organic carbon (DOC) ance (WT). Feed pressure was monitored with a pressure concentration of 9.0 mg/l. The NOM in the raw water was transducer (PT), and allowed to follow the peristaltic concentrated to approximately 500 mg/l DOC by reverse pump curve uncontrolled. The signals from the analytical osmosis, and fractionated according to an ion-exchange balance and pressure transducer were processed with data procedure as described by Aiken et al. (1992), Croue et al. acquisition software to calculate the permeate ¯owrate as (1993) and Bolto et al. (1998) (Fig. 1). The raw water con- a function of permeate throughput. The duration of exper- centrate was ®ltered through a 0.45-mm membrane, iments ranged from 100 to 600 min depending upon the adjusted to pH 2, and fed onto a Supelite DAX-8 non- rate of fouling. Micro®ltration experiments were carried functionalised resin which retains strongly hydrophobic or- out on the four NOM fractions and on the unfractionated ganic matter attributed to humic and fulvic acids (Croue, raw water, subjected to following pretreatments; coagu- 1999). This fraction was eluted with NaOH and acidi®ed lation with alum (Al2(SO4)3Á18H2O, Akzo±Nobel), adsorp- on an IRC-120 cation exchange resin. The unadsorbed tion with a commercially available powdered activated concentrate from the DAX-8 resin was fed onto an carbon (PAC, Sigma), and ®ltration through a 0.2-mm Amberlite XAD-4 resin, which retains weakly hydrophobic membrane (Gelman GH Polypro). All micro®ltration ex- (or transphilic) organic matter (Croue, 1999). This fraction periments were done at pH 6 (optimal for NOM removal was also eluted with NaOH, and separately acidi®ed on an by alum). IRC-120 resin. The unadsorbed concentrate from the XAD-4 resin, which consists of hydrophilic (non-humic) Analytical methods organic matter attributed to proteins, amino acids, and The DOC was measured by wet chemical oxidation (O/I carbohydrates (Owen et al., 1995) was fed onto an Amber- Analytical 1010 Wet Oxidation TOC Analyser), and ultra- lite IRA-958 anion exchange resin, which retains charged violet absorbance was measured at 254 nm (Shimadzu material (Bolto et al., 1998). This fraction was eluted with UV-160 UV-visible spectrophotometer). All samples were a NaOH/NaCl mixture. The remaining neutral material is ®ltered through a 0.45-mm ®lter (Selby-Biolab HPLC-certi- not retained by any of the resins. ®ed) prior to analysis. Calcium and sodium concentrations were measured by inductively-coupled plasma spec- Membrane fouling troscopy (Jobin-Yvon JY24 ICP spectrometer) and con- centrations in the four fractions were made up to the raw Micro®ltration experiments were carried out on single water concentrations by adding calcium chloride and polypropylene hollow-®bre membranes using the ®ltration sodium chloride, respectively. Turbidity was measured apparatus shown in Fig. 2. The ®bres had a nominal pore using a Hach Ratio/XR Turbidimeter. size of 0.2 mm, an internal diameter of 250 mm, an outer diameter of 550 mm, and a length of 0.9 m. The ®bres had a nominal surface area of 700 mm2 and a nominal pure water ¯ux of 0.3 l s1 m2 bar1. Filtration occurred from the outside to the inside of the ®bre. Fibres were opened at both ends and sealed at the outer wall by threading the Table 1. DOC concentrations of Moorabool NOM fractions ends through a silicone septum. The ®bres were wetted and degreased with ethanol, and ¯ushed thoroughly with Fraction DOC (%) DOC (mg/l) ultrapure water (MilliQ). The feed water was prepared in a stirred vessel, and pumped onto the membrane at 80± Strongly hydrophobic 34 3.1 Weakly hydrophobic 18 1.6 100 kPa with a peristaltic pump. The feed was forced Charged hydrophilic 33 3.0 through the hollow-®bre membrane under pressure and Neutral hydrophilic 15 1.4 the permeate emerged from the opened ends. The perme- Unfractionated 100 9.0 ate was collected in a vessel mounted on an analytical bal- 2864 T. Carroll et al.

Table 2. The fate of calcium and sodium in the fractionation pro- cedure

Fraction Ca2+ (mg/l) Na+ (mg/l)

Strongly hydrophobic 0.00 (20.001) 0.1 Weakly hydrophobic 0.00 (20.01) 0.2 Charged hydrophilic 0.01 143 Neutral hydrophilic 3.72 70 Unfractionated 4.30 62

RESULTS

Characterisation of the Moorabool NOM fractions The DOC concentrations and their relative per- centages in the four Moorabool NOM fractions are shown in Table 1. The DOC distribution was ap- proximately 1/3 strongly hydrophobic, 1/6 weakly hydrophobic, 1/3 charged hydrophilic, and 1/6 neu- tral hydrophilic. Fig. 3. The declines in permeate ¯owrate with permeate It was necessary to follow the fate of both cal- throughput for the four Moorabool NOM fractions and cium and sodium from the raw water in the frac- for pre®ltered unfractionated water. tionation procedure, as ionic strength and calcium concentration can each in¯uence both adsorption of ponent of the raw water NOM without colloidal NOM onto membranes and aggregation of NOM material. through moderation of electrostatic e€ects (Yoon et In previous work, the Moorabool NOM fractions al., 1998; Yuan and Zydney, 1999). The concen- were characterized by size exclusion chromatog- trations of calcium and sodium in the four fractions raphy with DOC and ultra-violet absorbance detec- are compared to those of the raw water in Table 2. tion (Bolto et al., 1999). The size distributions of The concentrations of calcium and sodium were both hydrophobic fractions were similar to that of negligible in the hydrophobic fractions. The sodium the raw water. The charged hydrophilic fraction concentration in the neutral hydrophilic fraction was predominantly larger-sized material whereas was slightly higher than in the raw water because of the neutral hydrophilic fraction was predominantly the addition of NaOH for pH adjustment before smaller-sized material. feeding to the IRA-958 resin. The sodium concen- tration in the charged hydrophilic fraction was elev- Membrane fouling of the reconstituted Moorabool ated by the elution procedure used on the IRA-958 NOM fractions resin. The declines in permeate ¯owrates with permeate The four NOM fractions were reconstituted as throughput during micro®ltration of the four recon- stock solutions with DOC concentrations similar to stituted Moorabool NOM fractions are shown in those of each fraction in the raw water. The calcium Fig. 3. The rates of fouling were relatively low for and sodium concentrations of the stock solutions the strong and weak hydrophobic fractions, and for were adjusted to the approximate values of the raw the charged hydrophilic fraction. In these three water (and also in the neutral hydrophilic fraction), cases, the permeate ¯owrate declined by approxi- although the sodium concentration of the charged mately 16% after 800 g of the solution was ®ltered. hydrophilic fraction was not reduced. The DOC, However in the case of the neutral hydrophilic frac- turbidities, calcium and sodium concentrations in tion, the rate of fouling was considerably faster, the the reconstituted NOM fractions are given in permeate ¯owrate declining by approximately 40% Table 3. As turbidities were low and uniform, each after 800 g of the solution was ®ltered. In all cases, reconstituted NOM fraction re¯ected that com- the change in DOC after ®ltration was below the

Table 3. DOC, turbidity, calcium and sodium concentrations in the reconstituted NOM fractions

Fraction DOC (mg/l) Turbidity (NTU) Ca2+ (mg/l) Na+ (mg/l)

Strongly hydrophobic 3.0 0.2 4.3 70 Weakly hydrophobic 1.5 0.2 4.4 69 Charged hydrophilic 3.0 0.3 4.4 143 Neutral hydrophilic 1.4 0.2 4.4 70 Unfractionateda 9.0 (8.0) 3.9 (0.2) 4.3 62 aValues after ®ltration pretreatment are shown in parentheses. Fouling of a micro®ltration membrane by NOM 2865 limit of detection of the TOC Analyser (20.2 mg/l). water and the water after PAC adsorption pretreat- The rate of fouling by the unfractionated raw water ment are compared in Fig. 5. In the latter case, the after ®ltration pretreatment was only slightly faster water was treated with an excess of PAC (100 mg/l than for the neutral fraction, the permeate ¯owrate for 68 h) which resulted in 68% DOC removal. The declining by approximately 45% after 800 g of the rate of fouling by the PAC-treated water was simi- solution was ®ltered. The raw water after ®ltration lar to the raw water with the permeate ¯owrate pretreatment had already passed through a 0.2-mm declining by 83% after 800 g was ®ltered. A 100 mg/ membrane, so the fouling in this case can be attrib- l suspension of PAC in ultrapure (MilliQ) water did uted to dissolved NOM. The rate of fouling by the not cause an appreciable decline in permeate ¯ow- neutral hydrophilic fraction did not improve when rate, indicating that the PAC particles themselves the permeate was re®ltered through a fresh mem- did not contribute a resistance to ®ltration. brane. Membrane fouling of raw Moorabool water with co- DISCUSSION agulation pretreatment Micro®ltration of untreated Moorabool water The declines in permeate ¯owrates with permeate caused a severe decline in permeate ¯owrate with throughput during micro®ltration of the untreated permeate throughput, indicative of rapid fouling water and the water after alum coagulation pre- (Fig. 4). The rate of fouling was una€ected by treatment are shown in Fig. 4. In the latter case, adsorption pretreatment with PAC (DOC 3.0 mg/l, the water was treated with alum at the optimum Fig. 5), but was signi®cantly reduced by coagulation 3+ dose (3.2 mg/l as Al ) for both DOC and UV254 pretreatment with alum (DOC 4.6 mg/l, Fig. 4), removal (46% and 69%, respectively). The rate of and ®ltration pretreatment (DOC 8.0 g/l, Fig. 3). fouling for the untreated Moorabool water was These results indicate that for this low-turbidity, relatively high, with the permeate ¯owrate declining high-NOM surface water, colloidal material rather by 82% after 800 g was ®ltered. In contrast, the than dissolved NOM determined the rate of fouling. rate of fouling for the alum-treated water was con- Although partial NOM removal was achieved using siderably lower, with the permeate ¯owrate declin- either an adsorbent or a coagulant, only the latter ing by approximately 50% after 800 g was ®ltered. treatment option reduced the rate of fouling. The If the alum ¯ocs were settled rather than stirred rate of fouling after ®ltration pretreatment although before micro®ltration (so that only the supernatant lower than the untreated water, was still appreciable solution was ®ltered) the rate of fouling was not indicating a contribution from dissolved NOM reduced. under these conditions. The reduction in the rate of fouling after coagu- Membrane fouling of raw Moorabool water with lation could be caused by aggregation of ®ne par- adsorption pretreatment ticles, incorporation of particles into ¯ocs, or The declines in permeate ¯owrates with permeate adsorption or precipitation of dissolved material throughput during micro®ltration of the untreated

Fig. 4. The declines in permeate ¯owrate with permeate Fig. 5. The declines in permeate ¯owrate with permeate throughput for untreated and alum-treated Moorabool throughput for untreated and PAC-treated Moorabool water. water. 2866 T. Carroll et al. into ¯ocs (Wiesner and Laine, 1996). These mech- 3.0 mg/l of the alum-treated water), or a contri- anisms imply that fouling after coagulation pre- bution from unsettleable ¯ocs. In either case, re- treatment can be attributed to the ®ltration sidual NOM played an important role in resistance due to formed ¯ocs, or to the fraction of determining the rate of fouling in micro®ltration dissolved NOM which is not removed by a coagu- with coagulation pretreatment. The substantial lant. The contributions of these two ®ltration resist- removal of the hydrophobic and charged fractions ances to fouling can be separated by comparing the by alum had little impact on the rate of fouling by rates of fouling for stirred and settled alum pre- Moorabool water. treatments (Fig. 4). The rates of fouling for both The interpretation of the rates of fouling in hol- pretreatments were similar, indicating that the ®l- low-®bre micro®ltration membranes is presently tration resistance due to settleable aggregates and qualitative. The rate of fouling of hollow-®bres ¯ocs is small compared to that due to residual dis- depends upon the ®bre length, diameter and thick- solved NOM. Furthermore, the rates of fouling ness, as well as properties of retained material from after both ®ltration pretreatment (Fig. 3) and co- the feed stream (Carroll and Booker, 2000). Quanti- agulation pretreatment (Fig. 4) were also similar, tative determination of ®ltration resistances attribu- indicating that the ®ltration resistance due to dis- ted to colloidal material or dissolved NOM would solved NOM is similar to that due to residual require isolation of these contributions from those NOM and formed ¯ocs. The rate of fouling was due to ®bre dimensions. This is not a trivial under- determined by dissolved NOM, regardless of taking, requiring the development of a detailed whether colloidal material was coagulated or model signi®cantly more complex than existing removed by pre®ltration. Darcy's Law models. However, even in the absence Coagulation pretreatment selectively removes cer- of such a model, fouling can be attributed to col- tain types of dissolved NOM. Aluminium-based loidal substances in micro®ltration of untreated and iron-based coagulants are known to preferen- water, and to residual NOM in micro®ltration of tially remove hydrophobic rather than hydrophilic alum-treated water. substances (Dryfuse et al., 1995; Singer and Har- The extent to which the rate of fouling is deter- rington, 1993; White et al., 1997), charged rather mined by colloidal or dissolved NOM will depend than neutral substances (Bose et al., 1993; Korshin on the characteristics of both the raw water and co- et al., 1997), and larger-sized rather than smaller- agulant used in any pretreatment. The Moorabool sized substances (Dryfuse et al., 1995; Sinsabaugh water used in this study was low in turbidity and et al., 1986). These trends were also observed when high in DOC. Water of a di€erent quality may the four Moorabool fractions were treated with require a di€erent alum dose to meet NOM alum. The resulting DOC and UV254 removals are removal requirements. This may alter the relative given in Table 4 (Bolto et al., 1998). Alum removed contributions to fouling by residual NOM and upwards of 65% of UV absorbers of the hydro- ¯ocs. The relative concentrations of each NOM phobic and charged hydrophilic fractions, but only fraction in the raw water is also important. The 24% of the UV absorbers from the neutral hydro- rate of fouling by a raw water with a relatively high philic fraction. The relative concentration of neutral concentration of neutral hydrophilic substances, hydrophilic substances was therefore substantially and low turbidity may not improve substantially higher after coagulation pretreatment than for the after coagulation pretreatment. Finally, both the co- untreated water. As a ®rst approximation, the con- agulant and treatment conditions used will deter- tribution of dissolved NOM to fouling after coagu- mine the permeability, size and stability of ¯ocs lation pretreatment can be attributed to fouling by formed in pretreatment. These properties in turn neutral hydrophilic substances. Indeed the rate of determine the ®ltration resistance due to the ¯ocs. fouling by the neutral hydrophilic fraction (Fig. 3) Alum treatment of Moorabool water produces ¯ocs was only slightly lower than the rate of fouling by whose ®ltration resistance is less signi®cant than alum-treated water (Fig. 4). This di€erence may be that of the residual NOM. This ®nding may not be due to both the substantial removal of the other applicable to treatment of other water sources and three NOM fractions (comprising approximately by other coagulants. On the basis of results given here, the rate of fouling will not improve if alum is replaced by a similar coagulant. However it is im- Table 4. NOM removal from Moorabool fractions by alum treat- ment portant to consider other factors in coagulant assessment such as cost, backwash e€ectiveness and

Fraction DOC removal (%) UV254 removal (%) sludge properties, etc. The rates of fouling of the four Moorabool Strongly hydrophobic 48 71 Weakly hydrophobic 49 65 fractions were similar for the hydrophobic and Charged hydrophilic 64 83 charged hydrophilic fractions, and substantially a Neutral hydrophilic 0 24 faster for the neutral hydrophilic fraction (Fig. 3). Unfractionated 46 69 The fouling of a hydrophobic polypropylene aLow ultra-violet absorbance at 254 nm. micro®ltration membrane by small, neutral hydro- Fouling of a micro®ltration membrane by NOM 2867 philic substances is consistent with previous fouling a micro®ltration membrane, and the mech- reports of fouling of ultra®ltration membranes by anism by which this fouling occurs. low molecular weight model organic compounds in the absence of particulate material (Crozes et CONCLUSIONS al., 1993), and by residual dissolved NOM fol- lowing treatment with ferric chloride (Lahoussine- Natural organic matter can play a rate-determin- Turcaud et al., 1990). In both studies, fouling ing role in the fouling of drinking-water micro®ltra- was attributed to adsorption of organic com- tion membranes. The contribution of NOM to pounds, and could not be reversed by hydraulic fouling depends on the raw water quality, charac- cleaning alone. In the former study both hydro- teristics of the NOM, and the type and level of any phobic and hydrophilic ultra®ltration membranes pretreatment. Coagulation pretreatment improves were fouled by a tannic acid solution which had NOM removal and reduces membrane fouling com- a molecular weight well below the membrane pared to micro®ltration of the raw water. Residual cut-o€. In the latter study a polysulphone ultra- dissolved NOM is strongly implicated in controlling ®ltration membrane was fouled after a coagu- the rate of fouling after coagulation pretreatment. lation pretreatment which produced a water This residual NOM is composed primarily of small, consisting predominantly of low molecular weight neutral hydrophilic substances. Measurement of polysaccharides. the levels of this class of compounds in a water In contrast, a recent study attributed fouling source, in combination with traditional water qual- of a micro®ltration membrane by commercial ity parameters should improve bene®t analysis of humic acid solutions to formation of a retained combined coagulation±micro®ltration processes. surface deposit rather than adsorption within the Improvements in the rate of fouling may be possible pore space (Yuan and Zydney, 1999). In this if neutral hydrophilic substances are speci®cally tar- study pre®ltration through membranes with pro- geted for removal. Improvements may also result gressively tighter molecular weight cut-o€s from selection of a membrane material speci®cally reduced fouling as more smaller material was for low rates of fouling by these substances, or pre- ferential fouling by the NOM which is targeted by removed. Furthermore, fouling by pre®ltered a coagulant. NOM was increased when exposed to elevated temperatures or added calcium. This increase was AcknowledgementsÐThe funding for this work was pro- attributed to aggregation and subsequent reten- vided by the Co-operative Research Centre for Water tion of pre®ltered NOM at the membrane sur- Quality and Treatment. The authors also wish to thank face. Cunli Xiang for the DOC results after PAC treatment and In the present work the rate of fouling was high- Melissa Toi¯ for the DOC results after alum treatment of the four fractions. est for the smaller molecular-sized fraction. The size exclusion chromatogram for the neutral hydrophilic fraction was taken in the presence of the calcium REFERENCES from the raw water (Bolto et al., 1999), so any Aiken G. R., McKnight D. M., Thorn K. A. and Thur- aggregation would be re¯ected in the size measure- man E. M. (1992) Isolation of hydrophilic organic acids ments. 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