PII: S (98)

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1 PII: S (98) Wat. Res. Vol. 33, No. 11, pp. 2517±2526, 1999 Published by Elsevier Science Ltd Printed in Great Britain /99/$ - see front matter MEMBRANE FILTRATION OF NATURAL ORGANIC MATTER: INITIAL COMPARISON OF REJECTION AND FLUX DECLINE CHARACTERISTICS WITH ULTRAFILTRATION AND NANOFILTRATION MEMBRANES JAEWEON CHO 1 *, GARY AMY 1*M and JOHN PELLEGRINO 2 { 1 Civil, Architectural and Environmental Engineering, University of Colorado at Boulder, Boulder, CO 80309, U.S.A. and 2 Physical and Chemical Properties Division, National Institute of Standards and Technology, MS , Boulder, CO 80303, U.S.A. (First received June 1998; accepted in revised form November 1998) AbstractÐTwo source waters containing natural organic matter (NOM) with di erent physical and chemical characteristics were cross ow- ltered using four types of membranes having di erent material and geometric properties. Transport measurements of NOM rejection and ux decline were made. A resistances-in-series model was used to represent and quantitatively compare membrane ux decline and recovery. As anticipated, the resistance due to speci c adsorption depended on the concentration at the membrane interface. For the two membranes showing evidence of NOM adsorption, reducing the initial ux (which we infer to also reduce the interfacial NOM concentration) also lowered the measured resistance assigned to adsorption in our protocol. Relative molecular mass (RMM) distribution measurements (by size exclusion chromatography) were used to calculate the average RMM of the NOM and persuasively illustrated that the nominal relative molecular mass cut-o (MWCO) of a membrane is not the unique predictor of rejection characteristics for NOM compounds. Size exclusion, electrostatic repulsion, and NOM aromaticity all in uenced the NOM rejection. For a given water composition (including ph and ionic strength), membrane characteristics (such as the surface charge, hydrophobicity and nominal MWCO) can be combined with the NOM properties (such as total dissolved organic carbon, speci c UV absorbance at 254 nm and humic content) to provide a consistent qualitative rationale for the transport results. Published by Elsevier Science Ltd Key wordsðdrinking water, ux decline, MWCO, nano ltration, natural organic matter, NOM, rejection, ultra ltration INTRODUCTION *Present address: Kwangju Institute of Science and Technology, 1 Oryong-dong, Puk-gu, Kwangju , Korea ( choj@kjist.ac.kr). {Author to whom all correspondence should be addressed. [Tel.: ; fax: ; jjp@boulder.nist.gov] We made bench scale measurements using micro ltered surface waters and four membranes, including NF and UF. Metrics for the characterization of the water composition and the membranes were tabulated and the resistance-in-series model was used to analyze the ltration process. Our results are consistent with those of prior researchers with regard to the general in uences and mechanisms associated with rejection and ux decline during natural organic matter (NOM) ltration. In this report we present initial measurements from our protocol that may be useful for developing future correlations. Previous studies (for example, Taylor et al., 1987; Fu et al., 1994) have shown that NOM can be e ectively rejected during ltration by low and medium pressure membranes, including nano ltration (NF) and, to a lesser extent, ultra ltration (UF). Removal of NOM is important since they act as the precursors to disinfection by-products (DBPs) which, in turn, have recently received attention in drinking water regulations. Also, organic matter is often found to be a primary source of ux decline due to fouling in RO and NF systems. Heretofore, the most popular predictor of NOM rejection by membranes has been the nominal molecular mass cut o (MWCO). Nonetheless, often di erent relative molecular mass (RMM) rejections have been observed for di erent membranes with comparable nominal MWCOs and for the same membranes when applied to di erent solutes, including NOM source waters. Moreover, membrane ltration ux decline due to organic (NOM) fouling (and ux recovery after cleaning) is felt to be less well understood than that due to other colloidal, biological and scale-related fouling. 2517

2 2518 Jaeweon Cho et al. Table 1. E ects of chemical and ux conditions on ltration of humic substances (adapted from Hong and Elimelech, 1997) Chemical conditions Humics in solution Humics on membrane surface Flux decline (>critical ux) Flux decline (>>critical ux) Flux decline (<critical ux) High ionic strength low ph, or presence of divalent ions Low ionic strength high ph, and absence of divalent ions compact, coiled con guration stretched, linear con guration compact, dense, thick fouling layer severe severe insigni cant loose, sparse, thin fouling layer small severe insigni cant NOM is a general descriptor for a mixture containing a variety of organic, slightly water-soluble components. A speci c, sequential, two-column fractionation of a general NOM (Thurman and Malcolm, 1981; Leenher and Noyes, 1984; Aiken et al., 1992) separates it into (1) hydrophobic acids (which are adsorbed on XAD-8 resin) including strong (fulvic and humic) and weak (alkyl monoand dicarboxylic acids) acids; (2) hydrophilic acids (which are adsorbed on XAD-4 resin) including hydroxy and sugar acids and (3) strongly hydrophilic species (which are not adsorbed on either XAD- 8 or 4 resins) including polysaccharides, alkyl alcohols, amides and bases. A variety of prior studies have addressed the chemical and physical aspects of NOM ltration and ux decline with NF membranes. For the most part model solutions of humic acids obtained from commercial sources have been used to provide consistent measurement conditions. Fractionated NOMs and partially treated (with micro ltration and/or powdered activated-carbon adsorption) natural waters have also been used. Considering the highly heterogeneous nature of NOM, it is not surprising that many prior measurements (Laine et al., 1989; Jucker and Clark, 1994; Nilson and DiGiano, 1996; Braghetta et al., 1997) have indicated the in uence of hydrophobic and charge interactions between the membrane and NOM (in addition to MWCO) on the ltration gures of merit: water ux decline and solute rejection. Recently, Hong and Elimelech (1997) have studied the ltration of three classes of isolated humic substances with a thin lm composite (TFC) NF membrane (nominal MWCO < 100) based on crosslinked aromatic polyamide. Their study included e ects of divalent cation (Ca 2+ ) concentration, ph, total ionic strength, and interfacial concentration (controlled by changes in the permeation rate) on ux decline and the mass of humic substances adsorbed on the membrane. The e ects from changes in permeation rate were interpreted by a critical ux viewpoint, i.e. that upon start-up of a ltration process there is an initial ux below which a decline of ux does not occur (Field et al., 1995). Hong and Elimelech's results were consistent with much prior literature and are summarized in the following Table 1. It is important to keep in mind that there can always be several sources of ux decline in any application. In a broad sense we can designate ux decline due to (1) concentration polarization; (2) gel (precipitate) formation, reversible with mild cleaning; (3) gel (precipitate) formation, reversible with harsh cleaning; (4) surface adsorption, both reversible and irreversible; (5) pore adsorption, both reversible and irreversible and (6) reversible and irreversible physical changes to the membrane (for example, compression). Many of these sources of ux decline are directly related to the solute concentration at the membrane interface and therefore can be a ected by permeate ux, cross ow velocity, turbulence promoters, real-time cleaning regimes (back ushing, pulsing, etc.), bulk concentrations of solutes and membrane rejection qualities. The general mechanistic view of NOM interactions with membranes and the ltration process, including the e ects of charge, ionic strength, ph, chelation by divalent ions, and ux is analogous to that which has evolved for biological and other macromolecules. The following four points are major aspects of the ltration process. Similar issues are important even if there is no ltration (for example, static adsorption) but then the driving forces for solute interactions with the membrane material are simply the bulk concentrations. 1. The NOM mixture has an intrinsic chemical nature (aromaticity, polarity, ionizable groups, etc.) and molecular size. The actual charge, con- guration and chemical potential of the NOM in solution depend on the current solution environment (ph, ionic strength, ion compositions, temperature, pressure, etc.) which varies throughout the ltration process. 2. The combination of (i) the operating conditions of the ltration process (transmembrane pressure and hydrodynamic mass transfer at the membrane/feed interface); (ii) the membrane geometry (porosity and pore size distribution) and (iii) the membrane's rejection characteristics toward the NOM controls the NOM's concentration at the membrane surface and in the pores. 3. The chemical nature of the NOM; its concentration at the membrane uid±solid interfaces and the chemical and geometrical nature of the membrane (under the given solution conditions) control the amount (and degree) of gel or precipitate formation and reversible and irreversible adsorption that occurs. 4. The NOM's interfacial concentration; the interfacial solution's viscosity and the mass and porosity of the adsorbed layer in uence the

3 UF and NF membrane ltration of NOM 2519 Table 2. Feed water metrics DOC (mg/l) UVA 254 (cm 1 ) SUVA 254 (L cm 1 mg 1 ) Cond. (ms/cm) ph Humic (%DOC) Ca (mg/l) W_SL-SW R_SL-SW Twitchell hydrodynamic aspects of ux decline and the change in the ltration process's apparent rejection of the NOM through both porous media and physical property aspects. Clearly, when the degree of chemical complexity of NOM constituents is combined with the many physico-chemical aspects of the membranes and the ltration process (as outlined above) it will be very di cult to predict performance from rst principles. Correlations may be a useful rst step in matching water compositions, membrane properties, and ltration conditions. METHODS AND ANALYSIS Source waters Silver Lake surface water (SL-SW) and Twitchell water were used to perform bench-scale membrane tests. SL-SW is a Colorado drinking water source and Twitchell water is an agricultural drain feeding into the California State Project water. SL-SW samples were collected in winter (W_SL-SW) and runo (spring) seasons (R_SL-SW) to obtain seasonal in uences. Several analytical metrics are used for quantifying NOM content and their potential to form disinfection byproducts: total organic carbon (TOC), dissolved organic carbon (DOC), ultraviolet adsorption at 254 nm (UVA 254 ) and the speci c UV adsorbance (SUVA = UVA 254 /DOC). The UV absorbance of NOM is ascribed exclusively to aromatic chromophores. The SUVA is considered a measure of the relative aromatic content of the colloidal carbon and therefore the NOM. Additional molecular interpretations of NOM UV spectra have been presented (Korshin et al., 1997) but were not applied in this study. All membrane ltration tests were performed with source water that had been pre ltered using a 0.45 mm lter. The measurement of TOC on the permeate through this lter corresponds to the working de nition of DOC. For this study each source water was analyzed for DOC, UVA 254, SUVA, conductivity, ph, humic content of the NOM and Ca 2+. These results are tabulated in Table 2. High performance size exclusion chromatography (HPSEC) was used to determine the RMM distribution of NOM with a Waters* protein-pak column and a commercial UV spectrophotometric detector. The eluent for the HPSEC was composed of mq water (water that is ltered with two proprietary cation-exchange mixed beds, an anion-exchange bed, and a 0.2 mm lter) bu ered with phosphate (ph 6.8) and NaCl to increase ionic strength to 0.1 M (Chin et al., 1994). Standard solutions for the RMM calibration curve of NOM were made with sodium polystyrene sulfonates (PSS) (1.8, 4.6, 8.0 and 35.0 k) and *Such identi cation is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the equipment identi ed is necessarily the best available for the purpose. salicylic acid (RMM = ) was used to extend the lower range of the sodium-pss calibration curve. The RMM distributions (Fig. 1) and average RMMs (Table 3) of winter season and runo SL-SW and Twitchell water were determined by HPSEC. The humic fractions of NOM source waters were determined by performing a DOC mass balance across an XAD-8 resin column, with the column e uent representing the nonhumic fraction. Continuous cross ow at-sheet membrane unit A commercial bench scale cross ow membrane module was used to evaluate at sheet membrane specimens. Our system is composed of the membrane module and the feed, permeate, recycle, and waste lines. The module accommodates 60 cm 2 at sheet specimens (of which 056 cm 2 are active for ltration) under tangential feed ow conditions with a channel height of 0.04 cm. Figure 2 presents a schematic of the mass transfer dimensions and con guration of the ltration cell. For measurements in this report, the cross ow velocity was kept approximately constant at 08.6 cm/s by setting up a constant feed owrate of 200 ml/min. The temperature was maintained at 298 K and the transmembrane pressure was kept constant at approximately 345 kpa (50 psi). At these conditions the Reynolds number is decidedly laminar, nominally 36. Each new membrane was soaked in mq water for 1 day to clean any coatings and/or pore stabilizers. Fresh mq water was ltered through a membrane specimen until approximately constant ux was obtained, then the NOM solution was ltered. The NOM-containing natural waters (stored under refrigeration) were kept at room temperature for 1 day prior to permeation measurements to assure thermal equilibration. The permeate ow, UVA and DOC of the permeate were measured over time. NOM rejection, based on bulk concentration, was calculated by C p R j bulk ˆCb, 1 C b where C b is NOM concentration in the bulk uid in the feed channel and C p is the NOM concentration in the permeate. Flux decline and adsorption tests using cross ow ltration unit A membrane resistances-in-series model was used to quantify ux decline by obtaining the di erent series resistances. At any pseudo-equilibrium condition the ux can be evaluated by DP J v ˆ m r m r c r g r a1 r a2, 2 Table 3. Relative molecular mass parameters of the NOM in the feed waters M w M n Polydispersity W_SL-SW R_SL-SW Twitchell M w =mass averaged relative molecular mass. M n =number averaged relative molecular mass. Polydispersity = M w /M n.

4 2520 Jaeweon Cho et al. Fig. 1. Fractional relative molecular mass distribution of the NOM contained in the feed waters used in these ltration tests: Twitchell drainage water (w), winter Silver Lake (q) and spring-runo Silver Lake (r). where J v is water ux through the membrane (cm/s), DP is the transmembrane pressure (kpa), m is the dynamic viscosity (kpas), r m is the clean membranes's hydraulic resistance, r c is the resistance due to concentration polarization (CP), r g is any gel layer resistance, r a1 is weakly adsorbed foulant's resistance and r a2 is ``irreversibly'' adsorbed foulant's resistance (all r's have units of cm 1 ). r c and r g are related to the osmotic pressure and viscosity of the phases immediately proximal to the membrane surface. The numerical value of the resistances were obtained by using the protocol depicted in Fig. 3, described as follows: Step 1: mq water was ltered through the membrane until a constant ux was obtained. Step 2: the NOM-containing water was introduced and the permeate rate was monitored over time until it reached a constant value, the permeation rate through the fouled membrane. Step 3: the applied pressure was released (to remove any concentration polarization that resulted from the membrane's rejection of NOM under forced permeation) and mq water replaced the NOM-containing water and permeation was again measured. Step 4: the fouled membrane was then vigorously ushed (volumetric ow rate of 450 ml/min, cross ow velocity of 19.3 cm/s) for 10 min with mq water so that loosely-held gel layer (concentrated NOM) could be removed from the membrane surface and mq water permeation was again measured. Step 5: the membrane was then soaked in 0.1 N NaOH solution for 24 h so that weakly adsorbed NOM on the membrane surface could be desorbed, then mq water was again ltered through the chemically cleaned membrane. Using the ux values from steps 1±5, r m, r c, r g, r a1 and r a2 could be calculated. These resistances are speci c to the protocol that is used to create them (i.e. water composition and ltration conditions) and measure them. The gel layer resistances are in uenced by any solid phase structure. The weak adsorption can be de ned as the NOM adsorption which can be removed with chemical cleaning by 0.1 N NaOH, while strongly adsorbed NOM is not. Fig. 2. Schematic of the ltration cell with dimensions of the available mass transfer area.

5 UF and NF membrane ltration of NOM 2521 zeta potential measurements we made and the existence of ionizable groups (carboxylic acid) in the polymer (which is information provided by manufacturers). Zeta potential measurements were made with a commercial instrument (Elimelich and Childress, 1996) using mq water with a conductivity of 02±3 ms/m (KCl) and ph varied between 5.5 to 8.5. RESULTS AND DISCUSSION Fig. 3. Schematic representation of the ltration protocol used to determine resistances-in-series. The RMM distribution of the NOM foulants removed by 0.1 N NaOH was analyzed for comparison with the RMM distributions of XAD-8 isolation solutions. Contact angle measurements The water contact angle on the membranes was measured with the sessile drop method (Adamson and Gast, 1997) using a goniometer to measure the contact angle between the water droplet, the membrane surface and air. Each membrane was cleaned of any coating materials by oating it skin-side down in a container of mq water for 24 h, changing the water three times. The rinsed membranes were dried in a closed desiccator for 24 h and stored in closed petri dishes before measurements. Membrane samples were cut into small pieces and mounted on glass supports. A 2 ml mq water droplet is placed on the sample and the contact angle is immediately measured. A low contact angle is associated with high water a nity. Membranes Four di erent membranes, including NF45, YM3, GM and PM10, were used for transport measurement tests. NF45 and GM membranes are made of crosslinked, polyamide thin- lm-composite (TFC), YM3 membrane is made of regenerated cellulose and PM10 membrane is made of polyethersulfone. The nominal MWCOs of the membranes were provided by the manufacturers. Table 4 provides a detailed listing of the characteristics and identi- cation code for the membranes. According to the contact angle measurement it could be concluded that YM3 membrane is relatively hydrophilic, while NF45, GM and PM10 membranes are relatively hydrophobic. The solute-free water permeability (PWP) of the PM10 membrane is especially high compared with the other membranes. NF45 and GM membranes are thought to have a signi cant negative surface charge based on the Transport measurements W_SL-SW and Twitchell water permeation was done with all four membranes to determine DOC rejection. As shown in Table 2, W_SL-SW represents relatively low DOC, low RMM and low aromaticity, while Twitchell represents relatively high DOC, high RMM and high aromaticity. NF45 membrane exhibits signi cantly high rejection of DOC in both waters, because it has low MWCO and negative charges. YM3 and PM10 membranes do not reject DOC in W_SL-SW, but reject some of the larger RMM fraction in Twitchell water. Even though GM membrane has larger MWCO than YM3 and almost the same MWCO as PM10, it exhibits relatively high DOC rejections in both waters compared with YM3 and PM10. This can be rationalized by the fact that GM membrane has a negative charge (like the NOM macromolecules) so that there are charge repulsions between the membrane surface and NOM. YM3 membrane does not have any signi cant negative charge on its surface (which is consistent with its nominal cellulose composition). The PM10 membrane has the highest permeability (or PWP) and nominal MWCO and is expected to allow NOM macromolecules to freely pass through the membrane pores. E ective MWCO determination of GM membrane The charged GM membrane had higher NOM rejection than expected based on the manufacturer's speci cation MWCO of We postulate that the e ective MWCO for a charged membrane with a charged solute (e.g. NOM) will be di erent than what is obtained in measurements with neutral (uncharged) solutes. We determined the NOM fractional rejection (Mulder, 1991) using Table 4. Nominal transport and physical characteristics of the membranes Membrane ID DOC rejection (% m) a MWCO b Contact angle (H 2 O a nity) W_SL-SW, low DOC, low RMM, low aromaticity Twitchell, high DOC, high RMM, high aromaticity Ionizable groups c d PWP NF (low) yes ( ) 2.6 YM (high) no 4.5 GM (low) yes ( ) 8.0 PM (v. low) no a DOC rejection is not equally sensitive to all NOM fractions. For example, a decrease in UVA 254 can be observed without a similar change in DOC. b Manufacturer's data. c Determined with zeta potential measurements. d PWP (L d 1 m 2 kpa 1 ) is water permeability determined with ltered, deionized water.

6 2522 Jaeweon Cho et al. Fig. 4. Relative molecular mass distributions of R_SL-SW before and after ltration through GM membrane. R Mi ˆ WM i feed W Mi perm 1 R overall, 3 W Mi feed where R Mi is fractional rejection for a certain RMM ``i'', W Mi is the mass fraction of that RMM in the speci c stream and R overall is overall NOM rejection by the membrane (based on DOC measurements). RMM distributions of the R_SL-SW (spring runo ) feed and permeate through the GM membrane are shown in Fig. 4. The e ective MWCO (de ned as the RMM when R Mi =0.90) of the GM membranes for R_SL-SW and Twitchell water NOMs was determined using equation 3 and are listed in Table 5. The lower (than nominal) e ective MWCOs of the GM membrane for R_SL-SW and Twitchell water is attributed to charge and hydrophobic interactions between the membrane surface and NOM. The data in Table 5 were obtained in a separate set of measurements than those listed in Table 4. The DOC rejection for the Twitchell water is consistent between the two measurements. Interestingly, the DOC rejection for the R_SL- SW>Twitchell>W_SL-SW, even though the nominal DOC molecular mass distribution is similar for both R_SL-SW and Twitchell water. The main di erence is that R_SL-SW has the highest Table 5. E ective relative molecular mass cut o (MWCO) of GM membrane against representative surface waters at their natural conditions of ph and ion content (see Table 2) Water Overall DOC rejection (%) E ective MWCO of GM R_SL-SW Twitchell SUVA 254 and a low ionic strength. Its high molecular mass species therefore have greater aromaticity and can maintain an elongated con guration (due to low ionic strength) thus enhancing exclusion from the membrane's pores. Some self aggregation and/or adsorption at the membrane surface is possible as the interfacial concentration increases which would further increase the apparent rejection. Flux decline (W_SL-SW water) Flux declines from ltration of W_SL-SW water, which has relatively low DOC, low RMM and low aromaticity, using NF45, YM3, GM and PM10 membranes were monitored over time. These data are presented in Fig. 5. Table 6 lists the calculated resistances for each of the series resistances discussed in the context of equation 2. The presence and magnitude of these resistances support the following interpretations. As a general observation, NF45 and YM3 membranes did not exhibit any signi cant ux decline over the time period of our measurements. The NF45 experiences minimal concentration polarization (CP) probably because of the low permeation rate and low feed DOC concentration. Also, apparently no adsorption on the NF45 occurs over the time scale of these measurements. YM3 does not reject any signi cant amount of DOC (therefore no CP) and similarly does not experience any apparent adsorption of the NOM from this water. The GM apparently rejects enough of the NOM in this water (probably due to the membrane surface charge) to create a weakly adsorbed gel layer that can be cleaned by water ushing and NaOH cleaning. The PM10 is a high ux membrane and resistance builds up quickly even though there is no discernible DOC rejection during the measurement period.

7 UF and NF membrane ltration of NOM 2523 Fig. 5. Relative ux decline (and recovery for GM and PM10) experienced during ltration of W_SL- SW by YM3 (w), NF45 (q), GM (+) and PM10 () using protocol in Fig. 3. DOC measurements are not as speci cally sensitive to the aromatic components as the UVA 254 measurement. UVA 254 measurements were made on the overall feed and permeate reservoirs after the ltration tests were performed and are presented in Table 7. (The NOM UV spectrum in the area of this peak is broad so we used the value of absorbance at the speci c wavenumber.) All the membranes provided some rejection of aromatic components. The PM10 membrane probably rejected aromatic NOM components by adsorption on both the outer surface and in the pores. We can speculate that since the largest pores carry the most ow, they become the most quickly fouled (blocked) and signi cant relative ux decline occurs when the membrane's pore size distribution is broad (or bimodal). The PM10 may have this attribute. Adsorption of aromatic components by the GM may also occur but the mechanism is likely to be more complex due to the GM membrane's intrinsic charge and the presence of the other (charged) rejected NOM components at the interface. Flux decline (Twitchell water) Figure 6 presents the ux declines using the Twitchell water, which has relatively high DOC, high RMM, and high aromaticity, with the four di erent membranes. The calculated resistances are listed in Table 8. The NF45 and YM3 still exhibit insigni cant CP. This is probably because of the low permeation rate. Apparently, no adsorption occurs over the time scale of these measurements. GM exhibits somewhat faster ux decline kinetics when compared to the results with W_SL-SW, which is probably due to the di erence in DOC feed concentration. PM10 also exhibits faster and larger ux decline than W_SL-SW and the magnitudes of all resistances are greater. The high aromatic content of the feed NOM in the Twitchell water and the hydrophobicity of PM10 (according to contact angle measurements) likely combine to cause greater driving force for adsorption of mass on the PM10. Table 6. Resistance-in-series from ltration of W_SL-SW water NF45 (cm 1 ) YM3 (cm 1 ) GM (cm 1 ) PM10 (cm 1 ) r m r c 0.9 r g r a r a2 1.4 Table 7. UVA 254 (aromatic components) measurements after cross- ow ltration tests (arbitrary units) W_SL-SW water Twitchell water feed reservoir permeate feed reservoir permeate NF YM GM PM

8 2524 Jaeweon Cho et al. Fig. 6. Relative ux decline (and recovery for GM and PM10) experienced during ltration of Twitchell water by YM3 (w), NF45 (q), GM (+), and PM10 () using protocol in Fig. 3. General discussion of measurement uncertainties Based on systematic uncertainties from resolution of the mass balance, volumetric standards and timing devices we estimate the uncertainties in the permeate ux measurements to be 0.2±1% of the reported value. The lower uncertainties are for the initial periods when the permeation rates are higher. Carrying this uncertainty into the calculation of the tabulated resistances-in-series, we estimate an uncertainty of between 1 and 10% of the reported value, again depending on its magnitude. The expanded uncertainties (coverage factor of 2) due to random and systematic e ects (based on replicate analyses) in the reported NOM rejections are 22 to 5% when based on DOC and 20.5 to 2% when based on UVA 254. The expanded coverage on the RMM values are estimated to be 260 based on the variance of peak times observed for replicate measurements with the RMM standards. Concentration boundary layer mass transfer e ects Solute rejection by the membrane leads to CP, that is a higher concentration of solute nearer to the membrane surface than in the bulk uid (which Table 8. Resistance-in-series from ltration of Twitchell water NF45 (cm 1 ) YM3 (cm 1 ) GM (cm 1 ) PM10 (cm 1 ) r m r c 20.6 r g r a r a2 4.3 has a more uniform concentration). This concentration gradient provides for back mass transfer of rejected solute away from the membrane surface. This mass transfer coe cient (k) in the concentration boundary layer for our experimental protocol was estimated after assuming an NOM di usion coe cient (D) of cm 2 /s. This value is in the range reported (Cussler, 1984) for a variety of medium RMM proteins in aqueous solution at 298 K. The classical (Porter, 1972) mass transfer coe cient correlation for laminar ow in a channel, ub D 2 1=3 k ˆ 1:177, 4 hl (with h = half channel height, L = length of channel and u b =average bulk velocity) was used to determine that k = cm/s in the measurements presented in the previous sections. If we assume D =310 6 cm 2 /s (a preliminary value for humic acids measured by a collaborator) then k = cm/s. These values of k may be compared to the initial pure water uxes (in Table 9) for the membranes. The lack of signi cant CP and adsorbed-layer formation during ltration with the NF45 and YM3 Table 9. Initial pure water ux, J 0 Membrane W_SL-SW, J o 10 3 (cm/s) Twitchell, J o 10 3 (cm/s) NF YM GM PM

9 UF and NF membrane ltration of NOM 2525 Fig. 7. Relative ux decline (and recovery) for GM (+) and PM10 () experienced during ltration of Twitchell water at two di erent transmembrane pressures (TMP) using protocol in Fig. 3. (Larger symbols are at lower TMP.) membranes is consistent with the estimate that solute mass transfer away from the membrane interface is very close to the ux toward the membrane. In the case of the GM membrane, the initial ux may have been 050% higher than the back transport. Therefore, early CP could have led to some adsorption and subsequent ux decline. This decline could then have retarded further CP, in such a way that our resistance-in-series analysis did not detect it. The PM10 membrane probably had high concentration polarization and susceptibility to surface and pore adsorption. This led to its severe ux decline and contributions from all the resistances included in our analysis. The role of interfacial solute concentration on the ltration resistances was further illustrated by ltering Twitchell water with new samples of GM and PM membranes, but using a transmembrane pressure (TMP) such that the initial pure water uxes would be approximately equal to each other and closer to the estimated mass transfer coe cient for the concentration boundary layer. The average velocity in the membrane test cell was kept the same as in the previous measurements. Figure 7 presents a comparison of the ux decline measurements for both sets of GM and PM10 membranes and Twitchell water and Table 10 lists the initial water uxes, TMP, %DOC rejection and calculated resistances-in-series. The lower value of J 0 corresponds to very small or nonexistent initial concentration polarization at the membrane interface. Therefore the dependency of the relative ux decline and the resistances-inseries on the concentration of NOM at the membrane interface is illustrated by the two levels of J 0. Both membranes exhibit qualitatively similar ux decline responses but the magnitudes are signi cantly lower when the NOM concentration at the membrane interface is lower. The main contribution to ux decline remains weakly adsorbed NOM but Table 10. Mass transfer e ects from ltration of Twitchell water by GM and PM10 membranes GM PM10 GM PM10 TMP (kpa) J 0 (cm/s 10 3 ) % DOC rejection Resistances-in-series (cm 1 ) r m r c r g r a r a

10 2526 Jaeweon Cho et al. both its apparent adsorption rate and its overall ow resistance are less when J 0 (and presumably interfacial NOM concentration) is lower. The average DOC rejection was also lower when J 0 was set closer to k (the concentration boundary layer mass transfer coe cient) and did not signi cantly change over the course of the ltration. The ux decline results (for both initial ux cases) seems to support a hypothesis that signi cant pore blockage occurs quickly. The continued gradual ux decline is unlikely to be due to buildup of a gel or surface layer, more likely to be the gradual narrowing and closing of further pores. This observation might suggest that NOM adsorption at the largest pores can occur very quickly to restrict both the convective ow and further NOM entry, thus increasing rejection and ux decline. Note that the individual resistances for the PM10 membrane are less than those of the GM membrane at the lower J 0 condition, even though the ux decline is greater. The net ux decline is given by the ratio r m / (r m +r c +r g +r a1 +r a2 ), which is larger for the PM10 membrane. Additional measurements, to create a larger database obtained under a consistent protocol, will provide us with a basis for better coupling of the underlying mechanisms to the observed ux resistances. CONCLUSIONS Considering the literature already cited and our own measurements, NOM rejection, based on DOC, is clearly controlled by size exclusion, electrostatic repulsion and aromaticity/hydrophobicity interactions between NOM and the membrane surface and pores. Feed NOM concentration and NOM aromaticity were less important factors in ux decline with relatively low ux membranes such as NF45 and YM3 at the permeate rates we used. However, the ux decline of relatively high ux membranes (for example UF) can be in uenced by NOM aromaticity and membrane hydrophobicity. We suggest that a tabulation of e ective resistances in cross ow ux decline measurements should be obtained using a consistent means of controlling the initial interfacial concentration. Correlations for boundary layer mass transfer coe cients are signi cant approximations but using them allows us to more consistently compare measurements on membranes with di erent permeabilities and operating under di erent conditions than not using them. Currently, measures of the solutes' and membrane's physical and chemical properties only provide a qualitative means of rationalizing the transport measurements. In the case of the natural surface waters we studied, the combination of total DOC, SUVA and % humic were consistent indicators of the NOM ltration properties when coupled with the available data on membrane material properties. 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