Coupled Effects of Scaling and Colloid Deposition on the Performance of Reverse Osmosis Membrane Filters

Transcription

1 FINAL PROJECT REPORT Coupled Effects of Scaling and Colloid Deposition on the Performance of Reverse Osmosis Membrane Filters Principal Investigator: Dr. Volodymyr Tarabara, Michigan State University

2 NWRI Final Project Report Coupled Effects of Scaling and Colloid Deposition on the Performance of Reverse Osmosis Membrane Filters Prepared by: Volodymyr Tarabara, Ph.D. Department of Civil and Environmental Engineering Michigan State University East Lansing, Michigan Published by: National Water Research Institute Ward Street P.O. Box 8096 Fountain Valley, California USA December 2007

3 About NWRI A 501c3 nonprofit organization, the National Water Research Institute (NWRI) was founded in 1991 by a group of California water agencies in partnership with the Joan Irvine Smith and Athalie R. Clarke Foundation to promote the protection, maintenance, and restoration of water supplies and to protect public health and improve the environment. NWRI s member agencies include Inland Empire Utilities Agency, Irvine Ranch Water District, Los Angeles Department of Water and Power, Orange County Sanitation District, Orange County Water District, and West Basin Municipal Water District. For more information, please contact: National Water Research Institute Ward Street P.O. Box 8096 Fountain Valley, California USA Phone: (714) Fax: (714) by the National Water Research Institute. All rights reserved. NWRI This NWRI White Paper is a product of NWRI Project Number 05-TM-007. ii

4 Acknowledgments This Final Research Project Report was prepared by Volodymyr Tarabara, Ph.D., of Michigan State University and sponsored by the National Water Research Institute of Fountain Valley, California. Special thanks are extended to the NWRI Research Advisory Board subcommittee, who provided valuable suggestions and insight in the preparation of this report, including David H. Furukawa, Ph.D. (Separation Consultants, Inc.), Douglas R. Lloyd, Ph.D. (University of Texas at Austin), and Robert L. Riley, Ph.D. (Separation Systems Technology). iii

5 Introduction This NWRI-funded research project aimed at a) enhancing the fundamental understanding of the process of reverse osmosis (RO) membrane fouling by feed waters of complex composition and b) developing predictive tools for the description of permeate flux under such conditions. The fouling could be due to residual particulate matter that passed the pretreatment stage or due to large colloidal loadings in cases when pretreatment fails or does not exist. By recognizing possible interferences between rejected salts and colloids deposited on the membrane surface, the work explored the phenomenon of coupling between colloidal fouling, concentration polarization, and scaling as main factors limiting the application of RO membranes. The research was conducted in three stages: 1. Stage A: Study on coupled effects of colloidal fouling and concentration polarization of soluble salts on RO membrane performance. 2. Stage B1: Calcium sulfate dihydrate (gypsum) scaling in the presence of silica particles in a batch system. 3. Stage B2: Calcium sulfate dihydrate (gypsum) scaling in the presence of silica particles in a RO membrane system. Stage A: Study on Coupled Effects of Colloidal Fouling and Concentration Polarization of Soluble Salts on RO Membrane Performance First, the simpler case of the interaction of colloids with soluble salts only was considered. The schematic of the bench-scale crossflow RO unit is shown in Figure 1: Two identical Sepa CF II high-pressure membrane modules in medium/high foulant configuration (GE Osmonics, Minnetonka, MN) were connected in parallel. A positive displacement pump (M03 Hydracell, Wanner Engineering, Minneapolis, MN) equipped with a flow rate control unit (KBMD-240D, KB Electronics Inc., Coral Springs, FL) was used to deliver feed water to the filtration cell. A pulsation dampener (Model H1020V, Blacoh Fluid Control Inc., Riverside, CA) was installed immediately downstream from the pump outlet. A back pressure regulator (BP-3, Circle Seal Controls Inc., Corona, CA) was used to maintain the transmembrane pressure differential at a constant predetermined value in the range of 150 pounds per square inch (psi) (1.034 megapascals [MPa]) to 500 psi (3.477 MPa); the pressure was continuously monitored by an analog pressure transducer (PX G5V, Omega, Stamford, CT). 1

6 Figure 1. Schematic of the experimental apparatus. In this part of the project, we proposed and implemented a novel approach to quantifying the degree of concentration polarization in the presence of a membrane deposit. Based on measurements of osmotic pressure and salt flux in the absence of colloidal foulants, the membrane salt permeability constant (B) of the membrane was experimentally determined first: J = B( C C ) = JC s m p p Then, the constant B, together with measured values of permeate flux and observed rejection, were used to calculate the concentration of rejected salt at the membrane wall and the resulting osmotic pressure when colloidal particles were present in the feed: JC Δπ = = m 2RT ( C m C p ) 2RT B This approach allowed for a clear identification of individual contributions of concentration polarization and colloidal fouling to the permeate flux decline (Figure 2). The observed transient behavior of permeate flux, salt rejection, mass of deposited colloids, and effective porosity of the deposited layer unequivocally pointed to the importance of the two-way coupling between salt concentration polarization and colloidal deposition. p 2

7 Figure 2. (a) Normalized permeate flux at three ionic strengths; (b) observed salt rejection, R obs ; (c) salt concentration at membrane surface, C m ; (d) mass of deposited particles, M d ; (e) effective porosity, ε ; and (f) hydraulic resistance of deposited colloidal layer, R d. After the initial increase in the effective porosity of the fractal colloidal deposit, the porosity reached a maximum, and gradually decreased to a steady state value that depended strongly on the solution ionic strength. Both the resistance of the colloidal deposit and osmotic pressure, which could be enhanced in the presence of colloids, contributed significantly to the permeate flux decline (Figure 3). A short-term increase in salt rejection was observed upon introduction of colloidal particles into the feed in the absence of membrane channel spacer. This increase could be sustained over the long term in the presence of spacers and was attributed to the depolarization of the salt boundary layer due to local mixing by the concentrated flowing layer of colloidal particles (Figure 4). 3

8 Figure 3. Energy consumption analysis for the system: (a) at 10 2 M; (b) at 10 1 M. Figure 4. Effect of the feed channel spacers on membrane performance: (a) Normalized permeate flux; (b) Observed salt rejection. Individual contributions of osmotic pressure and colloidal fouling to the RO permeate flux decline can be identified by determining salt permeability constant and measuring salt transport across the membrane. By using this approach, we demonstrate that the twoway coupling between salt concentration polarization and colloidal deposition is essential in determining both permeate flux and rejection. On one hand, porosity of the deposit and its resistance to the permeate flux are measured to be strong functions of the solution ionic strength. The initial growth of the colloidal deposit on the surface of the salt-rejecting membrane is found to be fractal, with the fractal dimension of 2.1 corresponding to reaction-limited aggregation. After the breakup and restructuring of the deposit, its resistance increases dramatically and contributes significantly to overall flux decline. On the other hand, deposition of colloidal particles on the membrane surface influences salt transport to and across the membrane in a complex way. Formation of the stagnant 4

9 colloidal deposit results in the hindrance of the back diffusion of salt away from the membrane surface. Under certain conditions, however, the presence of colloidal particles improves membrane performance. Specifically, the introduction of colloidal particles into the feed results in a short-term increase in salt rejection; this increase can be sustained over the long term when feed channel spacers are used. Stage B1: Calcium Sulfate Dihydrate (Gypsum) Scaling in the Presence of Silica Particles in a Batch System Although both particular and precipitation fouling have been subjects of extensive studies, surprisingly little is known about how these processes influence one another when they occur simultaneously (Sheikholeslami, 2000). With no research published to date on the effect of particles on precipitation fouling of membranes, most of the available knowledge comes from studies of heat exchangers, where the effect has been demonstrated to be complex (Hasson, 1999). The effect of suspended particles on scaling appears to depend on the crystallizing ability of particles (Bansal et al., 1997; Andritsos and Karabelas, 1999). Bansal et al. (1997) showed that crystallizing Calcium Sulfate (CaSO 4 ) particles greatly enhanced gypsum formation and attributed this effect to the increase in the number of nucleation sites. In the study on the deposition of colloids on heat exchanger plates, McGarvey and Turner (1999) found that the rate of particle deposition was enhanced by Calcium Carbonate (CaCO 3 ) precipitation. The enhancing effect of crystallizing colloids was also reported by Andritsos and Karabelas (1999), who showed that CaCO 3 precipitation rate was greatly enhanced by fine aragonite (CaCO 3 polymorph) particles. There are conflicting reports on the effect of noncrystallizing particles, which were shown to either have no effect on scaling (studies of CaCO 3 precipitation from silica [Andritsos and Karabelas, 1999], hematite [McGarvey and Turner (1999)], and silt [McGarvey and Turner (1999)] suspensions) or to reduce the amount of scale formed (research on gypsum precipitation from suspensions of alumina colloids [Bansal et al., 1997]). The objective of this stage of the project was to elucidate the effect of colloidal silica a common membrane foulant on the scaling of calcium sulfate dehydrate also a common membrane foulant under well-defined conditions of a simple batch reactor. Silica is a noncrystallizing particle with respect to gypsum. Model feed waters were prepared by suspending silica colloids in calcium chloride and sodium sulfate solutions of different degrees of supersaturation for gypsum. The effect of colloids on gypsum crystallization was examined in terms of induction time, rates of nucleation and crystallization, and the morphology of gypsum crystals. The nucleation of gypsum scale was studied in a batch reactor system to elucidate colloid-induced changes in the gypsum crystallization process for cases of different degrees of solution saturation. The induction time for gypsum crystal formation was determined based on measurements 5

10 of light transmittance through the sample. The rate of gypsum crystallization was determined by measuring the conductivity of the feed suspensions; that is, by determining how quickly dissolved calcium sulfate was removed due to its incorporation into suspended gypsum crystals. Suspension samples were taken periodically, dried in methanol, and subjected to examination by scanning electron microscopy (SEM) to evaluate size distribution and the morphology of grown gypsum crystals. The experimental batch system is shown in Figure 5: A 1.5-liter Pyrex beaker with a 1-liter feed suspension was placed in a water bath of a multi-stirrer (Model No.1286, Lab-Line Instruments Inc., Melrose Park, IL). The temperature of the water in the bath was maintained at (20 ± 0.2)ºC using a programmable circulating chiller (Model 9512, PolyScience, Niles, IL) equipped with an external resistance temperature detector (RTD). The water in the bath was mixed by two 3-inch magnetic stirrer bars at a rate larger than 200 revolutions per minute (rpm). The feed suspension in the beaker was mixed by a 3-inch stirrer bar at a rate of 120 rpm. A gear pump (Model No. EW , Cole-Parmer Instrument Company, Vernon Hills, Illinois) was used for diverting a small portion of the sample into a flow-through UV-vis sample cell (Model QS, Hellma GmbH & Co KG, Müllheim) for online transmittance measurement using a spectrophotometer (Multi-Spec 1501, Shimadzu, Kyoto, Japan). The suspension was continuously circulated by the pump at a flow rate of 1.6 liters per minute (L/min). The transmittance in the wavelength range of (400 to 800) nanometers (nm) was recorded in real time. The probe (Model A, Thermo Electron Corp. Beverly, MA) of a conductivity meter (Orion 550, Thermo Electron Corp.) was used for real-time measurement of the suspension s conductivity with 30 s measurement interval. A 5-percent decline in transmittance was considered to correspond to the beginning of gypsum crystallization (Lancia et al., 1999). The absorption at 600 nm wavelength was used for determining the induction time. As Figure 6 indicates, the value of the induction time determined based on transmittance measurement was lower than that determined based on conductivity measurements. This result indicates that the transmittance is a more sensitive indicator of when the processes nucleation and the crystal growth commence. The dependence of the induction time on the degree of solution saturation in experiments without or with particles is shown in Figure 7(a). It was found that the presence of particles resulted in a longer induction time. This effect was more evident at low saturation indices. 6

11 Figure 5. Schematic of the batch system. 100 (a) 80 No Particles With Particles Transmittance (%) S=6.0 S=5.0 S=4.0 S=3.0 S= Conductivity (ms/cm)_no Particles 0 16 (b) 15 S= S=5.0 No Particles 13 With Particles 12 S= S=3.0 9 S=6.0 S= Time (min) Conductivity (ms/cm)_with Particles Figure 6. The transmittance (a) and conductivity (b) of the feed solutions as a function of time. 7

12 Induction time (min) With Particles No Particles Saturation index (S) (a) Induction time Crystallization rate (min -1 ) With particles No particles Saturation index (S) (b) Crystallization rate Figure 7. Effect of particles on gypsum crystallization: (a) Induction time; (b) Crystallization rate. To determine the surface energy of gypsum crystals, log(t ind ) was plotted against 1/(log S) 2 for different saturation indices. From the slope, the surface energy was calculated to 2 be ± 0.96 J/(mol K) ( R = 0.98) for the crystals formed in the absence of silica 2 particles and ± 1.15 J/(mol K) ( R = 0.98) for the crystals formed in the solution that contained 50 milligrams per liter (mg/l) suspended silica. The gypsum crystallization rate was determined based on the transient behavior of solution conductivity following the beginning of gypsum crystal nucleation. The decline of conductivity is proportional to the decrease of [Ca 2+ ] and [SO 4 2- ] due to the formation of gypsum crystals. 8

13 Thus, the decline rate of conductivity can be used for determining dc/dt based on the standard curve of conductivity and CaSO 4 concentration: dc dt = k Where k b is the crystallization rate (min -1 ). b ( Cb Cs Gypsum crystallization rates at different saturation indices are shown in Figure 7(b). For all saturation indices, the presence of silica particles resulted in a higher gypsum crystallization rate. A 10-milliliter (ml) sample of the feed suspension was taken when transmittance decreased to 50 percent at a saturation index of 5. The samples were taken at t=26.5 minutes (min) and at t=28.0 min for the experiments without particles and with particles, respectively. The sampling time was different because of the difference in induction time (27.2 min ± 1.0 min and 28.4 min ± 0.8 min for the experiments without particles and with particles, respectively). This choice of sampling times ensured that the crystals underwent growth over the same period of time. After the samples were filtered through a 0.1 micrometer (µm) polycarbonate (PC) tracketch membrane, 5-mL methanol was used to remove the residual water from membrane surface, thereby preventing the further growth of the crystals on the membrane surface. SEM imaging demonstrated that predominantly needle-like gypsum crystals were formed in the absence of colloids in the feed. In contrast, when particles were present, most crystals had plate-like morphologies with a typical length-to-width ratio of ca. 2 (Figure 8). There was no evidence of the epitaxial growth wherein colloids particles would be incorporated into the gypsum crystals. In summary, it was found that in the presence of silica particles, it took longer for gypsum crystal nuclei to form, with the delay being more pronounced at lower saturation indices. At the same time, the gypsum crystallization rate was increased in the presence of colloids. SEM imaging revealed that larger crystals with predominantly plate-like morphology were formed when colloids were present in the feed. Based on the totality of light transmittance, conductivity, and SEM data, we hypothesize that colloids influence the process of gypsum crystallization via the mechanisms of collisions and local mixing. The collisions between colloids and nascent nuclei should result in a delay in nucleation while enhanced local mixing should promote, by relaxing diffusion gradients, the growth of a nucleus into a crystal once the nucleus is formed. The reasons for the predominant formation of crystals with plate-like morphology in the presence of colloids remain unclear. ) 9

14 (a) (b) (c) (d) (e) (f) (g) (h) Figure 8. Gypsum crystals morphology in the absence of silica colloids (a, c, e, g) and in the presence of silica colloids (b, d, f, h). The size of silica colloids is ca. 56 nm. 10

15 Stage B2: Calcium Sulfate Dihydrate (Gypsum) Scaling in the Presence of Silica Particles in a RO Membrane System At this stage of the project, experiments similar to Stage B1 batch reactor studies were carried out within pressurized membrane cell under conditions (geometry, crossflow, etc.) that closely simulated those of a commercial RO membrane module. Two gypsum crystallization regimes were studied: Surface crystallization only, when less concentrated feed solutions were used (saturation index: 0.9). Concomitant surface and bulk crystallization, when supersaturated solutions were used (saturation index: 4.0). The colloid loading was the same in all experiments. The experiments were conducted by sequentially adding highly soluble Calcium Chloride Dihydrate (CaCl 2 2H 2 O) and Sodium Sulphate (Na 2 SO 4 ) instead of directly adding Calcium Sulfate Dihydrate (CaSO 4 2H 2 O). This method of solution preparation was chosen for two reasons: The amount of undissolved salts was minimized (Bansal et al., 1997). Filtration of Calcium Chloride (CaCl 2 ) solution for 12 hours (h) conditioned membrane and, thus, reduced the effect of the salt addition-initiated changes in membrane properties on permeate flux decline. Experimental protocol is similar to what we used in our previous study (Wang and Tarabara, 2007). After compacting the membrane for 36 h and determining its hydraulic resistance, CaCl 2 2H 2 O was dissolved in the 20 L feed suspension to condition the membrane for 12 h. For gypsum scaling experiments, Na 2 SO 4 was dissolved and added into the feed suspension. In control experiments with only soluble salts and silica colloids, Sodium Chloride (NaCl) was used to replace Na 2 SO 4 to obtain solutions of similar ionic compositions. The initial permeate flux was controlled to be in the 53 liters per square meter per hour (L/m 2 /h) to 54 L/m 2 /h ( meters per second [m/s] to m/s) range by adjusting the transmembrane pressure. In experiments with silica colloids, 1.90 ml ST-XL stock solution was added into the feed solution to achieve a 50 mg/l silica particle loading. The ph of the feed suspension during the fouling experiments was between 5.8 and 6.2 and was not adjusted. The filtration experiment was stopped after 12 h. Membrane coupons were taken out and prepared for SEM imaging, as mentioned earlier. After each experiment, the crossflow system was cleaned thoroughly with a detergent solution. The ph of detergent solution was increased to 10 using a Sodium Hydroxide (NaOH) solution. Then the system was flushed with deionized water four times. The Teflon tubes in the retentate line were replaced by new ones to remove the possible silica colloids deposited. 11

16 To determine the mass of silica colloids deposited on the membrane, absorbance of the feed suspension was measured every 2 h. The suspension was diluted two times to dissolve the possible gypsum crystals. The concentration of silica colloids deposited was calculated based on the absorption of the suspension at 220 nm using a standard curve recorded beforehand (p 2 =1). A simple mass-balance equation was applied to calculate the mass of particles deposited on membrane surface (M p ). In the tests with supersaturated solutions, it was found that the silica colloids increased the induction time of gypsum crystallization, but also increased the crystallization rate. When transmembrane pressure was applied, the induction period was shortened and the crystallization rate was enhanced (Figure 9); these were attributed to the formation of gypsum crystals in the concentration polarization layer or directly on the membrane surface Transmittance (%) With Pressure, No Particles With Pressure, With Particles No Pressure, No Particles No pressure, With Particles Conductivity (ms/cm) Filtration time (min) (a) With Pressure, No Particles With Pressure, With Particles No Pressure, No Particles No pressure, With Particles Filtration time (min) (b) Figure 9. Transmittance (a) and conductivity (b) of feed suspensions at different operating conditions (Silica colloid loading: 50 mg/l; Transmembrane pressure (if applied): 300 psi; Experiments were conducted with spacers using supersaturated solutions with SI = 4). It is clearly shown that the colloids increased the rate of gypsum crystallization. These results were anticipated based on the data obtained in experiments in a batch reactor (Stage B1). The crystallization rates under different operating conditions were as follows: Without silica colloids, without transmembrane pressure: k 1 = liters per second (1/s). With silica colloids, without transmembrane pressure: k 2 = /s. Without silica colloids, with transmembrane pressure: k 3 = /s. With silica colloids, with transmembrane pressure: k 1 = /s. Altogether, (246.1 ± 4.7) milligrams (mg) gypsum was formed in the experiment with gypsum scaling (indicated as CaCl 2 + Na 2 SO 4 in Figure 10c), and (242.9 ± 4.5) mg 12

17 gypsum formed from the experiment with gypsum scaling and colloidal fouling (indicated as CaCl 2 + Na 2 SO 4 + Particles). The amount of calcium ion for the gypsum on membrane surface accounted for less than 0.6 percent of calcium in the feed solution, which indicates that the amount of gypsum formed on membrane surface did not significantly affect the CaSO 4 concentration in feed solution Normalized pemeate flux CaCl2 + Na2SO4 only CaCl2 + NaCl + Particles CaCl2 + Na2SO4 + Particles Permeate conductivity (us/cm) CaCl2 + NaCl + Particles CaCl2 + Na2SO4 + Particles CaCl2 + Na2SO4 only Filtration time (h) (a) Filtration time (h) (b) 1 Mass of particles deposited (mg) CaCl2 + Na2SO4 + Particles CaCl2 + NaCl + Particles Filtration time (h) (c) Normalized permeate flux CaCl2 + NaCl + Particles CaCl2 + Na2SO4 only CaCl2 + Na2SO4 + Particles Filtration time (h) (d) Flux can not be measured in-line Figure 10. Normalized permeate flux (a and d), permeate conductivity (b), and particles of particles deposited (c), as a function of filtration time (Silica colloid loading: 200 mg/l; Experiments were conducted without spacers using less saturated solutions with SI = 0.9 for a-c, and with SI = 4.0 for d). In summary, the most dramatic characteristic of the studied system was the rapid flux decline observed for both undersaturated (see Figure 10a) and supersaturated (see Figure 10d) solutions. In each case, the flux decline was significantly higher than could be expected based on the simple summation of flux declines due to colloidal deposition and gypsum scaling. The drastic decline in permeate flux in the case of colloids present in the feed was observed despite the fact that the amount of gypsum formed on the membrane was approximately the same as that in the case of colloid-free feed water. It 13

18 appears that the main reason for this flux drop is the different distribution of gypsum precipitate along the membrane surface. Visualization of gypsum deposit along the membrane surface (Figure 11) confirmed more homogeneous coverage of the membrane surface by the scale in the case when colloids were present in the feed. Also, based on the batch reactor study of the formation process and morphology of gypsum crystals (stage B1), we hypothesize that under the colloidinduced conditions of predominant formation of plate-like crystals, the lateral growth of surface crystals is favored, which translates into an increase in the membrane area blocked by surface precipitates and a corresponding decrease in membrane surface area available for permeation. (a) (b) (c) (d) Figure. 11. Gypsum scale in the absence (a, b) and in the presence (c, d) of silica colloids in the feed. Experiments were conducted using less saturated solutions with SI =

19 Summary 1. By measuring salt transport across the membrane, individual contributions of osmotic pressure and colloidal fouling to the RO permeate flux decline can be identified. 2. Two-way coupling between salt concentration polarization and colloidal deposition is essential in determining both permeate flux and rejection. 3. The initial growth of the colloidal deposit on the surface of the salt-rejecting membrane is found to be fractal. After the breakup and restructuring of the deposit, its resistance increases dramatically and contributes significantly to the overall flux decline. 4. The introduction of silica particles increased the initial observed salt rejection by depolarizing the previously stabilized layer of rejected salts near the membrane surface. When feed channel spacers are used, the initial increase in salt rejection can be sustained over the long term. 5. The concentrated flowing layer of colloidal particles is hypothesized to be responsible for the local mixing of the concentration polarization layer of salt; the overall effect of colloidal deposition on permeate flux and rejection is determined by the balance between the stagnant and flowing parts of the colloidal deposit. These findings point to the potential of using particles with low deposition propensity as mobile mixers to complement feed channel spacers as means of improving the performance of saltrejecting membranes. 6. The presence of colloids does not cause more gypsum scale to form, but contributes to a much faster flux decline. The flux decline is greater than what could be expected based on the simple summation of flux declines due to colloidal deposition and due to gypsum scaling. 7. The main reason for the drastic decline in flux is the difference in the distribution of gypsum precipitate along the membrane surface. It is also hypothesized that under the colloid-induced conditions of predominant formation of plate-like crystals, the lateral growth of surface crystals is favored, which translates into a faster decrease in the membrane surface area available for permeation. 8. A layer of surface crystals with interstitial spaces filled with colloidal particles forms on the surface. The distribution of colloids in the surface layer is not uniform. Silica colloids attach onto the surface of precipitated gypsum crystals, increasing resistance of the membrane deposit and making hydraulic cleaning inefficient. 15

20 Conclusions A straightforward method for the quantification of the relative contribution of the concentration polarization to the overall permeate flux decline was proposed. The method was used to study interrelationships between a) colloidal deposition, b) concentration polarization (primarily due to highly soluble salts, such as NaCl), and c) formation of bulk and surface scale (due to precipitation of sparingly soluble salts, such as gypsum) on the performance of RO membranes challenged by colloid-bearing feed waters. The study aimed at simulating scenarios of inadequate or failed feed water pretreatment. The uncovered complex interdependencies between the three processes directly implicate colloidal fraction as a major contributor to the permeate flux decline. The contribution of colloidal fraction is threefold: Enhancement of concentration polarization. Formation of a low permeability deposit on the membrane surface even in the absence of (gypsum) scale. Dramatic enhancement of membrane surface blockage of membrane by (gypsum) precipitate via: a) Colloid-induced changes in the morphology of (gypsum) scale. b) Redistribution of the surface-blocking (gypsum) scale along the membrane. c) Formation of an irreversible, dense colloid-scale cake on the membrane surface. By detailing the mechanisms of colloid-mediated fouling of RO membranes, the present study shows that and explains why even relatively small colloidal loadings could lead to drastic and irreversible decline in permeate flux. 16

21 Publications Based on this Project Journal Papers Wang F., and V.V. Tarabara (2007). Coupled effects of colloidal deposition and salt concentration polarization on reverse osmosis membrane performance. J. Membr. Sci., 293: Wang F., and V.V. Tarabara (Pending). Calcium sulfate crystallization in the presence of colloidal silica. I: Stirred batch reactor study. In preparation for submission to Water Res. Wang F., and V.V. Tarabara (Pending). Calcium sulfate crystallization in the presence of colloidal silica. II: Implications for precipitative fouling of reverse osmosis membranes. In preparation for submission to J. Membr. Sci. Wang F., and V.V. Tarabara (Pending). Permeability of fiber-filled phase inversion polymeric membranes. In preparation for submission to J. Membr. Sci. Presentations/Conference Papers Wang F., and V.V. Tarabara (2006). Coupled Effects of Salt Concentration Polarization and Colloidal Deposition on the Performance of Reverse Osmosis Membranes AIChE Annual Conference (paper #541c), Conference held on San Francisco, CA, November 2006, American Institute of Chemical Engineers, New York, NY. Wang F., and V.V. Tarabara (2007). Coupled effects of scaling and colloidal deposition on the performance of reverse osmosis membranes. The 233rd ACS National Meeting (paper #SUST111), meeting held in Chicago, IL, March 2007, American Chemical Society, Washington, DC. 17

22 References Andritsos, N., and A.J. Karabelas (1999). The Influence of particulates on CaCO3 scale formation. J. Heat Transfer, 121: Bansal, B., H. Muller-Steinhagen, and X.D. Chen (1997). "Effect of suspended particles on crystallization fouling in plate heat exchangers." J. of Heat Transfer, Transactions ASME, 119: 568. Hasson, D. (1999). Progress in precipitation fouling: a review in Understanding Heat Exchanger Fouling and Its Mitigation, T.R. Bott et al. (Eds)., Begell House, New York, NY. Lancia, A., D. Musmarra, and M. Prisciandaro (1999). "Measuring induction period for calcium sulfate dihydrate precipitation." AIChE J., 45: McGarvey, G.B., and C.W. Turner (1999). "Synergism models for heat exchanger fouling mitigation" in Understanding Heat Exchanger Fouling and Its Mitigation, T.R. Bott et al. (Eds), Begell House, New York, NY, Sheikholeslami (2000). "Composite fouling of heat transfer equipment in aqueous media- A review." Heat Transfer Engineering, 21: Wang, F., and V.V. Tarabara (2007). "Coupled effects of colloidal deposition and salt concentration polarization on reverse osmosis membrane performance." J. Membr. Sci., 293: