- Email: [email protected]
Fouling mitigation in membrane processes
DESALINATION Desalination 123 (1999) 45-53
ELSEVIER
www.elsevier.com/locate/desal
Fouling mitigation in membrane processes Report on a Workshop held January 26-29, 1999", Technion- Israel Institute of Technology, Haifa, Israel
R. Sheikholeslami School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney 2052, Australia Tel. +61 (2) 9385-4343; Fax +61 (2) 9385-6955; email: [email protected] Received 25 February 1999; accepted 3 March 1999
Abstract
Membranes have become one of the most sought-after techniques in separation processes. They are used in various industries and plants such as desalination, water purification and wastewater treatment plants. There has been a great degree of advancement in development of membrane systems; the bottleneck remaining is the fouling of membranes. A workshop on fouling mitigation processes was recently coordinated as a joint effort by several organizations attracting various groups from the five continents and presenting and discussing recent initiatives in this area. The topics were mostly related to desalting and associated inorganic and possible biofouling. Recent and ongoing techniques were discussed and a road map for future research directions in this area was set forward. This paper reviews information presented in this workshop; it aims to provide updated information (current activities and the future work required) in this area on the topic of wide concern - - t h e problem of fouling mitigation. Keywords: Membrane fouling; Fouling mitigation; Inorganic fouling; Desalting
I. I n t r o d u c t i o n
While there has development of desalination, water disposal, fouling
been a great advance in the m e m b r a n e systems for purification and wastewater still remains as a major
bottleneck. In view of its major impact on process efficiency and economics, the fouling problem is receiving wide attention from membrane manufacturers, plant designers, plant operators and researchers. Naturally, there is considerable
*The workshop was organized under the auspices of the Fairleigh Dickinson University, Teaneck, N J, USA; The Technion Water Research Institute, Haifa, Israel; Ben-Gurion University Water Center, Beer Sheva, Israel; and the Middle East Desalination Research Center, Muscat, Oman. 0011-9164/99/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S O 0 1 1 - 9 1 6 4 ( 9 9 ) 0 0 0 5 8 - 2
46
R. Sheikholeslami / Desalination 123 (1999) 45-53
interest in getting reliable and well-balanced information on the progress generated by these persistent efforts. This objective can be best achieved through a meeting allowing in-depth deliberations on new developments. Such a meeting, initiated by the newly established Israeli Desalination Society, was held on January 26-29, 1999, at the Technion-Israel Institute of Technology, Haifa, Israel. A workshop, Fouling Mitigation in Membrane Processes, was organized under the auspices of the Water Research Institute of the Technion (Israel), the Water Center of Ben-Gurion University (Israel), the Middle East Desalination Research Center (Oman) and the Fairleigh Dickinson University (USA). The organizing committee was chaired in Israel by Professors David Hasson and Raphael Semiat of the Technion's WRI Rabin Desalination Laboratory and was chaired in the US by Professor Harvey Winters, Program Director inthe School of Natural Sciences of Fairleigh Dickinson University. The workshop was attended by worldrenowned fouling experts from both industry and academia. The meeting was originally intended to be a local event, but the quality of the program attracted international participation. The workshop was attended by 130 Israeli participants and over 40 visitors from 12 countries which included 11 participants from Jordan and the Palestinian Authority. The day devoted to pre-workshop activities started with tours of the research and teaching facilities of the Technion's Rabin Desalination Laboratory. This was followed by a lecture, delivered by Professor Uri Shamir, Director of the Technion's Water Research Institute on "Water Issues of the Middle East". Professor Shamir not only presented a clear and highly informative review of the complex mid-East water issues, but also gave a fascinating glimpse on the intricacy of water issue negotiations from his personal experience as a member of the official Israeli
negotiating team. In the afternoon, a fully loaded bus took visitors to Kibbutz Maagan Michael to visit three membrane field installations: an RO plant desalting brackish water operated by the Mekorot Water Co., an experimental RO unit of the electric company drawing its power from solar panels and a Mekorot pilot plant engaged in the development of a UF pretreatment technique for RO processing of the nearby highly polluted fishpond wastewaters. In the opening session chaired by Professors Hasson and Semiat, greetings were delivered by the Technion President, Maj. Gen. (Res.) Amos Lapidot; Dan Hoffman, on behalf of the Acting President of the Israel Desalination Society, Shmuel Kantor; the president of the International Desalination Association, David H. Furukawa; the Research Director of the Middle East Desalination Research Center, Prof. Klaus Genthner; the Chairman of the Technion Chemical Engineering Department, Prof. Moshe Sheintuch; and by the Director of the Technion Water Research Institute, Prof. Uri Shamir. The workshop opening lecture, chaired by Professor Winters, was presented by the manager of the US Desalination Research and Development Program, Kevin Price. A total of 17 lectures was presented in four sessions. The session on Fouling Control was chaired by Professors Genthner and Semiat, the session on Advanced Pretreatment Processes was chaired by Professor Kedem and Mr. Priel, the session on Anti-fouling Membranes and Modules-I was chaired by Drs. Amjad and Glueckstern, and the session on Anti-fouling Membranes and Modules-II was chaired by Mr. Mansdorf and Dr. Wilf. The author of this paper was charged with presented a critical review of the main workshop disclosures. This paper is based on the review presented by the author at the workshop closing session chaired by Professor A. Fane, Mr. K. Price and Mr. D. Furukawa. The final part of the closing session was devoted to a general discussion on
R. Sheikholeslami / Desalination 123 (1999) 45-53
R&D priorities relating to membrane fouling mitigation. Following three break presentations of the views held by the chairmen of the closing session, a lively discussion ensued, generating many interesting views. The following sections elaborate on the substantive information provided by the various speakers on the central workshop themes.
47
In the membrane area, the US Bureau has spent $3.7 million in 1998. The support has been for pretreatment technologies and development of new or modified membrane materials to combat the fouling propensity of membranes. The Bureau also supported programs for better module design, longer life material, modelling and economics, and technology transfer. This support has partially declined in 199 and is expected to have a further marginal decline until 2001.
2. Background W a t e r - - its production and its s o u r c e s - - have always been an important issue in the arid areas of the world. Due to its importance and the fact that water scarcity is detrimental to the human condition and agricultural purposes, it is fair to consider it one of the most important items of negotiation. This clarifies the need for a technologically viable method of water desalination, purification and/or reclamation in arid areas. This is especially true of the arid areas with accompanying political problems such as the Middle East. As was discussed in this workshop, efforts in collaborative research to jointly solve water issues can actually lead to better political relationships between the conflicting countries. In addition, in the environmental age that we are living in, the efficient use and reuse of water have become significant issues even in non-arid locations attracting both scientific and political attention. Research in this area is highly favoured by national and intemational granting agencies; the support given by US government agencies is an indication of this. In his opening lecture Kevin Price, manager of the US Desalting R&D program [1], reported that the US Bureau of Reclamation has allocated significant amounts to joint industry (75%)/government (25%) research projects in the area of desalting in 1998. Th support has been in all associated areas such as traditional desalting techniques (membranes, thermal), non-traditional techniques, water recycling and reuse, and economic assessments and design improvements.
3. Current research and projects in fouling mitigation As mentioned above, research in fouling mitigation has been carried out in various areas which can be classified into three categories: fouling control, pretreatment technologies, and anti-fouling membranes and modules. Below some of the projects in these areas will be discussed. 3.1. Fouling control
Fouling mechanism almost always is preceded by an induction period. The induction period is a function of the system and operating conditions. If one can increase the induction period indefinitely, fouling would never occur. Various projects are concerned with defining an operating parameter or with modifying the solution conditions by addition of minute amounts of chemicals to control fouling in various systems. Some of these are discussed below. 3.1.1. Critical flux
Since fouling propensity is a function of flux and cross flow velocity, there has been work on determining a relationship between these two parameters and incipient of fouling. Previous work [2,3] had established the existence of a critical flux, below which fouling resistance remained
48
R. Sheikholeslami / Desalination 123 (1999) 45-53
Particle~/~
Flux
Cake
No Cake
Cross Flow Velocity Fig. 1. Critical flux curves for various particle sizes.
negligible. More recent work reported by Prof. Tony Fane of the UNESCO Centre for Membrane Science and Technology [2] has visually confirmed this phenomenon. This was done by a non-invasive, in situ, continuous direct observation through the membrane (DOTM) technique. The technique used transparent microfiltration membranes (60% porous) both in the presence and absence of spacers using latex, yeast, algae and bacteria particles. Transparency of the membranes is due to the straight pores in membrane material, and therefore the technique at this stage is not applicable to non-porous reverse osmosis membranes. Fig. 1 shows some of the experimental results. 3.1.2. Critical conversion
Stability of colloidal particles depends on salinity, pH, flux, cross flow velocity, temperature, pressure, repulsion forces, and type of particles. Another parameter to consider is the critical conversion which is a function of critical flux, critical cross flow velocity and other parameters affecting the stability of solution. In his lecture, Professor Harvey Winters of Fairleigh Dickinson University, discussed the crucial role of critical
conversion values on the onset of fouling [3]. Control of colloidal fouling is largely dependent on control of a critical flux, critical cross flow velocity, and other parameters affecting the stability of solution. A study of several desalination plants in the Middle East indicated that almost all these plants were designed at above the critical conversion value and were therefore doomed to foul.
3.1.3 Antiscalants/antifoulants
Another method for fouling control is use of antiscalants/antifoulants. Latest developments in these fouling control agents for RO treatment processes were discussed by Dr. Zahid Amjad of the BF Goodrich Company [4]. There are four different mechanisms through which these chemicals can control fouling. They can act as scale inhibitors, particulate dispersants, scale crystal modifiers, or sequestrates (for Fe, Mg, etc.). It is difficult to categorize the fouling control chemicals under one specific category because they may act in a combination of the above mechanisms. Polyphosphates, which are actually polymers of phosphate, were the first scale inhibitors used. These, however, can cause problems due to hydrolysis and also due to enhancement of eutrophication. In addition, they do not show good performance as metal ion stabilizers and dispersants. Synthetic polymers developed at later stages tried to improve these shortcomings. Calcium and silica were identified as some of the important water constituents that need to be properly managed to prevent scale formation. There are some important points to notice when using fouling-control agents. Firstly, these controlling agents may have an interfere effects with each other or may control one foulant while adversely affecting the control of another foulant. For example, it has been shown that 1 ppm of a cationic polymer called DADMAC reduces
R. Sheikholeslami / Desalination 123 (1999) 45-53
calcium carbonate inhibition by 30--40%. On the other hand, cationic polymers are shown to be effective in silica scale inhibition but they were poor dispersants. Some other control agents show good dispersion but poor inhibition capabilities. Another example of the interfering effect are the humic and fulvic acids that have been shown to act as calcium scale inhibitors while they promote biological fouling. The second issue in the use of fouling-control agents is their safety and environmental aspects. This is important both in terms of the permeate as well as the brine. If the membrane is permeable to these agents, their passage even in very minute quantities might be harmful, especially in the case of ultrapure water production. Unfortunately, it seems that research in this area is very limited. The third issue is that the costs involved with the use of these agents can be prohibitive. 3.2. Pretreatment technologies
Since RO fouling mitigation has become a necessity, a series ofpretreatment technologies has been integrated into the system. Some of these are already implemented in the industry and some are in their infancy stage. This category of fouling mitigation in reality relocates the fouling process; it does not alleviate it but only mitigates the RO fouling, not the fouling process. 3.2.1. Ultrafiltration/microfiltration
These two filtration techniques have been progressively used to reduce fouling of RO membranes. They are being applied both in the deadend and the cross-flow arrangements. Since the fouling takes place on these membranes, there is a need for fouling control at these stages. These membranes are subject to both surface and pore fouling and are normally cleaned by intermittent backwashing and less frequent chemical cleaning. Chemical cleaning is required, as backwashing does not restore the flux to the initial value and
49
therefore gradually the flux would decline to a level requiring a chemical wash; of course, chemical wash would not necessarily restore the original flux either. They are usually backwashed every 40 rain and chemically cleaned every 6 months. The discussion with workshop attendees indicated that the cost of these units (capital and operation) is estimated to range between $0.5/m 3 to $1/m 3 [5]. The water recovery (in several stages) ranges between 80-90%. The fiber integrity is essential to effective operation; even two compromised fibers have shown significantly to increase the passage of particles. A comparison of UF and MF made by Jean Michel Lain6 of Lyonnaise des Eaux-CIRSEE [6] showed UF to be the more effective of the two methods. Paul Gagliardo of the San Diego Manger Aqua 2000 Research Center described projects carried out in the San Diego Metropolitan Wastewater Department aiming to test MF/UF pretreatment processes on tertiary treated wastewater effluents that would enable RO purification to drinking water standards [7]. MF might not be effective in separation of pathogens because colliphage was detected downstream of MF 50% of the time but none downstream of RO. The San Diego tests also have shown that low-pressure membranes are becoming practical, but further tests are required for confirmation. Pretreatment of municipal effluents after secondary treatment was discussed by Dr. Mark Wilf, Technical Support Director of Hydranautics [8]. The new pretreatment technology recently introduced by Hydranautics for RO processing consists of backwashable MF and UF membrane elements in a capillary configuration. It is capable of maintaining stable performance of filtrate flow and of operating pressure in operation with heavy fouling .The capillary filtrate has a much lower concentration of colloidal and suspended particles than can be produced in a conventional pretreatment process. The fouling rate has been
50
R. Sheikholeslami/ Desalination 123 (1999) 45-53
dramatically reduced by introduction of new generation of low fouling composite membranes in which the salt rejection layer has been modified to make it more hydrophilic and to reduce its affinity to organics. The intriguing observation is that the capillary membrane pretreatment does not reduce in any significant extent organic material; the TOC concentration in the feed water is almost the same as obtained after conventional pretreatment. However, permeate flux decline of RO membranes is significantly lower than experienced in the RO system utilizing conventional pretreatment. 3.2.2. Dynamic membranes Marc Altman of the Technion's Rabin Desalination Laboratory described a project aiming to integrate dynamic membranes in UF pretreatment processes so as to relieve the load on UF membranes [9]. Dynamic membranes are conveniently formed in situ by colloidal deposition on a porous support. They are an attractive method for relieving the fouling load on the downstream membranes because once fouled, they can be removed chemically and reformed in place at a fraction of the cost of replacing conventional membranes. Data were reported on dynamic membranes formation conditions favouring high fluxes and good separation properties. The best dynamic membrane achieved to date exhibited 85% retention ofovalbumin, a model foulant, at a permeation rate in the range of UF fluxes (about 10 L/m 2 h bar). These are encouraging results that are being currently extended. 3.2.3. Compact accelerated pretreatment softening (CAPS) This novel pretreatment method for removal of inorganics, colloids and organic materials was reported by Dr. Jack Gilron of the Ben-Gurion University Center for Water Science and Technology [10]. In this method a combination of softening agents plus a MF membrane is used for
pretreatment. There is a need for backwashing that usually occurs in 40-min intervals, and the specific fluxes are high. Tests performed on 500L of highly turbid waters have shown good removal for calcium, dissolved organic carbon and bacteria as well as turbidity reduction. The challenges with this treatment technique are pore plugging, packing density, robustness, and scale up; in addition, it is in its infancy stage. 3.3. Anti-fouling membranes and modules There have also been numerous efforts to develop membranes and modules that are resistant to fouling. The activity in this area is very diverse and ranges mainly in three different categories of modifying the feed flow or physical properties, the membrane properties, and the module and spacer design. 3.3.1. Modifications to feed flow and physical properties The effect of ultrasound and flow pulsations on the hydrodynamics of flow has been the subject of several investigations. In this workshop, David H. Furukawa, President of Separation Consultants, described a new RO unit that is drawing wide attention [11]. The unit, developed by Mineral Water Development Company of South Africa, incorporates two novel features that are intended to reduce fouling to a level far below that achieved with current RO units. One feature is a patented flow distribution that greatly improves hydrodynamic conditions by reducing the boundary layer thickness at the membrane surface, thus enhancing both membrane flux and apparent rejection. Although not fully understood at this time, it is believed that the flow distributor causes gases to be expelled. Under pressure, the gases (CO2 and 02) form microspheres, which enhance scrubbing at the membrane surface reducing the boundary layer thickness, hence concentration polarization. In addition, the bicarbonate equili-
R. Sheikholeslami / Desalination 123 (1999) 45-53
brium is shifted to the right, greatly reducing the quantity of remaining bicarbonate, negating the need for scale inhibiting chemicals or acid. Research has been planned for 1998 and 1999 to substantiate this hypothesis. Another innovative feature is the incorporation of an electromagnetic device which allows an electromagnetic field to surround the membrane elements. It is claimed that the electromagnetic field deters the precipitation of sparingly soluble salts and also affects organic components either by negating or partially affecting their ability to interact with the charged membrane surface, which surrounds the membrane elements and affects both ionic and organic species in the feedwater, deterring both precipitation and membrane fouling. These phenomena will also be substantiated in tests being planned. The controversy surrounding the effectiveness of the anti-fouling elements in the new Metropolitan Water District unit is likely to be resolved in the not-too-distant future. Mr. Furukawa reported that several desalination companies are planning to subject these units to rigorous performance tests. 3. 3.2. Modifications to membrane properties
In general, membrane surface modification attempts to change the physical and chemical properties of the surface to reduce primary fouling and adsorption of molecules. It has limitations as it does not impact rejection of colloidal inorganic particles and as it cannot be used in conjunction with disinfectants. Ideal surface modifications should be resistant to cleaning and disinfectants, have no adverse effect on performance, and are stable and not prone to leaching. The ideal outcome would be a smoother surface with longer cleaning cycle, reduced fouling impact, more uniform local flux, and lower cleaning costs. Modifications to the membrane surface can be made by various means such as change of charge or hydrophilicity, redox radical grafting and also
51
application of composite coating to electrodialysis membranes. The redox radical grafting, described by Dr. Sofia Belfer of the Ben-Gurion University Center for Water Science and Technology [12], was reported to increase hydrophilicity of membranes as well as reducing their surface roughness. The modified membranes have been characterized to determine the effect of the modification on surface and transport properties. Specific flux declined, but salt rejection increased with this modification. Results show that specific flux was reduced by up to 20%, but the rejection was often increased by 1% or more. Fouling studies are underway to correlate these data with resistance to fouling and ease of cleaning of fouled layers on RO membranes that have been modified. An investigation of the anti-fouling modification of ED membranes was reported by Charles Linder of the Ben-Gurion University Center for Water Science and Technology [13]. The novel membranes developed comprise a commercial heterogeneous membrane with a stable, thin, multilayer composite coating to minimize polarization and fouling without significantly increasing resistance. Polarization is decreased by the conductance of the charged hydrophilic layer next to the membrane. A superficially thin negatively charged coating on top of this layer suppresses fouling. The anti-fouling properties of the novel membranes are characterized by model fouling solutions of dodecyl benzene sulfonic acid and/or humic acid. Some manufacturers have commercially available low-fouling membranes. It was mentioned earlier that Hydranautics recently developed a new generation of low-fouling composite membranes that is more hydrophilic and reduces the affinity of the surface to organics [8]. The flux is slightly lower, but it significantly reduces fouling, and in addition, the flux is recoverable after washing. New FILMTECH fouling-resistant BWRO elements were described by Jorge A. Redondo,
52
R. Sheikholeslami / Desalination 123 (1999) 45-53
Project Manager of Dow Europe [14]. Biofouling resistance is provided by a proprietary surface modification of FT30 membranes. Two products are available: FILMTEC BW30-365 FR1 is recommended where the permeate water is designated for potable use and FILMTEC BW30365 FR2 for non-potable water application. John Waring, International Sales Director of the TriSep Corporation, described a relatively new polyamide-urea RO membrane, commercially known as the X-20 T M , purported to possess unique fouling resistant properties [15]. The polyamideurea chemistry of this new barrier layer is said to be superior to standard membranes in water treatment applications with respect to fouling resistance, rejection of certain dissolved species, and chemical and mechanical durability. Case studies were presented from which it was concluded that benefits derived included consistent overall salt rejection of 99%+, consistently high silica rejection of 99%+, lower and stable system Ap, compatibility with up to 4ppm continuous exposure to chloramines, easier system maintenance and ease of cleaning and tolerance to cleaning chemicals. Wil F. Pergande, Vice President, OSMONICS, described the Ultrafilic® membrane used in a closed system application for water recovery in an oily water application [16]. The new material has been developed and packaged into a spiral element configuration which expands its commercial applications. The oil-repelling characteristics make the product unique in its applications and likewise its abilities to reject organics when operating at significantly higher than normal flux rates and low driving pressures. The product has the capabilities of providing an alternative to hollow membranes for municipal water treatment applications and pretreatment for brackish or seawater RO plants. 3.3.3. Membrane anti-fouling cleaning techniques
Costas Pappas, Managing Director representing
DuPont, referred to the hypochlorite process, a cleaning method developed by DuPont to remove biofouling, primarily from seawater membranes [ 17]. Internal flow path differences between older devices and newer Twin® and Hollow Fiber Cartridge® permeators were described, and their expected impact on improved cleaning was discussed. 3.3.4. Modifications to module and spacer design
Several papers pointed out that module and spacer design affect the performance and fouling propensity of membrane systems by affecting the hydrodynamics of flow and contact time. Dow, DuPont, Osmonics, and Trisep [13-16] have all been active in design modifications. The modifications include reduction in leaf length, spacing adjustments, alternative backing material, new feed and permeate spacer design, and even special glue which results in a module with better packing, higher density, and easy cleaning.
4. Future R&D needs
Further membrane research is required to develop more effective pretreatment to RO, more robust and selective membrane materials with broader applications, and lower capital and operating costs. There is also a need for developing more robust fouling control techniques. Brine disposal is an issue that has not received enough attention. This is especially significant for inland plants that cannot discharge retentate in the water systems. In general, attention should be paid to developing environmentally friendly and costeffective technologies. It also seems to be of significance to consider life-cycle analysis in the assessment of technologies. A more systematic approach is required to investigate the effect of critical flux, critical conversion, anti-scalants, flow instabilities and
R. Sheikholeslami / Desalination 123 (1999) 45-53
distribution, bubbles, ultrasound and electromagnetic force. There have been various studies to relate the fouling propensity to various operating parameters individually. However, no correlation that embodies the effect of various parameters simultaneously has yet been developed. Now that membranes are coming out o f their infancy, there seems to be a need for systematic investigation for obtaining a comprehensive correlation or characteristic curves for operational purposes. Many aspects o f the transport phenomena in membrane systems are still not known nor are they yet being investigated. There is a need to investigate these fundamental principles that can help in the development of practical models that in turn can be used for fouling prediction, appropriate process control techniques and efficient and timely cleaning and backwashing cycles. O f course the ultimate success in any technological development lies with its economic competitiveness and environmental impact. It is recommended to use life-cycle analysis to assess technological viability.
Acknowledgements First and foremost, I would like to thank the workshop organizers (Technion's WRI, Fairleigh Dickinson and Ben-Gurion Universities, and MEDRC) and especially its chairmen, Professors David Hasson, Raphael Semiat and Harvey Winters, for their great efforts that resulted in such a successful and fruitful workshop. Many thanks are due to the Israel Desalination Society for taking such an initiative. Sincere thanks to the distinguished lecturers for their participation and immense contributions to the success of this workshop. The contributions of all the participants who enriched the workshop with their attendance and in-depth discussions and deliberations, and without whom such a forum was not attainable are greatly appreciated. Unfortunately, the interest of space does not allow naming all the attendees and
53
listing their contributions individually in the paper; this is no way detracts from their immense contribution.
Presentations [1] K. Price, Latest developments in membrane research of the US Bureau of Reclamation. [2] T. Fane, Critical flux values in ultra- and microfiltration with regard to control of fouling. [3] H. Winters, Critical conversion with regard to control of fouling. [4] Z. Amjad, Latest developments in anti-fouling chemicals for RO pretreatment processes. [5] Private discussions of the author with workshop attendees. [6] J.M. LaTn6and K. Glucina, Assessment of combined membrane processes: UF/RO for surface water treatment. [7] P. Gagliardo, San Diego pilot studies using MF, UF and RO to treat wastewater. [8] M. Wilf, RO membrane fouling processes in reclamation of municipal water. [9] M. Altman, D. Hasson and R. Semiat, Removal of organic foulants from feed waters by dynamic membranes. [10] J. Gilron, D. Chaikin and N. Dalthrophe, A novel pretreatment approach. [11] D. Furukawa, A new approach for mitigating membrane fouling. [12] S. Belfer, R. Feinstein, Y. Purinson, N. Gara and J. Gilron, Antifouling modifications of RO membranes. [13] C. Linder, G. Savelief, Y. Mirsky and O. Kedem, Anti-fouling modifications of ED membranes. [14] J. Redondo, FILMTEC fouling resistant BW RO elements - - a successful two-year development product. [15] J. Waring, Operational experience with a unique fouling resistant RO membrane. [16] W. Pergande, Surface water pretreatment applications using a new Ultrafilic® membrane. [17] C. Pappas, PERMASEP® hollow fiber RO membrane cleaning techniques to eliminate biofouling.
ELSEVIER
www.elsevier.com/locate/desal
Fouling mitigation in membrane processes Report on a Workshop held January 26-29, 1999", Technion- Israel Institute of Technology, Haifa, Israel
R. Sheikholeslami School of Chemical Engineering and Industrial Chemistry, The University of New South Wales, Sydney 2052, Australia Tel. +61 (2) 9385-4343; Fax +61 (2) 9385-6955; email: [email protected] Received 25 February 1999; accepted 3 March 1999
Abstract
Membranes have become one of the most sought-after techniques in separation processes. They are used in various industries and plants such as desalination, water purification and wastewater treatment plants. There has been a great degree of advancement in development of membrane systems; the bottleneck remaining is the fouling of membranes. A workshop on fouling mitigation processes was recently coordinated as a joint effort by several organizations attracting various groups from the five continents and presenting and discussing recent initiatives in this area. The topics were mostly related to desalting and associated inorganic and possible biofouling. Recent and ongoing techniques were discussed and a road map for future research directions in this area was set forward. This paper reviews information presented in this workshop; it aims to provide updated information (current activities and the future work required) in this area on the topic of wide concern - - t h e problem of fouling mitigation. Keywords: Membrane fouling; Fouling mitigation; Inorganic fouling; Desalting
I. I n t r o d u c t i o n
While there has development of desalination, water disposal, fouling
been a great advance in the m e m b r a n e systems for purification and wastewater still remains as a major
bottleneck. In view of its major impact on process efficiency and economics, the fouling problem is receiving wide attention from membrane manufacturers, plant designers, plant operators and researchers. Naturally, there is considerable
*The workshop was organized under the auspices of the Fairleigh Dickinson University, Teaneck, N J, USA; The Technion Water Research Institute, Haifa, Israel; Ben-Gurion University Water Center, Beer Sheva, Israel; and the Middle East Desalination Research Center, Muscat, Oman. 0011-9164/99/$- See front matter © 1999 Elsevier Science B.V. All rights reserved PII: S O 0 1 1 - 9 1 6 4 ( 9 9 ) 0 0 0 5 8 - 2
46
R. Sheikholeslami / Desalination 123 (1999) 45-53
interest in getting reliable and well-balanced information on the progress generated by these persistent efforts. This objective can be best achieved through a meeting allowing in-depth deliberations on new developments. Such a meeting, initiated by the newly established Israeli Desalination Society, was held on January 26-29, 1999, at the Technion-Israel Institute of Technology, Haifa, Israel. A workshop, Fouling Mitigation in Membrane Processes, was organized under the auspices of the Water Research Institute of the Technion (Israel), the Water Center of Ben-Gurion University (Israel), the Middle East Desalination Research Center (Oman) and the Fairleigh Dickinson University (USA). The organizing committee was chaired in Israel by Professors David Hasson and Raphael Semiat of the Technion's WRI Rabin Desalination Laboratory and was chaired in the US by Professor Harvey Winters, Program Director inthe School of Natural Sciences of Fairleigh Dickinson University. The workshop was attended by worldrenowned fouling experts from both industry and academia. The meeting was originally intended to be a local event, but the quality of the program attracted international participation. The workshop was attended by 130 Israeli participants and over 40 visitors from 12 countries which included 11 participants from Jordan and the Palestinian Authority. The day devoted to pre-workshop activities started with tours of the research and teaching facilities of the Technion's Rabin Desalination Laboratory. This was followed by a lecture, delivered by Professor Uri Shamir, Director of the Technion's Water Research Institute on "Water Issues of the Middle East". Professor Shamir not only presented a clear and highly informative review of the complex mid-East water issues, but also gave a fascinating glimpse on the intricacy of water issue negotiations from his personal experience as a member of the official Israeli
negotiating team. In the afternoon, a fully loaded bus took visitors to Kibbutz Maagan Michael to visit three membrane field installations: an RO plant desalting brackish water operated by the Mekorot Water Co., an experimental RO unit of the electric company drawing its power from solar panels and a Mekorot pilot plant engaged in the development of a UF pretreatment technique for RO processing of the nearby highly polluted fishpond wastewaters. In the opening session chaired by Professors Hasson and Semiat, greetings were delivered by the Technion President, Maj. Gen. (Res.) Amos Lapidot; Dan Hoffman, on behalf of the Acting President of the Israel Desalination Society, Shmuel Kantor; the president of the International Desalination Association, David H. Furukawa; the Research Director of the Middle East Desalination Research Center, Prof. Klaus Genthner; the Chairman of the Technion Chemical Engineering Department, Prof. Moshe Sheintuch; and by the Director of the Technion Water Research Institute, Prof. Uri Shamir. The workshop opening lecture, chaired by Professor Winters, was presented by the manager of the US Desalination Research and Development Program, Kevin Price. A total of 17 lectures was presented in four sessions. The session on Fouling Control was chaired by Professors Genthner and Semiat, the session on Advanced Pretreatment Processes was chaired by Professor Kedem and Mr. Priel, the session on Anti-fouling Membranes and Modules-I was chaired by Drs. Amjad and Glueckstern, and the session on Anti-fouling Membranes and Modules-II was chaired by Mr. Mansdorf and Dr. Wilf. The author of this paper was charged with presented a critical review of the main workshop disclosures. This paper is based on the review presented by the author at the workshop closing session chaired by Professor A. Fane, Mr. K. Price and Mr. D. Furukawa. The final part of the closing session was devoted to a general discussion on
R. Sheikholeslami / Desalination 123 (1999) 45-53
R&D priorities relating to membrane fouling mitigation. Following three break presentations of the views held by the chairmen of the closing session, a lively discussion ensued, generating many interesting views. The following sections elaborate on the substantive information provided by the various speakers on the central workshop themes.
47
In the membrane area, the US Bureau has spent $3.7 million in 1998. The support has been for pretreatment technologies and development of new or modified membrane materials to combat the fouling propensity of membranes. The Bureau also supported programs for better module design, longer life material, modelling and economics, and technology transfer. This support has partially declined in 199 and is expected to have a further marginal decline until 2001.
2. Background W a t e r - - its production and its s o u r c e s - - have always been an important issue in the arid areas of the world. Due to its importance and the fact that water scarcity is detrimental to the human condition and agricultural purposes, it is fair to consider it one of the most important items of negotiation. This clarifies the need for a technologically viable method of water desalination, purification and/or reclamation in arid areas. This is especially true of the arid areas with accompanying political problems such as the Middle East. As was discussed in this workshop, efforts in collaborative research to jointly solve water issues can actually lead to better political relationships between the conflicting countries. In addition, in the environmental age that we are living in, the efficient use and reuse of water have become significant issues even in non-arid locations attracting both scientific and political attention. Research in this area is highly favoured by national and intemational granting agencies; the support given by US government agencies is an indication of this. In his opening lecture Kevin Price, manager of the US Desalting R&D program [1], reported that the US Bureau of Reclamation has allocated significant amounts to joint industry (75%)/government (25%) research projects in the area of desalting in 1998. Th support has been in all associated areas such as traditional desalting techniques (membranes, thermal), non-traditional techniques, water recycling and reuse, and economic assessments and design improvements.
3. Current research and projects in fouling mitigation As mentioned above, research in fouling mitigation has been carried out in various areas which can be classified into three categories: fouling control, pretreatment technologies, and anti-fouling membranes and modules. Below some of the projects in these areas will be discussed. 3.1. Fouling control
Fouling mechanism almost always is preceded by an induction period. The induction period is a function of the system and operating conditions. If one can increase the induction period indefinitely, fouling would never occur. Various projects are concerned with defining an operating parameter or with modifying the solution conditions by addition of minute amounts of chemicals to control fouling in various systems. Some of these are discussed below. 3.1.1. Critical flux
Since fouling propensity is a function of flux and cross flow velocity, there has been work on determining a relationship between these two parameters and incipient of fouling. Previous work [2,3] had established the existence of a critical flux, below which fouling resistance remained
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R. Sheikholeslami / Desalination 123 (1999) 45-53
Particle~/~
Flux
Cake
No Cake
Cross Flow Velocity Fig. 1. Critical flux curves for various particle sizes.
negligible. More recent work reported by Prof. Tony Fane of the UNESCO Centre for Membrane Science and Technology [2] has visually confirmed this phenomenon. This was done by a non-invasive, in situ, continuous direct observation through the membrane (DOTM) technique. The technique used transparent microfiltration membranes (60% porous) both in the presence and absence of spacers using latex, yeast, algae and bacteria particles. Transparency of the membranes is due to the straight pores in membrane material, and therefore the technique at this stage is not applicable to non-porous reverse osmosis membranes. Fig. 1 shows some of the experimental results. 3.1.2. Critical conversion
Stability of colloidal particles depends on salinity, pH, flux, cross flow velocity, temperature, pressure, repulsion forces, and type of particles. Another parameter to consider is the critical conversion which is a function of critical flux, critical cross flow velocity and other parameters affecting the stability of solution. In his lecture, Professor Harvey Winters of Fairleigh Dickinson University, discussed the crucial role of critical
conversion values on the onset of fouling [3]. Control of colloidal fouling is largely dependent on control of a critical flux, critical cross flow velocity, and other parameters affecting the stability of solution. A study of several desalination plants in the Middle East indicated that almost all these plants were designed at above the critical conversion value and were therefore doomed to foul.
3.1.3 Antiscalants/antifoulants
Another method for fouling control is use of antiscalants/antifoulants. Latest developments in these fouling control agents for RO treatment processes were discussed by Dr. Zahid Amjad of the BF Goodrich Company [4]. There are four different mechanisms through which these chemicals can control fouling. They can act as scale inhibitors, particulate dispersants, scale crystal modifiers, or sequestrates (for Fe, Mg, etc.). It is difficult to categorize the fouling control chemicals under one specific category because they may act in a combination of the above mechanisms. Polyphosphates, which are actually polymers of phosphate, were the first scale inhibitors used. These, however, can cause problems due to hydrolysis and also due to enhancement of eutrophication. In addition, they do not show good performance as metal ion stabilizers and dispersants. Synthetic polymers developed at later stages tried to improve these shortcomings. Calcium and silica were identified as some of the important water constituents that need to be properly managed to prevent scale formation. There are some important points to notice when using fouling-control agents. Firstly, these controlling agents may have an interfere effects with each other or may control one foulant while adversely affecting the control of another foulant. For example, it has been shown that 1 ppm of a cationic polymer called DADMAC reduces
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calcium carbonate inhibition by 30--40%. On the other hand, cationic polymers are shown to be effective in silica scale inhibition but they were poor dispersants. Some other control agents show good dispersion but poor inhibition capabilities. Another example of the interfering effect are the humic and fulvic acids that have been shown to act as calcium scale inhibitors while they promote biological fouling. The second issue in the use of fouling-control agents is their safety and environmental aspects. This is important both in terms of the permeate as well as the brine. If the membrane is permeable to these agents, their passage even in very minute quantities might be harmful, especially in the case of ultrapure water production. Unfortunately, it seems that research in this area is very limited. The third issue is that the costs involved with the use of these agents can be prohibitive. 3.2. Pretreatment technologies
Since RO fouling mitigation has become a necessity, a series ofpretreatment technologies has been integrated into the system. Some of these are already implemented in the industry and some are in their infancy stage. This category of fouling mitigation in reality relocates the fouling process; it does not alleviate it but only mitigates the RO fouling, not the fouling process. 3.2.1. Ultrafiltration/microfiltration
These two filtration techniques have been progressively used to reduce fouling of RO membranes. They are being applied both in the deadend and the cross-flow arrangements. Since the fouling takes place on these membranes, there is a need for fouling control at these stages. These membranes are subject to both surface and pore fouling and are normally cleaned by intermittent backwashing and less frequent chemical cleaning. Chemical cleaning is required, as backwashing does not restore the flux to the initial value and
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therefore gradually the flux would decline to a level requiring a chemical wash; of course, chemical wash would not necessarily restore the original flux either. They are usually backwashed every 40 rain and chemically cleaned every 6 months. The discussion with workshop attendees indicated that the cost of these units (capital and operation) is estimated to range between $0.5/m 3 to $1/m 3 [5]. The water recovery (in several stages) ranges between 80-90%. The fiber integrity is essential to effective operation; even two compromised fibers have shown significantly to increase the passage of particles. A comparison of UF and MF made by Jean Michel Lain6 of Lyonnaise des Eaux-CIRSEE [6] showed UF to be the more effective of the two methods. Paul Gagliardo of the San Diego Manger Aqua 2000 Research Center described projects carried out in the San Diego Metropolitan Wastewater Department aiming to test MF/UF pretreatment processes on tertiary treated wastewater effluents that would enable RO purification to drinking water standards [7]. MF might not be effective in separation of pathogens because colliphage was detected downstream of MF 50% of the time but none downstream of RO. The San Diego tests also have shown that low-pressure membranes are becoming practical, but further tests are required for confirmation. Pretreatment of municipal effluents after secondary treatment was discussed by Dr. Mark Wilf, Technical Support Director of Hydranautics [8]. The new pretreatment technology recently introduced by Hydranautics for RO processing consists of backwashable MF and UF membrane elements in a capillary configuration. It is capable of maintaining stable performance of filtrate flow and of operating pressure in operation with heavy fouling .The capillary filtrate has a much lower concentration of colloidal and suspended particles than can be produced in a conventional pretreatment process. The fouling rate has been
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dramatically reduced by introduction of new generation of low fouling composite membranes in which the salt rejection layer has been modified to make it more hydrophilic and to reduce its affinity to organics. The intriguing observation is that the capillary membrane pretreatment does not reduce in any significant extent organic material; the TOC concentration in the feed water is almost the same as obtained after conventional pretreatment. However, permeate flux decline of RO membranes is significantly lower than experienced in the RO system utilizing conventional pretreatment. 3.2.2. Dynamic membranes Marc Altman of the Technion's Rabin Desalination Laboratory described a project aiming to integrate dynamic membranes in UF pretreatment processes so as to relieve the load on UF membranes [9]. Dynamic membranes are conveniently formed in situ by colloidal deposition on a porous support. They are an attractive method for relieving the fouling load on the downstream membranes because once fouled, they can be removed chemically and reformed in place at a fraction of the cost of replacing conventional membranes. Data were reported on dynamic membranes formation conditions favouring high fluxes and good separation properties. The best dynamic membrane achieved to date exhibited 85% retention ofovalbumin, a model foulant, at a permeation rate in the range of UF fluxes (about 10 L/m 2 h bar). These are encouraging results that are being currently extended. 3.2.3. Compact accelerated pretreatment softening (CAPS) This novel pretreatment method for removal of inorganics, colloids and organic materials was reported by Dr. Jack Gilron of the Ben-Gurion University Center for Water Science and Technology [10]. In this method a combination of softening agents plus a MF membrane is used for
pretreatment. There is a need for backwashing that usually occurs in 40-min intervals, and the specific fluxes are high. Tests performed on 500L of highly turbid waters have shown good removal for calcium, dissolved organic carbon and bacteria as well as turbidity reduction. The challenges with this treatment technique are pore plugging, packing density, robustness, and scale up; in addition, it is in its infancy stage. 3.3. Anti-fouling membranes and modules There have also been numerous efforts to develop membranes and modules that are resistant to fouling. The activity in this area is very diverse and ranges mainly in three different categories of modifying the feed flow or physical properties, the membrane properties, and the module and spacer design. 3.3.1. Modifications to feed flow and physical properties The effect of ultrasound and flow pulsations on the hydrodynamics of flow has been the subject of several investigations. In this workshop, David H. Furukawa, President of Separation Consultants, described a new RO unit that is drawing wide attention [11]. The unit, developed by Mineral Water Development Company of South Africa, incorporates two novel features that are intended to reduce fouling to a level far below that achieved with current RO units. One feature is a patented flow distribution that greatly improves hydrodynamic conditions by reducing the boundary layer thickness at the membrane surface, thus enhancing both membrane flux and apparent rejection. Although not fully understood at this time, it is believed that the flow distributor causes gases to be expelled. Under pressure, the gases (CO2 and 02) form microspheres, which enhance scrubbing at the membrane surface reducing the boundary layer thickness, hence concentration polarization. In addition, the bicarbonate equili-
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brium is shifted to the right, greatly reducing the quantity of remaining bicarbonate, negating the need for scale inhibiting chemicals or acid. Research has been planned for 1998 and 1999 to substantiate this hypothesis. Another innovative feature is the incorporation of an electromagnetic device which allows an electromagnetic field to surround the membrane elements. It is claimed that the electromagnetic field deters the precipitation of sparingly soluble salts and also affects organic components either by negating or partially affecting their ability to interact with the charged membrane surface, which surrounds the membrane elements and affects both ionic and organic species in the feedwater, deterring both precipitation and membrane fouling. These phenomena will also be substantiated in tests being planned. The controversy surrounding the effectiveness of the anti-fouling elements in the new Metropolitan Water District unit is likely to be resolved in the not-too-distant future. Mr. Furukawa reported that several desalination companies are planning to subject these units to rigorous performance tests. 3. 3.2. Modifications to membrane properties
In general, membrane surface modification attempts to change the physical and chemical properties of the surface to reduce primary fouling and adsorption of molecules. It has limitations as it does not impact rejection of colloidal inorganic particles and as it cannot be used in conjunction with disinfectants. Ideal surface modifications should be resistant to cleaning and disinfectants, have no adverse effect on performance, and are stable and not prone to leaching. The ideal outcome would be a smoother surface with longer cleaning cycle, reduced fouling impact, more uniform local flux, and lower cleaning costs. Modifications to the membrane surface can be made by various means such as change of charge or hydrophilicity, redox radical grafting and also
51
application of composite coating to electrodialysis membranes. The redox radical grafting, described by Dr. Sofia Belfer of the Ben-Gurion University Center for Water Science and Technology [12], was reported to increase hydrophilicity of membranes as well as reducing their surface roughness. The modified membranes have been characterized to determine the effect of the modification on surface and transport properties. Specific flux declined, but salt rejection increased with this modification. Results show that specific flux was reduced by up to 20%, but the rejection was often increased by 1% or more. Fouling studies are underway to correlate these data with resistance to fouling and ease of cleaning of fouled layers on RO membranes that have been modified. An investigation of the anti-fouling modification of ED membranes was reported by Charles Linder of the Ben-Gurion University Center for Water Science and Technology [13]. The novel membranes developed comprise a commercial heterogeneous membrane with a stable, thin, multilayer composite coating to minimize polarization and fouling without significantly increasing resistance. Polarization is decreased by the conductance of the charged hydrophilic layer next to the membrane. A superficially thin negatively charged coating on top of this layer suppresses fouling. The anti-fouling properties of the novel membranes are characterized by model fouling solutions of dodecyl benzene sulfonic acid and/or humic acid. Some manufacturers have commercially available low-fouling membranes. It was mentioned earlier that Hydranautics recently developed a new generation of low-fouling composite membranes that is more hydrophilic and reduces the affinity of the surface to organics [8]. The flux is slightly lower, but it significantly reduces fouling, and in addition, the flux is recoverable after washing. New FILMTECH fouling-resistant BWRO elements were described by Jorge A. Redondo,
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Project Manager of Dow Europe [14]. Biofouling resistance is provided by a proprietary surface modification of FT30 membranes. Two products are available: FILMTEC BW30-365 FR1 is recommended where the permeate water is designated for potable use and FILMTEC BW30365 FR2 for non-potable water application. John Waring, International Sales Director of the TriSep Corporation, described a relatively new polyamide-urea RO membrane, commercially known as the X-20 T M , purported to possess unique fouling resistant properties [15]. The polyamideurea chemistry of this new barrier layer is said to be superior to standard membranes in water treatment applications with respect to fouling resistance, rejection of certain dissolved species, and chemical and mechanical durability. Case studies were presented from which it was concluded that benefits derived included consistent overall salt rejection of 99%+, consistently high silica rejection of 99%+, lower and stable system Ap, compatibility with up to 4ppm continuous exposure to chloramines, easier system maintenance and ease of cleaning and tolerance to cleaning chemicals. Wil F. Pergande, Vice President, OSMONICS, described the Ultrafilic® membrane used in a closed system application for water recovery in an oily water application [16]. The new material has been developed and packaged into a spiral element configuration which expands its commercial applications. The oil-repelling characteristics make the product unique in its applications and likewise its abilities to reject organics when operating at significantly higher than normal flux rates and low driving pressures. The product has the capabilities of providing an alternative to hollow membranes for municipal water treatment applications and pretreatment for brackish or seawater RO plants. 3.3.3. Membrane anti-fouling cleaning techniques
Costas Pappas, Managing Director representing
DuPont, referred to the hypochlorite process, a cleaning method developed by DuPont to remove biofouling, primarily from seawater membranes [ 17]. Internal flow path differences between older devices and newer Twin® and Hollow Fiber Cartridge® permeators were described, and their expected impact on improved cleaning was discussed. 3.3.4. Modifications to module and spacer design
Several papers pointed out that module and spacer design affect the performance and fouling propensity of membrane systems by affecting the hydrodynamics of flow and contact time. Dow, DuPont, Osmonics, and Trisep [13-16] have all been active in design modifications. The modifications include reduction in leaf length, spacing adjustments, alternative backing material, new feed and permeate spacer design, and even special glue which results in a module with better packing, higher density, and easy cleaning.
4. Future R&D needs
Further membrane research is required to develop more effective pretreatment to RO, more robust and selective membrane materials with broader applications, and lower capital and operating costs. There is also a need for developing more robust fouling control techniques. Brine disposal is an issue that has not received enough attention. This is especially significant for inland plants that cannot discharge retentate in the water systems. In general, attention should be paid to developing environmentally friendly and costeffective technologies. It also seems to be of significance to consider life-cycle analysis in the assessment of technologies. A more systematic approach is required to investigate the effect of critical flux, critical conversion, anti-scalants, flow instabilities and
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distribution, bubbles, ultrasound and electromagnetic force. There have been various studies to relate the fouling propensity to various operating parameters individually. However, no correlation that embodies the effect of various parameters simultaneously has yet been developed. Now that membranes are coming out o f their infancy, there seems to be a need for systematic investigation for obtaining a comprehensive correlation or characteristic curves for operational purposes. Many aspects o f the transport phenomena in membrane systems are still not known nor are they yet being investigated. There is a need to investigate these fundamental principles that can help in the development of practical models that in turn can be used for fouling prediction, appropriate process control techniques and efficient and timely cleaning and backwashing cycles. O f course the ultimate success in any technological development lies with its economic competitiveness and environmental impact. It is recommended to use life-cycle analysis to assess technological viability.
Acknowledgements First and foremost, I would like to thank the workshop organizers (Technion's WRI, Fairleigh Dickinson and Ben-Gurion Universities, and MEDRC) and especially its chairmen, Professors David Hasson, Raphael Semiat and Harvey Winters, for their great efforts that resulted in such a successful and fruitful workshop. Many thanks are due to the Israel Desalination Society for taking such an initiative. Sincere thanks to the distinguished lecturers for their participation and immense contributions to the success of this workshop. The contributions of all the participants who enriched the workshop with their attendance and in-depth discussions and deliberations, and without whom such a forum was not attainable are greatly appreciated. Unfortunately, the interest of space does not allow naming all the attendees and
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listing their contributions individually in the paper; this is no way detracts from their immense contribution.
Presentations [1] K. Price, Latest developments in membrane research of the US Bureau of Reclamation. [2] T. Fane, Critical flux values in ultra- and microfiltration with regard to control of fouling. [3] H. Winters, Critical conversion with regard to control of fouling. [4] Z. Amjad, Latest developments in anti-fouling chemicals for RO pretreatment processes. [5] Private discussions of the author with workshop attendees. [6] J.M. LaTn6and K. Glucina, Assessment of combined membrane processes: UF/RO for surface water treatment. [7] P. Gagliardo, San Diego pilot studies using MF, UF and RO to treat wastewater. [8] M. Wilf, RO membrane fouling processes in reclamation of municipal water. [9] M. Altman, D. Hasson and R. Semiat, Removal of organic foulants from feed waters by dynamic membranes. [10] J. Gilron, D. Chaikin and N. Dalthrophe, A novel pretreatment approach. [11] D. Furukawa, A new approach for mitigating membrane fouling. [12] S. Belfer, R. Feinstein, Y. Purinson, N. Gara and J. Gilron, Antifouling modifications of RO membranes. [13] C. Linder, G. Savelief, Y. Mirsky and O. Kedem, Anti-fouling modifications of ED membranes. [14] J. Redondo, FILMTEC fouling resistant BW RO elements - - a successful two-year development product. [15] J. Waring, Operational experience with a unique fouling resistant RO membrane. [16] W. Pergande, Surface water pretreatment applications using a new Ultrafilic® membrane. [17] C. Pappas, PERMASEP® hollow fiber RO membrane cleaning techniques to eliminate biofouling.