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Electrokinetics Methods To Control Membrane Fouling S. Nadh Jagannadh and H. S. Muralidhara* Cargill, Inc., P.O. Box 5699, Minneapolis, Minnesota 55440-5699
The efficiency of a separation process using membranes is often hindered by membrane fouling and concentration polarization. As a consequence, membrane cleaning costs increase, membrane life is shortened, and/or more capital is spent on purchasing more membrane area. The current practices to minimize the effect of these factors on membrane performance are classified into four general groups based on their mechanisms and are briefly reviewed in this paper. One of the approaches, the external fields approach, utilizes dc electric field and its electrokinetics as one of the means to control membrane fouling. The electric field approach is the major focus of this paper. During the electric field application, the phenomena of electrophoresis and electroosmosis occur as a result of electrokinetics. Besides these phenomena, electrolysis, Joule’s heating, ion migration, etc., also occur. One or more of these mechanisms simultaneously act on the applied membrane system and improve its performance. The electric field’s application, its mechanism, and examples are discussed in this paper. 1.0. Background
Table 1. Typical Pretreatment Methods
During the past few years, membrane separations have become more widely used, replacing some of the conventional concentration techniques. The membrane separations are becoming an increasingly important tool for separation and concentration of a variety of materials ranging from oil/water emulsions to waste sludge. Unlike distillation and evaporation, membrane separation is a nonthermal technique and its separation efficiency is normally higher (Humphrey and Seibert, 1992). Hence, its applications can be found in food and pharmaceutical industries as well. Retention of flavors and aromas in juices or maintaining enzyme or drug activity is possible using membrane techniques. The efficient use of membrane technology is, however, hindered by two factorssnamely, fouling and concentration polarization. These two membrane performance factors often dictate that significant pretreatment (see Table 1) and/or in-process cleaning be employed which contribute to the total cost of a membrane application. Fouling involves the adsorption or trapping of particles (foulant) that are present in the fluid being transported across the membrane and can be a physical and/or chemical phenomenon. Some typical foulants are proteins, lipids, bacteria, mucopolysaccharides, salts, etc. pH and concentration of the feed material being processed also influence fouling. Membrane fouling can result from plugging the pores of a membrane by foulant or agglomeration of foulant and eventual blockage of membrane pores. The foulant may adsorb very strongly to the membrane surface and, in some cases, chemically react with the polymeric membrane. Although several membrane materials such as polymeric, metal, or ceramic are present, fouling is still a critical problem during any membrane processing. The second factor, concentration polarization, is the accumulation of the solute species at the upstream surface of the membrane. This hydrodynamic/diffusion phenomenon can be controlled by means of high shear on the membrane surface, if high shear can be tolerated in an operation. The concentration polarization will always be present during membrane processing due to the fundamental limitations of mass transfer and the existence of a boundary layer. Concentration polarization and (especially) fouling issues contribute to costs of a process and performance 0888-5885/96/2635-1133$12.00/0
method
ref
removal of fats by centrifugation clarification of suspended particles by gravity settling and filtration demineralization by ion exchange pH adjustment of feed heat treatment or pasteurization
Merin and Cheryan, 1980 Delaney and Donnellly, 1977
diatomaceous earth filter aid prefiltration for RO
an industrial practice Glover and Brooker, 1980 DeBoer and Hiddink, 1980; Marshall, 1982 a Johns-Manville method
of membranes by reducing membrane flux (permeate flow rate per unit area) and decreasing the useful “life” of membranes. This results in frequent cleaning or requires replacement of the expensive membrane. Thus, these two factors clearly can limit the application of membranes in industrial sectors. The negative impact of membrane fouling is roughly estimated at $500 million on a yearly basis (Smolders and Boomgard, 1989). 2.0. Approaches To Improve Membrane Performance The various approaches to minimize the effects of membrane fouling and/or concentration polarization on membrane performance can be grouped into four categories: boundary layer (or velocity) control; turbulence inducers/generators; membrane modification and materials; and combined (external) fields. The approaches in these categories are discussed in detail in the present paper. The first two approaches mainly address concentration polarization issues, while the other two mainly focus on membrane fouling. 2.1. Boundary Layer (or Velocity) Control. This approach is based on the fact that boundary layer thickness or resistance on a surface depends on velocity of flow on the surface. Using continuity and X,Y components of Navier-Stokes equations and Prandtl’s boundary layer theory, for a flow over a flat plate, the boundary layer thickness (δ) can be expressed as in eq 1:
δ ) 4.6052ν/V
(1)
where ν is the kinematic viscosity of the permeate and V is the velocity of the flow across the membrane due © 1996 American Chemical Society
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to pressure drop. This equation suggests that, by increasing the velocity of flow, one can decrease the boundary layer thickness or the resistance due to the boundary layer. Much of the definitive work in this area was in the mid and late 1960s (Brian, 1965; Michaels, 1968). An interesting application of natural convection for boundary layer control during protein separations was reported (Huffman, 1970). This velocity approach was the major reason for the development of crossflow or tangential flow membrane units that are now widely used in industry. The use of velocity to control fouling also was (and continues to be) the motivation for the use of high agitation in dead-end membrane units that are widely used. The velocity-induced turbulence helps minimize membrane fouling by continually “sweeping” the membrane surface and/or dislodging or conveying away fouling material from the surface. Some investigators introduced pulses in the feed flow to better “sweep” the membrane surface. One recent study utilized flow pulsations along with a periodic back wash from the permeate side to minimize concentration polarization (Gupta et al., 1991). This aspect is one of the features of the turbulence promoters approach discussed below. The above investigations and others on reverse osmosis in the 1970s clearly demonstrated that high velocity and/or small flow channels resulted in sufficient turbulence that concentration polarization could be kept at a minimum. One outgrowth of this knowledge was the development of hollow fiber membranes that are used in many applications today; of course, this module type also provides significant membrane area per unit volume and flow which also helps mitigate the impact of concentration polarization. Conversely, hollow fiber membranes are highly susceptible to plugging of the internal tubes if the necessary precautions are not taken. Further, the fouling and concentration polarization can be significant problems on the shell side due to flow maldistribution. Industrial membrane manufacturers, of course, give considerable attention to the boundary layer control approach. This aspect is illustrated by the fact that numerous flow channel designs exist and are often cited as being an important factor in the relative performance of competing designs. Also in many ways, velocity or flow pattern control is a major distinguishing factor in the relative performance of the basic module designs, i.e., flat plate, hollow fiber, tubular, and spiral wound designs for the same application. The 1987 Business Communications Conference on membranes provides many such examples. Additional insight on the boundary layer control approach can be found in Belfort (1988) and Belfort et al. (1994). Figure 1 shows several designs to control the boundary layer (i.e., to decrease the boundary layer thickness) on the membrane surface. As mentioned before, this can be accomplished by increasing the flow rate, by physical inserts to generate Taylor vortices or to create surface roughness, or by pulsatory flowsall to induce turbulent flow conditions on the membrane surface. Despite all of the early academic and continuing industrial attention to boundary layer control, membrane fouling remains a major deficiency in many industrial applications, and this approach, by itself, is insufficient to meet the demands of industrial separations and concentration. 2.2. Turbulence Inducers/Generators. As discussed in section 2.1, turbulence inducers/generators are
Figure 1. Methods for controlling boundary layer thickness (Belfort, 1988).
an extension of boundary layer control approaches. Some examples of the inducers are shown in Figure 1. Some more specific examples of inducers/generators include ribbed spacers and channels, added particles or spheres of different densities, and ribbed or wavy membranes themselves. Most of these developments are aimed at creating turbulence near the membrane boundary, when high velocities cannot be employed. The influence of the turbulence inducers on membrane performance (increased throughput, i.e., reduced membrane fouling rate) is shown in Figure 2 from Rios et al. (1987). These investigators introduced fluidized bed particles into a tubular membrane as turbulence inducers. As a result of the particles, the membrane throughput for a given ∆P increased severalfold. Much of this developmental area is proprietary to membrane manufacturers and individual inventors; thus, significant information is not available. Some examples of turbulence inducers are discussed by Belfort (1988). In spite of such inventions, fouling still continues to be a major problem. In general, velocity or induced turbulence has not been an adequate solution to the fouling problem because of (1) pressure drop limitations, (2) feedstock sensitivity, (3) membrane materials issues, and/or (4) failure to address the fouling mechanism(s). 2.3. Membrane Modification and Materials. Velocity or turbulence on the membrane surface minimizes the contact of fouling material with the membrane. However, ultimately, the foulant does react or interact with the membrane. If this interaction can be prevented by the membrane material itself, then fouling can be minimized. Thus, the development of new membrane materials and/or surface modification thereof is another way to address the fouling problem.
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Figure 2. Effect of turbulence inducers (fluidized-bed particles) on membrane performance (Rios et al., 1987). Table 2. Examples of External Fields Approach topic or area
ref
development of an electrofilter combination of membrane and an electrical field for colloids electrical field with hollow fiber module for chromatography crossflow electrofilter for clay and oil suspensions high-frequency vibration and excitation during RO prevention of protein and paint fouling using an electric field optimization of an electrical field for crossflow MF prevention of protein clogging using electrophoresis prevention of MF flux decline with electricity
Beechold, 1926; Manegold, 1937 Bier, 1959 Reis et al., 1976 Henry et al., 1977 Hermann, 1982 Mullon et al., 1985 Wakeman and Tarleton, 1986 Hong et al., 1988 Wakeman, 1988; Muralidhara and Jagannadh, 1989
The development and commercial availability of both hydrophilic (e.g., cellulose acetate) and hydrophobic (e.g., Teflon, polypropylene, etc.) membranes has been a major step in this area. In addition to fouling minimization, these membrane materials have other advantages such as resistance to solvents or chemical deterioration. A paper by Light (1987) illustrates that changing membrane material affects the surface charge, which results in reduced biological fouling. The use of conducting polymer materials in the manufacture of membranes and electrochemical control of the transport through membranes are being investigated by several researchers [e.g., Mirmohseni et al. (1993, 1995), Zhao et al. (1993)]. In the recent past, many investigators in industry and academia found that some surfactants used in fermentation or cell culture would completely blind or clog a membrane. In this case, a simple substitution of the surfactant was sufficient to solve the problem. This fact appears to have rekindled interest in the use of surfactants to minimize fouling as discussed during the 1988 North American Membrane Society meeting in Syracuse, NY. Another additive approach is the work of Michaels (1987). It is clear that membrane manufacturers and polymer scientists will continue to make very important contributions to this approach. However, the several causes of fouling indicate that different materials or additives may be needed to address the different causes. The new materials or additives will have to be balanced against other material compatibility factors such as product stability, chemical resistance, product quality, and cost of manufacturing. Such balancing of material needs is thought to be a difficult barrier to overcome. In other words, introducing new materials or additives to a system may require extensive regulatory evaluations that the end-user or the manufacturer may find to be a significant cost barrier, particularly for the medical, health care, food, and beverage sectors. Thus, such
material development will have to be made, in many areas, on a case-by-case basis. Numerous membrane materials have already been researched and evaluated. Hence, it will be economical to develop a minimum fouling approach that augments the existing, basic types of membranes that are currently commercial. 2.4. Combined (External) Fields. The general approach of using external fields (e.g., electrical, magnetic) addresses most of the limitations of the other approaches discussed above. This approach can be developed and implemented independent of velocity field and membrane materials. Further, this approach is not limited to the intentional or direct introduction of new components (e.g., turbulence promoters) to the feed flow and is independent of the feed stream as long as certain electrokinetic properties, such as electrical conductivity and ζ potential, are present. Of course, such electrokinetic property enhancing materials could be added to feed streams if warranted and feasible from a product end-use and/or an economic standpoint. Some examples of the external fields approach are listed in Table 2. These examples clearly show the use of an electrical (dc) field and/or other fields such as vibration and magnetic fields in a variety of areas such as colloids, proteins, and oil. Some reports indicated the use of an ultrasonic field on the membrane surface to bounce off particles from the membrane pores, thus allowing permeate to pass through them. Table 2 also indicates the general applicability of the external fields approach to ultrafiltration (UF), microfiltration (MF), and reverse osmosis (RO). The electric field application leading to the reduction of membrane fouling and its mechanism are discussed in detail in this paper. 3.0. Electric Field Application and Membrane Fouling An analysis of the references (from Table 2) and others indicates that most of the investigators are
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attempting to address the fundamental issues of membrane fouling. These fundamental issues are surface activity and particle charge, neither of which can be effectively controlled by the velocity or turbulence approaches discussed above. Some of these fundamental aspects could be and are currently being addressed by investigations in membrane materials and surface treatments. However, the external fields approach is believed to have a greater potential and broader applicability to address such issues. Within the scope of the membrane fouling, the application of dc electric fields to improve the efficiency of cross flow membrane filtration processes has been in practice for a long time. However, corrosion of anodes is inherent with dc applications, and the lack of proper electrode material discouraged the practice of these electric field (dc field) enhanced technologies on a larger, commercial scale. The recent developments in material science and the availability of corrosion resistant electrode material inspired tremendous research, development, and commercialization activities in the domain of electric field enhanced technologies including electromembrane applications. 3.1. Electrokinetics and General Principles. dc electric field enhanced technologies utilize a phenomenon generally referred to as electrokinetics. By definition, electrokinetics is “the liquid flow that occurs along a solid/liquid interface as a result of an applied potential gradient, or conversely to the potential developed when a liquid is made to flow along an interface” (Sennett and Oliver, 1965). The electrokinetics phenomenon, defined for a solid/liquid interface, can be applied to membrane surfaces. This interfacial phenomenon was first recognized in 1809 by Reuss. Electrokinetics is perhaps widely referred to in the separation of colloidal solids from solution, i.e., solid/liquid separations. The colloid materials are generally electronegative in nature and are often surrounded by a layer of bound water oriented to neutralize this charge. This forms the so-called “double layer” which forms at the solid/liquid interfaces of the colloid particles. A detailed discussion on electrokinetics can be found in Krishnaswamy and Klinkowski (1986) and Hunter (1995). The most easily measured electrokinetic parameter defining the double layer is called the ζ Potential. This parameter provides a quantifiable basis for estimating the effectiveness of using an external electric field to enhance the separation of a mixture. Modification of this potential using an external electric field allows for the separation of colloidal particulates from otherwise stable suspensions. Electrophoresis and electroosmosis often are a result of the electrokinetics (Figure 3). As shown in the figure, electrophoresis is related to the movement of the solids or charged species (such as proteins), e.g., the migration of particles through a relatively stationary fluid. On the other hand, the electroosmosis phenomenon is about the movement of the fluid such as the permeation of fluid through a porous medium (membrane). Besides the occurrence of electrophoresis and electroosmosis, electrolysis, Joule’s heating, ion migration, etc., also happen due to the application of an electric field. One or more of these mechanisms simultaneously act on the applied system to improve the performance of membrane filtration. One or more of the above-mentioned mechanisms alter the mass transport rates during the electrolytic processes, depending on the physical properties of the
Figure 3. Electrokinetics and the associated phenomena (Krishnaswamy and Klinkowski, 1986). Table 3. Framework of Some of the Mathematical Equations for the Electric Field Enhanced Separation Processes equation of motion:
FDU/Dt + grad p ) (µ/3) grad div U + µ div grad U +
∑F F
i i
i
equation of continuity: ∂F/∂t + div (FU) ) 0 electric energy equation: E‚j ) j2/σ convective diffusion equation (minor ionic species in excess supporting electrolyte): ∂ci/∂t + U grad ci ) Ri div grad ci Faraday’s equation: N ) Iτ/f Helmholtz-Smoluchowski equation: (VE) ) ζE/(4∏µ) Kirchoff’s (first) law: div j ) 0 Ohm’s law: j ) σE
feed material (electrolyte). A mathematical analysis of the mass transport depends on electrochemical as well as the associated mass transport phenomena. A rigorous mathematical treatment is not in the scope of the present paper. However, a framework of some of the mathematical equations for the electric field enhanced separation processes is given in Table 3. The list of symbols used in Table 3 are listed at the end of the paper. A discussion on the modeling of electrophoretic systems is available in Hunter (1995). Mathematical equations for electric plus other combined fields can be found in the literature (e.g., Fahidy, 1983). 3.2. Electric Field Mechanisms Applied to Membrane Fouling. In the case of electric fields application on membranes, the dc field is applied either continuously or intermittently. The applied field strength depends on the conductivity of the feed material being processed, electrode arrangement and placement, and electrode material. ζ potential and charge of the feed particles should be determined prior to the field application. The field strength, electrode placement and design, and distance between the electrodes all depend on the electroproperties of the feed material. In the case of feed materials with low condutivity and ζ potential values, the electric field may not improve the membrane performance. Most of the work on electromembranes was conducted using tubular membranes as it was convenient to have one electrode (normally cathode) in the center of the tube and the other electrode away from the membrane. The centrally placed electrode attracts particles from the feed stream and keeps them away from the mem-
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brane surface. However, such an arrangement will be ineffective, especially when the distance between the electrode and the membrane surface is large. Some investigators placed electrodes directly over the membrane surface. In this case, the purpose of the electrode is to repel the particles away from the membrane surface. Electrode arrangement for flat membrane plates and the spiral configuration were also discussed in the literature (Muralidhara, 1991). The design of the electrode is important for uniform distribution of field strength on the membrane. If the field is properly established from the beginning of a membrane filtration, then the fouling particles in the feed could stay in the bulk flow without fouling the membrane at all. Typically, the cathode is designed to have a larger surface area, and the area of anodes is minimized. Note that corrosion due to electrolysis is more at anodes, and hence anode materials are expensive. Similarly, the distance between the two electrodes is critical as well. All these design considerations will have an influence on the energy consumption during electric field application on membranes. During membrane filtration, the particles in the feed stream tend to concentrate near the membrane surface, at which the feed velocity is zero (assuming “no slip” on the membrane). The concentrated particles form another resistance layer (e.g., concentration boundary layer) and could block the transport of fluid across the membrane. Application of an electric field (with appropriate electrode placement) on the membrane surface would potentially lift the particles from the membrane surface and allow for the bulk flow to carry them over. Simultaneously, electroosmosis would allow for more fluid to flow across the membrane. In all cases, the improved membrane performance is believed to be due to the electrophoretic or electrofiltration effect. These effects have been observed by a number of investigators; some of them are listed in Table 2. The other mechanisms that can be suggested are due to hydrodynamics, electrolysis, and/or electrocoagulation of the foulants. The velocity (V) mentioned in eq 1 will now be a combination of the velocity (V1) due to pressure drop across the membrane and the velocity (V2) due to the electric field (osmotic velocity), thus resulting in a decrease in (momentum or concentration) boundary layer thickness (δ). Hence, during electric fields application, the boundary layer thickness on a membrane surface can be expressed as:
δ ) 4.6052ν/(V1 + V2)
(2)
The gases generated during the resulting electrolysis may have a “sweeping” action on the membrane surface, thus keeping the membrane surface clean. A thorough experimentation is needed to verify these hypotheses. The influence of the applied fields on membrane performance is, however, well documented. As shown in Figure 4 from Rios et al. (1988), upon the application of the field, the membrane clearly was shown to be unfouling. The unfouling seems to be proportional to the level of the applied field. Whenever a dc field is applied, the phenomena of electrophoresis, electroosmosis, electrolysis, and Joule’s heating occur. The extent of contribution of each of these phenomena to the increased membrane performance is not completely known yet. Depending on feed material properties, it is desirable to focus on electrophoresis and osmosis, while minimizing electrolysis and Joule’s heating.
Figure 4. Effect of electric field on minimizing membrane fouling (Rios et al., 1988).
The performance improvement of membranes due to electric field application can be explained to some extent using Maulik’s model (Maulik, 1971). The model is based on a modified Darcy’s equation that contains an additional term to account for the electrokinetic effects. According to the model, a plot of permeate volume against the reciprocal of filtration rate yields a straight line. The decrease in the slope of the line is an indication of the decreasing filter cake buildup on the membrane surface. The decrease in the intercept values corresponds to the decrease in filter medium resistance. Maulik used a modified Darcy’s equation as:
t/q ) (µr/2pA2)q + µRm/pA
(3)
where q is the permeate volume collected over a time period t and Rm is the fouling layer resistance. Under constant-pressure filtration, a plot of t/q vs q should yield a straight line. When an electric field is applied, electroosmosis results in an additional flow in the same direction as bulk permeate. As reported by Maulik, the additional flow should reduce the fouling layer resistance (Rm) by the same proportion of the increased permeate flow. Hence, the new fouling layer resistance (Rm′) will be:
Rm′ ) Rm/[(Qv)0 + Q0)/Qv)0] ) Rm/[1 + kζI/σQv)0] (4) As an example, the model is applied to vegetable oil/ water data in Figure 5. The fit of the model to the data is shown in Figure 6. These tests were done using a Millipore Minitan unit. As shown in Figure 6, in the first 1 h of operation, both control and electromembranes are fouling at the same rate. However, after 1 h, the fouling rate for a control membrane increased significantly compared to the electromembrane. 3.3. Limitations. An extremely important limitation of the electric field approach is that most investigators have had to employ uneconomic power levels. For example, Bier (1959) used up to 70 kwh/1000 gal of water or nearly $3.50/1000 gal. Such high cost would limit the use of this approach in water treatment using RO. Wakeman and Tarleton (1987) employed electrical fields at a level of 100 V. The dominant mechanisms that were employed in these references were electrophoresis and electroosmosis; the energy required to decouple the solutes from the permeate or bulk stream was evidently high as suggested by the operating conditions.
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Figure 5. Effect of dc on membrane performance (Muralidhara and Jagannadh, 1989).
Figure 6. Improvement in membrane performance due to the external fields (dc field)sMaulik’s model (Muralidhara and Jagannadh, 1989).
Another limitation of the external fields approach, especially the electric fields application, is the cost of electrodes. The electrode materials chosen should be compatible with the membrane material as well as with the feed stock being processed. Further, the electrode areas should be such that they would not interfere with the feed and permeate flows. 4.0. Future for Electric Field Enhanced Membrane Technologies The trend in the electric field enhanced separation technologies shows that the next 5-10 years should produce a significant activity in this area. Similar to advances such as micellar-enhanced electrophoresis, which offers better resolution than was previously possible, other advances will come as we learn more about the theoretical basis for the electric field technologies. This class of separation has the potential to improve process selectivity, resolution, and purity. For example, a recent paper by Chee and Cho (1994) demonstrates the use of electrophoresis to separate intact chromosomal DNA molecules by size. Such a separation is not feasible using any existing conventional separation techniques. The greatest impact of such electric technologies is anticipated in the environmental areas for waste minimization and waste treatment due to a climate of increasing restrictions and mounting fuel costs. Membrane application will become the choice unit operation
due to its potential for energy savings and its ability to reduce waste (no need to add additional chemicals). As more and more membrane applications become commercial, the need for an efficient membrane operation becomes inevitable and the electric field application to minimize membrane fouling could become popular. The greatest benefit of the electric field enhanced technologies, however, will come to the potential applicators or users who anticipate specific problems and solutions and develop this kind of an approach to resolve their problems. Equipment manufacturers will probably be able to customize devices for the variety of emerging applications. Such tailored systems will be the basis for what promises to be an expanding market for purified substances needed for numerous emerging applications. One of the major hurdles of the commercial implementation of the electric field enhanced technologies is the availability of the suitable corrosion resistant and inexpensive electrode material. More research needs to be done to develop such a material. The design and placement of the electrodes in commercial units to maximize the benefits of the electric field should be further investigated. With advancements in material science and developments in clever engineering designs, we expect a bright future for the electric field enhanced separation technologies. The success of electrokinetic technologies would also depend on the development of novel types of materials which not only behave as membranes but also will have the ability to conduct electricity. They would perform as a cathode or anode and provide the separation mechanism as a membrane as well. Currently, there are some ceramic materials available where electrical conductivity is noticed, but only at high temperatures. It is also conceivable that the electrokinetic technology could open or close the membrane pores when the dc field is applied or shut off by having a proper charge on the polymeric membrane. A “electrokinetic membrane gate” concept could become a wave of the future in the years to come with the advances in materials and an understanding of interfacial science and engineering. Such a “gate” concept can find applications in areas such as medical (controlled drug release), chemical (catalysis), efficient transport and metering of chemicals, etc. Significant advances in molecular biology and biotechnology could pose new problems/opportunities for novel downstream processing options. Extra cellular or intercellular proteins are an integral part of any bioprocessing; hence, electrokinetic methods to control fouling by proteins could become an enabling technology to perform unique separation or purification processes. 5.0. Summary Two factorssnamely, membrane fouling and concentration polarizationsnegatively effect the membrane performance. The various approaches being practiced, either at lab scale or at industrial scale, to minimize the effect of these factors on membrane performance are briefly reviewed in this paper. The various approaches were grouped into four categories: boundary layer (or velocity) control, turbulence inducers/generators, membrane modification and materials, and external fields. The efficiency and applicability of these approaches depend on feed properties and the types of membranes used. In the external fields approach, the use of dc field to control membrane fouling, its mechanism, potential,
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and future for such electric membrane technologies are discussed in this paper. Nomenclature A ) membrane area c ) concentration D/Dt ) the substantial derivative ) (∂/∂t) + U‚del E ) electric field vector with magnitude E f ) Faraday’s constant F ) external force per unit mass I ) applied current j ) electric current density vector N ) ion flux p ) pressure q ) cumulative permeate volume Q0 ) electroosmotic flow rate ) kζI/σ, where k is a constant QE)0 ) filtration rate at voltage (E) ) 0 r ) average specific resistance of the fouling layer t ) time of filtration U ) velocity vector V ) flow velocity across membrane VE ) electrophoretic velocity Greek Letters R ) electrolyte diffusivity δ ) boundary layer thickness ) dielectric constant ζ ) zeta potential µ ) viscosity ν ) kinematic viscosity F ) density σ ) electrolyte conductivity τ ) transport number Subscripts i ) pertaining to species i
Acknowledgment The authors acknowledge the encouragement from Cargill management and Dr. W. J. Huffman of Gulf Science and Technology Corp. Literature Cited Beechold, H., Ultrafiltration and Electro-Ultrafiltration. In Colloid Chemistry; Alexander, J., Ed.; The American Catalog Company: 1926; Vol. I. Belfort, G. Membrane Modules: Comparison of Different Configurations using fluid mechanics. J. Membr. Sci. 1988, 35, 245270. Belfort, G.; Davis, R. H.; Zydney, A. L. The behavior of suspensions and macromolecular solutions in crossflow microfiltration. J. Membr. Sci. 1994, 96, 1-58. Bier, M., Ed. Electrophoresis, Academic Press: New York, 1959; Vol. I, 263. Bowen, R. W.; Kingdom, R. S.; Sabuni, H. A. M. Electrically enhanced separation processes: The basis of In-Situ Intermittent Electrolytic Membrane Cleaning (IIEMC) and In-Situ Electrolytic Membrane Restoration (IEMR). J. Membr. Sci. 1989, 40, 219-229. Brian, P. L. T. Concentration polarization in Reverse Osmosis desalination with variable flux and incomplete salt rejection. Ind. Eng. Chem. Fundam. 1965, 4, 439-445. Chee, D. W.; Cho, Y. I. DNA orientation during electrophoresis in a viscoelastic solution. Sep. Technol. 1994, 4, 55-61. Crull, A. Growth markets for membranes. Fifth annual membrane technology/planning conference; Business Communications Co., Inc.: Norwalk, CT, October 21-23, 1987; pp 88-95. DeBoer, R.; Hiddink, J. Membrane processes in the Dairy industry. Desalination 1980, 35, 169-192. Delaney, R. A. M.; Donnelly, J. K. Application of Reverse Osmosis in the Dairy industry; NRC: Ottawa, Canada, 1977, pp 417443.
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Received for review June 19, 1995 Accepted December 21, 1995X IE9503712
X Abstract published in Advance ACS Abstracts, March 15, 1996.
