Ind. Eng. Chem. Res. 1994,33, 1245-1249
1245
Electroosmotic Membrane Backwashing W. Richard Bowen’and Hoze A. M. Sabuni Biochemical Engineering Group, Department of Chemical Engineering, University College of Swansea, University of Wales, Swansea, SA2 8PP,U.K.
Membrane fouling is a limitation on the efficient use of membrane microfiltration and especially if it occurs within the pores, as the use of cross-flow of the process feed is then not effective. In-pore foulants may be removed by backwashing membranes. The paper reports an experimental study of the effect of backwashing induced by the use of applied electric fields, electroosmotic backwashing. It is shown that electroosmosis is a very effective means of producing high shear stress a t the pore walls of microfiltration membranes. This can result in the restoration of up to 97% of the initial water flux for the case of membranes which have become severely fouled by the in-pore deposition of proteins. Comparison shows that electroosmotic backwashing is more effective than pressuredriven backwashing. The advantages become more marked with repeated filtration/backwashing cycles. Electroosmotic membrane backwashing is a promising process meriting further investigation. 1. Introduction
Membrane microfiltration is an important means for the industrial separation of particles in the size range 0.110pm. As with all filtration processes, it is found that the rate of filtration declines with time. One reason for this decline is the build-up of a deposit, or filter cake, on the membrane surface. A number of means are available for reducing the amount of cake forming, the most widely used being cross-flow filtration. In such a process, the stationary membrane surface is swept by flowing the process fluid tangentially across it, normally at a velocity in the range 2-8 m s-1. The rate of filtration can then be kept high while the particles in the process feed are concentrated. However, filter cake formation is not the only reason for the decline in rate of microfiltrationwith time. Another significant factor is the gradual build-up of components of the process feed inside the pores of the membrane: pore blocking. The results of such internal pore blocking can be very deleterious even for solute components much smaller than the pore size of the membrane. For example, filtration of solutions of the protein bovine serum albumin (BSA), which has a characteristic dimension of 10 nm, can lead to rapid blocking of membranes with effective pore diameters of -0.2 pm (Franken et al., 1990; Bowen and Gan, 1991). Similar effects have been found in the filtration of solutions of the enzyme yeast alcohol dehydrogenase (YADH) (Bowen and Gan, 1992). Unfortunately, a high cross-flow velocity has no effect on such a phenomenon. The most usual means of combating such pore fouling is backwashing, reversal of flow through the membrane (Cheryan,1986). This can be effectivein many cases, but it increases the complexity of the equipment and may reduce membrane lifetimes as it stresses the membrane in a direction in which it is not supported. Another promising means of controlling membrane fouling is the application of external electric fields, giving rise to a group of processes collectively termed “electrofiltration” or “electricallyenhanced membrane processes” (Bowen,1992). Such processes utilize the surface electrical charge which all materials acquire when immersed in polar media. In particular, such processes make use of two electrokinetic phenomena (Hunter, 1981): firstly, electrophoresis, the transport of a charged surface relative to a liquid in an electric field, for example, the movement of
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* To whom correspondence should be addressed.
particles; secondly,electroosmosis,the transport of aliquid relative to an immobile charged surface by an electric field, for example, the movement of an electrolyte solution through a membrane pore or filter cake. Most studies of electrofiltration have made use of electrophoresis resulting from the application of continuous electricfields to reduce concentration polarization and membrane deposition (Henry et al., 1977). Such an approach is also applicable to biological materials (Yukawa et al., 1983; Iritani et al., 1992). Materials deposited on membrane surfaces often retain a surface charge and, hence, an electrophoretic mobility. Therefore, it is also possible to remove such materials from the surface by periodic application of electric field pulses (Bowen and Sabuni, 1991, 1992; Robinson et al., 1993). Reversal of the electrode polarity can induce electroosmotic flow through the membrane, an electrically driven analogy of pressure backwashing. Such a process was first shown to be effective in the control of fouling at reverse osmosis membranes during the desalination of a process stream contaminated with ferric hydroxide (Spiegler and Macleish, 1981). There have been two reports of the application of electroosmoticbackwashing to membrane microfiltration. One described the use of the phenomenon in a multiple stack filtration cell across which an alternating field was applied (Visvanathan and Ben Aim, 1990). The second described application of pulsed electric fields to remove deposited particles from a membrane surface by means of the resulting electroosmosis (Bowen and Sabuni, 1992). This latter also compared the effectiveness of such pulsed electroosmotic membrane cleaning with that of pulsed electrophoretic membrane cleaning, finding that the electrophoresis-based process was the more effective. The aim of the present paper is to present an experimental investigation of electroosmoticbackwashing under conditions where it is most likely to find application to microfiltration. The case to be investigated is the loss in filtration rate occurring during the microfiltration of protein solutions. Such a process finds practical application in the sterilization of such solutions as one of the last steps in the downstream recovery of protein from a fermentation broth. Such a loss in filtration rate has been interpreted as being due to the deposition of protein in the internal pore structure of the membrane (Bowen and Gan, 1991,1992),a conclusion recently confirmed by the use of atomic force microscopy (AFM) studies (Bowen and Hall, 1994). This phenomenon is quite different from
0SS8-5885/94/2633-1245$04.50/00 1994 American Chemical Society
1246 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994
the adsorption of proteins at such surfaces under low or zero flow, this latter giving only sub-monolayer or monolayer coverageunder the conditions studied. Neither crossflow nor electrophoresis (as it occurs in free solution) is appropriate for controlling such a decline in filtration rate as the protein is not rejected by the membrane. Indeed, the passage of protein through the membrane is an essential part of the process. In the present work, the backwashing was carried out with clean electrolyte solutions, as the permeate itself contained essentially the same protein concentration as the process feed. To provide a comparison with an existing technique, the effectiveness of electroosmotic backwashing was compared to that of pressure-driven backwashing. 2. Materials and Methods
Protein solution filtration and water flux measurements were carried out in a 10-mL cell (Amicon Corporation, Model 8081) connected to a solution reservoir with a maximum capacity of 1.0 L. The effective membrane area was 4.1 cm2. The system was pressurized with nitrogen gas at 100 kN m-2. The temperature was maintained constant at 20 OC. Rates of filtration were determined by continuously weighing the permeate on an electronic balance connected to a microcomputer. The solution pH for all filtration experiments was 5.0. For electroosmotic membrane backwashing the membranes were transferred to a "dipped cell" (Bowen and Clarke, 1984). In such a cell the membrane is held a t the end of a tube. Application of a constant current between an electrode positioned behind the membrane and a counterelectrode induces electroosmotic flow into the tube. A small peristaltic pump transfers the electroosmotically transported fluid to an electronic balance connected to a microcomputer. A power supply capable of delivering up to 100 V at up to 1 A was used (Farnell Instruments). Solutions were thermostated. Pressure-driven backwashing was carried out in the Amicon cell after reversal of the membrane using pressures of up to 600 kN m-2. The solution pH for all backwashing experiments was 5.0. The membranes were hydrophilic poly(viny1idene fluoride) (Durapore, Millipore (UK)) of nominal pore size 0.22 pm and used as 25-mm-diameter disks. The membrane thickness was 110 pm, and the membrane porosity was 0.75. Bovine serum albumin was obtained from Sigma Chemical Company (Grade A3912). It is a well-characterized protein with a molecular weight of 6.7 X lo4,size 11.6 X 2.7 X 2.7 nm (Norde and Lyklema, 1978), and an isoelectric point at pH 4.7-4.9 (Malamud and Drysdale, 1978). All other chemicals were AnalaR. Solutions were prepared with water produced by reverse osmosis, ion exchange, carbon adxrption, and then microfiltration. 3. Theoretical Background The effect of an applied electric field gradient and and applied pressure gradient on a charged capillary pore can be described by combining the Poisson equation with the momentum equation (Probstein, 1989),
where u is the fluid velocity through the pore, r is the radial position in the pore, x is the axial position in the pore, p is the pressure, is the potential, E is the permittivity of the fluid, and p is the solution viscosity. The equation can be integrated subject to the electrokinetic and flow boundary conditions,
+
where it is assumed that the potential at the wall of the pore (of radius a) is given by the zeta-potential (0. The result is (3)
In the case of small potentials and small Debye length, the potential at any radial position in the pore may be approximated with the DebyeHiickel linearization, +=
ce-(a-r)/Xo
(4)
where AD is the Debye parameter. This then gives for the velocity
It is also possible to integrate across the pore cross section to obtain an expression for the volume flow rate,
In the present case for protein deposited on the walls of the membrane pores it is likely that the efficacy of backwashing is related to the wall shear stress. Two limiting cases are important: purely pressure-driven flow and purely electrically-driven flow. For pressure-driven flow, from eq 5, (7) and for electrically driven flow, 70
e{d@ e{ I = -= -AD dx ADA,
where I is the applied current density and X, is the electrolyte conductivity. It should be noted that T~ does not depend on pore radius for the case of electrically driven flow, though this analysis is subject to the condition that a/hD > 1. Also, from eq 6, in the limit that a/AD >> 1,then eq 8 may be expressed as 7,
QJAD
(9)
where Qeo is the electroosmotic flow rate for unit pore area. Q, is a parameter which can be directly measured with relative ease (Bowen and Clarke, 1984). 4. Results and Discussion 4.1. Filtration of BSA Solutions. Typical data for the filtration of a solution of 1g/L BSA in le2M NaCl through Durapore microfiltration membranes is shown in Figure 1. There is a substantial decline in the permeation rate as the process prwedes, even though the pore diameter is about 20 times greater than the maximum linear dimension of the protein molecule. Under the conditions used, there was no measurable decline in pure electrolyte flux during the collection of equivalent volumes of permeate.
Ind. Eng. Chem. Res., Vol. 33, No. 5,1994 1247 5.0
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Figure 2. Volume of electrolyte transported by electroosmosis at Durapore membranes as a function of time. ( 0 )Clean membrane; ( 0 )membrane previously used for filtration under the conditions of Figure 1. 10-2 M NaCl, 250 mA.
The data shown were collected with no stirring in the cell, but even rapid stirring had an insignificant effect on the flux. This is strong evidence that there was no accumulationof BSA molecules in the solution at the front face of the membrane, in other words, that there is no concentration polarization. The data may be successfully analyzed using the "standard blocking filtration law" which describes a decrease in pore volume due to deposition of protein on the walls of the pores (Bowen and Gan, 1991). That is, fouling is occurring within the structure of the membrane. This analysis has recently been directly confirmed using atomic force microscopye (AFM) (Bowen and Hall,1994). 4.2. Electroosmotic Flow Rates. Figure 2 shows the rate of electroosmosis at a clean Durapore membrane and at a Durapore membrane which has been used for filtration of a BSA solution for 1 h under conditions identical to those of Figure 1. The conditions for measurement of electroosmotic flow were identical for both sets of data presented, the current being the maximum which was readily achievable for the experimental conditions with the equipment available. The electroosmotic flow rate was essentially constant over the period studied in both cases. The rate of electroosmotic flow was substantially greater for the membrane which has been used for fiitration. Theory predicts that such flow rates are directly proportional to the zeta-potential (0of the appropriate
Figure 3. Effect of the duration of electroosmotic backwashing on water flux for membranes previously used for filtration under the conditions of Figure 1. Backwashing in le2M NaCl at 250 mA. Water flux determined at 100 kN m-2.
plane of shear (eq 6). Hence, the results indicate that the deposition of protein during filtration led to an increase in the magnitude of the effective (negative) zeta-potential of the membrane pores. Changes in the electroosmotic properties of ultrafitration membranesfollowing exposure to protein solutions have also been reported recently (Lentsch et al., 1993). 4.3. Effect of Electroosmotic Backwashing on Water Flux. A series of experiments was carried out in which 1 d/L BSA solutions were filtered for 1 h in an Amicon cell under experimental conditions identical to those of the data in Figure 1. The membranes were then transferred to the "dipped cell" and a current was applied so that the membranes were electroosmotically backwashed. After backwashing was completed, the membranes were transferred to a second Amicon cell so that the water permeation rate of the membrane could be determined. This water permeation rate gives a measure of the effectiveness of the electroosmotic backwashing technique. 4.3.1. Effect of Duration of Electroosmotic Backwashing. From a process point of view, one of the parameters of interest in assessing electroosmotic membrane flushing is the effect of duration of application of the electric field. Data are presented in Figure 3 for the water flux after flushing in M NaCl solution, again at the maximum achievable current under these conditions of 250 mA. The water flux for a clean membrane under these conditions was 6.0 m h-l, and the water flux of the fouled membrane immediately after protein filtration but before backwashing was 0.25 m h-l. It may be seen that electroosmotic backwashing for only 1 min provided a substantial recovery of water flux. Increasing the backwashing time led to further improvements in the recovery of water flux, with backwashing of 20-min duration resulting in a water flux within 3% of that of the clean membrane. It may, therefore, be concluded that the process is a most effective means of removing protein deposita from the pores of microfiltration membranes. 4.3.2. Effect of Ionic Strength. The solubility of proteins and their extent of adsorption at interfaces can be a complex function of electrolyte ionic strength. To test the effect of ionic strength on membrane cleaning, membranes which have been used for the filtration of 1.0 g/L BSA solutions under conditions identical to those of Figure 1were electroosmotically backwashed in solutions
1248 Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 4.4
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Figure 4. Effect of ionic strength on the effectivenessof restoration of water flux by electroosmotic backwashing for membranes previously used for filtration under the conditions of Figure 1. Backwashing in NaCl solutions at 10-8 (A),1F2(O), and 10-1 M (0) for 10 min. Water flux determined a t 100 kN m-2.
of W, lk2,and 10-9 M NaCl, respectively. In each case the maximum current or voltage attainable with the power supply was used (voltage limited in le2and 10-9 M solutions and current limited in 10-l M solutions). In all cases electroosmotic backwashing was carried out for 10 min. Typical resulta are presented in Figure 4, which shows the resulting water flux as a function of wall shear stress. The latter was chosen as it is the hydrodynamic parameter most likely to be important in removing foulant material from a surface. It was found that at low wall shear stress the effectiveness of restoration of water flux occurred in the sequence 103 M > le2M > 10-l M. This is partly related to the solubility of the protein, which decreases in the same sequence, and partly due to the way in which the electroosmoticflow rate depends on ionicstrength, through the influence of ionic strength on voltage gradient and zeta-potential. An interpretation of the effectiveness of restorationbased on the direct force exerted on the charged adsorbed protein by the electric field is also possible. This would have given a similar correlation of data as such a force is proportional to the group lIJL, which also occurs in eq 8 for the wall shear stress. However, wall shear stress is more useful for comparative studies as it can also be calculated for pressure-driven backwashing. As a consequence of these dependencies and the maximum current and voltage attainable with the power supply, it is found that the highest shear stresses and the highest recoveries of water permeation rate are attainable in M solution. Optimum cleaning at intermediate ionic strengths has also been found in the removal of particles from the upper face of membranes by means of applied electric fields (Bowen and Sabuni, 1992). In order to compare electroosmotic backwashing with the conventional pressure-driven backwashing, a set of experiments has been carried out using pressure-driven backwashing at applied pressures of 100, 200, 300, and 400 kN m-2. The latter values represent backwashing pressures in excess of those normally used in practice, as such repeated reverse pressurizing can cause membrane damage as the force is applied in a direction in which the membrane is not supported. Typical resulta are presented in Figure 5. It may be seen that, except at the lowest applied pressure, the effectiveness of restoration of water flux occurred in the sequence 10-9 M > 10-2 M > 10-1 M, as occurred for the electroosmoticbackwashingat low shear stress. It is of interest that, in the range studied, the
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Wall shear stress/N tn-? Figure 5. Effect of ionic strength on the effectivenessof restoration of water flux by pressuredriven backwashing for membranes previously used for filtration under the conditions of Figure 1. Backwashing in NaCl solutions at 10-8 (A),1k2(O),and 10-1M (0) with an applied pressure of 400 kN m-2 for 10 min. Water flux determined at 100 kN m-2. Y
3
0
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Figure 6. Water flux after repeated filtration/backwashing cycles with backwashing by electroosmosis (0)and applied pressure ( 0 ) . Backwashing in M NaCl for 10 min with 250 mA or 400 kN m-2, respectively.
effectiveness of restoration of water flux for pressuredriven backwashing only shows strong dependenceon wall M solution. This probably relates to shear stress in the variation in the strength of adhesion of adsorbed protein with solution conditions. However, even at the highest pressures used, the resulting wall shear stress was less than that attainable electroosmotically in M solution. As a result, it is the electroosmoticcleaningwhich gives the greatest recovery of water permeation rate. 4.3.3. Effect of Repeated Filtration/Backwashing Cycles. From a practical point of view it is important that any cleaning technique gives good results with repeated use as it is a major advantage to be able to clean a membrane many times in-situ and while a filtration process is running. This gives longer run times and greater throughput before chemical cleaning is required. A comparison of the relative effectivenessof the repeated use of electroosmotic backwashing and pressure-driven backwashing is given in Figure 6. For each time of use, a 1.0 g/L solution of BSA was filtered for 1h. Backwashing was then carried out in 1k2M NaCl solution at a current of 250 mA for 10 min or at a pressure of 400 kN m-2 for 10 min. The water flux was then determined before the same membrane was used for a further filtration experi-
Ind. Eng. Chem. Res., Vol. 33, No. 5, 1994 1249 ment and the cycle repeated. It may be seen that electroosmotic backwashing offers increasing advantages over pressure-driven backwashing as the number of cycles increases. 5. Conclusions
Fouling is one of the more important limitations to the more widespread use of membrane processes. Fouling inside membrane pores, such as deposition of proteins inside microfiltration membranes, is the type of fouling which is most difficult to avoid. For example, use of high cross-flow velocities has little effect on deposition inside pores. The development of effective means of ameliorating the effects of such deposition is likely to have a major impact on the use of membrane processes. Application of electric fields to induce electroosmosis is a very convenient way of producing high wall shear stresses in the pores of microfiltration membranes and hence of removing deposited materials. Such application can be more effective than the use of pressure-driven backflushing. Indeed, electroosmotic backwashing can restore up to 97 % of the water flux to membranes which have been fouled by in-pore protein deposition. Electroosmotic backwashing is a useful addition to the range of electrically enhanced membrane processes. It is a promising technique and ita applications merit further investigation, in particular with regard to its optimization in larger scale membrane equipment.
Acknowledgment We thank the UK Science and Engineering Research Council for support, including a Research Assistantship for H.A.M.S. Literature Cited Bowen, W. R. Design of electrically enhanced membrane separation processes. Colloid and surface engineering: applications in the process induetries; Williams,R. A,, Ed.;Butterworth-Heinemann La.: Oxford, 1992; pp 215-247. Bowen, W. R.; Clarke, R. A. Electro-osmosis at microporom membranes and the determination of zeta-potential. J. Colloid Interface Sci. 1984,97,401-409. Bowen, W. R.; Gan, Q,Properties of microfitration membranes. Flux losa during constant pressure permeation of bovine serum albumin. Biotechnol. Bioeng. 1991, 38,688-696.
Bowen, W. R.; Sabuni, H. A. M. Pulsed electrochemical cleaning of inorganic microfitrationmembranes. Chem.Eng. Commun. 1991, 110,199-216.
Bowen, W. R.; Gan, Q.Properties of microfiitration membranes. The effects of adsorption and shear on the recovery of an enzyme. Biotechnol. Bioeng. 1992, 40,491-497. Bowen, W. R.; Sabuni, H. A. M. Pulsed electrokinetic cleaning of celluloaenitrate microfiltration membranes. Ind. Eng. Chem.Res. 1992,31,515-523.
Bowen, W. R.; Hall, N. Properties of microfiitration membranes. Mechanisms of flux loss in the recovery of an enzyme. To be submitted to Biotechnol. Bioeng. 1994. Cheryan, M. Ultrafiltration Handbook; Technomic Publishing Company: Lancester, 1986. Franken, A. C. M.; Sluya, J. T. M.; Chen, V.; Fane, A. G.; Fell, C. J. D. Role of protein conformation on membrane characteristics. F'roceedings ofthe Fifth WorldFiltrationCongress, Nice, France; Soci6t.4 F r a n ~ s de e Filtration: Cachan, 1990, pp 207-213. Henry, J. D.; Lawler, L. F.; Kuo, K. H. A. A solid/liquid separation process based on cross-flow and electrofiitration. AIChE J. 1977, 23,851-859.
Hunter, R. J. Zeta-potential in colloid science; Academic Press: London, 1981. Iritani, I.; Ohashi, K.; Murase, T. Analysis of filtration mechanism of dead-end electroultrafiltrationfor proteinaceous solutions. J. Chem. Eng. Jpn. 1992,25,383-388. Lentach, S.;Aimar, P.; Orozoco,J. L. Enhanced separation of albuminpoly(ethy1ene glycol) by combination of ultrafiltration and electrophoresis. J. Membr. Sci. 1993,80, 221-232. Malamud, D.; Drysdale, J. W. Isoelectric point of proteins. Anal. Biochem. 1978,86,620-647. Norde, W.; Lyklema, J. The adsorption of human plasma albumin and bovine pancreaa ribonucleaseat negatively charged polystyrene surfaces. J . Colloid Interface Sci. 1978, 66, 267-265. Probstein, R. F. Physicochemical hydrodynamics; Butterworth Boston, 1989; p 197. Robinson, C. W.; Siegel, M. H.; Condemine, A.; Fee, C.; Fahidy, T. Z.;Glick, B. R. Pulsed-electric-field crossflow ultrafiltration of bovine serum albumin. J. Membr. Sci. 1993,80, 209-220. Spiegler, K. S.; Macleish, J. H. Molecular (osmotic and electroosmotic)backwash of celluloseacetate hyperfitration membranes. J. Membr. Sci. 1981, 8, 173-191. Visvanathan, C.; Ben Aim, R. Enhancing electrofiitration with the aid of electro-osmoticbackwashingarrangement. Filtr. Sep. 1990, 27,42-44.
Yukawa, H.; Shimura, K.; Suda, A.; Maniwa, A. Cross flow electre ultrafiltration for colloidal protein solution. J. Chem. Eng. Jpn. 1983,16,305-311.
Received for review August 25, 1993 Accepted February 22, 1994. e Abstract publishedin Advance ACSAbstracts, April 1,1994.
