current research Physical Aspects of Electrofiltration Satya P. Moulik' Dept. of Physical Chemistry, Jadavpur University, Calcutta 32, West Bengal, India ~
The primary physical phenomena involved in electrofiltration have been discussed, and a n acceptable theory for the process has been put forward. The theory has been compared with experimental electrofiltration data on bentonite clay suspensions. A quantitative agreement between the two has been observed.
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n recent years forced-flow electrophoresis has been gaining attention in the field of water and waste purification. In a number of publications (Bier, 1959, 1966; Bier et al., 1967; Bier and Cooper, 1967; Bier and Moulik, 1967; Cooper et ai., 1965), the wide range of applications of this method has been reported. As specific examples concentration of proteins and bacteriophage, and separation of colored materials (asphalt catchment area runoff), algae (sewage effluent), silt (flood and river runoff), oil emulsion waste (aluminum rolling mill effluent), etc. can be cited. The underlying principle of forced-flow electrophoresis is electrofiltration. The filter-cake formation, encountered in ordinary filtration, is avoided in electrofiltration by the application of a n electric field of suitable polarity and magnitude. An elementary theory of electrofiltration has been proposed (Bier, 1962) for the separation of filterable colloids. Its application in the case of coarse suspensions has been discussed (Cooper, 1967). Recently, the various phenomena involved in the electrofiltration process have been discussed (Moulik et al., 1967) in a limited manner. In this paper a detailed analysis is offered and proper equations are developed by combining the elements of filtration with the inherent electrokinetic phenomena. Actual electrofiltration results have been tested quantitatively against the theoretical results. Theory and Results
The Electrofiltration Cell and the Apparatus. A filter placed by means of appropriate spacers between two nonpermaselective membranes constitutes the cell (Figure 1). Solution loaded with particles enters into the top of the input half-cell on the left, and the filtered solution leaves the cell from the top of the other half-cell. In case of normal filtration, suspended particles and the liquid medium both move toward the filter a t equal velocities. To minimize the transfer of the solid, an electric field of proper polarity and magnitude has to be applied, so that it will initiate motion of the suspended particles in the counter direction. This rate will depend on the particles' electrophoretic mobility and the hydraulic flow rate. Present address: Carbohydrate Research Laboratory, Dept. of Food Technology and Biochemical Engineering, Jadavpur University, Calcutta 32, West Bengal, India.
In absence of a n electric field (normal filtration), a deposit of filter-cake of thickness, L , is observed (top diagram). This filter-cake gradually grows thicker and thicker, obstructing the passage of the liquid more and more. Therefore, at constant pressure, filtration rate declines with time. The direction of the fluid flow through the filter and the cake has been shown by the solid arrows. Application of an electric field of suitable polarity can force the suspended particles (supracolloids) entering into the cell to migrate in the direction of the broken arrows. Thus, a decrease in the filter-cake deposit will result (middle diagram). The bottom diagram depicts a situation under a n applied voltage, when the rate of migration of the particles is equal to the linear rate of the liquid flow through the filter but opposite in direction. This voltage is called the critical voltage. At or above this voltage no filter-cake formation is possible, and continuous filtration a t constant rate ensues. For all practical purposes, working at this voltage is necessary t o give maximum efficiency to the process. In a closed cell the migrating colloids eventually get deposited on the membrane opposite to the filter. This can be prevented by using a rapid downward flow of the processed liquid through the input spacer. Thus, the depositing particles are swept through the bottom opening of the cell which is not shown in the diagram. The laboratory-scale electrofiltration apparatus resembles a filter press containing three cells with parallel flow distribution.
SOLUTION
FILTER CAKE
MEMBRANE
MEMBRANE WALL
x -
Figure 1. Schematic representation of an electrofiltration cell and its inner events Top: no voltage applied Center: insufficient voltage applied Bottom: adequate voltage prevents all filter-cake deposit Volume 5, Number 9, September 1971 771
The cells are interconnected through spacers and compressed between end plates (made of laminated layers of Plexiglas) which contain the platinum-coated reversible electrodes used to establish a direct current field across the unit. The apparatus has been previously described (Bier, 1959, 1962, 1966). Similar equipment is available from Canal Industrial Corp., Rockville, Md. A flow diagram is shown in Figure 2 with brief details of the experimental method. The solution under processing was fed under gravity into the cell pack, c, from the Mariotte supply bottle, s, via the channel, x. The solution could also be fed by means of a pump. The channel, y , delivered the filtrate into the collector, T . The differential in pressures across the filters was measured by the water manometers, M and M'. During the filtration process, channels I, k , and z remained closed. After each operation, the trapped solid material inside the cells was washed out by reverse flushing of the cells with electrolyte solution from the vessel, W , via the channel, I , and by collecting the washed solution into the beaker, B. Short reversal of current polarity was found to be helpful in removing the collected material more efficiently. In most of the laboratory experiments channel k remained functionless, but, if desired, can be used for the continuous removal of the accumulated solids during filtration. Gelman AN 800 membrane filters (porosity 0 . 8 5 ~having ) a total filter area of 300 cmz were used. The temperature of experimentation was 25°C. At low voltages heating was negligible. Continuous circulation of electrolyte solution through the electrode compartments was adopted to minimize the heating effects at higher voltages. The pressure remained fairly constant throughout the experiment ; only occasional minor adjustments were necessary (pressure drop 5 1 cm). Various Physical Phenomena. The processes of electrophoresis and filtration mainly constitute the electrofiltration process. Electrophoresis is the migration of colloid particles under an applied electric field. This occurs because the surface of every particle is charged. In the process of colloid removal by coagulation, this charge has to be minimized to a certain level (Black et al., 1963; Black and Chen, 1965; Black and Walters, 1964; Riddick, 1961). Clarification by coagulation is effective only in the case of hydrophobic colloids. Hydrophilic colloids are normally removed by adsorption, Electrophoresis, on the other hand, is capable of taking care of both types of colloids, because it operates only on the basis of the existing surface charge. For a good separation, low specific conductance of the supporting medium, and high electrophoretic mobility of the colloids are advantageous. Filtration is the transport of the fluid part of suspension through a porous medium (filter) under an applied gradient of pressure. The suspended particles get deposited on the filter surface. The greatest difficulty of normal filtration is the decrease in its efficiency owing to this deposited solids (filtercake). The solid particles can block as well as clog the filter pores, Regeneration of the pores by washing or by chemical treatment is tedious. This problem can be dramatically overcome in the electrofiltration process. While filtration is proceeding, an applied electric field of suitable polarity and magnitude can electrophoretically direct the charged suspended particles to move away from the filter surface. Under such a condition filter surface is not blocked or clogged by the solids, and therefore, it may transfer the liquid at equal rates under the applied gradient of pressure. It has been noticed that the applied electric field develops 772 Environmental Science & Technology
several secondary phenomena. These are electrokinetic in origin because one has to deal with a colloid, a porous filter medium, and a filter-cake, whose surfaces are all charged. The phenomenon of electroosmosis through the filter medium and the filter-cake significantly contributes to the process. Other associated and inherent phenomena such as polarization, electrodecantation, surface conductance, etc., may also appear. However, their contributions are considered minor since the equations developed without such considerations fits the experimental results well.
Figure 2. Flow diagram of the filtration assembly
n
n
u
C
D
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4
E>Ecr c
E
Figure 3. Various forces acted on colloid particles 1 = normal filtration force
2 = filter medium electroosmotic force 3 = cake electroosmoticforce 4 = electrophoretic force
A=E=O B = E E,, (no filter-cake formation, particle flow exceeds filtration flow)
I\ w
TIME
Figure 4. Filtration flow rate and time relation for different applied voltages (arbitrary scale)
The various forces that act on a suspended particle under varied experimental conditions are depicted in Figure 3. In situation A , that is, at no applied voltage, solid particles are carried over to the filter surface forming a filter-cake. Situations B and C (in which the applied voltages have not reached the critical voltage value) both warrant some filter-cake formation. In situation D, that is, at the critical voltage, a complete balance between the forward and the backward forces has been established. Here, no filter-cake formation is feasible as also in situation E, where the applied voltage has exceeded the critical value. The variation of the filtration rates under these varied electrical situations has been exemplified in Figure 4.‘In this the flow rate has been plotted against time. It is seen that increasing strength of the electric field is stabilizing the filtration rate more and more. At sufficiently high voltage ( E 3 Ecr), a complete stabilization is attained. In this figure, h,, ho’, and ho” represent the filter medium electroosmosis flow rates at the three stages of the increased field strength. Filter-cake electroosmosis flows at time tT are represented by he and he’ for the two applied voltages E Ec7and E E,,, respectively. Both tc and t,’ represent the respective cake electroosmosis commencing times for the same two voltages. Competitive solution and particle flows are shown in Figure 5 . The flow rate of the electrolyte solution is represented by line A , whose continuous increase denotes increasing filter medium electroosmosis with increasing voltage gradient. The same driving force causes the suspended particles to have increasing flow rates in the opposite direction along line C. Evidently, particles of a suspension should follow the flow rate line B at a decreasing rate up to the voltage gradient, E,,, at the point D shown in the figure. D ,therefore, is the point of critical voltage where the particle flow rate has been totally balanced by the solvent flow rate. In other words, the particle flow rate is zero at this point. Past this point the particle flow rate should increase in the negative direction. Through a cell of filter area A , particles having mobility pcm/sec/V/cm should have a flow rate equal to PEA. If we consider filtration flow at any voltage E as V’ (represented in the figure as VE),which is opposite in direction to the electrophoretic flow rate PEA, simple subtraction between the two has actually constructed the line. At the critical voltage Ecr-i.e., at the point D-this result (V’ PEA) is zero.
