In the case of binary particulate mixtures, initially the plots lie on the horizontal line and are consistent with those for single-component suspension of larger particles because the filter cake comprised of larger particles forms on the membrane surface. Subsequently, d2θ/dv2 increases with increasing dθ/dv since smaller particles becomes trapped in the filter cake formed. The value of d2θ/dv2 is eventually reached the limiting value and thereafter it remains constant. At this stage of filtration, it is inferred that smaller particles are fully rejected and that the mixed cake of both larger and smaller particles grows.
3.2. Particle rejection behaviors of mixed suspension
In filtration of mixed suspension, the increase in the filtration resistance strikingly depends on the solid mass of particles deposited, particularly smaller particles. Each apparent rejection R∗ of both larger and smaller particles in filtration of binary particulate mixtures illustrated in Fig. 3 is plotted in Fig. 5 against the filtrate volume v per unit membrane area. The rejection of larger particles is nearly equal to unity because the pore size of the membrane is smaller than the diameter of larger particles.However, the rejection of smaller particles is not quite as simple as that of larger particles. Smaller particles permeate through the membrane in the incipient period of filtration. However, once the value of v is over about 15 cm, the rejection R∗ significantly increases and approaches the value of unity. This means that smaller particles become trapped in the pores of the filter cake consisted of larger particles. Initially, the rate of increase in the rejection R∗ increases with v and then it decreases with v as it approaches the value of unity.
It is assumed that the capture of smaller particles is accelerated by the increase in the number of smaller particles trapped in the pores of the filter cake: the rate of increase in the rejection, View the MathML source, is directly proportional to View the MathML source, where the subscript ‘2’ indicates the value for smaller particles. In contrast, as the upper limit of the rejection is set to unity, it is assumed that View the MathML source is directly proportional to View the MathML source. Hence, View the MathML source is related to View the MathML source by
equation(6)
View the MathML source
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where r is the empirical constant associated with the rate of increase in the rejection. Integrating Eq. (6) with respect to v under the boundary condition that View the MathML source is 0.5 when v = v0, the relation between View the MathML source and v is represented as
equation(7)
where v0 is the filtrate volume per unit membrane area when View the MathML source is 0.5. Eq. (7) is the same formula as the logistic function, which is widely used in describing the population growth behavior represented by the S-shaped curve (sigmoid curve).
Fig. 6(a) illustrates particle rejection curves represented by Eq. (7) with the different values of the parameter r. As r increases, View the MathML source rises more rapidly. As indicated in Fig. 6(b), the smaller v0 is, sooner the particle rejection View the MathML source increases. Consequently, it may be possible to fit the experimental data of particle rejection with Eq. (7) by using appropriate values of r and v0. It can be seen that the plots of the rejection of smaller particles shown in Fig. 5 are well approximated by the solid curve described by Eq. (7).