Direct observation of particle deposition on the membrane surface during crossflow microfiltration

In the crossflow microfiltration of particles, a deposit cake layer tends to form on the membrane and this usually controls the performance of the filtration process. This paper presents observations of particle deposition on membrane surfaces using a non-invasive, in situ, continuous direct observation through the membrane (DOTM) technique. The particles used in the experiments were typical of microfiltration processes, yeast (mean diameter 5 mu m) and latex beads (3, 6.4 and 12 mu m). The filtration tests were conducted in the imposed flux mode, so that the flux could be controlled at, below, or above the "critical flux". Below the critical flux, the particle deposition was negligible; near the critical flux the particle deposition was significant; and above the critical flux, particle layers were formed on the membrane surface. Rolling of the particles was observed during the filtration of 6.4 mu m latex near the critical flux whereas a flowing cake layer was observed during the filtration of 3 mu m latex. The particle size distribution of the deposited particles changed with the crossflow velocity, with smaller particles deposited on the membrane at higher crossflow velocity. Comparison of the normalised flux (J/Delta P) with the membrane area coverage by the particles revealed that for filtration of latex particles "flux percentage (with respect to the clean membrane)" was marginally greater than the percentage of uncovered membrane area, whereas for filtration of yeast, the "flux percentage" was significantly less than the uncovered area percentage due to the deposition of smaller cell debris species. This paper demonstrates that DOTM is a powerful technique for the study of fundamentals of particle deposition and interactions between the particles and the membrane. (C) 1998 Elsevier Science B.V. All rights reserved.