EFFECTS OF SIZE AND COMPRESSIBILITY OF SUSPENDED PARTICLES AND SURFACE PORE-SIZE OF MEMBRANE ON FLUX IN CROSS-FLOW FILTRATION

Suspensions of model particles were filtered by microfiltration (MF) and ultrafiltration (UF) membranes in crossflow filtration (CFF). Spherical particles of polymethyl methacrylate (PMMA) and Saccharomyces cerevisiae were used as incompressible and compressible particles, respectively. The actual surface pore sizes of several MF membranes were measured from scanning electron microphotographs, and those of UF membranes were estimated by calculation from molecular weight cur-off values. The relationship between the average surface pore size of a membrane (d(m), in diameter) and the particle size (d(p), in diameter) proved to be an important factor that affected the steady state flux (SSF). The hydrodynamic resistance determining SFF was split up into those of the membrane (R(m)), of the cake (R(c)) and of pore blocking (R(b)). With the incompressible particles, PMMA, at a low applied pressure (low DELTAP), R(m) and R(b) mainly resulted in a decrease in flux, and SSF showed a maximum value when d(m) was similar to d(p). As DELTAP increased, the control of flux by R(c) gradually increased, and consequently, SSF showed a tendency to become independent of d(m). In filtration of compressible particles, S. cerevisiae, results were different from those of PMMA. When d(m) was about one-tenth of d(p), R(b) reached a peak and SSF showed a minimum. Biological particles surrounded by biopolymers were easily compressed, and they blocked the membrane pores. On the other hand, when d(m) was nearly equal to d(p), the compression of particles was weak and thus R(b) was not very large and SSF stayed relatively high.