MODELING OF FOULING OF CROSS-FLOW MICROFILTRATION MEMBRANES

Steady-state and transient models are reviewed for predicting flux dec line for crossflow microfiltration under conditions in which both exte rnal cake buildup and internal membrane fouling are contributing facto rs. Experimental work is not covered in the scope of this review, alth ough reference is made to a few recent studies which have compared exp erimental measurements with theory. The steady state cake thickness an d permeate flux are governed by the concentration polarization layer a djacent to the cake of rejected particles which forms on the membrane surface, Depending on the characteristic particle size and the tangent ial shear rate, Brownian diffusion, shear-induced diffusion, or inerti al lift is considered to be the dominant mechanism for particle back-t ransport in the polarization layer. For typical shear rates, Brownian diffusion is important for submicron particles, inertial lift is impor tant for particles larger than approximately ten microns, and shear-in duced diffusion is dominant for intermediate-sized particles. For shor t times, it is shown that the transient flux decline due to cake build up is closely approximated by deadend batch filtration theory, indepen dent of the tangential shear rate. For long times, however, the steady or quasi-steady flux increases with shear rate, because the tangentia l flow sweeps particles toward the filter exit and reduces cake buildu p.