In microporous membrane filtration, formation of a cake of solids results in flux decline. Pumping the permeate can remove the deposited layer and improve the flux. This technique allows operation at low transmembrane pressure and provides stable flux if a ＇critical flux＇ is not exceeded. Slow fouling can occur in time if the initial flux is beyond the critical value. Critical flux is dependent on the factors that influence concentration polarization. such as nature and concentration of the feed and shear rate, being higher for higher crossflow velocity and lower concentration. These effects have been observed fur activated sludge biomass. However, the complexity of the biomass can affect the dependency of critical flux on operating conditions. Well-characterized polystyrene latex particles were used to explain the factors influencing critical flux in a colloidal suspension without taking into account the complex nature of the feed. The main difference between these feeds was that the pumped permeate flux was significantly lower in the activated sludge than in the pure water flux. in the latex suspension the same flux was obtained for the feed and for the pure water. However, the same trend for factors affecting critical flux was observed in the activated sludge biomass and latex suspension.