Factors influencing critical flux in membrane filtration of activated sludge

Membrane filtration of biomass is usually accompanied by significant flux d ecline due to cake-layer formation and fouling. Crossflow filtration with f lux controlled by pumping the permeate can produce stable fluxes if a (crit ical flux) is not exceeded. Below critical flux the transmembrane pressure is typically very low and increases Linearly with imposed flux. Above the c ritical flux the transmembrane pressure rises rapidly signifying cake-layer formation which is usually accompanied by a continued rise in transmembran e pressure and/or a drop in delivered flux. A range of microfiltration and ultrafiltration membranes with pore sizes from 0.22 to 0.65 mu m and molecu lar weight cutoff of 100 kDa was used. The feed was an activated sludge mix ed Liquor with concentration in the range of 3-10 g dm(-3). The results sho w that the critical flux depends on feed concentration and crossflow veloci ty, being higher for higher crossflow velocity or lower feed concentration. Critical flux was also dependent on membrane type, being lower for hydroph obic membranes. Although the transmembrane pressure was higher for the larg er pore size membrane, no significant difference in critical flux was obser ved among different pore size membranes. (C) 1999 Society of Chemical Indus try.