Pore fouling of four microfiltration membranes was investigated for the filtration of activated sludge using sonication as the means to remove the cake layer from membrane surface. Results show that pore fouling was affected by membrane's microstructure, pore openings and hydrophilicity. For the track-etched polycarbonate (PC) membrane, there was no measurable pore resistance (R-p) to filtration. For the three membranes of sponge-like microstructure, the respective R-p values were 0.3 x 10(11), 1.3 x 10(11), and 16.4 x 10(11), m(-1) for polyvinylidene fluoride (PVDF), mixed cellulose esters (MCE), and polyethersulfone (PES). The PES membrane had the highest Rp due to its large pore openings (18-20 mu m), whereas NICE had higher Rp than PVDF because of its higher hydrophilicity. The R-p of PES membrane accounted for 86% of the total resistance for its filtration of activated sludge; the corresponding values were merely 2% for PVDF and 11% for NICE. It increased with the suspended solids level of the activated sludge to the power of 0.56. Results further demonstrated that R-p-dominant type of membranes, such as PES, was unsuitable for the membrane bioreactors. (c) 2005 Elsevier B.V. All rights reserved.