The effect of biosolids concentrations on permeate flux during the membrane filtration of activated sludge suspensions with different mixed liquor of suspended solids (MLSS) concentration was investigated. The cake resistance (R-c) decreased as MLSS concentration decreased. However, specific cake resistance too increased as the MLSS concentration decreased, i.e., R-c and alpha behaved inversely even though they have a similar meaning conceptually. This suggests that a should not be a proper criterion for estimating the degree of cake fouling, especially at low MLSS concentration. Tertiary treatment of secondary effluents with a membrane process was expected to exhibit a better flux performance than the membrane bio-reactor (MBR) process because biosolids in the secondary effluent have been already clarified through the secondary settling tank, i.e., the secondary effluent has a low MLSS concentration. Flux performances of both systems-submerged MBR and tertiary treatment-were compared and flux improvements for the tertiary treatment with a membrane were not observed. Due to the small particle size in the secondary effluent the flux was not better than that of the MBR even though its MLSS concentration was significantly lower than that of the MBR. From the point of view of membrane fouling, tertiary wastewater treatment with membrane does not have any beneficial effects compared to MBR. (C) 2004 Elsevier Ltd. All rights reserved.