The feasibility of using flat-sheet membrane for simultaneous thickening and digestion of activated sludge (MSTD) was studied in this paper, and the membrane fouling mechanisms of this process under high and changing mixed liquor suspended solids (MLSSs) were investigated. About 30 g/L concentrated sludge and 45% mixed liquor volatile suspended solids (MLVSSs) destruction efficiency were achieved at the end of each period of sludge treatment. Test results showed that viscosity, capillary suction time (CST), soluble microbial products (SMPs) and chemical oxygen demand concentrations of colloids (COD,) had significant influences on membrane fouling among various sludge variables. CST was then chosen as an indicated parameter, and the empirical relationship between CST and membrane permeability was established. It was also found that the filtration cake and the irreversible foulants consisted of large numbers of carbohydrate and protein, and membrane fouling of membrane modules was not homogeneous. (c) 2008 Elsevier B.V. All rights reserved.