The influences of mixed liquor properties on critical flux were investigated in the membrane for simultaneous thickening and digestion of activated sludge (MSTD) process, and the sustainable operation of the MSTD process was examined by employing sub-critical operation. The critical flux significantly decreased with the increase of operation time due to the variations of sludge properties in the MSTD process. Test results showed that mixed liquor suspended solids (MLSS), capillary suction time (CST), soluble microbial products (SMP) and chemical oxygen demand concentrations of colloids (CODc) were found to have significant influences on critical flux while the extracellular polymeric substances (EPS) had mild impacts. MLSS was then chosen as an indicated parameter, and an empirical relationship was established to predict the variation of critical flux in the MSTD process. The variable sub-critical flux operation was proposed to realize the sustainable operation according to the changing law of the critical flux during the MSTD process. The result showed that the membrane fouling potential in the MSTD process could be controlled at an operationally accepted condition by the variable sub-critical flux operation. (C) 2008 Elsevier B.V. All rights reserved.