The effect of sludge characteristics on membrane fouling in a pilot-scale submerged membrane bioreactor (MBR) during long-term performance was investigated. Viscosity, mixed liquor suspended solids (MLSS), extracellular polymer substances (EPS), soluble chemical oxygen demand (SCOD), dewaterability in terms of capillary suction time (CST), proteins and carbohydrates in the mixed liquor and their correlations with membrane fouling were studied. Among these variables, CST, SCOD, EPS and carbohydrate were found to have significant influence on membrane fouling by statistical analysis method. CST was then chosen as an indicator parameter and its relationship with critical flux (J(c)) was established. The concept of proper operational flux (J(p)) was introduced and further determined through parallel experiments. The difference between critical flux and proper operational flux in submerged MBRs was analyzed and discussed. Test results in this study show that the proper operational flux is only 56% of the critical flux value. (c) 2006 Elsevier B.V. All rights reserved.