This study focused on the physicochemical and biological characteristics of sludge in submerged membrane bioreactors (MBRs) at various sludge retention times (SRT) and their effect on microfiltration and membrane fouling. Three lab-scale submerged MBRs at SRT of 20, 40, and 60 days were performed at a constant permeate flux of 91/(m(2) h) with hollow fiber microfiltration membranes (hydrophilized polypropylene; nominal pore size = 0.4 mum). In order to evaluate the relative contribution of microbial floc and supernatant to the membrane fouling, supernatant was separated from the sludge by centrifugation at 366 rad/s for 5 min. Batch filtration experiments with the same configuration showed that the relative contribution of supernatant to overall membrane fouling was higher at SRT of 20 days (37%) than at SRT of 40 (28%) and 60 days (29%), whereas the overall fouling resistance increased as SRT prolonged. Hydrophobicity (correlation coefficient, r = 0.86; significant value, P < 0.05), surface charge (r = 0.87, P < 0.05) and microbial activity (r = -0.87, P < 0.05), which were related to composition and properties of extracellular polymeric substances (EPS), appeared key parameters relating to fouling by microbial floc. However, no remarkable factor was found in fouling caused by supernatant. (C) 2003 Elsevier Science B.V. All rights reserved.