The effects of aeration intensity, mixed liquid of suspended solids (MLSS) concentration, and sludge viscosity on membrane fouling were investigated on the basis of rheology and hydrodynamics concepts. This work was carried out with the help of statistical analysis. The results show that the membrane fouling resistance was increased exponentially with an increase of MLSS concentration. At low MLSS concentration, the aeration intensity had small impacts on membrane fouling resistance. There was an exponential relation between MLSS concentration and sludge viscosity. Compared with aeration intensity (r(p) = -0.337), the sludge viscosity (r(p) = 0.833) and MLSS concentration (r(p) = 0.793) had more remarkable impacts on membrane fouling resistance. Cross flow velocity (r(p) = -0.871) and Reynolds number (r(p) = -0.796) of sludge suspension were two significant factors affecting membrane fouling process. The decrease of cross flow velocity and Reynolds number of sludge suspension resulted mainly from the increase of MLSS concentration and sludge viscosity. The aeration intensity, however, had relatively small influence on cross flow velocity and Reynolds number of sludge suspension. Moreover, the increase of MLSS concentration or sludge viscosity could result in low dissolved oxygen (DO) concentrations. During the operation of membrane bioreactors, the MLSS concentration and sludge viscosity in bioreactors should be controlled in order to minimize membrane fouling. (c) 2007 Published by Elsevier B.V.