In an attempt to alleviate membrane fouling, a kind of cylindrical rigid suspended carrier was added in a submerged membrane bioreactor (SMBR) treating synthetic domestic wastewater. Its effect was investigated at different mixed liquor suspended solid (MLSS) concentrations and carrier doses. It was found that the suspended carriers added in the SMBR had two effects on membrane fouling: first was the positive effect by mechanically scouring membrane surface, thereby mitigating cake layer fouling; the second was the negative effect by breaking up sludge floes that caused an increase in the amount of small particles and supernatant total organic carbon and thus accelerated membrane fouling. The whole effect of suspended carriers on membrane fouling depended on the relative intensity of these two effects. Based on the experimental results of all runs and the inertial lift theory, therefore, it was concluded that there existed an effective carrier dose range for mitigating membrane fouling on the whole. When MLSS concentrations were 5 g/L, 8 g/L, and I I g/L, then the effective carrier doses (carrier volume vs. total volume) were about less than 4.4%, 7.3%, and 9.5%, respectively. (C) 2007 Elsevier B.V. All rights reserved.