In this paper, the influences of air bubbling mode, air flow rate, air bubble size and feed water quality on the membrane fouling of immersed hollow-fiber (HF) membrane for ultrafiltration of river water were investigated. The membrane was operated in the intermittent suction mode of 9 min on/1 min off. The results showed that continuous air bubbling was more effective for mitigating membrane fouling than intermittent bubbling (1 min on/9 min off in the study) at the same air flow rates of 1.0, 2.5, 5.0, and 7.5 m(3)/m(2)h, respectively. Furthermore, an optimum air flow rate was observed for immersed HF membrane system when considering the alleviating effect on membrane fouling and energy consumption simultaneously, with the optimum value determined as 5.0 m(3)/m(2)h in this investigation. It was also found that the smaller the air bubble was (4 dia of 3.5, 5.0, 6.5, and 8.0 mm were investigated), the more effective the air bubbling would be for mitigating the membrane fouling of immersed HF membrane. However, even if air bubbling was performed, eliminating membrane foulants from the feed water before ultrafiltration was still necessary for the reduction of membrane fouling. (C) 2010 Elsevier B.V. All rights reserved.