Membrane bioreactors (MBRs) have been widely used as advanced wastewater treatment process in recent years. However, MBR system has a membrane fouling problem, which makes the system less competitive. Thus there have been great efforts for fouling mitigation. In this study, two types of TiO2 immobilized ultrafiltration membranes (TiO2 entrapped and deposited membranes) were prepared and applied to activated sludge filtration in order to evaluate their fouling mitigation effect. Membrane performances were changed by addition of TiO2 nanoparticles to the casting solution. TiO2 entrapped membrane showed lower flux decline compared to that of neat polymeric membrane. Fouling mitigation effect increased with nanoparticle content, but it reached limit content above which fouling mitigation did not increase. Regardless of polymeric materials, membrane fouling was mitigated by TiO2 immobilization. TiO2 deposited membrane showed greater fouling mitigation effect compared to that of TiO2 entrapped membrane, since larger amount of nanoparticle was located on membrane surface. It can be concluded that TiO2 immobilized membranes are simple and powerful alternative for fouling mitigation in MBR application. (C) 2004 Published by Elsevier B.V.