The membrane bioreactor (MBR) process has now become an attractive option for the treatment and reuse of industrial and municipal wastewaters. However, the MBR filtration performance inevitably decreases with filtration time due to membrane fouling. Over the past two decades, increased interests in improving the performance of filtration membranes (i.e., reducing membrane fouling) have encouraged the development of new classes of chemically modified membranes. In this study polyethersulfone (PES) membranes and membranes with five different weight ratios of ZrO2 to PES of 0.01, 0.03, 0.05, 0.07 and 0.1, were prepared by the phase inversion method and applied to activated sludge filtration in order to evaluate their fouling characteristics. The membranes were characterized using field-emission scanning electron microscope (FESEM). The ZrO2/PES membrane strengths were higher than those of the neat membrane. The membrane molecular weight cut-off (MWCO) and membrane thickness were slightly affected by the ZrO2 addition. ZrO2 entrapped membranes showed lower flux decline compared to the neat PES membrane, with fouling mitigation increasing with ZrO2 particles content. The optimum load of ZrO2 immobilized membranes for membrane bioreactor (MBR) application in terms of highest membrane permeability and lowest fouling rate was the 5% weight fraction of ZrO2 with PES. (C) 2010 Elsevier B.V. All rights reserved.