Novel nanocomposite ultrafiltration (UF) membranes were designed and fabricated using hydrophilic polycitrate-Alumoxane (PC-A) nanoparticles and polyethersulfone (PES) to achieve desirable performance for membrane bioreactor (MBR) system. Pure water flux (PWF) and fouling tests were employed to examine the effects of PC-A nanoparticles on the UF performance. FTIR spectra of the PC-A nanoparticles revealed that carboxylic acid and hydroxyl groups are created on the surface of the PC-A. The presence of these hydrophilic groups in PC-A resulted in greater hydrophilicity for the PC-A-modified UF membranes. Therefore, the PC-A-modified UF membranes exhibited higher PWFs compared to the unfilled PES membrane. Fouling resistance of the membranes was evaluated by activated sludge suspension filtration disclosed that the membrane modified with only 0.5 wt% of PC-A had the best antifouling characteristic as well as the greatest PWF. Hence, this optimal nanocomposite membrane was used in the MBR and variations of fluxes exanimated at different mixed liquor suspended solids (MLSS) concentrations of 6000, 10,000 and 14,000 mg/L. During filtration of the activated sludge, the fluxes increased with an increase in MLSS. A decrease in soluble microbial products, extracellular polymeric substance (EPS) from the bacterial cells, and cake formation fouling may result in gentler permeation flux decline over time.