The performance of a wastewater bench-scale ultrafiltration membrane bioreactor (MBR) treatment plant using pure oxygen to supply the aerobic conditions for 95 days was studied. The results showed the capacity of the MBR systems to remove organic material under a hydraulic retention time of 12 h and a sludge retention time of 39.91 days. Aeration represents its major power input; this is why the alpha-factor of the aeration and kinetic parameters (design parameters) were determined when the mixed liquid suspended solids (MLSS) was increased from 3420 to 12,600 mg/l in order to understand the system. An alpha-factor in the range 0.462-0.022 and the kinetic parameters measured with the respirometric method (K(M) of 73.954-3.647 mg/l, k(d) of 0.0142-0.104 day(-1), k(H) of 0.1266-0.655 day(-1), and the yield mean coefficient of 0.941) were obtained. Our study suggested significant changes in the behaviour of the biological system when the concentration of MLSS was increased. (C) 2011 Elsevier Ltd. All rights reserved.