Membrane bioreactors (MBRs) have been increasingly used for municipal wastewater treatment. Systems where the membranes are submerged in the bioreactor, and scoured by the aerating gas stream, are very common and the "Kubota process" is a classic example of this technology [S. Churchouse, Membrane bioreactors: going from laboratory to large scale-problems to clear Solutions, in: Presented at Membranes and the Environment, University of Oxford, July 14-17, 2002]. Current treatment plants are designed to cope with three times the average dry weather flow (DWF) of wastewater to accommodate variable flow rates, and hence are over-sized compared to average or "normal" conditions. If throughput in submerged MBRs can be changed readily by manipulation of the aeration rate, due to increased scouring of the membrane and a corresponding increase in critical flux, a smaller plant may be designed compared to the "3 x DWF" rule. High throughput operation may be achieved by using a high aeration rate to generate a high crossflow velocity to minimise fouling, and at average or low throughput a low aeration rate may be used to minimise energy consumption. The aim of this work was to explore the feasibility of designing smaller membrane plants by using the aeration rate to manipulate critical flux in situ, and thereby allow variable throughput. It is concluded that (a) permeate flux and aeration rate are important hydrodynamic parameters that must be controlled to avoid excessive membrane fouling; (b) intermittent permeation is ail effective membrane cleaning technique for long-term sustainability of flux; (c) manipulation of the critical flux through variation of aeration rate is possible in situ in submerged MBR systems. (C) 2004 Elsevier B.V. All rights reserved.