This study shows the changes in transmembrane pressure in a membrane bioreactor (MBR) operating at a constant flux and in well-defined biological conditions. Despite the initial choice of filtration conditions at sub-critical flux, gradual fouling is seen to develop in membrane material which, after long periods of experimentation without intermediary membrane regeneration, proves to be hydraulically irreversible. Thus a single parameter model was set up, whose value could be derived from monitoring the changes in transmembrane pressure. It works on the hypothesis that the open surface is reduced due to a successive blocking of membrane pores. The development of this model has shown that, despite operating conditions held at constant flux, an increase occurs in the rate of circulation in the pores which remain open, which may reach critical local filtration conditions, resulting in the formation of deposits which are reversible but induce an abrupt change in transmembrane pressure. (C) 2003 Elsevier B.V. All rights reserved.