The aim of this work was to model the biological activity and anticipate the kinetic behaviour of microorganisms and the overall performance of the process according to a speci®c model and running parameters. The bacterial inoculum used in these experiments was a mixture of cultures taken from the wastewater treatment plant in Montpellier. The fermentor, used in association with an ultra®ltration separation stage (with a ®ltration area of 0.2 m 2 ) had a working volume of 15.8 l. For various working conditions (different solid retention times, different hydraulic retention times and substrate concentrations), the biomass concentration and the residual substrate concentration, expressed in terms of dry weight and chemical oxygen demand, respectively, were measured. The basic idea of modelling was related to the concept of maintenance. The coef®cient of maintenance, E, and the theoretical conversion yield, y, were therefore calculated. The values of E and y, measured for total cell recycling experiments and for experiments with various solid retention times, remained similar and were found to equal 0.040 mgCOD mgVSS h À1 and 0.36 mgVSS mgCOD À1 , respectively. Determining these two constants and modelling the treatment process made it possible to anticipate the optimal biomass concentration for a de®ned removal ef®ciency under different steadystate operating conditions.