The study of fouling for tubular ceramic ultrafiltration (MWCO 300 kD) membranes during activated sludge filtration under constant flux conditions led to the experimental identification of two fouling processes : (1) At low recirculation velocity (0.5 m/s; Re similar to 1200), sludge flee particles were deposited on the membrane surface and this cake layer provoked a very quick increase of hydraulic resistance (>10(9) m(-1) s(-1)). (2) However, under the usual recirculation conditions (4 m/s; Re similar to 9000), floc was not deposited. Filtration was stable for several days even with high fluxes (75-150 l/h m(2)). Progressive membrane fouling resulted in a linear increase of transmembrane pressure over a certain time, and could be described by an increase in the hydraulic resistance, with dR/dt varying between 10(5) and 10(8) m(-1) s(-1) depending on hydrodynamic and biological conditions.Mechanism models (shear induced diffusion, inertial lift, surface transport) led to a very good qualitative description of these results, and revealed the major role of convective backtransport phenomena in fouling processes.