Hydrodynamic control of bioparticle deposition in a MBR applied to wastewater treatment

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. (C) 1998 Published by Elsevier Science B.V. All rights reserved.