Comparison between filtrations at fixed transmembrane pressure and fixed permeate flux: application to a membrane bioreactor used for wastewater treatment

Membrane bioreactors for wastewater treatment must operate for long periods without chemical cleaning. This paper investigates the critical flux conce pt introduced by Field et al. as a means for achieving this goal. Experimen ts were conducted on a membrane bioreactor containing 6001 of activated slu dge, equipped with a 0.25 m(2) ceramic membrane and located in Compiegne wa stewater treatment plant. Hydraulic retention time was set at 24 h and slud ge retention time at 60 days, so that suspended solids concentration stabil ises at 10 g/l. We conducted two series of tests: at fixed transmembrane pr essure (TMP) and at fixed permeate flux, set by a volumetric pump on the pe rmeate. In both cases, velocity was varied from 1 to 5 m/s. In fixed flux t ests, the Aux was increased by 10 l/h m(2) increments and the TMP was obser ved to rise moderately first and then stabilise in about 15 min until a cri tical value of the flux is reached. Above this critical flux, the TMP rises rapidly and does not stabilise, as in dead-end filtration. The critical fl ux was found to increase approximately linearly with velocity, reaching abo ut 115 l/h m(2) at 4 m/s. These data were reproducible at various dates bet ween 30 and 120 days of continuous operation of the bioreactor and permit t o know at which flux a membrane bioreactor must be operated. Comparison of constant pressure and constant flux tests under same conditions showed that the critical flux is almost identical to the limiting or pressure independ ent flux obtained in constant pressure. More generally, constant flux proce dure below the critical flux avoids overfouling of the membrane in the init ial stage and is more advantageous for membrane bioreactor operation. (C) 1 999 Elsevier Science B.V. All rights reserved.