Critical flux based on the TMP increase is the flux below which no or negligible membrane fouling occurs. A method was tested for enhancing the critical fluxes by injecting air into a shell-side feed organic hollow fiber membrane module. A range of aeration intensities was tested to filtrate the biologically treated wastewater. Air sparging promoted turbulence, resulting significant enhancements of critical flux. Three different zones for the membrane bioreactor operation can be determined, supra-critical zone. critical zone and sub-critical zone. Sub-critical operation was a valid strategy for microfiltration of the biologically treated wastewater to work over an extended period with a small gradual linear rate of TMP increase. But it followed a sudden jump probably due to the redistribution of the local fluxes. (C) 2003 Elsevier B.V. All rights reserved.