As one of the largest wastewater sources, domestic sewage needs particular attention. This study focuses on how hydraulic retention time (HRT) affects a membrane-based process treating digested sewage. A laboratory-scale combined system, including an anoxic tank (AT) and an aerobic membrane bioreactor (MBR) was operated under decreasing HRTs (8 h, 6 h, 4 h and 2.5 h) for 218 days. HRT variation had a very limited effect on carbon removal but enhanced nitrogen (ammonium and total nitrogen) removal capacity. The biomass concentrations increased by five times until the end of HRT 2.5 h. The 16S rDNA fingerprinting analyses indicated that the microbial diversity decreased and the species belonging to the genera of Nitrospira and Variovorax became to dominate at short HRTs. The filtration resistance at HRT 2.5 h was almost three times higher than the one at HRT 8 h; also, the fouling rate was faster at short HRTs. (C) 2011 Elsevier B.V. All rights reserved.