A micro-aerobic side-stream reactor (MSSR) was inserted in returned sludge line of a membrane bioreactor (MBR) for sludge reduction. Three MSSR coupled MBRs (M-MBRs) and an anoxic/oxic MBR (AO-MBR) were operated in parallel to investigate effects of micro-aerobic condition and hydraulic retention time (HRT) of MSSR (HRT M ) on process performance. M-MBRs and AO-MBR were equally effective in COD and NH 4 -N removal. Rising HRT M from 3.3 to 6.7 h increased sludge reduction from 10.8% to 47.5%, and promoted dissolved organic matters release and utilization by enriching hydrolytic, fermentative and predatory bacteria. M-MBR at HRT M of 3.3 h obtained the highest nitrogen removal, and enriched denitrifiers ( Thauera and Hyphomicrobium ) in the MSSR. The highest relative abundance of polyphosphate-accumulating organisms was observed in M-MBR at HRT M of 5.0 h, which achieved the highest phosphorus removal. Prolonging HRT M to 6.7 h improved sludge dewaterability by enriching floc-formation bacteria ( Nannocystis ) and inhibiting filamentous bacteria ( Candidatus_Alysiosphaera ). [ABSTRACT FROM AUTHOR]