Sludge retention time (SRT) can produce significant effects on biomass properties in a membrane bioreactor (MBR) system. In this study, the membrane separation process was coupled to a sequencing batch reactor (SBR), which is one of the biological nutrient removal (BNR) processes, and the influence of SRT on membrane fouling and biological activity was investigated. Membrane fouling increased with SRT since sludge particles were more severely deposited on the membrane surface at longer SRT. Regardless of SRT change, COD removal efficiency was high and stable (over 92%) throughout the experiment. Nitrogen removal efficiency also attained a high treatment level. However, it was not proportioned to SRT increase and rather decreased at the longest SRT. Phosphorus removal decreased at prolonged SRT since excess sludge was reduced. Biological activity such as specific oxygen uptake rate (SOUR), specific nitrification rate (SNR), and specific denitrification rate (SDNR) did not increase with SRT but decreased at prolonged SRT. (c) 2004 Elsevier Ltd. All rights reserved.