In this study, the effect of sludge retention time (SRT) on oxic-settling-anoxic (OSA) process was determined using a sequencing batch reactor (SBR) attached to external aerobic/anoxic reactors. The SRT of the external reactors was varied from 10 to 40 d. Increasing SRT from 10 to 20 d enhanced volatile solids destruction in the external anoxic reactor as evidenced by the release of nutrients, however, increasing the SRT to 40 d did not enhance volatile solids destruction further. Relatively short SRT (10–20 d) favoured the conversion of destroyed solids into inert products. The application of an intermediate SRT (20 d) of the external reactor showed the highest sludge reduction performance (>35%). Moreover, at the optimum SRT, OSA improved sludge dewaterability as demonstrated by lower capillary suction time and higher dewatered cake solids content.