Oxic-settling-anaerobic process (OSA) has established that anaerobic followed by aerobic process in a cyclic reaction reduces the biological sludge growth. However, the mechanism behind the process of reduction is still undefined. The recirculation of aerobic sludge from a sequencing batch reactor (SBR) to an anaerobic side reactor at the rate of 3.8-9.8 gVSSrecycled/gVSS resulted in sludge reduction from 3% to 51% (Average = 14.6-39.8%) in comparison to absence of OSA process in conventional SBR. The observed sludge reduction was in proportion to specific oxygen uptake rate (SOUR) gradient between sludge recycle and no recycle condition. The SOUR gradient is the ratio of its difference between fasting when sludge came out of anaerobic reactor and feasting conditions during aeration, in comparison to SOUR difference between aeration and termination of settling in the absence of any sludge recycle. SOUR kept on increasing with increasing sludge recycle during feasting (in aeration) in comparison to fasting (in anaerobic). This gradient resulted from high substrate oxidation during aeration (feasting), which may be induced by increased energy uptake stimulated by prolonged fasting. Increased cell maintenance also validated the effect of SOUR gradient in direct proportion. (C) 2015 Elsevier B.V. All rights reserved.