This study investigated the role of sludge retention time (SRT) in aerobic granulation under negligible hydraulic selection pressure. Results showed that no successful aerobic granulation was observed at the studied SRTs in the range of 3-40 days. A comparison analysis revealed that hydraulic selection pressure in terms of the minimum settling velocity would be much more effective than SRT for enhancing heterotrophic aerobic granulation in sequencing batch reactor (SBR). It was shown that SRT would not be a decisive factor for aerobic granulation in SBR. (C) 2008 Elsevier Ltd. All rights reserved.