The effect of hydraulic selection pressure on the development of aerobic granules was studied in four column-type sequencing batch reactors (SBRs). The main feature of SBR is cycle of operations, and settling time acts as hydraulic selection pressure imposed on the microorganisms present. Aerobic granules were successfully cultivated in the reactors operated at a settling time less than of 15 min, while only bioflocs appeared in the reactor run at the longest settling time of 20 min. The characteristics of the aerobic granules in terms of settling ability, hydrophobicity, polysaccharide and protein content as well as specific oxygen utilization rate were examined. Results showed that a short settling time could significantly improve the production of cell polysaccharide, cell surface hydrophobicity and microbial activity. These selection pressure-induced microbial changes favour the formation of aerobic granules. (C) 2003 Elsevier Ltd. All rights reserved.