To evaluate the effect of sludge removal methods on the stability of aerobic granules, three different removal methods of sludge were investigated in sequencing batch reactors (SBR). Results demonstrated that (i) in R1, removing fresh granules effectively maintained granules with sizes of 1-2 mm and SVI of 31-45 ml g(-1) for more than 432 cycles (one cycle length was 3 h); (ii) in R3, removing aged granules led to disintegration of granules; (iii) in R2, removing completely mixed sludge resulted in granules with good settleability. However, the morphology of granules in R2 was similar to that of granules in R3 with sizes of about 4-5 mm. It was also found that removing fresh granules built up more ash solids in granules than removing aged granules did and removing aged granules significantly led to lysed granules with void shells. Results suggest that removing fresh granules is an effective approach of selective force for aerobic granulation, and the mature phase plays an important role on the stability of granules by rearranging granule structure (e.g. the buildup of ash solids). (c) 2006 Elsevier Inc. All rights reserved.