Two sequencing batch reactors (SBRs) were concurrently operated to investigate the effect of Mg2+ augmentation on aerobic granulation. Augmentation with 10 mg/l Mg2+ in R2 significantly decreased the sludge granulation (defined as that over 15% of granules were larger than 0.6 mm) time from 32 days to 18 days, at the same time, the mean diameter of the granules in R2 was 2.9 mm after the granulation, which was consistently larger than that (1.8 mm) in R1. Mg2+-fed granules were denser and more compact, showed better settling and had higher polysaccharide contents, but it did not result in a difference in microbial morphology. The results demonstrated that Mg2+ enhanced the sludge granulation process in the sequencing batch reactor. (c) 2008 Elsevier Ltd. All rights reserved.