Two sequential aerobic sludge blanket reactors were concurrently operated to examine the effect of Ca2+ augmentation on aerobic granulation. Augmentation with 100 mg Ca2+ l(-1) significantly decreased the time to cultivate aerobically grown microbial granules from 32 d to 16 d. Ca2+-fed granules were denser and more compact, showed better settling and strength characteristics, and had higher polysaccharide contents.