Well-formed denitrifying granular sludge with a biomass concentration of 24.8 gVSS L-1 and a specific nitrate removal rate of 0.19 gNO(3)-N gVSS(-1) d(-1) was obtained in an upflow sludge blanket (USB) reactor by cultivating seeded aerobic flocculent sludge for 6-8 weeks. Regularity phenomena exist in the granulation including flotation of flocculent sludge, formation of fine granules, occurrence of channelling, and formation of mature granular sludge. The granulation is similar to crystal growth, that the non-denitrifying bacteria evolve into the carriers (fine granules), on the surface of which denitrifying bacteria proliferate and develop into mature granular sludge. There are several key parameters that must be considered when developing a good denitrifying granular sludge. First, the proper seed sludge must be chosen (VSS/SS at 0.65-0.75, SRT over 25 days) to accelerate the granulation process. Secondly, any floating sludge should be stirred, and the sludge loading rate should be within the range of 0.05-0.15 gNO(3)-N gVSS(-1) d(-1) until fine granules emerge. Additionally, spontaneous gas agitation or interval air-blowing should be used to effectively eliminate channelling; Finally, the sludge loading rate should be less than 0.25 gNO(3)-N gVSS(-1) d(-1) until dense, mature granular sludge appears. This study could support and promote the full-scale application of denitrifying granular sludge. Crown Copyright (c) 2012 Published by Elsevier Ltd. All rights reserved: