The objective of this study was to define minimum size of bioparticles that could be classified as granules, to offer all advantages of granular sludge. Based on the theory of sedimentation, the minimum diameter bioparticles, which should be considered as granules was found out for specific gravity of sludge ranging between 1.01 and 1.05. For example, for specific gravity of 1.035 the minimum diameter of granules required for better sludge retention was 0.34 mm. The diameter based on this theory was evaluated by carrying out settling column analysis of a granular sludge obtained from lab-scale UASB reactor and verified with microscopic observation. To find out the effect of granules size on the nature of biodegradability, specific methanogenic activity (SMA) was carried out. It was observed that SMA increased with size of bioparticles tested in the range of 0.27-3.03 mm. The change in VSS/SS ratio and specific gravity was observed with size of granules. Consideration of variation in specific gravity with size of granules increased the degree of validation of sedimentation theory for the calculation of granules diameter. (c) 2006 Elsevier Ltd. All rights reserved.