The structural and hydrodynamic properties of aerobic granules, such as their morphology, fractal dimension, porosity, size distribution, settling velocity, permeability, and shear strength, were characterized. Microbial granules were formed in two sequencing batch reactors (SBRs) that are used to treat glucose-based synthetic wastewater. The first SBR (R1) had a low pH of about 3.0, due to low influent alkalinity (28.7 mg CaCO3/L), and produced fungi-dominated granules. The second SBR (R2) had a pH of around 8.1, due to high influent alkalinity (301 mg CaCO3/L.), and produced bacteria-dominated granules. The fungal granules were larger and weaker, with a loosely packed fluffy structure, whereas the bacterial granules were smaller and stronger and had a compact structure. The granules from both RI and R2 were fractal aggregates, and they had fractal dimensions of 2.23 and 2.42, respectively. The settling velocities in water for the granules from R1 ranged from 0.38 to 2.67 cm/s. Those from R2 ranged from 0.42 to 3.21 cm/s. This is in good agreement with the settling velocities predicted by Stokes' law for porous but impermeable spheres. The fungal granules were almost completely impermeable, with an average fluid collection efficiency of 0.006, whereas the bacterial granules were slightly permeable, with an average fluid collection efficiency of 0.052. The results demonstrate that biomass enrichment in bioreactors can be achieved by the generation of dense and fast-settling sludge granules. By controlling the feeding condition, different types of aerobic granules can be produced with different structural features and hydrodynamic properties. (c) 2008 Elsevier B.V. All rights reserved.