The influence of hydraulic retention time (HRT, 24, 12, and 6 h) on the physical characteristics of granules and performance of a sequencing batch reactor (SBR) treating rubber wastewater was investigated. Results showed larger granular sludge formation at HRT of 6 h with a mean size of 2.0 +/- 0.1 mm, sludge volume index of 20.1 mL g (1), settling velocity of 61 m h (1), density of 78.2 g L (1) and integrity coefficient of 9.54. Scanning electron microscope analyses revealed different morphology of microorganisms and structural features of granules when operated at various HRT. The results also demonstrated that up to 98.4% COD reduction was achieved when the reactor was operated at low HRT (6 h). Around 92.7% and 89.5% removal efficiency was noted for ammonia and total nitrogen in the granular SBR system during the treatment of rubber wastewater. (C) 2014 Elsevier Ltd. All rights reserved.