The phenomenon of bacterial wash-out frequently occurs in the traditional continuous stirred tank reactor (CSTR) systems at low hydraulic retention time (HRT). In this study, the effect of different aspect ratios, height (H) to diameter (D) of 1:1, 3:1 and 5:1, of a CSTR with immobilized anaerobic sludge on hydrogen (H-2) production were investigated. The pH, volatile suspended solids (VSS) and total solids (TS) concentrations of the seed sludge were 6.8, 33.3 and 65.1 g/L, respectively. Thermally treated sludge was immobilized by silicone gel entrapment approach. The entrapped-sludge system operated stably at a low HRT without suffering from cell wash-out. Hence, the hydrogen production rate (HPR) was enhanced by increasing organic loading rates. The immobilized sludge CSTRs were operated at 40 degrees C with sucrose (10, 20, 30 and 40 g COD/L) and Endo nutrient medium at different HRTs (4, 2, 1 and 0.5 h). It was found that the granule formation enhanced HPR. The maximum HPR and the H-2 yield were found to be 15.36H(2) L/h/L and 3.16 mol H-2/mol sucrose, respectively, with the H-2 content in the biogas above 44% for all tests runs. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.