This study was carried out to assess the efficiency of a mesophilic up-flow anaerobic staged reactor for continuous H-2 production from pretreated rice straw waste. The reactor was operated at different hydraulic retention times (HRTs) of 20, 16, 12, 8 and 4 h. The organic loading rate and sludge residence time were kept constant at 30 g chemical oxygen demand (COD)/L/day, and 1.9 days, respectively. The results showed that increasing the HRT from 4 to 20 h increased the H-2 production from 0.4 +/- 0.1 to 3.6 +/- 0.3 LH2/day, respectively. This corresponds to a H-2 yield of 2.1 +/- 0.2 mol H-2/g CODremoved at an HRT of 20 h and 0.03 +/- 0.002 mol H-2/g CODremoved at an HRT of 4 h. Likewise, carbohydrate and COD removal efficiency was strongly dependant on HRT. The removal efficiency decreased from 76.5 +/- 3.4% to 40 +/- 2.2% for carbohydrate and from 77.7 +/- 4.3% to 12.2 +/- 2.1% for COD when the HRT is reduced from 20 to 4 h, respectively. The addition of presettled sewage sludge to pretreated rice straw at a mixing ratio of 1:4 (v/v) increased the volumetric H-2 production from 3.6 +/- 0.3 to 8.2 +/- 2.5 L/day and the H-2 yield from 2.1 +/- 0.2 to 2.8 +/- 0.3 mol H-2/g CODremoved. Moreover, the removal efficiency of COD, volatile solids and carbohydrate was significantly improved. Copyright (C) 2013 John Wiley & Sons, Ltd.