BACKGROUNDThe efficiency of H-2 production by dark fermentation described in the literature is around 50% of the maximum theoretical value and the experiments are mostly carried out over short-term operations. This present study evaluated H-2 production in a UASB (upflow anaerobic sludge blanket) reactor operated at 55 degrees C for 366 days using a synthetic substrate based on sucrose. The reactor response to an increased organic loading rate (OLR) was assessed by applying three different hydraulic retention times (HRTs): 12h (211days), 6h (71days), and 2h (84days). RESULTSCH4 was observed in the biogas during startup. The highest average values of the molar H-2 flow rate (MHFR), volumetric H-2 production rate (VHPR), and H-2 yield (HY) were achieved at a HRT of 2h, and were, respectively, 14.005.24mmolh(-1), 75.33 +/- 24.65 mL H-2 L-1.h(-1), and 1.73 +/- 1.14mol-H-2 mol-suc(-1). Considering only the unstable steady-state period, HY average at a HRT of 2h was 2.56 +/- 0.84mol-H-2 mol-suc(-1). CONCLUSION Operational conditions were found to be the main factors responsible for removing methanogenic microorganisms from the system. Increase in the OLR enhanced H-2 production and the ethanol pathway proved to be favorable for H-2 production. (c) 2015 Society of Chemical Industry