and acetate were the main fermentation products along with little amount of butyrate and propionate. The pH of the fermentation was automatically maintained at 4.0–4.6 by adding 5 M NaHCO3.That pH range is considered to be the suitable values for ethanol type fermentation. Therefore, the bacterial metabolism in this study was following ethanol type fermentation which is regarded as the best fermentation type for fermentative hydrogen production (Renet al.,2008). Using Eqs. (3)–(5), theoretical CO2 and H2 yields could be calculated from ethanol and volatile acid concentrations measured in the effluent of the reactor. The carbon recovery ranged from 71.2% to 87.4% based on ethanol and volatile acid production with the balance assumed to be converted to biomass (Table 5). There was no loss of H2 via methanogenesis because no methane was detected during the whole operation period. In an idea hydrogen production system, the ratio of the measured H2 production in the gas products (HG) and the theoretical H2 production in the liquid phase (HL) should be close to 1. However, it was difficult to realize that because of the absorption of the liquid compounds and the high partial pressures of hydrogen. The HG/HL obtained from this study ranged from 71.4% to 74% which was comparable to Tapia-Venegaset’s (2013) result (HG/HL, 61–80%) probably due to the continuous release of hydrogen gas from the liquid phase caused by the high stirring speed (300 rpm) in this study. Ramírez-Morales