The two-stage hydrogen methane fermentation process with different patterns of recirculation was investigated. Operations with the circulation of heat-treated sludge performed considerably better than those with the recirculation of raw sludge with respect to both the hydrogen production rate and yield. In addition, the results of the batch tests demonstrated that circulated sludge was capable of consuming hydrogen under acidogenic pH while the heat-treated sludge was not. These results suggest that the recirculation of active methanogenic sludge had an inhibitive effect on the hydrogen production, which can likely be attributed to the high hydrogen-consuming activity of microorganisms present in the circulated sludge. On the other hand, operations without any sludge recirculation did not perform well in terms of hydrogen production or carbohydrates degradation compared to those with recirculation, perhaps due to a shortage of available nitrogen. This suggests that sludge recirculation in effect supplemented the NH4+ in the hydrogen reactor. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.