The effect of temperature and effluent recycle rate on hydrogen production by undefined bacterial granules

► Rapid bacterial granulation was induced under thermophilic conditions, 45 and 70 °C. ► The 45 and 70 °C treatments gave % H2 contents of 45% and 67%, respectively. ► Extreme rates of effluent recycling up to 3.5 L/min were applied to the bioreactor. ► At 70 °C high effluent recycling and low HRTs resulted in volumetric rates up to 14.8 L H2/L/h.