Hydrogen instead of methane was produced during Zn-inhibited anaerobic digestion. Hydrogen is a clean fuel gas with a high energy content and similar to biogas can replace fossil fuels in motors. Under anaerobic conditions, hydrogen can be produced by two fermentation pathways (Eq. 3 and 4, respectively)According to the above equations, hydrogen production per mole of glucose monohydrate at 20 C (gas analysis temperature) and at a glucose concentration of 2 g/L would be 971.3 mL/L (Eq. (3)) or 485.6 mL/L (Eq. (4)), respectively