Hydrogen is produced in the first stage as an intermediate metabolite which in turn is used as an electron donor by many methanogens at the second stage of the anaerobic fermentation. It might be feasible to harvest hydrogen at the acidification stage leaving the remaining acidification products for further methanogenic treatment.Therefore, it is possible that only acidogens are left to produce hydrogen gas, CO2 and volatile fat acids (VFAs) if the final stage or methanogenesis and other hydrogen-consuming biochemical reactions are inhibited during the dark fermentation.