Bio-hydrogen and bio-methane production from food waste in a two-stage anaerobic digestion process with digestate recirculation

Bio-H2 and bio-CH4 production from food waste in a two-stage temperature phased system were investigated to determine the effects of digestate recirculation on energy efficiency and process stability. Different recirculation ratios (RR), i.e. 0.3, 0.5, and 1.0, were tested. Maximum H2 production of 3 L-H2 L−1d−1 and yield of 135 L-H2 kg−1VSin were achieved for an RR of 0.3 at HRT 5 d and OLR of 18 kg-VS m−³d−1. The RR of 0.3 was also the best for producing CH4 and gave results of 2.9 L-CH4 L−1 d−1, i.e. 510 L-CH4 kg−1VSin at HRT 9 d and OLR of 5.7 kg-VS m−³ d−1. The energy recovered from the recirculation process increased the H2 production by 8% and decreased the CH4 production by 3%; the total energy production did not change. Digestate recirculation in comparison with a no-recirculation system reduced the need for alkali addition to maintain pH in the H2-reactor by 54%.