A pH- and temperature-phased two-stage process for hydrogen and methane production from food waste

A pH-phased two-stage fermentation process combined thermophilic hydrogen production and mesophilic methane production with recirculation of the digested sludge was suggested in this study and its treatment performance was demonstrated by a continuous experiment treating organic fraction of municipal solid wastes (OFMSW). A thermophilic hydrogen production reactor was operated at a hydraulic retention time (HRT) of 1.3 days, corresponding to a loading rate of 38.4 kg-VS/m3/d or 64.4 kg-CODCr/m3/d. The short HRT and low pH applied here were enough to separate hydrogen production from methanogenesis. The mixed slurry with a high concentration of organic acids was then converted into CH4 by the mesophilic methane production reactor at a short HRT of 5.0 days, corresponding to a loading rate of 6.6 kg-VS/m3/d or 16.3 kg-CODCr/m3/d. The experiment was continued for over 150 days, and a stable performance for the simultaneous hydrogen and methane production was obtained. The yields of hydrogen and methane in this study were 205 ml H2/g VS added and 464 ml CH4/g VS added, respectively.