Garden and food waste co-fermentation for biohydrogen and biomethane production in a two-step hyperthermophilic-mesophilic process.

Co-fermentation of garden waste (GW) and food waste (FW) was assessed in a two-stage process coupling hyperthermophilic dark-fermentation and mesophilic anaerobic digestion (AD). In the first stage, biohydrogen production from individual substrates was tested at different volatile solids (VS) concentrations, using a pure culture of Caldicellulosiruptor saccharolyticus as inoculum. FW concentrations (in VS) above 2.9 g L-1 caused a lag phase of 5 days on biohydrogen production. No lag phase was observed for GW concentrations up to 25.6 g L-1. In the co-fermentation experiments, the highest hydrogen yield (46 ± 1 L kg-1) was achieved for GW:FW 90:10% (w/w). In the second stage, a biomethane yield of 682 ± 14 L kg-1 was obtained using the end-products of GW:FW 90:10% co-fermentation. The energy generation predictable from co-fermentation and AD of GW:FW 90:10% is 0.5 MJ kg-1 and 24.4 MJ kg-1, respectively, which represents an interesting alternative for valorisation of wastes produced locally in communities.