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.