Gas biofuels from solid substrate hydrogenogenic–methanogenic fermentation of the organic fraction of municipal solid waste

The objective of this work was to evaluate the performance of a two-stage hydrogenogenic–methanogenic (H–M) semi-continuous process in terms of mass retention time (MRT) for hydrogenogenic stage (H-stage), feed source for methanogenic stage (M-stage) and thermal regime (35 and 55 °C) for both stages. The substrate was a model organic fraction of municipal solid wastes (OFMSW) at 35% total solids.In H-stage, mesophilic temperature had a positive significant effect on higher hydrogen productivities and lower amounts of hydrogen sinks compared to thermophilic operation. Calculations based on mass balances and biochemical stoichiometry confirmed that acid fermentation deviation was linked to low biohydrogen yields. The M-stage performance was influenced by both the temperature and feed source. Bioreactors in thermophilic regime performed better than mesophilic ones. Maximum methane productivity was 341 NmL CH4/(kgwmr d) that corresponded to the thermophilic bioreactor fed with fermented solids from H stage at 14 d MRT. The two-stage process showed higher gross energetic potential when compared to an only methanogenic process operated at equivalent MRT (control); this was due to a higher methane productivity in the M-stage of the series process. The main contribution of H-stage seemed to be associated to hydrolysis of the complex substrate thus generating metabolites for the M-stage rather than the hydrogen production itself.