Investigation of anaerobic digestion in a two-stage bioprocess producing hydrogen and methane

Hydrogen has received wide attention in the last decades as a clean energy vector. The major
advantage of energy generation from hydrogen is the near-zero carbon emissions, since the
utilization of hydrogen, either via combustion or via fuel cells, results in pure water. Recently,
there has been increasing interest on the biological production of hydrogen gas from renewable
biomass such as carbohydrates from agriculture or agro-food industries. This specific anaerobic
digestion is called dark fermentation and is involved in the classic anaerobic digestion producing
methane. Indeed, in a two-step process, i.e. when acido- and aceto-genesis are carried out in a
different bioreactor than methanogenesis, it is feasible to generate separated biogas flows
containing either H2 or CH4 depending on specific operating parameters.
This paper deals with the comparison of the first stage performances whether operated in
optimum conditions for H2 production with a mixed culture or a pure Clostridium butyricum
strain. Hydrogen yields of about 1.75 and 2.3 mol H2/mol glucose were achieved respectively.
The metabolites, mainly acetate and butyrate, contained in the spent medium were efficiently
converted to methane in a second anaerobic digester with a methane yield of about 170 ml/g
COD initially fed in the first stage.