The environmental biorefinery: state-of-the-art on the production of hydrogen and value-added biomolecules in mixed-culture fermentation

The environmental biorefinery consists of recovering and adding value to waste, possibly through a multi-product approach. A first implementation of such a concept is the production of methane and nutrient-rich digestate by anaerobic digestion in biogas plants. However, methane and digestate have only a low added-value and biogas plants still require feed-in tariff policies to be economically viable. The aim of this article is to provide a meta-analysis of current biomass recovery technologies compatible with environmental applications (i.e. non-sterile conditions and carried out by microbial mixed culture). The focus was particularly on those able to produce high value-added fermentation metabolites. To achieve this objective, both qualitative (e.g. substrates and pretreatments) and quantitative data (e.g. yields, productivities, and process parameters) were retrieved from 624 manually-checked research articles, excluding review papers, and 134 patents published after 1997. In addition, a straightforward market study was carried out for nine promising biomolecules: H-2, ethanol, acetate, propionate, lactate, 1,3-propanediol, butyrate, caproate and polyhydroxyalkanoates (PHAs). Finally, the feasibility of producing each biomolecule in the context of an environmental biorefinery is discussed in light of current process performances and their related bottlenecks.