Waste and biorefinery - a new boost for dark fermentation?

Sustainable management options for organic waste foresee the possibility of using industrial processes to produce energy or commodities from biodegradable materials. Limits on landfill disposal of biodegradable organics and green house gas emission reduction targets posed by the European Union represent an incentive to this approach. Wastes are treated before their final disposal to simultaneusly produce or recover bio-fuels, bulk chemicals, biopolymers or bioproducts for further industrial or commercial applications in a scheme of circular economy. Over the last years biological hydrogen production has arised a growing interest. The production of bio-hydrogen and other biofuels from organic residues could in fact well fit a sustainable energy economy combining use of hydrogen, electricity and batteries to serve as both energy carrier and storage systems. In this strategy hydrogen can be produced from renewable sources with decentralized on-site small scale plants to both minimize the problems related to storage and transport and to match the wide distribution of feedstock. Heterotrophic processes for hydrogen production are considered more attractive to couple bio-hydrogen production and organic waste stabilization and for this reason dark fermentation has been receiving considerable attention over the last decade, as demonstrated by the increasing trend of scientific papers published in the field.Also the concept of biorefinery has faced a growing interest in recent years (Figure 2). The concept of biorefinery involves the integration of different thermal, chemical or biological processes to produce various biofuels, materials or value-added products from renewable and biogenic feedstock. Dark fermentation is a process where complex organic matter is hydrolyzed and further converted to a pool of organic acids and alcohols by various bacteria. Within the concepts of biorefinery and sustainable waste management, bio-hydrogen production from organic waste has the potential to play the central role of primary treatment followed by and integrated with biofuel or biomaterial generation processes to maximise waste valorisation. Multiple processes and technologies can be combined to dark fermentation to produce energy (heat or electricity), biofuels (hydrogen, methane, ethanol and other alcohols), chemical products (sugars, organic acids, biopolymers) and other valuable materials for reuse (soil amending products, fertilizers), where bio-hydrogen production has the role to provide the starting feedstock to next processes thanks to its hydrolytic and fermentation capacities. Some of the possibilities to be combined with dark fermentation as port-treatment are: methane production by methanogenic digestion; hydrogen production by photofermentation or microbial electrolysis cell; electricity by microbial fuel cell; volatile fatty acids (acetic, propionic, butyric, succinic or lactic acid), ethanol or 1,3-Propanediol recovered from digestion liquid, ammonia and phosphate recovered from digestion liquid. Biohydrogen production process therefore can be considered an important treatment step in biorefinery as it can produce biofuels, fine chemicals and biomaterials. This perspective could contribute to reduce the overall costs of the dark fermentation process and stimulate its full-scale application thank to the economic value of multiple products obtained from the bioprocess.