Dark fermentation as an environmentally sustainable WIN-WIN solution for bioenergy production

The current energy and environmental crisis, linked to increasing industrialisation, has progressively driven the adoption of solutions focused on circular life cycle thinking, such as waste management with resource recovery and decarbonization of technologies. In this context, this study was built to quantify the environmental performance of a two-stage wastewater treatment process (dark fermentation with anaerobic digestion) in which three feedstocks (sugar beet molasses, cheese whey and wine vinasses mixed with wastewater treatment plant sludge) from the food industry were valorized as hydrogen. In this regard, several environmental profiles were created using the Life Cycle Assessment methodology with two system boundaries (cradle-to-gate and gate-to-gate) and two methods (ReCiPe Midpoint and Endpoint). Furthermore, this research was synergistically complemented with an energy analysis including indicators and input-output flow balances to provide a win-win solution for food waste utilization. The results have taken different directions depending on the methodological assumptions considered but, in general terms, the sugar beet molasses scenario can be claimed in all cases as the energetically sustainable process with the best environmental profile. With an energy surplus of 155%, the cradle-to-gate scenario recorded the best environmental impact in 4/8 midpoint categories and an overall reduction of 67% and 94% (excluding co-products) for the ReCiPe damage single score compared to the wine vinasses and wastewater treatment plant sludge and cheese whey scenarios, respectively. In this sense, the viability and competitiveness of these two scenarios is compromised by the lack of energy self-sufficiency (there is a 53% deficiency in the wine vinasses and wastewater treatment sludge scenario) and the lack of climate-neutral outcomes (a result of 5510 mPt/Nm³ H₂ shows that the cheese whey scenario is far from being a zero-emission process).