Integrated Biorefinery Routes for Hydrogen and Methane Co-production

Biohydrogen production and integration possibilities are imperative for energy security and environmental sustainability. Acidogenic fermentation is receiving increasing attention and is considered one of the foremost pathways to enhance energy productivity. However, drawbacks such as low hydrogen yield and low organic matter consumption, together with the conversion of nearly 60% of the organic matter into secondary products, attenuate its benefits. These products are appropriate substrates for secondary bioprocesses due to their high biodegradability. This short review provides insights into the lessons learned from using downstream dark fermentation (DF) rich in volatile fatty acids for generating additional energy. Moreover, the valorization of the DF effluent by integrating DF reactor with other reactors would be highlighted. Results regarding the energy recovery and the effluent characteristics in terms of organic matter consumption through various possible combined processes have been also investigated. Overall, the integrated biorefinery concept is a promising way to reduce the entire cost of the process while gaining extra environmental profits. To boost the DF technology, additional research should focus on implementing it on pilot or industrial scale in order to evaluate the process efficiency, operating cost, and reliability.