Biohydrogen production through dark fermentation: Recent trends and advances in transition to a circular bioeconomy

Fossil fuel reserves have drastically decreased over the past two decades, contributing to a worldwide decline in energy sources. The volatility of energy prices and their serious ecological consequences have drawn the attention of researchers to alternative carbon-neutral renewable energy sources. Furthermore, ineffective waste management contributes to environmental contamination, harms human health, and negatively affects the global economy. Biohydrogen (Bio-H2) derived from waste biomass via dark fermentation is a valuable green fuel to solve the issues of clean energy production and waste management. Bio-H2 has several advantages over other biofuels, including carbon-neutral production, ease of generation, low carbon dioxide (CO2) emissions, and high energy efficiency. Besides, by using biowaste to produce bio-H2, global warming and CO2 emissions associated with biowaste incineration can be reduced. Although dark fermentation produces the most efficient and clean bio-H2, the yield and rate of production of the process are very low. This review article summarizes the recent developments in dark fermentation processes to enhance bio-H2 yield and productivity, including pre-treatment, additives inclusion, and integrated solutions. Additionally, this article highlights the metabolic pathways of Bio-H2 production and inhibitory substances that negatively affect dark fermentation. Challenges related to bio-H2 technology transfer from lab to industry are discussed, and prospects for future research to realize the technology transfer are briefed. Further, an integrated carbon-neutral approach by dark fermentation-assisted bio-H2 production is also discussed as a contribution to circular bioeconomy.