Advances in the biomass valorization in dark fermentation systems: A sustainable approach for biohydrogen production.

[Display omitted] • Pretreatment is a critical process for biohydrogen production from biomass. • A distinction should be made between the initial pH and the pH of the process. • Dissolved bio-H 2 concentrations are not always in equilibrium with the gas phase. • The combined fermentation methods offer advantages compared to dark fermentation. • Iron catalysts are attractive due to high abundance and relatively low toxicity. Biohydrogen is a green energy carrier with the potential to reduce our dependency on fossil fuels. The production of biohydrogen from lignocellulosic materials may be low cost, but commercial production has not been realized. The issues to be addressed include screening and construction of microorganisms that meet industrial requirements, optimization of bioreactor designs, and novel hybrid fermentation systems such as combining dark fermentation (DF) with methanogenic systems and doping nanomaterials to increase biohydrogen yield. This paper focuses on the biohydrogen production, highlighting various fermentative bioreactor types, influential factors in dark fermentation, and pretreatment techniques. Besides, current strategies to produce biohydrogen, including synthetic biology approaches and metabolic engineering, are assessed for their potential to contribute to net zero carbon emission. Pretreatment of biomass using dilute acids is the most common technique to destroy the complicated structure of lignocellulosic biomass. Mesophilic temperatures (37 °C), HRT of 72 h, and pH near 7.0, are recurrent parameters of biomass fermentation. The combined approach of electro-fermentation and dark fermentation produces a higher yield of biohydrogen compared to the single dark fermentation. Furthermore, additives (mainly iron-based materials) have been investigated to improve the efficiency of biomass pretreatment as well as biohydrogen production. Finally, this paper points out that the vigorous development of novel engineered strains is an important part of achieving commercial-scale production of biohydrogen. Overall, considerable efforts are needed from both technical and managing aspects to achieve a full-scale application of biohydrogen production to make this technology economical, efficient and sustainable. [ABSTRACT FROM AUTHOR]