Can bio-H₂ compete with bio-CH₄-unequal chances of anaerobic digestion metabolic pathways

Biogas, a mixture of biomethane (bio-CH₄) and carbon dioxide (CO₂), is the product of methanogenesis, the final stage of anaerobic digestion (AD). Biohydrogen (bio-H₂), on the other hand, is produced during the acidogenesis stage through dark fermentation (DF) processes and the transformation of intermediates from other fermentation types, such as the conversion of lactate and acetate into butyrate. Bio-H₂ and CO₂ together form fermentation gas, whose volume and bio-H₂ content depend on the dominant type of acid fermentation. Understanding the complexity of metabolic pathways and the interactions between microorganisms at each stage of AD permits process optimization in two-stage systems, facilitating the production of both bio-H₂ and bio-CH₄. Theoretical calculations indicate that separate recovery of bio-H₂ and bio-CH₄ through a two-stage AD process yields a higher energy output from the degraded substrate. However, DF is characterized by limited bio-H₂ yields due to inherent metabolic constraints and competition among microbial pathways. In contrast, acetogenic pathways consistently generate substrates for methanogenesis. Paradoxically, analyses based on simple substrates such as glucose facilitate a deeper understanding of metabolic pathways and chemical reactions in bacterial cells. Such knowledge is essential for the rational design and critical evaluation of integrated multiproduct AD systems, supporting mindful resource management and sustainable development. This review focuses on (i) the biological limitations of bio-H₂ generation during acidogenesis; (ii) physiological constraints that keep bio-H₂ technologies at the research and development stage, compared with the commercially established biogas technologies; and (iii) prospects for bio-H₂ recovery within integrated multiproduct AD systems.