Evolution of Biohydrogen Reactor Technologies: From Conventional Bioreactors to Advanced and Integrated Systems

Biological hydrogen (biohydrogen) production has been investigated as a renewable route for converting sunlight, water, and organic residues into an energy carrier. Since the 1980s, research has progressed from laboratory studies of photobiological hydrogen evolution and anaerobic fermentation toward continuous, high-rate systems designed to retain active biomass, tolerate variable feedstocks, and integrate hydrogen recovery with wastewater treatment. Reactor design is central to this progress because it controls mass transfer, substrate and product concentrations, microbial retention, light delivery, heat transfer, and gas removal. This review synthesizes the development of biohydrogen reactor technologies from 1980 to October 2026, with emphasis on batch and continuously stirred-tank reactors (CSTRs), packed/fixed-bed reactors, up flow anaerobic sludge blanket (UASB) reactors, anaerobic fluidized-bed reactors (AFBRs), membrane bioreactors, immobilized-cell systems, and photobioreactors used for direct or indirect bio photolysis and photo fermentation. It compares their operating principles, strengths, limitations, and appropriate application domains. The literature indicates that no single configuration is universally optimal: CSTRs remain useful for flexible laboratory studies and mixed feedstocks, while attached-growth and biomass-retention systems can support higher loading rates by reducing washout. Photobioreactors must balance optical path length and illuminated area against mixing, oxygen inhibition, and pumping demand. Hybrid dark–photo fermentation and waste-to-hydrogen systems offer opportunities to improve substrate utilization, but process stability, hydrogen partial pressure, downstream gas purification, and techno-economic performance remain barriers to commercial deployment. Future research should prioritize standardized reporting, long-duration operation, microbial ecology, energy-positive gas separation, life-cycle assessment, and pilot-scale validation.