Waste to Hydrogen: Transforming Food Waste Into Biohythane (Bio‐H ₂  + Bio‐CH ₄ ) in a Two‐Stage Reactor With the Aid of a Metal‐Ion Catalyst

ABSTRACT This study aimed to enhance biohythane (biohydrogen [bio‐H 2 ] + biomethane [bio‐CH 4 ]) production from food waste (FW) via dark fermentation (DF) using a two‐stage anaerobic bioreactor system with specially developed seed cultures and metal‐ion catalysts. The primary objective was to maximize sequential bio‐H 2 and bio‐CH 4 production through phase‐separated DF and methanogenesis. Seed culture was prepared by selectively enriching Clostridium thermocellum from digestate slurry, using 2‐bromoethanesulfonic acid. The bioreactors were operated for 45 days in batch mode and 50 days in continuous mode. Following the initial loading of 10 L substrate into the Stage‐1 bioreactor, a continuous substrate loading rate of 0.08 L/day was maintained from day 36 onward under continuous mode. The digested effluent was subsequently transferred to the Stage‐2 bioreactor at a loading rate of 0.16 L/day for bio‐CH 4 production. The parameters impacting bio‐H 2 yield have been analyzed, and the optimal pH and temperature were found to be 5.5°C and 40°C, respectively. The highest bio‐H 2 production was 142 ± 5.48 L/day under batch mode and 212 ± 3.37 L/day under continuous mode, both observed at a NiCl 2 + FeCl 2 catalyst concentration of 75 mg/L. Gas chromatography analysis revealed a hydrogen (H 2 ) fraction of 75.81% at the optimal catalytic concentration under continuous mode. H₂ concentration and the total gas yield increased by about 30.8% and 77%, respectively, compared with the catalyst‐free FW substrate. Microbial community structure and biochemical transformations in the FW substrate during DF were investigated using field‐emission scanning electron microscopy and Fourier transform infrared spectroscopy. The integrated system at the optimal conditions demonstrated highly efficient performance in continuous mode. The first‐stage DF reactor achieved 40.13% chemical oxygen demand (COD) removal efficiency with a specific bio‐H 2 yield of 6005 L/kg COD. The second‐stage methanogenic reactor attained a COD removal efficiency of 89.70%, corresponding to a specific bio‐CH 4 yield of 2004 L/kg COD. Together, the dual‐stage system reached an overall COD removal efficiency of 93.83% and delivered a total bioenergy recovery of about 75.21 MJ/kg COD. These results demonstrate effective phase separation, high substrate conversion efficiency, and superior reactor performance under continuous mode. The associated enhancement in renewable energy recovery and waste valorization firmly establishes the feasibility of large‐scale biohythane production.