Biohydrogen production using sequential two-stage dark and photo fermentation processes

A two-stage process combining dark/photo fermentation was used to increase the overall hydrogen yield from sucrose and also to reduce the chemical oxygen demand (COD) in the effluent. Dark-H₂ fermentation was conducted using Clostridium pasteurianum CH₄, giving a maximum H₂ production yield of 3.80 mol H₂/mol sucrose. The soluble metabolites resulting from dark fermentation, consisting of butyric and acetic acid, were further used for H₂ production in the subsequent photo fermentation. Using soluble products from dark fermentation as substrate, Rhodopseudomonas palustris WP3-5 could produce H₂ phototrophically, elevating the total hydrogen yield from 3.80 (dark fermentation) to 10.02 mol H₂/mol sucrose (dark/photo fermentation). Meanwhile, a 72.0% COD removal was also achieved. When the photobioreactor was illuminated with side-light optical fibers and was supplemented with 2.0% (w/v) of clay carriers, the overall H₂ yield of the two-stage process was further enhanced to 14.2 mol H₂/mol sucrose with a nearly 90% COD removal. Continuous photo fermentation was also carried out at 96 h HRT using effluent from dark fermentation as the feed. The continuous culture maintained stable for nearly 10 days with an average H₂ yield of 10.21 mol H₂/mol sucrose. This demonstrates the feasibility of using the two-stage process combining dark and photo fermentation for simultaneous hydrogen production and COD removal.