Photo fermentative hydrogen production using dominant components (acetate, lactate, and butyrate) in dark fermentation effluents

Engineering strategies were applied to promote the phototrophic H ₂ production of an indigenous purple nonsulfur bacterium Rhodopseudomonas palustris WP3-5 using major components (i.e., acetate, butyrate, and lactate) of dark fermentation effluents as carbon sources. First, performance of cell growth and photo-H₂ production on each carbon source was examined individually. It appeared that acetate was the most effective carbon source for photo-H₂ production, giving an overall H₂ production rate and H₂ yield of 12.68 ml/h/l and 67.1%, respectively. Next, the effect of substrate concentration of each carbon source on photo-hydrogen production was investigated. Kinetic models were developed to describe the correlation between maximum specific growth rate/specific H ₂ production rate and the substrate concentration. The results show that using acetate and lactate as the carbon source, the kinetics for the cell growth and photo-hydrogen production can be described by Monod-type and Michaelis-Menten models, respectively, whereas substrate inhibition occurred when using butyrate as the carbon source. The continuous cultures were also conducted at a hydraulic retention time of 96 h using synthetic dark fermentation soluble metabolites (with a 5 and 10 fold dilution) as the influent. The phototrophic H₂ production efficiency was stably maintained for over 30 days with an overall H₂ yield 10.30 and 11.97 mol H₂/mol sucrose, when using 5-fold and 10-fold diluted dark fermentation effluent, respectively, as the substrate for dark fermentation. This demonstrates the feasibility of using the sequential dark and photo fermentation for high-yield biohydrogen production.