Sequential dark and photo-fermentative hydrogen gas production from agar embedded molasses

In this study, molasses and dark fermentation effluent were solidified using agar and used for H2 production by dark and photo-fermentation. During dark fermentation, the solid jelly form of molasses enabled a slow release of the substrate to the liquid broth hindering fast pH decreases. The initial total sugar concentration, H2 yield, H2 rate and lag phase in dark fermentation were 36.2 g/L, 226.24 mL H2/g TS, 29.85 mL H2/h and 4.37 h, respectively. Photo-fermentation of 5.77 g TVFA/L embedded dark fermentation effluent did not lead to efficient H2 production. The best performance in photo-fermentation was obtained with 1.55 g TVFA/L containing diluted dark fermentation effluent. The H2 yield, H2 rate and lag phase in photo-fermentation were 870.26 mL H2/g TVFA, 0.913 mL H2/h and 54.07 h, respectively. Embedding concentrated substrate using agar can enhance H2 production performance but only if the release of the substrate does not exceed inhibitory levels and if the rate of diffusion is tolerable for microbial activity. •Molasses and dark fermentation effluent were solidified using agar for H2 production.•Slow-release of sugar in DF enabled efficient H2 production.•TVFA release in photo-fermentation from the solid matrix was not convenient.•Embedding of molasses enabled to start DF with 36.2 g total sugar/L.•Yields of DF and PF were 226.24 mL H2/g TS and 870.26 mL H2/g TVFA, respectively.