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 H-2 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, H-2 yield, H-2 rate and lag phase in dark fermentation were 36.2 g/L, 226.24 mL H-2/g TS, 29.85 mL H-2/h and 4.37 h, respectively. Photo-fermentation of 5.77 g TVFA/L embedded dark fermentation effluent did not lead to efficient H-2 production. The best performance in photo-fermentation was obtained with 1.55 g TVFA/L containing diluted dark fermentation effluent. The H-2 yield, H-2 rate and lag phase in photo-fermentation were 870.26 mL H-2/g TVFA, 0.913 mL H-2/h and 54.07 h, respectively. Embedding concentrated substrate using agar can enhance H-2 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. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.