Hydrogen and co-polymer production via mixed culture photo fermentation of dark fermentation effluents: Effect of the feeding mode

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Recent advancements in waste-to-energy technologies have emphasized the potential of integrating dark and photo fermentation processes to enhance hydrogen yield while valorizing residual organic matter. In this context, the simultaneous production of hydrogen and polyhydroxyalkanoates (PHAs) using non-axenic mixed cultures represents a promising and cost-effective strategy for improving both the environmental and economic sustainability of biorefinery systems. The present study examines the efficiency of non-axenic mixed culture photo fermentation under continuous, semi-continuous, and batch modes, using dark fermentation effluents as substrate. Additionally, it assesses, for the first time in the literature for this kind of process, the composition of the biopolymers accumulated by cells during hydrogen production. Results show that the semi-continuous operation mode yields better hydrogen production, achieving a productivity of about 170 mL/g COD , accounting for more than 85 % of the total biogas produced. Additionally, more than 75 % of the substrate chemical oxygen demand can be consumed under this mode. The composition analysis of the PHAs accumulated by the microbial consortium also revealed that mixed culture photo fermentation can produce the copolymer polyhydroxybutyrate-co-hydroxyvalerate, with a productivity of 0.105 g/L (0.49 g PHBV /g TSS ) under the semi-continuous feeding mode.