Biohythane production from marine macroalgae Sargassum sp. coupling dark fermentation and anaerobic digestion

•Biohythane production from Sargassum sp. is feasible and has a high potential.•Hydrogen yield of 91.3±3.3L H2 per kg (VS) of Sargassum sp. was attained.•It was achieved a methane yield of 541±10L CH4 per kg (VS) of Sargassum sp.•Potential energy production from Sargassum sp. was estimated in 242GJha−1yr−1.•The value of estimated energy could result in 600EJyr−1. Potential biohythane production from Sargassum sp. was evaluated in a two stage process. In the first stage, hydrogen dark fermentation was performed by Caldicellulosiruptor saccharolyticus. Sargassum sp. concentrations (VS) of 2.5, 4.9 and 7.4gL−1 and initial inoculum concentrations (CDW) of 0.04 and 0.09gL−1 of C. saccharolyticus were used in substrate/inoculum ratios ranging from 28 to 123. The end products from hydrogen production process were subsequently used for biogas production. The highest hydrogen and methane production yields, 91.3±3.3Lkg−1 and 541±10Lkg−1, respectively, were achieved with 2.5gL−1 of Sargassum sp. (VS) and 0.09gL−1of inoculum (CDW). The biogas produced contained 14–20% of hydrogen. Potential energy production from Sargassum sp. in two stage process was estimated in 242GJha−1yr−1. A maximum energy supply of 600EJyr−1 could be obtained from the ocean potential area for macroalgae production.