Biogas Production and Evaluation of the Potential of Sargassum Digestate as an Agricultural Substrate

The purpose of this study was to evaluate the production of biogas and the digestate obtained by means of the anaerobic digestion of sargassum, and its anaerobic co-digestion with municipal solid waste, while considering the effect of particle size and the carbon–nitrogen ratio (C:N) on methane generation. Physicochemical analyses of both Sargassum and the digestate were performed, including ultimate analysis and heavy metal content. The highest methane yield (92.62 mL CH4/gVS) was achieved with a 2 mm particle size and a C:N ratio of 15. Digestate characterization revealed the presence of arsenic and zinc, indicating the need for additional treatment before agricultural use. The agronomic potential of Sargassum digestate was assessed by comparing it with livestock waste, humus, and garden soil in tomato seedling growth trials. The Sargassum-based digestate significantly enhanced seedling biomass and development, supporting its potential as a sustainable soil amendment. Overall, the findings confirm the viability of Sargassum as a feedstock for biogas and biofertilizer production, emphasizing the importance of contaminant monitoring to ensure environmental safety. This study supports the integration of Sargassum into circular economy strategies and regenerative agricultural systems.