Improving biomethane production from biochar-supplemented two-stage anaerobic digestion of on-farm feedstocks

On-farm feedstocks such as grass silage and cattle slurry present recalcitrant characteristics that can limit microbial conversion in biofuel production. Introducing a biochar supplement in two-stage anaerobic digestion may facilitate feedstock hydrolysis and improve energy yields in biohydrogen and biomethane production. The biomethane potentials were first investigated in batch trials without biochar supplement; results indicated a biomethane yield of 230 L per kilogram (kg) volatile solid (VS) in single-stage digestion and 275 L/kg VS in two-stage digestion. In continuous trials, operated at an organic loading rate of 4.0 g VS/L/d, the second-stage digester in two-stage digestion showed a methane yield of 237 L/kg VS with 10 g/L biochar addition; this was 7% higher than the second-stage digester without biochar addition. At the same loading rate of 4.0 g VS/L/d, the biomethane yield in continuous single-stage digestion with 10 g/L biochar addition was 212 L/kg VS; this was 3% higher than the single-stage digester without biochar addition. Biochar was found to enhance the hydrolysis of recalcitrant solid components in the hydrogen-producing phase, promote biomethane production in methanogenesis, and stabilize the digestion process. The highest energy yield of 8.5 MJ hydrogen and methane per kg VS was achieved in the two-stage digestion with 10 g/L biochar addition at a loading rate of 4.0 g VS/L/d. The results demonstrated that the application of a biochar supplement could effectively enhance gaseous biofuel production in two-stage anaerobic digestion.