Improving the efficiency of anaerobic digestion and optimising in-situ CO₂ bioconversion through the enhanced local electric field at the microbe-electrode interface

Integrating microbial electrolysis cells and anaerobic digestion (MEC-AD) improves upon conventional anaerobic digestion for biomethane production due to the in-situ provision of electrochemically produced reducing power (such as hydrogen). However, the electron transfer behaviour at the microbe-electrode interface remains unclear. This study assessed the micro-scale interface modification of a carbonaceous bioelectrode, leading to an enhanced local electric field, which was postulated to stimulate electro-methanogenesis. The effectiveness of incorporating electro-conductive biochar to enhance interspecies electron transfer was also evaluated for its potential to boost biomethane production. The findings revealed a notable increase in biomethane yield and methane content within the biogas of the MEC-AD system, with improvements of 96.8 % and 32.5 %, respectively, compared to conventional anaerobic digestion when co-digesting grass silage and cattle slurry. The enhancement was ascribed to the accumulation of charges and an intensified local electric field on the surface of the etched biocathode, thereby facilitating interfacial electron transfer. Incorporating 10 g/L of biochar to conventional anaerobic digestion resulted in a 7.9 % increase in biomethane yield compared to conventional anaerobic digestion. Overall, the heightened energy yield of biomethane by the MEC-AD system (featuring the modified graphite cathode) resulted in a 6.5-fold increase compared to the additional electrical energy input. This underscores the catalytic significance of the electricity in AD system.