Enhancing photosynthetic CO2 fixation in microbial electrolysis cell (MEC)-based anaerobic digestion for the in-situ biogas upgrading

Biogas from anaerobic digestion usually contains 30%-50% of CO2, which needs to be upgraded before industrial utilization. However, current upgrading techniques typically consume large amounts of chemicals and energy. This study designed a green and low-energy-consumption microbial electrolysis cell (MEC)-based anaerobic digester for in-situ biogas upgrading, in which the cathode with photosynthetic bacteria (PSB) was used to fix CO2. Results showed that charged MEC-AD increased photosynthetic CO2 fixation by 83.3% and CH4 production by 62.8% compared with uncharged open-circuit MEC-AD. Adding an extracellular electron uptake inhibitor to the cathode decreased the CO2 fixation, accompanied by the decline of current between the two electrodes, which suggested that the cathode electron uptake was the main reason for the facilitated CO2 fixation of PSB. The electrically-stimulated PSB fixed CO2 more efficiently than unstimulated PSB even under power off, likely related to their enhanced electroactivity and photoactivity that could acquire more extracellular electrons and light energy for CO2 fixation. The carbonyl-related hydrogen bonds in the LH1 complex of photosynthetic reaction centers increased under electrical stimulation, resulting in a red shift of absorbance spectra to improve the en-ergy utilization of PSB. This upgrading technique consumed about 0.37 kWh/Nm3 CO2removed of electric energy, much lower than other MEC-based techniques, providing a green and feasible strategy for in-situ biogas upgrading.