Electrode-attached cell-driven biogas upgrading of anaerobic digestion effluent CO2 to CH4 using a microbial electrosynthesis cell

Abstract

Upgrading biogas from anaerobic digestion (AD) has been highlighted as an alternative renewable energy source to replace geopolitically limited natural gas. The CO2 content of AD effluent is more than 40%, making it necessary to separate or increase the CH4 content to 95%. This study examined microbial electrosynthesis (MES) to convert CO2 directly to CH4 by a cathode electrode-attached cell. The MES with a −1.0 V (versus Ag/AgCl) applied cathodic potential exhibited a maximum methane production rate of 10.55 L CH4/m2 cat/day and achieved a 96% final CH4 content. Applying real biogas from a field AD plant resulted in a comparable production rate of 8.8L CH4/ m2 cat/ day with 95% CH4. The scaled-up bench MES reactor (total volume of 6L) was evaluated, and the energy efficiencies of the laboratory- and bench-scale MES were compared. The next-generation sequencing (NGS) revealed most methanogens (e.g., Methanobacterium, Methanothrix, and Methanobrevibacter) to be associated with the cathode surface rather than suspension. Cyclic voltammetry and field emission scanning electron microscopy showed that the electrode-associated cell predominantly controls the performance of the MES system. These results suggest that electrode-attached cells play a major role in the biogas upgrading of CO2 to CH4 in the MES system.