Optimization of CO2 feeding strategy for synthetic biogas upgrading in a three-cell electromethanogenesis stack

Electromethanogenesis (EMG) is a bioelectrochemical carbon capture and utilization technology that can upgrade biogas into biomethane (CH₄) while enabling CO₂ recycling and renewable energy storage. So far, most EMG studies focused on electrode materials, applied cathode potential/cell voltage, or microbial community enrichment strategies, while the CO₂ feeding strategy received comparatively less attention [1]. In most systems, CO₂ or biogas are continuously supplied to the cell without controlling empty bed gas residence time (EBRT) or minimizing the fraction of unreacted CO₂ in the outlet gas. However, the CO₂ supply strategy can strongly influence its availability at the biocathode, the CH₄ production rate and its purity in the outlet gas mixture.

The objective of this study was therefore to optimize the CO₂ feeding strategy in a 3-cells EMG stack, to maximize the CO₂ conversion efficiency and improve CH₄ production. Three double-chamber cells with stainless-steel wool electrodes (85 cm² each) were operated at 35ºC and hydraulically connected in parallel to a common catholyte recirculation tank, functioning as a gas-liquid separator. The bioanodes were continuously fed with an acetate-based synthetic wastewater, while the biocathodes were supplied with CO₂ as carbon source. The cells were individually controlled using a potentiostat with the anode poised at -0.10 V vs Ag/AgCl. The CO₂ feeding was regulated through an automated control system, to adjust the EBRT of CO₂ and minimize the amount of unreacted CO₂ in the produced gas.

Operation with pure CO₂ allowed the identification of the optimal feeding conditions of the EMG stack, based on the semi-continuous CO₂ injection at a low flow rate (0.21 mL/min), which increased the specific CH₄ content in the produced gas while maintaining its production rate (Table 1). Starting under these baseline conditions, the system was subsequently supplied with synthetic biogas (40:60% CO₂:N₂) to simulate anaerobic digestion biogas effluent and evaluate the feasibility of EMG for its upgrading. The 3-cell stack allowed the conversion of the residual CO₂ present in the synthetic biogas, producing an additional 0.2 m³-CH₄/m³_cat/d and upgrading the biogas to a final CH₄ concentration of 88 ± 1% with a specific energy consumption of 13 ± 7 kWh/m³ of treated gas. These values fall within the range reported for bioelectrochemical biogas upgrading systems, where CH₄ contents typically range between 85 and 98%, depending on reactor configuration and operating conditions [2].  

To further investigate the biological response to the different gas feeding strategies, cathode biofilm samples were analyzed. Metagenomic sequencing together with qPCR quantification of methanogenesis-related genes revealed shifts in microbial community structure and functional gene abundance when switching from pure CO₂ to synthetic biogas conditions. The results suggest that the lower CO₂ partial pressure in synthetic biogas, compared to pure CO₂ operation, may affect the activity of the methanogenic biocathode under the tested conditions.

Overall, this work demonstrates that controlling CO₂ supply is a key operational parameter for improving EMG biogas upgrading and provides new insights into the interaction between gas feeding strategy, process performance, and microbial community adaptation in an EMG stack.

Table 1. Comparison of reactor performance using pure CO₂ and synthetic biogas feeds