Biological upgrading of anaerobic digestion biogas into renewable natural gas : integrating CO₂ electrolysis with syngas biomethanation using a hollow fibre membrane bioreactor

Anaerobic digestion is a common organic waste treatment technology that produces biogas composed of about 40% carbon dioxide (CO₂) and 60% methane (CH₄). Upgrading this biogas to over 90% CH₄ to meet requirements for use as renewable natural gas (RNG) typically discards CO₂, rather than utilizing it as fuel. This thesis presents a novel bio-electrochemical system (BES) that integrates CO₂ electrolysis with a hollow fibre membrane (HFM) biomethanation reactor for biogas upgrading, a configuration not previously reported. The BES consisted of an upstream electrolyzer targeting a 1:3 CO:H₂ syngas ratio coupled with a downstream HFM supplying syngas to a biomethanation reactor where microbes converted the syngas to CH₄. The electrolyzer was optimized based on current density, cathode catalyst, single-pass versus gas recycle-loop configuration, inlet gas humidity, acid humidification, and anolyte concentration. Operation at a current density of 125 mA/cm2 using a silver catalyst and a gas recycle-loop configuration achieved complete CO₂ utilization at the target 1:3 syngas ratio. Cobalt phthalocyanine demonstrated faster reaction kinetics, confirming the viability of CO₂RR electrolysis for generating a syngas suitable for downstream biomethanation. Extended operation revealed a key limitation, cathode flooding, which mitigation strategies targeting inlet gas humidity, acid humidification, and reduced anolyte concentration were insufficient to control independently. Yet, the outcomes of these mitigation strategies provided evidence and insight for future co-optimization strategies. The HFM biomethanation reactor upgraded syngas to CH₄, achieving 67% and 79% CH₄ in 8- and 34-day experiments, respectively. Mass balance modelling of the 8-day run showed CO utilization efficiency improving nearly six-fold to 81%, with CO converted to CH4, acetate and biomass. Reactor performance was limited by low gas-liquid mass transfer due to biofilm accumulation on the HFM surface, and pH instability, underscoring the need for CH₄ optimization via HFM design and pH control. Together, these results demonstrate the technical feasibility of coupling electrolysis with HFM biomethanation for biogas upgrading, identifying cathode flooding, gas-liquid mass transfer and pH control as priority targets for future system optimization.