Biogas upgrading using Clostridium autoethanogenum for value-added products

Biogas production from anaerobic digestion is a well-established bioprocess for energy generation, nutrient recovery, and valorisation of waste resources. Typical biogas contains 60 % methane and 40 % carbon dioxide (CH₄/CO₂ of 1.5). Removing or altering carbon dioxide content facilitates biogas use as biomethane due to its impact on the calorific value. Technologies that effectively remove carbon dioxide to convert biogas to biomethane could play a central role in transforming waste resources. This is important as the world transitions into a low carbon and circular economy. Gas fermentation utilising acetogens and renewable H₂ offers an under-explored biological alternative for biogas upgrading. To demonstrate the use of gas fermentation for biogas upgrading, we used Clostridium autoethanogenum to ferment CO₂ from synthetic biogas supplemented with H₂ to produce ethanol and acetate (2.8 ± 0.1 and 3.9 ± 0.1 g/L, respectively). The bioprocess was able to enrich the biogas (CH₄/CO₂ of 2.20 ± 0.05) and transform the CO₂ into valuable fuels and chemicals in a one-step process. Strikingly, ∼50 % of the off-gas was inert, meaning there is potential for optimising acetogenic fermentation to achieve complete valorisation of CO₂ from waste streams. Increasing the gas to liquid mass transfer of H₂ improved the culture's growth rate and stability, but biomass and ethanol titres remained unaffected during chemostat operation. This represents a step towards upgrading biogas and other low-grade CO₂ waste streams by utilising renewable hydrogen and gas fermentation as a sustainable platform technology.