Using carbon dioxide for bioproduction combines decreased greenhouse gas emissions with a decreased dependence on fossil carbon for production of multicarbon products. Microbial electrosynthesis (MES) enables this, using renewable energy to drive the reduction of CO 2 at the cathode of an electrochemical cell. To date, low product concentrations preclude cost-effective extraction during MES. Here we present an approach that couples production and recovery of acetate in a single, three-chamber reactor system. Acetate was produced at 61% Coulombic efficiency and fully recovered as an acidified stream containing up to 13.5 g L −1 (225 mM) acetic acid, the highest obtained thus far. In contrast to previous MES studies, a single separated acidic product was generated through in situ membrane electrolysis enabling further upgrading.

Coupling decentralized biomethane generation to centralized CO production via gas grid redistribution can be an economically viable CO2 utilization strategy.

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The carrying capacity of the planet is being exceeded, and there is an urgent need to bring forward revolutionary approaches, particularly in terms of energy supply, carbon emissions and nitrogen inputs into the biosphere. Hydrogen gas, generated by means of renewable energy through water electrolysis, can be a platform molecule to drive the future bioeconomy and electrification in the 21 st century. The potential to use hydrogen gas in microbial metabolic processes is highly versatile, and this opens a broad range of opportunities for novel biotechnological developments and applications. A first approach concerns the central role of hydrogen gas in the production of bio-based building block chemicals using the methane route, thus, bypassing the inherent low economic value of methane towards higher-value products. Second, hydrogen gas can serve as a key carbonneutral source to produce third-generation proteins, i.e. microbial protein for food applications, whilst simultaneously enabling carbon capture and nutrient recovery, directly at their point of emission. Combining both approaches to deal with the intermittent nature of renewable energy sources maximises the ability for efficient use of renewable resources.

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Anaerobic digesters produce biogas, a mixture of predominantly CH 4 and CO 2 , which is typically incinerated to recover electrical and/or thermal energy. In a context of circular economy, the CH 4 and CO 2 could be used as chemical feedstock in combination with ammonium from the digestate. Their combination into protein-rich bacterial, used as animal feed additive, could contribute to the ever growing global demand for nutritive protein sources and improve the overall nitrogen efficiency of the current agrofeed/food chain. In this concept, renewable CH 4 and H 2 can serve as carbon-neutral energy sources for the production of protein-rich cellular biomass, while assimilating and upgrading recovered ammonia from the digestate. This study evaluated the potential of producing sustainable high-quality protein additives in a decentralized way through coupling anaerobic digestion and microbial protein production using methanotrophic and hydrogenotrophic bacteria in an on-farm bioreactor. We show that a practical case digester handling liquid piggery manure, of which the energy content is supplemented for 30% with cosubstrates, provides sufficient biogas to allow the subsequent microbial protein as feed production for about 37% of the number of pigs from which the manure was derived. Overall, producing microbial protein on the farm from available methane and ammonia liberated by anaerobic digesters treating manure appears economically and technically feasible within the current range of market prices existing for high-quality protein. The case of producing biomethane for grid injection and upgrading the CO 2 with electrolytic hydrogen to microbial protein by means of hydrogen-oxidizing bacteria was also examined but found less attractive at the current production prices of renewable hydrogen. Our calculations show that this route is only of commercial interest if the protein value equals the value of highvalue protein additives like fishmeal and if the avoided costs for nutrient removal from the digestate are taken into consideration.

Turning raw biogas into biomethane as energy carrier requires the selective removal of CO2 in a biogas upgrading process or a total conversion of CO2 to CH4 which is generally energy intensive.During membrane electrolysis, electrical energy can be used to simultaneously remove CO2 (and H2S) and produce H2 as side product. Biogas is thus scrubbed with catholyte and the captured HCO3and HSmigrate towards the anode. Simultaneously, cathodic H2 mixes with residual biogas in a ratio that can be fine-tuned. We obtained in one step an ideal 4:1 H2:CO2 ratio in the reactor off gas. Subsequently the gas could be further upgraded via chemoautotrophic microbial conversion of CO2 to CH4. Biomethanation delivered biomethane with 98.9 ± 0.9 % purity. The electrochemically-assisted scrubbing and stripping of CO2 and H2S resulted in high CO2 removal efficiencies (up to 100%), without addition of chemicals. The system was flexible depending on temporarily available power. Electrochemical biogas upgrading (EBU) can be envisaged as a scalable and decentralized storage of excess or off-peak renewable power, making better use of the power input used to drive a biological CO2 conversion.