Realizing the practical application of CO₂ electroreduction for urban wastewater denitrification

The artificial recycling of CO₂ into value-added feedstocks and chemicals provides a sustainable approach to mitigate its greenhouse effect and realize a carbon-neutral economy, for which the direct and efficient utilization of CO₂ reduction products without additional separation and purification remains challenging. Here an electrochemical–biological hybrid system has been developed to merge CO₂ electrolysis with municipal wastewater treatment. In this set-up, the formate produced electrocatalytically (formate-e) in neutral electrolyte (1.0 M KHCO₃) is directly applied as a carbon source and energy carrier for biological denitrification using activated sludge from municipal wastewater treatment plants, exhibiting an excellent nitrate nitrogen (NO₃⁻-N) removal rate of ~3.06 mg l⁻¹ h⁻¹. Moreover, after long-term continuous operation of the tailored denitrification bioreactor, the formate-e displayed high denitrification rate of 1.08 mgNO₃⁻-N per gram suspended solids per litre per hour, surpassing that of acetate, widely used as a commercial carbon source. Further environmental and techno-economic analyses suggest that integrating this electrochemical–biological hybrid system with an electrochemical recovery and separation system can significantly lower the cost of the electrolyte, thereby showing promise for the direct use of formate-e in industrial applications for wastewater treatment. © The Author(s), under exclusive licence to Springer Nature Limited 2025