The combination of water scarcity and increasing pollution of available water poses significant challenges to governance and society. The problem is exacerbated in rural areas due to limited access to water management services. In these regions, modern agricultural and intensive livestock production practices contribute significantly to nitrate pollution of water bodies. Therefore, there is a pressing need for the development of sustainable decentralized treatment solutions.

This doctoral thesis focuses on nitrate removal by means of water electro-bioremediation, which harnesses the ability of certain microorganisms to exchange electrons with a solid material. Specifically, nitrate is reduced to dinitrogen gas (innocuous) using a cathode as an electron donor. This innovative process eliminates the need to introduce chemicals as reducing agents into the water (a common practice in conventional treatments) and prevents the generation of any residue. Moreover, the energy required is minimal.

Alba Ceballos-Escalera’s has investigated the development of electro-bioremediation processes for nitrate removal at different scales. The experimental work started with laboratory experiments and concluded with the treatment of polluted groundwater at an on-site pilot plant in Navata (Alt Empordà, Catalonia). Main outputs are:

• Improvement of the nitrate reduction rate up to 5.0 kg NO3-m-3d-1 (HRTcat7 h) at laboratory-scale and up to 1.0 kg NO3- m-3d-1 (HRTcat 0.7 h) at pilot-scale level (treating nitrate-contaminated groundwater on-site). Optimal nitrate removal occurs at pH 6 and 35ºC with relatively low energy demand (4-6 kWh kg-1 NO3–). The OPEX cost is 0.40 € m-3. To achieve this milestone, the researcher studied the reactor design, the water matrix and the challenges posed by on-site operation.
• Demonstration of the possibility of addressing the coexistence of arsenite and nitrate water contamination by means of electro-bioremediation processes. Arsenite was efficiently oxidised to arsenate as a critical step for its removal, thereby reducing its toxicity.
• Integration of electrochemical disinfection into a single-step electro-bioremediation system, allowing for the achievement of an effluent that met drinking water standards not only in terms of nitrates and nitrites thresholds but also with regard to pathogens.
• Development of an electrochemical water softening membrane-less reactor as a suitable pre-treatment for nitrate electro-bioremediation. The presence of hardness in the groundwater matrix can significantly affect electro-bioremediation performance.

These results constitute a significant step towards unlocking the full potential of electro-bioremediation in addressing nitrate-groundwater contamination, and demonstrate its environmental sustainability and economic competitiveness. Moreover, the technology has proved to be versatile in treating different contaminants such as arsenite and pathogens. This paves the way for further research and development that could lead to full-scale implementation and commercialisation of the technology. The thesis, which was directed by Dr Sebastià Puig, Dr Maria Dolors Balaguer and Dr Narcís Pous from the Laboratory of Chemical and Environmental Engineering of the University of Girona (LEQUIA), will be defenced on March 20^th at Aula Magna of UdG Faculty of Sciences (10:00h). The event is open to the public.

Publications: Electro-bioremediation of nitrate and arsenite polluted groundwater, Ceballos-Escalera, A. et al, Water Research, 2021, 19015, 116748 // Nitrate electro-bioremediation and water disinfection for rural areas, Ceballos-Escalera, A. et al, Chemosphere, 2024, 352, 141370