Several regions around the world present high levels of nitrate in groundwater. Due to its toxicity, nitrate must be removed before the groundwater is used as drinking-water. This study assessed how a denitrifying bioelectrochemical system could be operated to treat nitrate-polluted groundwater. It evaluated the cathode potential (from +597 to -703mV vs SHE) and the anode electron donor (acetate and water). Similar trends were found regardless of the anode electron donor. The nitrate removal rate increased from 1.05 to 5.44mgN-NO3 -LNCC -1h-1 when the cathode potential was lowered from +597 to -403mV vs SHE, where it stabilized. The nitrate reduction end-products (nitrite, nitrous oxide and dinitrogen gas) also changed with the different potentials of the cathode electrode. The World Health Organization nitrates and nitrites standards for drinking-water were reached at cathode potentials between -103 and -203mV vs SHE. The highest rate of nitrate conversion to N2 (2.59mgN-NO3 -LNCC -1h-1, 93.9%) occurred at -123mV using water as anode electron donor, with an estimated operational cost similar to conventional technologies (0.68·10-2kWhgN-NO3-removed-1). The long-term stability of proposed operation was demonstrated during 96days, and the rate of nitrate conversion to N2 even increased to 4.09mgN-NO3 -LNCC -1h-1. A carbon-free operation for a bioelectrochemical system has been developed to treat nitrate-polluted groundwater at a competitive costThis research was financially supported by the Spanish Government (CTQ2011-23632, CONSOLIDERCSD2007-00055) and the Catalan Government (2014-SGR-1168). Narcis Pous was supported by a project grant from the Catalan Government (2012 FI-B 00941

The use of biocathodes in bioelectrochemical systems (BES) for the removal of nitrate in wastewater has become a vital field of research. However, the elucidation of the underlying extracellular electron transfer (EET) fundamentals of denitrifying biocathodes is still lacking, but required for a deeper BES understanding and engineering. This study reports for the first time on the thermodynamics of microbial cathodes for nitrate and nitrite reductions using microbial microcosms isolated from a running denitrifying BES. Cyclic voltammetry showed that nitrate and nitrite reduction proceed at - 0.30 V, and - 0.70 V vs. Ag/AgCl, respectively, by surface associated EET sites. The biocathodes were predominantly covered by Thiobacillus sp. contributing with a nitrate reductase (narG) to the major function of the microcosms. In conclusion, the EET characteristics of denitrifying biocathodes are demonstrated for the first timeThe authors acknowledge the scientific assistance of Prof. M.D. Balaguer. F.H. acknowledges support by the BMBF (Research Award "Next generation biotechnological processes-Biotechnology 2020+") and the Helmholtz Association (Young Investigators Group). This work was supported by the Helmholtz Association within the Research Programme Terrestrial Environment and by the Spanish Government (CTQ2011-23632). N.P. was supported by the Catalan Government (2012FI-B00941) and the University of Girona (MOB2014-ref22