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 cost

This 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)