Selective reduction of nitrate to nitrogen gas in a biofilm-electrode reactor

Microbial denitrification and the occurrence of neutralization in a denitrifying biofilm-electrode reactor (BER) using an amorphous carbon anode has been experimentally demonstrated. In this study, the BER was operated over one year and measurements of influent and effluent ionic species were made at different electric currents to evaluate the predominant electrochemical and biological reactions. The ionic species measured were NO3−, NO2−, SO42−, Cl−, PO43−, NH4+, Na+, K+, Ca2+ and Mg2+, most of which are common constituents of surface water or groundwater. Concentrations of Na+, K+, SO42− and Cl− were almost the same in the influent and effluent. Removal efficiency of nitrate (NO3−) varied in the range of about 0 to 100%, depending on the electric current. Complete denitrification to N2 gas was readily achieved without accumulation of NO2−, N2O and NH4+. The concentration of Ca2+ and Mg2+ decreased due to deposition onto the surface of the electrode, but the calculation result from the solubility equilibrium of CaCO3, MgCO3 and CaMg(CO3)2 using the saturation index (SI) showed that the deposition could be hampered by the electrochemical neutralization in the reactor. Furthermore, the deposited calcium and magnesium could be redissolved immediately by changing the polarity of electrodes. From these results, it is concluded that a highly selective reduction of nitrate is operationally possible in the present BER, hence this process is a feasible alternative for the treatment of various nitrate-contaminated water.