SELECTIVE REDUCTION OF NITRATE TO NITROGEN GAS IN A BIOFILM-ELECTRODEREACTOR

Microbial denitrification and the occurrence of neutralization in a de nitrifying biofilm-electrode reactor (BER) using an amorphous carbon a node 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 pred ominant electrochemical and biological reactions. The ionic species me asured were NO3-, NO2-, SO42--, Cl-, PO43-, NH4+, Na+, K+, Ca2+ and Mg 2+, most of which are common constituents of surface water or groundwa ter, Concentrations of Na+, K+, SO42- and Cl- were almost the same in the influent and effluent. Removal efficiency of nitrate (NOS) varied in the range of about 0 to 100%, depending on the electric current. Co mplete denitrification to Nz gas was readily achieved without accumula tion of NO2-, N2O and NH4+. The concentration of Ca2+ and Mg2+ decreas ed due to deposition onto the surface of the electrode, but the calcul ation result from the solubility equilibrium of CaCO3, MgCO3 and CaMg( CO3)(2) using the saturation index (SI) showed that the deposition cou ld be hampered by the electrochemical neutralization in the reactor. F urthermore, the deposited calcium and magnesium could be redissolved i mmediately by changing the polarity of electrodes. From these results, it is concluded that a highly selective reduction of nitrate is opera tionally possible in the present BER, hence this process is a feasible alternative for the treatment of various nitrate-contaminated water. (C) 1998 Elsevier Science Ltd. All rights reserved.