Nitric and nitrous oxides from denitrifying activated sludge at low oxygen concentration

Abstract

To determine the net-production of the denitrification intermediates nitric oxide (NO) and nitrous oxide (N2O), two batch experiments with different nitrite concentrations have been performed (A: 3.5 g NO2−-Nm−3, B: 12 g NO2−-Nm−3). In these experiments the concentration of oxygen in the reactor has been varied from 4 g O2m−3 to 0 g O2m−3. As NO and N2O might be produced by nitrifiers as well, the sludge used was unable to nitrify due to a low sludge age. High nitrite as well as aerobic conditions favor the production of N2O but not the production of NO. Furthermore, NO was accumulated at very low concentration and the highest emission was under completely anoxic conditions. A mathematical model has been formulated describing the reduction of nitrate, nitrite, and nitrous oxide. Noncompetitive inhibition of these processes by oxygen and nitrite has been included in the kinetics. The estimated parameters indicate that nitrite serves mainly as a denitrification intermediate rather than as an inhibitor. The net-production of N2O at low oxygen concentrations can be explained by different oxygen inhibition of the three reduction steps. The fast reduction of N2O is most inhibited by oxygen. As a consequence, the reduction of N2O is a slower process under low oxygen pressure than the reduction of nitrate and nitrite.