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.

Two mathematical models have been formulated to describe the turnover of denitrification intermediates. Batch experiments with different nitrite concentrations have been performed to calibrate the models. In these experiments the concentration of oxygen in the reactor has been varied from 4 g O2m−3 to 0 g O2m−3. Model I includes the reduction of nitrate, nitrite, and nitrous oxide. Non-competitive inhibition of these processes by oxygen and nitrite has been included in the kinetics. The estimated parameters indicate that the net production of N2O at low oxygen concentrations can be explained by different oxygen inhibition of the three reduction steps. Model II works with structured biomass to improve the description of the data from the experiments. In addition to Model I, the synthesis and decay of denitrification enzymes are described to allow correct predictions of nitrate, nitrite and N2O concentrations. Model II has been used to simulate the effect of cell saturation with enzymes in a wastewater treatment process. Low dissolved oxygen concentrations in the anoxic reactor reduce the denitrification efficiency equally by inhibiting enzyme activity and enzyme synthesis: at 0.5 gm−3 O2 enzyme decay causes a cell saturation of below 40%. The benefits of modelling with structured biomass are shown: The comprehension of experimental observations is improved, and plant design and operation can be optimized. However, the multitude of unknown parameters still may restrict the application of complex models.

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