NITRIC AND NITROUS OXIDES FROM DENITRIFYING ACTIVATED-SLUDGE AT LOW-OXYGEN CONCENTRATION

To determine the net-production of the denitrification intermediates n itric oxide (NO) and nitrous oxide (N2O), two batch experiments with d ifferent nitrite concentrations have been performed (A: 3.5 g NO2--Nm( -3) B: 12 g NO2--Nm(-3)). In these experiments the concentration of ox ygen in the reactor has been varied from 4 g O(2)m(-3) to 0 g O(2)m(-3 ) As NO and N2O might be produced by nitrifiers as well, the sludge us ed 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 product ion of NO. Furthermore, NO was accumulated at very low concentration a nd the highest emission was under completely anoxic conditions. A math ematical model has been formulated describing the reduction of nitrate , nitrite, and nitrous oxide. Noncompetitive inhibition of these proce sses by oxygen and nitrite has been included in the kinetics. The esti mated parameters indicate that nitrite serves mainly as a denitrificat ion intermediate rather than as an inhibitor. The net-production of N2 O at low oxygen concentrations can be explained by different oxygen in hibition of the three reduction steps. The fast reduction of N2O is mo st inhibited by oxygen. As a consequence, the reduction of N2O is a sl ower process under low oxygen pressure than the reduction of nitrate a nd nitrite.