Kinetic Modeling of inhibition of ammonia oxidation by nitrite under low dissolved oxygen conditions

In recent years the application of partial nitrification techniques has been denoted as very promising. These methods are based on the oxidation of ammonia to nitrite and the inhibition of the nitratation using different strategies. In most cases, this inhibition causes an increase in the concentration of nitrite. However, the effect of high nitrite concentrations under low dissolved oxygen (DO) conditions on the nitrification process is not well understood. In this paper, the effect of ammonia, nitrite, and nitrate concentrations on the nitrification process under low dissolved oxygen concentrations were studied using respirometric techniques. Results showed that the specific oxygen uptake rate (SOUR) followed a Monod-type equation with respect to the DO concentration. The coefficient SOUR was constant with respect to the ammonia, nitrite, and nitrate within the tested concentrations; in addition, K-O was constant with respect to ammonia and nitrate but it increased linearly with the nitrite concentration, suggesting that nitrite was a competitive inhibitor of the SOUR. The inhibitory effect of nitrite was reverted by washing, in accordance with a competition model. From the data obtained using the open respirometer, the ratio between the oxygen consumption (OC) corresponding to each pulse of ammonia at different nitrite concentrations and the OC in the absence of nitrite (OCO) was calculated. The experimental ratio OC/OCO was almost constant with respect to the nitrite concentration and it was close to the literature value. Finally, simulation results agree with the experimental data confirming that the proposed competition model represented adequately the inhibitory effect of nitrite on the respiration rate of ammonia-oxidizing bacteria.