Improvement of nitrate and nitrite reduction rates prediction

Reported models of denitrification rates integrate in an unique parameter the pH-dependent inhibition by HNO₂ and the pH effect on the bacterial metabolic activity; furthermore, they do not quantify separately the pH effect on the nitrate and on the nitrite reduction rates. The goal of this work was to quantify both effects on the kinetics of nitrate and nitrite reduction to improve the models' predictive value. Assays were performed at a pH range of 6.5-9.0 in batch reactors at 37°C with an activated sludge. At the studied pH range and at below the HNO₂ inhibitory concentration (0.004 mg L^-1), the maximum nitrate reduction rate diminished 23% and 50% by decreasing or increasing, respectively, one pH unit from 8.0. The maximum nitrite reduction at pH 8.0 diminished 15% at pH 7.0 and 40% at pH 9.0. At HNO₂ concentrations over the inhibitory concentration, except at pH > 8.0, the maximum nitrate reduction rate diminished 50% upon decreasing the pH from 8.0 to 7.0 or increasing it from 8.0 to 9.0. Inclusion of the pH effect in the reported models improved their predictive value; average deviations from the experimental data were reduced from 53% to 10.7% or 33.8% to 10.5% for nitrite and nitrate reduction rates, respectively.