Modeling kinetics of ammonium oxidation and nitrite oxidation under simultaneous inhibition by free ammonia and free nitrous acid

Inhibition of ammonium oxidation and nitrite oxidation by free ammonia (FA) and free nitrous acid (FNA) was studied using three different sludges. An uncompetitive inhibition model fit the experimental data well when the reactions were under FA inhibition, whereas a noncompetitive model fit well under FNA inhibition. The estimates of the inhibition constant (KI) of nitrite oxidation were 46 μM for FA and 1.7–6.8 μM for FNA, each of which was significantly smaller than that of ammonium oxidation, which were 290–1600 μM for FA and 12 μM for FNA. The much smaller values of KI for nitrite oxidation reflected the susceptibility of that reaction to inhibition by FA and FNA, which could lead to accumulation of nitrite during nitrification. A kinetic model for simultaneous inhibition by FA and FNA was derived. The model predicted that nitrite oxidation should be affected more seriously than ammonium oxidation by the simultaneous inhibition, which would accelerate the accumulation of nitrite in a strong nitrogenous wastewater treatment. It also indicated that a complete removal of ammonia could be achieved with high accumulation of nitrite in a sequencing batch reactor, which is impossible in a continuous-flow reactor.