To understand nitrite build-up in the nitritation process, the inhibition kinetics of free ammonia (FA) on nitrite oxidation were investigated. FA inhibition on nitrite oxidation was the key factor of nitrite build-up and its type was mixed inhibition. Therefore, the mixed inhibition model should be adapted to a Michaelis-Menten equation to set up a process rate equation for nitrite oxidation in the nitritation process.