Abstract: A lab-scale two-stage UASB-A/O combined process for treating the real municipal waste leachate is used to investigate the inhibition kinetics of free nitrite acid(FNA) to denitritation using nitrite as electron acceptors in wastewater.Under the premise of achieving short-cut nitrification-denitrification stably(running 130 days),a number of denitrification batch tests are carried out in different concentrations of NO₂^--N and under different pH gradient with sludge,which is from an A/O reactor and has good biological short-cut denitrification nitrogen removal efficiency.A denitritation inhibition kinetics model is established based on a large number of experiments and the Nitrous denitrification inhibition kinetic constants under different pH are deduced by function match.The relationship curves of NO₂^--N concentration and specific rate of denitrification match the Andrews inhibition kinetics model well at constant temperature and pH conditions.The maximum specific denitrification rate R_max and half-saturation constant K_S significantly vary at different pH,R_max and K_S being 17.8 mg/(g·h) and 6.40 mg/L under pH 8.0,respectively.When pH is in the range of 6.5-8.0,the inhibition constant KI significantly increases with the increase of pH.However,the corresponding value of ρ(FNA)(0.177-0.225 mg/L) remains the same level of magnitude,which proves that FNA is the true inhibitor from the view of kinetics point.