[Inhibitory effect of free nitrous acid and cross inhibition in denitrification]

Nitrite and nitrate have been generally recognized to have an inhibitory effect on metabolism of denitrifiers. The nitrite inhibition on nitrate reduction under various pH conditions and the cross effect of the both electron acceptors on each other were investigated through a series of batch tests by using a biological nutrient removal (BNR) sludge. The results showed that the nitrate reduction activity had a much stronger relationship with the free nitrous acid (FNA) than that of nitrite concentration, suggesting that FNA, rather than nitrite is likely the actual inhibitor on nitrate reduction. Sixty percent inhibition of nitrate reduction was observed at an FNA concentration of 0.01-0.025 mg x L(-1), while total inhibition occurred when the FNA concentration was greater than 0.2 mg x L(-1). Furthermore, nitrite reduction by the BNR sludge was also found to be inhibited by HNO2. The reduction rate decreased by approximately 80% when the FNA concentration was increased from 0.01 mg x L(-1) to 0.2 mg x L(-1). The inhibitory effect of nitrate on nitrite reduction was found to be insignificant, with the most recovery rates under different nitrate concentrations larger than 90%. However, the nitrate reduction rate was observed to recover only 3.04%-72.54%. The recovery rate from inhibition was independent of the duration of the inhibition and the feeding mode of inhibitor, but strongly dependent on the concentration of inhibitor the biomass was exposed to during the inhibition period.