Effect of Free Nitrous Acid as Inhibitor on Denitrification Process

Denitrification process is an important step of the global nitrogen cycle. As an intermediate of this process, nitrite always can be found accumulated when the nitrite reduction rate falling behind the rate of nitrate reduction. This study investigated the influence of nitrite/free nitrous acid (FNA) on denitrification process. The nitrate reduction rate, nitrite accumulation rate, nitrite reduction rate and chemical oxygen demand (COD) removal rate were measured under different FNA concentrations, and different levels of inhibition were observed in the nitrate reduction, nitrite reduction, nitrite accumulation and COD removal process. The nitrite accumulation rate decreased by 82% when initial FNA concentration was increased from 0.022 mg/L to 0.045 mg/L, and no nitrite accumulation was observed when the initial FNA concentration was greater than 0.072 mg/L. The nitrate reduction process in this experiment was almost completely inhibited and COD removal process was totally halted when the initial FNA concentration was increased to 0.18 mg/L; in the meantime, the phenomenon of bacteriolysis might occur in this test, which led to a slight upward tendency of COD concentration. The impact of FNA on nitrate reduction process was greater than that on nitrite reduction process, though nitrite reduction was also inhibited by FNA, it could not be completely halted. The amended Monod model was chosen to describe the inhibitory effect and the inhibition of FNA concentration on nitrate reduction, nitrite reduction and COD removal process could all be well described by this model.