BNP test to evaluate the influence of C/N ratio on N2O production in biological denitrification

With sufficient carbon sources in the influence to the anoxic stage, the total gas production rate in the BNP (Biolofical Nitrogen potential) system is rapid with a specific gas production of 3.83 mL/g-VSS-hr. The conversion of nitrate to nitrogen gas is accomplished through three steps: nitrate (NO₃^-) to nitrite (NO₂^-) to nitrous oxide (N₂O), and nitrogen gas (N₂). The BNP test results indicate that the optimal C/N ratios are 4.5±0.2, 3.1±0.4, and 2.0±0.2 for these three steps with carbon consumptions being 30%, 24%, and 46% of the total denitrification carbon consumption. With different concentrations of the influent nitrate, the optimal C/N ratios are 5.3∼5.6, 4.3∼4.7, 3.9∼4.0, and 2.5∼2.7 for 25, 50, 100, and 200 mg/L NO₃^--N, respectively. The conversion rate of N₂O under conditions of sufficient carbon source is 0.005%. If the carbon source becomes insufficient, the N₂O conversion rate may increase 100 times to 0.5%.