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%.

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