Ammonia oxidizing bacteria-enriched sludge was obtained in a sequencing batch reactor with synthetic wastewater. Batch tests of ammonia oxidation were carried out with the sludge, in the presence of only trace NO2 and in the presence of O2 and trace NO2, respectively. The Andrews model is used to describe NO2-dependent ammonia oxidation. The maximum ammonia oxidation rate of 139.11 mg N·(g sludge COD h)−1, occurred in the presence of 21% O2 and 100 ppm NO2, which was 3 times higher than the aerobic ammonia oxidation rate without NO2. The kinetic model of ammonia oxidation in the presence of O2 and trace NO2 is developed, and the function of NO2 apparently enhancing ammonia oxidation is suggested. The maximal nitrogen removel of 34.19% occurred at the 21% O2 and 100 ppm NO2 in the mixed gases. Nitrogen removal principally depended on the denitrification activity and NO2-dependent ammonia oxidation activity of ammonia-oxidizing bacteria (AOB).

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