Detection of nitrogen gas During the nitrogen removal process, the gradual reduction in TN concentration and very low content of nitrification products suggested the possible occurrence of aerobic denitrification. To verify this possibility, gas chromatographic tests were conducted to identify the production of N2 by strain Y1. Using 100 mg/L NH4 þ-N, NO2 -N or NO3 -N as the sole nitrogen source, N2 levels increased gradually to a relatively steady value 53.74%, 50.21% and 55.13% within 36 h. N2 did not increase in the controls.