Abstract: A heterotrophic nitrifying bacterium, Alcaligenes faecalis strain NR, was isolated from a membrane bioreactor. Citrate and ammonium were used as the sole carbon and nitrogen sources respectively to investigate nitrification characteristics of the new isolate. Under the aerobic condition of 30 °C and 120 r/min, the removal rates of NH+4N are 94.8%, 93.5% and 94.5% respectively when the initial concentrations are 20, 40 and 60 mg/L. And strain NR shows nitrification capability of producing nitrite and nitrate. To further understand the heterotrophic nitrification, enzyme assay was conducted. Sonication was used to obtain the key enzymes of AMO and HAO during nitrification process. Based on the singlefactor and orthogonal experimental results, the optimal ultrasonic conditions are: cell OD600 1.876, total working time 600 s, work/interval time 4, 6 s and power 300 W. The AMO and HAO enzyme activities under aerobic conditions are 0.011 U/mg protein and 0.016 U/mg protein respectively.

Key words: heterotrophic nitrification, ammonia monooxygenase, hydroxylamine oxidoreductase, ultrasonic disruption, ammonium sulfate precipitation