[Achieving the stability of nitrosation under low DO concentration].

With A/O phosphorus removal process of secondary effluent as raw water, the stable accumulation of nitrite was achieved under low DO. Total nitrogen loss, hydraulic retention time (HRT) and sludge recycle ratio(R) was separately studied on the effects of stable nitrosation. According the result, apparent nitrosation rate was affected by the concentration of COD influence, when COD < or = 50 mg/L, apparent nitrosation rate was reduced; when COD > 50 mg/L, it was increased. Extension and shortening of HRT has positive and negative effects on nitrosation efficiency, dynamic controlling was an effective method. The system stability would be undermined leading to the risk of nitrification by elevating sludge recycle ration. In addition, low DO concentration did not reduce nitrosation rate. Under the condition of HRT = 6 h, R = 0.5, T = 22-24 degrees C, the average ammonia nitrogen removal rate reached 83%, the ammonia nitrogen removal load reached 0.28 kg/(kg x d), the nitrite accumulation ratio was close to 100%.