The accumulation of nitrite and the removal of TN during aerobic denitrification in the four systems were inspected, the results are shown in Fig. 2C for nitrite accumulation and Fig. 2D for TN removal. The TN removal of the free cell system was only 29.7%, with 168.98 mg/L NO2−-N accumulated in the water. In addition, the higher TN removals of 43.78%, 42.31% and 57.25%, along with NO2−-N accumulation of 137.4, 142.6 and 89.02 mg/L, were determined in the MP, PFC and SAB systems, respectively. Thus, the TN removal PR-619 closely bound with the accumulation of nitrite during aerobic denitrification, as proven in the previous study (Sun et al., 2015). In the present study, the same result occurred in the free cell system and immobilized cell systems. The more nitrite nitrogen was accumulated, the lower TN removal was achieved. Fortunately, the immobilization resulted in varying improvements for restricting nitrite accumulation. The limiting of the oxygen transfer through immobilization was effective (Uemoto et al., 2000). The bacteria inside the carriers could express the high activities of nitrite reductase, which could be inhibited under a vectors high dissolved oxygen condition (Körner and Zumft, 1989).