To control naidid pollution in water treatment conducted by O3-BAC, the removal effects of coagulation and sedimentation to naidid were estimated by field sampling of water plant, jar test and simulation study. The results showed that both coagulation and sedimentation of water plant and jar test had obvious removal efficiency on naidids. In the former the mean population density of naidid was decreased from 0. 52 ind/m3 to 0.17 ind/m3, while in the later removal efficiency, which did not be influenced by operation condition of coagulation and sedimentation, reached nearly 100%. Drift migration of naidid from sediment to over-lying water were observed in simulation study and the drift efficiency could be influenced by both temperature and water flow. The drift efficiency of 20 degrees C was 18.5%, much higher than that of 30 degrees C and 10 degrees C. While the velocities of water flow were 2, 4 and 8 mm/s, the number of drifting naidid were 11, 25 and 39 ind respectively. Because of the existence of drift migration, the settlement in sedimentation tank does not mean the real remove of naidid and the thoroughly separating of naidid from water treatment process can only be realized by sludge discharge of sedimentation tank. The naidid removal efficiency of coagulation and sedimentation can be increased by optimizing sludge discharge and restraining drift migration of naidid in sedimentation tank.