Flow characteristics have been studied with the tracer simulation for a N-clearwell using computational
fluid dynamics(CFD) in this research. The flow is separated in the path of clearwell, showing the complicated turbulence physics. Thus, the behavior of tracer particles can be analyzed in the distribution, and especially the sampling location at the outlet can affect on the staying time of particles. The result of this study can be applied to the prediction for the performance of sanitization as well as evaluation and improvement for the water purification plants with the technology of CFD.