Abstract: Aeration tank is a core device in treatment of activated sludge waste water, and understanding its flow structure is crucial to improvement of its oxygen transfer and sewage treatment. An Euler-Euler multiphase flow model combined with a standard k-ε turbulence model (FLUENT 6.3) has been used in this study to simulate a typical plug-flow aeration tank. We analyzed the velocity distributions for two cases of different aeration tube installation, located at the tank centerplane and near a tank wall, and compared their velocity uniformity coefficients and velocity differences between the gas and liquid phases at characteristic cross sections. Our findings are that in the near-wall case the flow is more uniform and the velocity difference of two phases smaller, which is a favorable condition for flow mixing in the tank. In this case, the distribution of gas-phase volume fraction over the central longitudinal section at different time is more uniform. Thus, aeration tube installed near a tank wall is a case where both the flow structure and gas content are beneficial to oxygen and mass transfer between the two phases, leading to a higher rate of oxygen utilization and lower energy consumption. The results are useful for optimization of waste water treatment designs.