Under the Euler-Euler framework, an Euler-Euler two-fluid model coupled population balance model (PBM) was used for numerical simulation of gas-liquid two-phase flow in the activated sludge as non-Newtonian fluid in a lab-scale tower type aeration tank to study the dynamic behaviors, including gas-liquid two-phase flow velocity field, gas hold-up, and dynamic viscosity, etc., at different mixed liquid suspended solids (MLSSs). It was found that gas hold-up distribution was inverted-trapezoid and the time-averaged flow field and velocity distributions were symmetric at low velocity. With increasing MLSS, the high gas hold-up distribution zones gradually narrowed and the velocity peak of the liquid phase in y-direction increased in the middle and upper parts of the aeration tank. For low concentration sludge, the liquid velocities in x-direction periodically oscillated similarly to a Newtonian fluid. With increasing MLSS, the time-averaged gas hold-up declined in the aeration tank. For water as the liquid phase, total gas hold-up in the aeration tank was overestimated. At the sludge concentration of 10.2g/L, maximum dynamic viscosity peak occurred and the low dynamic viscosity zone distributed in the "V" shape in the center of the aeration tank.