Abstract: A major factor in deviation from optimal performance of the activated sludge secondary clarifier is the presence of density currents,resulting from the density difference and temperature difference. In this study,the density flow caused by the activated sludge and surface heat loss in radial clarifier is modeled with the Boussinesq approximation,the multiphase Eulerian model and the standard k-ε turbulent model on a two-dimensional grid. The water surface heat loss is estimated with the experiential equations.The calculations are reported to estimate the flow pattern,and the temperature and sludge distribution for activated sludge secondary clarifier and the results are compared with the experimental data.It is suggested that Fr_p²Gr/Re² can be used for analyzing the major cause of the density currents,buoyancy force resulting from the temperature difference,or the gravitational force from the density difference.