A computational fluid dynamics model that predicts the sedimentation of activated sludge in a circular secondary clarifier with activated sludge is developed. The axisymmetric single-phase flow is simulated by using a CFD code that has been written with Intel Visual Fortran. First, sludge withdrawing by suction-lift in the near-bottom region of the clarifier is simulated using suction at the bottom of clarifier. The flow and settling processes are simulated using k-ε turbulence model on a two-dimensional and orthogonal grid. A convection.dispersion equation that is extended to incorporate the sedimentation of activated sludge in the field of gravity is used. The computational domain includes the sludge blanket where the viscosity is affected by the rheological behavior of the sludge. Experimental data provided by Weiss et al. show that the relationship between shear stress and shear rate follows the Casson law for the shear rates lower than 50 s^(-1). Plastic viscosity of activated sludge is not removed from the concentration diffusion, so using regular non-Newtonian models leads to overestimation of blanket height. Modified Casson model is introduced to overcome the blanket height overestimation problem. Results show that the local sludge distribution in the clarifier has excellent agreement with concentration profile measurements of Weiss et al., for different treatment plant loadings. Alternative sludge withdrawing methods include withdrawing from pipes position at the bottom of clarifier and withdrawing by using sink terms in governing equations are used. Results show that the first withdrawing method gives less error comparing to these withdrawing methods.