A mathematical model for predicting the flow field, suspended solids and dye transport in secondary circular clarifiers is presented. The model is based on a coarse grid, numerical-diffusion-free third-order-upwinding explicit technique (Skew Third-Order-Upwinding Scheme STOUS). The computational domain of the model includes the inlet zone, settling zone, withdrawal zone and sludge zone. Density stratification effect has been incorporated in the model. The model can be used to simulate the density waterfall phenomenon at the clarifier entrance. Turbulent stresses are calculated using the eddy viscosity concept and the k-e turbulence model. Double exponential relationship to describe the settling process is used. The model has been calibrated and verified using field data obtained through the application of the Clarifier Research Technical Committee (CRTC) protocol. Model predictions have been compared with the flow fields, suspended solids distributions, Flow Through Curves (FTC) and dye profiles in the clarifier for different hydraulic and solids loading. Good agreement was obtained.