Abstarct: A three-dimensional numerical model has been developed to simulate flow and tracer transport processes in disinfection contact tanks. The model has two options, i.e. for laminar and turbulent flow, which for turbulent flow is baxsed on the time dependent Reynolds-Averaged Navier-Stokes equations, with a hydrodynamic pressure distribution being incorporated in the model. The used numerical method for solving equations is similar to that of MAC structures. For modeling of the water surface, the rigid lid approximation was considered. Two methods were used for simulation of turbulence. The first method involves prandtl’s mixing length with zero equation and second method was the widely used k െ ε model. The model was applied to laboratory model of Embsay contact tank, which is operated by Yorkshire Water. The results showed good agreement between numerical and experimental data. For model efficiency the computer code were developed for conservative tracer transport in which advection and diffusion terms were considered. Different configuration of baffling and also the inlet arrangement were analyzed and their efficiency indexes were computed. Results showed that the tank efficiency was highly dependent upon the baffle structure. Finally the new configuration was proposed which outperforms other previous numerical model considering better efficiency index.