The study was executed in two years. The objective of the first year is to develop a two-dimensional setting numerical model, to calibrate and validate the proposed model, and then to simulate the deposition efficiency of the settling basins. On the basis of the simulation results, the advantages and drawbacks of the existing design methods will be evaluated. In the second year, the economic sizes of the settling basins are determined through the numerical simulations based on the conditions of assumed designed basin dimensions, flow fields, and sediment properties. The obtained results may provide as a design reference to the associated agencies.