A three-dimensional numerical model was developed for the proposed primary settling tank (PST) for the Calumet Water Reclamation Plant (CWRP) in Chicago, IL. The numerical model can deal with three-dimensional multi-phase turbulent flow. Two-fluid model is used to capture the behavior of the solid-water mixture. Hindered settling was modeled by an empirical exponential relationship between the settling velocity and the solids concentration. The sludge was modeled as a Bingham plastic with parameters estimated from experiments. During the design, the most important parameters are the tank's depth and diameter. These two parameters are also the controlling factors for the construction cost. Other parameters include the inlet/outlet structure, feed well depth/diameter, bottom slope, etc. The tank diameter is fixed due to the space restriction. The tank depth and feed well depth were analyzed by comparing different options. Once these two parameters were fixed, other parameters were fine tuned to optimize the performance of the tank. Due to the length of this paper, only the simulations for the tank depth and the feed well depth are introduced.