The lack of knowledge on water flow and particle behavior of sandstone wastewater in sedimentation tanks of hydropower stations is a significant challenge for the design of these processes. In this study, the Euler-Lagrange two-phase model was applied to study the flow, sedimentation, and particle trajectories in a two-stage rectangular tank that is divided into two sections with different hydraulic retention times. The interfacial momentum transfer, buoyant force, and the collision force were considered, and the Lagrange approach was employed to determine the particle trajectories of the particles. The flow patterns, removal efficiency of the particles, and particle trajectories along the tank were investigated. The experimental and numerical results revealed that the two-stage rectangular tank is efficient in dealing with high turbidity sandstone wastewater. In addition, the simulated removal efficiencies were in good agreement with the experimental results. Last, the sensitivity of particle behavior to the collision force was studied, and the effect of the baffle length on particle settling in the rectangular sedimentation tank was evaluated.