High-Rate Settler Modules. The modules are placed in the downstream end of rectangular horizontal flow tanks below the launders as shown in Figure 7-14. They occupy approximately 75-95 percent of the tank area. The remaining area is left as open space. As with the rectangular sedimentation basin, a diffuser is placed at the inlet. A solid baffle wall covers the entire front end of the settler if the flow is countercurrent. Sufficient space is provided below the settler for the sludge collection mechanism.