What is the depth of the sedimentation tank described in the plant layout?

• A. 9 ft
• B. 10 ft 6 in
• C. 11 ft
• D. 12 ft

Answer: (B)

The depth of a sedimentation tank is chosen to give enough time for suspended solids to settle while keeping the tank size and cost reasonable. In the plant layout, the sedimentation tank is shown with a depth of 10 feet 6 inches. That depth provides a detention time that matches the expected flow and the tank’s surface area shown in the diagram, allowing most settleable particles to drop out before the clarified water overflows to the next stage.

If the tank were shallower, like 9 feet, the water would spend less time in the tank and fewer particles would settle, reducing removal efficiency. If it were deeper, such as 11 or 12 feet, the same surface area would contain more volume, increasing construction costs and structural requirements without a proportionate gain in performance. The 10 ft 6 in depth balances performance with cost and space, and aligns with what the layout specifies for the tank.