What is the length of each sedimentation tank?

• A. 40 ft
• B. 60 ft
• C. 90 ft
• D. 100 ft

Answer: (C)

Sedimentation tanks are sized to give water enough time to settle by gravity. The key is detention time, which depends on the tank volume (length × width × depth) relative to the plant’s flow rate. When the geometry is chosen to meet the required detention time at the design flow, the length becomes a critical factor in providing the necessary volume and surface area for effective settling. In this typical design scenario, a length around ninety feet yields the right balance of sufficient detention time and practical land use for the tank width and depth. Shorter lengths would reduce volume and surface area, risking inadequate clarification, while a significantly longer length would waste space and cost.