Approximately how many gallons are in one sedimentation tank?

• A. 150,000 gallons
• B. 282,600 gallons
• C. 500,000 gallons
• D. 1,000,000 gallons

Answer: (B)

Sedimentation tanks (clarifiers) are sized to provide enough detention time for solids to settle, so their capacity is tied to the tank’s geometry. For a circular clarifier, volume is V = π (D/2)^2 × depth. In many plants, a single tank is about 60–70 ft in diameter and 10–12 ft deep, which typically yields a few hundred thousand gallons.

If you take a common example around 63 ft in diameter and 12 ft deep, the radius is 31.5 ft and the area is about π × 31.5^2 ≈ 3,118 ft^2. Multiply by depth 12 ft gives roughly 37,400 ft^3. Converting to gallons (1 ft^3 ≈ 7.48 gallons) gives about 279,000 gallons, which is very close to 282,600 gallons. So, a sedimentation tank commonly holds on the order of a few hundred thousand gallons, making 282,600 gallons a reasonable approximate capacity.