Circular clarifier flow is 1.2 MGD, diameter 55 ft, and depth 8 ft. What is the detention time?

• A. 0.04 hr
• B. 1.2 hr
• C. 0.05 hr
• D. 2.8 hr

Answer: (D)

Detention time is the time a volume of water spends in the tank, calculated as tank volume divided by the flow rate through the tank. For a circular clarifier, volume equals the circular area times the depth. The area of a circle is π times the radius squared, and the radius here is half the diameter: 27.5 ft. Area ≈ π × (27.5)^2 ≈ 2376 ft^2. Multiply by the depth of 8 ft to get a volume of about 19,000 ft^3.

Convert to gallons: 1 ft^3 is about 7.4805 gallons, so volume ≈ 19,000 × 7.4805 ≈ 142,000 gallons. The flow is 1.2 MGD, which is 1,200,000 gallons per day, or 1,200,000 / 24 ≈ 50,000 gallons per hour.

Detention time ≈ 142,000 gallons ÷ 50,000 gallons per hour ≈ 2.84 hours, i.e., about 2.8 hours.