Calculate the average detention time in days for a rectangular basin 90 ft long, 40 ft wide, and 15 ft deep with a flow rate of 2 MGD.

• A. 0.02 days
• B. 2.00 days
• C. 0.20 days
• D. 4.00 days

Answer: (C)

Detention time is the time water spends in the basin, calculated as T = V / Q, where V is the basin volume and Q is the flow rate. For a rectangular basin, V = L × W × D. So the volume is 90 ft × 40 ft × 15 ft = 54,000 ft³. Converting to gallons (1 ft³ ≈ 7.48052 gal) gives V ≈ 54,000 × 7.48052 ≈ 404,000 gallons. The flow rate is 2 MGD = 2,000,000 gallons per day. Therefore T ≈ 404,000 / 2,000,000 ≈ 0.202 days, which rounds to 0.20 days (about 4.8 hours).