A chlorine contact tank with volume = 200,000 gal, flow = 2 MGD, and residual = 1.5 mg/L has a CT of what?

A chlorine contact tank with volume = 200,000 gal, flow = 2 MGD, and residual = 1.5 mg/L has a CT of what?

• A. 100 mg·min/L
• B. 50 mg·min/L
• C. 150 mg·min/L
• D. 200 mg·min/L

Answer: (A)

The main idea is that CT = C × T, where C is the chlorine residual in mg/L and T is the contact time in minutes. To get T, use Volume ÷ Flow with consistent units. The flow of 2 MGD equals 2,000,000 gallons per day, which is 2,000,000 ÷ 1440 ≈ 1,388.9 gallons per minute. Detention time T = 200,000 gallons ÷ 1,388.9 gal/min ≈ 144 minutes. Multiply by the residual: CT = 1.5 mg/L × 144 min = 216 mg·min/L. So the calculated CT is 216 mg·min/L, which isn’t exactly among the options provided.

The main idea is that CT = C × T, where C is the chlorine residual in mg/L and T is the contact time in minutes. To get T, use Volume ÷ Flow with consistent units. The flow of 2 MGD equals 2,000,000 gallons per day, which is 2,000,000 ÷ 1440 ≈ 1,388.9 gallons per minute. Detention time T = 200,000 gallons ÷ 1,388.9 gal/min ≈ 144 minutes. Multiply by the residual: CT = 1.5 mg/L × 144 min = 216 mg·min/L. So the calculated CT is 216 mg·min/L, which isn’t exactly among the options provided.