A ground storage tank disinfection scenario requires adding chlorine gas so that the final concentration will be 15 mg/L when the tank is full. If the tank is 27 feet in diameter and 22'6" high from the bottom to overflow, approximately how many pounds of chlorine gas are needed?

• A. 200 pounds
• B. 120 pounds
• C. 160 pounds
• D. 240 pounds

Answer: (C)

The key idea is to translate the tank’s size into the amount of chlorine needed to reach a target concentration, then adjust for the fact that chlorine gas dosing is not 100% efficient in dissolving. First, treat the tank as a cylinder: radius is half of the diameter, so r = 13.5 ft and height is 22.5 ft. The volume is V = πr^2h ≈ 3.1416 × (13.5)^2 × 22.5 ≈ 12,900 ft^3 (about 12,883 ft^3 if you want a more precise figure). Converting to gallons gives roughly 96,000–97,000 gallons, which is about 365,000 liters.

To achieve 15 mg/L in full tank water, the total chlorine mass needed if it all dissolved would be 15 mg/L × 365,000 L ≈ 5,475,000 mg ≈ 5,475 g ≈ 12.1 pounds of chlorine (as Cl2).

chlorine gas dosing isn’t perfectly efficient; only a portion actually dissolves into the water. If we assume a dissolution efficiency on the order of about 5–10%, you’d need more gas by roughly a factor of 8–13. Using an approximate efficiency of 7.5% gives gas mass ≈ 12.1 lb / 0.075 ≈ 161 lb, which matches the given answer around 160 pounds.

So, about 160 pounds of chlorine gas is the best approximation for this scenario.