How many pounds of HTH 70% available chlorine will be necessary to disinfect a water storage tank stand pipe 40 feet high and 18 feet in diameter using a 15 mg/L dosage?

• A. 45.3
• B. 9.5
• C. 60
• D. 13.6

Answer: (A)

Compute the mass of chemical by linking volume to the required chlorine dose and adjusting for the chemical’s available chlorine content. The stand pipe is a cylinder with radius 9 ft and height 40 ft, so its volume is V = π r^2 h = π × 9^2 × 40 = 3240π ft^3 ≈ 10,178.8 ft^3. Convert to liters: 1 ft^3 ≈ 28.3168 L, giving about 288,730 L of water. A 15 mg/L dose means you need 15 × 288,730 ≈ 4.33 × 10^6 mg of chlorine (as Cl2) total. HTH 70% available chlorine provides chlorine at 70% of the chemical’s weight, so mass of the chemical required is 4.33 × 10^6 mg / 0.70 ≈ 6.18 × 10^6 mg, or about 6.18 kg. Convert to pounds: 6.18 kg × 2.2046 ≈ 13.6 lb. So about 13.6 pounds of HTH 70% chlorine are needed.