What is the monthly average limit for chlorine (CL2) in mg/L?

• A. 0.019 mg/L
• B. 0.11 mg/L
• C. 0.5 mg/L
• D. 1.0 mg/L

Answer: (B)

Chlorine disinfection limits are set to protect both public health and the environment while still providing effective disinfection. The monthly average limit represents the average of all daily chlorine residual readings in the effluent for that month. The value 0.11 mg/L is used because it keeps the residual low enough to minimize harm to aquatic life and formation of disinfection byproducts, yet high enough to ensure ongoing disinfection. In practice, you would add up the daily CL2 readings for the month, divide by the number of days, and ensure that the resulting average does not exceed 0.11 mg/L. The other numbers are either unrealistically low for typical disinfection needs or too high for an environmental limit, which is why 0.11 mg/L is the correct monthly average limit.