What are the chlorine (CL2) regulatory values and calibration frequency as given?

• A. Monthly average .019 mg/L; daily max .011 mg/L; calibrated daily
• B. Monthly average .011 mg/L; daily max .019 mg/L; calibrated weekly
• C. Monthly average .011 mg/L; daily max .019 mg/L; calibrated daily
• D. Monthly average .011 mg/L; daily max .022 mg/L; calibrated daily

Answer: (C)

Chlorine residual limits are defined using two measurements and a practical maintenance action: a monthly average that the effluent must meet and a daily maximum that cannot be exceeded, with the instrument calibrated daily to ensure accuracy. The correct values specify a monthly average of 0.011 mg/L and a daily maximum of 0.019 mg/L, and the analyzer should be calibrated every day. The monthly average represents the mean of all samples for the month, while the daily maximum is the highest reading in any single day; having the daily max higher than the monthly average ensures both sustained compliance on average and protection against peak spikes. Calibrating daily is standard for chlorine analyzers because small measurement drift from temperature, reagent changes, or sensor fouling can affect readings quickly, and precise control of disinfection relies on accurate values. The other options mix up the relative sizes of the monthly average and daily max or shorten the calibration frequency, which would risk misrepresenting the actual chlorine residual and undermine compliance.