If finished water chlorine residual is 45 mg/L, what should an operator do first?

• A. Adjust the chlorine dose
• B. Sample again later
• C. Flush the line
• D. Re-run the test

Answer: (D)

When you see an unexpectedly high finished water chlorine residual, the first step is to re-run the test to verify the result. A reading like 45 mg/L is far beyond normal operating values (typically around 0.2–0.5 mg/L), so it almost always points to a testing or sampling error, instrument calibration issue, or reagent problem rather than actual system conditions. Confirming with a repeat measurement ensures you don’t make unnecessary or unsafe changes based on faulty data. If the repeat test still shows an unusually high value, then you would investigate the testing procedure, calibration, and sampling method and take appropriate corrective actions. Until the result is verified, hold off on adjusting doses or flushing based on that reading.