What is a typical disinfectant residual test used in drinking water plants?

• A. Free chlorine or chloramine residual measurement (mg/L).
• B. pH measurement.
• C. Turbidity measurement.
• D. Temperature measurement.

Answer: (D)

Disinfectant residual testing for drinking water plants centers on measuring the amount of disinfectant that is still present—specifically the free chlorine residual or the chloramines residual—in milligrams per liter (mg/L). This residual value shows how much disinfection capability remains as water moves through the plant and into the distribution system, helping ensure ongoing protection against microbial carryover. In practice, a colorimetric method (like the DPD test) is typically used to quantify this residual. While pH, turbidity, and temperature are important factors that influence disinfection effectiveness, they do not provide the actual remaining disinfectant concentration, which is why the residual test focuses on free chlorine or chloramines.