Which indicator signals disinfection effectiveness in drinking water testing?

• A. Total coliforms
• B. E. coli
• C. Turbidity
• D. Free chlorine residual

Answer: (D)

Disinfection effectiveness is signaled by a measurable free chlorine residual. This measures how much disinfectant remains in the water after treatment, indicating that disinfection is continuing as water moves through the distribution system. A detectable residual means the water still has the protective chlorine in place to inactivate pathogens.

Total coliforms and E. coli show whether contamination is present, not how much disinfectant remains, so they indicate potential microbial issues rather than current disinfection status. Turbidity reflects particle load and can influence how well disinfectants work, but it does not directly measure the ongoing disinfection level. In short, the presence of free chlorine residual directly reflects the disinfection status of the water.