Why is residual chlorine monitored in potable water systems?

• A. To confirm ongoing disinfection and detect system contamination
• B. To measure taste
• C. To regulate temperature
• D. To test for heavy metals

Answer: (A)

Residual chlorine is the amount of chlorine that remains in water after the disinfection step. Monitoring it shows that disinfection is still active as water moves through the distribution system, providing ongoing protection against microbes that could enter or persist in pipes. If the residual drops too low, the water may no longer be protected as it travels to consumers, indicating potential contamination or excessive disinfectant decay. Keeping a measurable residual also helps operators detect issues that could compromise safety, such as leaks, biofilm growth, or issues with treatment effectiveness.

This monitoring isn’t about taste, temperature, or heavy metals. Taste can be affected by chlorine but is not the purpose of residual monitoring; temperature has no relation to chlorine levels, and heavy metals require separate, specific tests.