What is the minimum residual needed to kill viruses?

• A. 0.2-0.5
• B. 3.0-4.0
• C. 5.0-6.0
• D. 1.2-2.0

Answer: (D)

Maintaining a disinfectant residual in the distribution system is what keeps viruses from surviving as water moves to consumers. Viruses tend to be more resistant to chlorine than many bacteria, so a higher residual is needed to provide enough disinfection over time and through varying water quality.

The range 1.2–2.0 mg/L of free chlorine is used as a conservative level to ensure that the chlorine-based disinfection remains strong enough to inactivate viruses under typical distribution conditions, accounting for factors like contact time, organic matter, and pH. This keeps protection in place even if some of the chlorine is consumed along the way.

Lower residuals, such as 0.2–0.5 mg/L, may not reliably inactivate viruses throughout the system, while much higher residuals (3–4 or 5–6 mg/L) are generally unnecessary and can lead to taste and odor issues and regulatory or operational drawbacks.