Which range represents the virus inactivation residual requirement?

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

Answer: (C)

Maintaining a chlorine residual in the distribution system that is enough to inactivate viruses is essential for protecting public health. Viruses tend to be more resistant to disinfection than many other microbes, so the residual must be high enough to ensure continued inactivation as water travels through pipes and as the disinfectant decays over time. The recommended virus inactivation residual is typically in the range of about 1.2 to 2.0 mg/L, which provides a balance between effective protection and acceptable taste, odor, and regulatory limits. This range helps ensure that even with water aging and varying conditions, there’s enough disinfectant present to keep viruses inactivated.

Ranges that are much lower, like 0.2–0.5 mg/L, may not provide consistent virus inactivation across the system, especially during longer residence times or challenging water quality. Ranges that are much higher, such as 3.0–4.0 or 5.0–6.0 mg/L, are generally unnecessary and can lead to taste and odor problems and higher production of disinfection byproducts, without a proportional safety benefit in typical distribution systems.