What is the minimum disinfectant residual that must be maintained in water served to a population greater than 3,300 for no more than 4 hours?

• A. 0.05 mg/L
• B. 0.1 mg/L
• C. 0.2 mg/L
• D. 0.3 mg/L

Answer: (C)

The correct answer, which is a minimum disinfectant residual of 0.2 mg/L, reflects regulatory standards established for public water systems. Maintaining a residual disinfectant level is essential for ensuring water safety, especially in larger populations where the distribution system is more extensive and could present higher risks for contamination.

In the context of public health, a residual of at least 0.2 mg/L ensures that any pathogens present in the water are adequately inactivated during distribution. This level helps maintain the potability of the water throughout the system and prevents microbial regrowth, which can occur if the disinfectant level drops too low.

For populations greater than 3,300, the requirements are designed to be stringent to minimize health risks, and the chosen threshold of 0.2 mg/L is a balance between effectiveness in disinfection and managing the characteristics of the water. Lower residuals may not provide sufficient protection, while excessively high levels can lead to other issues such as taste and potential health concerns from the disinfectant itself.

Overall, the standard of 0.2 mg/L reflects both regulatory guidance and practical considerations for managing water quality in larger distribution systems.