Mains that have been repaired or cut into also require disinfection, except for which condition?

• A. those that have not been drained
• B. those that have remained full of water and under pressure
• C. those that are new and not yet in service
• D. those that have been repaired with a temporary line

Answer: (B)

When you repair or cut into a water main, you normally need to disinfect the section to kill any microbes that could be introduced during the work. The exception is when the main has not been drained and remains full of water under pressure. In that situation, there’s no air-water interface created and the water inside stays within the existing disinfectant residual of the system, so there isn’t an opportunity for contamination to enter or for new bacterial growth during the repair. Therefore, disinfection after repair isn’t required for mains that stay full of water and under pressure.

In contrast, if the line has been drained, or if a new line or temporary line is involved, disinfection is typically required to ensure the water remains safe.