After a new water main is installed and pressure tested it should be:

• A. Flushed with clean water for 24 hours and put into service
• B. Filled with a solution of 25 ppm to 50 ppm of free chlorine for at least 24 hours prior to flushing
• C. Filled with clean water and allowed to sit for 5 days at full pressure before turning the water into the system
• D. Photographed so that mapping can be avoided until the system is complete

Answer: (B)

Disinfecting the new main before it goes into service is essential to prevent microbial contamination. The tested approach uses a chlorinated solution to sanitize the pipe interior, providing enough free chlorine and time for disinfection. Filling the main with water containing 25 to 50 parts per million of free chlorine for at least 24 hours ensures a solid contact time to inactivate potential organisms. After this sanitizing period, the line is flushed with clean water to remove the disinfectant, and service commences once safety checks, including bacteriological sampling, confirm the water is safe.

Flushing with clean water alone doesn’t address potential pathogens inside the new pipe. Holding clean water at full pressure for several days serves no disinfection purpose and could permit growth. Wiring it up or photographing the layout doesn’t resolve the contamination risk either. The chlorine-based disinfection step is the practical, code-compliant method to protect public health before the system is put into service.