What is the minimum chlorine residual required at the extreme end of a newly disinfected main after standing for 24 hours?

The minimum chlorine residual required at the extreme end of a newly disinfected main after standing for 24 hours is indeed 25 mg/L. This requirement is established to ensure that the disinfection process effectively kills pathogens and maintains an adequate level of chlorine to prevent recontamination.

When chlorine is added to a water distribution system for disinfection, it can react with organic matter and other compounds, which can reduce the amount of free chlorine that remains. A residual of 25 mg/L ensures that even after 24 hours of standing, there is still enough chlorine in the water to safeguard against microbial growth.

This residual level is a guideline to provide consumer safety, as it indicates that any water drawn from the system will have a sufficient chlorine concentration to minimize health risks associated with waterborne pathogens. Monitoring chlorine levels is a crucial part of water quality management, and the target of 25 mg/L reflects a balance between effective disinfection and the safety of the water for community use.