According to AWWA C651 standard, what is the required free chlorine level and soak time for hyperchlorination?

The AWWA C651 standard outlines the procedures and requirements for the disinfection of water mains, particularly emphasizing the use of hyperchlorination as an effective method. The correct answer focuses on a free chlorine level of 100 ppm combined with a soak time of 3 hours. This concentration and duration are designed to ensure that sufficient chlorine is present to effectively eliminate pathogens like Legionella and other bacteria that may be present in the water system.

At the specified chlorine level, the chemical action is significantly enhanced, promoting a rigorous disinfection process that allows the chlorine to penetrate biofilms and other potential harboring spots for microorganisms. The 3-hour soak time ensures that the disinfectant has adequate contact time with the surfaces inside the water mains, thereby increasing the likelihood of effective pathogen removal.

In practice, maintaining these parameters is essential because insufficient chlorine levels or inadequate exposure time may lead to inadequate disinfection, allowing harmful bacteria to persist and potentially cause health risks in water supply systems. The combination of 100 ppm and 3 hours is thus specifically validated for rigorous disinfection efforts aligned with public health standards indicated in the AWWA guidance.