Which chlorine residual is known to last the longest time in the distribution system?

Combined chlorine is recognized for its longevity in a water distribution system due to its chemical stability and interaction with organic matter. When free chlorine reacts with ammonia or organic nitrogen compounds in the water, it forms combined chlorine, primarily in the form of chloramines such as monochloramine and dichloramine. These chloramines are less reactive than free chlorine, allowing them to persist longer in the system before being consumed or broken down.

Free chlorine, on the other hand, is more reactive and primarily serves as a strong disinfectant. It can quickly react with various contaminants, leading to a shorter lifespan in the distribution system. Dichloramine, a specific type of combined chlorine, also lasts longer than free chlorine but is less stable than monochloramine, which is the primary form of combined chlorine. Hypochlorous acid, while effective as a disinfectant, also tends to dissipate more rapidly compared to combined chlorine due to its higher reactivity.

The ability of combined chlorine to remain stable while providing disinfection makes it particularly suitable for maintaining a residual throughout extended networks of water distribution, ensuring long-term microbial protection within the system.