Residual Chlorine

Definition - What does Residual Chlorine mean?

Residual chlorine is the amount of chlorine that remains in the water after a certain period or contact time.

Testing for residual chlorine is one of the most common tests used by water treatment plants. Through the residual chlorine test, the remaining chlorine amount is determined in water that has finished testing and is ready to be released in the distribution system.

Corrosionpedia explains Residual Chlorine

There are three forms of residual chlorine in water treatment:

  • Free - Residual chlorine composed of dissolved hypochlorite ions, hypochlorous acid and chlorine gas
  • Combined - Composed of chloramines that can kill bacteria and oxidize organic matter
  • Total - The sum of free and combined residual chlorine

Hypochlorous acid and other strong acids are used as oxidizing agents for disinfecting drinking water. However, these acids can also react as oxidants, especially with lead, which increases the chances of lead corrosion. Therefore, operators in water treatment and distribution plants must ensure that there is the proper amount of residual chlorine as the water reaches the end of the system.

In different settings and situations, there are sometimes required residual chlorine levels to prevent the occurrence of corrosion in water. This level must be determined and carefully studied by water plants for safe and efficient water processing and distribution.

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