Is there a difference between Total Chlorine and Total Residual Chlorine (TRC)?

Document ID

Document ID TE10573

Version

Version 1.0

Status

Status Published

Published Date

Published Date 10/09/2018
Question
Is there a difference between Total Chlorine and Total Residual Chlorine (TRC)?
Summary
An explanation of what the term residual refers to in Total Residual Chlorine.
Answer
Total Residual Chorine is a phrase often used in disinfection applications. A sample will initially have some Chlorine Demand which exists in the form of microorganisms and organics. When the Chlorine is added as a disinfectant, some of that Chlorine is consumed by that Chlorine Demand, which can often be considered an initial disinfection of the sample. The Chlorine that exists after this initial disinfection is called the Residual. As this Residual Chlorine left after initial disinfection is consumed by additional disinfection, then Total Residual Chlorine can continuously be used to refer specifically to the amount of Chlorine that is remaining after the disinfection that has happened so far. This is often viewed relative to the amount of Chlorine initially added to get an indication of how much chlorine has been consumed in the disinfection process. It is measured to ensure that some Chlorine is still there to continue to disinfect the sample in the future.

So the term "Residual" in Total Residual Chlorine is not referring to the type of test being performed, but rather when that test is being performed. Total Residual Chlorine is the result of a Total Chlorine test that is performed after disinfection has occurred. Any Total Chlorine test can be considered a Total Residual Chlorine test based on when the test is performed.

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