Chorine contact time

This is one of the key items in predicting the effectiveness of chlorine on microorganisms. It is the interval (usually only a few minutes) between the time when chlorine is added to the water and the time the water passes by the sampling point, contact time is the “T” in CT. CT is calculated based on the free chlorine residual prior to the first customer multiplied by the contact time in minutes.

Source: Handbook of water and wastewater treatment plant operations