Chlorine residual

This is the amount of chlorine (determined by testing) that remains after the demand is satisfied. Residual, like demand, is based on time. The longer the time after dosage, the lower the residual will be, until all of the demand has been satisfied. Residual, like dosage and demand, is expressed in milligrams per liter. The presence of a free residual of at least 0.2 to 0.4 ppm usually provides a high degree of assurance that the disinfection of the water is complete. Combined residual is the result of combining free chlorine with nitrogen compounds. Combined residuals are also called chloramines. Total chlorine residual is the mathematical combination of free and combined residuals. Total residual can be determined directly with standard chlorine residual test kits.

Source: Handbook of water and wastewater treatment plant operations