What is meant by "residual" in water treatment terminology?

In water treatment terminology, "residual" refers to what is left over after a process has occurred. This can encompass various substances remaining in the water after treatment steps have been completed. For example, after disinfection processes, there may be chlorine residual which indicates the amount of chlorine still present in the water to ensure continued disinfection as the water moves through the distribution system.

Understanding this concept is crucial because the presence of a residual can indicate the effectiveness of treatment and help in monitoring the system's performance. It helps water treatment operators ensure that treated water meets health and safety regulations while also providing insight into maintaining water quality throughout the distribution network.