What is defined as chlorine residual in water treatment?

Chlorine residual in water treatment refers to the amount of chlorine remaining in the water after the disinfection process has taken place. It consists of both free uncombined chlorine, which is the active form of chlorine available for disinfection, and combined chlorine, which is chlorine that has reacted with organic matter or ammonia in the water and still possesses some disinfecting properties.

This is important in water treatment, as having a residual helps ensure that water remains disinfected as it travels through the distribution system, thereby preventing the regrowth of pathogens. The combination of free and combined chlorine provides a more comprehensive understanding of the sanitizing effectiveness and safety of the water supply.

The other options do not encapsulate the complete concept of chlorine residual accurately. They either focus only on one specific type of chlorine or fail to acknowledge the role of combined chlorine in maintaining disinfection in the water supply.