What does residual chlorine refer to?

Residual chlorine refers to the amount of chlorine that remains in water after chlorination has taken place and after a specified contact time. This measurement is critical as it indicates the effectiveness of chlorination in controlling pathogens and maintaining water quality in the distribution system.

Residual chlorine is important because it ensures that there is a sufficient level of chlorine present to provide ongoing disinfection as the water moves through the distribution system and until it reaches the consumer. A necessary aspect of water treatment, this residual level helps prevent recontamination after the initial treatment.

The other choices pertain to different aspects of chlorine use or regulatory measures, but they do not accurately define the concept of residual chlorine, which specifically focuses on the amount left after chlorination has done its job.