What does residual chlorine refer to?

Residual chlorine refers to the amount of chlorination chemical that remains in the water after the chlorine has reacted to disinfect the water and has met the demand created by organic compounds and other materials present in the water. This residual is important for ensuring that there is sufficient chlorine available to continue providing disinfection as the water moves through the distribution system.

In essence, the correct answer highlights the chlorine that is still present to maintain a disinfecting effect, which is crucial for water safety as it travels to consumers. Monitoring residual chlorine levels is a key practice in water treatment operations, as it indicates that the water remains adequately sanitized.

The other choices do not align with the definition of residual chlorine. The amount of chlorine applied refers to the initial dose added to the water, while the amount of chlorine used pertains to how much was consumed in the process of disinfection. Total chlorine in the tank would account for both free and combined chlorine but does not specifically focus on what remains after meeting demand.