If chlorine was added to water but there is no chlorine residual, which statement is true?

When chlorine is added to water and there is no chlorine residual detected, it indicates that the chlorine demand has not been met. This situation typically occurs when all the available chlorine is consumed by reacting with contaminants in the water, such as organic matter or other substances that require disinfection. Since there is no residual left, it signifies that there are still remaining contaminants in the water that have not been effectively dealt with.

The absence of chlorine residual means that the water did not undergo a successful disinfection process, as chlorine normally leaves a measurable residual in treated water, which is necessary to ensure ongoing protection against pathogens. Therefore, while it might be assumed that the water could be safe to drink based on the initial chlorination, the lack of residual suggests it may still harbor harmful microorganisms.

In this context, assessing chlorine demand is critical. This measurement helps determine if enough chlorine has been applied to fully address existing contaminants and achieve proper disinfection. Hence, it is essential for water treatment operators to monitor and understand chlorine residual levels to ensure the safety and quality of the water supply.