What condition would indicate that breakpoint chlorination is being used?

Breakpoint chlorination is a process utilized in water treatment to effectively eliminate contaminants, primarily targeting organic matter, and to achieve a desired chlorine residual. The correct choice illustrates a key characteristic of this process: when an increase in chlorine dosage by a specific amount results in a corresponding increase in the chlorine residual, it indicates that the chlorine is no longer reacting with other compounds such as ammonia or organic materials.

At the breakpoint, all reactive substances like ammonia have been adequately chlorinated and converted into chloramines, at which point free chlorine can begin to establish a measurable residual. Therefore, if adding 1 mg/l of chlorine leads to a 1 mg/l increase in residual chlorine, it shows that the chlorination has reached a point where it is effectively disinfecting the water, and excess chlorine is now available as a free residual.

Other conditions such as a reduction in chlorine taste, presence of disinfectant odor, or changes in color may suggest other aspects of water quality or chlorine reactions but do not specifically indicate the successful application of breakpoint chlorination.