What does breakpoint chlorination involve?

Breakpoint chlorination is a water treatment process that focuses on adding chlorine to water until the chlorine demand is satisfied. This stage is crucial because it allows for the effective removal of organic contaminants and ammonia from the water, thereby ensuring that free chlorine remains available for disinfection purposes.

When chlorine is introduced to water, it first reacts with any pollutants present, such as ammonia and organic matter. The amount of chlorine needed to react with these substances is known as chlorine demand. Once all contaminants have been addressed, additional chlorine will lead to a point where the remaining chlorine is free to perform its disinfection role, which is referred to as "breakpoint." Therefore, the correct answer highlights the necessity of adding chlorine until this demand is fully met to maintain water quality and safety.

Other options, while related to the treatment process, do not accurately describe the target process of breakpoint chlorination. For instance, merely stating that chlorine is added until the water is clear does not account for the need to satisfy chlorine demand specifically. Continuous addition of chlorine may not necessarily indicate the equilibrium of demand being met, and mixing chlorine with other disinfectants diverges from the singular focus on chlorine's role in breakpoint chlorination.