During which stage of the Chlorine Demand Curve does breakpoint chlorination occur?

Breakpoint chlorination occurs during the final stage of the Chlorine Demand Curve, which is referred to as Stage 4. This stage is characterized by the point at which all forms of chlorine demand, including oxidizable substances and chloramines, have been met and the free chlorine concentration begins to rise significantly.

During Stage 1, the chlorine is reacting with various substances such as organic matter and ammonia, which represent the initial chlorine demand. As you progress to Stage 2 and Stage 3, the demand continues to be satisfied, but the free available chlorine remains low.

Stage 4 is crucial as it signifies that sufficient chlorine has been added to overcome the chlorine demand, and any additional chlorine results in a noticeable increase in the free chlorine residual, indicating that disinfection is effectively underway. This stage is critical for ensuring adequate disinfection of the water supply, as it provides the necessary residual that can help maintain water quality.