What does chlorine demand indicate?

Chlorine demand is a critical concept in water treatment, particularly in disinfection processes. It refers to the amount of chlorine that is necessary to achieve effective disinfection by "making the kill." This demand arises from various reactions that chlorine undergoes in water, including its reaction with organic matter, ammonia, and other contaminants.

When chlorine is added to water, it first combines with these substances to neutralize them before reaching the point where it can effectively kill pathogens. Therefore, chlorine demand is directly associated with the quantity of chlorine needed to ensure that a sufficient residual remains in the water to provide ongoing protection against microorganisms. This residual is crucial for maintaining water safety over time.

Understanding chlorine demand helps operators determine the appropriate dosage of chlorine needed to treat water effectively, ensuring that it meets safety standards for public health. Other options, such as the total chlorine supplied, evaporation losses, or concentration levels, do not specifically address this essential aspect of chlorine's role in disinfection and therefore do not accurately reflect what chlorine demand indicates.