What does chlorine demand refer to?

Chlorine demand refers to the total amount of chlorine that needs to be added to water to achieve a desired level of disinfection. This includes both the chlorine used to disinfect the water and the chlorine that reacts with various substances in the water, such as organic matter, ammonia, and other contaminants.

When chlorine is added to water, a portion of it is used up by these reactions, meaning that the remaining free chlorine is available for disinfection. Thus, understanding chlorine demand is crucial for water treatment processes, as it helps operators determine how much chlorine needs to be dosed to ensure adequate disinfection while preventing potential issues with water quality.

In contrast, the other options describe different concepts related to chlorine but do not define chlorine demand itself. Total chlorine in water refers to both free and combined forms of chlorine. The effectiveness of chlorine in disinfection pertains to how well chlorine works rather than the amount needed. The rate of chlorine evaporation is related to how chlorine may be lost from water but does not reflect the demand for chlorine in the water treatment context.