Which of the following best describes "chlorine demand"?

Chlorine demand refers to the amount of chlorine that is used by various substances present in water before a measurable residual is achieved. In this context, it is defined as the difference between the amount of chlorine added to a water sample and the amount of residual chlorine that remains after a specified contact time. This is significant because it quantifies how much chlorine is "consumed" by reactions with organic matter, bacteria, or other contaminants in the water.

Understanding chlorine demand is crucial for water treatment processes, as it helps operators determine the right dosage of chlorine necessary to achieve desired levels of disinfection and ensure that an effective chlorine residual is maintained in the water supply. By measuring this difference, water treatment professionals can assess the efficacy of chlorination in removing pathogens and maintaining water quality.