What is the difference between the amount of chlorine added to water and the residual chlorine at the end of the contact time?

The correct answer is the difference between the amount of chlorine added to water and the residual chlorine at the end of the contact time. This concept is known as chlorine demand. Chlorine demand represents the amount of chlorine that has been consumed by the organic and inorganic materials present in the water, including pathogens, microorganisms, and other substances that react with chlorine.

When chlorine is added to water, not all of it remains in the water as free residual chlorine after treatment. Some of the chlorine is consumed through these reactions, which is why understanding chlorine demand is essential for water treatment professionals. It indicates how much chlorine is needed to effectively disinfect the water and ensure that sufficient residual chlorine is maintained for ongoing disinfection.

Chlorine residual refers specifically to the amount of chlorine that remains in the water after the chlorination process is complete. Chlorination rate and chlorine concentration are not terms directly related to the difference between the amount added and what remains. Chlorination rate might relate to how quickly chlorine is added, while chlorine concentration refers to its level in the water without indicating the change during the treatment process. Understanding these terms helps in effective water treatment management.