What does chlorine demand represent?

Chlorine demand represents the difference between the amount of chlorine added to the water and the residual chlorine that remains after a certain period of time or treatment process. This measurement is essential for understanding how much chlorine is actually used up during the disinfection process, as chlorine reacts with various substances in the water, including organic matter, bacteria, and other impurities.

When chlorine is added to water, it does not all remain as free chlorine available for disinfection; some of it is consumed in chemical reactions with contaminants. The difference, or "demand," indicates the effectiveness of the chlorine in treating the water. A high chlorine demand suggests that there are substantial impurities in the water that require more chlorine to achieve effective disinfection, while a low chlorine demand implies cleaner water requiring less chlorine.

In contrast to this correct answer, the total amount of chlorine available in the system refers to the total calculated without accounting for what has been consumed, while the minimum amount for taste and maximum allowable limits pertain to aesthetic and regulatory standards but do not directly measure chlorine's effectiveness in disinfection.