Wastewater Treatment Operator Certification Practice Test

Chlorine demand refers to the amount of chlorine that is required to effectively disinfect water or wastewater, accounting for the chlorine that is consumed by organic matter, ammonia, and other substances present in the water. The fundamental relationship that defines chlorine demand is based on the difference between the amount of chlorine added (the chlorine dose) and the amount of residual chlorine remaining after disinfection has occurred.

When chlorine is added to water, some of it reacts with contaminants, thereby reducing the amount of chlorine that remains in the water. The chlorine residual is the amount of chlorine left in the water after these reactions have taken place. Therefore, to determine the chlorine demand, one must subtract the chlorine residual from the chlorine dose. This calculation gives the total amount of chlorine that was utilized in the reactions, effectively indicating how much demand was placed on the chlorine by the constituents in the water.

Thus, the correct formulation for chlorine demand—chlorine dose minus chlorine residual—accurately reflects this relationship, demonstrating that the total dose minus what remains unreacted gives the total consumption or demand for chlorine in the treatment process.