What term describes the difference between chlorine added and residual chlorine after the contact period?

Prepare for the American Water Works Water Treatment Plant Operator Test. Study with multiple choice questions, each with hints and explanations. Get ready to excel!

When chlorine is added to water, some of it is consumed by reactions with organic matter, inorganic compounds, and microorganisms. The amount that is used up during these reactions is called the chlorine demand. After a contact period, the chlorine that remains is the residual chlorine. The difference between what was added and what is left as residual is the chlorine demand. This value reflects how much chlorine was required to react and be consumed to reach that residual level. The remaining residual can be free chlorine (the active disinfectant form) or combined chlorine (chloramines), but the term describing the depletion from added chlorine to the residual level is chlorine demand. Breakpoint chlorination refers to a process of adjusting chlorine dosing to overcome chloramines and reach free chlorine, not to the simple difference between added and residual chlorine.