What is Chlorine Demand?
Chlorine chemicals (Table 1) interact (react) with many things
found in water, including: organic matter, metals, bromide,
manganese, hydrogen sulfide, and ammonium. Chlorine
chemicals will react with chemicals in solid forms (such as scale),
dissolved in water, or attached to sediment particles. At the same
time, chlorine chemicals will kill or inactivate microorganisms by
chemically disrupting (oxidizing) their tissues. These chemical
reactions occur together and create a chlorine (chemical) demand.
Therefore, the chlorination process starts with an initial amount
of chlorine added to water, it reacts with many things in the
water until all the chlorine is used up or until all reactions are
satisfied, and may leave some unused or partially used chlorine
in the water called residual chlorine. This residual chlorine is
divided into two forms: combined chlorine (composed mostly
of nitrogen-chlorine chemicals such as chloramines, which are
mild disinfectants) and free chlorine (available for additional
disinfection if needed) (CDC, 2012).