Swimming and spa pools control bacteria using disinfectants like chlorine or bromine, or sometimes UV light. When chlorine is added to dirty water, it reacts with organic waste and ammonia from things like urine. This reaction forms chloramines. When all the chlorine turns into chloramines, this is called marginal chlorination. Chloramines can still disinfect the water, but they work much slower than free chlorine.

If you add enough chlorine so that some stays in its free form, disinfection becomes up to 50 times more effective. Free chlorine also helps break down the chloramines already in the water. However, as long as people are using the pool, new chloramines will keep forming because of ongoing contamination.

By adding enough chlorine at the end of each swimming session, chloramines can be gradually reduced. When all chloramines are destroyed, the free chlorine and total chlorine readings will be the same. This is called breakpoint chlorination and it’s the most effective way to keep pool water free from harmful germs.

Chlorine demand is simply the difference between how much chlorine is added to the pool and how much remains available as a disinfectant.