Chlorine Demand is a pool water chemistry topic that is getting more attention due to changing climates, consumer's water chemistry and understanding of the problem. It is also becoming a greater issue in many private & commercial pools.
Chlorine Demand is defined as "the quantity of chlorine reduced or converted to inert or less active forms of chlorine by substances in the water." Faust and Aly's Chemistry of Water Treatment further state that, "since chlorine is a non-selective oxidant, almost any substance in the water...will react and consume chlorine."
In other words, the more "stuff" dissolved in the pool water, whether organic or inorganic; chemical, vegetable or mineral; heavy, constant rainfall can cause a chlorine demand.