When chlorine is added to water it not only reacts with bacteria, but also with other impurities, such as hydrogen sulfide, soluble metals, particles of organic matter and other microorganisms. The chlorine demand must first be satisfied, before a residual chlorine concentration can be established.
Once there is a residual chlorine concentration, this residual concentration has to be maintained during the required contact time to kill pathogenic microorganisms. To adequately disinfect the water it is therefore required to supply the water with a higher chlorine concentration than the concentration required to kill bacteria.
Chlorine demand is simply how much available or “free” chlorine in mg/L or PPM is needed to kill the bacteria, remove odor, and/or oxidize iron, in order to achieve your goals.