Most water treatment plants are required to disinfect the water, a process used to kill harmful bacteria. The most frequently used method of disinfection is the addition of chlorine. Here, we will briefly introduce three terms used during chlorination - chlorine dose, chlorine demand, and chlorine residual. These three characteristics are related to each other using the following equation:
(Chlorine demand) = (Chlorine dose) - (Chlorine residual)
The amount of chlorine added to the water is known as the chlorine dose. This is a measured quantity chosen by the operator and introduced into the water using a chlorinator or hypochlorinator.
As the chlorine reacts with bacteria and chemicals in the water, some of the chlorine is used up. The amount of chlorine used up by reacting with substances in the water is known as the chlorine
demand. If nothing reacts with the chlorine (as would be the case in distilled water), then the
chlorine demand is zero. However, in most cases the operator should count on some of the
chlorine dose being used up when it reacts with substances in the water.
The amount of chlorine remaining in the water after some of the chlorine reacts with substances in
the water is known as the chlorine residual. This lab introduces a test which can be used to
calculate the chlorine residual. The chlorine residual is the most important of these three values -
dose, demand, and residual - because it represents the actual amount of chlorine remaining in the
water to act as a disinfectant.