What is Cl2 demand calculated as?

Chlorine demand represents the amount of chlorine that has reacted with substances in water, such as organic matter, bacteria, or other contaminants. It is essential for water treatment because it indicates how much chlorine is consumed during the disinfection process and informs operators of the appropriate dosage needed to achieve the desired residual.

To calculate chlorine demand accurately, one must take the dose of chlorine added to the water (known as Cl2 Dose) and subtract the residual amount of chlorine that remains in the water after treatment (known as Cl2 Residual). This subtraction gives the total chlorine that has been consumed or reacted with other substances in the water, which is the chlorine demand.

In this context, the rationale is that the Cl2 Dose indicates the initial amount of chlorine intended for use in disinfection, while the Cl2 Residual indicates the amount that is left after the disinfection process has occurred. Therefore, by subtracting the residual from the dose, you identify how much of the chlorine was used up in reaction with the contaminants, thus accurately reflecting the Cl2 demand.