If an operator measures a total chlorine residual of 1.6 mg/L and a free chlorine residual of 0.9 mg/L, what is the chlorine demand?

To determine the chlorine demand, you first need to understand the relationship between total chlorine, free chlorine, and combined chlorine. The total chlorine residual is the sum of free chlorine and combined chlorine (chlorine that has reacted with organic matter and other compounds in the water).

In this case, the total chlorine residual is measured at 1.6 mg/L, and the free chlorine residual is measured at 0.9 mg/L. To find out the combined chlorine, you subtract the free chlorine from the total chlorine:

Total Chlorine - Free Chlorine = Combined Chlorine

1.6 mg/L (total chlorine) - 0.9 mg/L (free chlorine) = 0.7 mg/L (combined chlorine)

The chlorine demand is defined as the amount of chlorine that is consumed when it reacts with contaminants in the water before it can achieve a free residual. In this instance, since the free chlorine residual represents the available chlorine after some has reacted and combined with contaminants, the chlorine demand can be interpreted as the amount of total chlorine minus the remaining free chlorine available after the reaction.

However, in the context of simplifying this for practical purposes in water treatment operations, whenever the combined chlorine is calculated as that between total and free,