If chlorine is fed at 3mg/L and the residual after 30 minutes is 0.5 mg/L, what is the chlorine demand?

To determine the chlorine demand, you calculate the difference between the amount of chlorine added and the residual chlorine remaining after a specified contact time. In this case, chlorine was fed at a concentration of 3 mg/L, and after 30 minutes, the residual concentration was measured at 0.5 mg/L.

The chlorine demand is calculated as follows:

Chlorine Demand = Chlorine Added - Residual Chlorine

Substituting the values from the question:

Chlorine Demand = 3 mg/L - 0.5 mg/L Chlorine Demand = 2.5 mg/L

Thus, the chlorine demand is 2.5 mg/L, which indicates how much chlorine has been consumed in the water treatment process. This value is important for understanding how effective the chlorine is at disinfecting the water and can inform adjustments to chlorine dosing in future applications.