If you added 3.5 mg/L of chlorine to a storage tank and measured a residual of 1.2 mg/L after 30 minutes, what was the chlorine demand?

Determining chlorine demand involves calculating the difference between the amount of chlorine added to the water and the residual chlorine measured after a specified time. In this case, you initially added 3.5 mg/L of chlorine to the storage tank. After 30 minutes, the residual chlorine measured is 1.2 mg/L.

To calculate the chlorine demand, subtract the residual amount from the initial amount added:

Chlorine Demand = Initial Chlorine Added - Residual Chlorine Chlorine Demand = 3.5 mg/L - 1.2 mg/L = 2.3 mg/L

This result indicates that 2.3 mg/L of chlorine was consumed by the water through reactions such as oxidation of organic matter, bacteria, or other substances present. The correct answer reflects the effective consumption of chlorine in the water, which is critical for assessing disinfection efficiency and ensuring water quality.