Illinois EPA Class C Practice Test

To determine the correct chlorine dose, it's important to understand the concept of chlorine demand and residual. Chlorine demand refers to the amount of chlorine that is consumed by the water before a measurable residual is left. To meet the chlorine demand and achieve the desired residual measurement in the water, you must add both the chlorine demand and the desired residual together.

In this case, the chlorine demand is 2.5 mg/l, which is the amount needed to react with contaminants in the water. The desired residual is 0.6 mg/l, which is the amount of chlorine that should remain in the water after treatment to ensure disinfection.

To find the total chlorine dose required, you add the chlorine demand (2.5 mg/l) to the desired residual (0.6 mg/l):

Chlorine Dose = Chlorine Demand + Desired Residual

Chlorine Dose = 2.5 mg/l + 0.6 mg/l

Chlorine Dose = 3.1 mg/l

Therefore, the correct chlorine dose needed to effectively treat the water while ensuring that the desired residual is achieved is 3.1 mg/l. This calculation confirms why the answer is appropriate for the problem presented.