What dosage is required if the chlorine demand is 2.3 mg/l and the desired chlorine residual is 0.2 mg/l?

To determine the required dosage of chlorine, you need to consider both the chlorine demand and the desired chlorine residual. The chlorine demand represents the amount of chlorine that is needed to react with contaminants and to achieve disinfection. After satisfying this demand, any additional chlorine will remain as a residual, which is important for ongoing disinfection.

In this case, the total dosage required can be calculated by adding the chlorine demand to the desired residual. The chlorine demand is 2.3 mg/l, and the desired residual is 0.2 mg/l. When you sum these two values (2.3 mg/l + 0.2 mg/l), you get a total dosage requirement of 2.5 mg/l.

Therefore, the correct dosage needed to achieve the desired residual while satisfying the chlorine demand is indeed 2.5 mg/l. This amount ensures that there is sufficient chlorine not only to meet the needs of disinfection but also to maintain a safe and effective level of chlorine in the water system.