What target disinfection dose (mg/L) was used in the calculation?

• A. 50.0 mg/L
• B. 25.0 mg/L
• C. 75.0 mg/L
• D. 100.0 mg/L

Answer: (A)

The idea being tested is choosing the target disinfectant dose that you plan to apply in the calculation to achieve the needed inactivation under the given conditions. The target dose sets the starting point for how much disinfectant you’re aiming to deliver, before accounting for chlorine demand, temperature, pH, and contact time.

Fifty milligrams per liter is used as the target dose because it provides a conservative, practical starting point that helps ensure adequate disinfection even after the water’s chlorine demand is met and under typical operating conditions. It balances effectiveness with reasonable residuals and byproduct considerations.

A smaller dose, like twenty-five mg/L, might not achieve the required inactivation once demand and system conditions are considered. A larger dose, such as seventy-five or one hundred mg/L, would generally be more than necessary for this scenario and could unnecessarily raise taste/odor issues and disinfection byproducts while not providing additional meaningful benefit for the calculation at hand.