Initially, the main should be treated to achieve a chlorine residual of at least how many milligrams per liter?

• A. 10 mg/L
• B. 50 mg/L
• C. 5 mg/L
• D. 25 mg/L

Answer: (D)

Maintaining a chlorine residual from the start is essential to keep water disinfected as it moves through the main. The idea is to dose enough chlorine at the outset so the chlorine demand of the water and the pipe network is met and a protective free chlorine level remains by the time the water reaches users. In this item, the minimum target for the main is 25 mg/L. This level provides a buffer against consumption by organics, biofilms, and other reactions in the system, helping to ensure ongoing disinfection. Lower residuals, like 5 or 10 mg/L, are more likely to drop below protective levels before reaching customers, while a very high residual such as 50 mg/L isn’t typically necessary and could cause taste or odor issues. So, 25 mg/L is the best balance to maintain disinfection throughout the distribution.