After 24 hours, a chlorine residual of at least how many milligrams per liter must remain in the water?

• A. 5 mg/L
• B. 10 mg/L
• C. 15 mg/L
• D. 20 mg/L

Answer: (B)

Chlorine residual is the amount of free chlorine still present in water to keep it disinfected over time. The question uses a 24-hour time frame to check that the disinfectant level remains above a protective threshold as water sits in the system. In this context, the minimum that must remain after 24 hours is 10 mg/L. This level provides enough remaining disinfectant to counter decay and potential microbial regrowth during storage and distribution. A lower value like 5 mg/L could drop too low after a day, compromising safety, while higher values such as 15 or 20 mg/L are more than necessary for this scenario and could cause taste, odor, or corrosion issues.