In the tank disinfection problem, what units are used to express the target residual chlorine concentration?

• A. ppm
• B. percent
• C. mg/L
• D. grams per liter

Answer: (C)

The main idea is that residual chlorine concentration is a mass-per-volume measurement. The standard unit used here is milligrams per liter (mg/L). This directly tells you how much chlorine remains in each liter of water, which is exactly what you need to judge disinfection effectiveness and to perform dosing calculations. In practice mg/L and ppm convey the same value for water, since 1 mg/L equals 1 ppm, but problems typically specify the target in mg/L. Percent would describe a portion of a solution and isn’t used for residuals, and grams per liter would be far larger than typical residual levels.