Chlorine demand is defined as which of the following?

Chlorine demand is defined as which of the following?

• A. Residual minus applied dose
• B. Applied dose minus residual
• C. Free chlorine concentration
• D. The maximum allowed dose

Answer: (B)

Chlorine demand is the amount of chlorine that gets used up by reacting with substances in the water. It’s found by subtracting the chlorine residual that remains after a contact time from the chlorine that was added. In other words, it’s the difference between the applied dose and the residual chlorine. For example, if you add 5 mg/L of chlorine and 2 mg/L remains after mixing, the chlorine demand is 3 mg/L—the amount consumed to react with contaminants and other substances. The other ideas don’t capture this consumed portion: residual chlorine isn’t the amount used up, free chlorine concentration is simply what’s left, and the maximum allowed dose is a regulatory limit, not the consumed amount.

Chlorine demand is the amount of chlorine that gets used up by reacting with substances in the water. It’s found by subtracting the chlorine residual that remains after a contact time from the chlorine that was added. In other words, it’s the difference between the applied dose and the residual chlorine. For example, if you add 5 mg/L of chlorine and 2 mg/L remains after mixing, the chlorine demand is 3 mg/L—the amount consumed to react with contaminants and other substances. The other ideas don’t capture this consumed portion: residual chlorine isn’t the amount used up, free chlorine concentration is simply what’s left, and the maximum allowed dose is a regulatory limit, not the consumed amount.