An increase in which substance will raise chlorine demand?

• A. Organic matter
• B. Temperature
• C. Turbidity
• D. Alkalinity

Answer: (A)

Chlorine demand is the amount of chlorine that gets consumed by reactions with substances in the water before any free chlorine remains. Organic matter is oxidizable by chlorine, so when more organic matter is present, more chlorine is used up in those reactions and a larger residual must be added to achieve disinfection. Temperature, turbidity, and alkalinity can influence how chlorine behaves, but they don’t introduce oxidizable material themselves in the same direct way; the primary driver of higher chlorine demand is increased organic matter.