Operator terms · Water and wastewater
What is chlorine demand?
Chlorine demand is the amount of chlorine the water consumes as it reacts with organisms and other material. It is the chlorine used up before any residual is left. Dose equals demand plus residual, so knowing the demand tells an operator how much chlorine to add.
Last reviewed 2026-08-03
Part of the dose equation
Dose equals demand plus residual. The demand is consumed first; whatever is left is the residual that proves disinfection is working.
Why it varies
Dirtier water with more organics and organisms has a higher chlorine demand, so it needs a higher dose to leave the same residual.
Common questions
- How does demand relate to dose?
- Dose equals demand plus residual, all in mg/L.
- What raises chlorine demand?
- More organic matter and organisms in the water.
- Is demand the same as residual?
- No. Demand is consumed; residual is what remains after it.