Operator terms · Water and wastewater
What is chlorine residual?
Chlorine residual is the amount of chlorine still active in the water after some has been used up killing organisms and reacting with material. A measurable residual proves disinfectant is present all the way to the customer. Dose equals demand plus residual: what you add, minus what is consumed, is what remains.
Last reviewed 2026-08-03
Dose, demand, and residual
Chlorine demand is what the water consumes reacting with organisms and material. Whatever is left over is the residual. Dose equals demand plus residual, which is the key disinfection relationship.
Why it is measured
A residual carried through the distribution system shows that disinfectant is still working when the water reaches the customer, not just at the plant.
Common questions
- What is the dose formula?
- Dose equals demand plus residual, all in mg/L.
- Why keep a residual?
- It proves disinfectant is present all the way to the tap.
- What is chlorine demand?
- The chlorine consumed reacting with organisms and material in the water.