Operator terms · Water and wastewater
What is chlorination?
Chlorination is adding chlorine to water to kill disease-causing organisms. It is the most common disinfection method in water and wastewater, done with chlorine gas, liquid bleach, or calcium hypochlorite, and it leaves a residual that keeps protecting the water downstream.
Last reviewed 2026-08-03
How it is done
Chlorine is added as gas, as liquid sodium hypochlorite, or as solid calcium hypochlorite. It must have enough contact time in the water to inactivate organisms before the water reaches the customer.
Why a residual matters
Chlorination is used because it leaves a lasting residual. That leftover chlorine keeps working in the distribution system, so contamination that enters after treatment still meets a disinfectant.
Common questions
- What does chlorination do?
- It disinfects water by killing disease-causing organisms.
- What forms of chlorine are used?
- Chlorine gas, liquid sodium hypochlorite, or solid calcium hypochlorite.
- Why leave a residual?
- So the water stays protected as it moves through the distribution system.