Operator terms · Water and wastewater
What is combined chlorine?
Combined chlorine is chlorine that has bonded with ammonia in the water to form chloramine. It disinfects more slowly than free chlorine but lasts longer, which makes it useful for keeping a residual far out in the distribution system.
Last reviewed 2026-08-03
How it forms
When chlorine meets ammonia in the water, the two combine into chloramine. That bonded form is combined chlorine. Some systems add ammonia on purpose to build a lasting chloramine residual.
Why systems use it
Combined chlorine is weaker per unit than free chlorine but does not fade as fast, so it holds a measurable residual at the far ends of the distribution system where free chlorine would be gone.
Common questions
- What is chloramine?
- The compound formed when chlorine combines with ammonia; it is combined chlorine.
- Why use combined chlorine?
- It lasts longer in the mains, holding a residual farther from the plant.
- Is combined chlorine part of total chlorine?
- Yes; total chlorine is free plus combined.