Chlorine
The most widely used drinking water disinfectant, and the intervention most responsible for the decline of waterborne disease in the twentieth century. A residual concentration is deliberately maintained in the network to guard against contamination between the works and the tap.
Key facts
- Contaminant group
- Disinfection by-product
- World Health Organization — guideline
- 5 mg/lWHO notes this is conservative and that taste becomes objectionable well below it.
- United Kingdom — legal limit
- No numerical limit; managed operationallyAcceptability to consumers is the practical constraint.
- United States — legal limit
- 4 mg/l maximum residual disinfectant level
Where it comes from
- Deliberate dosing at the treatment works
- Booster dosing within the distribution network
How it behaves in water
Dissolves to form hypochlorous acid and hypochlorite, whose balance depends strongly on pH. Hypochlorous acid is the far more effective disinfectant, which is why chlorination works better in slightly acidic to neutral water than in alkaline water. The residual decays with time, temperature and organic load.
How it is measured
Measured as free and total chlorine in milligrams per litre, typically by the DPD colorimetric method. Taste and odour are detectable by many people well below any concentration of health concern.
Regulatory position
| Jurisdiction | Value | Status | Notes |
|---|---|---|---|
| World Health Organization | 5 mg/l | guideline | WHO notes this is conservative and that taste becomes objectionable well below it. |
| United Kingdom | No numerical limit; managed operationally | legal limit | Acceptability to consumers is the practical constraint. |
| United States | 4 mg/l maximum residual disinfectant level | legal limit | — |
Table sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- European Commission — Drinking Water Directive (EU) 2020/2184 and reporting. Open — attribution required · European Commission reuse policy (Decision 2011/833/EU)
- Drinking Water Inspectorate — Drinking water quality in England and Wales. Open — attribution required · Open Government Licence v3.0
- United States Environmental Protection Agency — Safe Drinking Water Information System. Open · US Government work — public domain
Health context
At the concentrations used in drinking water, chlorine itself is not a health concern for the general population. The disinfection by-products it forms are the subject of regulation, not the residual.
What removes it
| Row | Effective | Not reliably effective |
|---|---|---|
| 1 | Activated carbon filtration | Sediment filtration |
| 2 | Standing in an open container for several hours | Water softening |
| 3 | Boiling | — |
| 4 | Ascorbic acid, used in aquarium and brewing contexts | — |
Relationships
Related intelligence
Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- Peer-reviewed scientific literature — Open-access hydrology, water chemistry and water use research. Public but restricted · Publisher terms — public access, reuse not clearly granted