Treatment process
Chlorination
The intervention most responsible for the collapse of waterborne disease in the twentieth century. Chlorine kills most pathogens quickly and, uniquely among common disinfectants, leaves a residual that protects water all the way to the tap.
Key facts
- Scale
- Municipal and household
- Treatment stage
- Disinfection
How it works
Chlorine gas, sodium hypochlorite or on-site generated hypochlorite is dosed to achieve a target contact time multiplied by concentration. Hypochlorous acid is the active species, and it dominates below about pH 7.5.
What it removes — and what it does not
| Row | Removes | Does not remove |
|---|---|---|
| 1 | Bacteria including E. coli, Salmonella, Vibrio cholerae | Cryptosporidium, at drinking water doses — the critical limitation |
| 2 | Most viruses at adequate contact time | Dissolved chemicals, metals, nitrate or PFAS |
| 3 | Provides ongoing protection within the distribution network | Turbidity, which actively shields organisms from it |
Trade-offs
- Forms trihalomethanes and other by-products by reacting with organic matter
- Efficiency falls sharply as pH rises
- Taste and odour complaints well below health-relevant concentrations
- The by-product risk is small compared with the risk of not disinfecting, and every regulator has reached that conclusion
Related intelligence
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Sources
- World Health Organization — Guidelines for Drinking-water Quality. Public but restricted · CC BY-NC-SA 3.0 IGO
- NSF International / American National Standards Institute — NSF/ANSI drinking water treatment unit standards. Public but restricted · Publisher terms — public access, reuse not clearly granted
- American Water Works Association — AWWA standards, water audits and utility benchmarking. Public but restricted · Publisher terms — public access, reuse not clearly granted