What chlorination does
Adding free chlorine (Cl₂, or hypochlorite as a delivery form) to treated water destroys bacteria, viruses and protozoan parasites. The benchmark targets are coliforms, including E. coli, and waterborne pathogens such as cholera and typhoid that historically killed at scale before chlorination became standard practice in the early twentieth century. A second purpose is residual disinfection: a small concentration of chlorine (typically 0.2-1.0 mg/L at the tap) is maintained throughout the distribution network so that any pathogen entering downstream of the treatment plant — a cracked pipe, a back-siphon event — is inactivated before it reaches a consumer.
The regulated thresholds
- WHO guideline. Up to 5 mg/L free chlorine; no health-based guideline below that.
- U.S. EPA. Maximum residual disinfectant level (MRDL) of 4.0 mg/L; below detection at the tap is generally undesirable in the distribution system.
- EU Drinking Water Directive (recast 2020). Member states set residual levels nationally; typical operating range 0.2-0.5 mg/L.
- Health Canada. No MRDL but a guideline of 0.04-2.0 mg/L free chlorine residual throughout the distribution system.
Disinfection by-products
Free chlorine reacts with natural organic matter in source water to produce a family of by-products including trihalomethanes (THMs) and haloacetic acids (HAAs). These are regulated:
- Total THMs. U.S. EPA limit 0.080 mg/L (80 µg/L) as a locational running annual average. WHO sets individual guideline values for chloroform, bromodichloromethane, dibromochloromethane and bromoform.
- HAA5. U.S. EPA limit 0.060 mg/L (60 µg/L), same averaging basis.
Long-term studies (notably IARC's monograph on chlorinated drinking water) have looked at possible associations with bladder cancer. The position of the major public-health agencies is that the documented benefits of chlorination — preventing acute, mass-casualty waterborne disease — outweigh the modeled long-term risk at regulated by-product levels.
Alternatives and complements
Chloramine (chlorine + ammonia) is used in many large U.S. systems as a residual disinfectant because it produces fewer THMs than free chlorine. Ozonation and UV are common at the plant for primary disinfection, often followed by a small chlorine dose for residual. No single approach is universal; the choice depends on source water chemistry and the size and topology of the distribution system.
Sources
- World Health Organization, Guidelines for Drinking-water Quality (4th edition incorporating the 1st addendum).
- U.S. EPA, National Primary Drinking Water Regulations (40 CFR 141).
- Health Canada, Guidelines for Canadian Drinking Water Quality — Chlorine.
- IARC Monograph Vol. 52, "Chlorinated drinking-water; chlorination by-products."