Drinking water standards and WHO guidelines

Water quality standards for drinking water

Water quality standards are the maximum (or minimum) permissible values of the physical, chemical and biological characteristics of drinking water that make it safe for human consumption. They are set by law so that water supplied to the public does not harm health and is acceptable to the user. In Nepal, the National Drinking Water Quality Standards (NDWQS) 2062 BS apply; many countries follow the WHO guidelines.

Examples (Nepal standard): turbidity 5 NTU (maximum 10), pH 6.5 to 8.5, TDS 1000 mg/l, hardness 500 mg/l, chloride 250 mg/l, iron 0.3 mg/l, arsenic 0.05 mg/l, nitrate 50 mg/l, residual chlorine 0.1 to 0.2 mg/l, and E. coli 0 per 100 ml.

Significance of WHO guidelines

1. Scientific basis: The WHO "Guidelines for Drinking-water Quality" give health-based guideline values drawn from toxicology and epidemiology reviews and are updated periodically.
2. Basis for national standards: Countries, including Nepal, adapt them to local conditions, cost and risks.
3. Public health protection: Guideline values for E. coli (zero per 100 ml), arsenic (0.01 mg/l), fluoride (1.5 mg/l), nitrate (50 mg/l), lead (0.01 mg/l) and other substances help prevent acute and chronic disease.
4. Risk-based approach: They promote Water Safety Plans, covering catchment to tap, instead of only end-product testing.
5. Guides for monitoring and treatment: They help in selecting treatment, monitoring frequency and emergency response.
6. Basis for comparison and planning: They permit comparison of supplies and set targets for developing countries.