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Keep a disinfection quick reference and check chlorine with a DPD kit

Disinfection numbers are spread across this module. Keep them on one card by the kitchen tap, and measure chlorine instead of trusting the dose.

How

  1. Boil: a 1 minute rolling boil; 3 minutes above 1,000 m covers both CDC and US EPA (water-25).
  2. Bleach for drinking water: 2 drops per litre of 5 to 9 percent bleach, or 10 drops per litre of 1 percent bleach. Wait at least 30 minutes (CDC).
  3. Routine chlorination: at least 0.5 mg/L free chlorine left after 30 minutes at pH below 8 (WHO benchmark, UK DWI). Keep 0.2 to 0.5 mg/L at the tap (Sphere).
  4. Ceiling: never above 5 mg/L free chlorine in drinking water (WHO).
  5. Shock dose for new or contaminated tanks and pipes: about 200 mg/L, held at least 12 hours, then flushed (water-19). Do not drink shock-dosed water.
  6. UV: NSF/ANSI 55 Class A units only, after prefiltration (water-16).
  7. DPD test: put a tablet in the test chamber with a few drops of treated water. Crush it, then fill the chamber (WHO Technical Note 11).
  8. Compare the colour with the chart on the comparator to read free chlorine in mg/L (WHO Technical Note 11).
  9. Test after the full contact time, and again at the tap the household drinks from.
  10. Use the chlorine dose calculator for any volume, target dose and bleach strength.

Safety: Store bleach and DPD tablets away from children. Never mix chlorine products with acids or ammonia.

Do these first

Sources

  1. How to Make Water Safe in an Emergency. US Centers for Disease Control and Prevention (CDC), accessed 2026-11
  2. Emergency Disinfection of Drinking Water. US EPA, accessed 2026-11
  3. Sphere Handbook, Water supply standard 1: access and water quantity (interactive edition). Sphere Association, accessed 2026-11
  4. How to ensure sufficient disinfection (private water supplies treatment guide, citing the WHO chlorine benchmark). Drinking Water Inspectorate (UK), accessed 2026-11
  5. Technical Note 11: Measuring chlorine levels in water supplies. World Health Organization and WEDC, Loughborough University, accessed 2026-11
  6. Chlorine in drinking-water: chemical fact sheet (2022). World Health Organization, accessed 2026-11
  7. Standards for Water Treatment Systems. NSF, accessed 2026-11
  8. Protecting Water Supply Springs. NC State Extension, accessed 2026-11

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