Design the treatment train to potable
No single device makes water safe. WHO promotes managing risk from catchment to consumer, and practical home systems stack several barriers so one failure does not make anyone ill.
How
- Test the raw water first (water-20) and design for what is actually in it. CDC advises choosing treatment that removes the specific contaminants present.
- Stage 1, sediment: a typical rainwater train in the Texas manual uses a 5 micron sediment filter, then a 3 micron activated carbon filter for taste and some chemicals.
- Stage 2, cysts and bacteria: Giardia and Cryptosporidium need an NSF certified absolute 0.85 micron filter (Texas manual) or a treatment proven to inactivate them.
- The CDC Yellow Book says filters under 1 micron remove bacteria and protozoa but do not reliably remove viruses. Ultrafilters of about 0.01 micron also remove viruses. Check this rating on ceramic filters too.
- Stage 3, disinfection by UV: use a unit certified to NSF/ANSI 55 Class A (at least 40 mJ/cm2) with an alarm or automatic shutoff. Ohio guidance advises prefiltration when turbidity is 5 NTU or more, and pretreatment for iron (0.3 ppm or more) and hardness (103 ppm or more), because particles and deposits block UV.
- Or disinfection by chlorine. Chlorinate only clear water, below 5 NTU turbidity (Sphere, WHO Technical Note 11). Above that, settle or filter first.
- Chlorine target: at least 0.5 mg/L free chlorine left after at least 30 minutes contact, at pH below 8. This is the WHO benchmark, quoted by the UK Drinking Water Inspectorate.
- Colder water needs longer contact (WHO Technical Note 11). At the tap, keep 0.2 to 0.5 mg/L free chlorine (Sphere, WHO Technical Note 11).
- Never exceed 5 mg/L free chlorine. That is the WHO guideline value, and most people can taste chlorine at that level (WHO).
- Measure, do not guess. Check free chlorine with a DPD test kit after the contact time and at the tap (WHO Technical Note 11). The dose alone does not tell you what is left.
- Dose example only: the Texas manual adds about 59 mL of 6 percent bleach per 3,785 L (2 fl oz per 1,000 US gal). Its 2 to 5 minute contact time assumes a 2 ppm residual, so follow the 30 minute rule above.
- NC State suggests a 3 to 5 ppm residual for continuous chlorination; stay under the 5 mg/L ceiling. Size any dose with the chlorine dose calculator on this page.
- Chlorine does not reliably kill Cryptosporidium (CDC), so keep a cyst filter stage even when you chlorinate.
- Treat only the potable line if you have a dual system. Note that boiling and chlorine do not remove metals, salts or most chemicals (US EPA), so a chemical problem such as nitrate or arsenic needs a specific treatment chosen from test results.
Safety: Store bleach and other chemicals away from children and never mix chlorine products with acids or ammonia.
Sources
- The Texas Manual on Rainwater Harvesting, 3rd edition. Texas Water Development Board, accessed 2026-11
- Collecting Rainwater and Your Health: An Overview. US Centers for Disease Control and Prevention (CDC), accessed 2026-11
- How to Make Water Safe in an Emergency. US Centers for Disease Control and Prevention (CDC), accessed 2026-11
- Emergency Disinfection of Drinking Water. US EPA, accessed 2026-11
- Ultraviolet (UV) disinfection fact sheet. Greene County Public Health, Ohio, accessed 2026-11
- Guidelines for drinking-water quality, 4th edition, incorporating the 1st addendum. World Health Organization, accessed 2026-11
- Sphere Handbook, Water supply standard 1: access and water quantity (interactive edition). Sphere Association, accessed 2026-11
- How to ensure sufficient disinfection (private water supplies treatment guide, citing the WHO chlorine benchmark). Drinking Water Inspectorate (UK), accessed 2026-11
- Technical Note 11: Measuring chlorine levels in water supplies. World Health Organization and WEDC, Loughborough University, accessed 2026-11
- Chlorine in drinking-water: chemical fact sheet (2022). World Health Organization, accessed 2026-11
- CDC Yellow Book: Water disinfection for travelers. US Centers for Disease Control and Prevention (CDC), accessed 2026-11