Make drinking water safe

When do I need to treat water?

If the water supply is interrupted (power outage, burst pipe) or your municipality has issued a boil notice, tap water must no longer be drunk without checking. In Switzerland, tap water is normally excellent – you only need to treat it if the supply is disrupted or the water is visibly or reportedly contaminated.

Important: boiling, filters and tablets eliminate pathogens – but NOT chemical pollutants or heavy metals. If there is suspected chemical contamination, only safe water from another source (bottled water, water tanker) helps. If in doubt about poisoning risk, Tox Info Switzerland provides information 24/7: 145.

This article is for general information and does not replace official instructions. In an event, always follow the announcements of your municipality and Alertswiss.

Filtration, disinfection, clarification – the terms

Three terms are often confused, but mean different things: clarification removes suspended solids (e.g. by cloth or settling), but does not make the water germ-free. Filtration mechanically retains particles and – depending on pore size – pathogens. Disinfection kills pathogens (boiling, chlorine, silver, iodine, UV).

Turbid water should first be clarified, then filtered or disinfected – suspended solids protect pathogens from the effect. The multi-barrier principle (pre-clarify → filter → disinfect) is the safest approach.

How do I purify drinking water in an emergency?

There are several proven methods. Which one is suitable depends on the pathogen, water quality and equipment available. The matrix below shows what works against what.

Boiling: bring to a rolling boil – at least 1 minute, longer at high altitude (CDC: above 2000 m at least 3 minutes; EPA recommends 3 minutes from 1000 m as a precaution – in case of doubt use the longer time). Kills bacteria, viruses and protozoa including Cryptosporidium. Does not work against chemicals, heavy metals, salt or turbidity.

Chlorine/bleach (unscented, 5–9%): about 2 drops per litre, allow at least 30 minutes to work; for turbid or cold water double the dose and time. Does not reliably work against Cryptosporidium.

Micropur Forte (Swiss product, silver + chlorine): 1 tablet per litre, about 30 minutes against bacteria and viruses, about 2 hours against Giardia. Also preserves stored water. Unreliable against Cryptosporidium.

Iodine (2%): about 5 drops per litre, about 30 minutes. Does not work against Cryptosporidium and is unsuitable for pregnant women, those with thyroid disease or long-term use.

Filters: micro/hollow-fibre/ceramic filters with 0.1–0.4 µm retain bacteria and protozoa but not viruses. Only ultrafilters or purifiers capture viruses; the ~0.02 µm quoted for this is a manufacturer specification – there is no official pore size for virus removal, the CDC names only reverse osmosis. Activated carbon improves taste and binds some chemicals but is not germ protection.

UV disinfection (e.g. SteriPEN): works against bacteria, viruses and protozoa including Cryptosporidium – but only in clear water, needs electricity and provides no residual protection.

SODIS (Eawag method): clear PET bottles of up to 2 litres in full sun for at least 6 hours; if cloud cover is more than 50%, expose them on two consecutive days. For clear water only – very turbid water above 30 NTU cannot be used for SODIS and must first be pre-clarified or filtered. Simple but slow.

Distillation: the only method that also works against salt and heavy metals and many chemicals – but laborious and energy-intensive.

Dosage: chlorine, silver and iodine

In chemical disinfection, dose and contact time determine effectiveness. The values below are guidelines – your product's package information is binding and takes precedence.

What should I watch for when filtering water?

A filter is practical because it needs no energy and no waiting time. What matters is pore size: standard filters (0.1–0.4 µm) retain bacteria and protozoa but not viruses. In Switzerland, where viruses in water are rarely the main problem, a good filter combined with boiling or tablets is usually sufficient.

Water storage

1. Store drinking water in food-safe, closable containers cool, dark and separate from chemicals
2. Unopened bottled water (six-pack) keeps for many months; self-filled water should be changed about every 6 months
3. Label containers with filling date and keep water for hygiene separate from drinking water
4. Don't forget house reserves: boiler and line water provide additional water in an emergency

Specific material and quantity examples

Plan for at least 3 litres per person per day for drinking and cooking; hygiene needs additional water. The federal government recommends at least 9 litres per person as emergency supplies.

1. Boiling: pot with lid + heat source (gas cooker with spare cartridges, as the cooker fails in a power outage)
2. Disinfection: Micropur or similar (Forte additionally contains chlorine for viruses/bacteria; Classic/silver mainly for storage) – package instructions are binding
3. Filter: hiking/outdoor filter with 0.1–0.4 µm or hollow-fibre drinking straw filter
4. Pre-clarification: coffee filter or clean cotton cloth for turbid water
5. Storage: food-safe, closable canisters/bottles, labelled with filling date

Common mistakes

1. Boil when chemical contamination is suspected – it does not remove chemicals, heavy metals or salts
2. Choose contact time for disinfection tablets too short
3. Re-contaminate treated water in unclean containers
4. Use filters for viruses when not designed for them
5. Rely only on chlorine tablets when Cryptosporidium is resistant to chlorine
6. Only pay attention to boiled drinking water but wash teeth or dishes with contaminated water

Frequently asked questions

Official sources

• SVGW Drinking water knowledge
• FONES Emergency supplies & water
• Tox Info Switzerland Emergency number 145 if poisoning suspected
• Guide: build emergency supplies Water is part of the week's supply
• Guide: survival times without water & food How long can humans survive without water?
• Scenario: water contamination Boil notice & behaviour
• Scenario: flooding Common cause of water contamination
• Emergency supply calculator How much water your household needs – calculate