To make water safe to drink in an emergency, the most reliable method is to bring it to a rolling boil for at least one minute. If you cannot boil it, use purification tablets or a certified filter, and always start from the clearest water you can find. Sweden's Agency for Civil Defence (MCF, formerly MSB) recommends every household be able to manage at least one week on its own, and water is the first thing to secure. Here is how to treat it safely.
Reviewed by Oskar Bjork, defence engineer (Swedish Armed Forces) and adviser to Kapsel. Last updated 2026-06-23.
How much water you need
Plan for at least 3 litres of drinking water per person per day, and 3 to 5 litres including hygiene. Over a week that is roughly 20 to 35 litres per person. Store as much as you reasonably can, and keep a way to treat more, because stored water runs out faster than people expect.
The three reliable methods
1. Boiling, the most reliable
- Bring water to a rolling boil and keep it there for at least one minute (three minutes at high altitude).
- Boiling kills bacteria, viruses and parasites. Let it cool and store it in a clean, covered container.
- Boiling does not remove chemicals, fuel or heavy metals, so start from a clean source.
2. Purification tablets
- Chlorine dioxide or chlorine tablets are compact and ideal when you cannot boil.
- Follow the dose and the wait time on the pack, usually around 30 minutes, longer if the water is cold or cloudy.
3. A certified filter
- A microbiological filter removes bacteria and protozoa. Check whether it also handles viruses if you need that.
- Pre-filter cloudy water through a clean cloth first so the filter lasts longer.
Start from the clearest water
Whatever method you use, begin with the cleanest water available. Let sediment settle and pour off the clear water, or strain it through a cloth. Cleaner input gives a safer result and makes filters and tablets work better.
Common mistakes to avoid
- Treating water you do not need to. If the mains supply is still safe, use it.
- Using water from pools, radiators, or anything that may contain chemicals or fuel.
- Assuming boiling fixes everything. It kills microorganisms but does not remove chemical contamination.
- Storing treated water in a dirty container, which can recontaminate it.
Where this fits in your preparedness
Water is one of the five basics, alongside heat, light, communication and food. Build it into your kit from our complete emergency-kit checklist and plan your amounts with our food and water stores for a week. For the food side, light, long-life options like freeze-dried food pair well with treated water. The Kapsel Core covers water in its Water and Health capsule, with a filter and purification built in, so there is less to source and track. See the Kapsel Core.
Frequently asked questions
How long should I boil water to make it safe?
Bring it to a rolling boil for at least one minute, or three minutes at high altitude. That kills bacteria, viruses and parasites.
Are water purification tablets safe?
Yes, when used as directed. Follow the dose and the wait time on the pack, and allow longer if the water is cold or cloudy.
Does boiling remove all contamination?
No. Boiling kills microorganisms but does not remove chemicals, fuel or heavy metals. Start from the cleanest source you can and avoid contaminated water.
How much water should I store per person?
At least 3 litres per person per day for drinking, and 3 to 5 litres including hygiene. Over a week that is roughly 20 to 35 litres per person.
Sources: MCF (mcf.se), Livsmedelsverket, krisinformation.se.
Purifying water matters most during a water shortage or irrigation ban, when supply is cut or quality drops.