Water · 8 min read
Water filter vs purifier: what actually makes water safe in an emergency
Filters remove bacteria and protozoa; viruses and chemicals need something more. Here is how to combine filtering, boiling and tablets without over-buying.
Updated 20 August 2026
Key takeaways
- A hollow-fibre filter of 0.1-0.2 microns removes bacteria and protozoa but not viruses.
- Boiling for one minute at a rolling boil is still the most reliable treatment when you have fuel.
- Chlorine dioxide tablets cover viruses and store for years — the ideal backup to a filter.
- Pre-filter cloudy water through cloth first; sediment ruins filter flow rate faster than anything else.
The three threats, and what beats each one
Emergency water carries three categories of risk: biological contamination, chemical contamination, and sediment. No single lightweight device handles all three, which is why serious kits carry a filter and a chemical treatment rather than one 'purifier'.
Mechanical filters trap anything bigger than their pore size — that covers protozoa such as Giardia and Cryptosporidium and bacteria such as E. coli. Viruses are smaller and pass through most hollow-fibre filters, so they need chemical treatment, UV or heat.
- Bacteria and protozoa: 0.1-0.2 micron filter, boiling, or tablets
- Viruses: boiling, chlorine dioxide tablets, or a certified purifier
- Chemicals and taste: activated carbon stage, or avoid the source
- Sediment: cloth or coffee-filter pre-filter before anything else
Where your water will actually come from
In a European water interruption, the realistic sources are stored water at home, a tanker or standpipe set up by the utility, rainwater, and finally surface water from a stream or pond. The first two need no treatment; rainwater needs filtering because of roof debris; surface water needs the full sequence.
Plan storage first. Two litres per person per day for drinking, plus roughly the same again for cooking and basic hygiene, means a household of four needs around 50 litres for three days. That is a stack of containers, not a couple of bottles.
The survival scenario: a burst main in winter
Taps run dry with no warning and the repair takes two days. Toilets are the first surprise — each flush is several litres you no longer have. Households that cope have containers filled in advance and a rule that stored water is for drinking and cooking only.
The moment a warning appears — storm, flood, planned works — fill everything: containers, bottles, the bath, cooking pots. Water already in the house needs no filter and no fuel, which makes it the cheapest preparation you can make.
- Fill containers and the bath at the first warning
- Keep one filter accessible rather than packed deep in a bag
- Store tablets as the long-life backup — they outlast filter cartridges
- Label containers with the fill date and rotate every six months
Frequently asked questions
Does a water filter remove viruses?
Most hollow-fibre filters at 0.1-0.2 microns do not. Pair the filter with chlorine dioxide tablets or boiling if the source could contain sewage or run-off.
How much water should I store per person?
Plan two litres per person per day for drinking and a similar amount for cooking and hygiene. For a household of four over three days that is roughly 50 litres.
How long do purification tablets last?
Chlorine dioxide tablets typically keep for several years in sealed foil, which makes them the best long-term backup to a filter whose cartridge can clog or freeze.
Water gear in stock
Dispatched from EU warehouse, delivered in 5-8 days.
Ready-made kits for this
Prefer not to shop item by item? These kits already cover the gear in this guide, built from EU stock and priced by budget.
- Apartment · StarterApartment starterLight, cheap, fits a hallway cupboardView kit
- House · ReadyHouse readyOutage, storm or water cut for several daysView kit
- On the go · ReadyGo-bag ready72-hour evacuation bagView kit