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Water filters
A filter turns questionable water into drinking water, with no fuel. They are not all the same, and the differences are the whole point.
By Walter Wolf · Updated November 2026
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In short
- Stored water comes first. A filter is the backup for when it runs low.
- Most filters remove bacteria and parasites. Only purifiers also deal with viruses.
- None of them remove fuel or most chemicals.
- For a household, a gravity filter is far easier than a straw.
- If a hollow-fibre filter freezes after use, it is ruined and you cannot tell by looking.
What is in the water
| Problem | Filter (0.1 to 0.2 micron) | Purifier | Boiling |
|---|---|---|---|
| Parasites such as Giardia | Yes | Yes | Yes |
| Bacteria such as E. coli | Yes | Yes | Yes |
| Viruses | No | Yes | Yes |
| Chemicals, fuel, heavy metals | No | Mostly no | No |
In the back country, parasites and bacteria are the main risk and a filter is enough. In a town after a sewage backup or flood, viruses matter too, so use a purifier, or filter and then boil or disinfect. If officials warn of chemical contamination, no home method fixes it. Use stored water.
The types
- Squeeze and straw filters. Small, cheap, light. Good for one person or a go-bag. Slow for a family.
- Gravity filters. Hang a bag of dirty water and clean water runs out the bottom. No pumping, several litres at a time. The best fit for a household.
- Purifier bottles and pumps. Remove viruses as well. Cost more, and cartridges need replacing sooner.
- Tablets and drops. Chlorine dioxide treats viruses, bacteria and parasites with enough waiting time. Tiny, long shelf life, good backup to a filter.
Picks
Each button goes straight to that product on Amazon.
Sawyer Squeeze (SP129)
A 0.1 micron hollow-fibre filter that comes with two squeeze pouches and also fits standard bottles. Cleaned by backflushing, and rated for a very large volume of water. The best value here.
Platypus GravityWorks 4-litre
A two-bag gravity system that filters 4 litres in about two and a half minutes with no pumping. The household choice. The cartridge is good for about 1,500 litres.
LifeStraw Personal
A simple straw for drinking directly from a container. Cheap enough to put one in every go-bag and the car.
Grayl GeoPress 24 oz
A press-style purifier bottle that handles viruses as well as bacteria and parasites. Good for one or two people where the water may be contaminated with sewage. Each cartridge treats about 250 litres.
Katadyn Micropur MP1 tablets, 30 count
Chlorine dioxide tablets, individually sealed, with a five-year shelf life. One tablet treats one litre. Waiting time runs from 15 minutes up to 4 hours for cold or dirty water, so read the pack.
The cold-country problem
- Keep a used filter inside the heated room, or in an inside pocket.
- A brand-new filter that has never been wet can be stored cold.
- If a used filter may have frozen, replace it, or boil everything.
- Tablets are not harmed by freezing, which makes them a good winter backup.
Using one well
- Start with the cleanest water you can find. Let muddy water settle and pour it through a cloth first, or the filter will clog.
- Keep dirty and clean containers separate and labelled.
- Backflush as the maker says when the flow slows.
- Practise once before you need it.
What about big countertop filters?
Large stainless gravity units are popular with preppers. Some make claims that have not been independently certified. If you buy one, look for testing to recognised standards such as NSF/ANSI 53 or P231, from an independent lab, for the exact model.
Sources
Official guidance this page was checked against. If your local authority says something different, follow them.