How to choose a water filter for emergencies

What it is for

A portable water filter lets you make water from a doubtful source safer to drink when your mains supply is interrupted or you are away from home. In the UK, most households will rely on stored water and their water company’s advice first. A filter is a useful backup, not a replacement for a water store.

Before buying one, ask yourself:

• Do you already have enough stored water? If not, start there with our water storage calculator.
• Where would your water come from? Rain butts, streams and ponds carry different risks.
• How many people would you need to supply, and for how long?

Types

Personal straw filters

You drink directly through the filter. They are small, light and cheap, which makes them good for a grab bag. But they only serve one person at a time, and you cannot easily fill a pan or bottle with them.

Squeeze and bottle filters

You fill a soft pouch or bottle with dirty water and squeeze or drink it through the filter. They are still compact and can fill other containers, so they are more flexible than a straw.

Pump filters

You pump water through the filter by hand. They can draw from shallow sources and move a reasonable volume, but they have more moving parts that can wear or break.

Gravity filters

You hang a bag of dirty water above a clean container and let gravity do the work. They are good for groups and base camps, since they filter while you do other things, but they are bulkier.

Countertop and jug filters

Many kitchen jug filters are designed to improve the taste of water that is already safe mains water. Most are not designed to make untreated water safe. Check the specification carefully.

What to look for

What it removes

This is the most important thing to check. Look for clear claims, ideally backed by independent testing standards, for each of these:

Contaminant	Notes
Bacteria	Most portable filters claim to remove these.
Protozoa (parasites such as Giardia and Cryptosporidium)	Most portable filters claim to remove these.
Viruses	Viruses are much smaller. Many filters do not remove them. Look for a specific claim, or plan to boil or disinfect the water as well.
Chemicals	Activated carbon can reduce some chemicals and improve taste. No portable filter should be relied on for water that may contain fuel, chemicals or floodwater.

If a product does not say what it removes and to what standard, treat its claims with caution.

Pore size

Filters are often described by their pore size in microns. A smaller pore size stops smaller organisms. Look for the manufacturer’s stated pore size and whether it is “absolute” (the largest hole) or “nominal” (the average). Absolute figures are more meaningful.

Capacity

Capacity is how many litres the filter can treat before it needs replacing. Compare this with your household’s needs. A household of four using 4 litres per person a day needs 16 litres a day, so a filter rated for a few hundred litres would last only a few weeks of full-time use.

Flow rate

How quickly it filters, usually in litres per minute or per hour. A slow filter is fine for one person, but frustrating for a family.

Maintenance

• Can the filter be back-flushed or cleaned to restore its flow?
• Are replacement cartridges easy to buy?
• Does it show when it has reached the end of its life?

Freezing

Many hollow-fibre filters can be damaged if water freezes inside them, and the damage may not be visible. If you store a filter somewhere cold, check the manufacturer’s advice.

Shelf life

Check how long an unused filter can be stored, and whether it has an expiry date.

Who each type suits

Situation	Types that may suit
A grab bag for one person	Straw or squeeze filter
A family at home during a water outage	Gravity filter, plus a way to boil water
Walking or camping	Squeeze or pump filter
Improving the taste of safe mains water	Jug filter (not for untreated water)

Limitations

• Most portable filters do not remove all viruses.
• Filters do not make water with chemicals, fuel or floodwater safe.
• Cloudy water clogs filters quickly. Settle and strain it first.
• A filter does not replace the water company’s advice. If there is a “do not drink” notice, do not drink the water even if filtered.

For the safest results, combine filtering with boiling or purification tablets. Our guide to emergency water purification explains how.

Price bands

These bands are our own rough estimates to help you set a budget. They are not the result of a price survey, and prices change, so check current retailers before you buy.

Type	Typical price band
Straw filter	Low (often under £30)
Squeeze or bottle filter	Low to mid (roughly £20 to £60)
Pump filter	Mid to high (roughly £50 to £150 or more)
Gravity filter	Mid to high (roughly £50 to £150 or more)

Replacement cartridges add ongoing cost. Factor them in when you compare products.

Care and storage

• Read and keep the instructions with the filter.
• Store it clean, dry and in its packaging until needed.
• Keep it somewhere that will not freeze.
• Once used, keep “dirty” and “clean” sides separate to avoid recontaminating your water.
• Label it with the date you bought it and its rated capacity.

Checklist before you buy

• Clear claims about what it removes, including viruses
• Stated pore size and testing standard
• Capacity that matches your household’s needs
• A flow rate you can live with
• Replacement parts are easy to get
• Storage and freezing advice from the manufacturer

Sources and further reading

• Guidelines for drinking-water quality: small water supplies (World Health Organization)
• Boil water (World Health Organization)