The government's Be Ready guidance puts stored bottled water first. A filter or a pack of tablets is a back-up, for water that might not be clean. Each one deals with some things and not others, as the table shows.
Buying through our links costs you nothing extra. Some of them earn us a small commission, and that's what keeps Survival Bunker going: it pays for the time it takes to keep our picks and figures up to date. Thank you for helping. How we choose and how we earn.
If there's a problem with tap water, your water company issues one of three notices, and the notice decides what helps.
In an outage, Be Ready says boiling or purification tablets can make other water safer to drink. Water-butt water is for the garden and the toilet, not for drinking.
| Method | Bacteria | Parasites (Giardia, Cryptosporidium) | Viruses | Chemicals and metals |
|---|---|---|---|---|
| Boiling (a rolling boil) | Yes | Yes | Yes | No |
| Microfilter (most portable filters, such as LifeStraw and Sawyer) | Yes | Yes | No | No |
| Purifier (such as GRAYL) | Yes | Yes | Yes, maker's claim | Some, not all; not salt |
| Chlorine tablets | Yes | Some (Aquatabs claims Giardia) | Yes | No |
| Chlorine dioxide tablets | Yes | Better than chlorine | Yes | No |
Filters and tablets are the makers' claims for their own products, and they differ by model. Sources: LifeStraw, Sawyer, the UK's travel health service (NaTHNaC), UKHSA.
A microfilter strains out bacteria and parasites through very fine holes, but viruses are small enough to pass through. Most portable filters, including the LifeStraw Peak and the Sawyer Squeeze, are microfilters. A purifier also removes viruses, and some reduce certain chemicals. Each has a stated range: GRAYL, for example, lists salty water, industrial or nuclear incidents and blue-green algae as outside what its purifier is for. None of the filters here removes salt.
A tiny micron figure on the box is about germs, not chemicals. Sawyer says its Squeeze doesn't remove metals, chemicals, pesticides or salt.
Each pack gives its dose by volume and a waiting time; tablets work best in clear water, and some packs give longer times for cold or cloudy water. Shelf life differs: Lifesystems gives up to five years unopened for its chlorine tablets and two for chlorine dioxide, and Aquatabs prints the expiry on the pack.
Filters and cartridges have a stated life (up to 2,000 litres for the LifeStraw Peak SOLO, about 250 litres or three years for a GRAYL cartridge), and each maker gives its own cleaning and replacement schedule. Sawyer says to replace a Squeeze that may have frozen while wet.
The portable filters here are built for one person outdoors, a bottle at a time. For a household, stored water is the main thing in the government's guidance, and a pack of tablets is a cheap back-up. How much water to store.
Prices checked 4 November 2026. Specs are the makers' published figures.
A small, light microfilter that screws onto most bottles. It removes bacteria and parasites, but not viruses or chemicals. A good, cheap back-up for one person; not a household supply.
A long-standing microfilter you squeeze water through from a pouch or bottle. Bacteria and protozoa, not viruses, metals, chemicals or salt. This shop ships from the Netherlands, adding £9.95; Sawyer Europe's own shop was sold out when we checked.
A purifier: you press the water through the cartridge, and GRAYL says it removes viruses as well as bacteria and cysts. It needs a firm press, and it was sold out on GRAYL's UK shop when we checked.
Chlorine tablets, widely used and cheap. For clear water; the dose depends on how much water and how risky the source, set out in a chart on the pack, with a 30-minute wait.
Cheap chlorine tablets, one per litre (60 litres a pack). Chlorine dioxide does more against parasites. The pack gives the waiting time.
Chlorine dioxide, which works better against parasites than chlorine; Lifesystems says it treats Cryptosporidium too. Dearer, with a shorter shelf life, and the waiting time varies with the water, as the pack sets out.