Clean water is the one thing you cannot go long without, and it is also the supply most likely to be interrupted — by a burst main, a boil-water notice, flooding, or simply being out on the hills further from a tap than you meant to be. A water filter is cheap insurance against all of that, but the market is full of jargon that makes it hard to tell a genuine safeguard from a gimmick. This guide explains, in plain terms, what a water filter actually does, the difference between a filter and a purifier, and which type earns its place in a home kit, a car or a rucksack.
Filter or purifier — and does it matter here?
The two words are used loosely, but the distinction is real. A filter physically strains out particles, protozoa (like Cryptosporidium and Giardia) and bacteria — the things that cause most waterborne illness from streams and rivers. A purifier goes further and also deals with viruses, which are too small for most filters to catch. In the UK backcountry, viruses in water are rare, so a good filter is enough for stream and lake water on a walk. If you are collecting water from an unknown or contaminated source in a genuine emergency — floodwater, standing water, or anywhere sewage might be present — you want purification as well, which usually means chemical tablets or boiling on top of, or instead of, a filter.
What microns and log reduction actually mean
Two numbers matter. Pore size, in microns, tells you the biggest gap the filter lets through: 0.2 microns or smaller reliably stops bacteria and protozoa, and most reputable hollow-fibre filters sit around 0.1 to 0.2 microns. Log reduction is how manufacturers state effectiveness — a 'log 6' or '99.9999 per cent' reduction of bacteria means it removes all but one in a million. Look for something that quotes both a small micron rating and a high log reduction against bacteria and protozoa. Ignore vague claims like 'filters 99.9 per cent of impurities' with no organism or micron figure — that is marketing, not a spec.
The main types, and who each suits
Squeeze and straw filters are small, cheap and light — you drink through them or squeeze water through into a bottle. They are ideal for a rucksack, a grab bag or the car, and the popular hollow-fibre models will do thousands of litres before they need replacing. Gravity and pump filters move larger volumes and suit a family or a base camp: you fill a bag, hang it, and let clean water run into a container without effort. Bottle filters build the element into a flask so you fill from a tap or stream and drink filtered — convenient for everyday carry and travel. For the home, a filter plus a stock of purification tablets and a way to boil covers you across a boil-water notice, a supply failure and flooding alike.
Chemical tablets and boiling — the backup that never clogs
Purification tablets (chlorine dioxide or chlorine-based) are the cheapest, lightest and most foolproof water treatment there is, and they kill viruses that a filter may miss. The trade-offs are a wait of 30 minutes to a few hours and a faint taste, and they do nothing about cloudy or dirty water — you still need to filter or let sediment settle first. Boiling is the gold standard when you have the fuel: a rolling boil for one minute (three minutes at altitude) kills everything living in the water. A sensible UK home kit pairs a filter for grit and protozoa with tablets and the ability to boil for the belt-and-braces case. During an official boil-water notice, follow your water company's advice — usually a rolling boil, cooled before use.
Cold weather, maintenance and shelf life
Hollow-fibre filters are ruined if the water inside them freezes and cracks the fibres, and the damage is invisible — so never leave a used filter in a freezing car or porch over winter; store it indoors and dry it out after use. Back-flush squeeze filters as the maker directs to keep the flow rate up, and replace elements at the stated litre limit rather than waiting for them to clog. Purification tablets have an expiry date and lose strength over time, so check them when the clocks change and rotate old stock. None of this is onerous, but a filter you never maintain is one that lets you down on the day you finally need it.
What we'd buy
For most UK households: a compact hollow-fibre squeeze or straw filter for the grab bag and car, a tub of purification tablets as a virus-capable backup, and a water carrier to store treated water at home. If you regularly walk or camp, add a bottle or gravity filter sized to your group. Buy from recognised brands that publish micron ratings and independent test data, keep the filter from freezing, and rotate your tablets — do that and you have covered nearly every way a UK water supply can let you down. See the related gear below to build it out.
