How to purify water in an emergency: safe UK methods

Start with the contamination, not the kit

There is no single thing called "purifying" water. The right method depends entirely on what you are trying to remove, and the honest starting point is that most people reading this in the UK do not have a water problem at all — mains tap water is treated to a high standard and needs no further treatment.

Emergencies split water risks into two families. The first is microbiological: bacteria, viruses and parasites such as Cryptosporidium, which cause diarrhoea and sickness. This is what a boil-water notice is about, and it is what boiling, tablets and filters are designed to deal with. The second is chemical: nitrates, heavy metals like lead, pesticides, or fuel and solvent spills. No home method fixes those, and trying to treat them can make matters worse.

So before choosing a method, decide which problem you actually have. If a water company or council has issued guidance, follow it — it tells you which family you are dealing with and what to do.

Boiling: the method the NHS and DWI actually recommend

For microbiological contamination, boiling is the gold standard, and it is what UK guidance points to first. The Drinking Water Inspectorate's instruction is simple: bring the water to a rolling boil, remove it from the heat, and let it cool naturally. As it notes, most bacteria and viruses are killed well below 100°C, and boiling is also effective against parasites such as Cryptosporidium — the one that chemical tablets struggle with.

Store the cooled water in a clean, covered container in the fridge and discard anything unused after 24 hours. Use boiled-then-cooled water for drinking, brushing teeth, making ice, preparing food and cleaning baby-feeding equipment. During a mains notice you can still shower and bathe (keep it out of your mouth), wash dishes in hot water and dry them thoroughly, and flush the toilet as normal.

Two limits matter. Boiling kills organisms but removes nothing chemical — and because water evaporates as it boils, it can slightly concentrate contaminants like nitrates and lead. And boiling water scalds, so make up a batch in advance rather than being tempted to use it hot.

Chemical tablets: disinfection you can carry

Purification tablets — chlorine-based (sodium dichloroisocyanurate), chlorine dioxide, or iodine — are the pocket-sized option when you cannot boil. They disinfect reliably against bacteria and most viruses, are light, and are the mainstay of travel and off-grid use.

Their weak point is Cryptosporidium. Its oocysts are notably resistant to ordinary chlorine, which is why UK water companies rely on filtration and UV rather than chlorine alone; chlorine dioxide tablets do better but need a long contact time. So follow the dose and waiting time on the packet exactly, and pre-filter cloudy water through a cloth first, as sediment shields organisms and blunts the dose. Iodine tablets are not suitable for pregnant women, people with thyroid conditions, or prolonged use.

Tablets treat only living organisms. They do nothing for dissolved chemicals, metals or nitrates, and they leave a taste some people dislike.

Filters: straws, pumps and gravity systems

Physical filters push water through a hollow-fibre membrane, typically around 0.1 microns, that physically strains out bacteria and protozoa — including Cryptosporidium and Giardia, the very organism tablets miss. That makes filtration a strong partner to chemical treatment rather than a rival.

The catch is viruses: they are far smaller than bacteria and pass straight through most portable filters. So for genuinely unknown natural water, the belt-and-braces approach is to filter first (removing sediment and protozoa) and then chemically disinfect or boil (dealing with viruses). Straw and squeeze filters suit one person on the move; gravity systems process larger volumes hands-free and are better for a household. An activated-carbon stage improves taste and captures some chemicals, but does not remove dissolved salts, nitrates or most heavy metals.

For everyday UK tap water none of this is necessary — filters are for travel, off-grid living and unknown sources, not a temporary mains notice.

What no method fixes — and where you must not drink

Boiling, tablets and filters all target living organisms. None of them make chemically contaminated water safe. That means nitrates, lead and other heavy metals, pesticides, fuel and solvent spills, and anything radiological pass straight through — and boiling actively concentrates some of them.

There are places you should never drink from, treated or not. Do not drink flood water: it is often contaminated with sewage and chemicals, and if your mains supply is disrupted, use bottled water instead (UKHSA / Food Standards Agency). Never drink seawater — no home method removes the salt. Steer clear of water with a chemical or fuel smell, an oily sheen, or from swimming pools and industrial sources.

For any suspected chemical contamination the only safe response is a different water source — bottled, or an official alternative supply — not a filter or a pan on the hob.

A simple order to work through

Put the methods in sequence and the decision becomes easy. First, if your water company or council is providing bottled water or an official alternative supply, use that — it is the simplest safe option.

Second, if you are on mains water under a boil-water notice, boil: a rolling boil, cooled, in a clean container in the fridge, discarded after 24 hours. Third, for an unknown natural source — off-grid or abroad — filter to remove protozoa and sediment, then boil or chemically disinfect to deal with viruses. Fourth, if you suspect chemical contamination or the water has been through flooding, do not drink it and do not try to treat it; find an alternative supply. Storing a few days of clean water in advance means you rarely have to purify anything at all.