How to purify water when you have to

In a UK emergency the answer is nearly always bottled water or a tanker from your water company. If you are asked to boil, the Drinking Water Inspectorate says to boil the water, “allow it to cool naturally”, store it “in a clean container, in the fridge” and discard it after 24 hours (checked 30 August 2026).

First, the honest answer: you probably will not be purifying anything

This page is a backstop, not a plan. In this country, when the water goes off or goes wrong, your water company is obliged to sort it out. The Drinking Water Inspectorate is explicit: “It is also your local water company’s responsibility to provide alternative supplies should the main supply be contaminated” (checked 29 August 2026).

In practice that means bottled water at collection points, a bowser or tanker in the street, and home delivery for people who cannot get out. Even during long incidents, the households affected drank bottled water rather than treating their own.

The second most likely thing is a boil water notice, which is not a filtering problem either. It is a kettle problem. So the order of usefulness is: stored bottled water, then what the water company brings, then boiling, then everything below.

Before any of it, know how much you actually need. GOV.UK Prepare cites the World Health Organization minimum of “2.5-3 litres of drinking water per person per day”, and notes that “10 litres per person per day will make you more comfortable by also providing for basic cooking and hygiene needs” (checked 29 August 2026). The water storage page covers how to keep that at home.

Boiling: what a boil notice actually asks for

A boil water notice means, in the Drinking Water Inspectorate’s words, that “the water in your tap might contain bacteria, viruses or parasites which could be harmful to your health” (checked 29 August 2026).

The method, quoted from the DWI exactly as the page has it (checked 30 August 2026):

“You should boil your water until it reaches a rolling ball. Remove the water from the heat and allow it to cool naturally. The water should be stored in a clean container, in the fridge and should be discarded if not used within 24 hours.”

The same page gives the reason: “To be safe, bring water to the boil so any bacteria or viruses that were in your water are killed or deactivated, making the water safe to drink.” It adds that “Boiling water is also effective against parasites such as Cryptosporidium”.

Separately, and as a gloss rather than the instruction: “a rolling ball” appears to be a slip for the standard phrase, a rolling boil. That is the wording the World Health Organization uses, and it explains why boiling works: “Heating water to a rolling boil, as recommended in the WHO Guidelines for Drinking-water Quality (GDWQ), is sufficient to inactivate pathogenic bacteria, viruses and protozoa” (WHO, checked 29 August 2026).

So: bring it to a proper boil, take it off the heat, let it cool on its own, and put it in something clean. A kettle that clicks off at the boil does the job. The DWI adds the obvious warning that is easy to forget in a busy kitchen: “Please be cautious that boiling water can cause scalds.”

Use boiled water for drinking, brushing teeth, making ice, preparing food, cleaning feeding equipment and for pets. You do not need it for everything: the DWI says “The water is still safe to shower and bathe in, but make sure it does not get into your mouth”, that it is “safe to wash dishes, if you use hot water and dry the dishes thoroughly before use”, and that “You can continue to wash clothes and flush the toilet as normal”.

Our fuller walkthrough is on the water’s off, or there’s a boil notice and is water safe after a boil notice.

Boiling when the power is off too

A gas hob will usually still light by hand, which is covered on cooking without electricity. If you have no way to boil water safely indoors, do not improvise one: never bring a camping stove, barbecue or generator into the house. Use bottled water instead, and if there is a baby in the house, ready-to-feed formula is the answer rather than untreated water.

What boiling does not fix

Boiling deals with the living things in water. The Drinking Water Inspectorate frames a boil notice entirely around them: the tap water “might contain bacteria, viruses or parasites which could be harmful to your health”, and boiling means “any bacteria or viruses that were in your water are killed or deactivated” (checked 30 August 2026). Nothing on that page claims boiling addresses anything else.

That is why a boil notice and a “do not drink” notice are different instructions. If your water company tells you not to drink the water at all, boiling is not a workaround, and you should not treat it as one. Follow exactly what they say, and ring them if the notice does not make clear which one you are under.

Purification tablets

Chemical tablets, usually chlorine-based or chlorine dioxide, are the pocket-sized version of treatment. They are what walkers and aid agencies use, and they are cheap and light enough to sit in a go bag without anyone noticing.

They are also the part of this page where the honest answer is “read the packet”, and we are not going to print a dose. Products differ from one another, and the dose, the contact time and the water temperature are set by the manufacturer for that specific tablet. Follow the instructions on the packet you bought, exactly, and do not carry a number over from a different brand.

The World Health Organization runs a formal evaluation programme for exactly this reason: its International Scheme was established “to evaluate the microbial performance of household water treatment (HWT) technologies against WHO health-based criteria” (checked 29 August 2026). If you buy tablets, buy a known brand with clear instructions in English, keep the packet with them rather than tipping them loose into a tin, and check the expiry date once a year when you check your smoke alarms.

Filters, and what they do and do not remove

A filter is not the same thing as a purifier, and the marketing does not always make that clear. We had a table here comparing product classes by pore size and the pathogens each removes. We took it out, because we could not source those claims to anything better than the manufacturers themselves, and a table on a page like this reads as settled fact.

Here is what you can rely on instead. Every filter has a manufacturer’s claim about what it removes, and that claim is the whole of what it promises. Read it before you buy, keep it with the filter, and treat anything outside that claim as untreated water. A filter sold for taste is not making a safety claim at all.

That last point is the one that matters during a boil notice. A kitchen jug filter does not make a boil notice go away: the instruction from your water company is to boil, and boiling is what the Drinking Water Inspectorate asks for. If you own a camping filter and you are thinking of relying on it, the honest test is whether its own maker says, in writing, that it deals with the thing you are worried about.

Rainwater, streams and butts: a last resort

There is no realistic UK household emergency in which the sensible move is to drink from the water butt. Rainwater off a roof picks up bird droppings, moss, dust and whatever the gutter has been holding; a water butt is warm, dark and open. A stream carries whatever is upstream of it, including farmland run-off.

If you are ever genuinely out of options, that water is for flushing the toilet and washing, and the drinking water comes from bottles or from the water company. Keep a couple of buckets of butt water for the loo during an outage and you have used it for the thing it is actually good for.

If your home is on a private supply rather than the mains, the Drinking Water Inspectorate’s advice after flooding applies: “If your water comes from a private supply network such as a well or spring, then check that this hasn’t been affected by flooding before any usage. If you believe that the supply network has been affected or unsure, avoid all usage and use alternative supplies while waiting for confirmation or in doubt.”

Floodwater: never

Floodwater is not a water source under any circumstances, and it is not made into one by boiling or by a filter.

UKHSA is direct about what is in it. Its flooding and health guidance says “Floodwater can contain harmful pollutants or contaminants hazardous to humans and animals” (checked 30 August 2026), and its overview for responders explains where that comes from: “floods can damage the sewage system when groundwater, rivers and coastal waterbodies overflow, resulting in sewage, human and animal waste entering flood waters, and can be a health risk to affected populations if not cleaned up properly”. It adds that “Floodwaters can contain contaminants that cause infections or irritation, that may remain even after the water has receded”.

So the rule extends past drinking. Public Health England’s advice during flooding was a short list: “avoiding contact with flood water, washing hands and food preparation surfaces and not eating food that has touched flood water” (checked 30 August 2026). Keep floodwater out of your mouth, wash your hands and your worktops, and throw away food it has reached.

What the Drinking Water Inspectorate does say about flooding is reassuring about the tap: “In most flooding circumstances the main supply is unaffected and so is safe to drink, however if you notice a change in quality – such as discolouration or a change in taste or smell, or unsure, call your water company” (checked 29 August 2026). And while you wait for an answer: “While waiting for confirmation if the main supply has been affected, boil all intended water (and let it cool to avoid scalding) or use bottled water.”

What this asks of your body

Water is heavy. Ten litres is 10 kg, and a collection point is rarely at your front door. Being able to carry two full containers a few hundred metres, or lift a case of bottles into the boot, is the physical side of this page. The five ready body tests include the carry for exactly this reason.

Frequently asked questions

How do I make water safe to drink at home?

Boil it. The Drinking Water Inspectorate’s instruction is: “You should boil your water until it reaches a rolling ball. Remove the water from the heat and allow it to cool naturally. The water should be stored in a clean container, in the fridge and should be discarded if not used within 24 hours.” The WHO separately confirms a rolling boil inactivates bacteria, viruses and protozoa.

How long do I need to boil water for?

Bring it to a rolling boil. The WHO states that “Heating water to a rolling boil… is sufficient to inactivate pathogenic bacteria, viruses and protozoa”. A kettle that switches itself off at the boil has reached that point, so you do not need to hold it there for minutes.

How long does boiled water stay safe?

The Drinking Water Inspectorate says boiled water “should be stored in a clean container, in the fridge and should be discarded if not used within 24 hours”. Boiling does not protect water from being recontaminated afterwards, so the container matters as much as the boiling.

Can I still shower during a boil water notice?

Yes. The DWI says “The water is still safe to shower and bathe in, but make sure it does not get into your mouth”. You can also “continue to wash clothes and flush the toilet as normal”, and wash dishes “if you use hot water and dry the dishes thoroughly before use”.

Do water filter jugs make tap water safe in an emergency?

No. A jug filter sold for taste is not making a safety claim, and it does not replace boiling during a boil water notice. Whatever filter you own, read the manufacturer’s own written claim about what it removes, and treat anything beyond that claim as untreated water.

Is rainwater safe to drink in the UK?

Treat it as a last resort and use it for flushing and washing instead. Roof run-off collects bird droppings, dust and debris, and a water butt is an open, warm container. In a UK emergency your water company is required to provide an alternative supply, which is a far better answer.

Can I drink floodwater if I boil it?

No. UKHSA says “Floodwater can contain harmful pollutants or contaminants hazardous to humans and animals”, and that flooding can put “sewage, human and animal waste” into it. Public Health England’s advice includes “not eating food that has touched flood water”. Use bottled water, or what your water company brings.

Sources & review

Drinking Water Inspectorate: Receiving a boil water notice. Checked 30 August 2026. What a boil notice means; the instruction verbatim, “You should boil your water until it reaches a rolling ball. Remove the water from the heat and allow it to cool naturally. The water should be stored in a clean container, in the fridge and should be discarded if not used within 24 hours”; “To be safe, bring water to the boil so any bacteria or viruses that were in your water are killed or deactivated”; effective against Cryptosporidium; scald warning; showering, dishes, laundry and flushing.
Drinking Water Inspectorate: Flooding. Checked 29 August 2026. Mains supply usually unaffected; call your water company if quality changes; boil or use bottled water while waiting; water company must provide alternative supplies; private supplies after flooding.
World Health Organization: Boil water technical brief. Checked 29 August 2026. “Heating water to a rolling boil, as recommended in the WHO Guidelines for Drinking-water Quality (GDWQ), is sufficient to inactivate pathogenic bacteria, viruses and protozoa.”
World Health Organization: Household water treatment and safe storage. Checked 29 August 2026. The International Scheme evaluates the microbial performance of household water treatment technologies against WHO health-based criteria.
UK Health Security Agency: Flooding and health: advice for the public. Checked 30 August 2026. “Floodwater can contain harmful pollutants or contaminants hazardous to humans and animals”; cleaning and disinfecting the kitchen before using it with food; bottled water for handwashing during water disruption.
UK Health Security Agency: Flooding and health: an overview. Checked 30 August 2026. “Floods can damage the sewage system when groundwater, rivers and coastal waterbodies overflow, resulting in sewage, human and animal waste entering flood waters”; contaminants may remain after the water has receded.
Public Health England: Flooding – health advice on floodwater. Checked 30 August 2026. Avoiding contact with flood water, washing hands and food preparation surfaces, and not eating food that has touched flood water. This is a 2014 press release, quoted for the public advice it gives rather than for its incident detail.
GOV.UK Prepare: Get prepared for emergencies. Checked 29 August 2026. 2.5-3 litres of drinking water per person per day (WHO minimum); 10 litres per person per day for cooking and hygiene.

Two things this page deliberately does not tell you: a dose and contact time for purification tablets, and a table of what each class of filter removes. Neither could be sourced to anything better than the manufacturer, so both point you at the packet instead. This page carries health-adjacent instruction and is awaiting review (reviewedBy: TBC-first-aid).