Emergency water purification: boiling, filters and tablets

In the UK, tap water is treated and tested, and you should always follow your water company’s advice first. But if the supply is interrupted or you are told water may be unsafe, you may need to treat water before drinking it. This guide covers the three main methods and their limits.

Stored water is always the safest option. If you have not yet built a water store, start with how much water should you store?

Follow official advice first

If your water company issues a notice, follow it exactly. There are different kinds:

• Boil water notice: the water may contain germs. Boil it before drinking, brushing teeth or preparing food.
• Do not drink notice: the water may contain something that boiling cannot remove, such as a chemical. Do not drink it, even if boiled. You can usually still use it for flushing toilets.
• Do not use notice: do not use the water for anything except flushing toilets.

Boiling will not make chemically contaminated water safe. If you do not know what is wrong with the water, use stored water.

Method 1: boiling

Boiling is the most reliable way to kill germs (bacteria, viruses and parasites) in water. The World Health Organization’s guidance is that bringing water to a rolling boil is enough.

How to do it:

1. If the water is cloudy, let it settle, then pour the clearer water off the top. You can also strain it through a clean cloth.
2. Bring the water to a rolling boil, where large bubbles break the surface continuously.
3. Let it cool, covered, in a clean container.
4. Store it in a clean, covered container and use it within a day if you can.

Boiled water can taste flat. Pouring it between two clean containers a few times adds air and improves the taste.

What boiling does not do

• It does not remove chemicals, fuel, heavy metals or salt.
• It does not remove dirt or cloudiness.
• It uses fuel. In a power cut, a gas hob may still work. Never use a camping stove, barbecue or generator indoors: they produce carbon monoxide.

Method 2: filters

Portable water filters work by pushing water through a material with very small holes, and sometimes through activated carbon.

What to know:

• Filters are rated by the size of particle they stop. Many remove bacteria and parasites, but most do not remove viruses. Check the manufacturer’s specification.
• Activated carbon can improve taste and reduce some chemicals, but it does not make chemically contaminated water safe.
• Filters clog. Pre-filter cloudy water through a cloth first, and follow the instructions for cleaning or replacing the cartridge.
• A filter that has frozen may be damaged. Check the instructions.

Our buying guide to emergency water filters explains the specifications to look for.

Method 3: purification tablets

Water purification tablets release a chemical, usually chlorine-based, that kills germs over time.

How to use them:

• Follow the instructions on the packet exactly, especially the dose and the waiting time before drinking.
• They work less well in cloudy or very cold water. Filter or settle cloudy water first.
• They do not remove chemicals or dirt.
• They can leave a taste. Some products include a second tablet to reduce it.
• Check the expiry date. Store them in their original packaging.

Do not improvise with household bleach unless official advice in your situation tells you to and gives a dose. Scented, thickened or “splash-less” bleaches are not suitable, and getting the amount wrong is dangerous.

Combining methods

Each method has gaps, so combining them works best:

1. Settle and strain to remove dirt.
2. Filter to remove most germs and improve taste.
3. Boil or use tablets to deal with what the filter might miss.

When not to drink the water

Do not drink water that:

• may contain floodwater, sewage, fuel or chemicals
• smells of fuel or chemicals, or has an oily sheen
• you have been told not to drink

Floodwater in particular can carry sewage and chemicals. Avoid contact with it where possible and do not use it for drinking even after treatment.

A small treatment kit

• A pan with a lid, and a way to boil water safely
• A few clean cloths or coffee filters for straining
• Clean, lidded containers for treated water
• A portable filter, if you choose one
• Purification tablets, in date, with their instructions
• A marker pen to label treated water

Key points

• Stored water is safest. Treat water only when you have to.
• Boiling kills germs but does not remove chemicals.
• Filters vary: check what they actually remove.
• Tablets need the right dose and time.
• Never drink water you suspect is contaminated with chemicals, fuel or floodwater.

Sources and further reading

• Boil water (World Health Organization)
• Guidelines for drinking-water quality: small water supplies (World Health Organization)
• Water (Ready.gov (US Government))