How do I get safe drinking water in the outdoors?

In most of Britain you can plan a day walk around carrying what you need and refilling from taps, which is simpler and safer. Once you are camping, walking long distances or in the Scottish hills, taking water from streams becomes normal — and the general practice is to choose the source carefully and then treat it anyway.

Safety

What you'll need

• A filter, chemical treatment or a stove for boiling
• A collapsible bottle or bladder for dirty water
• A clean bottle kept for treated water only

Step by step

1. Choose the source before you choose the treatment.Fast-flowing water from a small stream high on a hill, above any grazing, buildings, tracks, mine workings or forestry operations, is the best you will find. Look uphill: what is up there is in the water. Avoid standing water, ponds, water with algae, water with a smell, and anything below a farm, a road or a settlement.
2. Understand what you are treating for.Protozoa — Cryptosporidium and Giardia — are the common cause of illness from UK surface water, and they come from livestock and deer. Bacteria from farm runoff and sewage. Viruses are rare from UK upland sources but present downstream of habitation. Chemical contamination from old mine workings cannot be filtered out at all, which is why the source choice matters more than the kit.
3. Boil if you want the most reliable answer.Bringing water to a rolling boil kills protozoa, bacteria and viruses. In the UK, reaching a rolling boil is sufficient — the temperature does the work, not the duration. It uses fuel and takes time, and it does nothing about sediment, taste or chemical contamination.
4. Filter if you want speed and volume.A hollow-fibre filter with a pore size around 0.1 to 0.2 microns removes protozoa and bacteria, works instantly and improves clarity. It does not remove viruses, which is acceptable for clean upland UK sources and less so downstream of habitation. Filters clog with silty water, and they must not be allowed to freeze — ice destroys the fibres and the failure is invisible.
5. Use chemical treatment for light weight and viruses.Chlorine dioxide tablets or drops kill bacteria, viruses and — with a longer contact time — Cryptosporidium. Iodine and simple chlorine are less effective against Cryptosporidium. Everything chemical needs time: read the packet, which typically specifies substantially longer for protozoa and longer again in cold water.
6. Combine methods if the source is poor.Filter first to remove sediment and protozoa, then treat chemically for viruses. A pre-filter of a coffee filter or a clean buff removes silt and stops a proper filter clogging.
7. Keep clean and dirty separate.One container for untreated water, one for treated, and never mix them up. Clean the threads of the treated bottle after filling, because untreated water in the thread recontaminates it.
8. Plan your refill points on the map.Streams shown on the map, taps at campsites, village halls, churches, pubs and cafes. In much of Britain the practical answer is a tap rather than a stream, and a planned route with refills needs far less kit.
9. Carry enough for the dry sections.Ridges and limestone country can have no water at all for many miles. Two litres for a full day is a common figure and it rises sharply in heat.
10. Look after the kit.Backflush a filter as the maker instructs, dry it before storing, and never let it freeze. Check the expiry on chemical tablets, which do degrade.

Tips

• Water from a fast stream on open hillside above the grazing line, boiled or filtered, is about as good as it gets in Britain.
• Never take water from below a mine or spoil heap in old mining areas — heavy metals pass through every filter and every chemical treatment.
• A collapsible one-litre bottle for dirty water weighs nothing and makes filtering far easier than trying to fill a rigid bottle in a shallow stream.
• In freezing conditions, sleep with the filter inside your sleeping bag. A frozen filter is a broken filter and it gives no sign of it.
• Peaty water is brown and usually harmless in itself, though it tastes of the moor and will stain. Filtering improves the taste; it does not need to.

Common mistakes

• Judging water by how it looks — Cryptosporidium and Giardia are invisible and utterly clear water from below a sheep pasture is a common cause of illness. Clarity tells you about sediment, not about contamination.
• Not waiting the full contact time with chemical treatment — Chlorine dioxide needs a specified time to be effective against protozoa, and considerably longer in cold water. Drinking after ten minutes because you are thirsty defeats the purpose.
• Letting a hollow-fibre filter freeze — Ice crystals rupture the fibres and create channels that pass everything straight through, and there is no way to tell from the outside that it has happened.

Alternatives

• Squeeze or straw filter: Lightweight hollow-fibre filters that you drink through or squeeze into a bottle. Fast, cheap and very popular. No protection against viruses, and vulnerable to freezing.
• Pump filter: Heavier and slower per litre but handles silty water better and fills bottles for a group. Ceramic elements can be cleaned and reused for years.
• Gravity system: Hang a dirty bag and let it filter into a clean one. Excellent for a group at a camp, no effort, and needs somewhere to hang it.
• Chlorine dioxide tablets or drops: Almost no weight, kills viruses as well as bacteria and protozoa, and needs patience. The standard backup even for people carrying a filter.
• UV pen: Ultraviolet light in clear water, effective and quick. Needs batteries, needs the water to be clear, and is fragile.
• Just carry it: For a day walk in most of the UK, carrying two litres and planning taps is simpler, lighter than it sounds, and completely reliable.

Questions people ask