How to purify water in the wild: 6 ways to make water safe to drink

Never be thirsty again with this handy guide to water purification in nearly all outdoor situations

woman scooping mountain water from a pool
(Image credit: Getty)

You’re outdoors and thirsty, and you’ve managed to find some water, but you’re worried about how clean it really is. Ideally you’d look for a better, cleaner water source, but if that’s not practical (or there isn’t one) then you’ll need to purify what you’ve got, either using technology (if you head out armed with one of the best water purifiers, you're sorted), chemicals, or various processes. Read on for the low-down on what to do next. Here are 6 ways to purify water in the wild. 

#1. Boil it

 Sometimes the oldest ways are the best, and the oldest water-purifying trick in the book is one of the simplest: boiling. Whether over an open fire, or on a gas stove, heating your water to boiling point for more than one minute will kill off bacteria, viruses, protozoa and cysts alike. For this, you'll of course require some basic equipment and fuel – there are plenty of super lightweight options in our best camping stove guide, if portability is your priority, as well as clever stoves that run on twigs, so you don't need to bring gas along with you. The other potential down-side is that you’ll end up with boiling water that can’t be drunk immediately (although tea is a good way to use the hot water). Although boiling water will kill nasties, it won’t remove man-made pollutants or debris from turbid water, so it’s not perfect.

 #2. Use chemicals 

There are a variety of tablets available that purify water, or at least kill off any lurking nasties. They usually work by adding chlorine to the water, which then needs neutralising – unless you're happy to put up with the not-entirely-pleasant taste. Apart from the taste, the disadvantage is that each tablet only cleans a certain number of litres, so you’ll need a lot of them for an extended trip. In addition, where Giardia might be present, some tablets need to be left for several hours to be effective, which means a lot of planning ahead. As with boiling, chemicals won’t remove dirt or other solids, and they won’t remove toxins either.  

#3. Try a forced filter

Many of the water purifiers on the market use a pressure filter system, which involves forcing water through a very fine filter mesh, removing the vast majority of nasties. These are very effective and lightweight, sometimes being built into water bottles or in a straw style-format. They’ll usually filter and purify thousands of litres of water, so perfect for an average camping trip, and many have a charcoal filter as well, to combat unpleasant tastes and smells too. The downside is that you have to suck or pump the water through the filter mesh, and not all systems can generate water for cooking, for example. Although filters will remove mud and all types of gunk from the water, very dirty water will gum up the filters much faster than usual, so using relatively clean water to begin with is a good idea. 

#4. ... or a gravity filter

The professional expedition’s choice, gravity filters are similar to forced filters in the filtration area, but take advantage of gravity to do the work for you. They tend to be designed for group use, so can produce clean water in quite large volumes, and are very easy to use once set up. They’re not very mobile once set up though, so best for a basecamp-style arrangement, rather than trekking or through-hiking type situations. 

#5. Try UV light

UV is a well-tried and tested method of killing off unwanted nasties in water, often used in home or community-grade water treatment plants. Available in small pen-style devices, a simple sweep or several through your water should kill off most viruses and bacteria, although there are caveats. Any solids in the water will block the UV rays, allowing some to escape, and it’s vital to have the right size container for the UV device being used: too large and you’ll also miss a few. Needless to say, you’ll need batteries or a power source to run a UV purifier, something worth considering on a longer trip. 

#6. Still got it

A final way to purify water in the wild is to use a solar still. It's not something that you’ll want to build on a day trip to the Cairngorms, but a useful technique in other situations. Solar stills are particularly useful in situations where there’s lots of non-drinkable water and sunshine, making them ideal for offshore sailing situations. At its simplest, a solar still is a hole in the ground with a cup in the middle, and a sheet of plastic covering the hole in a cone shape. Add non-drinkable water to the hole, and even green foliage, the sun evaporates the water but not any pollutants, which then condenses on the plastic and drips into the cup. Commercial, inflatable versions are available, and can create up to a couple of litres a day for no effort or power. Like the Gravity filters, solar stills are much better in a static situation, rather than as a portable solution. 

Mark Mayne

Mark Mayne has been covering tech, gadgets and outdoor innovation for longer than he can remember. A keen climber, mountaineer and scuba diver, he is also a dedicated weather enthusiast and flapjack consumption expert.