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As a general rule of thumb, the average person can only survive for three days without water—which is why it is so important to know how to purify water. It’s easy to take for granted how much water we really drink when you’re at home, but when you’re doing any kind of physical activity outdoors, you can become dehydrated quickly. Because of this, any plans for an outdoor adventure should include a way to ensure you have a clean source of drinking water. In a survival situation, after you’ve taken the first steps to prevent hypothermia (securing a survival shelter and starting a fire), finding a fresh source of water is next on your list. 

Even after you find water, however, you still need to make sure it’s safe to drink. Two parasites to be mindful of when drinking water from lakes, rivers, and streams are Giardia—aka, “beaver fever”— and Cryptosporidium, both of which can lead to severe diarrhea, nausea, and stomach cramps. This story will teach you the best way to filter water. Because certain techniques make more sense from one scenario to the next, we’ll explore a variety of water purification methods—from natural water filters (i.e., the sun) to knowing how long to boil water to purify it. Here’s how to purify your water so it’s safe to drink.

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boiling is an effective way to purify water
Purifying water the old-fashioned way—boiling over a fire. Jim Baird

Water Purification Method 1: Boiling

Does boiling water purify it? Yes. Boiling is the good, old-fashioned way of purifying water. Typically, letting water come to a rolling boil for one minute will do the trick, but if you’re in an area where the water is in really question, let it boil for a full three minutes just to be safe. You can do this in a pot either on a camping stove or over the fire. Don’t have a pot or metal container? The following survival hacks of boiling water to purify it will work in a pinch. 

1. How to Purify Water with Hot Rocks

Do this by heating up golf ball-size rocks in a fire and dropping a couple of them into your water bottle at a time. You’ll have to periodically swap out the rocks for hotter ones by using a couple of sticks as a pair of makeshift tongs.

2. Boiling Water in a Plastic Bottle Over Fire

Carefully place the water bottle on the coals of a fire while it’s about half full of water. Try leaning it up against a rock for support so it won’t tip over and extinguish your fire. Tying a string around the top of the bottle and hanging it over the fire’s coals or low flames will work, too. The plastic won’t burn when there’s water in the bottle—trust me on this. Make sure you leave the lid open while you’re boiling the water, however, or the cap will explode off the top, and you could get burned with the hot steam. 

3. Purify Water in a Carved-Out a Basin

Do this with the help of whatever survival knife or cutting tools you may have on hand. (To save energy, consider using coals from a fire to help burn out a concave dent in the side of a log or in the top of a stump.) Once a significant indent is made, fill it with water and transfer hot rocks from the fire into the water, rotating them out for hotter rocks to keep the boil going.

4. How to Purify Water With a Tarp

Line a shallow hole with a tarp or plastic sheeting and fill it with water. Then use the hot-rock technique described above to get the water boiling. 

Water Purification Method 2: Filtration

There are many portable water-filtration devices on the market today made specifically for purifying water outdoors. The most common styles are pump filters, squeeze filters, press filters, and gravity filters. Only a few of them can withstand freezing and thawing, so be sure to check the specs on the filter you buy and don’t use it in winter if it’s not made for it. The ones that can withstand freezing are a lot more expensive. Also keep in mind that although they’re not typically an issue in true wilderness areas, filters typically don’t remove viruses from the water. The mechanism of all of these products (squeeze, pump, press, or gravity) just refers to the different method that a given type of water purifier uses to push the water through the device’s filter. Here’s a little more on each one. 

Pump Filters 

Pump filters do the trick well and some are faster than other styles of water-filtration devices. Due to the fact that you’re often pumping in shallow water near a shore, the intake often sits in silt, which means the filter clogs easily. Counter this by filling a pot or bucked up with water, and let the sediment settle before pumping out of it, while keeping the intake suspended out of any of the settled debris. Filters can be replaced and cleaned in the field as well. Consider using an elastic band to secure a coffee filter over the intake if the water you’re pumping is murky. 

Our Expert’s Gear Pick

The Kayadyn Vario is an efficient pump filter that won’t break the bank. 

Squeeze Filters and Straws 

These are a good option for when you’re on the go. The filter mechanism is attached to the bottom of the cap, which is secured onto a playable bottle. After filling the bottle and affixing the cap, the water is filtered as you squeeze it into your mouth. You can also use it to fill up other water bottles. Filter straws, such as the Life Straw, good in a pinch and can be used to drink directly out of a stream—or you can fill up a water bottle and drink out of it with a filter straw. The straw is slower than the squeeze so you can’t get as satisfying of a chug when you’re thirsty. It’s a good idea to bring a back-up filter cap on longer trips as they get clogged with heavy use. Also, make sure you have a rugged squeeze bottle, because many are not durable and if they get a hole in them they don’t work. 

Our Expert’s Gear Pick

A Katadyn BeFree, one of the best backpacking water filters, top of a sturdy Hydrapack Flux Soft Bottle Hydrapack Flux Soft Bottle is what I use. The Hydrapack Flux is durable and it has a built-in loop so it can be clipped down. 

Press Filters

These are two-piece devices that purify water and also serve as a water bottle. Essentially they consist of a cup and a bottle that has a filter built into the bottom of it. To operate it, you fill up the cup and then press the bottle slowly down into the cup. As you push down, the water in the cup will be forced into the bottle passing through the filter. Press filters are sturdy, can be clipped down easily, and act as a two-in-one because they’re a bottle and a filter—so you can fill them up at home before heading out. Drawbacks are that, for them to work, you need to find a flat, solid surface to push down on them. They also work relatively slowly and take more acute physical effort than the other options. 

Our Expert’s Gear Pick

Your best bet for a press filter is the Grayl Geopress 24oz Water Purifier.

Gravity Filters

These are great as they’re passive and do a lot of water at once. They consist of a large bag with a tube coming out the bottom. The tube runs into a filter and a water bottle or another bag connects to another tube that runs out of the bottom of the filter. The top bag is filled and hung in a tree, and gravity filters the water into the container below. Most have an on/of mechanism allowing clean water to be accessed like a tap once it’s hung. They are great to have a round camp, particularly for larger groups but are definitely not ideal to pull out and use while on the go. 

Our Expert’s Gear Picks

A reliable gravity filter is the MSR Auto-Flow. For an option that can handle the cold, go with the MSR Guardian Gravity Purifier.

Read Next: Tips for Backflushing a Water Filter

Water Purification Method 3: Water Tablets 

Tablets might be the easiest way to purify water—particularly while you’re on the go. It’s always good to have a few extras in your survival kit. All you have to do is fill up your water bottle and drop in a tab or two. (The number of tabs depends on the type of tabs you’re using and how much water you want to treat.) After the tabs are in, you wait a given amount of time for them to take effect, which can be anywhere from 30 minutes to 4 hours.

Tabs are a great option, because while they’re doing their thing, you can get to work cutting wood or working on a survival shelter. They also allow you to treat a larger amount of water with less time and effort than if you had to sit in one spot using a filtration pump for example. It’s important to note that water tabs have a shelf life and won’t be as effective after their expiration date. Here are a couple recommendations for the best tablets for purifying water:


Aquatabs are among the top tablets for water purification. They are relatively inexpensive tabs work in half an hour and require one tab per liter; however, adding two tabs to a 1.5 liters (the large sized Nalgene bottles) is okay. A package of them comes with 40 tabs. They make the water taste like strong tap water, but they get the job done in most situations and work quickly. One package will treat 40 liters. These tabs work agains viruses, bacteria, and Giardia. However, they are not effective against Cryptosporidium. 

Katadyn Micropure

These are more expensive than other options, but leave almost no taste in the water. Chlorine dioxide is the main ingredient, and one tab will treat one liter of water while a pack will treat 30 liters total. The main drawback to these tabs is that they take 4 hours to properly activate. This wait time is standard with other brands of chlorine-dioxide tablets. If you’re on a tough trek and you need to consume a lot of water, this wait time can lead to dehydration. In addition to better flavor than other options, another positive is that they are effective against Cryptosporidium. 

Potable Aqua Tabs

Iodine treatment for water has been around a long time. Iodine treatment stains water red and leaves a bad taste, however, Potable Aqua offers a taste neutralizer component that also takes away the red color in the water. Drawbacks are that treating your water with this product is a two-step process, because the taste-neutralizer tabs are added 30 minutes after the Iodine tabs. They take about 35 minutes total before your water is ready to drink. Another drawback is that over-consumption of Iodine-based-treated water cause thyroid issues. 

Water Purification Method 4: Ultraviolet Light

Ultraviolet systems sanitize water by rendering bacteria, viruses, and protozoa inactive, meaning they can’t reproduce after treatment. A drawback to these backcountry UV water purifiers is that they require batteries. If you’re in a bit of a jam on a sunny day and don’t have many options for purifying your water at all, you can actually use the sun to help do the job. Here’s a closer look at both these options. 

How to Purify Water with a Steripen

Steripen makes several options but they all work in a similar way. This device is basically a handle with a long light coming out of the end. It runs off batteries and needs to be used according to the instructions for it to work effectively. To use the device, you first fill up your water bottle or cooking pot and insert the UV light into the water up to the point where the sensors at the handle are touching the water and then you start stirring with it. A light will flash on the device or the UV light will go off in 50 to 90 seconds depending on the model. They destroy viruses, bacteria, and protozoa, including Giardia and Cryptosporidium. 

How to Purify Water With the Sun

The sun can help kill organic matter in your water if you leave it in direct sunlight during the summer. To do this effectively, your water should be as clear as possible. If it’s not, consider letting any sediment in it settle and/or strain it through a piece of fabric. A cotton shirt will work. You will also need a water bottle that’s as clear and thin as possible; thin plastic works best. Put the bottle in direct sunlight and leave it for at least five to six hours—or as long as 36 hours if the water is more questionable. You want the water to get as hot as possible, so if you have any reflective material at your disposal, use it to put behind the bottle. This is definitely a passive technique, which is good if you’re remaining in one spot for a long time. But the length of time it takes—and depending on how dire your situation is—can make it impractical in some situations. 

boiling water over fire
A pot of water boils (and purifies) over a campfire. Jim Baird

What is the Safest Way to Purify Water?

While UV options, such as the Steripen, work and are faster options than kill-all tabs when treating smaller amounts of water, they don’t completely remove protozoa like a filter does. Therefore, the safest and best way to filter water is to filter your water with one of the options above to effectively remove all bacterial and protozoan—and then treat the water with a 4-hour, chlorine dioxide-based water tab to kill any viruses and potential remaining bacteria. As long as the water you are treating doesn’t have any industrial chemical pollution in it, this will make it as safe to drink.


Does boiling filter water?

Boiling water does not filter it—but it will purify your water. Best practices suggest letting the water come to a rolling boil for one minute. However, if you’re in an area where the water looks very dirty or contaminated, let it boil for a full three minutes just to be safe.

How long does it take to purify water?

The length of time it takes to purify water depends on the method you use—and how contaminated the water may be. You can purify water by boiling it in as little time as one minute. Conversely, you can purify water by leaving a clear plastic bottle in direct sunlight—but this take several hours (possibly up to 36 hours) for it work effectively.

What are 3 ways to purify drinking water?

Three of the most common ways to purify water are: boiling, filtration, and water tablets. The use of ultraviolet light is another effective way to purify water. Review the water purification methods detailed in the story above to determine which method will work best for you.

Final Thoughts on How to Purify Water

All of the water purification methods detailed above have worked well for me over the years. However, my trips don’t cover all geographical regions and types of adventures that others may take. Before making a decision on what method to go with, a good place to start is by researching the water quality in the areas you’ll be traveling. (There’s a chance that, due to contamination from heavy metals or harmful chemicals, you may need to bring in enough water for your whole trip). After you do your research, choose a water-treatment method to suit whatever contaminants are likely to be in the water there and the types of adventures you have planned. Now get out there and stay hydrated!

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