How to Purify Water Backpacking: Filters, Tablets, UV and Boiling Compared

The simplest way to purify water backpacking in the US is a hollow-fiber filter rated 0.3 micron or smaller, which removes the protozoa (Giardia and Cryptosporidium) and bacteria that cause most backcountry illness. A filter does not stop viruses, so where water may be contaminated by human waste, add a chemical disinfectant such as chlorine dioxide, or use a UV pen or a product sold as a purifier. Boiling kills everything: the CDC recommends a rolling boil for 1 minute, or 3 minutes above 6,500 feet.

No single method is best for every trip. The right choice depends on what’s likely to be in the water, how cold it is, how much you drink, and how much weight and fuss you’ll accept. The rest of this guide sets out what each method actually removes and when to combine two of them.

What’s in backcountry water

Three groups of germs make hikers sick, and they differ enormously in size, which is why methods work on some and not others.

• Protozoa, mainly Giardia and Cryptosporidium. The largest, roughly 3 to 15 microns. They are spread by animal and human feces and survive a long time in cold water. Cryptosporidium has a tough shell that resists chlorine.
• Bacteria, such as E. coli, Campylobacter and Salmonella. Smaller, around 0.2 to 5 microns.
• Viruses, such as norovirus and hepatitis A. Tiny, far below 0.1 micron, so most backpacking filters can’t catch them. They come mostly from human waste.

In remote US wilderness with little human traffic, protozoa and bacteria are the main concern. Viruses become a bigger risk near popular campsites, trailheads, huts and anywhere people may not bury waste properly. That’s the practical reason to know which camp you’re in.

What each method removes: the CDC comparison

The table below follows the CDC’s backcountry water treatment chart.

Two things stand out. Chemicals alone are weakest against Cryptosporidium, and filters alone miss viruses. That’s why the CDC suggests filtering first and then disinfecting when you need protection against everything.

Filters: the default for most backpackers

A filter pushes water through a membrane or ceramic element with pores small enough to trap protozoa and bacteria. Most popular backpacking models use hollow fibers with pores of about 0.1 to 0.2 micron, well inside the CDC’s 0.3 micron threshold.

Types of filter

• Squeeze filters. A small cartridge screws onto a soft bottle or pouch. You fill the pouch and squeeze clean water into your bottle. Light (2 to 3 ounces), cheap, and the most common choice on trails like the Appalachian Trail and Pacific Crest Trail.
• Gravity filters. A large dirty-water bag hangs from a branch and drains through the filter into a clean bag. Best for groups and for camp, since it works while you do something else.
• Pump filters. You pump water from a hose dropped into the source. Heavier, but they reach shallow puddles and seeps that are hard to scoop.
• Filter bottles and straws. Convenient for day hikes. Straws are awkward for filling a pot or bottle for camp.

What goes wrong with filters

• Freezing. Water trapped in hollow fibers can freeze, crack the fibers and leave the filter passing untreated water with no visible sign. Manufacturers warn against letting them freeze. On cold nights, sleep with the filter in a zip-top bag inside your sleeping bag, and keep it in an inside pocket by day.
• Clogging. Silty water slows the flow to a trickle. Backflush as the maker describes, and let muddy water settle or pour it through a bandanna first.
• Cross-contamination. A drip of raw water on the clean side, or a dirty bottle mouth, defeats the filter. Keep dirty and clean containers clearly separate.
• Taste and chemicals. Most hollow-fiber filters don’t remove chemicals such as agricultural runoff or mining residue. Carbon elements help with taste, but they aren’t a fix for polluted water.

Chemical treatment: tablets and drops

Chemicals are light, cheap and good as a backup or a second layer on top of a filter.

• Chlorine dioxide (tablets or two-part drops) is the most useful for backpackers. It kills bacteria, viruses and Giardia, and is somewhat effective against Cryptosporidium, but the CDC Yellow Book notes that tablets and drops need a contact time of several hours to handle it. Follow the label: waiting times differ by product and by the germ you’re targeting, and cold water slows the reaction.
• Iodine works on bacteria and viruses and is only somewhat effective on Giardia. It does nothing against Cryptosporidium. The CDC says people who are pregnant, have thyroid problems or are sensitive to iodine should avoid it, and that iodine use should be limited to a few weeks.
• Household bleach (unscented, plain chlorine bleach) works on bacteria and viruses in an emergency, but shares iodine’s weakness against protozoa. Use the dose the CDC gives on its emergency water page and let it stand.

Cloudy water uses up disinfectant, so it needs a longer contact time or a higher dose. Filter or settle it first. If the chemical taste bothers you, the CDC Yellow Book says a pinch of vitamin C powder after the contact time is finished removes it.

UV purifiers

A UV pen or bottle cap shines ultraviolet light through the water, scrambling the DNA of bacteria, viruses and protozoa so they can’t reproduce. The CDC rates UV effective against all of them, but only in clear water, because particles shade germs from the light. Most devices treat about 1 liter (1 quart) at a time in a minute or two.

UV is fast and leaves no taste, which makes it popular for international travel and for trips where viruses are a concern. Its downsides are batteries, which drain faster in cold weather, and a small screen or bulb that can break. Carry tablets as a backup and pre-filter anything murky.

Boiling

Boiling is the most reliable single method. The CDC calls for a rolling boil of 1 minute, or 3 minutes above 6,500 feet, which covers much of the Rockies, the High Sierra and the Cascades. It kills every germ type, including Cryptosporidium, and works in cold and murky water where chemicals struggle.

The cost is fuel and time. Boiling all your drinking water for a full day can use a lot of canister fuel, so most hikers only boil what they’re heating anyway for meals and coffee, and filter the rest. It’s also the go-to fallback if your filter freezes or breaks. Boiling doesn’t remove sediment or chemicals, and the water tastes flat until you pour it back and forth between containers.

Which method for which trip

Whatever you pick, carry a backup. A few chlorine dioxide tablets weigh almost nothing and save the trip when a filter clogs or a UV battery dies. Keep them in your first aid kit so they come on every hike.

Collecting water well

How you collect water matters almost as much as how you treat it.

• Choose the best source available. Flowing water from a spring or stream is usually cleaner than a stagnant pond, but still needs treatment.
• Collect upstream of trail crossings, campsites and places where livestock or wildlife gather.
• Keep the clean side clean. Wash your hands before handling the clean bottle, and don’t let the dirty bag touch it.
• Carry enough capacity. Desert trips and dry ridges can mean 10 miles or more between reliable sources. Check recent water reports with the land manager and plan how much to carry, using our guide on how far you can hike in a day.
• Protect the source. Wash dishes and bodies at least 200 feet from lakes and streams and bury waste well away from water, as the Leave No Trace principles describe. That’s how everyone downstream stays healthy.

Your dog drinks from the same streams and can pick up the same parasites, so carry treated water for pets too (see hiking with dogs). On paddling trips, the lake is your supply for days, and our guide to planning a first canoe trip covers how to fit water treatment into camp routine. For cold-weather trips, winter camping for beginners explains melting snow, which also needs treating, and the day hike packing list shows where a filter fits for shorter outings.

If you get sick after a trip with diarrhea, cramps or bloating that lasts more than a few days, see a doctor and mention that you drank backcountry water. Giardia symptoms often start one to two weeks later, and a stool test is the only way to know what you picked up.