How to purify water in the wild

Jun 08, 2023

By Cliff JacobsonPhotographs by Rachid DahnounFrom the September-October 2015 issue of Scouting magazineBackpacking & Hiking, Camping, Cooking, Ground Rules, Health, Magazine, Outdoors

THERE ARE FEW places in North America where you can safely drink water from a lake or stream. So unless you camp where safe drinking water is readily available, you’d best bring your own. But at about 8 pounds per gallon, water can be a real drag if you have to carry it far. The alternative is to go light and purify what you need.

Examine the methods described here and determine the best option for the type of trip, the geographic area you will be visiting and the number of people in your group.

BoilingThe good news is that gastrointestinal pathogens are very sensitive to hot water. Boiling water from an in-the-wild water source will kill almost everything, even parasites like giardia and cryptosporidium that can survive long exposure to iodine and chlorine. (Technically, a water temperature of 150 F will kill all GI pathogens, but it's easier and simpler to just bring it to a boil.)

Chemical TreatmentChlorine bleach or iodine will kill most waterborne pathogens, but if you get the dosage wrong, they can be dangerous. Water-treatment tablets are safer and easier to use. Chlorine dioxide and sodium dichloroisocyanurate tablets are more effective against giardia than chlorine or iodine tabs. However, water-treatment tablets will not reliably kill cryptosporidium, which, fortunately, is uncommon in surface waters.

Some negatives about using water-treatment tablets: They require at least 30 minutes to work if the water temperature is above 68 degrees. Much more time is needed for cold or cloudy water. Heat speeds chemical reactions, so you’ll shorten the purification time if you set your water in the sun while the tablets work. Tablets lose potency over time; fresh ones work faster. And to reduce the bitter taste after treatment, aerate your water by pouring it back and forth between two canteens.

A Filter or Purifier?Filters strain out microorganisms, but they don't kill them. If hiking in the U.S., filtration is typically considered to be sufficient.

A filter should remove the smallest infectious agents. For instance, a filter with a 1-micron absolute pore size will trap giardia and cryptosporidium. Viruses fly through the smallest pores, but they can be killed with water-treatment tablets after filtering. Fortunately, harmful viruses are extremely rare in surface waters.

Because the "bugs" become trapped inside the filter, this can eventually become clogged and must be cleaned or replaced.

Filters may have pumps or be gravity-fed. Gravity-fed units are bulky, but they’re as effective as pumps and are easier to use — particularly with groups of Scouts. They’re great if you need to filter a lot of water fast and don't want to labor with a pump.

Purifiers, on the other hand, typically use microfiltration and either chemical treatment or ultraviolet light to kill microorganisms, including viruses. This higher level of water purification is necessary when traveling outside of the U.S. in regions where viral contaminants are a risk.

Ultraviolet PurifiersPeople who live near bodies of freshwater have long relied on ultraviolet light to kill microorganisms. The popular SteriPen was among the first to harness this technology for use in the wilderness. Just insert the UV bulb into a water bottle and activate the light. When the light turns off (after about a minute) the water is safe to drink. UV purifiers will destroy giardia, cryptosporidium and viruses. But they won't remove sediment or work in cloudy water. And they can purify only small amounts (usually one liter or less) of water at a time.

Before You GoDon't buy more protection than you need. Talk to park rangers or other hikers or campers who are experienced in the area where you’re traveling. Once you find out more about the types of water sources available, you’ll be able to choose the best system for your needs.

What's best for you depends on the microbes you expect to encounter, how light you want to travel and the volume of water you will treat.

Tips for Treating H2O

TAKE A QUIZ TO TEST YOUR KNOWLEDGE OF HOW TO PURIFY WATER.