Purifying Wild Water: A Guide to Staying Hydrated in the Outdoors

21 February 2023

As an Emergency Medicine Physician Assistant based on the Island of Unalaska/Dutch Harbor in the Aleutian Islands, Alaska, Sarah Spelsberg knows the value of being prepared in remote and challenging environments. One lesson she learned the hard way was the importance of properly purifying water while out in the wild. After falling ill with giardia for two weeks due to improperly purified water, she gained a newfound appreciation for the significance of this crucial skill. 

In this blog, Sarah will share her insights on the essential nature of water purification and the techniques you can use to stay safe in any wilderness setting.

Water Sources

Water sources in the wilderness can be vulnerable to various contaminants such as animal carcasses, faecal matter, agricultural waste, heavy metals from mining (even if it ended decades ago), algal overgrowth, and bacterial or viral infections when travelling to an endemic area. 

To make such water safe to drink, three key steps are required: decontamination, sterilisation, and beautification. Decontamination involves removing heavy metals, toxins, or harmful substances. Sterilisation is the process of eliminating harmful microorganisms such as viruses, bacteria, and giardia. Finally, beautification involves removing unpalatable substances like algae and tannins. This can be achieved through a combination of gross filtration to remove larger particles, fine filtration using gravity or pressure, and sterilisation. A filter with a pore size of 0.22 microns or less can effectively remove virus particles in addition to bacteria, giardia, and other organisms.

In the absence of a filter, there are various methods of purifying water:

Gross Filtering/Beautification

To beautify/gross filter water, begin by selecting the clearest possible water source, ideally in a sunny location and away from agricultural and wildlife areas. Moving water tends to be cleaner than stagnant water. Next, use a t-shirt or similar material to filter out large impurities and prevent the fine filter from becoming clogged. Store the gross filtered water in a clean container.

Fine Filtering/Decontamination

For fine filtering/decontamination of water, a gravity filter such as a Sawyer Mini can be used to purify impure water. Simply fill the filter with impure water, hang it up, and let the water drip through into another bag or container, producing clean water. Alternatively, pumps can be used where individuals manually pump water through a filter using pressure. As a personal preference, I always carry a LifeStraw or similar handheld and portable water filtration and sterilisation system. The LifeStraw filters and purifies water but requires considerable suction force to pull water through it.

Purifying Sterilisation

In the absence of a filter, there are various methods of purifying water:


Once the water has been filtered, it can be further purified through boiling. Boiling the water at a rolling boil for at least one minute, even at high altitudes, can kill most harmful organisms. Ideally, a pot and camp stove or fire will be available. However, if these are not available, you can build a fire and create a tripod or suspension device using sticks to hang a water bottle or Gatorade bottle filled with water over the fire until it boils for at least one minute. The bottom of the bottle may become blackened from the fire, but it should still be sturdy enough to hold and boil the water. To make a camp stove, you can use any alcohol (such as everclear, vodka, or isopropyl alcohol) or gas, and a soda or tuna can, by cutting two rows of holes along the top.


Although iodine tablets are a convenient and lightweight option for water purification, they can have an unpleasant taste. It’s important to note that iodine, when consumed in high levels, can lead to thyroid issues.


In my experience, adding a capful of bleach to a gallon of filtered water is a more palatable method of chemical water treatment when compared to iodine tablets. It’s recommended to let the treated water sit for 30 minutes before consuming.

UV Light

Using a UV light device is a tasteless and effective way to sterilise water. Alternatively, leaving filtered water in the sun for three days provides enough UV exposure to purify the water. 

Another method of harnessing the sun’s energy is by constructing a solar still. You can create a solar still using any type of plastic material, which can be suspended over water, allowing the evaporated water to be collected in a clean receptacle. There are several functional designs available for building solar stills, which can be found online.

Morning Dew/Condensation

If you search for larger leaves, you can often find condensation on them, which can be collected. Additionally, condensation can be collected from the tarp or tent.

Top Tips

Consider these tips before heading out on your trip to ensure you have access to safe drinking water. First, inquire with locals about any water concerns in the area. Next, consider your specific needs during the trip. Do you need to keep your gear lightweight, are you providing water for a larger group, or is taste important? 

While diarrhoea is often caused by unfamiliar bacteria and typically resolves on its own, prolonged symptoms lasting over a week should be assessed by a medical professional. 

Bottled water is impractical and environmentally unsound when travelling in the wilderness, but using lightweight, safe, and reliable water purification devices is a practical alternative.

Survival Situation

If you find yourself in a survival situation without any water filtration or purification tools, there are still ways to make water safer to drink. One method is to use cloth, such as a bag or t-shirt, to filter the water, along with sand, stones, or charcoal to remove larger sediment.

Another option is to let the collected water settle so that the sediment drops to the bottom, then carefully collect the clearer water from the top. Boiling water after filtering out sediment can be done by hanging a Gatorade or other water bottle from a stick tripod over a fire. 

Remember to always hope for the best, but be prepared for the worst! When it comes to purifying water – a wise physician once told me – two is one and one is none. It is safest to utilise two purification methods. 

 → Join a World Extreme Medicine training course to learn how to take care of yourself and others during an expedition. 

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