Two Ways to Purify Water

Visitor Filtering Water from Cosley Lake in Glacier National Park
Visitor filtering water at Cosley Lake in Glacier National Park

NPS/Jacob W. Frank

Are you going to explore the great outdoors? It is very important that you plan for your water needs as potable water may not always be available, especially in backcountry and wilderness areas. “Potable water” is clean water that’s safe to drink, brush your teeth with, wash your hands with, and use for preparing food. Before you head out, check out the “Plan Your Visit” section on the park’s website or contact the park to find out if there are potable drinking water sources in the park and along your adventure route. Learn about any water quality alerts, such as harmful algal bacteria or chemical spills, that would affect drinking it.

Never drink water from a natural source that you haven’t purified, even if the water looks clean. Water in a stream, river or lake may look clean, but it can still be filled with bacteria, viruses, and parasites that can result in waterborne diseases, such as cryptosporidiosis or giardiasis. It is essential that you purify natural water. Purifying water involves filtering to remove large particles and treating by boiling or with chemicals to kill organisms such as bacteria, viruses and parasites.

If you plan to use natural water along your adventure, follow these steps to collect and purify water from a natural water source.

The first step in the process is to collect the water you will drink. Here are some tips on how to safely collect water from a natural source:

  1. Start with a clean container that preferably has been disinfected prior to use.

  2. Wash your hands with soap and water or use hand sanitizer before collecting water so you don't contaminate it.

  3. Chose a collection spot that is:

    • On higher elevations or near the water's source

    • Away from established campsites

    • Away from animal grazing areas.

  4. Collect water from areas of moving water in rivers and streams, or the top few inches of a lake. Stagnant (standing or non-moving) water is a breeding ground for insects, bacteria and viruses and should be avoided.

  5. Dip your bottle just under the surface and fill from there.
Hiker filters water from spring at dead horse spring
Hiker filters water from spring at dead horse spring
NPS Photo

The next step in the water purification process is filtration. Filtration by itself does not purify water. It must be followed by boiling or disinfection to purify water for drinking.

  • Most water filters are made of a screen with many tiny holes in it. These filters can remove protozoa and some bacteria, but they cannot filter out viruses because viruses are too small.

  • Filters also remove bigger contaminants like leaves, silt, dirt, and sand. If the water is cloudy or has floating material in it, you should filter it even if you plan to boil or disinfect it.

  • Filtration systems with absolute pore size less than or equal to 1 micron filter (NSF Standards 53 or 58 rated “cyst reduction / removal) have a high effectiveness in removing cryptosporidium and giardia.

  • Be sure to use and care for your filter according to the manufacturer’s instructions. Filters don’t work as well if they aren’t taken care of over time.

The final step of purification is disinfecting the water which can be done by either boiling or treating with a disinfectant. This is the most important step as these methods will kill any remaining organisms in the water, especially those that could make you sick.

Boiling is the best way to kill disease-causing organisms, including viruses, bacteria, and parasites. The high temperature and time spent boiling are very important to effectively kill the organisms in the water. Boiling will also effectively treat water if it is still cloudy or murky.

  • If you’re at an elevation below 6,500 feet, put the water in a container over a heat source, such as a campfire or propane stove, and bring to a rolling boil for 1 minute.

  • If you’re at an elevation over 6,500 feet, bring the water to a rolling boil for 3 minutes.

Your water is now ready for drinking! You can make tea or coffee if you like it hot. Let it cool down first before you put it into your container.


Disinfection happens when a chemical or UV light is added to water to kill bacteria, viruses, and other potentially harmful organisms. Many factors can impact the effectiveness of these methods including water temperature, pH, and cloudiness. With disinfectants, it is important to allow the chemical or UV light enough time to treat the water and kill the organisms before drinking – this is called contact time.

Chemical Disinfection
Chemical disinfection involves adding one or more chemicals to your filtered water that are effective at killing waterborne organisms.

  • Chemical tablets or liquid drops are the most common ways to disinfect natural water. Iodine or chlorine dioxide are the most frequently used disinfection agents. National Sanitation Foundation (NSF) approved products are recommended

  • Follow the manufacturer’s instruction for disinfecting the water. Contact time to disinfect the water varies by product (example: 30 minutes to 4 hours). If the water is cloudy or has floating debris, it will be more effective to pre-filter the water before disinfecting.

  • WARNING: Do not use pool-cleaning tablets to disinfect drinking water! Pool-cleaning tablets are not intended to be consumed.

  • WARNING: Consult with your physician before using any disinfection products. Some tablets or drops, especially iodine, may not be safe for pregnant women, people with thyroid issues or iodine hypersensitivities, or for user over long periods of time.

UV Light Purifiers
Visitors using UV purifiers to disinfect their water
Visitors using UV purifiers to disinfect their water
NPS Photo

Portable battery-operated UV purifiers can be used to reduce bacteria, viruses and protozoa in water from natural sources. However, these purifiers are only effective in disinfecting small quantities of clear water, are battery dependent and require correct contact time with the water.

To achieve maximum disinfection, make sure you:

  • Pre-filter your water as small particles and sediment may shield microorganisms from the UV light.

  • Check that you have enough battery power. If the battery power falls below a certain level, it will not be able to safely disinfect water.

  • Make sure you allow for enough contact time with the water. Follow the manufacturer’s instructions because inadequate exposure to the UV light may fail to disinfect your water.

  • As an extra precaution: Add a small amount of your preferred chemical disinfectant to the water in your container to maintain water quality and reduce the growth of organisms in the container.

Stay Hydrated

Remember – it takes time to prepare your water for drinking. Plan ahead so it’s ready for drinking before you get thirsty, especially in hot weather. If you think you have the symptoms of water-borne disease or were exposed to potentially contaminated water, be sure to consult with your healthcare provider after your trip.

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Last updated: August 24, 2022