How to Purify Water Outdoors

Purifying water outdoors from mountain rivers and streams is essential. Here we’ll talk about the different options available (filters, Steripen, and tablets for example) to get rid of viruses, bacteria and parasites.

Drinking water is essential for mountain activities. Photo: Katadyn

Water is essential for life. Hydration is always important, but when you combine dehydration caused by outdoor activity with a shortage of drinking water, it becomes critical.

Sometimes, you may not be sure of the quality of water and it will need to be purified. Although clean drinking water can be found in the mountains, what appears to be pure water may actually be contaminated by the presence of livestock or a submerged carcass at a higher altitude.

In drier climates, such as Mediterranean regions, water is harder to find and is usually of a dubious source.

For this reason, it is a good idea to carry water purification options to prevent intestinal problems that could ruin your plans or even worse, your health.

Some people choose to resist using water purification systems for mountain activities, but this is usually related to the fact that until recently, the only affordable and lightweight systems were tablets, which tended to take time to work and would leave a strong taste of chlorine in your mouth.

But things have evolved and modern systems now allow you to purify and drink on the go.

Basic rules for water intake

  • Even if you plan to purify water, you should always try to collect running water, rather than stagnant water from a pond or pool.
  • The nearer you are to the source of the water, the purer it will be. Spring water will be safer than river water.
  • In remote areas, snow water is a good option, but bear in mind that snow water lacks in minerals and this can lead to dehydration. For this reason it is convenient to add a mineral containing supplement, such as electrolytes or tea.
  • There is a common belief that snow water is safe to consume, but this is not entirely true. In most cases, it is safe when the snow collected has not been contaminated by animals, which is often the case in high mountain environments. An example of this is the legendary stomach problems caused by drinking snow water in the Goûter camp. However it is always important to remember the well-known phrase: never drink from yellow snow.
  • No matter how crystal clear it is, and no matter how close we are to the source of a river, you should avoid drinking water near livestock, dead animals upstream, excrements, etc.

What harmful microorganisms are found in water?

The pathogens that contaminate water may include:

1. Viruses

These are the smallest microorganisms. They measure between 0.02 and 0.2 microns.

The smaller microorganisms are not eliminated by most physical filtering systems, but these are not usually found European mountain water or areas far from human population. In any case, each system indicates the diameter of the virus it is able to filter and in most cases, they are able to filter common viruses. For example, the SARS-COv2 virus, which causes COVID-19, has a size between 100-160 nanometers, and many filters specify that this can be filtered.

Viruses are often confused with bacteria. We have all heard of the cholera virus, for example, but it is in fact a bacteria (Vibrio cholerae).

Why are viruses usually not a problem in the high mountains?

The first thing to keep in mind is that viruses rely on other transmission systems, such as air, etc. Water is not an element prone to viral transmission, and water-borne viruses are specifically human, not animal, such as Hepatitis A. The waterborne transmission system is human ingestion-feces-human ingestion.

Next: due to their characteristics, they do not reproduce by themselves: they require the living cells of a "host" which they penetrate to produce more viral particles.

In the water, unlike bacteria and other microorganisms, viruses do not reproduce and only survive for a certain length of time. If they survive long enough to be reintroduced into another human, they reproduce again. It is therefore unlikely that a remote mountain river, far away from any contamination such as untreated sewage, would be contaminated by a virus. Except in a few exceptional circumstances viruses are not transmitted by water.

In short: outside densely populated areas with contaminated, untreated water, viruses are rarely a problem. So the possibility of infection in high mountain waters in the Pyrenees, for example, is extremely unlikely, but it is easier to become infected in a densely populated area with a non-existent or ineffective wastewater treatment system.

2. Bacteria

Larger than viruses, but smaller than parasites, the size of bacteria is between 0.2 and 5 microns. They cause cholera, typhoid, etc. E. coli, Salmonella, etc. are eliminated by physical purifiers.

3. Parasites

The largest of the microorganisms measure between 1 and 15 microns. Amoebas, Protozoa such as Giardia, worms, etc. are also eliminated by physical purifiers.

Purifying Systems

1. Boiling

One of the simplest way to eliminate microorganisms is by boiling the water. But that is not always an option and even less so during activity.

Apart from having to wait for boiled water to cool down before you can drink it, it means you always have to carry a stove, fuel and saucepan, even for a short day trip, and boil it every time you run out of water during the excursion and have to refill the canteen in the middle of the trip.

It is still a good system for activities such as camping, where weight is not as much of an issue and you aren’t constantly on the move.

2. Physical Purification Systems

The most commonly used non-chemical systems are filters and membranes with a micron pore diameter of 0.1 to 0.2, which remove 99.9% of bacteria and parasites, as well as viruses larger than the filter size, which are the majority. With a diameter between 0.1 and 0.16 microns and no scientific evidence of its transmission by water, the COVID-19 virus is filtered by many physical systems.
Msr Miniworks EX
Until recently, physical systems were heavy and more suitable for camping than for use on the go, during activity. But in addition to the fact that the traditional ones are now lighter, this has changed with the advent of water bottle systems, such as the Katadyn Befree or Lifestraw Go.
Katadyn BeFree. As simple as drinking from a water bottle
This system consists of a water bottle which incorporates a 0.1-0.2 micron filtering system, depending on the model. They can be used like any other water bottle, for running, hiking, etc. but unlike normal water bottles, these guarantee the elimination of bacteria, parasites, and most viruses. This is a fast and efficient way of getting instant drinking water.

This system is similar to the water filter straw system, such as the Lifestraw Personal Water Filter. This is still an excellent system for on the go hydration.

The Katadyn Befree water filter, adapts to any Hydrapak flexible water bottle.

Traditional filters are used for quickly hydrating one or two people, depending on the size. The larger ones are group filters, and are more complete than the water bottle systems: many of them have a dual system, that not only filters, but also removes turbidity, bad taste and odor from the water.

3. UV Purification

A highly effective system that has revolutionised the world of water purification.

A small device weighing just 140 grams that eliminates 99.9 percent of bacteria, parasites and viruses with ultraviolet "shots" in a non-chemical way. There areseveral models by the brand Steripen, that also include a LED to inform when the water is ready to drink.

The benefits are evident and this system is surprisingly efficient.

It has 2 disadvantages:

  • The first is filtering. The water is clean enough to drink, but any floating residues are not filtered, (e.g. a leaf, ant or hair).
  • The second is the problem of large quantities. It purifies, depending on the model, from 1 litre to 4 litres at a time. This is more than enough for most, but a bit slow for groups, for example at base camp.

The Steripen Quantum solves the problem with residues, by incorporating a filter. Making the best of both worlds.

4. Chemical Purifying

This is most traditional system, consisting of water purification tablets or drops.

They usually consist of a blend of chlorine and silver ions. Some people prefer to use bleach suitable for consumption, but this is not easy to transport in small quantities when outside and water purification tablets are a much safer and more convenient system.


  • Very low in price
  • They take up no space or weight
  • They eliminate viruses in densely populated areas where this can be a problem (such as while travelling or backpacking)
Katadyn Micropur Forte MF 1T

  • They leave a taste of chlorine in the water (this can be hidden by isotonic powder, tea, etc)
  • If you mistakenly add more tablets than required, this can cause problems.
  • They can harm your body if used frequently. Consumption should be limited.
  • You have to wait for them to work: it usually takes thirty minutes for bacteria and viruses, but in some cases, such as eliminating Giardia, it can take up to two hours. It therefore does not perform as well as other systems when it comes to supplying water on the go and requires planning.

So this system is not very efficient for mountain activities, where hydration is needed on the go, but as they are so small and light, it is easy to pack a tablet in your pack, just in case. However, it is still an effective system for travelling, when bottled water is not easy to find.

If you are unsure if the water is clean, you can use a test strip, such as the Msr FAC test strips to measure the residual chlorine concentration. They are to be used thirty minutes after purifying the water and indicate if the chlorine has completed its work or if you need to wait longer.

Some manufacturers recommend adding more than the usual amount if the water is very cold. However, since these are chemical products, it is important to follow the instructions carefully.

Clean water is always recommended. In case of cloudy water, their effectiveness decreases, so it is a good idea to filter or decant the water in some way.

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