water treatment

Backcountry water treatment

All water sources in the backcountry, pristine-looking lakes, fast-flowing creeks and glacier-fed streams can be contaminated by birds, animals or humans. Simply put, if your water is from an un-treated source, you should treat it before you drink it, cook with it or use it to brush you teeth.

The cleaner the water is at the start, the more effective any treatment method will be. When you select a water source, avoid standing water if you can, and use the clearest water available. If you have to collect water that looks cloudy or muddy, let it settle in a pot or bucket, and skim or siphon off the clearer water to treat it.


Waterborne pathogens (disease-causing micro-organisms) of most concern:

  • Protozoa include cryptosporidium and giardia (the cause of giardiasis or beaver fever). Protozoa are increasingly widespread in North America. They are larger than one micron.
  • Bacteria exist in water all over the world. Most are harmless, but some cause sicknesses such as diarrhea and dysentery. Most bacteria are about 1.0 micron in size, but some, like disease-causing campylobacter, can be as small as 0.2 microns.
  • Viruses cause hepatitis, polio, and other diseases. They can be present in any water contaminated by human waste. Most viruses are in the 0.1 micron range, but can be as small as 0.002 microns.
Treatment methods compared protozoa bacteria viruses particulate
boiling yes yes yes no
iodine/chlorine most yes yes no
chlorine dioxide/mixed oxidants yes yes yes no
0.5mi filter and pre-filter yes most no yes
0.2mi filter and pre-filter yes yes no yes
0.1mi filter and iodine/chlorine yes yes yes yes
ultravioliet (UV) light yes yes yes no
UV light and pre-filter yes yes yes yes

Treatment methods: boiling

Enough heat will destroy all pathogens, but it’s not a popular method as it consumes time and fuel, and boiled water has a flat taste. So, except for cooking water, most people choose to treat their water with filters, chemicals, or both.

  • Recommended times range from 3 to 10 minutes.
  • Higher elevations require slightly longer boil times.

Chemical treatments

These don’t require any pumping to use, but they do require time to act – from a few minutes to several hours, depending on how cold or dirty the water is, and which pathogens you want protection against. Chemicals are effective against bacteria and viruses. But some protozoa, including some cryptosporidium, are resistant.

Chemicals are good for short-term use: a few days or a few weeks. For many, the health risk associated with small amounts of exposure to chemicals is outweighed by the benefit of protection against pathogens. But for pregnant women, young children or anyone with a pre-existing health problems, this may not be true. It’s best to consult your doctor before using chemical water treatments.

  • Chlorine kills many of the micro-organisms that help you digest food, resulting in digestive trouble. In water, chlorine also forms some by-products that are suspected carcinogens.
  • Iodine comes in easy-to-use drops or tablets, though many people don’t like the taste and odour it adds to the water.
  • Chlorine dioxide is unstable, so you prepare it in the field by mixing two component chemicals. Since the active ingredients in chlorine dioxide break down quickly, it usually won’t kill beneficial micro-organisms in your digestive system. It doesn’t leave any unpleasant taste or odour in your treated water.


Filters strain out pathogens when water is forced through them. They remove protozoa and some larger bacteria, and at 0.2 micron, also remove smaller bacteria such as campylobacter. Viruses are many times smaller than bacteria, so most filters cannot remove them.

  • Filters can fail or be damaged by impact. Many people carry compact water treatment tablets as a back-up.
  • Great where viruses are not a concern, or for occasional users, small groups, and short trips.
  • For protection against viruses, you can treat water with iodine then filter it. A filter with a carbon stage can remove between 90 to 98% of iodine.


These have one or more filter stages that trap larger pathogens, together with an iodine resin or another element that inactivates viruses.

  • High-end purifiers do not introduce chemicals into the water.
  • Economical over the long term. Suited to frequent users, extended trips, and larger groups.

UV light purifiers

Portable devices designed to treat 0.5 or 1L of water at a time. By beaming UV light through water, they damage the DNA of microbes, pathogens and viruses. It’s the same technology used in commercial bottling plants and municipal water systems.

  • They avoid the taste, health concerns, and waiting times of chemical treatments.
  • Don’t require pumping, and don’t need to be dissembled and dried when you return home.
  • Cloudy water must be filtered before you use UV.
  • Most use batteries, so you’ll need to carry spares or a solar re-charging system.


Filters should be taken apart and left to air-dry to help prevent mildew and bacteria growth. They can be cleaned many times with a brush before needing to be replaced, but clean them only when necessary to prolong their life.

Silty water (often found in glacial regions) clogs filters and makes pumps more difficult to operate. To make pumping easier, let any silt settle for an hour or two before pumping the water.

Cost effectiveness

Generally, the lower the initial cost for a treatment method, the higher the cost of each litre treated.

  • If you camp occasionally and in a small group, a moderately priced filter with a semi-disposable cartridge, combined with a small chlorine dioxide kit, may be fine.
  • If you camp frequently, or with a large group, it may be more economical in the long run to buy a more expensive filter.
  • Inexpensive filters clean a few dozen to a few hundred litres. A high-end filter might clean thousands of litres.
  • A UV purifier with rechargeable batteries can treat 3,000-8,000 litres.