Field Water Disinfection


You Are What You Drink… And What You Might Be, Is Very Very Sick.

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Ah, water - easily one of humanity’s top ten most important liquids!  “The stuff of life,” cry some.  “The universal solvent,” wrongly exclaim others.  Ensuring clean, safe, potable water is increasingly difficult, as polluting agents become more widespread, and danger may lurk within even seemingly pristine water sources.  This entry discusses some contaminants commonly found in the North American backcountry, some techniques for removing them, and the perhaps-not-surprising role of basic hygiene in preventing contamination and GI illness.

The information in this article is largely based on Dr. Howard Backer’s excellent presentation, Field Water Disinfection, published online through the Journal of Wilderness Medicine's presentation series.  



Water Treatment Terminology

Disinfection: Removal or destruction of pathogens.  Usually this term refers to the various methods of rendering bacteria, viruses and protozoa non-threatening, usually by reducing their numbers.  It is important to remember that disinfection does not necessarily destroy all the pathogens - only enough to make the water safe to drink.  For salmonella, this means bringing the viable population below 100,000, while for Giardia, this means bringing the population under 10.

Sterilization: Destruction of all life forms in the water.  Necessary for invasive medical instruments, but not necessary for drinking water.  All backcountry water sources are home to a plethora of microbiological life forms, most of which are of no concern to human health.

Purification: Removal of chemicals and other substances that impart a bad smell or taste to water.  True purification techniques might reduce pathogen populations, but they do not reliably disinfect water.

Filtration: Removal of sediment and debris to make water clear (or less cloudy).  Has little or no effect on pathogen populations.


Types of Water Contaminants

Types of water contaminants can be broken into 5 groups: bacteria, viruses, protozoa, parasites, and chemical contaminants. Examples of the first four contaminants include:


Agents in red are found in remote areas in North America; others are most commonly found in areas of dense human habitation without proper sanitation infrastructure, usually in developing countries.  Please take the time to familiarize yourself with local pathogens and contaminants if you intend to travel in those environments.  

The common effect of these infections is the same - butt stuff, or traveler's diarrhea - but the severity can vary wildly. Some cause only mild upset, while others are potentially fatal, if not treated.  The minimal infectious dose of each also varies wildly - 100,000 Salmonella bacteria is considered an infectious dose, and if that seems like a lot, then consider that as few as 10 Giardia protozoa are needed to set up shop in your GI system.  

Chemical Contaminants include herbicides and pesticides, run-off from industry, and leaching from landfills, tailings ponds, and deactivated mines.  It can be very difficult to know where these contaminated sites are - locations of older, decommissioned sites are often forgotten - and wilderness users should consider chemical as well as microbiological decontamination.  However, in the concentrations generally found in wilderness water, chemical contaminants generally don’t pose a danger to human health when ingested infrequently and in small amounts, so… good news… I guess...


Water Treatment Techniques

  • Heat
  • Clarification
  • Granular activated charcoal
  • Filtration
  • Chemical Disinfection
  • Ultraviolet Light

Each of these techniques has its advantages and disadvantages.  Best results can be achieved by combining techniques based on your situation, the source of water, and your individual preference.  The Need to Know section directly below gives a brief overview of each, while the Nerd to Know section that follows is… extensive.

Water Treatment Techniques - Need to Know


Water Treatment Techniques - Nerd to Know

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Ah, heat!  One of the body’s top ten most important temperature conditions!  Treating water with heat is a time-honored technique, and one that has many advantages - it is fast, effective, and relatively safe.  It doesn’t need any special equipment or knowledge, and its effective point - boiling - is easy to recognize.  The only disadvantage is the reliance on fuel, which might be scarce due to the environment, or limited by local fire bans.  Boiling also does nothing to improve the taste or the appearance of cloudy or dirty water.

To disinfect water, bring to a rolling boil - as seen in the photo above - rather than stopping when some little bubbles form and rise.  In North America, this alone generally provides adequate disinfection, and even sterilization.  Just for fun, here are the thermal death points of common pathogens:

  • Giardia: 2-3 minutes at 60º C
  • Cryptosporidium: 2 minutes at 64º C, 1 minute at 72º C
  • Enteric viruses: Seconds at 80º -100º C
  • Hepatitis A: 1 minute at 85º C
  • Enteric bacteria: 1 minute at 65º C, Seconds at 100º C

Also, since boiling point is affected by atmospheric pressure (Google it, kids) higher altitudes require longer boiling times.  However, as you can see, even at almost 6000 metres, a few minutes at a rolling boil should set you right.

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Clarification Treatments


Clarification methods improve the appearance of water, but are of limited use in disinfection.  Always use in conjunction with a true disinfecting technique, such as boiling, halogens, or UV light.  Although clarification methods do not disinfect, they have several advantages when combined with other treatment methods:

  • Improve the appearance, and sometimes the taste, of water.
  • Prolong filter life by reducing the amount of filter-clogging debris.
  • Improve the effectiveness of UV treatment by removing potential “hiding spots” for sneaky little buggers.
Naturally occurring brown coloration in rivers is primarily due to organic and inorganic sediment stirred up by the movement of water, and is worse during periods of storms or other turbulence.

Naturally occurring brown coloration in rivers is primarily due to organic and inorganic sediment stirred up by the movement of water, and is worse during periods of storms or other turbulence.

One method of clarification is sedimentation, which simply involves allowing large particles to settle by gravity.  This is particularly effective treatment for glacial water, which tends to contain inorganic compounds.  Let it sit for 30-120 minutes, then carefully pour off the clear water, leaving the sediment behind.  This water should now be subjected to a disinfecting treatment.


Not all material will settle by gravity, but coagulation-flocculation treatment can improve your clarification efforts.  C-F treatment involves adding some form of chemical that binds with suspended particles, forming clumps that sink to the bottom, or float to the top, for easy removal.  Alum - aluminum potassium sulfate - is a cheap and effective C-F option that coagulates inorganic and organic compounds, many chemicals, and up to 90% of giardia cysts and other protozoa and bacteria.  In a pinch, fine white ash from the campfire can be used as well.  Haven’t tried it myself, but the comments section is a good place to report your results, if you should feel so inclined.

Floc layering with alum - add about 1/8 teaspoon per litre of water, and allow 30-60 minutes before filtering.  If that doesn't do the trick, simply repeat - alum is non-toxic.

Floc layering with alum - add about 1/8 teaspoon per litre of water, and allow 30-60 minutes before filtering.  If that doesn't do the trick, simply repeat - alum is non-toxic.


Although C-F methods are somewhat effective against all types of contaminants except viruses, they are not effective enough to be solely relied upon.  Filtration, boiling, or UV treatment is highly advised.

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Activated Charcoal


Activated charcoal binds to many chemicals, and is a common treatment for many types of poisoning.  Many filters include a charcoal segment.  As a treatment option, it can be added to water (and then filtered out) after halogen disinfection or boiling.  It is particularly useful if you are sensitive to the after-taste of halogens, because it will remove these chemicals after they have done their job.

On the other hand, activated charcoal provides very limited disinfection - again, it is best used in conjunction with halogen treatment or filtration, and many commercial devices combine these treatments.

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Commercial filters have become very popular water treatment options among backcountry wanderers.  They are simple to use, and require no chemicals.  However, they are relatively bulky, prone to breaking, and have finite life spans that might not be recognized.

As a filter becomes clogged, flow rate decreases.  This might be treated as an inconvenience, but should be taken as a sign to change the filter - the increased pressure required to pass water through the filter can deform organisms, allowing them passage as well.  Bacteria also proliferate in filters, and they should be cleaned with a diluted bleach solution between trips.  Silver impregnated filters claim to prevent bacterial proliferation, but there is no evidence to suggest that they are effective - clean your filters!

The usefulness of filtration against microbiologicals depends on its pore size.


Few manufacturers offer filters with pore sizes small enough to filter viruses, and it must be remembered that there is no government regulatory oversight of any claims of filter effectiveness - they are based only on internal testing.  Decide for yourself the degree to which you trust the manufacturers, but a post-filtration halogen treatment… wouldn’t hurt.

Drink-through filters like LifeStraw are light, effective personal systems, but can’t be used to filter water for the group, or for purposes than personal hydration.

Drink-through filters like LifeStraw are light, effective personal systems, but can’t be used to filter water for the group, or for purposes than personal hydration.

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Chemical Disinfection Agents


Ah, halogens!  Easily one of the top ten groups of salt-producing chemicals on the periodic table.  Chlorine and iodine are traditional disinfecting agents, but each lends its own distinctively awful taste to treated water.  Furthermore, iodine can be harmful to the health of pregnant women and people with thyroid disorders.  These days, chlorine dioxide is de rigeur - it imparts negligible taste (although some claim greater sensitivity to it… of course…), plus its cheap, safe, and readily available.  

Chlorine dioxide water treatment is available in two forms: two liquids that are separately inert but activate when added to water together, and tablets.  The liquids are heavier, bulkier, slightly more complicated to use, and the bottles are prone to breakage.  The tablets take a little longer to work, but are very light, and there is very little risk of spoilage.

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The main advantage to halogen treatment is ease of use in any quantity of water.  The main disadvantage is that it takes time, and that time is affected by the water temperature.


Although chlorine dioxide is usually assumed to be a 30 minute treatment, this varies highly with the temperature of the water.  The following graph (lifted from Dr. Backer’s presentation) shows the effectiveness of iodine against Giardia at different concentrations at 5 degrees Celsius.  These results are the same for chlorine and chlorine dioxide.

Graph Iodine Effectiveness Against Giardia.jpg

Thus, for very cold water, 2-4 hours of exposure is needed to completely eliminate giardia.  As for cryptosporidium, 99.99% inactivation can be achieved by multiplying the amount of time needed for effective giardia treatment… by a factor of eight!  So, under optimal conditions, at least 4 hours of halogen treatment is necessary for effective cryptosporidium disinfection.  Even this might not be enough - if you are concerned about cryptosporidium contamination, boil or filter your water before applying halogen treatment.

Furthermore, other contaminants - organic and inorganic - can reduce halogen effectiveness by binding to the chemical.  Cloudy or turbid water should be clarified or filtered before treatment.

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Ultraviolet Light


Ultraviolet light water treatment units are just about the most disco piece of gear you can stick in your pack.  Apply your glowing purple sci-fi stick to contaminated water and watch your friends gasp in envy and amazement.  “Halogens are for suckers,” you declaim derisively, taking a long drink of your cool, sweet water, while those around you anxiously check their watches, waiting for thirty minutes to pass so they, too, can enjoy some refreshing hydration.  Yes, UV disinfection is the stuff of the future, rendering all other forms of water treatment archaic...

Slow your horses, buckaroo!

While UV treatment has definite advantages, there are other considerations - and, like the other treatment options on this list, it is best used in conjunction with other methods. On the pro side, UV treatment is fast and effective - in fact, it might even be most effective against the durable and dastardly cryptosporidium, due to that organisms relatively larger size, and therefore exposed surface area.  UV disinfection leaves no bad taste, acts fast, and the coolness factor cannot be overstated - you, too,  will be friendless no more!

On the other hand, while hand-crank options are available,  most UV treatment units require batteries, which means packing extras, and perhaps planning for resupply on particularly long expeditions - not to mention the environmental effects of using batteries at all.  Furthermore, like filters, UV units are prone to breakage, rendering them useless.  Finally, they are only truly effective in completely clear water - debris and cloudiness create shadowed areas where microorganisms can hide from the sterilizing effect of the UV light.  Use a clarification or filtration method prior to UV treatment for best results.

Solar Disinfection

Solar disinfection (also called SODIS… I mean… really…) is an improvised UV treatment option is which simply involves putting your water in a clear plastic - not glass! - bottle, and leaving it in the sun.  Disinfection occurs in 4-6 hours on a sunny day, but in cloudy conditions, you need TWO DAYS of exposure to achieve sufficient UV disinfection.

Water Treatment Summary

Heat, chlorine dioxide tablets, and ultra-violet can provide relatively reliable one-step disinfection for most remote areas of North America - just be aware of the limitations of each, and prepare a backup.

Two-step treatments are always better, and lead to more aesthetically palatable drinking water.  The two combinations that promise the best results are,

Coagulation-Flocculation followed by Halogenation (Chlorine Dioxide)
Halogenation followed by Filtration (With a charcoal matrix)

No matter which technique, or combination of techniques, you choose, be sure to familiarize yourself with the expected contaminants of the area you will be hiking in, and the manufacturers information on any equipment you will be using.  


The Role of Hygiene


Holy smokes!  That is a lot of information about water treatment!  Faced with such a wide variety of waterborne threats, and the many difficulties in confidently ridding your drinking water of them, one might be forgiven for rethinking the wisdom of one's next expedition, choosing instead to spend the week on the sofa with a Raspberry Pi Super Nintendo Emulator and a nice glass of clean municipal water.  

It is worth noting, though, that GI illness in the backcountry seems to be caused by waterborne pathogens… almost never???  A 2004 article in the Wilderness Medical Journal, which examined available evidence-based research (admittedly, it was - and continues to be - scarce), concluded “there is no good epidemiological evidence that North American wilderness waters are inherently unsafe for consumption.” (Welch, 237)  

This is not to suggest that field water treatment is unnecessary - rather, it is a reminder that most cases of “traveler's diarrhea” do not actually seem to be associated with waterborne pathogens, but rather originate in… wait for it… “hand to mouth fecal contamination.”  


As it turns out, while all these water treatment methods are useful, none are as effective at preventing GI illness in the backcountry as good camp hygiene and personal cleanliness.  Some tips for camp hygiene include:

Used tissue paper half-halfheartedly concealed under a nearby rock  is a bad example of camp hygiene, but an excellent way to get everyone sick!  Way to go.

Used tissue paper half-halfheartedly concealed under a nearby rock  is a bad example of camp hygiene, but an excellent way to get everyone sick!  Way to go.

  • Establish your latrine area at least 100 feet from camp and from any water drainage.
  • Bury waste at least 8-10 inches deep.
  • Establish a common trench latrine for large groups.

As for personal cleanliness, lean on alcohol based hand sanitizers, and good, old-fashioned hand washing with lots of soap to drastically reduce your chances of getting sick during your next backcountry expedition.  

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All of that, and it comes down to, "if you don't want to get sick, wash your hands."  Who would have thought?


Share your own ideas, experiences and especially mishaps with backcountry water treatment in the comments section.