Success in terms of drinking water can be defined as follows:
The quantity of water available is sufficient to prevent dehydration and the quality of the water is such that the health risk from consumption of the water is acceptable.
The next question automatically becomes what is an acceptable health risk? In general terms, most people would agree that so long as consumption does not create any unwanted side effects than the risk is acceptable.
This is where things become complicated . The ability of an individual's immune system to ward off waterborne pathogens varies greatly from one person to the next. For example a sewage system operator may be able to consume 1000's of pathogenic organisms with impunity due to workplace vaccination and repeated exposure to various pathogens. A normal healthy person may be able to consume only 100 of the same pathogenic organisms before becoming sick. Lastly a person with HIV, cancer or other immune system disorders may not be able to consume any pathogens.
Ultimately an acceptable risk level is determined individually. The following article examines the different types of pathogens commonly encountered, the methods available for disinfection and the pros and cons for each method. The article does not recommend any specific product but instead strives to provide you with the information necessary to make an educated choice regarding your drinking water in the backcountry.
A few words on my qualifications. I am a civil engineer with a specialty in water and wastewater treatment. I own a small company that specializes in the design, construction and operation of treatment facilities. I typically split my time between North American projects and projects in the developing world (usually Africa and the Asian "Stans").
Waterborne ThreatsWaterborne disease threats come in two forms:
Biological threats are a common occurrence in the backcountry, fortunately there are simple methods to reduce or eliminate this risk. The remainder of this article examines some of the pathogens that may be present in the backcountry and the methods available to mitigate the associated risk.
The Usual Suspects (A.K.A. Pathogens)There are three major classes of microbial pathogens. In order of increasing size they are as follows:
Essentially all waterborne microbial pathogens fall into one of these three categories. Specific characteristics and common examples are discussed further in the following sections. Not all specific pathogen species are described as this would make the article unsuitably long, however the control methods described in the "Treatment Options" section of the article are specific to each general group i.e. if a treatment method is described as being effective for viruses than it is effective for all species of virus, or if effective for bacteria than it is effective for all species of bacteria etc.
- Require a host cell to replicate
- Known viral pathogens are from human fecal sources
- Can only move via passive transport (e.g. stream current or the bottom of a boot)
- Very small size 0.020 to 0.100 microns (a micron is 1/1,000,000 of a meter - to put this in context, human hair is 40 to 50 microns in diameter and a hydrogen atom is about 0.0001 microns in diameter)
- Examples include norovirus(NLV), hepatitis A, polio and rotavirus
It is estimated that 90% of gastroenteritis cases not caused by bacteria are the result of NLV. NLV is of particular concern in areas such as base camps as humans are the only known host and close contact facilitates transmission. Other properties of NLV that are of concern:
- Can survive freezing
- Very infectious (low number of organisms required to cause illness)
- Very persistent in water
- Rapid onset (12 to 48 hours after exposure)
- Causes vomiting and diarrhoea
- Reproduce by binary fission, numbers can grow explosively
- Use the warm, nutrient-rich environment of the gut to reproduce and are shed in feces
- Size range is from 0.3 to 2.0 microns
- Some are mobile with a whip-like flagellum
- Examples include Camphylobacter, Escherichia coli O157:H7, Salmonella, and Vibrio Cholerae
- Survive best in cold water
- Incubation of 2 to 5 days
- Causes abdominal pain, headache, fever, nausea and vomiting
- Chronic complications occur in 1 to 2% of cases including arthritis, convulsions, and paralysis
- Fatality rate of 0.1% in the developed world
- Reproduction involves a complex life cycle
- Can form cysts (single parasite) or oocysts (multiple parasites) that are resistant to environmental conditions
- Reproduction occurs when a cyst or oocyst finds a favourable environment
- Range in size from 2 to 50 microns
- Examples include Giardia (beaver fever), Cryptosporidium and Ameoba
The largest documented outbreak of waterborne disease in the history of the United States was attributed to cryptosporidium. In the city of Milwaukee in 1993 over 400,000 people were sickened and over 100 died due to cryptosporidium that was transmitted via the municipal water system.
Source Water Selection
- Drilled well (not usually an option) - often (but not always) considered potable without treatment
- Snow in alpine areas - often (but not always) considered potable without treatment
- Ice in alpine areas - often (but not always) considered potable without treatment
- Clear cold (4C or 40F and less) alpine surface water from non-stagnant water bodies - may be potable without treatment but treatment is usually recommended
- Warm (above cold temperature limit) alpine surface water from non-stagnant water bodies - treatment is recommended
- Non-alpine surface water from non-stagnant water bodies - treatment required
- Surface water from stagnant sources such as swamps or beaver ponds - treatment required.
In all cases the source should be located upstream from any human or animal activity, if possible.
Springs are not mentioned in the above list as they can range from close to top of the list to just above the bottom of the list depending on local geology conditions. For example a spring may be fed from glacier melt water via rock fractures (good) or it may be fed from a perched stagnant swamp through rock fractures (bad).
Treatment OptionsTreatment of drinking water in the backcountry can be grouped into five general categories as follows:
- Boiling or pasteurization
- Enhanced Filtration (sometimes marketed as purification)
- Ultraviolet irradiation
- Chemical disinfection
An important point to note is that when used properly treatment provides disinfection. This is different than sterilization. Disinfection is a percent removal or inactivation of pathogens. Generally speaking the treatment process will aim to reliably remove/inactivate 99.9% to 99.99% of viable pathogens after treatment. If the source was properly selected than this will reduce the pathogen load in the water to a level that can be consumed by most healthy people. Sterilization on the other hand refers to the destruction/inactivation of all life in a given sample. An example of this would be an autoclave machine at a hospital used to sterilize surgical instruments. Due to the difficulty involved in trying to kill every last pathogen sterilization is considered impractical in the backcountry.
Boiling and Pasteurization
This is the simplest way to treat water and involves boiling the water (rolling boil required) for 2 minutes or heating the water to 65C (150F) for 5 minutes. This method is effective against all forms of pathogens and works even if the water appears dirty or cloudy. The unfortunate drawbacks to this method are the time and fuel required.
Portable water filters have been available from outfitters for decades and vast improvements have been made in efficiency and performance since they were first introduced. Filters are usually rated based on the pore (opening) size of the filter. The pore size is typically given in microns, however this number can be misleading as there are different methods for determining the pore size. The pore size is usually described as nominal, average or absolute.
Nominal pore size means that about 70% of particles, at the rating size and larger, will be retained by the filter. The remaining 30%, at the rating size and larger, will pass through the filter. Essentially nominal just means that at the rating size the filter operates at about 70% efficiency. Average means that openings can vary from much larger to much smaller than the rating. Essentially average tells you nothing about a filters ability to remove a particle of a given size. Absolute means that all particles above the rated opening size will be removed.
Therefore when choosing a filter the absolute pore size must be known. Filters currently on the market offer absolute ratings as low as 0.2 microns. At 0.2 microns, filtration will effectively remove protozoan and bacterial pathogens but is not effective against viral agents. If the source water you are using is not subject to contamination from human fecal sources than a filter (preferably 0.2 micron absolute) is a good choice as virtually all waterborne viral agents are from human sources. An absolute rating between 0.2 and 1.0 microns is effective against protozoan pathogens but is not considered effective against bacterial and viral pathogens. A filter in this category may be appropriate depending on the source quality and the immunity of the individual drinking the water. Other factors to consider when choosing a filter are:
- Flow rate capacity of the filter
- Filter capacity (how much water can be pumped before filter element requires replacement or cleaning)
- Robustness (The filter ratings were determined in a laboratory - not in the backcountry. How will the filter perform after being dropped, stepped on, freeze-thaw cycles, even water temperature can adversely affect the filter performance, etc.)
- Pre-filtration may be required for water containing a lot of suspended matter (turbidity). This will prevent premature filter clogging. I like to use coffee filters or paper towel for this as they provide about 15 micron pre-filtration and are inexpensive and light.
Enhanced Filtration (Purification)
Now for the caveats:
- the other factors mentioned for filtration also apply to enhanced filtration
- In order to guarantee their performance maintenance is critical as there are numerous other contaminants that also have a negative charge. The result is the resin or media can quickly become exhausted depending on the water quality. Manufacturers recommendations must be followed.
- Make sure that the chosen purifier is "certified to" or "meets the requirements of" the US EPA standard for virus removal/inactivation.
- One last item, if the water being treated is cold the performance of these type of purifiers is greatly reduced and it may be necessary to substanstially reduce the flow rate through the purifier or pump the water through twice.
Ultraviolet Irradiation (UV)
UV can be effective against all types of pathogens but does have limitations. The limitations are summarized below:
- The water must be free from suspended particulate. Suspended particles can shield pathogens by directly blocking light transmission
- If the water contains dissolved (not always visible - usually at the molecular level) matter the UV light can be absorbed (think of it as sunglasses that block UV light). This can in severe cases reduce the UV dose by a factor of 2 to 20. In these cases UV can still be effective by using the device for a period 2 to 20 times longer than would normally be required. If the water in question is visibly tinted than this is a sure sign that the water contains dissolved solids and a longer exposure time will be required. Make sure you read the manufacturers specifications/directions as this may be addressed in the manual.
- Check with the manufacturer to make sure that the recommended exposure is effective against viruses - if you are concerned about viral threats. If the manufacturer indicates that the recommended exposure only works for protozoa and bacteria than the exposure time must be increased by a factor of 8 to adequately inactivate viruses.
Disinfection using some type of chemical additive has been used in the water treatment industry for over 100 years. The common characteristic that all chemical disinfectants share is that they are oxidants. Addition of the oxidant causes inactivation of the pathogen through one of, or a combination of the following:
- DNA damage
- protein coat damage
- capsid function impairment
The different types of chemicals can come in liquid, powder or pill form. In all cases the chemical is added to and mixed into the water until dissolved. Once the chemical is dissolved a period of time must pass before the water is considered disinfected. The length of time required is a function of the chemical used and the dose applied. Make sure the manufacturers recommendations are followed as the time required is critical.
Some of the typical chemicals on the market include:
- Free chlorine - usually produced from pelletized or powdered calcium hypochlorite. Sometimes produced with sodium hypochlorite solution (this is essentially bleach but possibly at a different solution strength)
- Combined chlorine (also called chloramine) - can be in pellet, powder or liquid form. This is free chlorine that has been combined with ammonia. The causes the chlorine to persist in the water for much longer periods. The drawback is that it is a much weaker disinfectant and is not usually used for the initial disinfection. This may see use in a base camp where large quantities of water are required. In these cases the water would be initially disinfected using some other method and the chloramine would be added to prevent regrowth.
- Chlorine dioxide - can be in pellet, powder or liquid form. This is a very strong disinfectant that rapidly inactivates most pathogens
- Ozone - a powerful gaseous oxidant that can only be produced on site with electricity. Not usually done in the backcountry unless you are at a base camp that has a reliable electricity source.
- Iodine - a powerful oxidant that is available in liquid or solid form.
- Mixed oxidant - a mixture of oxidants produced by a battery powered probe and a chemical additive (usually salt).
There are numerous other chemical options on the market. These are just some of the more common ones. With the exception of chloramine all of the oxidants listed above are effective against viral and bacteriaI pathogens. Chloramine can be used after initial disinfection as means to prevent regrowth. The only oxidant considered effective against protozoa is chlorine dioxide. Even with chlorine dioxide 4 hours or more may be required to inactivate protozoa if the water is cold. In all cases follow the manufactures directions as temperature and suspended solids (dirt) can greatly affect the disinfection process.