The methods of
treatment available to backcountry travellers are boiling, adding chemicals, and
filtering. The latter two methods are sometimes used in combination. Some
commercial water treatment devices both filter and add chemicals. Such units
are properly called purifiers to distinguish them from filters,
which do not add chemicals.
Boiling | Iodine |
Filters | Purifiers
Boiling
- Kills all pathogens, including viruses
- Imparts flat taste to water
- No special equipment required
- Used fuel
- Time consuming
According to the U.S. Centre for Disease Control, a full, a rolling
boil for one minute will destroy all pathogens, regardless of your elevation
(water boils at lower temperatures at higher altitudes). Most pathogens are
killed at temperatures below 100 degrees Celcius and die while the water is
heating. Boiling's primary drawbacks are that it requires additional fuel, is
relatively time-consuming, adds an unpleasent or flat taste, and does not remove
debris such as silt from the water. For those who travel only one or two
weekends a year and are willing and able to carry a day's worth of water,
boiling is a good option.
BACK
Iodine
- Properly performed, effective against
almost all pathogens, including viruses
- Affects taste of water
- Inexpensive
- Temperature sensitive
- Not for long term use
Chemicals
The addition of iodine or chlorine is another option for rendering
contaminated water drinkable. Some books discuss the use of chlorine in the
form of bleach, and it is true that bleach has been successfully used. However,
in addition to its immediate poisonous effects, bleach can kill micro-organisms
that are a natural and vital part of your digestive system. Chlorine is also
available in purer forms, but it is chemically unstable, making it impractical
for backcountry use, which leaves iodine as the best choice of chemical
treatments.
Iodine crystals, tablets, and tincture are inexpensive, lightweight,
compact, and, when used correctly according to the manufacturer's instructions,
effective at destroying bacteria, viruses, and most protozoa (although
Cryptosporidia and some other protozoa highly resistant to iodine may require
filtration).
There are limitations to iodine, however. It takes time to work (The
St. John's Ambulance Guide cautiously recommends a contact time of eight hours
to be certain of killing Giardia). The effectiveness of iodine is
dependent on the acidity of water (ph), temperature (in water below 10 degrees
Celcius its effectiveness is reduced), and siltation level of the water;
determining the correct dosage and then waiting for the iodine to work is
incovenient to some people. In addition, many people find the taste to be
objectionable, although this can be masked by adding juice crystals. Add
flavour mixes after the iodine contact time is complete, to avoid interfering
with the chemical reaction. Also of concern are the possible health risks
asscociated with long-term ingestion of iodine. One generally suggested maximum
time for iodine use is two to three weeks. This is only a rule of thumb
however, as the safe limits will vary depending on the iodine concentration used
and the individual's physiology. Pregnant women and persons with thyroid
problems or iodine allergies should consult a physician before using iodine.
Lastly, iodine does not remove solids and debris from the water.
All in all, iodine is an option for those persons who will not be
using it for extended periods of time, and can live with its taste.
BACK
Filtration
- Not consistently
effective against viruses
- Physically blocks many pathogens
- Expense varies with different units
- May actually improve taste
- Effectiveness dependent on filter design and materials
Unlike boiling and chemicals, both of which attempt to destroy
pathogens, water filters attempt to remove various micro-orhanisms from the
water. Some filters also have components of carbon, which in addition to
removing some pathogens, remove many undesirable tastes and odours. The
pitcher-type filters often sold in supermarkets and drugstores, usually using
activated charcoal, are intended only to remove tastes and odours (such as the
taste of the chlorine added to municipal water systems) - they are not suitable
for disinfection of biologically contaminated water.
In order to remove micro-organisms, a water filter usually employs one
or more filters that will physically block the passage of the organism(s) and
any associated particulate debris through the filter. As a general rule, in
order to block pathogens and bacteria - which are measured in microns (a micron
is one thousand of a millimetre) - a filter must screen down to at least 0.2
microns. Although the bacteria Klebsiella terrigena (0.5+ micron) is commonly
used to test filters, some experts now maintain that 0.3 microns should be the
size filters are tested to. They point out that there is at least one disease
causing bacteria only 0.3 microns in size - Campylobacter. Campylobacter has
been responsible for outbreaks of disease in the Rocky mounatins of Wyoming and
may be moving into other North American watersheds. Check with public health
officials or park personnel in your travel area for up-to-date information.
Most water filter manufacturers do not claim their products will
protect against viruses since viruses are smaller than even the tiniest filter
pores. In practice, however, a filter with a 0.2 or 0.1 micron opening will
offer a degree of protection against viruses such as hepatitis since these
micro-organisms are usually attached to particles in the water. In addition,
carbon filter elements may trap by bonding many viruses that are smaller than
the actual rated pore size of the filter. To be on the safe side, you should
also treat the water with iodine, or use a purifier when travelling in areas
where viruses are a concern - most often Third World countries.
The primary advantages of using filters are that they are more time
efficient than either chemicals or boiling, lighter and more compact than the
fuel needed for boiling on longer trips, and do not add any undesirable
after-taste. In fact, some activated carbon filters actually improve the taste
by removing odours and chemicals from the water. On the negative side, compared
to boiling and chemicals, water filters are expensive, relatively heavy and
bulky, and require occasional maintenance.
Despite these limitations water filters are a popular choice for those
frequently in the backcountry. When used in conjunction with iodine treatment,
they are a viable choice for those travelling overseas for extended periods of
time, especially in Third World countries.
BACK
Purifiers (Filters with built-in iodination)
-
Potentially effective against all pathogens, including viruses
- Some designs eliminate odours and taste of iodine
- Expensive (iodine cartridges must be replaced)
- Simpler than adding iodine seperately
- Can reduce exposure to iodine
- May reguire more than one pass through unit
While devices sold only as filters do not have to meet any government
standards in either Canada or the U.S., any device marketed in the U.S. as a "water
purifier" must meet the EPA Guide Standard and Protocol for
Microbiological Purifiers, which mandates removal or inactivation of
99.9999% of Klebsiella terrigena (a 0.5+ micron bacteria), 99.99% of poliovirus
and rotavirus and 99.9% of Giardia.
As noted above, viruses are smaller than the tiniest filter pores, so
purifiers must add a chemical, usually some form of iodine, to inactivate
viruses. This is accomplished by passing the water through a material (often a
resin matrix) impregnated with the chemical. Many purifiers incorporate a
filter downstream of the iodine, which removes particles and iodine resistant
pathogens. These downstream filters may also have carbon beds to remove
chemicals, including the iodine - eliminating the objectionable taste and smell.
The last approach sounds like the best of all worlds - the security of
having inactivated the viruses, removed the iodine resistant pathogens, and the
bonus of making the water palatable to drink. However, as always, increased
sophistication brings increased complexity. As noted above, iodination is
temperature and time sensitive. Filtering out the iodine in a purifier's final
stage reduces the time the iodine has to work on the pathogens.
At least two purifier manufacturers acknowledge in their product
instructions that worst case water (in the case of a purifier "worst case
water" usually means water that is very cold (5 to 10 degrees Celcius),
highly alkaline, and very turbid) may require a second pass through the device
and an interval before drinking to ensure safety.
The other complication is that the iodine elements have a limited
useful life - measured in volume yeilded, not time. This makes it important to
keep track of the amount of water processed through an iodine cartridge -
trickier than you might expect since purifiers do not have water meters built
in. Iodine cartridges are also more expensive than drops or tablets. That
being said, a properly designed and operated purifier is a viable method for
water treatment.
Some treatment systems allow the user to choose whether or not to
iodinate the water, attaching or detaching an iodine cartridge (of course, when
used without the iodine cartridge, these systems do not meet the EPA's standards
for a "water purifier"). These systems function as a regular filter
when viruses are not considered a likely danger (usually the North American
backcountry), and as a purifier where viruses are a threat (anywhere human or
animal waste enters the watershed in large quantaties).
Filters without iodine resin cartridges offer the option of adding
iodine separately where the danger of viral infection is high. With this
method, there is no uncertainty about the iodine concentration or contact time.
The limitations of iodine in cold and turbid water discussed in "Addition
of Chemicals" section still apply. The iodinated water can then be
filtered to reduce odour and taste (provided the filter includes a carbon bed).
When risk of viral infection is low, use of iodine can be discontinued and the
water treated by filtration alone. This two-tiered approach requires slightly
more work and awareness, but is more adaptable to conditions.
Methods of Treatment
Water Filters