Backcountry water contamination, Giardia is the most
common, can stem from wild denizens such as beavers and moose, or from domestic
intruders such as livestock and people. Your favorite brook might be safe one
day but not the next, or it might be fine in one pool but undrinkable 100 yards
downstream. The only prudent choice is to purify all drinking water, and you
have three options.
l) Boiling: No
microscopic organism survives three minutes at a rolling boil. But the extra
fuel demand makes boiling impractical on longer trips unless you resort to
campfires. Even then, boiling is a time-consuming hassle.
This is the filtration process of choice for winter camping trips because
filters will freeze up after the first use.
Disinfection: Iodine treatment in tablet, crystalline, or concentrated
liquid form is light, easy to use, and effective. But it won't make funky river
water look any more appetizing, and gulping iodized water requires, an acquired
taste. Depending on water temperature, iodine tabs may require lengthy contact
times before you can safely drink. Those with thyroid conditions and pregnant or
nursing women should avoid iodine. Chlorine-based treatments such as Halazone
simply aren't reliable in backcountry situations.
3) Filtering: Is
considered by most as the best all-round water purification method available. A
filter physically removes most offending organisms. It strains out suspended
solids, turning coffee-colored murk into clear (or at least less murky) liquid.
Some filter models follow up with iodine-based chemical treatment to kill
critters too small to be filtered out. Despite potential breakdowns, a filter
remains the most convenient means of water purification in the field.
This filter relies on gravity to trickle water through the filter element. It is
a simple process using a reservoir (bucket) a filter, and connecting tubes. A
gravity filter works in slow motion, so it is best used for treating water in
Pump Filter: A
hand or foot pump speeds the water processing time. A pump filter allows you to
treat water directly out of a stream an drink it . However a pump has many
moving parts and may fail to operate. You should always have
a second method for water treatment just in case your pump filter fails.
Straw Filter: A
straw filter is a filter-filled tube that strains out some larger impurities.
But without chemical backup, the straw type can’t be relied upon for adequate
protection. These filters are not
recommended for backcountry use.
A filters task is to remove organisms and other particles
larger than a specified size from water. This mission isn't so easy, given the
teensy-weensy critters involved. Only an exceedingly fine filter or
microstrainer will catch them.
Here the water runs through a cylinder of unglazed ceramic material. A ceramic
filter generally exhibits the longest service life of any filter available
(back-wash or scrub the element surface to clean it), but the element will need
to be replaced eventually.
Depth Filter with Carbon: Activated carbon strains out live cooties and also removes organic chemicals like pesticides, herbicides, chlorine, and iodine. The carbon won't, however, remove dissolved minerals such as salt. It collects chemicals by a process called
"adsorption, the adhesion of molecules to a solid
surface. Problem is, after reaching the limit of adsorption for a particular
material, the filter will no longer remove that material, and previously
adsorbed material can be released. Accordingly, carbon filter manufacturers
recommend replacing these models on a specified schedule, whether clogged or
This filter consists of thick, porous materials with a complex structure to trap
the undesirables. A depth filter can be partially cleaned by backwashing or by
brushing the outer filter surface. Even with regular cleaning, a depth filter's
element will clog up eventually and will have to be replaced.
Depth Filter with
Iodine: This filter combines physical filtration with chemical treatment to
kill the little nasties. The filter snags Giardia and other larger bugs, while a
resin-bonded-iodine element kills bacteria and viruses on contact. Unlike
treatments using free" iodine (tablets), the resin-bonded iodine
contributes little of the chemical to the water.
Also called a membrane filter, a surface filter is perforated with precisely
sized holes that allow water through while blocking particles larger than the
openings. A surface filter is simple and effective. This type tends to clog
quickly but is generally easy to clean and has a long service life.
Pore Size: This
rating indicates the size of openings in a filter element, which determines what
size particles can be physically removed. Pore sizes are measured in microns,
and the period at the end of this sentence is about 600 microns across. We
requested that the manufacturers provide "absolute" pore-size ratings,
which means the filter element will pass no particles below a given size. These
absolute ratings are much more meaningful than the vaguely defined
"nominal" ratings that filter manufacturers sometimes use. These
ratings basically mean a filter removed, most, but not all, the particles below
a specific size.
Straining out the most common backcountry water-bogey,
Giardia cysts, requires a maximum pore size of 4.0 microns. Look for a
0.2-micron pore size to remove bacteria; however, a filter this fine is
subject to rapid clogging and will need frequent cleaning in murky or silty
water. Because viruses can be as small as .0004 microns, no field device that
relies entirely on filtration can reliably remove them.
This heading tells what impurities each filter is designed to remove. There are
generally three types of water borne diseases: protozoa, bacteria, viruses, plus
organic chemicals that are of concern to backcountry hikers.
Generally smaller than protozoa (.2 to 10 microns), bacteria travel in a
wide, wicked variety. E. coli indicates fecal contamination and gives you
the trots; Salmonella's various transmutations inflict intestinal fever,
food poisoning and typhoid fever; Staphylococcus and Streptococcus breed
big, painful boils.
Organic Chemicals: Pesticides, herbicides, diesel fuel, fertilizers, and strip-mine runoff make up this group. As a rule, suspect any water draining from an area where there’s agricultural or timber production, mining operations, or heavy industry. And beware of any water that is discolored or has an odor.
filter option fits on the end of the intake hose to strain out larger particles
that would otherwise clog the main filter. A prefilter is often an extra-cost
option but is worth its price to reduce cleaning hassles and extend filter life.
This figure reflects the manufacturer's stated output in pints per minute under
"ideal" conditions. Take it with a very large grain of salt. Turbid or
glacially silted water, a half-clogged filter, or kinked hoses can lengthen
processing times considerably.
weights are complete with standard accessories.
Cost: This is the bad news about how much you'll have to shell out for a new
filter element when cleaning won't coax any more water through a well-used unit.
Price: Costs for
filters range from $15 to $150.
References: Backpacker, March 1994
Copyright 2001 Northern Arizona University, ALL RIGHTS RESERVED