HOW TO CHOOSE A WATER FILTER
OR PURIFIER
Is it possible to drink straight from backcountry streams and never become ill?
Yes.
Is it possible to drive down a large city's main boulevard, ignore a few red
lights and never have a fender-bender? Yes.
Is either practice worth the accompanying risks? In our opinion, no.
Free-flowing mountain
streams, for all their beauty and clarity, are not always the fountains of
purity we imagine them to be. Backcountry water sources — crystal-clear rivers,
lakes and streams — sometimes harbor microscopic pathogens (disease-causing
agents) that are tough to pronounce, difficult to spell and, for many people,
awful to ingest.
Giardia lamblia. Cryptosporidium. Campylobacter jejuni. Hepatitis A. All are
members of an invisible fluvial zoo that may be present in pristine-looking
backcountry water.
How
do they get there? When water becomes tainted by animal or human feces. What impact
could such microbes have? They can leave you reeling with diarrhea, abdominal
cramps, nausea, weight loss and fatigue. How long might these symptoms last?
Between 4 and 6 weeks. Maybe longer. Ugh.
More details on waterborne pests, and techniques you can use to defeat them,
are explained in our Water Treatment section. In this
presentation our goal is to provide guidance on the water-treatment strategy
favored by most wilderness travelers — using a water filter or purifier.
Portable
water filters and purifiers both operate on the same mechanical principle.
Using a hand pump and intake hose, both slurp up "raw" water from a
lake or stream and force it through an internal element (a filtering
"medium"). This medium traps suspended elements — from fine sediment
to invisible microorganisms—before dispensing clean water into a container of
your choice.
What's the Difference?
Some definitions:
Water filter—A microbiological device that removes bacteria (e.g.,
Campylobacter jejuni) and protozoan cysts (Giardia lamblia, cryptosporidium)
from contaminated water.
Water purifier—A microbiological device that removes bacteria, protozoan cysts
and viruses (e.g., hepatitis A) from contaminated water.
Viruses
are infinitesimal organisms too tiny to be trapped by a filter. Devices
identified as "purifiers" usually cause water to interact with iodine
(often in the form of iodine resins), which can render viruses inactive.
Another purifier uses a positive electrostatic charge in its filter medium to
capture viruses.
Viruses:
Over time, filters have proven that they reliably protect wilderness travelers
from the most common waterborne pathogens found in the North American
backcountry: giardia and cryptosporidium. Still, purifiers and their antiviral
feature offer an elevated level of security.
To fully disinfect suspect water using a water filter, the Centers for Disease
Control and Prevention recommends 1) mechanically filtering the water, 2)
treating it with a halogen (chlorine or an iodine solution), 3) letting it sit
15 to 60 minutes, 4) then drinking. For more details on this process, and a
discussion of what pathogens may be found in backcountry water, refer to our Water Treatment Section.
In an ideal world, a water
filter or purifier will be:
-Simple to use
-Easy to pump
-Capable of sustaining a
steady, generous flow
-Effective against
waterborne pathogens
-Slow to clog, easy to clean
-Long-lasting
How can you tell if a
filter or purifier delivers in these areas? Look for clues in the product
description.
Here's how to interpret the
information:
Filter medium — This is the cartridge that actually traps pathogens
(plus silt and other debris). The composition of the medium contributes greatly
to the quality (and cost) of a device. Medium materials include:
Ceramic: This is an effective, high-quality earthen material
that can be cleaned many times before it needs a replacement. A ceramic
cartridge captures most particles within .005 of an inch of its surface, so
it's easy to brush away clogged pores and expose new ones. Cartridges
themselves are fragile and require careful handling. Ceramic elements are the
longest-lasting mediums and make a good choice for frequent backcountry
visitors.
Ceramic with a carbon core: This additional layer helps filter out the
taste of halogens (chlorine and iodine) plus some organic chemicals, herbicides
and pesticides.
Fiberglass (or glass fiber): As effective as ceramic in straining out
pathogens, but not as long-lasting.
Structured matrix, or labyrinth: A dense, honeycombed material that
effectively captures pathogens.
Iodine resin: A chemical layer integrated with a purifier's filtering
medium that deactivates viruses, though it does not actually remove them.
Field
cleanable
— A desirable feature. This means you may open the filter to brush or scrub the
filter medium and increase water flow. Clogging should not cause you alarm; it
shows the filter or purifier is working. Ceramic filter media can usually
accept dozens of cleanings. Some models can be cleaned through backwashing
(feeding clean water through the filter in reverse) but you need ample clean
water in order to do so.
Longevity: How long will a filter or purifier last? Ceramic
filters that can accept cleaning will last the longest, but the life of any
filter depends on the clarity of water you pump through it. If possible, seek
out clear water in still pools. You're likely to find less sediment in such
water than in rushing water. Use a prefilter if your device includes one.
Manufacturers sometimes include an estimate of the number of liters a filter or
purifier is expected to treat effectively.
Pump
force —
The higher the number, the harder it is to pump. The Katadyn Pocket Filter, for
example, has a pump force number of 16.5. While this is one of the
longest-lasting filters available, it really gives users a workout as they
pump.
A few additional considerations not listed in spec charts include:
Effectiveness — Nearly all commercial
filters will knock out larger microorganisms such as giardia and
cryptosporidia. So what do you get for choosing a more expensive filter?
Usually a longer-lasting filter medium, cleanability features and maybe a more
efficient pump handle. Which filter is right for you? Here's a basic guide:
Pore
size — A
familiar benchmark for determining a filter's effectiveness is to establish
that it is a "point-2 (0.2-micron) filter." The number refers to the
size of the pores (openings) in a filter medium. It's not a bad gauge, since
the smallest bacteria measure 0.2 microns, yet some microbiologists will tell
you it is a simplistic standard. Factors such as maximum flow rate, minimum
wall thickness and adsorptive capacity can influence such a conclusion.
Arguments can be made to show that a 0.3- or 0.4-micron filter can be as
effective at trapping the particles as a 0.2-micron filter.
Tip: Look for "absolute" pore size (the largest and
least effective holes) when evaluating filters, not "nominal" pore
size.
Adsorption — When filter media block
particles while clean water streams through, the process is known as
"sieving." When particles stick to the media in the manner of a
magnet, this is "adsorption." Activated carbon, found in some filters
and purifiers, is especially effective at adsorption.
Replacement cartridges are available
for nearly all units on the market. They cost roughly one-half of the original
unit's cost.
Some models attach directly to specific water bottles, which is a nice touch.
It can prevent a heartbreaking spill in the field.
If you're visiting places where turbid water is a factor (say, the desert
southwest), a cleanable ceramic filter should be tops on your list. The same
goes if you'll be filtering for a group.
Avoid filtering water in area where animal or human activity is obvious.
Try and filter water from still, clear water sources. Many microorganisms tend
to sink to the bottom of still water; a turbulent stream keeps them suspended.
Rather than filter directly from the stream or lake, put water in a pot and
filter from that. This gives you a chance to examine exactly how the water
looks before you send it through your filter. This helps prevent clogging. If
the water is cloudy, let it sit in the pot for an hour or so, then skim the
clearest water off the top.
Don't save the first few streams of output from your filter. They don't taste
as fresh.
When you clean your filter, recognize you are handling a potentially
contaminated object. Don't handle food or put your hands to your mouth after
cleaning your filter.
Follow manufacturer instructions for cleaning and storage. At home, consider
pumping a weak bleach-and-water solution through the filter to sterilize it. If
you can disassemble your unit, allow it to dry out completely before storing
it.
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