Chapter 27 - Food, Water and Outdoor Cooking
Water Filters

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.

2) Chemical 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.

Filter Types

Gravity-Feed Filter: 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 camp.

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.

Filter Design Types

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.

Ceramic Element: 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 not.

Depth Filter: 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.

Surface Filter: 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.

Water Filter Features

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.

Filters Out: 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.


Bacteria: 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.


  Protozoa: Hard-shelled, single-cell parasitic protozoa, or cysts, are the largest waterborne micro-organisms, ranging from 5 to 15 microns. The Giardia cyst causes most waterborne illnesses in the United States.


  Viruses: The smallest (.004 to 0.1 microns), least pervasive water-borne fauna are viruses, yet they are arguably the most                 dangerous because they can deal deadly ills like polio and hepatitis. Fortunately, viruses pose little danger in the surface   water throughout most of the United States and Canada.

Prefilter: This 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.

Rated Output: 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.

Weight: Listed weights are complete with standard accessories.

Replacement Filter 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

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