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Water Filtration Guide

     Filters physically strain out offending critters larger than a certain size, but here's the confusing part:  There are filters that remove, or filter out, protozoa's and bacteria, and there are filters that 'purify', which means they also remove viruses.  Which type you choose goes back to the degree of risk you're willing to take.  Obviously, 'purifiers' are the safer way to go, because they strain hard-to-get viruses.  But purifiers cost more, and while all filtering devices eventually clog, purifiers tend to clog quicker than non-purifiers.

Iodine Disinfection
Types of Filters
The Filter Element
Pore Size

Iodine Disinfection:
     Iodine treatments come in three forms: Tablets, crystals, and liquid. For many years, iodine has been the most efficient way for backpackers to treat their water, because it's lightweight, cheap, and as easy dropping a sugar cube into a cup of coffee. But iodine has its downsides. First, it does nothing to make murky, foul-tasting water more appealing. In fact, it compounds the bad taste for some people. Second, and more important, experts have recently discovered that iodine doesn't kill Cryptosporidium, a protozoan that's becoming increasingly common in North American backcountry water.

Types of Filters:
     Pump filter.  This category encompasses the majority of available filters.  Pump filters feature an intake hose, a filter element, a hand pump, and an outlet.  Simply stand over the stream and pump away.   They're fast, efficient, and the better ones don't require too much effort.   Be fore-warned, though, because pump filters rely on a host of mechanical parts that can break when you're miles from nowhere.
     Gravity-feed filter.  These are best for in-camp use when time is not of the essence.  You gather water in some sort of reservoir or bag, hang it on a tree, then tend to other chores while the water trickles through a filter element and into a receptacle.
     Straw filter.  These are inexpensive and easy to use.  Most consist of a filter-filled straw that you insert in the accompanying water bottle after you've dipped it in a water source.  For the most part, straw filters are rudimentary and remove larger microorganisms only.  The best kind incorporate some sort of iodine as back up.

The Filter Element:
     Carbon.  Activated carbon strains out organic chemicals like herbicides, pesticides, and chlorine.  It is always used in conjunction with another filter material.
     Ceramic.  By nature, ceramic is a porous, intricate material with lots of little nooks and crannies that capture microorganisms.   The best thing about ceramic filters is their longevity; they can be cleaned time and again before they need replacing.  The drawback:  Ceramic is fragile, especially in cold weather.
     Fiberglass or Glass Fiber.   Glass fibers are long and slick and can be molded into intricate structures that effectively catch contaminants.  Fiberglass doesn't last as long as ceramic, but it's more durable.
     Iodine.  See "Iodine Disinfection" for more information.  Some companies offer iodinated cartridges that can be attached to their filters to kill viruses.

Pore Size:
    The size of the openings in a filter, or pore size, determines what particles get trapped, and which ones slip through.  Pore size is measured in microns.  The period at the end of this sentence is about 500 microns.   Generally speaking, a filter needs a pore size of 4 microns or smaller to remove protozoa's, and 0.2 microns to remove bacteria.  Because viruses can be as small as 0.0004 microns, no field device that depends entirely on filtration can reliably remove them.
     Bacteria.  Generally smaller than protozoa's, bacteria range in size from 0.2 microns to 10 microns and can cause all sorts of unpleasantness like dysentery, food poisoning, and typhoid fever, to name a few.
     Cryptosporidium.  A type of protozoan, this is relatively new threat that's tough to kill because of its hard outer shell.   Iodine has no effect on crypto, even though it kills other similarly sized protozoa's.  Boiling or filtering with a 2 micron or less filter is the only way to put these bugs out of commission.
     Organic Chemicals.  Contaminates like pesticides, herbicides, diesel fuel, fertilizer, and strip mine runoff make up this group.   As a rule, suspect any water draining from agricultural areas, logging operations, mines, or heavy industry.  Beware of water that is discolored or foul swelling.
     Protozoa's.  Hard-shelled, single cell parasitic protozoa's, or cysts, are the largest waterborne microorganisms, ranging from 2 to 15 microns.  The Giardia lamblia cyst causes most waterborne illnesses that occur in the United States.
     Viruses.  These are the smallest waterborne fauna, yet they are arguably the most dangerous because they bring on deadly diseases like polio and  hepatitis.  Until recently, experts agreed that viruses didn't pose a serious threat to North American wilderness travelers (they are quite common in developing countries).  But according to Dr. Herbert DuPont, chief of internal medicine at St. Lukes Episcopal Hospital, viruses are even more prevalent than bacteria in North American waters, and are becoming an increasing concern.