How to Choose Water Filters or Purifiers, Part 2

What Really Matters

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 specification chart that accompanies each product description.

Understanding Specification Charts

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 wateryou 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— All of the filters and purifiers in REI's product mix 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:

• If you're a recreational backpacker, someone who takes 1 or 2 overnight trips per year, an inexpensive filter will serve you well. Still, be careful about what type of water you send through it. Make it as clear as possible and the filter will last longer.
• If you visit the wilderness regularly, seek out a field-cleanable model designed to provide years of service.
• People who explore terrain closer to urban areas, at lower elevations and who travel outside the United States and Canada are candidates for a purifier.

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.

Other Considerations

Replacement cartridges are available for all of the filters and purifiers REI carries. 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.