Water Filtration and Purification
Understanding the difference and how to choose a filter
The Murky Truth About Clear Water
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.
More details on waterborne pests, and techniques you can use to defeat them, are explained here. 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.
Explaining Water Filters and Purifiers
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.
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.
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 here.
To be identified as a water purifier, a device must conform to a US Environmental Protection Agency protocol (last revised in 1987). Purifiers are required to "remove, kill or inactivate all types of disease-causing microorganisms from the water, including bacteria, viruses and protozoan cysts so as to render the processed water safe for drinking." A device must inactivate 99.99 percent of viruses to be labeled as a purifier.
Does this mean purifiers are superiorto filters? Not necessarily. Depending on conditions (when water is very cold or obviously contaminated, for example), manufacturers of iodine-based purifiers may recommend that users double-filter their water or significantly reduce the rate at which water is pumped through the unit. (The slower flow exposes water to the iodine or iodine resins for a longer period of time.) Sometimes resins must be allowed time to "recharge" after treating a few quarts of water.
Filter-makers, meanwhile, contend that quality filters routinely capture 99 to 99.9 percent of viruses on the first pass since viruses (and bacteria) often become clumped with organic or mineral particles in water. These clumps are easy for filters to trap. Still, when these clumps are "smashed" into a the wall of a filtering element as you pump the water, it's possible a virus could separate from its clump and still slip through.
Note:Pregnant women and people with thyroid conditions often have adverse reactions to iodine. Consult a physician before selecting a purifier.
What Really Matters
In an ideal world, a water filter or purifier will be:
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 Filters trap microorganisms, and other debris, in inner elements or cartridges made from a variety of materials such as:
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.
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.
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:
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. Other factors, such as pore shape and water pressure, can also affect filter performance.
Other Considerations
-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.
Understanding the difference and how to choose a filter
- Remember the difference between a filter and a purifier. While both remove bacteria from water particles using a mechanical process of pumping then forcing water through a filtering device, only purifiers can render viruses inactive using either an additional chemical or electrostatic process.
- Waterborne viruses are believed to be less common in North American wilderness waters. But if you're traveling outside of the United States or Canada, you may want to opt for the more involved, and hence perhaps more expensive, purifying system.
- When comparing filters and purifiers, look for an absolute, not nomimal, pore size of 0.2 microns. This industry wide benchmark indicates the system's smallest filtering capability. No bacteria larger than 0.2 microns can get through. Though a larger pore size of 0.3 or 0.4 may suffice in most situations, why take chances with your health?
- Depending on its function, a water-treatment system can have numerous parts, some of which will need to be maintained or replaced on a regular basis. If your travel plans include many nights away from a reliable water source, you will not only have to pack the filter but the back-up and replacement parts that go with it.
- To get the cleanest water possible be sure to clean and dry your filtering system regularly. When in the outdoors, try to collect still, clear water. Whenever possible, boil the water before treating it. Then, just as if you were seasoning a skillet, let the first few streams of water pass before you begin collecting your treated water.
The Murky Truth About Clear Water
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.
More details on waterborne pests, and techniques you can use to defeat them, are explained here. 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.
Explaining Water Filters and Purifiers
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.
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.
- May exist in water wherever there is a reasonable chance of human fecal contamination;
- Are believed to be less prevalent in North American wilderness water sources than protozoan cysts or bacteria, but may be a greater threat in less developed countries.
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 here.
To be identified as a water purifier, a device must conform to a US Environmental Protection Agency protocol (last revised in 1987). Purifiers are required to "remove, kill or inactivate all types of disease-causing microorganisms from the water, including bacteria, viruses and protozoan cysts so as to render the processed water safe for drinking." A device must inactivate 99.99 percent of viruses to be labeled as a purifier.
Does this mean purifiers are superiorto filters? Not necessarily. Depending on conditions (when water is very cold or obviously contaminated, for example), manufacturers of iodine-based purifiers may recommend that users double-filter their water or significantly reduce the rate at which water is pumped through the unit. (The slower flow exposes water to the iodine or iodine resins for a longer period of time.) Sometimes resins must be allowed time to "recharge" after treating a few quarts of water.
Filter-makers, meanwhile, contend that quality filters routinely capture 99 to 99.9 percent of viruses on the first pass since viruses (and bacteria) often become clumped with organic or mineral particles in water. These clumps are easy for filters to trap. Still, when these clumps are "smashed" into a the wall of a filtering element as you pump the water, it's possible a virus could separate from its clump and still slip through.
Note:Pregnant women and people with thyroid conditions often have adverse reactions to iodine. Consult a physician before selecting a purifier.
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 Filters trap microorganisms, and other debris, in inner elements or cartridges made from a variety of materials such as:
Ceramic. Can be brushed clean repeatedly before needs to be replaced. 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.
Glassfiber. As effective as ceramic but not cleanable.
Structured matrix. A unique, chemical-free honeycombed material that effectively removes viruses from First Need.
Iodine resin. A chemical layer integrated with a purifier's main filter, which traps larger pathogens; its iodine inactivates any viruses in the water.
Hollow fiber. Hundreds of clustered, U-shaped tubes produce a rapid flow rate.
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.
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. Other factors, such as pore shape and water pressure, can also affect filter performance.
Other Considerations
-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.