The main difference lies in the level of protection they provide. Generally speaking, a water filter is designed to remove waterborne protozoa and bacteria, but not viruses. A water purifier is designed to combat all three classes of microbes, including viruses.
Because North American backcountries are regarded as low-risk for viruses, filters typically offer a sufficient water treatment method. In places heavily trafficked by humans or where sanitation systems are poor, the risk for viruses increases. In these areas, such as in developing countries, a purifier is recommended.
Additionally, filters physically remove matter and microbes from the water. In contrast, a "purifier" can employ a variety of methods to do so, from physically removing the contaminants to disinfecting the water of them, through UV or chemical treatments.
Why the two treatment device options? The reason is, viruses are just too small for filters to catch. Far smaller than protozoa or bacteria, viruses slip through the technologies used in backpacking filters. Traditionally, UV light, chemical treatments or boiling were required to deactivate viruses by scrambling their DNA or killing them. Today, new advancements in physical purifiers provide a convenient option to physically remove viruses quickly and easily.
The flow rates of physical purifiers are generally slower than that of filters, because the internal media that removes contaminants has to be small enough to capture tiny viruses. This slows the rate at which water passes through the media, decreasing overall flow rate.
Bacteria are an order of magnitude smaller than protozoa measuring in the 0.5 - 2 micron range. The most well-known bacteria are E. coli; others include Leptospira (which causes Leptospirosis), Campylobacter, Salmonella, and Shigella. Bacteria aren't as hearty, surviving in water only for weeks instead of months. However, unlike protozoa and viruses, which require a host to multiply, some bacteria can grow in water and so can be found in higher numbers than protozoa and viruses. Bacteria, like protozoa, are often carried by both animals and humans. Because of this, many water sources are contaminated with bacteria. Beware especially of sources near agricultural operations.
First, it will usually take more than one organism to make you sick—around 10 for protozoa and viruses and 10 to 1,000,000 for bacteria depending on the species. The onset of symptoms can start anywhere from 10 hours to several weeks from the point of infection depending on the type and amount ingested, and constitution of the individual. Symptoms can range from nothing to diarrhea, abdominal cramps and pain, nausea, vomiting, muscle cramps, fever, fatigue, headache, chills, loss of appetite, and, in the case of hepatitis, dark urine and jaundice. Symptoms usually dissipate in a couple of days or weeks, again depending on the three elements above. Healthy adults will generally knock the illness without needing medical attention and without lasting effects. However, infants, young children, elderly, pregnant, and immune-compromised individuals are at higher risk of needing medical attention or having complications from an illness. Your location—a remote basecamp in Pakistan or a day hike close to home— will play a key role in the severity of your illness, should you become infected. For more information on diseases caused by specific pathogens, please visit www.cdc.gov.
A filter actually removes matter and microbes from the water while a "purifier" can employ a variety of methods to disinfect the water (such as UV or MIOX® or combination system like the MSR Sweetwater® purifier). A purifier must meet the EPA Guide Standard for Testing Microbiological Purifiers, which requires inactivation of all three classes of microbes: protozoa, bacteria and viruses. Filters in general claim to remove only protozoa and bacteria, making it adequate for most travel in developed countries. Some filters alone can qualify as a purifier, but since they are filtering to such a small pore size, they tend to clog quickly. Essentially the classification as a "purifier" means that the device must be capable of inactivating viruses, as well as protozoa and bacteria.
To decide whether you need a filter or a purifier, two important factors must be evaluated: water sources and susceptibility.
When evaluating water sources it is important to keep the following in mind: In general, protozoa and bacteria infect both animals and humans and are transmitted to water sources by animal and human waste. In contrast, waterborne viruses are species-specific and therefore transmitted to water sources by human waste alone. As a rule of thumb, remote areas with few people require (at least) filtration while popular areas with many human visitors require purification. If traveling in developing countries with poor sanitation, purification will be your safest choice.
Susceptibility is another good way to determine needs. If the treated water is to be consumed by infants or young children, elderly, or pregnant or immunocompromised individuals, then it is essential to use a purifier for the highest level of protection. Purification is also best for people who tend to get sick. If you rarely get sick and do not fit into any categories mentioned above, then filtration may be adequate depending on the water source.
The Guardian Gravity Purifier features a purge hose and valve to self-clean the system, no backflushing required. Open the purge valve to flush dirt and air until the water color matches the source water and is free of bubbles. This helps increase the flow rate, because air trapped in the purifier cartridge can reduce water flow. Hold the cartridge horizontally or gently tap the cartridge as water flows to help release the air.
Please note, the purge valve releases highly concentrated dirty water from the purifier. Do not allow any drops from the purge hose to splash into your clean water.
Gravity filters and purifiers rely on the physics of gravity to pull water down through the filter/purifier media and into your clean water receptacle. The higher the dirty reservoir bag is above the clean water output, the better the flow rate will be.
To increase the flow rate, first try hanging the reservoir bag as high as you can above the clean water output. Second, make sure to purge air out of the system following the directions included with the filter or purifier (also found in the Videos section).
If your filter or purifier has seen a significant amount of use, it may be time for a replacement filter/purifier piece.
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