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.
Which do you need?
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.
Check out our blog post for more detailed information on this topic.