What To Look For
When it comes to purchasing a water filter, the most important thing to consider is its ability to filter dangerous contaminants from the water you drink. As such, you want to seek out a filtration system with small pores that can capture smaller bacteria. Look for filters that advertise effectiveness against cryptosporidium and giardia. Output is also an important consideration, i.e. how long it takes to produce a certain volume of water and how much physical effort is required. Some filters are gravity-based while others will require pumping. Along with ease-of-use should come considerations of a filter's maintenance needs. Some filtration systems, such as reverse-osmosis, are more effective but have much more delicate filters that must be cleaned or replaced often. Finally, look for a filter that fits your existing systems. For example, there is likely a filter that will attach directly to your water bottle.
Common Pitfalls
The most common pitfall is to trust a filter absolutely. Filters are bad for removing viruses, making them inappropriate as the sole method in certain high-risk areas. Many filters will advertise the ability to remove protozoa, but this typically does not include smaller bacteria, such as giardia cysts. While many people purchase filters so as not to deal with chemical tastes, a number of filtration systems depend upon an included chemical component designed to remove contaminants likely to make it past the filter.
Where To Buy
Filters should always be purchased at an outdoor outfitter store with a knowledgeable staff. Never buy a filter from a general big-box store since employees will likely not be in a position to navigate you through the many subtle differences between filters. Better outdoor stores will even allow you to try out the filters, giving you a sense of how much effort pump filters take or how long gravity-fed filters require.
Cost
Filters can vary widely in cost. Basic filtration systems, designed to clear out sediment and larger microorganisms, start at around $30. Filters built into water bottles typically fit into this lower price range. Pump filtration systems can vary in cost, typically beginning around $70 for a solid backpacking pump filter. More durable pumps, with the ability to filter thousands of gallons without much maintenance, can cost upwards of $200.
Comparison Shopping
Compare the raw stats outlined on the back of every filter. There may seem to be an overwhelming amount of data, but what is most important is the "absolute size filtered," which should indicate the largest object that can get through the filter. Try to avoid filters that allow objects larger than .3 microns through.
Accessories
Backup filters and filter-cleaning kits are the two most necessary accessories. More expensive filtration systems will have ceramic filter kits that are cleaned rather than replaced.
Insider Tips
Check out other sterilization methods to supplement your filter.
Gather your water from running sources rather than standing water.
