Although there are hundreds of brands of home water filters, they all rely on a small number of technologies to remove contaminants. That does not mean that every filter that uses a given
technology is as good as another, but it does mean that you can get a good idea of the general pros and cons of the different systems relatively easily.
A few tips to keep in mind:
Some filters use a combination of technologies, while others rely on just one.
To ensure that a filter removes a particular contaminant, verify that it is certified for that contaminant by a reputable, independent agency. For example, some carbon filters can remove chloramine but others cannot. Filters vary widely in quality.
Some filters are labeled "NSF certified." NSF is a reputable product evaluation company, but its certifications are not all the same. It may certify that a filter will improve water's taste and odor but not necessarily guarantee that it will remove any specific contaminants. Read the fine print.
EWG's water filter guide only includes filters that have been certified by the California Department of Public Health and/or NSF to reduce one or more common drinking water contaminants.
Pitchers or large dispensers are typically fitted with an activated carbon filter that can remove contaminants and improve taste and odor. Models vary, but many reduce chlorine, lead, mercury and
(less frequently) disinfection byproducts. This filter style works well for filtering drinking water and can be stored in the refrigerator.
Inexpensive. No installation required. Available in various sizes and styles.
If filters are replaced regularly, yearly cost may equal expense of faucet, countertop or under-sink filters. Can require frequent filter changes. Filtering is slow.
Faucet-mounted filters attach directly to the end of the faucet. Most can be pivoted to an "on" or "off" position, allowing you to collect filtered water for drinking and cooking. This filter
style typically uses an activated carbon filter that can remove contaminants and improve taste and odor. Models vary, but many reduce chlorine, lead, mercury and (less frequently) disinfection
Relatively inexpensive. Easy to install. Allows user to switch between filtered and unfiltered water. Filtration is fast enough to fill cooking pots.
Does not work with all faucet styles. May slow down faucet flow rate. Typically must change filter more frequently than with countertop or under-sink filters.
On-Counter filters typically sit on the counter, with a line connecting directly to the faucet. A diverter value allows you to switch between filtered and unfiltered water. You collect filtered
water from an extra spout or faucet on the filter unit. Models use a range of technologies, including activated carbon and reverse osmosis. Effectiveness varies widely between models, but many
on-counter filters will reduce a wide array of contaminants.
May allow user to switch between filtered and unfiltered water. Typically requires relatively infrequent filter changes. Ideal for filtering both drinking and cooking water.
Requires installation and possibly plumbing modification. Can be expensive, though not always.
Under-Sink filters are mounted underneath the kitchen sink, where they are fitted to the water supply line. Some models have a separate spout or faucet for water collection. Models use a range of
technologies, including activated carbon to reverse osmosis. Effectiveness varies widely between models, but many under-sink filters will reduce a wide array of contaminants.
Placed out-of-sight under the sink. Typically requires filter changes relatively infrequently. Ideal for filtering both drinking and cooking water.
Requires installation and possible plumbing modification. Can be expensive, though not always. http://saita.net/myblog/detail/22427/194355.html