Household water filters generally fall into one of two categories: point-of-entry units, which treat water before it gets distributed throughout the house; and point-of-use units, which include countertop filters (e.g. filter pitchers), faucet filters, and under-the-sink units. Some filters use more than one kind of filtration technology. As a general rule, look for filters labeled as meeting NSF/ANSI standard 53 and that are certified to remove the contaminant(s) of concern in your water. While the NSF certification program is not flawless, it does provide some assurance that at least some claims made by the manufacturer have been verified. NSF-certified filters have been independently tested to show that they can reduce levels of certain pollutants under specified conditions. Those that meet standard 53 are geared toward treating water for health, not just for aesthetic qualities.
For many people, an activated carbon filter bearing NSF Standard 53 certification will filter out most pollutants of concern. But if you've got perchlorate, a rocket fuel ingredient, in your water, for example, a simple countertop filter won't do the job. The list below will you help you determine what type of filter will work best for you. Once you've got a general idea, visit NSF International's drinking water treatment units online product database.
Activated Carbon Filter
How it works : Positively charged and highly absorbent carbon in the filter attracts and traps many impurities.
Used in : Countertop, faucet filters and under-the-sink units.
Removes : Bad tastes and odors, including chlorine. Standard 53-certified filters also can substantially reduce many hazardous contaminants, including heavy metals such as copper, lead and mercury; disinfection byproducts; parasites such as Giardia and Cryptosporidium ; pesticides; radon; and volatile organic chemicals such as methyl-tert-butyl ether (MTBE), dichlorobenzene and trichloroethylene (TCE).
Cation Exchange Softener
How it works : "Softens" hard water by trading minerals with a strong positive charge for one with less of a charge.
Used in : Whole-house, point-of-entry units.
Removes : Calcium and magnesium, which form mineral deposits in plumbing and fixtures, as well as barium and some other ions that can create health hazards.
How it works : Boils water and recondenses the purified steam.
Used in : Countertop or whole house point-of-entry units; can be combined with a carbon filter.
Removes : Heavy metals such as cadmium, chromium, copper, lead and mercury, as well as arsenic, barium, fluoride, selenium and sodium.
How it works : A semipermeable membrane separates impurities from water. (Note: This filtration technique wastes a substantial amount of water during the treatment process.)
Used in : Under-the-sink units; often in combination with a carbon filter or UV disinfection unit.
Removes : Most contaminants, including certain parasites such as Cryptosporidium and Giardia; heavy metals such as cadmium, copper, lead and mercury; and other pollutants, including arsenic, barium, nitrate/nitrite, perchlorate and selenium.
How it works: Ultraviolet light kills bacteria and other microorganisms.
Used in: Under-the-sink units, often in combination with a carbon filter and sediment screen.
Removes : Bacteria and parasites; class A systems protect against harmful bacteria and viruses, including Cryptosporidium and Giardia , while class B systems are designed to make non-disease-causing bacteria inactive.
Maintain Your Filter Properly
No filter will give you good performance over the long term unless it receives regular maintenance. As contaminants build up, a filter can not only become less effective, but actually can make your water worse, by starting to release harmful bacteria or chemicals back into your filtered water. To keep your filter working properly, follow the manufacturer's maintenance directions. Some filters only require a cartridge change, while others are better maintained by a certified professional. Many filter distributors offer maintenance and service contracts for their products. Before buying any water treatment system, compare not only filter prices, but also operating and maintenance costs for the different units.