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.
Gets rid of: 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.
Gets rid of: Calcium and magnesium, which form mineral deposits in plumbing and fixtures, as well as barium and some other ions that can create health hazards.
Distiller
How it works: Boils water and condenses the purified steam.
Used in: Countertop or whole-house point-of-entry units, sometimes combined with a carbon filter.
Gets rid of: Heavy metals such as cadmium, chromium, copper, lead, and mercury, as well as arsenic, barium, fluoride, selenium, and sodium.
Reverse Osmosis
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.
Gets rid of: 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.
Ultraviolet Disinfection
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.
Gets rid of: 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.
