EWG's Tap Water Database


Water Filter Technology

Choosing a home water filter can feel overwhelming. There are literally hundreds of them on the market.

The good news is that all filters rely on a small number of technologies. Once you understand how these technologies work, choosing the best option for your home will be a lot easier. And while some of the options are more expensive, even a small investment can improve the quality of your tap water.

A few tips to keep in mind:

Here are some basic water filtration technologies you should know.

Carbon/activated carbon: Activated carbon chemically bonds with and removes contaminants from your water as it flows through the filter. Keep in mind that carbon filters vary greatly in effectiveness. Some just remove chlorine, and improve taste and odor. Others remove contaminants including asbestos, lead, mercury and volatile organic compounds, or VOCs. However, activated carbon doesn’t remove common inorganic pollutants such as arsenic, fluoride, hexavalent chromium, nitrate and perchlorate. In general, carbon filters come in two forms: carbon block and granulated activated carbon.

Ceramic: Ceramic filters feature tiny holes that let water through but block solid contaminants such as cysts and sediments. They do not remove chemical contaminants from your water.

Deionization: These filters use an ion-exchange process that removes mineral salts and other electrically charged molecules, or ions, from your water. This process does not remove microorganisms or non-ionic contaminants, including trihalomethanes and other common VOCs.

Distillation: This technology heats your water enough to vaporize it and then condenses the steam back into water. The process removes minerals, many bacteria and viruses, and chemicals that have a higher boiling point than water. But it does not remove chlorine, trihalomethanes or VOCs from water.

Ion exchange: Water filters that use this technology pass your water over a resin that replaces undesirable ions with others that are more desirable. One common example is water softening, which replaces calcium and magnesium with sodium. Keep in mind that the resin must be periodically recharged with replacement ions to continue working.

Mechanical filters: Like ceramic filters, these filters feature tiny holes that let water through, but block contaminants such as cysts and sediments. They are often used in conjunction with other technologies, as they do not remove chemical contaminants.

Ozone: Ozone kills bacteria and other microorganisms in your water and often is used in conjunction with other filtering technologies. It is not effective at removing chemical contaminants from water.

Reverse osmosis: This process pushes tap water through a semipermeable membrane that blocks particles larger than water molecules. Reverse osmosis is effective at removing many contaminants not removed by activated carbon, including arsenic, fluoride, hexavalent chromium, nitrates and perchlorate. However, reverse osmosis does not remove chlorine, trihalomethanes or VOCs. Many reverse osmosis systems also include an activated carbon component than can remove additional contaminants from your water. The quality of both the membrane system and the carbon filter can vary greatly. Also, consumers should be aware that reverse osmosis systems waste a lot of water – they typically use three to 20 times more water than they produce. For this reason, EWG recommends that they be used for drinking and cooking water only.

UV (ultraviolet): These systems use ultraviolet light to kill bacteria and other microorganisms. They do not remove chemical contaminants from your water.

Water softeners: Water softeners typically use an ion-exchange process to reduce calcium and magnesium, which can build up in plumbing and fixtures, as well barium and certain forms of radium that can be found in tap water. They do not remove most other contaminants. Because water softeners usually replace calcium and magnesium with sodium, doctors may advise some people to avoid softened water. Softened water is not recommended for watering plants and gardens.