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EWG's Tap Water Database — 2019 UPDATE

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Water Filter Technology: A Primer

October 2019

There are hundreds of home water filters on the market, and choosing one can feel overwhelming.

The good news is that most filter products on the market use one of three primary technologies explained in this graphic: carbon filtration, reverse osmosis and ion exchange. Once you understand how these technologies work, selecting the best one is much easier.

When you're weighing your options, keep in mind these considerations:

About the technologies

Carbon and activated carbon

Activated carbon chemically bonds with and removes contaminants as water flows through the filter. Activated carbon filters' effectiveness varies significantly. Some are certified only to remove chlorine and improve taste and odor. Others remove contaminants such as asbestos, lead, mercury and volatile organic compounds, or VOCs. However, activated carbon does not 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.


Reverse osmosis

Reverse osmosis pushes tap water through a semipermeable membrane that blocks any 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, it does not remove chlorine, trihalomethanes or VOCs.

Many reverse osmosis systems also include a sediment filter stage and one or more activated carbon components that can remove additional contaminants. Additionally, reverse osmosis systems waste a lot of water: They typically use five times more water than they produce. For this reason, EWG recommends that reverse osmosis-treated water be used for drinking and cooking water only.

Ion exchange and 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, but some models can remove certain contaminants. Because water softeners usually replace calcium and magnesium with sodium, doctors may advise people with certain health conditions or susceptibilities to avoid softened water. Softened water is not recommended for watering plants and gardens.