Water Filter Technology: A Primer
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:
- Even filters of the same type can vary in their capacity to reduce the levels of specific contaminants. To ensure that a filter can remove a particular contaminant, check that it has been certified to do so by an independent third-party certification company.
- There may not be a third-party certification for some contaminants. Thus, a particular filter model that otherwise fits your needs might not carry a certification for a specific contaminant. But that filter can still help reduce the level of that contaminant in your water.
- EWG provides information on filter technologies that have been certified to reduce common drinking water contaminants. We also point to filter types that may reduce the level of contaminants, even if they don't carry a specific certification.
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.
- Carbon block: Carbon block filters contain activated carbon that is shaped into blocks under high pressure. They are usually more expensive than granular activated carbon but are generally more effective because they have more surface area that comes into contact with your water. However, they may need to be replaced more often. Their effectiveness depends in part on how quickly water flows through the filter.
Fibredyne is a proprietary type of carbon block filter. Manufacturers claim Fibredyne filters remove sediment more effectively than other carbon block filters.
- Granulated activated carbon: These filters contain fine grains of activated carbon. They are usually less effective than carbon block filters, because they have a smaller surface area of activated carbon that comes into contact with water. Their effectiveness depends on how quickly water flows through the filter.
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.