By Lahne Mattas-Curry, Science Communications Staff, U.S. Environmental Protection Agency (EPA)
Communities across the country have been challenged by per- and polyfluorinated substances (PFAS), a group of man-made chemicals that have been used for decades in consumer products to create non-stick and water-resistant surfaces and in firefighting foams and industrial processes. The characteristics that make them useful are also what make them a challenge. They don’t break down in the environment, and they can bioaccumulate, or build up, in our bodies and the bodies of animals.
For environmental and public health agencies, one concern with PFAS is that they dissolve in water, which, combined with their chemical properties, makes removing the chemicals through traditional drinking water treatment technologies nearly impossible. To solve this challenge and help our communities, EPA researchers have been studying a variety of technologies to determine which methods work best to remove PFAS from drinking water, a goal we all have in common to protect public health.
EPA’s Drinking Water Treatability Database is continuously updated with new information on the best technologies to deal with different contaminants. Recently, data pages were added to address PFAS chemicals.
The three technologies that researchers specifically concentrated on to address PFAS are activated carbon adsorption, ion exchange resins, and high-pressure membranes. These technologies have been found to remove PFAS from drinking water, especially Perfluorooctanoic acid (PFOA) and Perfluorooctanesulfonic acid (PFOS), which are the most studied of the PFAS. These technologies can be used in drinking water treatment facilities, in water systems in hospitals or individual buildings, or even in homes at the point-of-entry, where water enters the home, or the point-of-use, such as in a kitchen sink or a shower.
Activated Carbon Treatment
Activated carbon treatment is the most studied treatment for PFAS removal. This technology is commonly used to adsorb natural organic compounds, taste and odor compounds, and synthetic organic chemicals in drinking water treatment systems. Adsorption is the physical and chemical process of accumulating a substance such as PFAS at the interface between liquid and solids phases. Activated carbon is an effective adsorbent because it is a highly porous material and provides a large surface area to which contaminants may adsorb.
Granulated activated carbon (GAC) is made from organic materials with high carbon contents such as wood, lignite, and coal. GAC has been shown to effectively remove certain PFAS from drinking water when it is used in a flow-through filter mode after particulates have already been removed. GAC works well on longer-chain PFAS like PFOA and PFOS, but shorter chain PFAS like Perfluorobutanesulfonic acid (PFBS) and Perfluorobutyrate (PFBA) do not adsorb as well.
Another type of activated carbon treatment is powdered activated carbon (PAC). PAC cannot be used in a flow-through bed, but it can be added directly to the water and then removed with the other natural particulates in the clarification stage (conventional water treatment or low-pressure membranes, microfiltration or ultrafiltration). However, PAC is not as efficient or economical as GAC at removing PFAS.
Ion Exchange Treatment
Another treatment option is ion exchange treatment, or resins. Ion exchange resins are made up of highly porous, polymeric material that is acid, base, and water insoluble. The tiny beads that make up the resin are made from hydrocarbons. There are two broad categories of ion exchange resins: cationic and anionic. Cation exchange resins (CER) are effective for removing positively-charged contaminants. For PFAS, which is negatively charged, we are interested in anion exchange resins (AER). Ion exchange resins are like tiny, powerful magnets that attract and hold the contaminated materials from passing through the water system. AER has shown a high capacity for removing many, but not all, PFAS, but it is typically more expensive than GAC. The single-use mode AER is the most promising technology, followed by incineration of the resin. Like GAC, AER removes 100% of the PFAS for a time that is dictated by the choice of resin, bed depth, flow rate, the PFAS that needs to be removed, and the degree and type of organic matter and other contaminants constituents in the water.
High-pressure membranes, such as nanofiltration or reverse osmosis, have been extremely effective at removing PFAS. This technology depends on membrane permeability, and reverse osmosis membranes are tighter than nanofiltration membranes. A standard difference between the two technologies is that while nanofiltration membranes will reject hardness to a high degree but pass single charged salts like sodium chloride, reverse osmosis membrane will reject all salts to a high degree.
Our research shows that these types of membranes are typically more than 90% effective at removing a wide range of PFAS, including short chain PFAS. However, because of the waste stream created by this technology, it is best suited to be a point-of-use technology for the homeowner.
For more information about drinking water technologies available for removing PFAS, please visit EPA’s Drinking Water Treatability Database. This interactive literature review database contains more than 65 regulated and unregulated contaminants, including PFAS, and it covers 34 processes commonly employed or known to be effective. Users can search by contaminant or technology.