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Wastewater treatment facilities are a major source of PFAS contamination in drinking water in the US – they discharge enough of the “forever chemicals” to raise concentrations above safe levels for an estimated 15 million people or more. They can also release long-lasting prescription drugs into the water supply.

Even though these plants clean wastewater, they do not destroy all the contaminants added upstream – and the chemicals that remain behind are released back into the same waterways that supply drinking water. “It’s a funnel into the environment,” says at New York University. “You capture a bunch of things from a bunch of different places, and it’s all released in one place.”

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are of particular concern because they contain carbon-fluorine bonds, which make them extremely persistent in the environment. Regular exposure to several types of PFAS has been associated with increased risk for many health problems, from liver damage to various forms of cancer. The US Environmental Protection Agency (EPA) recently set strict limits in drinking water for six of the best-studied PFAS.

Wastewater treatment facilities are a known source of PFAS contamination in the they produce as a by-product, which is sometimes used for fertiliser. To find out whether similar contamination remains in the treated water, Ruyle and his colleagues measured the concentration of PFAS and other molecules that contain carbon-fluorine bonds in wastewater at eight large treatment facilities around the US.

Their findings suggest wastewater treatment plants across the US discharge tens of thousands of kilograms of fluorine-containing compounds into the environment each year, including a substantial amount of PFAS. Once treated wastewater is discharged from a facility, it mixes with natural waters in rivers and lakes. “That’s going to create a downstream drinking water problem,” says Ruyle.

Applying these figures within a model of the US drinking water system, the researchers estimated wastewater could raise concentrations of PFAS above EPA limits in the drinking water of around 15 million people. During droughts, when there is less natural water to dilute the wastewater, the model suggests concentrations would rise above the limit for as many as 23 million people. And Ruyle says these may be conservative estimates – their model assumes the natural waters do not already contain PFAS.

“It demonstrates that wastewater treatment plants are really important sources for these compounds,” says at Johns Hopkins University in Maryland, who was not involved with the study. There are ways to remove or destroy PFAS in water, and more drinking water facilities are installing such systems, but currently, “our wastewater treatment plants are not set up to deal with this”, he says.

Forever chemicals alone would be a problem, but the researchers also found PFAS made up only a small fraction of the total volume of fluorinated chemicals discharged from the facilities. Most were not PFAS at all, but other compounds used in common pharmaceuticals, such as statins and SSRIs. These pharmaceuticals are also of concern for ecosystems and people.

“Another person could be drinking a cocktail of fluorinated prescription medication,” says Ruyle. However, he says the consequences of long-term exposure to low doses of such compounds aren’t well understood.

“We need to start conversations about whether or not we should be using a lot of fluorine in pharmaceuticals,” says Ruyle. Fluorination is widely used in drugs to enhance their effect in the body, but “preventing widespread chemical contamination should also be important”, he says.