PFPRA and other Ultra-Short-Chain PFAS
CFPUA monitors raw and treated drinking water for 65 PFAS compounds. Some of these PFAS are known as "ultra-short-chain," classified by the number of carbon chains in the molecule. Ultra-short-chain are the smallest, with three or fewer carbon chains.
Smaller PFAS are often the hardest to remove during water treatment. Since bringing our new Granular Activated Carbon (GAC) filters online in 2022, CFPUA has seen some PFAS breaking through the filters, particularly two ultra-short-chain PFAS called PFMOAA and PFPrA.
This page provides an overview of CFPUA's ongoing work to remove these ultra-short-chain PFAS from drinking water. At the bottom of the page are definitions of some terms you may encounter to help you better understand this evolving, complicated topic. Watch this page for future updates.
What's going on?
The new GAC filters have been removing the majority of the 65 PFAS we monitor, including GenX, to levels at or near non-detection. A few PFAS compounds, particularly these really small ones called ultra-short-chain PFAS, have been very tough to treat. Some of these have been making it past the filters and showing up in treated water.
The GAC filters are doing an excellent job at treating the six PFAS EPA is considering for regulation. PFOA and PFOS are consistently not detected in treated water. EPA's hazard index, a calculated value which includes GenX, has consistently been at or very near zero at Sweeney.
What are the PFAS making it past the filter?
CFPUA built the GAC filters to treat PFAS in the Cape Fear River from Chemours, which operates a chemical manufacturing plant upstream from CFPUA's raw water intakes. PFMOAA and PFPrA are ultra-short-chain PFAS that have been tied to Chemours' operations, through the Consent Order or in compliance reports filed by Chemours itself. Although there have been a few other PFAS compounds that have been detected in treated drinking water, they have typically been in mid- to low single-digit parts per trillion concentrations or even less than 1 ppt.
When did CFPUA know About PFPRA breaking through the filters?
We've been monitoring PFAS at Sweeney since 2017 and publishing results of that monitoring on our website for all to see. We first noticed PFPrA in treated water last fall and issued a public notice right away. (November 21, 2022).
Is the water safe?
The filters are doing an excellent job at treating the PFAS we have the most health information about. These include PFOA, PFOS, and GenX. There isn't much, if any, health information available for PFMOAA and PFPrA. The State already has been studying PFMOAA, and we hope they can issue some guidance soon. On August 24, we asked the State to study PFPrA, too.
EPA recently issued something called a reference dose for PFPrA. The EPA indicated it had low-confidence in the reference dose it issued, in part because so little research has been done. By itself, a reference dose isn't enough to provide health or water treatment guidance. A reference dose can be used to develop guidance like health advisory levels or the health goal the State originally developed for GenX (140 ppt) or the health advisory level EPA issued for GenX (10 ppt).
What is CFPUA doing about it?
We have been working with several partners to find a way to address the issue:
- We're adjusting operations at Sweeney to enhance treatment. GAC's ability to remove PFAS decreases the longer it is used, so we will be exchanging the GAC media in the filters more often. This will add about another $1 million per year to operating costs but should result in even more effective treatment over the next few months.
- CFPUA asked the State and our university research partners to help provide health guidance on PFPrA.
- The new filters at Sweeney were designed to provide treatment flexibility. We're working with the N.C. Collaboratory and others to investigate the potential of other treatment media that could be used at Sweeney, along with the GAC, to provide additional treatment benefits for these harder-to-treat PFAS.
Ultra-short-chain PFAS: Per- and polyfluoroalkyl substances (PFAS) are a diverse group of compounds, and their classification by carbon chain length can help provide insights into their behavior, bioaccumulation potential, and removal efficiencies in treatment processes. Ultra-short-chain PFAS have three or fewer carbon chains. Examples are PFMOAA and PFPrA.
Granular Activated Carbon: In October 2022, new filters came online at the Sweeney Water Treatment Plant to treat Chemours' PFAS. These filters use granular activated carbon (GAC). These are small particles of carbon that have unique qualities that make them very powerful at removing PFAS from contaminated water. GAC works through a process called adsorption, where contaminants like PFAS adhere to the surface of the GAC. And GAC has a tremendous amount of surface area. The eight filters at Sweeney hold almost 3 million pounds of GAC, which collectively have enough surface area to cover the state of North Carolina three times over.
GAC media regeneration: GAC's ability to remove PFAS decreases the longer it is used. One feature of GAC is that it's PFAS adsorptive capacity can be restored. This occurs through a process called regeneration. The GAC media is removed from a filter and transported to our vendor's facility in the Buffalo, NY, area. There it is regenerated, a process that includes very high temperatures and destroys the PFAS adhering to the GAC. Once regeneration is complete, the GAC can be returned to Sweeney and reused as if it were brand new.
Bed-volume: A bed-volume is the volume of GAC in a GAC single filter (Sweeney has eight filters), which is equivalent to about 76,676 gallons. One of the triggers CFPUA is using to determine when to exchange GAC in a filter is at 10,000 bed-volumes in that filter, about about 766.8 million gallons. That’s the amount of water treated by a filter that would trigger an exchange of GAC media.
Parts per trillion: Contaminants in water such as PFAS are measured in terms of their concentration. For PFAS, this is usually in parts per trillion (ppt) -- a one part PFAS for every trillion parts of water. This can be difficult for most people to conceive so comparisons can be helpful: 1 ppt is equivalent to about a half-teaspoon of a substance in an Olympic-size swimming pool. Note that you may also see PFAS concentration expressed as nanograms per liter (u/L). That is equivalent to ppt.