Ion-Exchange PFAS Removal Expands in Northern California

Northern California water agency Zone 7 opened its second PFAS treatment facility in March 2025. (Zone 7 Water Agency)

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In August 2022, California water agency Zone 7 faced the prospect of taking its most productive well out of service during the driest three water years on California record.

Located about 15 miles east of the San Francisco Bay in Pleasanton, Calif., the agency’s Stoneridge Well had shown elevated levels of perfluorohexane sulfonic acid (PFHxS), a chemical that had drawn scrutiny for its adverse health effects at even very small concentrations. The state’s upcoming drinking-water regulations around PFHxS threatened to take the Stoneridge Well offline, pushing the water agency to the front lines of PFAS treatment.

Zone 7’s actions in 2022 would lead to it building the first PFAS treatment facility in Northern California in 2023 and the eventual opening of its second PFAS treatment facility, at its Chain of Lakes wellfield, in early 2025.

Per- and polyfluoroalkyl substances (PFAS or “forever chemicals”) describe a family of more than 12,000 synthetic chemicals previously favored by industries for their resistance to heat, oil and water. The chemicals have found a wide variety of commercial applications from non-stick cookware and fabric treatment to insulation and fire suppression.

But their suitability to these applications arises from the same chemical properties that cause them to resist degradation in both the environment and within human bodies, hence the forever-chemical moniker. Health problems, such as kidney and testicular cancers, restricted fetal growth, and immune-system suppression, have been linked to a handful of PFAS, with others currently under study.

One 2023 study by the U.S. Geological Survey estimated that at least 45 percent of the nation’s tap water contains one or more of the 32 PFAS it tested. In April 2024, the Environmental Protection Agency finalized its National Primary Drinking Water Regulation for six PFAS, establishing legally enforceable rules involving monitoring, notification and treatment.

Treatment Options

The EPA also identified three best-available technologies for removing PFAS from drinking water: granular activated carbon (GAC), ion-exchange (IX) resins and high-pressure membrane systems such as reverse osmosis (RO). The Stoneridge team ruled out RO early on in its design process because of the energy required to constantly push water through at high pressure, explained Brandon Woods, acting principal engineer at Zone 7. RO also exhibits other side effects: “It strips out everything from the water—PFAS and everything else,” adds Woods. “It can be kind of aggressive, so you have to blend it with other water that has minerals in it.”

But GAC and IX presented viable options at Stoneridge. GAC, with granules made from high-carbon organic materials such as wood, lignite or coal, traps PFAS molecules within its highly porous surfaces. It’s commonly used to adsorb organic compounds in drinking water treatment systems. Similarly, IX uses electrostatically charged beads to attract and collect PFAS molecules.

Moving Targets

Zone 7, like many other water agencies, had to make investment decisions within a regulatory landscape that evolved rapidly as data about the health effects of PFAS emerged.

The EPA first established lifetime health advisory limits for two PFAS—perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS)—in 2016 at 70 parts per trillion (ppt) combined, advising municipalities to notify their customers if they exceeded that limit. The agency later updated those lifetime limits in 2022 to 0.004 ppt for PFOA and 0.02 ppt for PFOS before bringing the maximum contaminant level goals down to zero for these chemicals in its 2024 regulations.

Meanwhile, the California State Water Board established its own interim notification levels for these two chemicals in 2018, requiring public notification if PFOA levels exceeded 14 ppt and PFOS 13 ppt, then lowered these notification levels the following year to 5.1 ppt for PFOA and 6.5 ppt for PFOS. In 2020, the Water Board also set response levels, where agencies were required to either take water sources offline or treat them to below response levels, at 10 ppt for PFOA and 40 ppt for PFOS.

California then set notification and response levels for a third PFAS, PFHxS, in October 2022 at 3 ppt and 20 ppt, respectively.

In summer 2022, the third year of a prolonged drought in the American West, Zone 7 was well aware of a PFAS plume underneath the Tri-Valley area, from where it sourced its groundwater. Its Stoneridge Well had averaged 26 ppt for PFHxS and its Chain of Lakes wells 24 ppt. Although Zone 7 normally receives up to 80 percent of its water supply from the State Water Project—California’s network of state-operated surface reservoirs and canals—its allocations drop precipitously during drought years, leaving agencies to rely heavily on local sources.

“We knew that in October 2022, the California Division of Drinking Water was going to be coming out with response levels for PFAS, particularly PFHxS,” says Woods. “Since we had been sampling our water at Stoneridge Well since 2018, we knew we had elevated levels of that, so we were not going to be in compliance (at) Stoneridge Well, our largest producing well. Especially at that time, it was not an option for us to just shut the well down and go back to the drawing board.”

While an investigation into the source of the plume is still ongoing, the Tri-Valley is home to several potential sources, including a fire-training facility, an airport and a military complex.

 

 

Ion-Exchange Technology

Zone 7 and its consultant, water engineering firm Carollo Engineers, opted to use IX at Stoneridge largely on the basis of available space, according to Carollo Vice President Peter von Bucher.

“GAC and ion exchange can be pretty comparable in cost,” he adds. “The big thing that drove the Zone 7 decision was actually footprint. A well itself doesn’t take up a lot of space; it’s a very small thing. These wells were built, and then cities built around them, and so a lot of these well sites are very small. One of the challenges is finding a treatment process that fits on whatever site you’re given. Otherwise, you have to buy more land.”

 

 

GAC treatments can require a 25- to 50-percent larger footprint than IX, notes von Bucher.

While Stoneridge uses six treatment vessels, each about 12 feet in diameter and 14 feet tall, the site would have needed four to six additional, larger vessels if using GAC. The larger footprint would have restricted their ability to access the site for normal maintenance.

IX treatments rely on electrostatic forces to remove contaminants from water. Tiny hydrocarbon beads form a porous, polymeric resin that is acid, base and water insoluble. The beads can be manufactured to collect either negatively or positively charged contaminants.

PFAS molecules consist of a fluorinated tail, which is both hydrophobic and lipophobic, attached to a hydrophilic terminal group. This structure results in a tendency for PFAS compounds to position themselves along interfaces, such as between air and fuel in the case of PFAS-based firefighting foams. IX resins work on PFAS by attracting their negatively charged terminal groups.

“A lot of times, the ion-exchange resin beads are coated in chloride ions, which are negatively charged, and they attach to the positively charged resin,” explains von Bucher. “When you then put PFAS-contaminated water over this resin, the chloride ion dissociates, and the PFAS ion, which is negatively charged, attaches to the resin.”

Stoneridge’s design includes multiple treatment trains, with each train consisting of two pressure vessels filled with IX resin.

“In that train, there’s a lead vessel and a lag vessel,” adds von Bucher. “The lead vessel is doing most of the work removing all the PFAS, and the lag vessel is there almost ‘just in case.’”

Each vessel includes multiple sample points down its side, allowing operators to test resin at progressive stages of treatment. Stoneridge operators test the sample points roughly monthly.

“When you have brand new resin, and you start testing the water at that top sample point, you might not detect any PFAS,” says von Bucher. “But after a couple of months, you may begin to see some presence of PFAS. Meanwhile, at the second sampling point, you might not find any at all.”

 

 

As the resin accumulates more PFAS, the position of first detection moves down the tank until eventually the first vessel fails to remove all the PFAS. At that point, that vessel’s media is replaced, and the train is reversed, with the vessel with the freshest media serving as the new lag vessel.

The choice of resin depends not only on the target contaminant, but also on what else is present in the water, as other solutes may compete with the contaminant for the resin. What’s more, the 12,000-plus types of PFAS differ from each other structurally, and some resins work better than others for certain compounds.

“For any given utility or water source, you’re trying to pick a resin that works best for their specific contaminants and their specific water quality,” notes von Bucher.

For Stoneridge in 2022, Carollo was luckily able to locate IX equipment in short order.

“At the time we were doing Stoneridge, there was a huge demand for those (treatment) vessels in California, because everybody was trying to get them to do PFAS treatment, and so we had to take the vessels that we could get,” adds von Bucher. “And the vessels that we could get were rated for 150 psi, and that well is more like 250 psi. That discrepancy required us to design an entire booster pump and pressure-control system so we could safely use the lower-pressure vessels without damaging them.”

Zone 7’s board of directors granted emergency approval of some $16 million in funding in August 2022, awarded the contract that October, broke ground the following January and brought the system online in September 2023.

Chain of Lakes

In March 2025, Zone 7 expanded with a second PFAS treatment facility at its Chain of Lakes wellfield, which Zone 7 and Carollo worked on somewhat concurrently with the Stoneridge project. Chain of Lakes’ design was completed in December 2022, and the Zone 7 Board awarded the construction contract in April 2023.

Like Stoneridge, Chain of Lakes faced space constraints and, like Stoneridge, the team decided on IX treatment for those reasons. According to Woods, with Zone 7’s Stoneridge facility already online, the choice of IX at Chain of Lakes also offered the agency a degree of operations and maintenance streamlining.

While Stoneridge faced time pressures, Chain of Lakes presented challenges in the form of porous soils at the facility site. Soil remediation work and pile driving were ultimately necessary to prevent the treatment structures from settling through time.

Looking Beyond

With two treatment facilities open, Zone 7 is in the process of designing a third facility at its Mocho wellfields. Mocho, however, already has an existing RO facility, originally installed to remove salt. The team will therefore consider modifying this facility along with its other PFAS treatment options.

As PFAS treatment becomes more prevalent across the country, von Bucher explained that facility operators need to be aware of the liability they incur by producing PFAS-contaminated treatment media, even if those operators carry no relationship to the original contamination source.

“Even though it’s not in your hands anymore, you’ve touched it, and it’s gone off somewhere else,” notes von Bucher.

While RO produces brine that must be sent off for wastewater treatment, GAC and IX media can be incinerated to destroy PFAS compounds, although the adverse effects of incomplete combustion or destruction remain under study.

With the EPA’s 2024 regulations, public water systems nationwide must complete initial PFAS monitoring by 2027 and begin updating the public with their results also beginning in 2027. Should testing reveal levels that exceed the national enforceable levels, agencies have until 2029 to implement solutions reducing those contaminant levels.