Testing the Waters: Insights from a Six-Month PFAS Pilot Study

Figure 2. PFAS Trends at the Bridge Street WTP.

Bridge Street WTP: PFAS Pilot Goals

The Bridge Street pilot program was recommended to ensure that full-scale PFAS treatment added downstream of the existing WTP operations would be effective and efficient. Granular Activated Carbon (GAC) and Ion Exchange (IX) resin were evaluated for PFAS removal at the sand/anthracite filter effluent prior to chemical treatment.

The pilot study aimed to:

1. Estimate PFAS breakthrough based on current and potential future regulations.
2. Estimate the performance of the treatment media.
3. Assist in selecting the most cost-effective and efficient PFAS removal system for full-scale design.
4. Estimate media replacement costs and frequencies.
5. Operate for six months to allow sufficient time for PFAS breakthrough and seasonal variation.

In addition to the piloting goals listed above, we also defined the following water quality goals, summarized in Table 1, as a standard for evaluation of treatment process effectiveness.

The bed volumes listed in Table 1 correlate to approximately one year of full-scale operation at the WTP. The goal, from an operational perspective, was to have media last for a minimum of one year before a changeout would be required.

Pilot Media Selection

Prior to selecting the media and finalizing the pilot design, the existing WTP effluent water quality was reviewed. The assessment focused on historical total organic carbon (TOC), iron, and manganese concentrations in the WTP effluent. Results are provided in Table 2.

The water quality review noted slightly elevated concentrations of TOC, which posed an initial concern regarding its effect on PFAS treatment. For this reason, we provided media vendors with the TOC concentrations to review and provide input on expected media performance. TOC concentrations at the levels provided were not expected to greatly affect PFAS treatment. Similarly, low iron and manganese concentrations were not expected to adversely affect PFAS treatment or hydraulics.

We reviewed the chlorine residual present in the existing sand/anthracite filter effluent and found concentrations were above 0.1 mg/L. With these concentrations, the structure of IX resin could degrade, therefore a dechlorination agent was utilized to neutralize chlorine residuals upstream of IX resin media. Chlorine residuals can be removed by GAC, taking up potential adsorption sites for PFAS. However, the low concentrations at the Bridge Street WTP filter effluent were not expected to impact PFAS treatability, so dechlorination upstream of the GAC columns was not necessary and not designed.

Calgon F400 GAC, Dowex PSR2+ IX resin, and Purolite PFA694E IX resin were selected as the media of choice for the following reasons:

1. Review of WTP effluent water quality parameters.
2. GAC’s potential to remove TOC.
3. IX resin’s potential to be unaffected by the TOC concentration present.
4. Success in other drinking water treatment facilities.
5. Included on the Massachusetts List of Approved Drinking Water Technologies.

Pilot Design and Setups

Figure 4 - Bridge Street Pilot Testing Setup.

Each pilot contactor was equipped with sample taps at the influent, 25%, 50%, and 75% bed depths and at the effluent. These taps allowed water quality to be tracked over the depth of the media, allowing the team to track and predict when PFAS breakthrough might occur. Figure 3 represents how the mass transfer zone migrates over the media bed depth over time. As shown, the goal is to collect samples from a given sample port until breakthrough takes place, at which point, samples are taken from the next sample port location.

The Bridge Street WTP pilot operated from July 2023 through January 2024. This provided time for PFAS to be present at various bed depths to track breakthrough more accurately for the full-scale treatment. Every three weeks, samples were collected and analyzed for PFAS and TOC. Differential pressure of each contactor, flow rates, and water temperature were also measured and collected weekly. Cartridge filters were installed upstream of each IX resin column to remove any remaining particulate matter from the sand/anthracite filter effluent water prior to entering the resin columns.

Sodium thiosulfate, a dechlorination agent, was injected into the resin influent water to reduce chlorine residuals from 0.3 mg/L to below 0.1 mg/L. Pilot setups were designed such that the hydraulic loading rate (HLR), empty bed contact time (EBCT), and influent water quality simulated full-scale operation. Figure 4 shows the pilot setup, and Table 3 outlines the setup and operating parameters.

PFAS Treatment Operational Data

Differential pressure across the cartridge filters upstream of the resin columns increased substantially due to solids buildup. The cartridge filters proved to be successful in protecting IX resin from such solids. During the pilot study, the cartridge filters were changed out twice. Substantial increases in differential pressures across each resin and GAC contactor were also observed.

Headloss across Purolite Contactor: Differential pressure (DP) remained relatively stable from the start of the pilot to the end of September across the Purolite media. At that point, we began observing substantial increases in differential pressure with a DP slope of 0.18 pressure per square inch differential (psid)/day. Within three months of the pilot, the differential pressure reached 10 psi and then continued to climb until reaching the maximum pressure of 30.4 psid. Within four months of the pilot, the targeted flow rate through the contactor was not achieved and did not meet the overall pilot design. Therefore, the pilot was shut down.

Headloss across Dowex Contactor: Differential pressure across the Dowex media also remained relatively stable from the start of the pilot to the end of September. At that point, we began observing substantial increases in differential pressure with a DP slope estimated at 0.12 psid/day. While this increase was slightly less than Purolite, within three months of the pilot the differential pressure reached 10 psi. By December 2023, the maximum pressure of 23.2 psid was recorded. Flow rates through the pilot became restricted and the vessel was not meeting pilot design goals. The pilot was shut down due to restricted flow.

Headloss across GAC Contactor: Beginning in December, the differential pressure across the GAC media began increasing drastically until it reached a maximum of 24.2 psid in January 2024. At that time, the flow rates were trending toward restricted flow through the vessel. The GAC contactor was able to stay online through the entire duration of the pilot study, with completion taking place on January 18, 2024. It is important to note that flow rates through the GAC filters were half of the flow rates through the IX resin contactors to provide the contact time required for GAC. This supports why the duration until restricted flow was delayed in comparison to the IX media.

Over the course of the pilot program, differential pressures in the contactors reached levels where the target flows could not be achieved or were trending towards restricted flow. This directly impacted the loading rate and contact time. Data collected under these circumstances were not representative of a full-scale design. A summary of the differential pressures reached, dates encountered, and the rate of accumulation are shown in Table 4.

Backwashing GAC can be performed in full-scale operations to alleviate headloss accumulation. However, backwashing with a rate too high can impact the ability to accurately track PFAS removal over the bed depth. The mass transfer zone is the section of media that is actively removing the majority of PFAS in the water. Over time, the mass transfer zone moves down through the media bed as it gets saturated. Backwashing at a rate higher than recommended can cause fresh media, saturated media, and media at the mass transfer zone to mix. As a result, sample results may not be representative of actual mass transfer zone migration. Figure 5 shows the potential changes to mass transfer zone migration if backwashing higher than vendor recommended rates occur.

Figure 5. Mass Transfer Zone after Incorrect Backwashing

Headloss Contribution

To understand the differential pressure increases, we reviewed the pilot’s influent water quality and operational data of the WTP. TOC, iron, manganese, biological growth, and upstream flocculant aid dosing that were all suggested to be possible contributors to the increased head loss within the media. However, after a review of influent water quality, we were not able to attribute the headloss to a single source. Iron and manganese were consistently non-detect in the pilot influent throughout the study. Upstream flocculant aid dosing logs indicated that the dose remained consistent throughout the duration of the pilot program, with no dosing spikes being recorded.

TOC was present in influent water and has been known to impact PFAS removal efficiency for GAC, as GAC can remove TOC. However, there was not efficient TOC removal by any media piloted. TOC removal is summarized in Table 5. At first, TOC samples were also collected from the 25% port, but TOC saturated the top 25% of the media beds after three weeks of operation. The effluent was then sampled for the remainder of the pilot study. TOC was not significantly removed by Dowex after the first sample event. TOC results from the Purolite effluent suggested the media was fully saturated with TOC, as effluent concentrations were greater than influent concentrations. This suggests that the TOC that had been removed began to "shed" off the media due to chromatic effects. GAC did remove TOC, however the percent removal decreased over the course of the pilot study. Based on low TOC removal, it is unlikely that it was the main contributor to headloss development.

From the influent water quality data collected and operational data reviewed, we did not believe there was a clear indication of what was causing the high-head loss in each of the contactors. We also believe that our pilot sampling program did not collect sufficient influent water quality samples to be able to indicate what was causing the issue.

Figure 6. 25% Port PFAS Breakthrough - Purolite.

Estimated PFAS Breakthrough

In addition to the headloss accumulation, the pilot program demonstrated early PFAS breakthrough in all media.

Figures 6, 7, and 8 show the 25% breakthrough for Purolite, GAC, Dowex when looking through the lenses of the EPA regulation and the PFAS6 regulation. The 25% port was graphed as it provided the most data.

Figure 7. 25% Port PFAS Breakthrough – GAC.

Figure 8. 25% Port PFAS Breakthrough - Dowex.

Based on flows and volume of media, we calculated the estimated bed volumes (BV) until PFAS breakthrough (Table 6). Time to reach breakthrough and estimated media replacement were calculated under the proposed EPA regulation and current MassDEP PFAS6 which told a very different story for media replacement. The pilot results projected that each media would need to be replaced approximately six times more often under the EPA regulation and that PFOA breakthrough would be driving media replacement.

As shown above, the regulation has substantial impacts on media replacement at the Bridge Street WTP. All media would require multiple replacements each year. Piloting each media helped to understand the implications that the new regulations would have on operational costs.

Summary

We are aware there are many factors that contribute to implementing an effective PFAS treatment system. Initially, the water quality assessment at the Bridge Street WTP did not raise concerns about the feasibility of installing a PFAS treatment system. However, our pilot program identified how upstream treatment processes and water quality can affect PFAS treatment. During the pilot, we encountered significant hydraulic issues and observed early PFAS breakthrough in all media.

Our pilot program also showed how the new and more stringent EPA regulation can greatly impact media replacement and operational costs of PFAS treatment.

Due to these challenges, we are currently unable to recommend an effective full-scale PFAS treatment system for the Bridge Street WTP. Although the pilot program did not obtain the results we were hoping for, piloting provided us with valuable information that we were fortunate to have obtained before moving forward with a full-scale design. A full water quality and treatment evaluation across each existing unit process will be conducted at the WTP. The results of the evaluation will be used to provide future direction as to how best to address the fouling issues experienced during pilot testing.

Acknowledgments

The authors would like to thank the following individuals for their support, guidance, and expertise. Without them, this project and this article would not have been possible.

• Blake Lukis, Executive Director, Dedham-Westwood Water District, blukis@dwwd.org
• Erik Grotton, P.E., President, Blueleaf Incorporated, egrotton@blueleafwater.com
• Fred Lusky, Blueleaf Incorporated, flusky@blueleafwater.com
• Weston & Sampson Engineers Project Team
• Dedham-Westwood Water District Operations Team