Theme: 07. Per- and Polyfluorinated Alkyl Substances (PFAS)

Topic Area: 7p. PFAS: Groundwater Treatment Case Studies

Formatted Title:

A Case Study Example of Optimizing an Existing Treatment System to Address PFAS

Background/Objectives:

PFAS sampling was performed at a CERCLA Site on the east coast of the US where an existing groundwater extraction and treatment system (GWETS) was being operated as an interim response action to address legacy impacts to groundwater. The sampling was performed in collaboration with the regulatory agencies after a suspicion arose as to the potential for PFAS in groundwater at the Site. Of the numerous existing treatment components of the GWETS (i.e., filtration, cationic exchange resin, air stripping, and granular activated carbon [GAC]), only the GAC was determined to be amenable to remove targeted anionic PFAS (PFOS, PFOA, and PFNA). As the GWETS was an interim response action that was in operation for several years, optimizations to the existing lead-lag orientation of the GAC vessels were spaced constrained. As a starting point, the existing GAC was changed out to provide a best-case assessment of its throughput to remove PFOS, PFNA, and PFOA to reference criteria. An unacceptable performance, specifically breakthrough, was observed after less than 1 month of continuous operation. At this point, an optimization study was initiated.

Approach/Activities:

The first recommendation was to review the associated geochemistry of the influent to the GWETS with competitive influences on sorptive PFAS removal. Various pre-treatment technologies were considered to reduce the influent total organic carbon (TOC) from the 1s milligram per liter range. The influent inorganic geochemistry was also suspected of a diminished extraction capacity (~30 to 40 gallons per minute [gpm]) versus a higher design flowrate because of fouling. Space constraints and the high effectiveness of the existing treatment components for the legacy impacts limited significant system changes, such as installing larger GAC vessels to increase the empty bed contact time (EBCT). Therefore, despite identifying geochemical challenges to GAC based removal of PFAS and recognizing that the EBCT was not optimal, a different source of GAC was recommended as well as optimization to the flow through the GAC vessel. A cost comparison was also performed to evaluate implementing anionic exchange resin.

Results/Lessons Learned:

After a field study of more than a year, the throughput of the GAC vessels improved from a change out within 30 days to a change out after more than 180 days of continuous operation. A GAC-to-GAC comparison revealed an improved removal of approximately 0.42 micrograms PFOS+PFOA+PFNA per gram of GAC (µg/g) to 3.8 µg/g. As the field study progressed and after three GAC changeouts, the incremental throughput to breakthrough was observed to decrease. This field-scale case study of optimizing an existing GWETS to remove PFAS to low regulatory criteria will highlight the thought process around troubleshooting. It will also explore the comparative cost difference between optimizing versus replacing/enlarging an existing GWETS for the purposes of mitigating PFAS in the effluent.

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John Hogue