Formatted Title

Nanofiltration Followed by Electrical Discharge Plasma for PFAS Destruction in Groundwater

Background/Objectives

Ongoing assessment of former fire/crash sites at DoD installations has identified concentrations of PFOA, PFOS, and other PFAS in groundwater well above (in some cases, several orders of magnitude above) established health advisory levels. Given the physicochemical properties of PFAS and the fact that they are often commingled with other contaminants, remediation of PFAS-impacted water best lends itself to a treatment train approach with integrated PFAS concentration and destructive steps. Since PFAS plumes are generally dilute (often a few ppb), reliable approaches that concentrate PFAS and co-contaminants for subsequent destruction are needed. This ESTCP-funded project aims to demonstrate and validate an integrated treatment approach using nanofiltration to concentrate PFAS and co-contaminant-impacted water and electrical discharge plasma to treat the concentrate derived from nanofiltration.

Approach/Activities

In late 2023, a field demonstration of the nanofiltration and plasma treatment systems was conducted at Fire Training Area 2 (also known as AFFF Area 21) at Wright Patterson AFB in Dayton, OH. The total PFAS concentration (sum of 18 detected PFAS) in groundwater was approximately 15,000 ng/L from extraction wells located 300-500 ft from the potential source zone. Of the PFAS quantified, 6:2-FTS, PFHxS and PFOS had the highest concentrations in groundwater. Elevated concentrations of calcium, alkalinity and dissolved iron were also observed, which required additional pre-treatment steps (e.g., green sand; water softening, pH adjustment) prior to the nanofiltration system. During the field demonstration, the performance of two membranes (CR100 and NF90) were tested at varying recovery setpoints (80%, 85%, and 90%). The effluent from the nanofiltration treatment system was split into two flows: i) the retentate (or concentrate) containing high concentrations of PFAS (10-20x concentration factor) and ii) the permeate (or filtrate) with very low or non-detect concentrations of PFAS. The retentate generated from each experimental condition was then treated using the mobile plasma treatment system under varying operating conditions (e.g., semi-batch vs. continuous flow operation; different flowrates; surfactant addition).

Results/Lessons Learned

Although the field demonstration is ongoing (as of the submission date of this abstract), preliminary results are very promising. Over a two-week period, the nanofiltration system processed 15,400 gallons of PFAS-impacted groundwater, generating approximately 2,400 gallons of retentate and 13,000 gallons of permeate.  At 80% recovery, the CR-100 membrane reduced concentrations of long-chain PFAS (including PFOA and PFOS) to below detection limits and achieved >90% reduction in concentrations of short-chain PFAS and precursors.  The results for the 85% and 90% recovery set points have yet to be received; however, based on the results of previous treatability studies using groundwater from Wright Patterson, we expect the NF90 and CR100 membranes to achieve performance objectives.

Treatability results for the plasma system were equally promising, achieving removal of all identified PFAS precursors and long-chain PFAS to below detection limits within 20 minutes of treatment. Short-chain PFAS removal ranged from 57% to >99% within 150 minutes of treatment. The slower removal observed for short-chain PFAS when compared to the long-chain PFAS is expected due to the simultaneous formation and destruction of these compounds during the plasma treatment. Extended treatment times and the addition of surfactant will be incorporated into the field demonstration to improve short-chain PFAS removal. Field results for the nanofiltration and plasma systems are anticipated by December 2023.