Per- and polyfluoroalkyl substances (PFAS) are a group of man-made chemicals that have been widely used in a variety of industrial and consumer applications due to their unique chemical properties. PFAS are surfactants used as active ingredients in stain repellents (e.g., Scotchgard) and firefighting foams. PFAS are of concern because of their toxicity and persistence in the environment. Researchers and practitioners have focused on the use of adsorbents such as granular activated carbon (GAC) and ion exchange (IX) and, to a lesser extent, nanofiltration (NF) or reverse osmosis (RO) for the treatment of PFAS-impacted water. 

The principal objective of this research was to assess the effectiveness of GAC adsorption, IX, and membrane processes for the removal of PFAS from different source waters and quantify the benefits and disadvantages (treatment capacity, life cycle costs, etc.) for each technology as either stand-alone processes or integrated treatment trains. Specific objectives were to: 

• Develop a comprehensive assessment framework for ex situ PFAS treatment technologies.
• Generate a PFAS treatment efficiency database to support development of a decision support tool.
• Develop a treatment technology decision support tool.

The project objectives were achieved through a systematic investigation of PFAS removal under various treatment scenarios and development of a life cycle impact and costing model to quantify and compare the technologies for different treatment scenarios. To address the objectives, the project team collected data from previously conducted studies and performed laboratory-based experimentation to fill in data gaps related to PFAS removal by a variety of competing technologies including GAC, IX, NF and RO. Along with technology information, this data was used to develop the life cycle assessment (LCA) and life cycle cost (LCC) and, ultimately, a decision support tool to aid in treatment technology selection.

All three technologies (GAC, IX, and high-pressure membranes) studied in this research showed great ability to remove PFAS from different source waters. However, the removal efficiency was impacted by various factors. Key findings include the following:

• Media use rates determine whether GAC or IX is more cost-effective.
• Membrane processes have higher costs and usually environmental impacts compared to adsorbents.
• Background organic matter negatively impacts adsorbent performance; IX is less impacted by background organic matter than GAC.
• Thermal reactivation is the most environmentally preferred management option for spent GAC.

Overall, total organic carbon (TOC) concentration had the most dramatic effect on GAC use rate. In groundwater with a TOC concentration of 1.3 mg/L, the GAC use rate was 10 times that in groundwater with a TOC concentration of <0.3 mg/L. Empty bed contact time (EBCT) and GAC type were also important parameters that affected GAC use rates. Doubling EBCT from 10 minutes to 20 minutes decreased GAC use rates by a factor of about two for the tested GAC and water matrix combination. Out of four major inorganic anions investigated (chloride, bicarbonate, sulfate, and nitrate), nitrate had the most pronounced impact, negatively impacting the adsorption of short-chain PFAS. Fresh and weathered aqueous film forming foam constituents had a negligible impact on the removal of targeted PFAS. GAC use rates were also strongly impacted by different regulatory scenarios; both lower concentration targets and inclusion of shorter-chain PFAS into treatment criteria led to substantial increases in GAC use rates.

Initial PFAS concentration and competition among co-occurring PFAS had a negligible effect on PFAS breakthrough curves for the selected IX resin. Doubling EBCT from 1.5 minutes to 3 minutes had a negligible impact on PFAS removal. TOC concentration negatively impacted IX performance, but not as strongly as GAC performance. Long-chain PFAS were more impacted by TOC than short-chain PFAS. Out of four major ions investigated (chloride, bicarbonate, sulfate, and nitrate), nitrate had the most pronounced impact, negatively impacting PFAS removal by IX. Short-chain PFAS were more strongly affected by nitrate than long-chain PFAS. Similar to GAC, IX use rates were strongly impacted by different regulatory scenarios; both lower concentration targets and inclusion of shorter-chain PFAS into treatment criteria led to substantial increases in IX use rates.

For the perfluoroalkyl acids (PFAAs) investigated, and over the range of operating conditions evaluated, tight NF and RO membranes provided higher separation or rejection than the loose NF membrane. For a deionized water matrix, the RO membrane provided greater than 99% rejection for all PFAAs, with permeate concentrations being near or below the limit of quantification. For waters with higher ionic strength, loose NF and to a lesser extent tight-NF and RO exhibited lower PFAA separation. In particular, PFAA separation by loose NF was observed to be significantly and detrimentally impacted by the presence of salts and subsequentially high product water recoveries. The addition of sodium sulfate negatively impacted the rejection of perfluorobutane sulfonic acid, perfluorooctanoic acid (PFOA), and perfluorooctane sulfonate (PFOS) by loose NF. Compared to the addition of sodium sulfate, calcium chloride had a minimal impact on the rejection of PFAAs with separation being marginally (5-10%) lower than baseline conditions across the recovery setpoints evaluated.

As part of the project scope, an Excel^®-based decision support tool was developed. The tool provides a pathway to comprehensively evaluate PFAS treatment technology options consisting of GAC, IX, and high-pressure membrane systems for groundwater remediation. Key points include the following:

• The decision support tool allows users to estimate relative life cycle costs (e.g., capital, operation, and maintenance), environmental impacts (e.g., greenhouse gas emissions), and logistics (e.g., land footprint, residuals management) of comparable PFAS treatment processes based on user data inputs.
• The tool allows for site-specific quantitative comparison of PFAS treatment technologies, enabling selection of the most appropriate approach based on user preferences.
• The tool can determine cost and environmental impacts of PFAS.

The results of this demonstration provided an assessment of the most promising technologies for PFAS treatment. Treatability data generated during this project was used to evaluate the impact of various factors on costs associated with PFAS treatment using GAC, IX or high-pressure membranes. For GAC and IX treatment, the highest treatment costs were associated with treating high TOC groundwater. High-pressure membrane treatment was found to be significantly more expensive than GAC and IX due to higher capital and operation and maintenance costs, with concentrate management costs being substantial depending on the disposal option. Improved insight into the life-cycle feasibility of various technologies for the ex situ removal of a variety of PFAS from dilute groundwater plumes will facilitate the adoption of appropriate technologies, thereby directly impacting the Department of Defense's ability to maintain operational readiness and bolster national defense capabilities. (Project Completion - 2023)

Murray, C.C., R.E. Marshall, C.J. Liu, H. Vatankhah, and C. Bellona. 2021. PFAS Treatment with Granular Activated Carbon and Ion Exchange Resin: Comparing Chain Length, Empty Bed Contact Time, and Cost. Journal of Water Process Engineering, 44:102342. doi.org/10.1016/j.jwpe.2021.102342.