Research on soils and waters impacted by per- and polyfluoroalkyl substances (PFAS) over the past several years have produced important findings and highlighted many of the challenges associated with treatment and elimination of these compounds from the environment. Among these challenges are the large number of PFAS species in aqueous film forming foam (AFFF) (i.e., anionic, cationic, and zwitterionic compounds), many of which may strongly sorb to surfaces and are not readily rinsed with water. PFAS treatment is difficult and most treatment approaches generate a PFAS residual requiring further treatment and/or disposal that is costly. These challenges apply to the cleaning of firefighting delivery systems; thorough cleaning is critically important before new PFAS-free firefighting agents can be used in these systems.
The first challenge is effectively removing AFFF residuals from the tanks and lines of the delivery systems. As simple water flushing will likely not be efficient for dissolving/desorbing all the AFFF constituents, the use of co-solvents may provide a promising solution. This demonstration will use water and a co-solvent to remove the residual AFFF constituents. The specific objectives of this project include:
- Develop procedures for cleaning of equipment used to convey and transport AFFF,
- Optimize water–co-solvent ratios and number of cleaning cycles needed to remove AFFF from equipment and produce minimal volumes of cleaning wastewater,
- Optimize (maximize water recovery and solute rejection, and minimize energy demand) a nanofiltration process for separation of PFAS and co-solvent mixtures, and
- Assess overall system performance with respect to energy demand and cost, and provide recommendations for treatment system modifications.
In this demonstration, propylene glycol, which is miscible with water, nonflammable at ambient temperature, and is a food grade chemical, is the selected co-solvent. After confirming removal of AFFF and PFAS from the system, the collected rinsate will be treated by a portable nanofiltration system; the system will be developed to achieve high system recoveries (95-98%) and will be operated in conventional and closed-circuit desalination nanofiltration (CCDNF) modes. The purpose of the nanofiltration system is twofold; 1) the permeate will serve as a means to separate the propylene glycol from the PFAS-impacted waters, thereby preventing most of the propylene glycol from entering the PFAS treatment stream and allowing the propylene glycol to be reused for future cleaning of equipment; and 2) the nanofiltration system will produce a low-volume of high-concentration PFAS stream that will be further treated (not included in this study).
This demonstration is expected to provide the DoD with a validated approach for cleaning AFFF residuals from small to moderate sized firefighting delivery systems. This demonstration will evaluate a combined flushing (water + propylene glycol) and rinsate treatment approach, but these approaches may also provide value independently. Validation of such approaches will save the DoD millions of dollars by avoiding costly replacement of firefighting equipment prior to implementation of new PFAS-free firefighting agents. (Anticipated Project Completion - 2022)