By Pete Antoniewicz
For more than 16.5 million water-utility customers in 33 different states, contamination caused by per- and polyfluoroalkyl substances (PFAS) is a source-water issue that will not go away for a long time. What are the practical options for community water systems currently confronting this challenge? Here is an overview of several treatments and their relative successes against a wide variety of PFAS compounds.
The Nature Of The Problem
Understanding common characteristics of the more than 4,000 PFAS variations is important for recognizing the difficulties of dealing with them and for appreciating the effectiveness of various treatment options for those variations.
- Sources Of The Problem. The U.S. EPA cites several well-known sources of PFAS contamination in drinking water supplies:
- Firefighting Foam. A history of firefighting-foam usage at commercial and military airports, other military sites, and oil refineries where firefighting training occurred accounts for many known PFAS-contamination sites, given that water runoff from those locations was not controlled in many cases.
- Manufacturing Locations. Many currently polluted sites are associated with past industrial use of PFAS, whether they are manufacturing sites that produced PFAS compounds or simply used them in their production processes.
- Roots Of The Problem. The root of PFAS pervasiveness and longevity is the nature of the carbon-fluorine bonds, which repeat in both long-chain and short-chain PFAS compounds. The EPA defines long-chain as eight or more carbons in a perfluoroalkyl carboxylic acid (PFCA) or as six or more carbons in a perfluoroalkane sulfonate (PFSF).
Because the carbon-fluorine bond is one of the shortest and strongest chemical bonds, PFAS is a persistent problem once it is introduced into the environment. PFAS compounds do not readily break down in the environment and, in certain soils, they exhibit mobility that can cause them to leach through soils during rainfall or irrigation and accumulate in the groundwater below.
The fact that PFAS substances are also both toxic and bio-accumulative means that their contamination in the environment can represent long-lasting problems for drinking water sources — either in groundwater or as runoff to surface water. Although the EPA characterizes short-chain PFAS compounds as being less toxic and less bio-accumulative, those short-chain compounds can also be more resistant to certain types of treatment.
- Millions Of Variables. Of the more than 4,000 PFAS compounds grouped into broad categories of perfluorooctanoic acid (PFOA), perfluorooctanesulfonic acid (PFOS), and their GenX replacements, the EPA has only studied a fraction of them. In addition, different combinations of those compounds can be intermixed in any given water source. Because not all combinations behave the same or respond to specific treatments in the same way, it is important to categorize and compare the specific compounds at hand against the full spectrum of proven PFAS-treatment options.
Granular Activated Carbon (GAC) — A Good Start
GAC was one of the earliest cost-effective treatments for PFAS removal from drinking water, offering greater than 90-percent effectiveness for removal of PFOA, PFOS, and perfluorononanoic acid (PFNA) compounds. As time went on, specific types of activated carbon sources were recognized for their different levels of effectiveness in specific PFAS removal challenges, based on their physical properties:
- Bituminous Agglomerated Activated Carbon. The mesopore (2 nm to 50 nm pore size) structure of this established GAC format is well-suited when high total organic carbon (TOC) levels are present.
- Coconut-Shell Activated Carbon. The micropore (< 2 nm pore size) structure and purity of this sustainable raw material has demonstrated excellent performance for PFAS removal but can be susceptible to TOC adsorption blocking the pores.
- Proprietary-Grade Activated Carbon. Proprietary-grade activated carbons that combine a unique distribution of micropores (< 2 nm) and macropores (> 50 nm pore size) offer an attractive alternative across a wider range of applications. In one full-scale PFAS removal trial, a proprietary type of GAC increased the run time between backwash cycles from a range of 18 to 28 hours for a coal-based carbon to a range of three to five days for the proprietary formula.
In addition to matching the best GAC format to the specific application, it is also important to allow for an appropriate contact time between the GAC media and the water being treated.
Ion Exchange (IX) Resin — The Next Step Forward
For all GAC’s success in certain PFAS treatment conditions, there are multiple circumstances that favor consideration of an IX resin treatment alternative.
The concept of using IX technology in water treatment is not new. It has served as a standalone treatment and as a complementary treatment (e.g., anionic and cationic resins) for a variety of drinking water applications. For PFAS treatment, IX shows promise in instances where:
- Total PFAS concentration is greater than 2 ppb.
- A high percentage of the PFAS compounds are short-chain PFAS (fewer than six carbon atoms).
- Total dissolved solids (TDS) levels are greater than 500 ppm.
- Competing volatile organic compounds (VOCs) are present (> than 0.5 ppb after competitive adsorption on GAC).
- A smaller treatment-system footprint is required or desired. (For example, a 3-minute empty-bed contact time [EBCT] for IX resin vs. a 10-minute EBCT for GAC allows an IX resin system solution to be installed within a smaller footprint.)
Combined GAC and IX Resin — The Potential Best Of Both Worlds?
For removing multiple types of PFAS chemicals from drinking water supplies, both activated carbon and IX resins have been shown to be viable treatments — independently and in hybrid combinations with other treatment technologies. Related studies include: side-by-side comparisons, PAC adsorption and IX, PFAS removal using combined GAC and IX, and nanofiltration with GAC or IX. With further study, the addition of an IX resin treatment as a complement to GAC treatment may offer the best attributes yet for optimum results under certain conditions. Access the EPA’s Drinking Water Treatability Database for more details about research on various treatments for PFAS in drinking water.