Existing filtration technology

While there are many different ways drinking water is treated in the United States, conventional treatment methods like sand filtration, disinfection, oxidation, and others remain largely ineffective at removing PFAS from water. Thanks to their unique physical and chemical properties as well as their ability to dissolve in water, PFAS can be efficiently removed from drinking water by only a select few different types of technology. The three main types of these technologies are activated carbon systems, ion exchange resin systems, and high pressure membranes.

Activated carbon systems

In granular activated carbon (GAC) systems, water flows along beds of highly porous organic material like coal, peat, wood, or coconut shells. If these systems are maintained properly, organic compounds and chemicals like PFAS that are found in the water can bind to the surface area of the material in the bed of the filter. 

However, GAC systems' effectiveness at removing PFAS is influenced by a multitude of factors. The technology can potentially be 100% effective for a length of time that depends on the type of carbon being used, the depth of the carbon bed, the time the water spends along the bed, temperature, the concentration of other contaminants or organic material already present in the water, and the specific PFAS that needs to be removed.

The North Carolina PFAS Testing Network's performance study of different removal technologies completed as a part of the research mandated by Session Law 2018-5, for example, found that while GAC was able to remove every different PFAS they tested, it quickly lost its effectiveness for short-chain PFAS like GenX chemicals or perfluorobutane sulfonic acid. So for water sources where higher concentrations of short-chain PFAS have been detected, a GAC system's filters would need to be replaced more often in order to maintain high efficiency. 

The factors determined by the system design, like the depth of the carbon bed and the length of time the water spends along the bed, are often dictated by the specific application the system is being used for. For example, some GAC systems are used to remove the organic compounds that affect the taste and odor of the water and remain far less effective with PFAS control than the GAC systems that have been optimized for that very purpose.  

In a 2020 performance study of different types of point-of-use filters, N.C. State University environmental engineering professor Detlef Knappe and other researchers from that school and Duke University found that activated carbon filters showed a wide variation of effectiveness, removing an average of 60 to 70% of long-chain PFAS but only 40% of short-chain PFAS. 

GAC's close cousin, powdered activated carbon (PAC), involves releasing small, powder-like particles of the same activated carbon material used in GAC directly into the water before they're filtered back out. Similar to GAC systems, PAC's effectiveness at removing PFAS also depends on factors like the type and amount of carbon being added to the water, the organic materials already present in the water, and whether or not the systems are designed and maintained with an eye towards PFAS filtration. 

While some water treatment facilities in North Carolina use PAC systems specifically for PFAS removal, the majority of the facilities Facing South examined in North Carolina use the technology only to regulate the taste and odor of drinking water. 

Mei Sun, an environmental engineering professor at the University of North Carolina at Charlotte, said that PAC systems utilized for taste and odor issues might likely require much less carbon than what would be needed for PFAS removal. And research from the Environmental Protection Agency shows that, even using the very best carbon at very high doses, PAC is unlikely to remove a high percentage of PFAS. 

In addition, Facing South found that many water treatment facilities that use PAC systems for taste and odor control add PAC to the water only during the times of the year they've received taste and odor complaints from the community.

Another version of activated carbon systems are known as carbon block filters. These filters function almost exactly like a GAC filter, but instead of holding the ground-up carbon inside a cartridge or container, the carbon is ground even finer and mixed with a food-grade binder before it's heated and compressed into a solid block. 

Ion exchange resin systems

These systems use resins made of highly porous and insoluble polymeric material that acts as a medium for ion exchange. In chemistry, the term “resin” refers to a solid or highly viscous substance of organic or synthetic origin, and a “polymer” is a substance consisting of large molecules composed of many repeating subunits. These resins come in the form of tiny bead-like objects that act like an army of magnets, attracting the contaminants in the water so they can be removed and disposed of. 

There are two major categories of these systems based on the type of charge the resins have. The negatively charged cationic exchange resins are effective at removing positively-charged contaminants, and the positively-charged anion exchange resins can effectively remove PFAS and other negatively-charged contaminants. 

Similar to GAC systems, ion exchange resin systems can remove 100% of PFAS for a length of time determined by the type of PFAS that needs to be removed, the type of resin being used, the rate at which water flows through the system, and the makeup of the organic material already in the water. 

The Testing Network's initial performance study of different PFAS-removing technologies found that the type of resin polymer is the most important factor for PFAS removal in ion exchange resin systems. 

High pressure membranes

The two major types of high pressure membrane technology are reverse osmosis (RO) and nanofiltration. RO membranes are slightly tighter than nanofiltration membranes and consequently remove PFAS and other material more efficiently. 

RO works by using pressure to force water through a semipermeable membrane that stops contaminants and other matter from flowing through. In the 2020 performance study of different point-of-use filters that Knappe and other researchers from Duke and N.C. State University published in Environmental Science and Technology Letters, RO filters consistently showed near-complete removal of all the different PFAS they evaluated. 

While RO is undoubtedly the most effective treatment method for PFAS that currently exists, it's also the most expensive and must be monitored closely for safety. 

For example, the RO units in Brunswick County Schools each cost $75 to install and $75 a month to maintain. In contrast, the GAC units in Granville County Schools each cost around $1,100 for installation and require $50 filter replacements every 6 months. 

Not only does RO require a significant amount of energy, it also creates a significant amount of wastewater. For example, a typical point-of-use RO system will produce five gallons or more of reject water for every gallon of treated water produced, according to the EPA. While membrane technology is improving and some manufacturers claim only one gallon of reject water is generated for each gallon of treated water, this can still create issues in areas where the drinking water source is limited.

In addition, the wastewater produced by RO systems is corrosive, and the system's flow rate must be monitored to prevent the corrosion of plumbing and fixtures.