With municipalities gearing up to reduce PFAS as part of recently announced U.S. Environmental Protection Agency (EPA) regulation, something called TOC will have a major impact on the filtration required to meet the limits. Even in their own homes, consumers may be impacted by TOC if they are using filters to reduce PFAS in their water.

What Is TOC?

Total organic carbon (TOC) is a measurement of the carbon in water that is part of organic molecules.

The water that ends up in your glass always originates in nature. Picture a river flowing slowly toward a reservoir. As the water flows in the river, it picks up natural organic matter. This can be from a variety of sources, including decaying leaves and microbes.

There are many thousands of molecules that fall under this category. They can range from large molecules, such as the humic acid molecule shown in Figure 1, to smaller molecules, such as the fulvic acid molecule in Figure 2. Even if the municipality uses groundwater, organic molecules like this are present.

Figure 1: Example of large humic acid molecule.
Figure 2: Example of a small fulvic acid molecule.

Unless you have well water, the water that reaches your tap does not contain these molecules. This is due to how municipalities treat their source water. In most cases, municipalities use oxidizers to kill any microbes present in water. Examples of oxidizers include chlorine, chloramine, and ozone. In the oxidation process, though, the oxidizing chemicals also react with the natural organic molecules in the water. These oxidation reactions break up the larger organic molecules into smaller ones. Examples of these smaller molecules are shown in Figure 3.

Figure 3: Examples of small molecules formed by the oxidation of natural organic matter molecules.

TOC comprises a mixture of molecules like these. And, just like natural organic matter, there are many thousands of possible molecules. These molecules are generally not harmful to humans at low levels and are left in the tap water delivered to homes.

Removing PFAS with Activated Carbon or Ion Exchange

PFAS, as a category, includes thousands of molecules. Currently, however, the focus is on about a dozen or so of those molecules.

Two of the main methods for reducing PFAS in drinking water, as promoted by the EPA, are activated carbon and ion-exchange resin.1 Both remove PFAS but through different mechanisms.

Activated carbon works through the principle of adsorption. The structure of activated carbon is highly porous, leading to an exceptionally large surface-area-to-mass ratio. In other words, a small amount of activated carbon has a deceptively large surface area, sometimes in the thousands of square meters per gram. To put this into perspective, less than two grams of activated carbon has the same surface area as a football field. When PFAS molecules or other organic molecules go through activated carbon, they are attracted to the surface of the activated carbon and get stuck.

PFAS reduction with ion-exchange resin works differently. To understand the mechanism, let’s look at the structure of a common PFAS, perfluorooctanoic acid (PFOA), seen in Figure 4.

Figure 4: Molecular structure of PFOA.

At the far left of the molecule is something called a carboxylic acid group. Specific ion-exchange resins have a complementary functional group that attracts and binds with the acidic group on the PFOA.

TOC Impacts PFAS Reduction

In general, TOC in water dramatically reduces the reduction capability of activated carbon and ion-exchange resin because of how similar TOC molecules are to PFAS molecules. The structures are compared in Figure 5, in which the last structure shown in Figure 3 has been drawn in the same way as the PFAS molecule from Figure 4.

Figure 5: Molecular structure of TOC molecule octanoic acid shown in Figure 3 (left) and PFAS molecule perfluorooctanoic acid (right).

Do you see how similar the molecules in Figure 5 look? These two molecules interact with activated carbon and ion-exchange resin through similar mechanisms. This is true for many TOC and PFAS molecules. So, TOC and PFAS are competing for the same space in your filter media.

To make matters worse, TOC is usually in water at the parts-per-million (ppm) level. PFAS, on the other hand, are usually found at the parts-per-trillion (ppt) level. This means the amount of TOC is on the order of one million times greater than the amount of PFAS in water.

Real-World Implications

What does this mean to the water-treatment industry? Let’s look at an example from a study that compared the water from three municipalities with different levels of incoming TOC. The water for each test was spiked with a mixture of PFAS to make the comparison consistent. For each water sample, a team assessed how many bed volumes of granular activated carbon could be used before PFAS broke through.

One of these municipalities was based in North Carolina and had an incoming TOC level of less than 0.3 ppm. On the other end of the spectrum was a municipality in Florida with a TOC level of 4.6 ppm. The water from the municipality in North Carolina was able to reach 84,000 bed volumes before PFAS breakthrough was observed (greater than 20 ppt in this experiment). The water from Florida was able to reach only 750 bed volumes before breakthrough of PFAS was observed. This means the municipality in Florida would have to use 1,000 times more activated carbon in a year to remove PFAS than the municipality in North Carolina.2

If a municipality does not yet remove PFAS, then the amount of TOC in the water will determine the life of a point-of-use or point-of-entry filter that contains activated carbon and/or ion-exchange resin. Unfortunately, there is no easy way to test for TOC on site; it requires special analytical instrumentation. However, you can check your municipality’s water report. Note, however, that TOC varies from season to season if your municipality is supplied by surface water.

Opportunities for Innovation

With such a challenge on our hands, there are many opportunities for innovation in the water-treatment industry:

Activated carbon and ion-exchange resin manufacturers can work on developing materials that are selective for PFAS or TOC.

Sensor manufacturers can work on affordable real-time PFAS sensors. This would prevent treatment systems from being under- or overengineered.

Point-of-use and point-of-entry water filter manufacturers can design their filters to handle some amount of TOC in water.

References

U.S. Environmental Protection Agency. “Reducing PFAS in Drinking Water with Treatment Technologies.” Science Matters, published August 23, 2018. https://www.epa.gov/sciencematters/reducing-pfas-
drinking-water-treatment-technologies.

Knappe, D. “PFAS: State of the Science and Research Needs.” American Water Works Association Contaminants of Concern Virtual Symposium, March 14-16, 2023.

About the author

Steven Woltornist is a senior manager of product development at Marmon Water Residential Filtration, where he helps original equipment manufacturers develop advanced filtration solutions for contaminants including PFAS, heavy metals, and more. He holds a PhD in chemistry from the University of Connecticut, where his research focused on graphene-based composite materials for applications in water purification, electronics, sensors, and energy storage.

About the company

Marmon Water Residential Filtration, a subsidiary of Berkshire Hathaway, brings together the expertise of KX Technologies and Filtrex Technologies. Our core mission revolves around point-of-use drinking water filtration. As a trusted partner, we supply more carbon blocks to OEMs than anyone else in the world. Furthermore, we lead in technology, with several products already certified to reduce PFAS in drinking water, and we continue to innovate with new solutions on the horizon.

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