Time, temperature, and turbulence: the fundamentals of removing PFAS from gaseous emissions

Per- and polyfluoroalkyl substances (PFAS) are chemicals with heat and water-resistant properties that have been utilized in manufacturing since the 1940s. Their historical use in the production of everyday materials has led to them being detected in surface water, groundwater, soil, and plants around the globe. PFAS compounds have been linked to various diseases, immune system disorders, and cancers leading to concerns about the environment and health risks associated with their presence. And drinking water contamination isn’t the only concern.

The PFAS challenges facing utility companies in the United States

Today, utility companies and private organizations in the United States operating municipal wastewater treatment plants are facing major challenges pertaining to their residual solids. The U.S. Environmental Protection Agency (EPA) finalized a rule in April 2024 to designate two widely used PFAS as hazardous substances under the Comprehensive Environmental Response, Compensation, and Liability Act. In January 2025, the EPA released the Draft Sewage Sludge Risk Assessment for Perfluorooctanoic Acid (PFOA) and Perfluorooctanesulfonic Acid (PFOS) for public comment. More than half of residual solids produced are land applied as a fertilizer alternative due to them being rich in carbon and nutrients. Now identified as potentially harmful to our food supply, organizations must rethink ways to repurpose their residuals or remove PFAS from them all together. New legislation in Maine and Connecticut has banned the use of land applied biosolids, and other states are considering it.

Potential solutions for one problem may create another. While technologies such as gasification and pyrolysis are being evaluated as alternative options for the processing of biosolids to remove PFAS from the residuals, questions remain about what happens to the PFAS compounds that have been introduced into the gas that goes up the stack after the combustion process.  

“Our clients want a solution that removes PFAS from biosolids and protects air quality,” says our water market leader in the eastern part of the United States, Amy Jablonsky. “This is something we are working to get ahead of for our partners through proper research and evaluation.”  

Mott MacDonald’s vice president and principal process engineer, Philip Pedros, has partnered with Northeastern University to study the fate of PFAS compounds in combustion using equilibrium computer model simulations. In this article, he provides his thoughts on the research and what needs to be evaluated for a successful outcome.

Why equilibrium modeling?

The fate of PFAS in a specific combustion process and thus in the exhaust gases is less understood and much more difficult and expensive to measure, in fact it can cost upwards of $150,000. To mitigate the cost to organizations, the use of validated models could be used to predict the products including PFAS derivatives that may be in the gaseous emissions.

The scope of the research

The first step in the use of equilibrium model simulations is to determine the temperature required for complete mineralization of each PFAS compound of interest.  This is accomplished by running multiple simulations at increasing temperatures until mineralization is indicated by the predicted products. 

It must be noted that the determination of properties and equilibrium models are only part of a much larger effort to understand what happens to PFAS in combustion processes.  In addition, kinetic studies as well as combustion testing (under highly controlled conditions) of individual PFAS compounds are required to better understand, and therefore design combustion processes that are effective and efficient in removing PFAS compounds and derivatives from the gas.  The larger effort is comprised of the following four primary tasks:

1. Determine the thermodynamic properties of the PFAS compounds of interest over a large temperature range
2. Conduct equilibrium computer model simulations to determine minimum temperatures for mineralization (destruction)
3. Perform kinetic studies of each of the compounds to determine the time required for mineralization at a given temperature  
4. Conduct combustion experiments and emissions testing and analysis to compare the results and validate the modeling as much as possible, while considering and accounting for products of incomplete combustion that will manifest in the actual process

The effort outlined above is significant and will require the efforts and expertise of experts from multiple disciplines, for several years or more. Mott MacDonald is currently working with Northeastern University on task 1, the determination of the thermodynamic properties, which has resulted in two publications and task 2, equilibrium modeling, which is currently underway. Given the depth and breadth of the research effort, Mott MacDonald looks forward to working with others in our industry to address these interesting challenges.