Conference Agenda

Rising concern of Perfluorinated alkylated substances (PFAS) contamination of our ecosystem has sparked interest in this pollutant as it pertains to water and waste management. While PFAS sources are numerous, it is widely believed that firefighting foam and extensive industrial uses are common pathways for PFAS compounds to proliferate into our ecosystem.

There is a pressing need to develop and validate advanced treatment technologies that can destroy PFAS in a variety of substrates. Supercritical water oxidation or SCWO for short, is one such advanced physical-thermal process that relies on the unique reactivity and transport properties of water above its critical point of 374 °C and 218 atm. At these conditions, organics are fully soluble in supercritical water, and with the addition of oxygen, all organics rapidly and completely oxidized to form carbon dioxide, clean water, and inorganic salts.

In an attempt to better understand the physio-chemical destructive mechanisms, and the fate and transport of PFAS compounds undergoing oxidation, a one (1) wet ton per day scale SCWO system was employed to study the elimination efficiencies of this process treating three distinct PFAS waste substrates from three different sources. The three wastes include (1) lime stabilized sludge from a municipal wastewater resource recovery facility; (2) aqueous film forming foam (AFFF) from a DoD facility; and (3) spent ion exchange resin from a ‘pump and treat’ water treatment facility.

The studies examined both targeted and non-targeted PFAS compounds, including PFOA and PFOS. These two compounds are the most studied PFAS due to their high toxicity and being most prolific in our ecosystem. SCWO, on average, was able to eliminate 99.95% of PFOA and 99.99% of PFOS across all waste substrates, and greater than 99.9% elimination of all other PFAS compounds combined.

Non-targeted PFAS was accounted for using laboratory scale verification of the fluorine mass balance. No hydrogen fluoride was found in the effluent gas, and all the fluorine from the destroyed PFAS was accounted for as fluoride in the effluent water.

The studies produced valuable data and design parameters to support design and deployment of SCWO for real world applications.

Many wastewater agencies are facing the dual challenge of trying to address PFAS within the treatment plant and facing limitations in biosolids disposal options. This presentation will address both of those challenges and give attendees a solid understanding of how PFAS enters wastewater, accumulates in biosolids, and can potentially be destroyed by different techniques that are being evaluated. We will specifically discuss PFAS characterization studies that have been done to-date in wastewater treatment plants and new innovative studies where Brown and Caldwell (BC) is partnering with utilities to better understand PFAS destruction using thermal processes. This presentation will highlight recent work that is underway at Silicon Valley Clean Water (SVCW), which has the only operational large-scale biosolids pyrolysis systems in the country. Because the fate of PFAS through pyrolysis is not well understood, BC has partnered with SVCW to perform special studies aiming to provide a comprehensive picture of the fate of different PFAS species, precursors, and transformation products through the biosolids pyrolysis processes. This work will provide valuable insights into the level of PFAS transformation and/or destruction within the pyrolysis system. Because thermal treatment is the only technology currently available to utilities to destroy PFAS, this research aims to characterize the extent of destruction and support the development of scientific data documenting their positive environmental impact.

For this study, parallel samples will be processed through SVCW’s pyrolysis reactor and a bench-scale pyrolysis reactor coupled with a thermal oxidizer at operating conditions resembling SVCW’s process. Samples of the dewatered biosolids, dried biosolids, biochar, and gas emissions will be collected for PFAS analysis, including targeted, non-targeted, and total organic fluorine to fully characterize PFAS fate through the system. Results of this study will demonstrate whether current sampling and analytical approaches approximate a mass balance for specific compounds while identifying others previously unknown. Testing is scheduled to take place this summer and results from this study may be ready to discuss prior to the presentation.