PFAS Can't Survive Pyrolysis, Two Studies to Prove It

[b]Introduction[/b] PFAS continues to garner widespread attention, driving concern and mitigation efforts from water utilities. Biosolids have been given special attention, especially since the USEPA's draft risk assessment stating that greater than 1 ppb of PFOA or PFOS poses human health risks from land application practices. As such, utilities have been searching for solutions. High temperature processes offer the potential to remove and potentially destroy PFAS contained in biosolids (Winchell et al. 2021). Pyrolysis is one promising technology. Pyrolysis thermally treats biosolids in the absence of oxygen, yielding a solid residual typically called char and hydrocarbon-rich-off-gas. While pyrolysis alone provides some PFAS treatment (Winchell et al 2022), coupling a thermal oxidizer to combust the off-gas oxidizes the remaining PFAS. Unfortunately, little information on the extent of PFAS treatment is available for pyrolysis coupled with thermal oxidizers. The two studies covered in this abstract and subsequent presentation will cover laboratory-scale and demonstration-scale pyrolysis and thermal oxidizer systems. Each system underwent extensive PFAS sampling and analysis to determine performance and evaluate correlation between the laboratory-scale and the larger system. [b]Methodology[/b] In 2021, Brown and Caldwell funded Western University Institute for Chemicals and Fuels from Alternative Resources (ICFAR) to fabricate a solids pyrolysis and thermal oxidizer laboratory reactor system. In May of 2023 the system was used to process biosolids from Silicon Valley Clean Water (SVCW) where the demonstration-scale system operates. Dried biosolids were fed at roughly 0.3 kg/h to the pyrolysis reactor operating at 600 °C for 75 minutes. Off-gas proceeded through a thermal oxidizer achieving 1,000 °C for two seconds. Samples of dried biosolids, char, flue gas from and combustion air for the thermal oxidizer were collected and analyzed for PFAS. More details on the study were published by Winchell et al (2024). In January of 2025 samples from the laboratory-scale system were collected from the same points as the demonstration-scale system. Additional sampling points included the dewatered cake fed to the dryer and stack emissions downstream of the air pollution controls (wet scrubber and activated carbon filter). Analyses included targeted PFAS and organic fluorine for all samples, plus volatile fluorinated compounds in the flue gas samples. The demonstration-scale pyrolysis reactor operated at 550 °C for the first two-thirds and 630 °C for the final third of the run with a residence time of approximately 20 minutes. The thermal oxidizer operated at approximately 985°C and 3-second residence time. [b]Results and Discussion[/b] The laboratory study (Winchell et al 2024) documented the presence of targeted PFAS in all inputs and outputs of the pyrolysis system coupled with a thermal oxidizer (Figure 1). Across the system, an average 99.4% PFAS (molar) removal was observed. In the solid phase, the dried biosolids PFAS content was reduced 99.0–99.9% after treatment to produce biochar. The flue gas contained reportable PFAS but less than 0.200% of that fed to the system in the dried biosolids. Drying altered the PFAS profile in the demonstration-scale study documented and was associated with a statistically significant reduction in extractable organofluorine, decreasing from 244 to 137 µmol per batch. Outputs from biochar, scrubber water, and stack emissions combined accounted for 0.026 µmol, corresponding to an overall removal efficiency of 99.98% for targeted PFAS. The wet scrubber demonstrated measurable PFAS uptake, though results were inconsistent across runs. Five volatile fluorinated compounds were detected at the stack, accounting for roughly seven percent of the initial targeted PFAS fluorine mass. [b]Conclusions[/b] These studies exhibit that pyrolysis coupled with thermal oxidation achieve PFAS treatment at levels that will protect human health. Both the laboratory- and demonstration-scale systems achieved similar PFAS treatment performance, demonstrating investigators can conduct similar experiments at the more economical laboratory-scale and expect comparable performance at larger scale operations. The results warrant further investigation into validation and investment in advancing the reliability of pyrolysis and thermal oxidation to manage PFAS from biosolids.