PFAS Fate in Pyrolysis System Reflecting Full-Scale Configurations: Thermal Oxidizer Impacts

Introduction
Per- and polyfluoroalkyl substances (PFAS) are becoming an increasingly scrutinized and studied class of pollutants due to their widespread use and environmental contamination leading to federal and state health-based guidance and regulatory values. PFAS resist environmental degradation and can accumulate in the urban water cycle, including wastewater systems (Winchell et al., 2021c). Furthermore, various PFAS will migrate to wastewater solids (termed biosolids when stabilized) where conventional treatment technologies have little to no effect on the PFAS content of the solids (Hamid et al., 2016, Yu et al., 2009, Loos et al., 2018, Gallen et al., 2018, Mailler et al., 2017, Kim Lazcano et al., 2019). PFAS in biosolids have the potential to derail beneficial reuse via land application due to the potential impacts to groundwater, and transfer into agricultural crops (Lee et al., 2014, Zhang et al., 2015, Navarro et al., 2017, Ghisi et al., 2019). Thermal treatment is the only process currently identified to reliably destroy PFAS (USEPA, 2020a), but destruction efficiency must be confirmed to avoid worsening atmospheric pollution and deposition (Shafer et al., 2020). While incineration shows promise for PFAS destruction (Winchell et al., 2021a), utilities are looking to thermal treatment alternatives that do not require the same permitting rigor and generate a higher value solid residual. Pyrolysis fits these requirements.
Current Understanding
Pyrolysis systems include more than the reactor thermally processing the feedstock in a reducing atmosphere, see Figure 1. Pyrolytic reactions consume and release heat energy, with an overall heat flow generally considered zero when processing dry feedstocks (Basu, 2013). Dewatered wastewater solids require drying to evaporate water prior to treatment in the pyrolyzer. Contemporary systems recycle heat for drying and pyrolysis by processing the fuel rich off-gas from pyrolysis through a thermal oxidizer. Subsequent processes include removal and handling of the solid residual, biochar, and further cleaning of the flue gas from the thermal oxidizer. The pyrolysis systems offers several opportunities to remove and possibly destroy PFAS. Pollutant destruction and removal efficiency (DRE) in high-temperature thermal processes can generally be benchmarked by the '3 T's' (i.e., time, temperature, and turbulence). Table 1 contains key operating parameters for a recent survey of technology suppliers. The pyrolyzer can reach temperatures up to 950°C, depending on the goals of the solid and gas residuals, with several second residence times for the off-gas and 15-20 minutes for the solids. The thermal oxidizer conditions can likewise be tailored, but typically process the off-gas for over two seconds at over 850°C. PFAS destruction requirements often quoted only refer to temperature (>1,000°C) based on limited data sets designed for hazardous waste incineration (Winchell et al, 2021a) which can lead to premature conclusions about currently available high-temperature processes. The wastewater industry requires a better understanding on the fate of PFAS through these systems. Several studies have started looking into the fate of PFAS through pyrolysis systems processing wastewater solids. Kim et al. (2015) observed no significant change of PFAS in the biochar after pyrolysis of wastewater solids at 300 and 700°C in a laboratory setup. Kundu et al. (2021), however demonstrated removal of all measured PFAS species in biochar at temperatures ranging from 500 – 600°C. Williams et al (2021) detected three PFAS of 28 evaluated in biochar at 500°C and none after 700°C. Williams et al (2021) also analyzed the off-gas stream and reported a combined mass removal efficiency of 84.4 and 95.6 percent of measured PFAS when including the levels in the biochar at the same two experimental temperatures, respectively. The body of research does not consider the impacts of the thermal oxidizer on the gas-phase outlet.
Experimental Approach
A research consortium partnered to begin evaluating the fate of PFAS through the entire pyrolysis process. The initial phase of the research considers the fate of PFAS through laboratory-scale drying, pyrolysis, and thermal oxidation systems. Collaborative funding of the partners supported construction of a laboratory-scale system at Western University's Institute for Chemicals and Fuels from Alternative Resources (ICFAR). Figure 2 shows the basic configuration of the laboratory system. Dried wastewater solids will be loaded into a 'biomass' feed hopper and metered into the mechanically stirred pyrolyzer. Off-gas can be processed through a condensing system, reflective of historical applications, or through a thermal oxidizer. The system will process roughly one kilogram per hour of dried material under conditions reflecting a currently operating full-scale system at the Silicon Valley Clean Water treatment facility in Redwood City, CA. Construction and commissioning of the laboratory system will be completed in December of 2021. PFAS sampling and analysis poses a significant challenge given the complexity, diversity, and novelty of the target substances (Winchell et al., 2021b). This research will employ the latest standardized and research methods. All streams into and out of the system will be sampled including solid and gas-phases at the laboratory scale. Using targeted, non-targeted analysis (NTA), total organic fluorine (TOF), and Fourier transform infrared spectroscopy (FTIR) this study will aim to close the PFAS mass balance across the system to the extent possible within current analytical constraints. Preliminary assessment of the donor wastewater solids and laboratory system operation suggest specific PFAS can be detected, above the method reporting limits, in the gas emissions at high DREs, see Figure 3. Furthermore, to our knowledge NTA has not been reported yet on wastewater solid pyrolysis systems. NTA identifies PFAS not currently evaluated for in the standardized USEPA Method 537 (2020b). This will help determine what 'other' PFAS are present in wastewater solids, biochar, and gas emissions.
Conclusions
This paper and presentation will provide the WEF audience a detailed summary of the current published literature on the fate of PFAS through pyrolysis systems. This summary identifies the gaps in knowledge and the proposed study takes the first step in evaluating the emissions from the system post thermal oxidation, consistent with existing configurations. An in-depth discussion on the study sampling and analytical procedures will provide the wastewater community a valuable resource on PFAS measurement. Additional value will be provided by way of the available study results.