Fate of PFAS through Incineration at Wastewater Reclamation Facilities

Introduction Per- and polyfluoroalkyl substances (PFAS) encompass a wide range of compounds, numbering in the thousands. These compounds, because of their properties, have been used in a variety of consumer and industrial products and, consequently, are widely distributed in the environment (Buck et al., 2011). Research focused on understanding the sources of PFAS in drinking water and have identified municipal wastewater reclamation facilities (WRF) as an important pathway (Clara, Scheffknecht, Scharf, Weiss, & Gans, 2008). Because PFAS are concentrated in wastewater solids (Rainey, 2019), WRFs introduce these compounds to the environment through land application of biosolids, potentially allowing PFAS to enter surface water and groundwater (Lindstrom et al., 2011). What has not been well studied is the potential for sewage sludge incinerators (SSI) to introduce PFAS to the environment or destroy it. The extent of PFAS thermal destruction (i.e., thermal degradation by-product formation or complete mineralization) is poorly understood. No published data currently exist on the fate of PFAS through an SSI, although limited information from other industries can be referenced. This paper documents the current understanding on the fate of PFAS through SSIs and the research currently underway to further our understanding. PFAS Thermal Behavior For a combustion process to achieve complete PFAS thermal destruction (mineralization), PFAS compounds would have to be driven to their thermodynamic endpoints of CO2, H2O, HF, or sulfur compounds, if present. Residence time, turbulence (mixing), and stoichiometry (the relative mixture of waste to fuel, oxygen, and other gas-phase constituents) within the flame zone impact the completeness of combustion (Lewis, 2008; Niessen, 2002). Published research and industry guidance indicate that complete destruction of PFAS requires high temperatures (Table 1) which exceeds typical temperatures in SSIs. Notably, this guidance is based on limited conceptual or laboratory-scale experiments and precedence on previous guidance established for hazardous waste incineration. The baseline research for these recommendations stems from USEPA and other international environmental agency activities. Taylor & Yamada (2003) published results of a simulated hazardous waste incineration experiment with limited air to account for non-ideal combustion conditions. Experiments showed less than 0.4% and 0.05% of the PFOS fed to the reactor were detected in the exhaust at tests conducted at 600°C and 900°C, respectively. Yamada, et al., (2005) published a similar laboratory-scale study that considered combustion of PFAS-impregnated textiles. Neither PFOA nor PICs were detected under non-ideal combustion performance. Taylor et al. (2014) found no PFOA emissions after combustion of gasified fluorotelomer-based polymers. Full-Scale Incineration Studies A few full-scale studies have been published on the fate of PFAS through incineration systems with only two considering an SSI. Loganathan et al. (2007) investigated eight PFAS through a wastewater treatment facility. PFAS were measured in dewatered solids fed to the incinerator and in the ash. No mention was made on the operating conditions or whether the bottom or fly ash was sampled. Findings indicated a significant transformation of the measured compounds (26 & 97 percent); however, some PFAS were still detected in the ash. A second study is currently underway at an SSI facility employing an fluidized bed (MacGregor, 2020). Samples were taken at all inputs and outputs of the SSI system and analyzed quantitatively for 28 PFAS. Preliminary results show that mass flows were reduced through the SSI processes for all quantitated PFAS, with the exception of 6:2 fluorotelomer sulfonate. The authors also noted that the inorganic fluoride content of the wet scrubber discharge water was over 10,000 times that of the influent flow plus the measured PFAS fed to the furnace, assuming mineralization. No other studies have been published on SSIs, but limited information can be found in literature investigating incineration in other industries. Lemieux et al. (2007) reported on the USEPA's study of feeding carpet treated with fluorotelomer products into a pilot-scale rotary kiln furnace. The emitted PFAS detected, primarily PFOA and PFHxA, did not change when feeding PFAS contaminated carpet or not. Aleksandrov et al. (2019) investigated emissions from a pilot-scale rotary kiln incinerator. No significant PFAS emissions were detected. Thermal By-Products Bench- and pilot-studies have examined decomposition of specific PFAS congeners and by-product identification; several studies documented thermal PFAS degradation by-products (Table 2). These studies show that PFAS will decompose at temperatures relevant to operating conditions of SSIs although by-product formation will be a concern. Funded Research The USEPA (2021) has recently sampled an SSI as part of a broader sampling program at a WRRF. Samples were taken from all major input and output streams of the multiple hearth for PFAS characterization. The samples are currently being analyzed for targeted, non-targeted, and total organic fluorine. Winchell et al. (2021b) provide a detailed discussion on these analytical techniques. A separate study led by Brown and Caldwell with support from the Water Research Foundation's Tailored Collaboration program will sample two SSIs, one multiple hearth and one fluidized bed. This study mirrors the USEPA from a sampling and analytical standpoint. The full-scale sample will take place during the winter and spring. Initial work has included site selection and workplan development. Including, estimating PFAS emissions at various destruction and removal efficiencies (DREs) based on targeted PFAS measured in initial dewatered cake samples. Figure 1 shows the PFAS emissions from one of the test sites remain above the reporting limits for the analytical method at relatively high DREs. This suggests specific PFAS will be detectable unless achieving high DRE, a question highly sought after by the industry. Conclusions Thermal treatment of PFAS through SSIs represents a potential wastewater solids process for destroying PFAS; although, significant questions remain regarding both the destruction efficiency and potential formation of undesirable by-products. Temperature is only one of the three key operational parameters that should be considered when assessing destruction capacity in combustion systems. The other two important factors include residence time and turbulence. A well-functioning SSI will submit PFAS to greater residence times and mixing (or turbulence) than the laboratory-scale research performed to date, further promoting PFAS destruction.