Is It Possible To Remove PFAS From Biosolids? A Review of Different PFAS Removal Technologies

Introduction
Emerging contaminants per- and polyfluoro alkyl substances (PFAS) have been labeled as forever chemicals due to their ubiquitous detection in nature and hard to degrade characteristics. Although EPA has established health advisory levels (70 ppt for PFOA plus PFOS) in drinking water, there hasn't been any federal or statewide regulations for treated wastewater or biosolids. Only Maine has imposed regulations for three PFAS compounds in biosolids (Sheets and Ledoux, 2021).
Problem
Long chain PFAS compounds have been banned but the PFAS replacement compounds have the capability of degrading into long chain PFAS precursors. The carbon fluorine bond in PFAS is one of the strongest bonds and does not breakdown easily. Thus, conventional wastewater treatment technologies are not capable of removing PFAS. This results in PFAS leaching into agricultural soils and entering crops after land application (Ebrahimi et al., 2021). The prevalence of these compounds in addition to their adverse health effects have raised concerns and necessitated the search of PFAS removal technologies (Ahrens, 2011; Gorrochategui et al., 2014).
Wastewater secondary sludge and biosolids have shown to be routes for PFAS emission to the environment. Class A and class B biosolids have both been used for land application and can help to enrich soils and prove to be a great source of fertilizer. PFAS precursors from biosolids have shown to be transformed into PFAS compounds even after 6 months of land application (WRF, 2021). Treatment techniques for removal of PFAS in biosolids have been evolving but the results are not publicly available and have not been compared to each other. The objectives of this research were to compare different PFAS removal technologies in biosolids in terms of their development and implementation, and further summarize the lessons learnt from different case studies employing these technologies.
Methods
Technologies for removing PFAS have been piloted and researched but are not widely implemented and need further understanding. This presentation will provide a deeper dive into the various technologies being developed along with considerations for each technology. The research team identified six technologies for PFAS destruction/removal from biosolids and reviewed case studies for each technology. These technologies are summarized in Table 1. The different technology readiness levels defined by EPA were used to evaluate the status of each technology in terms of development and testing (Watkins, 2021).
Results and Discussion
Table 2 describes the current status of each technology with a few examples of pilot or full-scale facilities. The key points for each technology are as follows: - Incineration has strong evidence demonstrating that it can destroy PFAS (Winchell et al., 2020; Loganathan et al., 2007), however the high temperatures of greater than 1000oC needed for PFAS destruction (Kucharzyk et al., 2017) might not be attained in conventional incinerators. A few lab-scale studies showed no detectable levels of perfluoroocatanoic acid (PFOA) after thermal destruction of fluorotelomer-based polymers (Yamada et al., 2005; Taylor et al., 2014). The biggest concern around this technology includes the possibility of PFAS being released into the atmosphere and transformation into other fluorinated byproducts (EPA, 2020). - Very few studies by technology manufacturers have documented PFAS removal by pyrolysis (Bioforcetech, 2020). Complete destruction of all PFAS compounds and precursors is yet to be verified. EPA is currently analyzing PFAS content in biochar produced by full-scale pyrolysis facilities (EPA, 2021). - Similar to pyrolysis, gasification has the potential for PFAS destruction based on a pilot-scale study (Aries Clean Energy, 2020). However, transformation of PFAS compounds and detection in gas phase has not been conducted. - SCWO has been developed only in the past couple of years by researchers at Duke University. Initial research shows potential for PFAS destruction in landfill leachate (Jama et al., 2020) but more research is needed to validate its ability to eliminate PFAS in biosolids. - Vitrification facilities operate at temperatures greater than conventional incinerators resulting in combustion of recalcitrant compounds and thus has potential for PFAS destruction (CDM Smith, 2020). A full-scale vitrification facility was operated in Illinois for 3 years but had to be decommissioned due to cost and performance issues. PFAS removal by this technology has not yet been established. - Hydrothermal liquefaction bench-scale studies showed greater degradation of carboxylated PFAS compounds over sulfonic compounds but original PFAS compounds were still present (Yu et al., 2020). This technology needs further modification and development before being implemented for biosolids PFAS removal.
Significance and Conclusions
Figure 1 shows the readiness levels for each technology. Out of the six technologies reviewed, incineration, pyrolysis and gasification have been studied in greater detail due to their widespread use and show potential for PFAS reduction. Other technologies are still being developed and have no full-scale implementations. Case studies for each of these technologies will be studied in detail in the final manuscript and will include lessons learnt from each bench or pilot-scale and full-scale installations. These case studies for PFAS removal from biosolids can improve existing knowledge and database for WRRFs trying to remove PFAS. In depth understanding of each technology, advantages, disadvantages and difficulties in implementing each technology can be crucial for future treatment optimization and modifications.