Challenges and Research NeedsPFOA and PFOS account for only a small fraction of the total organic fluorine present in water and wastewater samples. Certain polyfluoroalkyl substances, including FTOHs, EtFOSE, and cationic/zwitterionic PFAS, can transform to PFOA and PFOS during chemical, biological, and thermal treatments. However, there are also unidentified precursors present in drinking water (Boiteux et al. 2017). The removal of precursors of PFOA and PFOS is the key to safeguarding drinking-water quality. The source water chemistry, the dose and type of disinfectants, and adequate pretreatments prior to the biological treatment or chemical disinfection are likely the main factors for controlling the formation of PFOA and PFOS in water and wastewater treatment processes.Various advanced technologies have been developed to remove PFOA, PFOS, and their precursor compounds from drinking water. Although some of these treatments have been successful in research labs [see references in Espan et al. (2015)], most of these approaches have major economic or design challenges prior to implementation for treatment of natural waters containing colloidal particles and dissolved organic matter (NOM) at circumneutral pH. At present, GAC adsorption appears to be one of the very few scalable treatment options for PFAS. The author believes that PFOA, PFOS, and their precursor compounds could be controlled by preoxidation via ozone or advanced oxidation processes to transfer precursor compounds to perfluoroalkyl substances (e.g., PFOA and PFOS), followed by adsorption of PFAS by GAC and thermal treatment of PFAS-laden GAC at 700°C or above (Xiao 2019).Conversely, although the thermal treatment holds promise for degradation of PFAS chemicals and reactivation of PFAS-laden GAC (Duchesne et al. 2020; Sasi et al. 2021; Watanabe et al. 2018; Xiao et al. 2020), carbon loss, energy cost, and volatile decomposition products of PFAS are potential concerns. Further studies are suggested to investigate alternative reactivation methods, such as chemical (Huling et al. 2005) or electrochemical (Karimi-Jashni and Narbaitz 2005) approaches, for PFAS-spent GAC.Nanofiltration (NF) and reverse osmosis (RO) as advanced separation technologies are capable of removing PFAS from water (Appleman et al. 2013; Tang et al. 2006), although not every community can afford to include NF and RO in the water treatment system. The main residual produced from an NF or RO system is brine containing elevated levels of NOM and ionic strength. The removal of PFOA, PFOS, or their precursor compounds from NF/RO brine is rarely studied.Furthermore, only limited data are available on the reaction of the cationic and zwitterionic precursor compounds with chemical disinfectants (Xiao et al. 2018). Systematic studies are needed to understand the fate and transformation of cationic and zwitterionic PFAS in water and wastewater treatment processes.Treated water leaving the drinking water treatment plants is usually disinfected and stored prior to distribution. The reaction between precursor compounds and residual chlorine may contribute to a further concentration increase of PFOA and PFOS during water storage. Future studies are suggested to understand the fate and transformation of precursor compounds in storage facilities and distribution systems.References Andrews, D. Q., and O. V. Naidenko. 2020. “Population-wide exposure to per- and polyfluoroalkyl substances from drinking water in the United States.” Environ. Sci. Technol. Lett. 7 (12): 931–936. https://doi.org/10.1021/acs.estlett.0c00713. Appleman, T. D., E. R. Dickenson, C. Bellona, and C. P. Higgins. 2013. “Nanofiltration and granular activated carbon treatment of perfluoroalkyl acids.” J. Hazard. 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