### Conventional Water Treatment Methods  
There are various methods used for water treatment to remove contaminants, broadly categorized as separation and destructive methods. The separation process involves conventional techniques like flocculation, coagulation, sedimentation, aeration, disinfection, and filtration. It also includes non-conventional techniques such as sorption techniques using activated carbon, biomaterials, minerals, ion exchange resins, polymers, and nanomaterials. Additionally, non-conventional techniques that include more advanced techniques such as foam fractionation and oozofractionation, as well as membrane technologies like nanofiltration and reverse osmosis, are utilized for separation [17; 20; 21; 22; 23]. On the other hand, destructive methods involve advanced oxidation techniques, such as chemical, electrochemical, and photochemical processes, as well as ultrasonication and the conventional technique of biological remediation, aeration, and disinfection [17; 20; 21; 22; 23].  
Numerous reviews of the existing literature on PFAS treatment have consistently found that conventional treatment processes are ineffective for PFAS degradation. This is primarily due to the recalcitrant behaviour of PFASs, resulting from their high chemical and thermal stability, which is a consequence of the strong covalent C-F bond in their molecular structure, as well as their high negativity in F\({}^{-}\), low water concentration, and pronounced amphiphilicity and lipophobicity [16; 20; 21; 23; 24; 25; 26]. Biological treatments, both aerobic and anaerobic, are capable of breaking the C-C bond in PFAS compounds, leading to the formation of short-chain PFASs. However, it has been observed that only polyfluorinated forms very slowly transform into extremely persistent perfluoroalkyl acids (PFAAs), with some intermediate forms still unidentified [24]. [PERSON] et al. [25] also reported the ineffectiveness of both aerobic and anaerobic biological treatment, particularly when considering contact time constraints in drinking water facilities. When advanced treatment technologies are not utilized, biological wastewater treatment plants (WWTPs) can impact receiving water bodies, resulting in higher concentrations of PFASs detected in finished water compared to untreated water [26]. Under typical water treatment plant conditions, disinfection usingfree chlorine or UV irradiation and biological treatment alone is ineffective in removing PFASs [27; 28].  
[PERSON] et al. [25] reviewed the literature and found that conventional treatment technologies generally achieve PFAS removal rates of 0 to 5 percent. [PERSON] et al. [28] conducted a review on PFAS removal from water treatment plants in Japan, Australia, and other locations using conventional processes such as coagulation, flocculation, and sedimentation. They concluded that these techniques resulted in minimal removal of PFASs.  
In their study, [PERSON] et al. [27] evaluated 15 full-scale treatment systems in water treatment utilities across the US for the removal of PFASs. They reported that water treatment techniques such as ferric or alum coagulation, granular/micro-/ultrafiltration, aeration, oxidation (i.e., permanganate, ultraviolet/hydrogen peroxide), and disinfection (i.e., ozonation, chlorine dioxide, chlorination, and chloramination) were mostly ineffective in removing PFASs.  
Compared to conventional water treatment methods, advanced treatment approaches, such as high-pressure nanofiltration (NF) and reverse osmosis (RO), adsorption/anion exchange using specialized synthetic materials, and certain advanced oxidation methods, have shown better success in removing PFASs [8; 17; 20; 21; 23; 24; 25; 26; 27; 28; 29; 30]. However, these advanced methods can be costly, may not always be applicable in plant-scale operations, and can generate toxic waste. These aspects are further discussed in the following sections, including steps that can be taken to address some of these challenges (e.g., combining two or more processes in a treatment train).