## 2 Hybrid Treatment Methods
### Adsorption/Membrane Separation Treatment  
Adsorption is a commonly employed strategy to combat the presence of PFASs in contaminated water. However, reusing spent adsorbents requires effectively dealing with the adsorbed PFASs--typically by desorbing PFASs and incineration of the desorbed solution. The economic and safe viability of these approaches relies on minimizing the volume of desorbed PFAS solution. This is where membrane processes come into play, offering a practical way to reduce solution volumes.  
In a study conducted by [PERSON] et al. [38], three robust anion exchange resins (Purolite(r) A520E, A600E, and A532E) were utilized to remove trace levels of PFOA, PFOS, PFBA, and PFBS from drinking water-targeted groundwater (with concentrations in the hundreds of nanograms per litre range). Among these, A600E (non-hydrophobic) and A520E (moderately hydrophobic) exhibited lower sorption capacities compared to A532E (highly hydrophobic). They demonstrated that while A600E and A532E could be regenerated using solvent-free, dilute solutions of non-toxic NH4 Cl and NH4 OH. A532E required concentrated solutions of methanol or ethanol along with 1% NH4 Cl. Notably, they integrated reverse osmosis with under-vacuum evaporation, drastically reducing the volume of regeneration effluents requiring incineration by over 96.5% (Figure 4) [38]. In this setup, the reverse osmosis module operated at 20-22 bars with an average recovery rate of 65%. The volume of regeneration eluate was decreased by a factor of 2.9 (to 1.2 L) and subsequently concentrated in an under-vacuum evaporator operating at 35-38 \({}^{\circ}\)C until reaching a final volume of 0.11 L.  
An alternative sequence involves employing membrane treatment followed by adsorption. In this scenario, the adsorbent must either be disposed of in specialized zones post-usage or destroyed. Alternatively, the adsorbed PFASs could be desorbed into small-volume solutions for subsequent disposal or destruction. [PERSON] et al. [50] conducted a similar exploration in a Swedish NF pilot plant over a 6-month period, achieving over 98%  
Figure 4: Use of anion exchange resins followed by reverse osmosis (RO) vacuum evaporation and incineration to remove PFAS (modified from [38]).  
PFAS removal (Figure 5). After the pilot plant's concentration, the researchers subjected it to adsorption treatment in columns containing GAC materials and anion exchange resins. The latter outperformed the GACs in PFAS elimination based on the cumulative bed volumes each material could process before meeting predefined discharge targets. Previous observations have noted greater adsorption of PFAS on the anion exchanger, attributed to the presence of adsorption sites and functional groups that are more specific to PFAS adsorption compared to those of GAC [51; 52].  
Upon reaching bed volume limits, the GAC was regenerated and reused, while the resin was incinerated and replaced with new material. Cost considerations factored in the procurement of virgin GAC, its regeneration, and transportation to and from the treatment site. In contrast, anion exchange resin costs encompassed the procurement of new material, transportation, and the incineration of spent resin. An economic assessment for various concentrate discharge objectives revealed that using anion exchange resins for discharge goals was more economically viable than relying on GAC for concentrate treatment.  
In conclusion, these investigations underscore that anion exchange resins offer a more cost-effective and efficient method for removing PFASs from polluted water compared to GAC utilization. The ability of anion exchange resins to be regenerated with non-toxic solutions, combined with the substantial reduction in regeneration effluent volumes through reverse osmosis with under-vacuum evaporation, further reinforces their advantage. Moreover, these resins can be disposed of in an environmentally safe and sustainable manner.