## 3 Conclusions  
Poly- and perfluoroalkyl substances (PFASs) present a significant health concern at high concentrations in water, impacting both humans and animals. To address this issue, various treatment technologies have been applied to PFAS removal. Conventional techniques like ferric or alum coagulation, granular/micro-/ultrafiltration, aeration, oxidation (e.g., permanganate, ultraviolet/hydrogen peroxide), and disinfection (e.g., chlorine dioxide, chlorination, and chloramination) have proven mostly ineffective in PFAS removal. Advanced methods such as adsorption, ion exchange, ozone oxidation, and membrane processes also face challenges, particularly in the presence of other contaminants, mainly dissolved natural organic matter in water whose concentration is much higher than that of PFASs.  
The primary hurdle lies in not only removing PFASs from water but also degrading the removed PFASs. In the adsorption process, the adsorbent must selectively remove  
Figure 5: Removal of PFAS using nanofiltration followed by adsorption in a pilotplant study (modified from [50]).  
PFASs amidst other contaminants. Once removed, the adsorbed PFASs must be destroyed. This can be achieved by leaching the adsorbed PFASs into a smaller volume of solution, making it easier and less costly to handle during the incineration process for PFAS destruction. Furthermore, the adsorbent can be rejuvenated for repeated use. In membrane technology, ultrafiltration (UF) and microfiltration (MF) prove ineffective as their pores are larger than the molecular size of PFASs. Nanofiltration (NF) and reverse osmosis (RO) processes, employing higher pressures and smaller membrane pore sizes, effectively remove PFASs. However, these methods demand higher energy consumption and produce a concentrated retentate stream, requiring additional treatment before disposal. Membrane fouling presents an additional challenge, reducing the operational lifespan of the membrane. While oxidation processes are effective, they are typically expensive, time-consuming for complete PFAS degradation, and frequently yield undesirable byproducts.  
Given the limitations of individual methods in completely and cost-effectively eliminating PFASs from water, hybrid treatment methods have emerged as a successful approach. Hybrid methods that integrate individual methods have a synergistic effect in removing PFASs. In hybrid methods, one technique extracts PFASs from contaminated water and concentrates them, while the other degrades the extracted PFASs. Examples such as adsorption + oxidation, membrane separation + oxidation, and adsorption + membrane separation + incineration are discussed in the review, highlighting their effectiveness in separating and destroying PFASs from water. However, the majority of studies on these hybrid methods have been conducted under short-term laboratory conditions using synthetic solutions. It is imperative to extend these studies to long-term pilot-scale levels using real wastewater containing PFASs.  
Conceptualization, S.V. and P.L.; methodology, S.V. and P.L.; formal analysis, P.L.; resources, S.V.; writing--original draft preparation, P.L.; writing--review and editing, P.L., S.V., J.K. and H.R.; supervision, S.V.; project administration, S.V.; funding acquisition, S.V. and H.R. All authors have read and agreed to the published version of the manuscript.  
This research was funded by EU ERA-NET Water JPI-2018, Grant 776692: Closing the Water Cycle Gap--Sustainable Management of Water Resources 783 (Water Harmony) obtained by [PERSON].  
No new data were created or analyzed in this study.  
The authors declare no conflicts of interest.