Functional unit is a standardized measure used in various sustainability analyses to compare indicators across different technologies, systems, or processes. This measurement is crucial for comparisons on an equivalent basis, ensuring results are based on consistent performance metrics. In the context of sustainability of PFAS treatment, various functional units were identified across the literature, including the mass of PFAS and mass of sludge . However, Figure shows a predominant emphasis on the unit volume of water. Regional and global differences in domestic water consumption, influenced by climate, infrastructure, and societal factors, highlight the challenges of establishing a standardized functional unit. The majority of examined papers opted for a functional unit based on the volume of water, with 70% of the sampled literature employing 1 m 3 of treated water as the standard. Notably, one paper deviated from this trend, utilizing 800,000 gallons of groundwater per day as its functional unit. The chosen functional unit corresponds to the actual daily water consumption of Brighton's population, estimated at approximately 8000 people with an average daily consumption of 100 gallons per capita per day. Landfill leachate is a major source of PFAS in the environment. It is usually transported to wastewater treatment plants and treated with municipal wastewater. Therefore, this review also examines the sustainability of leachate treatment for PFAS removal. The importance of selecting a functional unit is highlighted in sustainability assessments of leachate treatment. When "1 g of PFAS removed" is used as the functional unit, the assessment reveals higher environmental and human health impacts compared to using "1 m³ of leachate treated" as the functional unit. Evaluation per mass of PFAS removed is more relevant to human and environmental health, while evaluation per volume of PFAS-impacted water treatment is more relevant to costs and engineering. This demonstrates that the choice of a functional unit significantly influences the overall sustainability outcomes of PFAS treatment technologies. However, the water treatment industry operates based on the volume of water treated, and associated costs are typically calculated per unit volume. Therefore, most of the reviewed papers have utilized water volume as the functional unit.