The present study investigated the changes in concentration and composition of five selected PFAS substances in the water treatment at the waste-to-energy plant in Umeå. The five PFAS were PFHxA, PFHpA, PFOA, PFHxS and PFOS. As PFAS exists in a variety of consumer products, they will to some extent end up in waste incineration. During treatment of the flue gas formed during incineration, water is gained from the acid scrubber and the condensate scrubber. The water is treated using techniques such as coagulation, flocculation, sedimentation, and sand filtration. PFAS has previously been detected in the condensate water, but no studies have investigated the fate of PFAS in the water treatment at a waste-to-energy plant. This study was done by taking samples at five different locations in the treatment plant on five different, non-consecutive, days. At the last step both water and foam were collected. For the sample clean-up and analysis, a solid phase extraction followed by LC-MS/MS was used. The results showed that PFAS was not removed in any treatment step. Studying the mass flow of PFAS, the condensate scrubber was a significant input of PFAS to the water treatment. The dominating PFAS in the water was PFHxA. In the foam, an accumulation of PFAS was seen as concentration reached 5300±1100 ng/L compared to 50±28 ng/L in the water. The long chained PFAS was clearly present in the foam. Based on effluent concentrations, 2.5 g per year is emitted to the environment through the treated wastewater. In the future it is important to evaluate all possible effluent streams from the plant, to determine which ones are the most important streams to limit spreading to the environment.