Foam fractionation has recently attracted attention as a low-cost and environmentally benign treatment technology for water contaminated with per- and polyfluoroalkyl substances (PFAS). However, data on the mass balance over the foam fractionation process are scarce and when available, gaps in the mass balance are often identified. This study verified the high treatment efficiency of a pilot-scale foam fractionation system for removal of PFAS from industrial water contaminated with aqueous film-forming foam. ΣPFAS removal reached up to 84 % and the removal of perfluorooctane sulfonic acid (PFOS) up to 97 %, but the short-chain perfluorobutanoic acid (PFBA) was only removed with a mean efficiency of 1.5 %. In general, mobile short-chain PFAS were removed less efficiently when the perfluorocarbon chain length was below six for carboxylic acids and below five for sulfonic acids. Fluctuations in treatment efficiency due to natural variations in the chemistry of the influent water were minor, confirming the robustness of the technology, but significant positive correlations between PFAS removal and influent metal concentration and conductivity were observed. Over all experiments, the mass balance closure did not differ significantly from 100 %. Nonetheless, PFAS sorption to the walls of the reactor was measured, as well as high PFAS emissions by the air exiting the reactor. PFAS emissions in aerosols correlated positively with mass balance closure. The elevated aerial PFAS concentrations measured in the experimental facility have implications for worker safety and prevention of PFAS-emissions to the atmosphere, and demonstrate the importance of installing appropriate filters on the air outlet of foam fractionation systems.@en