Real ultrahigh-temperature (UHT) flash cooler (FC) condensates obtained in the dairy industry were processed in situ using a reverse-osmosis (RO) membrane (Duratherm HWS 4040 HR; 8.2 m 2 filtration area). Semicontinuous trials were performed to estimate the maximum volume concentration rate (VCR) that fulfils the permeate specifications for reuse as boiler water. Then, long-term continuous experiments (100 h in duration) were carried out at a constant permeate flow rate (40 L h −1 m −2 ), and the conductivity, chemical oxygen demand (COD), pH, and Ca 2+ concentration were monitored. An activated carbon column (ACC) was used to reduce the organic charge of the permeate.