After their use for heating, e.g. in greenhouses, waste thermal waters may cause environmental problems due to their high contents of ions, and in some cases organic matter (associated with an oxygen demand) or toxic compounds. The aims of this work were to decrease the high organic content of waste thermal water by a combination of ozone treatment and membrane separation, and to investigate the accompanying membrane fouling. The results demonstrated that the chemical oxygen demand and the total organic content can be effectively decreased by a combination of ozone pretreatment and membrane filtration. Ozone treatment is more effective for phenol elimination than nanofiltration alone: with a combination of the two processes, 100% elimination efficiency can be achieved. The fouling index b proved to correlate well with the fouling and polarization layer resistances.