Abstract With the “plate-and-frame” reverse osmosis system and a new thin film composite membrane it has been possible to desalt very salty water (43.000 ppm). A minimum of pre-treatment has been necessary, because the membranes are very resistant to pH and temperature, and the flow rate in the plate-and-frame modules is very high. The raw water is pumped directly to the multi-media filter, followed by a 25 micron cartridge filter. The growth of algea and bacteria is prevented by irradiation with UV-light. After the pre-treatment the water is pumped into the modules and desalted. The economy of the process is good, thanks to the high capacity of the membranes and the small degree of pre-treatment.