The use of the reverse osmosis process in water treatment often requires the careful selection of methods of pretreatment. This especially applies to mineralised water, ferruginous water, water with a high carbonate hardness and also water with a tendency to precipitate sulphate and silica deposits. Increased feed water temperature may be a further factor which could encourage scaling of the membrane. To decrease the number of divalent ions, the nanofiltration process was tested with the use of a commercial NF-270 membrane (Dow Filmtec). Filtration was performed under a transmembrane pressure of about 10 bar with cross-flow filtration. Tests were carried out on two highly mineralised geothermal waters mineralisation (TDS 22-2.3 g/L), more than 600 mgCaCO(3)/L and a high silica concentration. Before laboratory tests, the tendency to precipitate mineral deposits was determined by means of geochemical modelling. In addition, appropriate doses of antiscalant were selected to avoid the precipitation of deposits on the NF membrane. The research was carried out on different variations of: 1) two temperatures: 30 degrees C and 17 degrees C and 2) the use of raw water and water with the addition of antiscalant (Hydrex). During tests the changes of membrane effectiveness with time were observed. Qualitative and quantitative identification of the precipitate deposits on the NF-270 membrane was made for each of the variants of the water pretreatment process. (C) 2016 Published by Elsevier B.V.