The study tested I he effectiveness of well water treatment in one step using nanofiltration (NF). The possibility of using a tight, unchanged NF membrane is discussed. In the initial stage of the study model solutions of NaCl, Na2SO4 and CaSO4 were filtered with a NF membrane, and the separation of monovalent and bivalent ions was studied. Next, separation properties of the membrane were determined for natural waters with lesser and greater mineralization. A bacteriological and sanitary analysis of well water purification is also presented.