Nanofiltration (NF) has attracted increasing attention during recent years due to the development of new applications. The advantage of NF compared with reverse osmosis (RO) and ultrafiltration (UF) is that it is possible to separate organic substances from some types of salts. It is, furthermore, also possible to separate different kinds of salts due to the negatively charged groups on the membrane. The retention of sulphate salts by the membranes used in this investigation was 88-96%, while the retention of chloride salts was only 12-47% at low salt concentrations. This difference in retention is due to the charge density of the anion. The retention of NaCl was found to be strongly related to the concentration. The retention decreased from 45% to 7% when the concentration was increased from 0.05 M to 1 M. Although the repulsion of the anion mainly determines the retention of salt solutions, it was observed that the cation can affect the retention, especially in salt solutions with a high concentration of monovalent anions. It was found that the retention of divalent cations was three times higher than that of monovalent cations in this type of salt solution. In chloride solutions with mixed cations, it was shown that a monovalent cation passed through the membrane preferentially to the divalent cations, to such an extent that the retention became negative. NF was utilized to treat a landfill leachate with an extremely high salt content from a waste cell containing mainly ash because of the good separation of cations. Most of the heavy metals, which are multivalent cations, are rejected while the monovalent cations, which are rather harmless substances, pass through the membrane. The retention of, for example, cadmium, zinc, lead and chromium was found to be higher than 70%, while the retention of potassium and sodium was less than 10%. Since the transmembrane osmotic pressure was low, due to the low retention of the monovalent ions, the flux was several times higher than for RO membranes. The flux of the leachate, with a conductivity of 6800 mS/m, was above 50 l/m(2)h at 3 MPa and 25 degrees C.