Use of nanofiltration (NF) for surface water treatment was taken into consideration as an efficient method for purification of natural waters. NF membranes exhibit the capability of retaining the organic compounds with a molecular weight higher than 200 g/mol as well as bacteria and viruses. The multivalent ions can be separated to a high degree whereas a majority of monovalent ions pass freely through the membrane (Donnan effect). The NF is more effective than the conventional methods due to a significantly higher degree of retention of natural organic matter (including the precursors of carcinogenic substances and mutagenic tri-halomethanes). A variety of membrane units with tubular, spiral or flat sheet membranes were recommended to remove the organic contaminants present in the surface water to a high extent (up to 80%).