Abstract: The invention relates to a method for treatment of liquid, that is meant particularly for filtering of pressurized liquid, such as water led from a water distribution system, by way of an apparatus, which has an essentially closed frame (1), in connection with which there has been arranged at least a filtering device (2) for purifying of liquid and a flow device (3), such as feed-flow and/or exhaust flow arrangements (3a, 3b) for leading of liquid. The filtering device (2) is made of thin filtering film (2a), advantageously made of plastic based material, such as PTM (Particle Track Membrane), TeM (Track-etch Membrane) or like, the thickness of which is preferably 7-25 &mgr;m and the size of pores of which is 0.05-10 &mgr;m. The filtering film (2a) belonging to a filtering device (2) are being purified essentially self-powered, such as by way of a purification device (5) exploiting the pressure (p) or the movement (v1) of the liquid to be treated and/or the liquid that has been treated.