A filter arrangement includes: a chamber including a chamber inlet for unfiltered liquid, a chamber outlet for filtered liquid and a wall arranged to keep the unfiltered liquid and the filtered liquid separated, a filter element having a longitudinal extension arranged inside of the chamber, wherein the filter element includes a semi-permeable filtration wall including a folded filter arranged in a zig-zag configuration between an inner perforated cylinder and an outer perforated cylinder and an internally located back-washing arrangement. The back-washing arrangement includes: an elongated and hollow body having a longitudinal extension, which is substantially parallel to the longitudinal extension of the filter element, a mechanism configured to drive the body in a rotational and/or an axial movement, a plurality of nozzles arranged in fluid communication with a cavity inside of the body, the plurality of nozzles projecting laterally from the body such that a distal end of each of the plurality of nozzles is arranged in close proximity to the inner perforated cylinder and an outlet arranged in fluid communication with the cavity of the elongated and hollow body and wherein the plurality of nozzles are arranged parallel to each other along a straight line on the body such that they all project in the same direction.