A filter arrangement comprising: - a chamber comprising 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 said chamber , wherein said filter element comprises a semi-permeable filtration wall comprising 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 ; wherein said back-washing arrangement comprises: - 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 said body in a rotational and/or an axial movement; - a plurality of nozzles arranged in fluid communication with a cavity inside of said body , said plurality of nozzles projecting laterally from said body such that a distal end of each of said plurality of nozzles is arranged in close proximity to said inner perforated cylinder ; and - an outlet arranged in fluid communication with said cavity of said elongated and hollow body; wherein said plurality of nozzles are arranged parallel to each other along a straight line on said body such that they all project in the same direction, wherein each one of said plurality of nozzles is arranged adjacent to at least another one of said plurality of nozzles , wherein the filter arrangement comprises a cap which covers the distal end of at least one of said plurality of nozzles . - a chamber comprising 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 said chamber , wherein said filter element comprises a semi-permeable filtration wall comprising 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 ; wherein said back-washing arrangement comprises: - 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 said body in a rotational and/or an axial movement; - a plurality of nozzles arranged in fluid communication with a cavity inside of said body , said plurality of nozzles projecting laterally from said body such that a distal end of each of said plurality of nozzles is arranged in close proximity to said inner perforated cylinder ; and - an outlet arranged in fluid communication with said cavity of said elongated and hollow body; wherein said plurality of nozzles are arranged parallel to each other along a straight line on said body such that they all project in the same direction, wherein each one of said plurality of nozzles is arranged adjacent to at least another one of said plurality of nozzles , wherein the filter arrangement comprises a cap which covers the distal end of at least one of said plurality of nozzles . - 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 said body in a rotational and/or an axial movement; - a plurality of nozzles arranged in fluid communication with a cavity inside of said body , said plurality of nozzles projecting laterally from said body such that a distal end of each of said plurality of nozzles is arranged in close proximity to said inner perforated cylinder ; and - an outlet arranged in fluid communication with said cavity of said elongated and hollow body; <EOS>