Water filter

A back-washing arrangement for cleaning a ballast water filter arrangement comprising a filter element configured to filter a liquid and having an inner perforated cylinder, wherein said back-washing arrangement comprises:

• an elongated and hollow body having a longitudinal extension and being configured to be located in said perforated cylinder of said filter element configured to filter a liquid;
• a mechanism configured to drive said elongated 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 when said plurality of nozzles are located in said inner perforated cylinder, a distal end of each of said plurality of nozzles is arranged in close proximity to said inner perforated cylinder of said filter element of said filter arrangement;
• 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 project in the same direction,
• wherein each one of said plurality of nozzles is arranged in contact with at least another one of said plurality of nozzles, and
• wherein said back-washing arrangement comprises a common cap which covers the distal ends of several of said plurality of nozzles, the common cap including a plurality of cap openings, each cap opening arranged over a respective nozzle opening of the plurality of nozzles.