In order to clean the strainer, the entire strainer pack is rotated by the external handle against a set of stationary cleaning knives interleaved with the main pack. During this rotation, accumulated debris builds up on the leading edge of the cleaning knife, and it is deposited into a solid, vertical groove formed in the outside surface of the strainer element by special packing pieces. As there is no flow through this part of the element there is no force holding the accumulated dirt against the element, and it falls into the sump at the bottom of the strainer.