The present invention discloses a purification method for purifying contaminated water, comprising at least one biological purification path as well as a filtration purification path, wherein the filtration purification path is operated at least partially in parallel to the biological purification path with a partial flow which is removed from the biological purification path. The purification method according to the invention can be used as, preferably last, stage of a purification process in a wastewater treatment plant prior to introducing the purified water into a river, lake or the ocean.