A method and a plant for waste water treatment include a method for purifying waste that has the steps of a) storing waste water; b) optionally removing phosphorous compounds; c) leading the waste water to a first aerated biodegrading zone; d) leading waste water from the first aerated biodegrading zone to a second aerated biodegrading zone to obtain purified water; and e) leading purified water to at least step a) or to step b) if step b) is present. Advantages of the method are that production of H.sub.2S is kept at a minimum, the necessary amount of chemicals used in phosphorous removal is reduced, an improved nitrification occurs, and the purification is improved during start up after a longer break.