NOVELTY - The method for the phosphorus precipitation from a waste water under formation of magnesium ammonium phosphate (MAP), in a waste water treatment plant, comprises displacing the waste water with magnesium chloride and ammonia in a continuous crystallization stage, and displacing to a stripping stage, in which the waste water is treated with air. The waste water (75-100%) is subjected to a treatment in the crystallization stage. The MAP pellets are removed from the base of a crystallization reactor (1) under utilization of the force of gravity and/or without use of energy from mechanical means.    USE - Method for the phosphorus precipitation from waste water under formation of magnesium ammonium phosphate (MAP), in a waste water treatment plant, where the magnesium ammonium phosphate is used as a fertilizer (claimed).    ADVANTAGE - The method ensures safe and continuous precipitation of phosphorus from waste water without sedimentation process in large-scale operation in an economic and ecological manner, thus producing fertilizers with high-quality.    DETAILED DESCRIPTION - The method for the phosphorus precipitation from a waste water under formation of magnesium ammonium phosphate (MAP), in a waste water treatment plant, comprises displacing the waste water with magnesium chloride and ammonia in a continuous crystallization stage, and displacing to a stripping stage, in which the waste water is treated with air. The waste water (75-100%) is subjected to a treatment in the crystallization stage. The MAP pellets are removed from the base of a crystallization reactor (1) under utilization of the force of gravity and/or without use of energy from mechanical means. In the crystallization stage: the pH is adjusted to 8.0-9.2; the adjustment of the pH takes place via the addition of caustic soda solution; the magnesium chloride in form of an aqueous solution with a concentration of 20 to 40% (w/v) is added; ammonia in form of an aqueous solution is added; the waste water displaced with magnesium chloride is moved by mechanical means; and/or the retention time in the reactor is up to 3 hours. The agitator is included in the stripping process, develops small seed crystals, deposits new precipitation products at the seed crystals, and disposes the formed MAP pellets in the reactor. The process is suitable for: phosphorus precipitation from the waste water quantity of more than 5 m3/h or phosphorus precipitation from a total waste water quantity of more than 2000 m3/d; phosphorus precipitation from waste water containing phosphorus/phosphate of more than 290 kg/d; and/or phosphorus precipitation from waste water containing phosphorus/phosphate of more than 120 mg/l. In stripping stage: the ventilation is carried out in a stripping reactor (2); carbon dioxide is removed from the waste water by the ventilation and the pH of the waste water is increased; the retention time in the reactor is 2-3 hours; and/or the ventilation takes place with a blower or a surface blower. The waste water is subjected to an anaerobic condition before the crystallization and stripping stages. The anaerobic treatment is carried out in a reactor system, which is an anaerobic simulator, a fixed bed, a fluidized bed, an upflow anaerobic sludge blank (UASB) with a space load of 7.5-8.5 (kgCSB/m3.D) or an expanded granular sludge bed (EGSB). The waste water from the reactor runs through a mixing- and an equalizing tank. In the anaerobic stage: the developing biogas from the reactor is removed; the developing biogas is used for power production; the temperature of the waste water is adjusted to 35-38 degrees C; and/or the temperature of the waste water is adjusted using heat exchanger systems. The waste water is subjected to an aerobic treatment for removing the residual concentration of carbon and nutrients. The aerobic treatment stage is carried out in an aerobic treatment tank and/or 3-4 sequence batch reactors (SBR). In the aerobic treatment stage, an aerobic activated sludge process takes place in full-continuous operation and/or a biological phosphorus precipitation is carried out using microorganisms and/or optionally using iron ions. INDEPENDENT CLAIMS are included for:    (1) a device for the phosphorus precipitation from waste water; and    (2) magnesium ammonium phosphate pellet.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic representation of a waste water treatment plant.    Crystallization reactor (1)    Stripping reactor (2)    Aerobic reactor (4)    Waste water inlet (11)    Waste water outlet. (11a)