Method for treating ammonia nitrogen waste water

A treatment method for waste water with ammonia nitrogen is characterized in that the waste water is divided into two parts, part A and part B, wherein, magnesium salt precipitant is added to the waste water of part A and the pH value thereof is adjusted to 7.0-10.5; phosphoric acid precipitant is added to the waste water of part B and the pH value thereof is adjusted to 1-4.0; then the waste water of the two parts is mixed and the pH value of the mixed waste water is adjusted to 8-10; after sedimentation the sediment of magnesium ammonium phosphate and supernatant liquor with ammonia nitrogen removed are obtained. The method of the invention improves the utilization rate of the precipitant and has high ammonia nitrogen removal efficiency; the obtained MAP is easy to deposit and separate and the turbidity is dramatically reduced, thereby reducing the difficulty of following flocculation treatment with short reaction sedimentation.