Precipitation of ammonium together with phosphate and magnesium is a possible alternative for lowering the nitrogen content of waste water and sludge liquor. Certain chemical conditions need to be satisfied, especially the molecular ratio NH4: Mg:P04 = 1:1:1 and a suitable pH in the range between 8 and 10. The salt MgNH44P04.6aq4 then crystallizes quickly and settles within a few minutes. The salt is rather insoluble and dries in air. The residual ammonium and phosphate concentrations are not, however, low enough for the waste water to be run off into a river. Biological treatment of the precipitation effluent is therefore necessary. However, neither nitrification nor denitrification is required. This saves costs and makes the process worth considering. Three process design variants in combination with biological stages are suggested.

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