At the example of the sewage water plant of the village Grammendorf the advantages of the sequencing batch reactor process for the aimed elimination of nitrogen even with small sewage plants are demonstrated The nitrification and denitrification can be realised chronological controled with the hyperboloid mixing and aeration system in the same basin. By the adaption of the mixing and aeration system to the geometry of the basin the efficiency of the treatment was increased markedly.