In the sewage plant the waste water is cleaned by biological waste water treatment . The cleaned water is discharged. The fermentation of organic residues in foultowers can generate energy in form of biogas. Sludge and cosubstrate will be mixed and fermented in foultowers. Thereby the production of biogas is increased. The biogas is a valuable energy source for thermal processes. Zymotic leavings (drained sludge with a water content of approximately 70%) will be used in agriculture by composting or alternatively by thermal sludge treatment. In the primary thichenertower, foultower and sludgepresser a fraction of unclearwater is separated from the sludge. After the sedimentation of suspended material in the unclearwaterbasin the water is recycled to the primary sedimentation basin. Due to the addition of sludge cosubstrates in the fermentation the nitrogene concentration in the unclearwater is increased. The unclearwater with high nitrogene content flows to the primary sedimentation basin and has a negative effect on the biological waster water treatment process. A stripping of ammonia from the unclearwater befor the backflow into the primary sedimentation basin is economically practicable. Thereby the energetically valuable cosubstrates can be used in the fermentation without burdening the aerobe biological treatment with increased nitrogen loads.