The general goals of anaerobic sludge stabilisation are degradation of organic substances, reduction of solids, decrease of pathogenic bacterias, improvement of dewatering capabilities and production of biogas. The anaerobic degradation process can be divided in several steps with different optimum operational conditions. This gives the possibility to design treatment plants as a two-stage process, optimizing the process conditions in each step and thereby leading to an overall increase in process efficiency. Research results in lab scale and pilot scale have shown that a two-stage digestion process with a high loaded first thermophilic (50-55°C) stage and a second stage under mesophilic (35-37°C) conditions with sufficient retention time will lead to the best results. After giving a short summary of the development of the two-stage thermophilic/mesophilic digestion process the paper will present full scale experiences with this system in Germany.

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