The biogas process could be divided into four steps, the hydrolysis, the acidogenic, the acetogenic and the methanogenic phase. Whereas the two last phases require absolute anaerobic condition with a redox potential under -400mV the first two steps could be anaerobic as well as aerobic. One common method to heat up the digester-input is to run the hydrolysis under aerobic conditions. The aerobic metabolism generates much more heat than the anaerobic metabolism, this leads to a self-heating process and thermal process energy could be saved. The disadvantage of the aerobic hydrolysis is the use of easily degradable organics to produce heat instead of biogas. Biogas, in particular the methane content, is chemically bounded energy which could be stored or converted into electric and thermal energy. The chemical energy in form of methane is of higher value than thermal energy for preheating processes. Thermal energy is normally provided by the rejected heat of the co-generator to heat the digester. The advantage of the aerobic hydrolysis besides the self-heating is an improved hydrolysis, due to the aerobic metabolic pathways which include different types of enzymes. Combing these two advantages, the aerobic metabolic pathway and the saving of easily degradable organics, was the main idea of the intermediate aerobic hydrolysis. Before the digester-input is aerated a first digestion with a very short hydraulic retention time is implemented in a fixed bed reactor. The aeration of the first digestion output is the second step. Hereby the use of aerobic metabolic pathway should improve the hydrolysis of medium and hardly degradable organics, like celluloses and hemicelluloses. Hydrolysed products are afterwards digested in a third process step.