To provide an organic sewage treatment method for biologically utilizing excess sludge, sewage sludge, garbage, and the other organic wastes derived from processes for biologically treating the organic sewage, such as night soil, septic tank sludge, sewage, and industrial wastewater.

In the reduction of the excess sludge generated in the biological treatment processes for the organic sewage, high active sludge is separated and pulled out from the biological treatment processes, and raw water, such as the night soil and the septic tank sludge, and organic substances improving biological activity are added thereto, thereby accelerating the metabolism of microorganisms to cause self-heating. Furthermore, the raw water, such as the night soil, and the organic substances improving the biological activity are added to raise the temperature to the killing temperature region, which solubilizes the sludge. The solubilized sludge is circulated to the biological treatment processes to reduce the sludge. In this invention, the self-heating caused by a high load of activated sludge is used to increase the sludge temperature, and the high load condition continues until the temperature reaches the killing temperature region, thereby solubilizing the sludge and controlling and reducing the amount of the generated excess sludge.