PURPOSE: To reduce the substantial thermal load of a combustion furnace by a method wherein a combustion process and an evaporation process are separated form each other, and an evaporation of the liquid component occupying almost all of the thermal load is eliminated from the combustion furnace.

CONSTITUTION: Raw night soil contained in a night soil tank 1 is transported by an operation of the pump 2 to a solid-liquid separator means 3. The separated solid material is fed to the combustion furnace 5 through a screw conveyor 4. The raw night soil having solid material separated is passed through the piping 6 into the raw night soil evaporation means 7. A liquid combustion burner 9 is used for evaporating the raw night soil and the generated vapour is passed through the piping 10 and mixed with the combustion gas from the combustion furnace. As described above, an evaporation of the liquid raw night soil occupying almost all of the thermal load is removed from the combustion furnace. In the combustion furnace, only combustion of the solid raw night soil is performed and then a thermal load of the combustion furnace may be reduced substantially.