PURPOSE: To reduce the cost of a drying heat source, and to dispense with a fan for supplying hot air into a dryer by utilizing a part of the air discharged from an aeration blower for drying the water-contg. solid matter generated in the treatment of organic sludge.

CONSTITUTION: A part 8" of the air 8 discharged from an aeration blower 6 is passed through a dryer 10 for drying dehydrated cake 9, etc., and the dehydrated cake 9 contg. about 80% water is effectively dried by the heat retained in the discharged air 8" and the flow of the air to obtain dried matter 11 contg. ≤40% water which is stored in a storage tank 12. Although dry waste gas 13 is discharged from the dryer 10 in said drying, the concn. of the odor is extremely low due to the low-temp. drying using the discharged air 8" as a heat source. Accordingly, the gas can be discharged as it is into the atmosphere, or the gas can be biologically deodorized by introducing the gas into an aeration tank 2, etc.