PURPOSE: To conserve the consumption amounts of a flocculant and auxiliary fuel in an incinerator, in the course of treatments for dehydrating and burning sewage sludge, by circulating incineration ash discharged from an incinerator to the side of a dehydrator to use it as a dehydrating assistant.

CONSTITUTION: Sewage sludge is poured into a mixer 5, mixed with FeCl3 as an inorganic flocculant at a ratio of about 10% on the basis of the sewage sludge and then sufficiently agitated. In succession, the sludge is transferred to a mixing tank 4, and incineration ash discharged from an incinerator 1 in the following step is concurrently added through a hopper 3 to the sludge at an approximately equal ratio and mixed with the sludge. Thereafter, the mixture is charged in a filter press dehydrator 2, compressed and dehydrated therein, supplied as dehydrated cake having a small water-retaining ratio to a fluidized-bed oven 1, and burnt by adding a small amount of auxiliary fuel. Since the incineration ash is utilized as a dehydrating assistant, the reduction of the capacity of the dehydrator into a small size, the save of the flocculant and the remarkable savings of the auxiliary fuel in the incinerator can be attained.