PURPOSE: To reduce the bulkiness of residual solids finally by the procedure in which silids present in raw water is treated with magnetite and a coagulant, and then the separated coagulations are heated, gasified, and then subjected to a magnetite separation treatment.

CONSTITUTION: An original water containing solids, e.g., sewage, etc., is first mixed with the magnetite 4 and then with the coagulant 7, e.g., aluminium sulfate, etc., and the high molecular coagulant 9 to convert the solids into mixed flocs which are, in turn, subjected to the magnetic separator 11 where the coagulant 16 is again charged to form a recoagulated sludge consisting of solids and magnetite. Then, the sludge is introduced into the fluidized heating furnace 22 where the organics present in the sludge are decomposed by combustion, and then only magnetite is taken out by a magnetic means in water in the magnetite separation tank 26. Thus, the bulkiness of the finally residual solids can be made smaller, permitting the simplification of treatment.