PURPOSE: To make easy the handling of collected dusts and to prevent the collected dusts from scattering again, by mixing dusts collected by an electric dust collector or the like with contaminated water in a mixing vessel, filtering the dusts by a solid- liquid separator, and congealing the separated solids to cakes having a predetermined water content.

CONSTITUTION: Collected dusts A are carried from a dust collector 1 through a hopper 2 to a mixing vessel 3, where the dusts are mixed with contaminated water B from a pump 4 by an agitator 5 to a mixture C. The mixture is carried through a pump 6 to a solid-liquid separator 7, where the mixture is divided to a solid part D and a liquid part E. The solid part D is discharged as a cake having a water content of 45W70%. Depending on the kind of collected dusts, a heavy metal sequestering agent F may be supplied from a hopper 8 for restricting the disolution of heavy metals.