To try to remove ammoniac nitrogen components and others, to prevent the generation of new harmful components, to make a processing facility compact, and to reduce installation costs besides the removal of harmful components such as cyanic components in scrubber drainage.

Scrubber drainage discharged from a scrubber 3 in which exhaust gas discharged from an incinerator 2 of sludge containing organic components such as sewage sludge is wet-treated, is filtered by filter column 4, the filtrate, whose pH is adjusted at 5-10, is mixed with an oxidizing agent. The mixture is introduced into a catalytic oxidation column 5 to be oxidized in a catalyst layer 51 filled with manganese dioxide as a catalyst.