PURPOSE: To prevent COD removal rate from lowering by adding an oxidizer and a metallic ion dissociating material to COD-contg. waste water and allowing these to react under acidic conditions then adding an alkali agent and subjecting the formed sludge to solid-liquid separation.
CONSTITUTION: In the 1st agitating tank 2, hydrogen peroxide 3 and a material 4 which dissociates metallic ions in an aqueous soln. such as ferrous sulfate are added to the COD-contg. waste water such as biologically treated water of night soil or the like, and these are agitated under acidic conditions. The reacting liquid 5 having completed oxidation reaction is flowed into the 2nd agitating tank 6, where a magnesium base alkali agent such as magnesium hydroxide or an alkali agent 7 of calcium carbonate is added thereto to neutralize, after which precipitate is allowed to deposit and sludge 10 is separated. The sludge 10 is transferred to the 3rd agitating tank 11, where a sulfuric acid is added thereto to dissolve the sludge, and this is conducted into the 4th agitating tank 14, where it is added with hydrogen peroxide 13 and this is recycled as return liquid 15 into the waste water 1.