PURPOSE: To improve the capacity to treat waste water by providing a dehydrator to dehydrate the water in the waste water and connecting a 1st waste water system contg. fine silicon powder and a 2nd waste water system separated and isolated therefrom to the dehydrator in such a manner that the systems can be selected via a pump.

CONSTITUTION: Milk 1a of lime formed by, for example, an oxygen waste water treatment in the waste water system 1 is introduced by a press feed pump 3 by opening a selector valve 2 and is fed into the dehydrator 4 by which the milk of lime layer is formed to several millimeter thickness on the filter cloth surface of the dehydrator 4. The waste water 5a contg. the fine silicon powder in the waste water system 5 is fed through the pump 3 into the dehydrator 4 by opening a selector valve 6 upon lapse of the prescribed time and is thereby treated. Since the fine silicon powder does not come into contact with the filter cloth at this time, the filter cloth is hardly clogged. The milk 1a of lime is again fed into the dehydrator in the final of the stage, by which the piping is substd. with the milk of lime and the danger of hydrogen generation, etc., is averted.