To provide a method for treating a CMP wastewater which, in the treatment of a CMP wastewater, comprising an ozone-accelerated oxidation treatment step under alkaline conditions for decomposing and removing a hardly biodegradable organic matter, can prevent the precipitation of a metal hydroxide in the ozone-accelerated oxidation treatment step and can realize stable and effective treatment.

The CMP wastewater is coagulated in a coagulation tank 1, and is then subjected to solid-liquid separation in a solid-liquid separation tank 2. An alkali is added to the separated water, and the water is rendered alkaline in terms of pH in a pH adjustment tank 3. The pH adjusted water is filtered through a filter 4. The filtrate water is subjected to ozone-accelerated oxidation treatment in an ozone-accelerated oxidation treatment tank 5 to obtain treated water. Rendering the CMP wastewater alkaline in terms of pH prior to the filtration in the filter 4 can allow metal such as manganese and copper to precipitate as a hydroxide which can be removed by the filter 4. Accordingly, troubles caused by these precipitates in the ozone-accelerated oxidation treatment tank 5 and treatment apparatuses provided behind the ozone-accelerated oxidation treatment tank 5 can be avoided.