PURPOSE: To enhance biological treatment efficiency, in subjecting semiconductor wasting waste water containing org. alkali to biological treatment while mixing the same with industrial water, by subjecting semiconductor washing waste water to cation exchange treatment using a cation exchange resin before mixing the same with industrial water.

CONSTITUTION: Semiconductor washing waste water containing org. alkali recovered from a semiconductor manufacturing process is introduced into an activated carbon column 21 from piping 31 to be subjected to hydrogen peroxide decomposing and removing treatment and the treated water is introduced into a weak anion exchange resin column 22 from piping 32 to be subjected to anion exchange treatment and subsequently introduced into a cation exchange resin column 23 from piping 33. The semiconductor washing waste water from which org. alkali is removed by cation exchange in the cation exchange resin column 23 is introduced into a mixing tank 24 from piping 34 to be mixed with industrial water introduced from piping 35. The mixed water is introduced into a biological treatment apparatus from piping 20 to be subjected to biological treatment and org. matter such as urea is decomposed and removed.