PURPOSE: To separate away the oil-component in waste water contg. emulsified mineral oil so that the oil-component can be stably released with ≤5ml/l water quality by making combination use of a biomembrane contact oxidation treatment with a gravity sepn. treatment and active carbon adsorption treatment.

CONSTITUTION: The waste water 1 contg. the emulsified mineral oil discharged from an automobile service shop, etc., is introduced through a mud reservoir tank 2 into an oil-water sepn. tank 3 where the oil-component in the waste water 1 is floated and removed by gravity sepn. during the passage through honeycomb tubes 4. The treated waste water flows into an oil-component coarse-granulator 5 where the microparticles of the oil-component in the waste water are coarse-granulated during the passage through net honeycombs and are removed by floating; thereafter, the waste water is supplied into a conditioned tank 6 where the inflow of the treated waste water is once conditioned. This waste water is sent to a biomembrane contact oxidation tank 7. The microorganisms in the waste water stick to the inside surface of the honeycomb tubes in the tank 7 and the biomembranes are formed. The fine particles of the oil-component in the waste water are successively removed by the biomembranes. The waste water is thereafter sent to an active carbon adsorption device 4 where the surfactants contained in the emulsified mineral oil in the waste water are removed.