PURPOSE: To simultaneously carry out the separation of foreign matters such as oil-water separation and treatment with anaerobic microorganisms and to efficiently and economically purify org. waste liquor by using an aggregate of chips of lighter plastic foam than water as a medium for contact of the anaerobic microorganisms with the foreign matters.

CONSTITUTION: Oil such as deep-frying oil and other matter having low specific gravity contained in waste liquor are converted into coarse grains on the surfaces of chips, lumps or small molded bodies without biodegrading in the case of a downward flow and they are separated from water and discharged together with gasified matter from an upper part. Emulsified oil contg. suspended solids (SS) and detergents is easily converted into a low molecular compd. such as acetic acid, butyric acid or propionic acid with anaerobic bacteria in an aggregate layer, that is, an anerobic filter layer. In the case of an upward flow, the emulsified oil is converted into a low molecular compd. with sufficiently propagated anaerobic bacteria in bottom sludge or in the filter layer. After ≥80% of the oil is removed, the org. waste liquor having high concn. is passed through an anaerobic bacteria layer having high concn. to remove ≥80% of SS and components increasing BOD and COD.