Abstract: A process is provided for treating waste liquor or waste water containing free phenols in excess of 10,000 ppm, harmful to aerobic bacteria present in the activated sludge for general purpose, comprising the steps of (1) reducing the phenol content to 100 ppm or less by means of solvent extraction or adsorption, (2) conditioning the waste water obtained in step (1) in a large conditioning tank to adjust the pH, temperature and BOD, (3) treating the waste water obtained in step (2) in an aerating tank by means of activated sludge for general purpose containing aerobic bacteria, and (4) separating the treated water from the sludge in a sedimentation tank. Preferably above 10 to 50 weight percent of the treated water is recycled to the conditioning step (2).