PURPOSE: To provide a method of treating waste water with active carbon, comprising the steps of applying physical and chemical treatment to maste water and regernerating active carbon chemically in a first active carbon adsorption tower, subjecting waste water to adsorption treatment in a second active carbon adsorption tower, said first and the second adsorption towers being connected in series, and regenerating deactivated carbon at a place located outside the second adsorption tower, whereby it is enabled to make the most use of adsorption capacity of active carbon.

CONSTITUTION: Waste water A is introduced into a first active carbon adsorption tower 1, where biological treatment and adsorption treatment are applied to the same, and at the same time, carbon source D is supplied from carbon source tank 3 to biologically regenerate deactivated carbon. Treated water B thus obtained is then introduced into a second active carbon adsorption tower 2, where adsorption treatment is applied by active carbon, whereafter it is discharged as treated water C. In the above system, deactivated carbon E in the second adsorption tower 2 is regenerated at active carbon regenerating means 4 to restore its adsorption capacity and to be reused as regenerated active carbon F. Thus, it is enabled to make an effecient use of the adsorption capacity of active carbon and to improve and stabilize the quality of treated water.