To provide a plant for treating water with ozone comprising a plurality of ozone generators and a plurality of contact aeration basins, characterized in that the qualities of the water treated in each basin are uniformized with one another.

The plant for treating water with ozone comprises the ozone generators 1a, 1b, 1c which generate the ozone and the contact aeration basins 2a, 2b, 2c, 2d. As for the ozone generated in the ozone generators 1a, 1b, 1c, the ozone concentration in the ozone-air mixture generated in the generators 1a, 1b, 1c is so adjusted as to be approximately uniform with one another by means of control devices 10, 11a, 11b, 11c and thereby the ozone concentration in the ozone-air mixture which flows from the generators 1a, 1b, 1c to the contact aeration basins 2a, 2b, 2c, 2d can approximately be uniformized mutually among the basins.