Abstract: Oxygen is separated from air by pressure swing adsorption in plant 2. The oxygen is passed through an ozone generator 4. The resulting ozonated oxygen is passed upwardly through a contactor 6 countercurrently to the flow of a side stream of water to be treated with ozone. The side stream containing dissolved ozone flows from the contactor 6 to a mixer 8 in which it is mixed with a main stream of water. The effluent gas from the contactor 6 contains air displaced from the water and undissolved oxygen. It is divided into two parts. One part flows to the pressure swing adsorption plant 2 and is separated therein with the air. The other part is dried by temperature swing adsorption in drier 20 and the resulting dried gas is ozonated in the ozone generator 4 with the oxygen from the plant 2.