7 with the process and device according to the invention it is now possible to mix water with a defined amount of oxygen without any oxygen loss whereby the water can be enriched with oxygen in a manner that is independent of the internal pressure of tank 1 as well as independent of the throughput volume of water the optimum oxygen content and operating pressure necessary for the operation of biological filters and chemical treatment stages can be generated without great effort this prevents loss of oxygen caused by out gassing of the oxygen due to oversaturation when it is reduced to atmospheric pressure in this manner the formation of bubbles in the cartridges and filters due to pressure fluctuations can also be avoided the process and device according to the invention are particularly advantageous when the operating pressure and thus the internal pressure of tank 1 have to be kept constant for technical reasons since in such cases the conventional method does not permit the use of the oxygen pressure in tank 1 for purposes of setting the oxygen concentration via the partial pressure and for setting the oxygen concentration in the water which is at a state of equilibrium with the partial pressure therefore according to the invention the incorporation of oxygen into the water can be dissociated from the internal pressure of the pressurized tank so that the internal pressure of the tank continues to be available as an adjustable parameter for other control systems the maximum possible oxygen concentration for saturation at atmospheric pressure namely approximately 40 mg l.