Abstract: Process and apparatus for the purification of water, oxygen/ozone-gas mixture is introduced under high pressure in the water to be purified, and the thus obtained oxygen/ozone-gas mixture enriched water is passed to a purification stage which comprises at least one reactor. Not or insufficiently degraded contaminants are retained by a membrane filter unit and recycled to the purification stage. Thus contaminants, such as hydrocarbon compounds and the like, are recirculated until they are converted to carbon dioxide and water.