Pure Oxygen Systems
In nature, biological oxidation is performed by a deep contact between air and water. The more is the contact surface, the more is the oxygen dissolution yield. Dissolved oxygen is utilized by the respiration of an aerobic bacterial flora, which, in turn, is fed by the organic substrate dissolved in the water, by oxidizing and transforming it into water and carbon dioxide, that are innocuous compounds.
Oxygen is contained in air in volume proportion - at saturation - of about one part to five parts of air. In natural basins and watercourses the dissolution is performed by natural diffusion through the contact surface between air and water.
Therefore, the more is the number of falls and rapids in a watercourse, the more is its depuration capacity. In fact, in a waterfall the waterflow is subdivided into myriads of fine drops (aerosols); the sum of the individual water fine drop surfaces is considerably bigger than the sheet of water made by the river in case the river course or the lake surface is flat.
Similarly, a deep contact surface can be obtained by introduction of fine bubbles of air in water.