Oxygen Sewage Treatment

Modern high efficiency sewage treatment uses a process termed aerobic digestion, the same process that occurs in the purification of natural waters. In nature, the purification begins when oxygen from the air dissolves in the water. Aerobic bacteria, those requiring oxygen in their life processes, consume the contaminants in the water for their growth. Part of the material each organism consumes is used for cell growth and part is reacted with oxygen to provide energy. Cells grow, die and are consumed by other organisms. Ultimately this biological activity converts most of the waste materials to harmless carbon dioxide and water. In the mechanization of this process, the contaminated water is forced to contact as much air as possible to speed the transfer of oxygen from the air into the water.

To be accessible to the waste-consuming bacteria the oxygen must be dissolved in the water. One of the limiting factors in aerobic water treatment is the rate of oxygen transfer from the air to the contaminated water. The rate of transfer is determined by the surface area of water in contact with the air and the partial pressure of the oxygen.

Many mechanical schemes are used to increase the surface area available for the transfer of oxygen. All depend on forcing large amounts of air into intimate contact with the sewage. Examples are: strong stirring in open tanks (allowing air to be whipped into the water), supplying compressed air into the bottom of a tank (allowing many bubbles to contact the water), and open porous beds of rocks (allowing the water to trickle through and contact the air in the spaces between the rocks). The air supplies oxygen to bacteria that consume the waste materials. After several cycles of aerobic treatment, settling and separation, most of the waste is converted to carbon dioxide and water by the metabolic process of the bacteria.

The mechanical schemes mentioned above improve the contact between the air and the water; however, the partial pressure of the oxygen is determined by its concentration in the air. Air is about 21% oxygen so the partial pressure of oxygen is 0.2 atmospheres. If the air contacting the contaminated water is replaced with pure oxygen, the oxygen partial pressure is one full (1.0) atmosphere. This pure oxygen partial pressure is five-fold higher than that obtained with air. The five-fold increase in the oxygen partial pressure results in a potential five-fold increase in the rate oxygen dissolves in the wastewater. Kriiger Inc. of Cary, North Carolina 224 designs and builds oxygen enhanced water treatment plants utilizing this technique.

The five fold increase in oxygen partial pressure will improve the rate water can be treated in existing plants by a factor of nearly five. The increased treatment rate can be utilized to improve the total capacity of treatment plants, without building any new facilities, or it can be used to provide a higher quality output. The mix of increased capacity or higher quality effluent water will be made on the basis of the operating conditions of each individual plant, taking into account the input and the need for high quality output. However this partitioning is made, the availability of large amounts of oxygen will result in much higher quality water from treatment plants.225

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