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Effect of Multiple Compartments on Oxygen Transfer in Postaeration Tanks

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National Pollution Discharge Elimination System permits for approximately 5900 wastewater treatment plants in the U.S. include a minimum dissolved oxygen requirement. Of these plants, approximately 2300 have an average flow exceeding 3900 m3/d (1 mgd). To achieve these dissolved oxygen standards, final effluent from some plants is aerated before discharge using cascades or postaeration tanks. Postaeration tanks are typically designed as completely mixed tanks with effluent concentration equal to concentration throughout the tank. The concentration difference that determines rate of oxygen transfer is the saturation concentration minus the effluent concentration.

Oxygen transfer can be made more efficient by dividing the tank into two or more compartments. Because the difference between oxygen deficit in the compartments before the last one is greater than the deficit in the last compartment, the standard oxygen-transfer rate (SOTR) required with multiple compartments is less than with one compartment. A decrease in SOTR causes the same percent decrease in number of diffusers and diffuser capacity (for diffused-air systems) or capacity of mechanical aerators. Also, the basin itself can be made smaller. Optimum configuration is obtained when the SOTR for each tank is equal. The most efficient system is a plug-flow regime, which can be achieved by means of multiple compartments or use of a tank with a high length-to-width ratio.


Document Type: Research Article


Publication date: 1999-09-01

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  • Water Environment Research (WER) is published monthly, including an annual Literature Review. A subscription to WER includes access to the latest content back to 1992, as well as access to fast track articles. An individual subscription is valid for 12 months from month of purchase.

    Water Environment Research (WER) publishes peer-reviewed research papers, research notes, state-of-the-art and critical reviews on original, fundamental and applied research in all scientific and technical areas related to water quality, pollution control, and management. An annual Literature Review provides a review of published books and articles on water quality topics from the previous year.

    Published as: Sewage Works Journal, 1928 - 1949; Sewage and Industrial Wastes, 1950 - 1959; Journal Water Pollution Control Federation, 1959 - Oct 1989; Research Journal Water Pollution Control Federation, Nov 1989 - 1991; Water Environment Research, 1992 - present.
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