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Effects of Air-Free Water Surface Mass Transfer and Horizontal Flow on Oxygen Transfer Rates in Microporous Aeration Systems

Author(s): Xiangju Cheng; Yuning Xie; Ran Li; Jun Xie

Linked Author(s): Xiangju Cheng, Ran Li

Keywords: Microporous aeration system; Oxygen transfer rate; Horizontal flow; Film cover

Abstract: Microporus aeration system, profited from its energy-saving, environmental protection, easy installation, adaptability and other good characteristics, has been widely used in lake, estuary, river and pond where water need to be increased the dissolved oxygen. Thus fully understanding its reoxygenation efficiency and improving its oxygen transfer rate become popular for researchers at home and abroad. Most existed studies generally took stagnant water as test object, which is out of natural phenomenon with the flow caused by wind or tide. Additionally, when investigating the oxygen transfer rates, many studies did not separate the oxygen transfer from air bubbles and atmosphere, which is bound to bring inaccuracy for microporous aeration system, since the mass transfer mechanisms of air bubbles and air-free water surface are different. In view of these, a series of reaeration tests were performed in a tank of 2m length, 0.8m width and 0.8m height at 1.2-2.0 m 3 /h diffused-air flow rate and 0.7m water depth, with and without horizontal flow, existing and isolating the mass transfer at air-free water surface. A water pump was used to make test water horizontal motion. Plastic film was used to cover on the above surface of the test tank to cut off the air-mass transfer of free water surface. The reaeration tests were performed according to the standard method from the American Society of Civil Engineering (ASCE). The results showed that, (1) the presence of plastic film cover made the volumetric mass transfer coefficient reduced, indicating that the film cover hampered the oxygen mass transfer from the atmosphere. Besides, the volumetric mass transfer coefficient was the lowest at no horizontal flow and with the plastic film cover on the above surface of test tank; (2) the horizontal flow promoted the volumetric mass transfer coefficient. But with the increase of diffused-air flow rate, the promoting percent was decreased, indicating the promoting level of horizontal flow on the volumetric mass transfer coefficient was reduced when the diffused-air flow rate reached to a certain extent in shallow water.


Year: 2016

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