RE-OXYGENATION EFFICIENCY OF DIFFUSED AIR AERATION

One technique used to increase the dissolved O2 concentration of polluted waters is the bubbling of air through a diffuser pipe located at depth, thereby producing a bubble curtain from which O2 transfer to the water occurs. The laboratory results on the aeration efficiency of a diffuser placed along the entire width of a flume, perpendicular to a cross flow are presented (2 dimensional aeration). Parameters investigated include diffuser type.sbd.porous materials with mean pore sizes of 40, 90 and 180 .mu.m and perforated pipes with 0.4, 0.6 and 1.0 mm diameter ports, air flow rate/unit width 3-53 m3 (m h)-1 and cross flow velocity (2.5-15 cm s-1). The effect of variation from the 2 dimensional situation, the consequence of using O2 instead of air and the sensitivity to discharge angle, port spacing and the free surface are discussed. The measured efficiencies are compared with theory, and available laboratory and field data. Aeration efficiencies using diffused air aeration are on the order of 2-13% m-1. The aeration efficiency increases with increasing cross flow velocity and decreasing air flow rate per unit width. Aeration efficiencies using porous filters, for air flow rates less than 40 m3 (m h)-1, are significantly higher (a factor of 2-3) than those achieved using perforated pipes. Changing the pore size from 40 to 180 .mu.m, the port size from 0.4 to 1.0 mm or the port spacing does not significantly effect the aeration efficiency. Aeration using O2 is somewhat less efficient than that using air. Since equivalent O2 bubbles containing approximately 5 times more O2 than air bubbles, more O2 is transferred on an absolute basis at the same gas flow rate using compressed O2 as opposed to air. Aeration efficiency resulting from aeration over a portion of the entire width can be reasonably predicted using the results of the 2 dimensional studies. The available laboratory and field data compare well with the results of these laboratory studies.