On the reoxygenation efficiency of diffused air aeration

One technique used to increase the dissolved oxygen concentration of polluted waters is the bubbling of air through a diffuser pipe located at depth, thereby producing a bubble curtain from which oxygen transfer to the water occurs.

The results of laboratory studies on the aeration efficiency of a diffuser placed along the entire width of a flume, perpendicular to a cross flow are presented (two dimensional aeration). Parameters investigated include (1) diffuser type-porous materials with mean pore sizes of 40, 90 and 180 μm and perforated pipes with 0.4, 0.6 and 1.0 mm diameter ports (2) air flow rate per unit width 3–53 m³ (m h)^?1 and (3) cross flow velocity (2.5–15 cm s^?1. The effect of variation from the two dimensional situation is also discussed as well as the consequence of using oxygen instead of air, and the sensitivity to discharge angle, port spacing and the free surface. The measured efficiencies are compared with theory as well as available laboratory and field data.

The major conclusions are (1) aeration efficiencies using diffused air aeration are on the order of 2–13%m^?1 (2) the aeration efficiency increases with increasing cross flow velocity and decreasing air flow rate per unit width (3) 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 (4) changing the pore size from 40 to 180 μm, the port size from 0.4 to 1.0 mm or the port spacing does not significantly effect the aeration efficiency (5) aeration using oxygen is somewhat less efficient than that using air. However, since equivalent oxygen bubbles contain approximately five times more oxygen than air bubbles, more oxygen is transferred on an absolute basis at the same gas flow rate using compressed oxygen as opposed to air (6) aeration efficiency resulting from aeration over a portion of the entire width can be reasonably predicted using the results of the two dimensional studies and (7) the available laboratory and field data compare well with the results of these laboratory studies.