PREDICTING OXYGEN TRANSFER IN ANNULAR DITCHES EQUIPPED WITH FINE BUBBLE DIFFUSERS AND MIXERS

The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 24 full scale annular ditches, were interpreted using dimensional analysis. Two relationships have been developed between the oxygen transfer efficiency per meter of submergence (SOTEs) and the parameters affecting oxygen transfer: air flow rate (QG), depth of submergence (h), surface area (S), membrane surface (Sp), aerated surface (Sa), width of the basin (W), angle between mixers and diffusers (Ang) and horizontal velocity (UC). These relationships differentiate annular ditches according to the diffuser placement: tank coverage higher than 50 % of the surface area (Type I) or at the maximum 50 % of the area (Type II). They can be used to predict oxygen transfer performances, at a design stage.Within the range of the parameters considered, and for a given tank geometry, the oxygen mass transfer increases with the number of diffusers, and, for an equal number of diffusers, with the total surface area of the zones occupied by the diffusers. A horizontal velocity increase induces an enhancement of the SOTEs in annular ditches of Type I, but has only a slight positive effect on the ditches of Type II. This can be attributed to the different hydrodynamic behavior induced by the diffuser placement. The SOTEs is a decreasing function of the air flow rate, the drop being higher when the diffuser grids are grouped (m = −0.327, Type II) than when they are spaced out (m = −0.249, Type I).