The horizontal rotation of water in an oxidation ditch by separate agitation improves the oxygen transfer of diffused air aeration systems. This can be explained by the modifications. due to the horizontal flow. of several parameters that play a role in the oxygen transfer. An experimental study was performed on a 1400m3 annular ditch fitted with fine bubble diffusers and mixers to quantify this empirical knowledge. The initial findings of this research are presented. Compared to the absence of an imposed horizontal flow, the O2 mass transferred per hour, in clean water, is increased by 40% to 50% for a horizontal water flow of 0.4 to 0.5 m/s. This improvement may be explained by the prolongation of air bubble contact time (modification of spiral flow phenomena) for 10% at the most, by the increase of the exchange surface for the most part, due, not to the shear or to the elimination of the coalescence of air bubbles, (insignificant phenomena in this case). but to the deformation of air bubbles for 17% at the most and especially to the decrease of the diameter of the nascent bubbles. Moreover, the renewal of the liquid layer around the air bubbles could be considered as insignificant.

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