Effect of air flow rate on oxygen transfer in an oxidation ditch equipped with fine bubble diffusers and slow speed mixers

The effect of air flow rate on oxygen transfer efficiency was examined in clean water and under process conditions in an oxidation ditch equipped with fine bubble membrane diffusers and large blade slow speed mixers. Under process conditions, an increase in the air flow rate resulted in a decrease of the oxygen transfer efficiency similar to that observed in clean water. Consequently, the value of the alpha factor was in the order of 0.58 independently of the air flow rate (between 45 and 110 m3 h−1 per m2 of membrane). The combined influence of the air flow rate and of the horizontal liquid velocity on the oxygen transfer efficiency was studied. Results have evidenced that regrouping the diffuser grids and applying a horizontal flow could lead to a significant limitation in the impact of the air flow rate on the oxygen transfer efficiency. Finally, the use of the off gas method to determine the oxygen transfer efficiency of an aeration system at a given air flow rate led to the definition of a relationship, that enabled the evaluation of the system oxygenation capacities at air flow rates different from the one at which the measurements were initially performed.