Prediction of mean circulation velocity in oxidation ditch

In wastewater treatment, oxidation ditches are used for the removal of carb on and nitrogen of activated sludge. The control of the single-phase flow i s essential to the optimisation of the whole process. Among the two global functioning parameters (mean liquid velocity U-c, power dissipated per unit of volume P/V), the mean circulation velocity can be recommended. Indeed, the values of the power dissipated per unit of volume P/V obtained in diffe rent scale plant show that the industrial criterion on P/V leads to an over design of channel. Therefore a mean liquid circulation velocity U-c created by horizontal impellers must be maintained inside the ditch. In order to p redict the velocity U-c, a model has been proposed based on the Equations o f the continuity and motion and using a few simple parameters. Experiments were carried out on pilot plant (1m(3)) and full scale ditches (860, 1400 a nd 2800m(3)) in which the characteristics of the mixing system and the dime nsions of channels were varied. A good agreement was observed between the m odel predictions and experimental data for the mean circulation velocity U- c.