Mixing and transport processes in wastewater basins

The hydraulic behavior and mixing processes in wastewater basins are studied experimentally in a laboratory scale model. The laboratory basin is modeled after a full scale aerated lagoon system operating in Saint-Julie, Quebec. Three experimental techniques are used to reach the objectives: (i) video flow visualization, (ii) tracer studies and (iii) two-dimensional flow measurements using hot-film anemometry. Experiments are conducted on non-aerated basins, as well as on artificially aerated basins.The flow visualization and flow measurements show that the hydraulic behavior of non-aerated wastewater basins is very complex, due to the formation of flow patterns such as stagnant zones and recirculation. The location and size of these flow patterns are determined using the measured velocity flow field and the flow visualization images. Mixing in these basins is highly non-uniform since flow is clearly segregated into high-velocity and low-velocity areas. The exchange between these areas is very low, affecting the distribution of solids and contaminants.When artificial aeration is introduced, mixing becomes more uniform. Flow measurements and visualization indicate that the size of stagnant zones and the extent of recirculation decreases. The tracer studies show that, in general, aeration tends to decrease the actual residence time of the waste material. Calculation of the dispersion number, d, and the dead volume, Vd, are not sufficient to characterize the effect of aeration on mixing.The understanding of mixing and transport mechanisms in wastewater basins is important, because such basins are often used in applications which require settling or mixing of solids and contaminants