Improvement of Flow in Final Settling Tanks

Abstract

Poor flow in final settling tanks may be the result of neglecting the effects of density current. This can be countered when inserting guiding flow elements, such as the inlet and the barrier walls in the tank. Based on a simple approach, the optimum height of the inlet aperture increases with both the overflow rate per unit width and the return sludge ratio, but decreases with the relative difference of density. Further, a central barrier wall is introduced to divide the tank into high and low sludge concentration units. More generally, the effect of density can be reduced by correctly positioning the inlet structure. These results are verified by selected prototype observations involving measurements of both velocity and concentration fields. A comparison of original and final designs where effects of density were reduced clearly demonstrates significant improvement in sedimentation and effluent characteristics. The present recommendations may easily be incorporated in an existing tank and, thus, are efficient in improving the final sedimentation.