Computational Fluid Dynamics Analysis for Improving Secondary Settling Tank Performance

The secondary settling tank (SST), also known as a secondary clarifier or secondary sedimentation tank, is a complicated and sensitive part of activated sludge process. In order to improve SST performance, computational fluid dynamics (CFD) models have been employed. In this study, a Fluent-based three-dimensional numerical model is applied to assess the capacity of the SSTs and improve their performance for the two large wastewater treatment plants (WWTPs) in southern California. Three different energy dissipating inlet (EDI) structures among the SSTs in these two WWTPs are evaluated based on the prediction of velocity profiles, sludge concentration profiles, and three different performance indicators (effluent suspended solids, recycle solids concentration, and sludge blanket height) over a range of loading rates. The results show that (1) the Orie Albertson EDI outperforms the original tangential EDI under the normal flow conditions but is inferior to the tangential EDI at high flow conditions; (2) the LA-EDI performance can be improved by optimizing the depth of the flocculating well; (3) SSTs are more sensitive to the increase of MLSS concentration than the increase of flow rate; (4) an alternative indicator needs to be developed to differentiate the SST's performance under the extreme SLR.