Smarter and Greener Design and Operation of Primary Sedimentation Tanks Using Three-Dimensional Computational Fluid Dynamic Modeling

The Drainage Services Department (DSD) of the Government of Hong Kong Special Administrative Region (HKSAR) is evaluating possible smarter and greener primary sedimentation tank (PST) design and operation. An advanced three-dimensional (3D) computational fluid dynamic (CFD) package incorporating principles of the water and solids movements in the PST in terms of flow, energy and hence the solids flocculation and settling patterns, and density currents, from the inlets to the outlets of both sewage and sludge lines, was selected and developed. When applied to the existing PSTs of the Shatin Sewage Treatment Works, the 3D CFD PST model showed that total suspended solids (TSS) removal could be improved by 17% if the existing PSTs were modified with the simulated hydraulic enhancement features including a set of each of inlet energy dissipating and flocculation baffles, mid-tank re-flocculation baffles and settled primary sludge hopper canopy baffles. The improvement is projected to lead to better than 16% OPEX savings just on the electricity costs resulting from less aeration power requirements due to reduced organic and nitrogen loading to the biological process. When taken together with an increased biogas production and less sludge management, the savings of this smarter and greener PST design and operation will potentially approach USD 700,000/year. Full-scale validation tests are being organized on the developed 3D CFD PST model with existing PSTs with and without the modelled modifications.