Numerical simulation of secondary sedimentation tank based on population balance model

Abstract

A technical method of grid flocculation wells is proposed to control the effects of heterogeneous flow on the flow regime and the removal efficiency of suspended solids in the secondary sedimentation tank. The flocculation and sedimentation of activated sludge in sedimentation tanks were simulated by a combination of population balance model (PBM) and computational fluid dynamics. Compared with the prototype of the flocculation well, the grid split the vortex under inertia and the turbulence of the grid, which increased water viscosity, improved the energy dissipation of the flocculation well by more than 40%. The energy consumed by the viscous action provided the power for flocculation of the activated sludge, and reduced the volume fraction of particles with a particle size of less than 98μm by over 50%. After the addition of No. 2.5 (grid interval is 25 mm) and No. 3.5 (grid interval is 35 mm) grids, the concentration of effluent suspended solids was reduced by 29.3% and 69.4% respectively, compared with the prototype. The average suspended solids concentration of the No. 3.5 model of the actual optimization was 9.2 mg L-1, which was similar to the simulation results, indicating the effectiveness of the optimization method.