Fluidization and Sedimentation Patterns in a Secondary Sedimentation Tank

Secondary sedimentation tank is an important unit for wastewater treatment processing. It often proves to be the bottleneck of the whole activated sludge wastewater treatment process. It is mainly used for the separation of sludge and water to guarantee the quality of effluent and the steady operation of biological system. However, many secondary sedimentation tanks encounter different problems due to inappropriate design. Existing facilities have to be utilized efficiently and economically to attain higher treating performance. The mixed liquor entering the secondary sedimentation tank should be sufficiently clarified in order to produce an effluent of acceptable quality. The sludge in the tank should also be adequately thickened so that the desired solids level in the bioreactors can be maintained through sludge re-circulation. Should any of these functions fail, suspended solids will be carried over the effluent weirs and escape with the effluent. Besides the resulting poor effluent quality, excessive loss of suspend solids may affect the whole biological process correspondingly. Hydrodynamic characteristic is inevitable to achieve the above goals. This paper employs both mixture model and discrete phase model to explore the fluidization and sedimentation of activated sludge flows in a horizontal secondary sedimentation tank. The influences of sludge particle diameter, density, entrance velocity are investigated and cross checked with two models. The flow and distribution of solid phase in the tank were obtained by the mixture model. Meantime, the DPM (Discrete Particle Method) was employed to explore different effects on particle paths. The particle flow paths at different situations were analyzed. The behavior of settled particle was also explored. The results from Euler-Euler model and those from DPM complement each other. Both of them draw a more complete picture of the flow and solid sedimentation in the secondary sedimentation tank. All these results could contribute to the design and optimization of secondary sedimentation tanks. It will also help to the separation and sedimentation of sludge in the tank.