Numerical Simulation of Horizontal Sedimentation Tanks

沉淀池是水处理中最基本的单元操作，但沉淀池的设计和运行大多靠经验，其行为非常复杂，研究难度很大。引入数值模拟而不是昂贵的中试研究，可以更好地了解沉淀池的特性。将水平沉淀池内的流动简化为二维非压缩非定常流动、颗粒均匀沉降、颗粒无划痕和恒温。数值方法有SMAC法、有限单元法和Crank-Nicholson法。进水流量迅速下降，在底部形成密流。如果沉降速度较快，则在颗粒沉降后，电流从底部向上攀升，并沿表面流向出口。在靠近进水口的上部有一个死流区，在靠近出口的下部有一个死流区。模拟示踪研究表明，实际滞留时间仅为名义滞留时间的10%。当颗粒很难沉淀时，密度电流就会到达水槽的末端。沉淀物从水流中分离出来，上升到水面，沿着水面流向出水口。滞留时间约为50%。密流被认为阻碍了沉积效率，但由于其深度小，颗粒到底部的移动距离变得很短，似乎对沉积很有帮助。如果有办法加强电流，效率应该会提高。较小的入口开度会产生较强的进水流动，从而产生较强的向下流动，并显著提高沉降效率。数值模拟是研究沉淀池的非常有效的工具。

A sedimentation tank is the most basic unit operation in water and wastewater treatment, however its design and operation are mostly done by experiences, as its behaviors are very complicated and hard to study. Introduction of numerical simulations instead of expensive pilot plant studies gave better insights of characteristics of sedimentation tanks. The flow in forizontal sedimentation tanks was simplified as two dimensions un-compressive unsteady flow, uniform settling velocity of particles, no scoring of settled particles, and constant temperature. Numerical Methos was SMAC, finite elements method, and Crank-Nicholson method. The influent flow went down rapidly forming density current along the bottom. If settling velocity was fast, the current climbed up from the bottom after the particles had settled and went along the surface to the outlet. There was a dead current zone at the upper part near to the inlet and another one at the lower part near to the outlet. Simulated tracer studies showed the actual detention time was only 10% of the nominal one. When the particles were hard to settle, the density current reached to the end of the tank. The settled flows separated from the current and rose to the surface and went along the surface to the outlet. Detention time was about 50%. The density current was considered as hinder sedimentation efficiency, however it seemed to help sedimentation very much as its depth was small and the travel distance of particles to the bottom became very short. If there was a mean to strengthen the current, the efficiency should improved. Small inlet opening, that created strong influent flow, generated a stronger doenward flow and improved the settling efficiency considerably. Numerical simulations were very effective tools for the study of sedimentation tanks.