Numerical Simulation of Horizontal Sedimentation Tanks

沉淀池是水和废水处理中最基本的单元操作，但由于其行为非常复杂，难以研究，因此其设计和操作大多依靠经验。引入数值模拟而不是昂贵的试点工厂的研究提供了更好的见解沉淀池的特点。将强化沉淀池内的流动简化为二维无压非定常流动，颗粒沉降速度均匀，沉降颗粒无划痕，温度恒定。数值方法采用SMAC、有限元法和Crank-Nicholson法。进水流量迅速下降，形成沿着底异重流。如果沉降速度快，则在颗粒沉降后，电流从底部向上爬，并沿着表面到达出口。在靠近进口的上部存在一个死流区，在靠近出口的下部也存在一个死流区。模拟示踪剂研究表明，实际滞留时间仅为标称滞留时间的10%。当颗粒难以沉降时，异重流到达槽底。沉淀下来的水流从水流中分离出来，上升到水面，然后沿着水面流向出水口。滞留时间约为50%。异重流被认为是一种阻碍沉降的因素，但由于异重流的深度很小，颗粒到达底部的距离也很短，因此异重流似乎对沉降有很大的帮助。如果有一种方法来加强电流，效率应该提高。小的入口开口，创造了强大的进水流量，产生了更强的向下流动，并大大提高了沉降效率。数值模拟是研究沉淀池的有效工具。

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.