Numerical Simulation of Horizontal Sedimentation Tanks

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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.