Numerical Simulation On The Rectangular Secondary Sedimentation Tank And Improved Optimization Using CFD Technology

Rectangular secondary sedimentation tank is an important part in the water treatment system.The design and operation of the tank are related to the whole sewage treatment effects.The existing design of the sedimentation tank is still based on the ideal sedimentation tank assumption, and there are many problems exposed in such a method.In this paper, the computational fluid dynamics is utilized to study simulate the characteristics of fluid motion in a water treatment reactor, and an improved optimization measures is provided a new train of thought and important method.This paper summarizes the usage of CFD technology in the home and foreign, and briefly introduces the characteristics of the Fluent software, as well as turbulence flow, solid-liquid phases flow model. A numercial simulation Imam's simplified rectangular sedimentation tanks experiment was established, and the comparison between the calculated velocity field and the measured velocity data showes that there is a good agreement. the consequences also validate the efficiency of the numerical simulation method.A rectangular secondary sedimentation tank designed according to the corresponding design specifications,for the single-phase steady state motion and the solid-liquid phases astable motion in the sedimentation tank studied respectively by numerical simulation.The simulation results show that there are obvious backflow zones and dead water regions in the single-phase flow, that is responsible for not high sedimentation efficiency. Meanwhile,whether a horizontal or vertical direction in the longitudinal section, their velocity distribution can not be satisfied with the ideal sedimentation tank assumption. In the solid-liquid phases flow, this paper analysed velocity distribution and concentration characteristic in the case of solid-liquid phases interaction, it is found that the analysis results are coincident with the theory of four district and dual interface in the secondary sedimentation tank actual working conditions well.For optimization of sedimentation tank, this paper considered the SS(suspended solid in the water outlet) and removal rate as major indicators, analysed emphatically the factors which could affect removal efficiency include particle diameter,return sludge ratio,slope and excess sludge.The results show that,①the more the particle diameter is, the better for the sediment process, and the higher for removal rate,②under different return sludge ratio, when return sludge ratio became larger gradually, it will enlarge tank interior surface velocity and shorten the hydraulic retention time(HRT), as a result, the SS increased and and the removal rate reduced obviously.③there is no significant influence for different slope on removal effect, high slope made high altitude difference and lead more sludge into the mud bucket, the efficiency of particle settling higher relatively, but overlarge slope wil increase the cost and depth, so the proper slope must be choiced between the removal rate and cost.④Enlarge excess sludge merely, the SS decreased gradually, this proved that enlarge excess sludge is positive for removal rate.