Numerical Simulation Of Gravity Flow In Rectangular Secondary Settling Tanks

As an important part of the sophisticated sewage treatment process, the role of secondary settling tanks is irreplaceable. Rectangular secondary settling tanks have the ability to adapt a shock load and temperature changes, simple construction and low cost, are applicable to the higher level or badly of geological and various types of sewage treatment plants. They are widely used.The concentration difference between influent water and tanks'water, or the different inlet temperature in different seasons will produce gravity flow phenomena in the sedimentation tanks. The flow pattern of the pool will be changed. In this paper, gravity flow phenomena in rectangular secondary settling tanks are further researched by numerical simulation. In aspect of mathematical model in rectangular secondary sedimentation tanks, the RNG k-εtwo-equation turbulence model and the Mixture model is selected through comparison and analysis. Two important factors of causing gravity flow (concentration and temperature difference between influent water and tanks'water) are taken into account, the effect on stream and concentration distribution causing by gravity flow should be studied, and then the method should be seeked to improve the flow pattern responsable for the sedimentation efficiency.In this paper, the study about flow in rectangular secondary sedimentation tanks is mostly aimed at the changes of flow patterns and concentration, so two different tests were selected to varify the validity of the model in aspects of flow patterns and sedimentation. After having confirmed the model validity, the numerical simulation of flow in rectangular secandary sedimentation tanks is conducted.First, the gravity flow causing by concentration difference will be simulated; Second, the gravity flow causing by temperature difference is simulated; Third, the gravity flow causing by both concentration and temperature difference is simulated; Last, some research is conducted about optimizing the structure of rectangular secondary settling tanks.The simulation results show that:the concentration difference would produce the gravity flow in the substrate. Low-temperature water inflow would also produce the gravity flow in the substrate, and high-temperature water inflow would produce the gravity flow on the top. The gravity flow in the substrate moves along the bottom of the tank, when reaching the end of the pool it flows up along the pool wall, excluds from the export, without sedimentation, even taking the lower concentrated sludge out. This seriously affects the efficiency of the secondary settling tanks. The gravity flow on the top moves along the pool surface, reaching the outlet without sedimentation completely and flowed out. Similarly, this will affect the sedimentation tank. In general, gravity flow in rectangular secondary sedimentation tanks does more harm than good. Gravity flow's formation has a close relationship with the Froude number. Optimizing the structure of outlet could weaken the concussion of gravity flow.