| Secondary sedimentation tanks in sewage treatment process is a very important one of the structures, but also a very sensitive structure.To ensure water quality and improve the quality of sewage treatment process efficiency, in this paper, rectangular secondary settling tanks are further researched by numerical simulation to aim at the assumption that currently the majority numerical simulation on secondary sedimentation tank are based on ignoring the impact of the water density difference and the cunent situation that the mathematical models including the impact of density difference mainly select single model and isn't unified and that the interacting mechanism of water and suspended solids also needs to be further researched.In aspect of mathematical model in rectangular secandary sedimentation tanks, the improved RNGk-εtwo-equation turbulence model and the Mixture model is selected through comparison and analysis. Considered the present problems and difficulties in physical model test, in this thesis, based on comparing with the Froude number theory about channel flow proposed by Esch and actual running state of secondary clarifiers, the validity of model is proved.After having confirmed the model validity, the following numerical simulation research in rectangular secandary sedimentation tanks is conducted. First of all, the structure and working case of secondary clarifiers is optimized which is used to prove the validity of model previously. Secondly, the simulation is conducted without baffle, then, the working state when concentration of inlet is changed with baffle or not. Finally, the case were simulated about the impact of bottom slope on running efficiency and state when the way of discharging sludge on the Gravity was used in rectangular secondary settling tanks.Comparison and analysis of the numerical simulation results indicate the following results. Firstly, the flow in secondary clarifiers is density current, and the flow field is impacted by the strong body force caused by density difference, so the impact of density difference should be taken into account when numerical simulation in secondary sedimentation tank is researched. Secondly, residence time is main influence factor of emission concentration among the design residence time, the export structure and sludge ratio, however, return sludge concentration was impacted by these three kinds of factors together. Thirdly, the concentration of suspended substances on the two outlet will change as well when the concentration of inlet changes, and along with the time lasting will tend to a new steady state, but the unstable state of inlet concentration will have adversee impacts on running. Fourthly, the baffle has a positive effect on optimizing water conditions to improve water quality, buffering the impact of changes in the inlet concentration. Finally, there are negative impacts if the bottom slope is too small when secondary clarifiers discharge sludge from the force of gravity. |
