Study On The Hydrodynamic And Wastewater Quality Transformation Model In Urban Sewer Networks

With the rapid development of urban economy and the rapid expansion of population, deterioration of urban water environment and water resources shortage have been becoming the bottleneck of constrainting urban economic and social development.In order to achieve the sustainable development of cities,we must redefine and set forth the tasks and functions of urban drainage system,so that a new model of urban drainage systems can be finalized.The functions of the new drainage systems should gradually change to recycle of wastewater and nutrient substances instead of previous water logging control and elimination of disaster as well as pollution prevention and elimination of disaster,so as to recover a good water environment and enable the recycle of water resource.In order to achieve the functions of the new drainage systems,we must hold the movement law of water,as well as the transport,dispersion and microbial transformation processes of wastewater quality along with the water flow in order to find the pollution prevention and treatment methods quickly.Based on the understanding of urban sewer networks and the needs of the research,the one-dimensional model is developed for the hydrodynamic and water quality transformation in urban sewer networks.Taking into account the differences of the water environment between urban sewer networks and rivers,the research results on water quality model of river(such as WASP) can not be directly applied to the simulation of water quality in sewer networks; therefore,based on the analysis of the physical and biochemical processes in sewer networks and combined with activated sludge model(ASM) theory,the integrated wastewater quality transformation model is developed in this thesis,in which the coupling calculation and the transformations and transport processes between 12 contamination variables is considered in aerobic amd anaerobic conditions.The one-dimensional hydrodynamic model is based on the Saint-Venant equations. Through the introduction of Preissmann fictitious slot at the top of a pressurized sewer,the model for pressure and surface flow in sewer networks is controlled by the same equations, achieving the simulation of free-surface-pressurized flow at the same time.The weighted four-point Preissmann implicit difference scheme is used to discrete the equations,and then the "Three Steps Method" of sewer-junction-sewer is applied to resolve the model,which reduces the number of variables and improves the computational efficiency through making use of the recursion relations.The model is applied to simulate flow of complicated dendriform and looped and Yahua district sewer networks.The simulated results are in good accordance with Literature results and corresponding measurements.It is shown that the present model is an effective tool for simulating the free-surface-pressurized flow in sewer networks.The one-dimensional water quality model is based on the advcction-diffusion equation, which is discrctcd by the implicit upwind difference scheme.In consideration of the characteristics of the unsteady flow with variable directions and unsteady nonuniform convection transport in sewer networks,a series of recursion equations and the numerical method have been derived to reslove the equations.In order to determine the variable water quality parameters efficiently,a modified genetic algorithm is proposed,and it is shown by the test results that the algorithm has higher searching efficiency and convergence.First of all, Baiziyi's experiment is selected to validate the water quality model,and then the model is applied to simulate transformation and transport processes in three real sewer networks.The simulated results are in good accordance with corresponding measurements.It is shown the model is credible,and it may be a practical tool for forecast and management of water quality in sewer networks.