| Source control and pollution interception is an effective measure to implement non-point source treatment,and the sewage interception trunk canal is the last key project,which plays a vital role in improving the water environment.Dianchi Lake is a plateau freshwater lake mainly caused by non-point source pollution under the State Council.In order to control the non-point source pollution of Dianchi Lake,the government and other departments have constructed a pollution control engineering system around the Dianchi Lake from source control to end interception,in which the full-length 101.5km trunk canalaround the lake is transported to the terminal water purification plant through maximum interception of non-point source pollution load.It has become an important project to control Dianchi Lake.At present,64% of the total pollution load of Dianchi Lake comes from rural agricultural non-point source pollution,of which total phosphorus accounts for 56%,and mainly exists in granular form.Therefore,phosphorus from rural agricultural non-point sources has become one of the main limiting factors in the treatment of eutrophication in Dianchi Lake.When the sewage interception trunk canal around the lake transports rural agricultural non-point source runoff sewage,the concentration of total phosphorus gradually decreases along the way and is lower than the designed influent concentration of the terminal water purification plant,which is not conducive to the benign operation of the sewage treatment process.According to hydrodynamics and sedimentation,and through the force analysis and dynamic analysis of granular matter in multiphase flow,it is found that the movement of granular matter is affected by critical velocity and resistance coefficient.The critical velocity and resistance coefficient are closely related to the shape of pipe section,the roughness of pipe canal,the hydraulic diameter of pipe canal and the slope of pipe canal.Therefore,the different design parameters of the sewage interception trunk canal may increase or decrease the concentration of total phosphorus in the process of transport,and the decrease or increase of the concentration of total phosphorus at the end of the main canal will affect the benign operation of the end treatment process.In order to reasonably select the design parameters and the corresponding end treatment process of the sewage interception trunk canal,this study used the three-dimensional design software FLUENT based on CFD calculation method to construct a multiphase flow model of rural agricultural non-point source sewage including particulate total phosphorus in the perfect interception canal on the east bank of Dianchi Lake,and simulated the flow process.On the basis of verifying the model with the measured total phosphorus concentration,the effects of different section shape,hydraulic diameter,slope and roughness of the canal on the concentration of total phosphorus in rural agricultural non-point source sewage transported by the trunk canal were investigated.The response surface analysis was used to analyze the significant effects of the above four factors alone and pairwise interaction on the concentration of total phosphorus in rural agricultural non-point source sewage transported by the sewage interception trunk canal,and then optimize the parameters to achieve the maximum and minimum removal rates of total phosphorus respectively.The main conclusions are as follows:(1)The difference between the simulated and measured values of TP at the end of the rural agricultural non-point source sewage transported by the interception trunk canal is not more than 0.1mg / L,which shows that the model used to simulate the concentration change of TP in the main sewage interception canal is consistent with the actual situation,and the model is accurate.(2)In rectangular,circular and trapezoidal trunks,the total phosphorus concentration at the end of circular cross-section is the lowest,and that of trapezoidal cross-section is the highest;In rectangular,circular and trapezoidal cross-sections,with the increase of the roughness of the trench,the concentration of total phosphorus at the end of rural agricultural non-point source sewage transport decreases with the increase of the roughness of the three cross-sections,and the trapezoid decreases the least.With the increase of hydraulic diameter,the concentration of total phosphorus at the end of rural agricultural non-point source increases with the increase of hydraulic diameter,in which the increasing range of trapezoidal cross-section is the largest and that of circular cross-section is the smallest.For the three kinds of cross-section main canals,with the increase of slope,the concentration of total phosphorus in the rural agricultural non-point source of each cross-section decreased gradually at the end of the main canal,and the change range of the rectangular cross-section was the biggest.(3)The hydraulic diameter has a significant effect on the TP removal rate of the circular,rectangular and trapezoidal interception canals,the slope has a significant effect on the TP removal rates of the three kinds of interception canals,and the roughness only has a significant effect on the TP removal rate of the circular interception canals.(4)The interaction between roughness and hydraulic diameter,hydraulic diameter and slope has significant influence on the change of TP concentration in rural agricultural non-point source sewage transported by rectangular,circular and trapezoidal interception channels,while the interaction between roughness and slope has no significant effect on the change of TP concentration in rural agricultural non-point source sewage transported by rectangular,circular and trapezoidal trunks.(5)The parameter design of the circular,rectangular and trapezoidal cross-section sewage interception main canal to achieve the maximum TP removal rate of 94.75%,92.6% and 90.9% are as follows: roughness 2mm,2mm,1.96 mm,pipe diameter 3.45 m,4.49 m,3.59 m,slope 0.6‰,1‰,0.2‰;The parameters with the minimum TP removal rates of 63.28%,56.2% and 64.7% are designed as follows: roughness 2mm,2mm,1mm,pipe diameter 6.5m,slope 0.2‰.(6)Through response surface optimization analysis,the TP influent water quality design index(TP=2mg/L)of the TP influent water purification plant at the end of the Dianchi Lake east interception main channel transportation rural agricultural non-point source sewage purification plant was reached.The TP removal rate was 75.9%,and the circular interception The target design of the main canal application is: roughness 1mm,hydraulic diameter 5.61 m,slope 0.7‰;rectangular sewage interception main channel:roughness 2mm,hydraulic diameter 5.64 m,slope 0.5‰;trapezoidal interception main channel: roughness 1.2mm,hydraulic diameter 5.69 m,slope 0.5‰. |
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