Research On Pollution Control Of Combined System Overflow In Old Urban Area Of Xi'an Based On SWMM

At present,the old urban areas of most cities in China are facing serious combined overflow pollution problems,which are mainly caused by the backward design return period of drainage pipe network,incomplete separation of rainwater and sewage,large hard impervious area and high difficulty in ground reconstruction.Aiming at the above problems,this paper takes the old urban area of Xi'an as the research object,and uses the drainage network model software EPA SWMM and ArcGIS geographic information system,based on the measured data,constructed the drainage network model of the old urban area,carried out the simulation evaluation of combined Overflow Pollution with short duration design rainfall and long duration annual rainfall respectively.Based on the simulation results,five transformation measures were put forward,and different reconstruction measures were simulated under different rainfall conditions to control the combined overflow pollution The optimal analysis provides guidance suggestions for the pollution control of combined system overflow in the old urban area of Xi'an.The main conclusions are as follows:(1)Based on the rainfall data of Xi'an city from 1980 to 2019,the overall rainfall situation in Xi'an city is analyzed.The results show that:the average rainfall of Xi'an in recent 40 years is 539.91mm,and the rainfall is mainly concentrated in July to September.The frequency of rainstorm is increasing.(2)Based on the data of the existing drainage network in the old urban area,the Overflow Pollution Model of the regional combined system pipeline is constructed based on SWMM.The calibration results of the model show that the error of the simulated comprehensive runoff coefficient is controlled within 5%,the ENS meets the requirements of the code,and all parameters are within the requirements of the design specification,which proves the reliability of the model in the hydraulic calculation of rainfall runoff and pipe network,which has a scientific guidance role in the selection of calculation method of drainage model.(3)Through calculation,the comprehensive runoff coefficient of the drainage basin in the old urban area of Xi'an is 0.782;the evaluation of the drainage capacity of the existing drainage network under different rainfall conditions shows that the drainage capacity of the current drainage network is low;the initial rainwater pollution is relatively serious,and the pollution load 60 minutes before overflow is more than 50%of the total;the total annual overflow of the study area in 2019 is 3.403 million m3,The discharge load of SS,COD,NH3-N and TP is 215.167 T,186.617t,14.614t and 1.262t respectively.A large number of overflow pollutants have caused serious pollution to the moat,which proves the necessity of the renovation of the combined Overflow Pollution in the old urban area.(4)In view of the current situation of the study area,five control measures were set up,including lid measures,end storage tank,expansion of river closure main pipe,combination measures of lid and end storage tank,and lid and expanded closure main pipe.Comprehensive comparison of overflow pollution control effect and investment amount shows that the combination of lid and expansion of river closure trunk is the best measure.The five transformation measures proposed in this study can meet the current demand of Xi'an for the combined system transformation of the old urban area,and have certain reference value for the pollution control of the combined system overflow in the old urban area of China.