Reconstruction Of Combined Drainage System And Project Benefit Evaluation In The Old City Of Hefei

An important cause of water environment pollution is urban non-point source pollution caused by the initial rain runoff.This paper aims at the characteristics of the combined drainage system in the old city,and starts with the rainfall characteristics and on the basis of full investigation,it proposes a combination strategy of "source control,process collection,and end dispose" to deal with the initial rainwater pollution problem in the old city.The project combines engineering design cases and discusses the feasibility and advantages of the combined wastewater treatment and storage technology in the old city from four aspects:current status investigation,standard argumentation,engineering design,and benefit evaluation.The main results were as follows:（1）The status quo and existing problems of the drainage system in the old city were analyzed,and the results of the research were integrated to study the characteristics of overflow pollution,rainfall characteristics,rainfall interception rate,domestic and international engineering standards option,and storage capacity of the old city’s pipe network.Determined the overflow pollution control standards in the old city.（2）The equivalent of added detention tank was 8 mm of rainfall.And the total service area was 5.2 km²,including the Xinghua detention tank,Xiaoyaojin detention tank and Baohe detention tank,with the service areas were 2.9 km²,1.05 km² and 1.25 km²,respectively.The surface runoff coefficient of 0.75.Therefore,the storage capacity of added detention tank was 31,500 m³ in this project,which including Xinghua drainage system（17500 m³）,Xiaoyaojin drainage system（6,500 m³）and Baohe drainage system（7,500 m³）.（3）Taking the demonstration project of the Xiaoyaojin storage tank as an example,the demonstration and project design of the storage tank scheme of the old city was completed.Through the design,construction and operation management of the storage tank,the demonstration applications of the closure and storage of the combined overflow pollution control and optimization of operation scheduling technology are carried out.（4）The overall benefits of the demonstration project were evaluated.The results showed that he cut-down rate of storage tank water was 57.8%,the COD pollution load reduction rate was 45.9%,the ammonia nitrogen pollution load reduction rate was 61.3%,the TN pollution load reduction rate was 56.3%,and the SS pollution load reduction rate was 57.3%,all the above reduction rates were more than 30%.（5）Comparing rainfall and overflow wastewater quality data between 2010 and this monitoring period（2016）,the amount of rainfall was equivalent,the quality of the overflow wastewater monitored this time was improved.In 2010,the storage tank was not constructed.The benefits of the storage tank are reflected in the reduction of pollution load.It can be seen that the inflow pollution load(SS,NH3-N,COD_Cr,TN)reduction rate of the demonstration area overflow port in the old urban area was more than 32%.