| Combined sewer overflow(CSO)is a key part of the improvement of urban water environment.With the development of sponge city construction,pilot cities which had CSO problems adopted the "major,minor,and micro" drainage system and CSO control system to achieve the goal of peak cut and pollution control,thereby reducing the occurrence of CSO,but there are still have problems need to be resolved urgently.For example,the CSO control standards are not clear,the response relationship of source volume control,CSO and receiving water body need to be studied.Firstly,analyzing the CSO control index and the basis of CSO standard formulation,and put forward the concept of the CSO pollution load proportion.In order to explore the best type and proportion of source facilities,the SWMM model was used to simulate and study the response relationship between source volume control,peak flow cutting and pollutant load reduction under the influence of rainfall duration and land scale differences.On this basis,further study the differences of combined regional characteristics parameters such as the capture ratio of annual rainfall runoff volume,impervious area rate,terrain slope,soil permeability,interception ratio,pipe network design return period,etc.The concept of overflow ratio is put forward,its meaning is the CSO volume of unit rainfall,which is used to study the response relationship between rainfall level and CSO.The SWMM model and WASP model are used to simulate the relationship between source volume control and CSO volume reduction and ammonia nitrogen concentration reduction,and the relationship between source volume control and the improvement of receiving water quality.The main research conclusions are as follows:(1)The existing CSO control indicators can be divided into two types: technology-based emission control indicators and control indicators based on the water quality assurance of receiving water body.CSO control standard formulation basis is mainly divided into three categories: analogy diversion pollutant load determination CSO pollutant discharge standards,CSO effect standards based on engineering practice effects,and standards of different control levels for CSO based on water body use.The concept of CSO pollution load proportion is proposed.CSO pollution load proportion is the percentage of the pollution load of runoff rainwater(in the pipe network)that overflows in a confluence area and the total surface rainwater runoff pollution load that produces confluence in the area,which is used to characterize runoff The contribution rate of rainwater to CSO and the ratio of rainwater pollution load in the combined pipeline network.(2)Selecting area with 5 rows and 5 columns,under the rainfall design return period higher than 20 yr,when the proportion of storage type goes up,what the peak flow and pollutant load reduction goes up.Under the rainfall design return period less than 10 yr,the three control types’ effect of all storage facilities,purification: storage =1:1,purification: storage =2:1 is better than all purification facilities,purification: storage =1:2,purification: storage =1:4,purification:storage =4:1.(3)Under the different conditions of the combined sewer systems,the source volume control corresponding to capture ratio of annual rainfall runoff volume of 30%,50%,70%,80%,and 85% can reduce the total overflow by 12.01%～ 38%,29.36%～95.59%,45.59%～98.93%,59.01%～100%,74.74%～100%;can reduce the CSO events by 0～5 times,1～4 times,1～4 times,1 ～6 times,2～6 times.(4)Put forward the concept of overflow ratio,under the scenario where the interception ratio is 3 and the capture ratio of annual rainfall runoff volume is 85%,the annual overflow ratio has a strong correlation with rainfall.The annual overflow ratio is relatively high in the extra high flow year and the partial abundant year.There is almost no overflow in extra dry years.When the rainfall level is large to heavy rain and above,it is easy to cause CSO,and the overflow ratio increases with the increase of rainfall grade.(5)Under the condition of rainstorm grade,the CSO volume can be reduced by 6.53%,31.43%,54.99%,65.27%,73.77%,84.17% and 93.72% under the capture ratio of annual rainfall runoff volume range from 40%,60%,70%,75%,80%,85% and 90%.(6)The peak concentration of ammonia nitrogen in the downstream water can be reduced by 3.33%,16.21%,30.58%,37.79%,44.21%,52.71% and 60.47% under the capture ratio of annual rainfall runoff volume of 40%,60%,70%,75%,80%,85% and 90%. |
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