Simulation On Combined Sewer Overflow Pollution In The Old City Of Chaohu Based On SWMM

Many cities in our country, combined sewer overflow turns to be one of the primary causes of water quality degradation. To control the water pollution around the city that caused by combined sewer overflow, the paper chose the old city of Chaohu as study area and researched the influence on combined sewer overflow pollution under different interception ratios ,rainfall recurrence periods and rainfall styles.First of all, combined sewer overflow pollution situations at home and abroad are analyzed and water qualities of combined sewer overflow effluent, dry weather flow and storm runoff in different citys are compared. After comparing different relative models, the paper chose the Storm Water Management Model (SWMM) developed by the United States Environmental Protection Administration as research tool. On the basis of surveying the terrain, pipelines trend, land utilization patterns and some other characteristics within the area of Huancheng river in the old city of Chaohu, the paper conceptualizes the drainage area and confluence mains. The study area is conceptualizing into thirty-two subcatchments, thirty-two nodes, thirty-seven pipe sections and five overflow outfalls. The paper choose TSS, COD, TN,TP as overflow pollution factors .On the basis of referencing the model parameters of domestic and overseas relative research and the characteristics of research area, then combined sewer overflow pollution model in the study area based on SWMM is set up . Secondly, rainstorm intensity formula is used to design two hours rainfall in Chaohu under the condition of different recurrence period (P = 0.25, P = 0.5, P = 1, P = 2, P = 3, P = 5) and different rain style (r = 0.3, r = 0.4, r = 0.5). And the Chicago rain style formula is adopted to distribute rainfall .Finally, simulation results of SWMM is analyzed.1) Under the condition of the recurrence period P= 1, rain style r=0.4 and interception ratio n0=1, each outfall occurs overflow when the rain appeared 35min later. The time peak flow appeared was about 10min later than the rainfall intensity reached peak. It shows great positive correlation between the concentration of TSS ,COD and the rainfall intensity . The peak concentration of TN and TP occurred when the rain began and reached the lowest concentration after the rain began 2 hours later.2) The influence of interception ratio on the combined sewer overflow pollution is analyzed.Overflow quantity and pollution load are analyzed under the condition of P=1, r=0.4 and different interception ratios. With the increase of n0, overflow duration,peak flow and sewer quantity decreased, and the reduction became slow. The cumulant load of overflow pollutant such as TSS,COD,TN and TP increased along with the rise of n0 ,but the increase speed became slow gradually.3) The influence of recurrence period on the combined sewer overflow pollution is analyzed. Overflow quantity and pollution load are analyzed under the condition of r=0.4,n0=1 and different recurrence period.With the increase of recurrence period, overflow duration,peak flow and wastewater quantity decreased, and the growth became slow. The cumulant of overflow pollutant such as TSS,COD,TN and TP increased along with the rise of P ,but the increasing speed became slow gradually.4) The influence of rain style on the combined sewer overflow pollution is analyzed. Overflow variation and overflow pollutant concentration are analyzed under the condition of P=1,n0=1 and different r. With the increase of the recurrence period, the more posterior the rainfall peak exists, the more posterior flow peak occurs, the higher the wastewater quantity will be. Generally, the concentration of pollutant varies little. Concretely, the more anterior the rain peak is, the lower pollutant concentration peak will be.