Research On The Optimization Of Rainwater Detention Tanks For Non-point Source Pollution Controlling In Separate System Drainage

Nowadays, the ratio of the pollution brought by rainfall in the total weightof pollutant emission was becoming higher and higher. The controlling ofnon-point source pollution becomes to an urgent task. Among all theengineering measures of controlling non-point source pollution, the rainwaterdetention tanks on the end of the system are important. During the design ofrainwater detention tank with the current formula, the determination of theparameters were difficult, mainly based on experiences, and it could not beoptimized according to the actual situation of the locality. It is very necessaryto find a convenient and accurate design method of the detention tanks forcontrolling pollution.In this study, based on previous studies, the improvement of the designformula was done. The system of computing technology supporting framewere established. It was convenient for the accurate calculation of engineeringdesign. The conclusions in this study were as follows: (1) Based on the current rainfall estimation formula, combined with the relationship function of rainfall-runoff coefficient and rainfall-runoff pollutant concentration, a new formula for the design of the rainwater retention tanks were established. The rate of pollutant reduction and the condition of the underlying surface was considered in this formula.(2) Based on the parameters set, the effects of five kinds of typical underlying surfaces on the volume of detention tank were investigated. In the same conditions of catchment area and pollutant reduction rate, the volumes of detention tanks were:SBS> Impervious brick> Cement pavement> Grassland> Permeable brick. The use of the underlying surface with better water permeability and the hysteretic capacity could be helpful for reducing the volume of tank.(3) By comparing the volume of tanks required in cement and grassland, which need by the reduction rate increased from70%to80%and from20%to30%separately, the reduction rate in the grassland surface was bigger than those in cement surface. This may be due to the decreased ability to contain lag.(4) In design of new engineering, the sewage pipeline should be determined with the sum of the amount of drain sewage and rainwater. When only sewage delivered, the velocity should be check with the minimum velocity of the pipe diameter. In engineering design, the check velocity can be quickly solved by hydraulic diagram under the relationship between flow ratio, velocity ratio and hydraulic diagram. The longest venting mode of detention tank can be analyzed by1440min hyetograph. The time of its emptying should not be longer than the beginning between the two interval rainfall periods, when the continuous cumulative drawdown reached the depth of tank contained firstly. And the length should be no longer than12h. The minimum time should be satisfied to the verification of the flow rate of dry sewage.The maximum capacity of the pump flow in the retrofit pump shall be accounted with the formula:(5) The design case was studied in XXX County in the west of Shanxi province, whose metro area was94.35ha. Two schemes were designed in this case, and the optimal scheme was selected by the optimization decision method established in this study. Under the premise of pollution reduction rate of50%, a2600m3volume tank was needed, and it costs2.83million yuan. Using the SS as the target pollutant, the cut benefit was about1347yuan per ton.(6) When the cut rate was increased from50%to70%, the increasing rate of investment environment efficiency was decreased by47.54%than that from30%to50%. The economics of further improving the rate of pollution controlling by improving the volume of tank should be further considered.(7) The impact degree of the volume by Pollutant reduction rate was higher than that of the types of underlying surface. Due to the service area was unable to change in real cases, the determination of a reasonable rate ofregional pollutant reduction was significantly important.(8) The optimization system was established based on engineeringinfrastructure investment, operating expenses, depreciation expenses and totalpollutant reduction. It could be used as the optimization method for the designof the pollution controlling system for the non-point source pollution.(9) A model of decision was established containing the new algorithm, thedetermine method of the parameters, the design method of the downstreampipeline, the principle of pollutant reducing ration determination, the optimizedecision system and the post evaluation mechanism.