Research On The Interception Ratio Of Combined Sewer Systems In Small Towns Of Chongqing Area

There are many small towns in Chongqing area facing the fact of drainagenetworks reconstruction and the construction of sewage treatment plants with theimplementation of the Three Gorges reservoir area planning. Studies have showed that itis able to meet the requirement of the water protection by usingcombined closuresystems and installing sedimentation tanks in the vicinity of the sewage treatment plants.However, the interception ratio,which is a significant design parameter in the reformprogram, can directly affect the overflow amount which will flow into the water bodyand the costs of proect investment and running. This study aimed to identify the optimalinterception ratio of society-economy-environment by using urban surface runoff model,optimization modeland evaluation methods.Firsty,this study analyzed and comparedthe the advantages and disadvantages ofdifferent hydraulic and water quality models on urban surface runoff. And the USAstorm water management model(SWMM), which had advantages in simulating thehydraulicsof collection systems, waschosenas the hydraulic and water quality model inthe research. Secondly, a small town named Baishiyi in Chongqing was selected as theresearch area through the analysis of the characteristics of Chongqing area and the smallmountainous towns in Chongqing.Then a hydraulic and water quality calculation modelof Baishiyi town based on SWMMwas built. The model was calibrated and validated bythe monitoring data of the actual rainfall events. Subsequently,the overflow process ofmain intercepting pipes in Baishiyi was simulated by the constructed model to work outthe overflow amount and pollution load,intercepted amount and pollution load indifferent interception ratios.Furthermore,the author estimated the construction costs andcorresponding operating costs of the required intercepted combined trunk sewer,hoistingpumping station and sewage treatment tanksin different interception ratios coupled withthe economic losses of the environmental pollution caused by overflows.Then,theoptimal model which used the economic costs and environmental loss as its evaluationindex was built. At last, both determined weighted method and scenario analysis wereused to indentify and optimize the interception ratio.The research results showed that the SWMM model was able to absolutelysimulate the hydraulic and water quality variation of urban surface runoff from itsformation to overflow. After calibration, the model had high simulation accuracy and pretty good simulation effects of the hydraulic and water quality.The efficiencycoefficients Ens between the measured and simulated results were all above0.8.Compared with interception of1,the total annualoverflows for interception of2,3,4,5,6,reduced by12.1%,22.7%,31.4%,38%and42.4%respectively.The optimal interceptionratio of the combined closure system in Baishiyi should be3.