Study On Types And Design Issues Of Intercepted Separate Urban Drainage System

In the last thirty years, separated drainage system has been widely applied in China.With the issue and implement of Code for Design of Outdoor Wastewater Engineering(GB50014-2006)(2011revision editon), separated drainage system will get furthermore attention. Pollution of first flush stormwater runoff has obtained widely attentionin developed coutries, and with many wastewater treatment plants built, point sourcepollution like municipal sewage is being gradually controlled, so first flush stormwaterrunoff pollution will become one of the main problems to urban water environment.Now related researches on pollution of first flush runoff have been carried out in Chinaand a few of cities are even planning, designing, and building intercepted separatedrainage systems. But in general, design issues of intercepted separate systems (ISS) havenot gained necessary attention and systematic design methods have been formulated. Soresearch on intercepted separate drainage system has become very necessary and urgent.In this paper, by reviewing and analysing the tethnological progresses for first flushstormwater runoff control and the practices, application types of intercepted separatedrainage system are proposed and some main design problems such as determination ofthe intercepting volume, types of intercepting well, design discharge of pipe andsystematical method of comparison and selection are studied. The key findings are asfollows:(1) Five types of intercepted separate sewerage system are proposed and theappropriate application conditons are discussed. The proposed types are: Type I,interception pipe+intercepting manhole; type II, double-layer sewer+double-layermanhole; Type III, traditional separate system+double-layer intercepting manhole;Type IV, Type I (or type II and Type III)+storage tank; Type V, Type I (or Type II，TypeIII and Type IV)+Low Impact Development (LID). These five kinds of ISS types can beapplied with reagard to the real condition and The Type II will be particularly suitablefor mountainous cities.(2) Caculation methods and issues for main flows or volumes are studied andpointed out respectively:1) interception volume: for regions with sufficient rainfall data,the frequency analysis method and post check-up with SWMM can be adopted; whilefor regions without enough data, empirical values and post check-up with SWMM.canbe used;2) pipe design flow: for the rainfall pipe after the intercepting manhole, if the manhole does not intercept the peak discharge, the design flow can be calculated byusing current method, if the manhole does intercept part of the peak discharge, thedesign flow can be calculated by deducting the intercepted flow; the design flow ofoutlet pipe of storage tank is subject to the receiving capacity of the downstreamwastewater pipes and facilityies, the flow balance among intercepting pipe, sewage pipeand storage tank inlet pipe by using remaining capacity of waste water tratement orsewage pipe need to be paid attention to; and3) Volume of storage tank can becalculated based on the formulaV t0dtandSWMMsimulation.(3) Types that can be selected of intercepting wells of intercepted separate drainagesystem are proposed, and the design method for weir-intercepting well (include jumpingweir-intercepting well) and three intercepting control methods for the double-layerintercepting well are proposed.(4) Cost function optimized model for two stages that are preliminary design stageand construction drawing design stage are proposed on intercepted separate drainagesystem.(5) The design processes are discussed on intercepted separate drainage system,and actual design calculations for separate drainage system are made in a region inChongqing by using SWMM model and the above theory, and the study methods areproved to be feasible.