The combined sewer system is a kind of drainage system in which domestic sewage and rainwater are mixed and drained in the same pipe culvert. In ancient times or in the early developing stages of the city, most drainage systems were the combined sewer system which has the characteristics of a low cost, simple pipeline and convenient construction. For example, in the 19th century, the domestic sewage of some countries in Europe were all discharged by the rainwater pipe network. However, with the continuous development of the city, the amount of the discharge of sewage is increasing day by day and the composition of the sewage is becoming more and more complex. Therefore, the pollution problem caused by the combined rainwater in the combined sewage system is becoming more and more serious. The pollution in combined sewage is one of the main factors that leads to black and odorous water in cities and it is prevalent in areas with rapid urbanization, dense population and the load of heavy surface pollution [
1,
2]. In addition to the overflow pollution in areas with combined sewerage system, the mixed flow of rainwater and sewage also exist in areas with separated drainage systems, which results from the mixed connection and the aging of the rainwater and sewage pipe network. Many studies have found that the mixed discharge of rainwater and sewage could damage the water environment of the downstream river and cause black and odorous water. At the same time, emissions of pollutants and toxic substances contained in the combined sewage would threaten the health of residents [
3,
4]. In China, although all regions have fully recognized the negative impact of combined sewage pollution on the water environment, gray measures are still used to deal with this problem, which is not effective and needs difficult construction and a high engineering cost. Therefore, combined sewage pollution is still an urgent problem to be solved [
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6].
Interception of mixed rainwater is the most common control measure used in the transmission process or the end of the pipe network [
7,
8]. At present, many regions in China still adopt the traditional design of intercepting weir, which intercepts all sewage to the sewage treatment plant in sunny days and intercepts urban runoff and initial rainwater on rainy days. Besides, on rainy days, the combined sewage exceeding the designed closure rate of the intercepting manhole is discharged directly into the water body through the overflow pipe [
9]. As we know, the interception ratio is a key parameter that affects the closure effect [
10]. Practice has proved that the traditional intercepting manhole is difficult to control the pollution of mixed rainwater economically and efficiently [
11,
12]. This can be explained by the fact that the traditional interception weir is mostly based on the empirical and artificial water quantity calculation, while the interception of mixed rainwater possesses the characteristics of large regional difference, diverse pollutant sources and large concentration variations [
8,
13], which could lead to the overflow pollution when the intercepting amount is too small. Moreover, when the intercepting amount is too large, the traditional interception weir could result in low inflow concentration and the large flow rate of a sewage treatment plant, thus giving rise to the imbalance of sewage treatment operations. In order to solve the problem of combined rainwater pollution, the regulation of flow or water level is adopted in most practical projects and related research. Andrea Zimmer et al. studied the real-time hydraulic simulations coupled with control algorithms and explored a large number of potential changes to control procedures at short time intervals to provide dynamic feedback and optimization [
14]. Meneses et al. investigated the combining traditional infrastructure solutions for urban drainage with real-time control (RTC) strategies and realized the reduction of the storage volume expansion otherwise needed to fulfill the environmental regulations for combined sewer overflow (CSO) discharge [
15]. However, the practice shows that, to solve the problem of combined rainwater, more attention should be paid to the change of pollution concentration in rainwater and more urban drainage models are needed to realize the scheduling mechanism based on water quality and flow [
16,
17]. By using the United States Environmental Protection Agency’s (USEPA) storm water management model (SWMM) for the hydraulic simulations, Rathnayake et al. proposed a new multi-objective evolutionary optimization method, which can realize the active control of intermittent dissatisfied discharged from combined sewer system [
18]. Relevant studies have shown that the foundation of RTC based on the pollution process is available [
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21]. For example, the Lowell Regional Wastewater Utility had constructed and installed a comprehensive supervisory control and data acquisition (SCADA) system to manage and remotely operate the CSO stations based on real-time conditions, which are fairly sophisticated—typically including some flow screening, flow monitoring and CSO discharge pumping (when necessary) [
22]. However, there are still rare studies or applications that use the RTC method based on the change of mixed rainwater pollutant concentration.
Therefore, in this study, Infoworks ICM (Infoworks ICM 7.5, Innovyze, UK) and ARCGIS (ARCGIS 10.2, Environmental Systems Research Institute Inc., Redlands, CA, USA) were used to build a mixed connection network model of a certain area in Fuzhou and the difference between the traditional interception scheme of mixed sewage and the water quality interception strategy based on pollutant concentration was compared. The water quality interception strategy based on COD and NH4-N concentration was proposed to improve the intercepting efficiency. At the same time, the relationship between the pollution concentration characteristics of combined rainwater and the intercepting process control was described quantitatively, which can improve the reliability and economy of the intercepting design and provide reference for the intercepting control technology based on changes in pollutant concentration.