Study on Combined Sewer Overflow (CSO) Pollution and Its Control Method Based on Sewage Interception Along Rivers

. Combined sewer overflow (CSO) pollution is the main factor that plagues the water environment in the area with combined sewer system in most cities of China. This paper analyzed CSO pollution, and reviewed the measures to solve CSO pollution and the latest research progress. The problems of traditional river interception projects were analyzed, and optimization and improvement ideas were proposed. Finally, a combined optimization scheme of source reduction-process control-end storage was proposed to control CSO pollution, which could provide reference for urban CSO pollution control in China.


Introduction
Due to the limitation and shortage of early planning and construction, combined sewer system is usually adopted in old urban areas in many cities, and there are a large number of mixed connections in the area where separate sewer system is adopted. In recent years, in order to solve the problems of confluence and mixed flow, the administrative departments have vigorously promoted the rainwater and sewage separation reconstruction from residential areas to municipal area. However, due to the narrow site, large traffic impact, wide social scope, construction difficulties and high investment, the CSO pollution problem has not been completely solved, which is still one of the most important problems of urban water environment.
Moreover, although China has developed a series of measures and projects for CSO pollution control, but the incomplete systems, single means, backward technology and other problems limit systematical treatment of the CSO pollution. Therefore, integrating the CSO control technology to comprehensively solve the CSO pollution is in urgent demand.

Several situations of CSO pollution 2.1 Overflow pollution in rainwater outlet
There are usually a large number of rainwater discharge outlets around urban water bodies, and upstream of these outlets there are combined rainwater and sewage from combined sewer system and mixed sewage from separate sewer system. Sewage interception pipes are designed and set up for outlet interception along rivers. The interceptors intercept the mixed rainwater in the combined system pipes, mixed sewage and initial rainwater in the diversion system rainwater pipes. During the dry season, the interception pipes transport all the intercepted sewage to the sewage treatment plant. During the rainy season, only a certain number of rainwater can be intercepted, and the excess is discharged through the overflow of the interception well, causing water pollution.

Overflow pollution of rainwater pumping station and combined sewage pumping station
The combined sewage pumping station is usually built in the combined sewer system area, which adopts the same forebay. In the dry season, the sewage pump is opened to transport the sewage, while in the rainy season, the rainwater pump is opened to discharge the mixed rainwater and sewage to rivers. In the area with serious mixed connections, a large amount of sewage will be mixed in the rainwater pumping station. Although it has a drainage pump, if the drainage is not timely or the sewage system does not work well, the sewage will not be transported and the perennial high water level phenomenon of the rainwater pumping station will occur. When rainfall occurs, sewage is discharged into the river with the opening of the rainwater pumping station, causing pollution in rivers.

Overflow pollution of sewage treatment plant
Due to the mixed connection of rainwater and sewage pipelines, rainwater enters the sewage pipeline, significantly increasing the amount of sewage collected by the drainage pipe networks. This mixed water far exceeds the processing capacity of the corresponding sewage treatment plant, and part of it is usually directly discharged into the receiving water bodies during rainfall.

Adopting separated sewer system
Changing the combined sewer system into the separate sewer system [1][2] can theoretically solve the problem of sewage overflow pollution completely, and the amount of sewage to be treated in the sewage treatment plant is greatly reduced after renovation, and the sewage plant has a stable influent concentration and influent volume. However, the renovation is often slow and difficult to advance due to the large investment and construction difficulties. Urban areas mainly adopted combined sewer system have the characteristics of narrow roads, tight pipe spaces, dense population and traffic, making the renovation more difficult. For example, in the Chicago metropolitan area of the United States, if all the combined-flow system is converted into the separate system, it requires an investment of $2.2 billion, and the roads may be destroyed by the new pipeline for several to a dozen years [3].
However, the separate sewer system does not completely solve the problem of discharging sewage into the river, even bringing more serious pollution. In areas where the separation of rainwater and sewage is not complete, the mixed sewage is directly connected to the water bodies through rainwater outlets, especially when flooded outflow is not easy to detect. In addition, the initial rainwater in the separate sewer system also causes serious pollution of urban water bodies.

Decentralized source control
Decentralized source control technology adopts decentralized facilities such as rain gardens, depressed green areas, vegetated shallow trenches, rainwater wetlands, and rainwater ponds at the source of the catchment surface to achieve the goals of peak reduction, emission reduction, and water quality purification, thereby reducing the frequency and pollution load of downstream CSOs [4][5] and intercepting more sewage and initial rainwater to the wastewater treatment plant for treatment. Tang et al [6] took an area in Beijing (catchment area of about 240 hm 2 ) as an example, and found that the implementation of source control measures delayed the onset of peak flows (by about 10 min and 5 min, respectively), significantly reduced total runoff (by about 74% and 57%, respectively) and peak flows (by about 61% and 29%, respectively), and had higher reduction efficiency for rainfall events with smaller return periods. The City of Portland has undertaken a series of sourcedecentralized rainwater control projects to address CSO pollution in addition to the construction of pump stations and storage tunnels, such as the Rainfall Pipe Disconnect Program, the Green Streets Program, and the Green Roofs Program, and will remain committed to the widespread promotion of LID and green infrastructure and the renewal and maintenance of the pipe system in the future to further improve the quality of the water environment [7]. Although the effect of LID facilities on CSO pollution control is relatively obvious, but there are not enough cases of large-scale application of LID facilities. The limitations include space constraints, geographical factors and management difficulties, which affects the widely promoting of LID in old urban areas in China.

Process (in-situ) control measures
More and more in-situ treatment equipment and technologies are widely used for CSO pollution control, which are applied in a site-specific, economical and efficient manner. In-situ treatment of combined rainwater can be a cost-effective method to remove floating materials, settleable solids, bacteria and other pollutants in situ to the maximum extent. In recent years, pipe flushing technology, flow regulating gate technology, real-time control technology, online monitoring technology and in-situ treatment technology for CSO pollution such as overflow interceptor, cyclone separator, wetland and pond have been widely researched and applied.
Considering the high water level operation of the pipes exhibited by serious siltation of urban drainage pipes in China, the use of pipe flushing technology can be used to flush the combined pipes regularly during the dry season and flush the pipe siltation to the sinkhole or the front end of the sewage treatment plant for treatment, which can not only improve the drainage capacity of the drainage pipes in the dry season, but also reduce the frequency of sewage overflow during the rainy season, thus reducing CSO pollution. This technical measure is relatively easy to install, and the equipment is installed in the pipe without additional land, which is important for improving the capacity of urban drainage network. Intelligent detection facilities are set up in the pipeline system to monitor the water level, water volume and siltation level in the pipeline, especially in the interceptor, and analyze this information through the control software to control the opening of pumps, valves, weir doors and other devices to make full use of the available volume in the pipeline to reduce CSO pollution is also the development direction of in-situ control. Cyclone separators are also widely used in foreign countries for CSO in-situ treatment. The treated combined sewage water quality is significantly improved, and SS and COD removal rates can reach 36% to 90% and 15% to 80%, respectively [8].
Although in situ treatment equipment and technology to reduce CSO pollution has a high efficiency, but the equipment is more scattered, part of the equipment by the site conditions and the impact of fluctuations in water quality, can not effectively play the best efficacy, but also difficult to make CS0 pollution control efficiency to further improve.

End control system
The end control system is mainly through the construction of sewage interception pipes along rivers serving as the last line of defense and set up storage facilities to effectively reduce CSO pollution. Sewage interception system along rivers mainly include the main interception pipe and end storage pools laid along rivers. Storage facilities can adopt storage pools or storage tunnels. The interception pipe along rivers can intercept the sewage and initial rain from the direct sewage outlet, the direct rainwater outlet and the mixed flow according to the actual situation. In the dry season, the sewage from the sewage outlets and mixed outlets is intercepted to the sewage treatment plant for treatment, and during rainfalls, the sewage from the sewage outlets, rainwater outlets and mixed outlets and initial rainwater are intercepted to the STPs, and the excessive combined sewage which cannot be received and treated by the STPs is discharged into the rainwater storage tank for storage.
Chen et al. [9] proposed to solve the overflow pollution problem of the combined flow system in the Dongsha Lake area of Wuhan City by implementing diversion system renovation and urban rainwater runoff management in the long term to solve the pollution problem at the source; in the near term, the method of "dry flow sewage and initial rainwater interception and treatment, medium term rainwater extended runoff time to the Yangtze River, and late term rainwater discharge into the lake for storage" was adopted. In the near future, the approach of "dry flow sewage and initial rainwater interception treatment, medium rainwater extended runoff time to the Yangtze River, and late rainwater discharge into the lake for storage" is adopted to alleviate water pollution by reducing the number of opening of outfalls and cutting the discharge. In order to solve the pollution problem of Bailang River in Weifang City, Zhu Wei [10] proposed to lay interceptor culverts on both sides of the river to intercept sewage and transport it to the downstream pretreatment station for centralized treatment to reduce the pollution to the river. At present, the transfer tank storage program has been widely carried out in China, tunnel storage in China is still less applied. Some cities in China plan to solve the flooding problem and combined flow system overflow (CSO) pollution by building a tunnel scheme. Lu et al [11] selected some tunnels in some countries, summarized and analyzed their functions, categories, construction and design principles, and selected three types of typical application cases, introduced their construction background, program characteristics, scale effects, etc., and made some suggestions for the application of rainwater control tunnels in China, in order to provide some reference for the decision making and planning of rainwater control tunnels in some cities in China.
Tthe interceptor system along rivers gives full play to its end interceptor protection function under the conditions of difficult urban storm water diversion, slow and cumbersome management of LID facilities, and limited in-situ treatment. Not only can guarantee the mixed flow sewage does not enter the river, but also can solve the problem of primary rain pollution, and easy to implement, in a certain period of time and some areas are still not lost as an effective solution.

Problems of intercepting sewage system along rivers
Interceptor combined flow drainage system, if set up unreasonably, often affects the operation of the entire pipe network system and reduces the water quality of sewage treatment plant influent, thus affecting the effect of water environment management. The following is a brief introduction to the more prominent common problems that exist in traditional interceptor pipes.

River backflow
Interceptor well as an important structure in the intercepting combined flow system drainage system, the commonly used forms of interceptor wells are trough, weir, and trough-weir combination [12]. In the plain river network area, because the rainwater pipeline needs to be laid in accordance with a certain slope plus the high water level in the urban water system, the bottom elevation of the rainwater discharge outlet is mostly below the normal water level, and the outlet is in a submerged state. When the river is flooded, the drawbacks of traditional interceptor wells are even more prominent, and river water often occurs to back up to the phenomenon of slotted interceptor wells. Although the weir type interceptor well can prevent the backflow problem, but the use of weir type interceptor well weir top is too high and will affect the normal drainage of the flooding area, local flooding. Traditional backflow prevention measures mostly use flapper door or duckbill valve, but in practice, due to the clogging of debris, sewage corrosion, rubber aging and other factors, the flapper door or duckbill valve does not close tightly, serious leakage and can not be opened normally, which seriously affects the effect of backflow prevention, and often requires repair and replacement due to clogging of debris, with a large maintenance workload.
Setting up hydraulic anti-backflow adjustment weir system.
By setting up hydraulic anti-backflow adjusting weir system and ultrasonic level meter to monitor the water level before and after the weir, the weir door will always be fully closed during the dry season and the first light rainfall, so that the sewage and the first rainfall will be intercepted to the interceptor trunk pipe; when the flooding is carried out, the hydraulic anti-backflow adjusting weir will be opened and the rainfall will overflow into the natural water body at the later stage. Monitor the liquid level behind the weir to ensure that the top of the weir is higher than the liquid level behind the weir to prevent the backflow.

Failure to accurately intercept pollution
In the traditional design of sewage interception pipes along rivers, the combined pipe and the interception pipe share the interceptor well, which is located at the intersection of the two pipes. The traditional interception method makes the already intercepted sewage diluted repeatedly by the rainwater intercepted afterwards, and eventually the sewage treatment plant cannot treat the influent water with high concentrations of pollutants, and most of the sewage overflows to the river through the interceptor well, especially in the area where the combined sewer system and the separate sewer system coexist, and the collected sewage is also diluted continuously by the interceptor well, being unable to achieve the expected effect of the separate sewer system.
(1) Improving the interception system. Through the small diameter connection pipe to separate the interceptor trunk pipe and the interceptor well, the intercepted sewage flows directly into the interceptor trunk pipe to avoid the collected sewage being diluted again by the downstream rainwater, and the interception amount of water can be determined according to the design depth of the connection pipe and pipe diameter. Although the improved interception system can intercept mixed sewage and initial rainwater to a greater extent, if the connection pipe cannot be controlled, a large amount of rainwater will still enter the interceptor trunk pipe and occupy the pipe space so that the downstream sewage cannot be connected properly. By setting an electric flow restriction valve on the connection pipe and using ultrasonic level meter to detect the difference of liquid level before and after the flow control valve, flow control is realized.
(2) Additional flow limiting valve. At present, there is also a new flow limiting device swirl valve. Swirl valve relies only on the flow of water to achieve the purpose of flow control, relying solely on the principles of hydrodynamics and aerodynamics of automatic operation. In the engineering projects such as Shenzhen's Futian River, Dasha River, Fengtang River interception projects, the valve was applied [13].

Sewage interception pipe clogging
The parameter of sewage interception pipe is usually calculated according to the high daily high time sewage volume in the dry season, and the diameter of pipe is determined according to a certain interception multiplier or a certain production flow in the rainy season. Due to the large difference between dry season and rainy season flow, the sediment in the interceptor trunk pipe is very serious, and most of the urban combined flow system pipes in China have not been flushed for a long time, so not only the quantity of sediment is large, but also its form and type are more complicated, and it may occur hardening, slabbing and forming larger particles, and even there are some construction garbage and stones that are difficult to be flushed out, so the influence of pipe sediment on CSO pollutant transport is more complicated and significant [14].
(1) Changing the form of pipe culvert. The use of egg-shaped or special shaped pipe design can effectively reduce pipe blockage. Studies of other countries have shown that when the design of the combined flow system pipeline is egg-shaped, there is almost no sediment [15][16].
(2) Carrying out the pipeline scouring and desilting Self-siphoning hydraulic dredging device was used for dredging. Peng et al. [17] studied that the installation of self-siphoning hydraulic desilting device in the interceptor wells in the river interception system project of Hebei Peninsula in Liuzhou City could effectively remove the deposition of high organic pollution sediments in the territory, restore the water crossing section of the intake pipe, and effectively reduce the risk of high organic pollution sediments scouring into the river under rainfall conditions. The barrage shield flushing system was used to form a flushing wave effect by suddenly opening the barrage shield, and the water stored in the upstream pipeline formed a strong sweeping flow to flush the sediment away from the bottom of the pipeline and transport it out. Fan et al. [18] studied the use of an interceptor storage flushing system to flush a DN1500 inverted siphon sewer trunk pipeline in a city, which was put into operation at the end of 2012 and was operated well.

Conclusions
Due to site conditions and management factors, it is difficult to control CSOs pollution by one technology alone, so a comprehensive control model of "sourceprocess -end" is proposed. Terminal sewage interception system is the last line of defense for the control of CSOs pollution, while the treatment measures need to be combined with rainwater and sewage diversion, LID facilities, in-situ treatment systems, etc. Additional interceptor trunk pipes and storage pond systems along both banks of urban water bodies can intercept mixed sewage during dry days, overflow sewage and initial rainwater during rainfalls and transport them to sewage treatment plants for treatment, thus significantly reducing the pollution of the urban rivers. In response to the problems including river backflow, failure to accurately intercept pollution, and pipeline blockage of the traditional interceptor, improving the traditional interceptor system by adopting targeted measures such as intelligent interceptor wells and intercepting storage shields were proposed to improve the concentration of incoming water from sewage plants and reduce sewage overflow pollution.