sanitary sewer overflow

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In the USA, most wastewater utilities collect wastewater through a collection system that is independent of runoff collection networks. Combined sewer systems that are common in Europe are seen less frequently in the USA. Thus, properly designed, operated and maintained sanitary sewer systems are meant to collect and transport all of the sewage that flows into them to a publicly owned treatment works. However, occasional unintentional discharges of raw sewage from municipal sanitary sewers occur in almost every system. A sanitary sewer overflow (SSO) is defined as discharge from a sanitary sewer system at any point upstream of a sewage treatment plant. Thus, an SSO is any release of untreated sewage into basements, out of manholes, onto city streets and playgrounds, and into streams, before it can reach a treatment facility. Health risks occur from direct and indirect exposure to the SSO.
The US Environmental Protection agency estimates that at least 40,000 SSO events occur each year. SSOs have a variety of causes including, but not limited to, blockages in sewer lines and breaks in the sewer lines, in addition to severe weather, improper system operation and maintenance, and vandalism. Some municipalities have found severe problems necessitating billion-dollar correction programs, and often communities have to curtail new development until problems are corrected or systems capacity is increased.
Better operation and maintenance practices will resolve many of the problems that lead to SSOs. Some approaches have been pointed out. However, they may still not be adequate to recognize a pending release or an ongoing SSO, especially ion poorly accessible areas. And it is possible to identify an ongoing SSO by analyzing flow a...

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...e-specified level of detection. As shown, the trade off between the number of detections and the number of meters is easily evaluated, and the approach allows the decision maker to define the confidence level of detection. An approach was successful for a highly metered small system. Clearly, some field testing and verification are needed to fine tune the control rules in the field as real-time data is collected. It would also be of interest to examine the effects of the system size and collector sizes and slopes on the metering network design. The impact of scaling of flows may also change the number of meters necessary to achieve a desired level of confidence in SSO or blockage detection. Finally, the set-covering methodology can likely be extended to assist in identifying meter locations for detecting combined sewer system overflows and water quality events.

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