Combined sewer

A combined sewer is a sewage collection system of pipes and tunnels designed to simultaneously collect surface runoff and sewage water in a shared system. This type of gravity sewer design is no longer used in almost every instance worldwide when constructing new sewer systems. Modern-day sewer designs exclude surface runoff from sanitary sewers, but many older cities and towns continue to operate previously constructed combined sewer systems.[1]

Combined sewers can cause serious water pollution problems during combined sewer overflow (CSO) events when combined sewage and surface runoff flows exceed the capacity of the sewage treatment plant, or of the maximum flow rate of the system which transmits the combined sources. In instances where exceptionally high surface runoff occurs (such as large rainstorms), the load on individual tributary branches of the sewer system may cause a back-up to a point where raw sewage flows out of input sources such a toilets, causing inhabited buildings to be flooded with a toxic sewage-runoff mixture, incurring massive financial burdens for cleanup and repair. When combined sewer systems experience these higher than normal throughputs, relief systems cause discharges containing human and industrial waste to flow into rivers, streams, or other bodies of water. Such events frequently cause both negative environmental and lifestyle consequences, including beach closures, contaminated shellfish unsafe for consumption, and contamination of drinking water sources, rendering them temporarily unsafe for drinking and requiring boiling before uses such as bathing or washing dishes.[2]

CSO diagram US EPA
Combined sewer system. During dry weather (and small storms), all flows are handled by the publicly owned treatment works (POTW). During large storms, the relief structure allows some of the combined stormwater and sewage to be discharged untreated to an adjacent water body.


Recent archaeological discoveries have shown that some of the earliest sewer systems were developed 2500 BC in the ancient city of Harappa. The primitive sewers were carved in the ground alongside buildings. This discovery reveals the conceptual understanding of waste disposal by the early civilizations.[3]

The earliest sewers were designed to carry street runoff away from inhabited areas and into surface waterways without treatment. Before the 19th century it was commonplace to empty human waste receptacles, e.g., chamberpots, into town and city streets, while the use of draft animals such as horses and herding of livestock through city streets meant that most contained large amounts of excrement. Open sewers, consisting of gutters and urban streambeds, were common worldwide before the 20th century. In the majority of developed countries, large efforts were made during the late 19th and early 20th centuries to cover the formerly open sewers, converting them to closed systems with cast iron, steel, or concrete pipes, masonry, and concrete arches. Most sewage collection systems of the 19th and early to mid 20th century used single-pipe systems that collect both sewage and urban runoff from streets and roofs (to the extent that relatively clean rooftop rainwater was not saved in butts and cisterns for drinking and washing.) This type of collection system is referred to as a combined sewer system. The rationale for combining the two was that it would be cheaper to build just a single system.[4]:8 Most cities at that time did not have sewage treatment plants, so there was no perceived public health advantage in constructing a separate "surface water sewerage" (UK terminology) or "storm sewer" (US terminology) system.[2]:pp. 2–3 Moreover, runoff was, pre-automobile, likely to be typically highly contaminated with animal waste. The widespread replacement of horses with automotive propulsion, paving of city streets and surfaces, and provision of mains water in the 20th century changed the nature and volume of urban runoff to be initially cleaner, include water that formerly soaked away and to include previously saved rooftop rainwater after combined sewers were already widely adopted.

When constructed, combined sewer systems were typically sized to carry three[2]:pp. 2–4 to 160 times the average dry weather sewage flows.[5] It is generally infeasible to treat the volume of mixed sewage and surface runoff flowing in a combined sewer during peak runoff events caused by snowmelt or convective precipitation. As cities built sewage treatment plants, those plants were typically built to treat only the volume of sewage flowing during dry weather. Relief structures were installed in the collection system to bypass untreated sewage mixed with surface runoff during wet weather, protecting sewage treatment plants from damage caused if peak flows reached the headworks.[6]

Combined sewer overflows (CSOs)

Anacostia combined sewer outflow 2018
Combined sewer outflow into the Anacostia River in Washington, D.C.
Manhole Brighton
Photo of the interior of a combined sewer in Brighton, England.

These relief structures, called storm-water regulators (in American English - or combined sewer overflows in British English) are constructed in combined sewer systems to divert flows in excess of the peak design flow of the sewage treatment plant.[6] Combined sewers are built with control sections establishing stage-discharge or pressure differential-discharge relationships which may be either predicted or calibrated to divert flows in excess of sewage treatment plant capacity. A leaping weir may be used as a regulating device allowing typical dry-weather sewage flow rates to fall into an interceptor sewer to the sewage treatment plant, but causing a major portion of higher flow rates to leap over the interceptor into the diversion outfall. Alternatively, an orifice may be sized to accept the sewage treatment plant design capacity and cause excess flow to accumulate above the orifice until it overtops a side-overflow weir to the diversion outfall.[7]

CSO statistics may be confusing because the term may describe either the number of events or the number of relief structure locations at which such events may occur. A CSO event, as the term is used in American English, occurs when mixed sewage and stormwater are bypassed from a combined sewer system control section into a river, stream, lake, or ocean through a designed diversion outfall, but without treatment. Overflow frequency and duration varies both from system to system, and from outfall to outfall, within a single combined sewer system. Some CSO outfalls discharge infrequently, while others activate every time it rains.[2]:pp. 2–3, 2–4

The storm water component contributes pollutants to CSO; but a major faction of pollution is the first foul flush of accumulated biofilm and sanitary solids scoured from the dry weather wetted perimeter of combined sewers during peak flow turbulence.[8] Each storm is different in the quantity and type of pollutants it contributes. For example, storms that occur in late summer, when it has not rained for a while, have the most pollutants. Pollutants like oil, grease, fecal coliform from pet and wildlife waste, and pesticides get flushed into the sewer system. In cold weather areas, pollutants from cars, people and animals also accumulate on hard surfaces and grass during the winter and then are flushed into the sewer systems during heavy spring rains.

Health impacts

CSO discharges during heavy storms can cause serious water pollution problems. The discharges contain human and industrial waste, and can cause beach closings, restrictions on shellfish consumption and contamination of drinking water sources.[9]

Comparison to sanitary sewer overflows

CSOs should not be confused with sanitary sewer overflows. Sanitary sewer overflows are caused by sewer system obstructions, damage, or flows in excess of sewer capacity (rather than treatment plant capacity.)[2]:Ch.4 Sanitary sewer overflows may occur at any low spot in the sewer system rather than at the CSO relief structures. Absence of a diversion outfall often causes sanitary sewer overflows to flood residential structures and/or flow over traveled road surfaces before reaching natural drainage channels. Sanitary sewer overflows may cause greater health risks and environmental damage than CSOs if they occur during dry weather when there is no precipitation runoff to dilute and flush away sewage pollutants.

CSOs in the United States

CSO map EPA 2008
Most of the US combined sewer systems are in the Northeast and Great Lakes regions, and the Pacific Northwest.

About 860 communities in the US have combined sewer systems, serving about 40 million people.[10] Pollutants from CSO discharges can include bacteria and other pathogens, toxic chemicals, and debris. These pollutants have also been linked with antimicrobial resistance, posing serious public health concerns.[11] The U.S. Environmental Protection Agency (EPA) issued a policy in 1994 requiring municipalities to make improvements to reduce or eliminate CSO-related pollution problems.[12] The policy is implemented through the National Pollutant Discharge Elimination System (NPDES) permit program. The policy defined water quality parameters for the safety of an ecosystem; it allowed for action that are site specific to control CSOs in most practical way for community; it made sure the CSO control is not beyond a community’s budget; and allowed water quality parameters to be flexible, based upon the site specific conditions. The CSO Control Policy required all publicly owned treatment works to have ″nine minimum controls″ in place by January 1, 1997, in order to decrease the effects of sewage overflow by making small improvements in existing processes.[13] In 2000 Congress amended the Clean Water Act to require the municipalities to comply with the EPA policy.[14]

Mitigation of CSOs

The United Kingdom Environment Agency identified unsatisfactory intermittent discharges and issued an Urban Wastewater Treatment Directive requiring action to limit pollution from combined sewer overflows.[15] In 2009 the Canadian Council of Ministers of the Environment adopted a Canada-wide Strategy for the Management of Municipal Wastewater Effluent including national standards to (1) remove floating material from combined sewer overflows, (2) prevent combined sewer overflows during dry weather, and (3) prevent development or redevelopment from increasing frequency of combined sewer overflows.[16]

Municipalities in the US have been undertaking projects to mitigate CSO since the 1990s. For example, prior to 1990, the quantity of untreated combined sewage discharged annually to lakes, rivers and streams in southeast Michigan was estimated at more than 30 billion US gallons (110,000,000 m3) per year. In 2005, with nearly $1 billion of a planned $2.4 billion CSO investment put into operation, untreated discharges have been reduced by more than 20 billion US gallons (76,000,000 m3) per year. This investment that has yielded an 85 percent reduction in CSO has included numerous sewer separation, CSO storage and treatment facilities and wastewater treatment plant improvements constructed by local and regional governments.[17]

Many other areas in the US are undertaking similar projects (see, for example, in the Puget Sound of Washington).[18] Cities like Pittsburgh, Seattle, Philadelphia, and New York are focusing on these projects partly because they are under federal consent decrees to solve their CSO issues. Both up-front penalties and stipulated penalties are utilized by EPA and state agencies to enforce CSO-mitigating initiatives and the efficiency of their schedules. Municipalities' sewage departments, engineering and design firms, and environmental organizations offer different approaches to potential solutions.

Sewer separation

Some US cities have undertaken sewer separation projects — building a second piping system for all or part of the community. In many of these projects, cities have been able to separate only portions of their combined systems. High costs or physical limitations may preclude building a completely separate system.[19] In 2011 Washington, D.C. separated its sewers in four small neighborhoods at a cost of $11 million. (The project cost also includes improvements to the drinking water piping system.)[20][21]

CSO storage

Another solution is to build a CSO storage facility, such as a tunnel that can store flow from many sewer connections. Because a tunnel can share capacity among several outfalls, it can reduce the total volume of storage that must be provided for a specific number of outfalls. Storage tunnels store combined sewage but do not treat it. When the storm is over, the flows are pumped out of the tunnel and sent to a wastewater treatment plant.[17] One of the main concerns with CSO storage is the length of time it is stored before it is released. Without careful management of this storage period, the water in the CSO storage facility runs the risk of going septic.

Washington, D.C. is building underground storage capacity as its primary strategy to address CSOs. In 2011 the city began construction on a system of four deep storage tunnels, adjacent to the Anacostia River, that will reduce overflows to the river by 98 percent, and 96 percent system-wide. The system will comprise over 18 miles of tunnels with a storage capacity of 157 million gallons.[22] The first segment of the tunnel system, 7 miles in length, went online in 2018. The remaining segments of the storage system are scheduled for completion in 2023.[23] (The city's overall "Clean Rivers" project, projected to cost $2.6 billion, includes other components, such as reducing stormwater flows.)[24] The South Boston CSO Storage Tunnel is a similar project, completed in 2011.

Expanding sewage treatment capacity

Some cities have expanded their basic sewage treatment capacity to handle some or all of the CSO volume. In 2002 litigation forced the city of Toledo, Ohio to double its treatment capacity and build a storage basin in order to eliminate most overflows. The city also agreed to study ways to reduce stormwater flows into the sewer system. (See Reducing stormwater flows.)[25]

Retention basins

Retention treatment basins or large concrete tanks that store and treat combined sewage are another solution. These underground structures can range in storage and treatment capacity from 2 million US gallons (7,600 m3) to 120 million US gallons (450,000 m3) of combined sewage. While each facility is unique, a typical facility operation is as follows. Flows from the overloaded sewers are pumped into a basin that is divided into compartments. The first flush compartment captures and stores flows with the highest level of pollutants from the first part of a storm. These pollutants include motor oil, sediment, road salt, and lawn chemicals (pesticides and fertilizers) that are picked up by the stormwater as it runs off roads and lawns. The flows from this compartment are stored and sent to the wastewater treatment plant when there is capacity in the interceptor sewer after the storm. The second compartment is a treatment or flow-through compartment. The flows are disinfected by injecting sodium hypochlorite, or bleach, as they enter this compartment. It then takes about 20‑30 minutes for the flows to move to the end of the compartment. During this time, bacteria are killed and large solid materials settle out. At the end of the compartment, any remaining sanitary trash is skimmed off the top and the treated flows are discharged into the river or lake.[17]

Screening and disinfection facilities

Screening and disinfection facilities treat CSO without ever storing it. Called "flow-through" facilities, they use fine screens to remove solids and sanitary trash from the combined sewage. Flows are injected with sodium hypochlorite for disinfection and mixed as they travel through a series of fine screens to remove debris. The fine screens have openings that range in size from 4 to 6 mm, or a little less than a quarter inch. The flow is sent through the facility at a rate that provides enough time for the sodium hypochlorite to kill bacteria. All of the materials removed by the screens are then sent to the sewage treatment plant through the interceptor sewer.[26]

Reducing stormwater flows

Communities may implement low impact development techniques to reduce flows of stormwater into the collection system. This includes:

Gray vs. green infrastructure

CSO mitigating initiatives that are solely composed of sewer system reconstruction are referred to as gray infrastructure, while techniques like permeable pavement and rainwater harvesting are referred to as green infrastructure. Conflict often occurs between a municipality's sewage authority and its environmentally active organizations between gray and green infrastructural plans.

The 2004 EPA Report to Congress on CSO's provides a review of available technologies to mitigate CSO impacts.[2]:Ch. 8

New approaches

Smart infrastructure

Recent technological advances in sensing and control have enabled the implementation of real-time decision support systems (RT-DSS) for CSO mitigation. Through the use of internet of things technology and cloud computing, CSO events can now be mitigated by dynamically adjusting setpoints for movable gates, pump stations, and other actuated assets in sewers and storm water management systems. Similar technology, called adaptive traffic control is used to control the flow of vehicles through traffic lights. RT-DSS systems take advantage of storm temporal and spatial variability as well as varying concentration times due to diverse land uses across the sewershed to coordinate and optimize control assets. By maximizing storage and conveyance RT-DSS are able to minimize overflows using existing infrastructure. Successful implementations of RT-DSS have been carried out throughout the United States [27][28][29] and Europe.[30]


As a product of the Industrial Revolution, many cities in Europe and North America grew in the 19th century, frequently leading to crowding and increasing concerns about public health.[31][32] As part of a trend of municipal sanitation programs in the late 19th and 20th centuries, many cities constructed extensive sewer systems to help control outbreaks of disease such as typhoid and cholera.[33]:29–34 Initially these systems discharged sewage directly to surface waters without treatment.[31] As pollution of water bodies became a concern, cities added sewage treatment plants to their systems. Most cities in the Western world added more expensive systems for sewage treatment in the early 20th century.[31][34]

As Britain was the first country to industrialize, it was also the first to experience the disastrous consequences of major urbanisation and was the first to construct a sewerage system as we know it today to mitigate the resultant unsanitary conditions.[31] Joseph Bazalgette designed an extensive underground sewerage system that diverted waste to the Thames Estuary, downstream of the main centre of population. Six main interceptor sewers, totalling almost 135 miles (217 km) in length, were constructed. The intercepting sewers, constructed between 1859 and 1865, were fed by 450 miles (720 km) of main sewers that, in turn, conveyed the contents of some 13,000 miles (21,000 km) of smaller local sewers.[35] With only minor modifications, Bazalgette's engineering achievement remains the basis for sewerage design up into the present day.[36]

In France, the Paris cholera epidemic of 1832 sharpened the public awareness of the necessity for some sort of drainage system to deal with sewage and waster water in a better and healthier way since the Seine received up to 100,000 cubic meters of wastewater per day. Between 1865 and 1920 Eugene Belgrand led the development of a large scale system for water supply and wastewater management.[31] By 1894 laws were passed which made drainage mandatory. The treatment of Paris sewage, though, was left to natural devices as 5,000 hectares of land were used to spread the waste out to be naturally purified.[37]

Society and culture

A combined sewer-pipe being laid by the city's sewerage company in Ghent, Belgium.

The image of the sewer recurs in European culture as they were often used as hiding places or routes of escape by the scorned or the hunted, including partisans and resistance fighters in World War II. Fighting erupted in the sewers during the Battle of Stalingrad. The only survivors from the Warsaw Uprising and Warsaw Ghetto made their final escape through city sewers. Some have commented that the engravings of imaginary prisons by Piranesi were inspired by the Cloaca Maxima, one of the world's earliest sewers.


United Kingdom

There is in the UK a legal difference between a storm sewer and a surface water sewer. You do not have a right of connection to a storm-water overflow sewer under section 106 of the Water Industry Act.[38]

These are normally the pipe line that discharges to a watercourse, downstream of a combined sewer overflow. It takes the excess flow from a combined sewer. A surface water sewer conveys rainwater; legally you have a right of connection for your rainwater to this public sewer. A public storm water sewer can discharge to a public surface water, but not the other way around, without a legal change in sewer status by the water company.

In fiction

The theme of traveling through, hiding, or even residing in combined sewers is a common plot device in media. Famous examples of sewer dwelling are the Teenage Mutant Ninja Turtles, Stephen King's It, Les Miserables, The Third Man, Ladyhawke, Mimic, The Phantom of the Opera, Beauty and the Beast, and Jet Set Radio Future. The Todd Strasser novel Y2K-9: the Dog Who Saved the World is centered on a dog thwarting terroristic threats to electronically sabotage American sewage treatment plants.

Sewer alligators

A well-known urban legend, the sewer alligator, is that of giant alligators or crocodiles residing in combined sewers, especially of major metropolitan areas. Two public sculptures in New York depict an alligator dragging a hapless victim into a manhole.[39]

Alligators have been known to get into combined storm sewers in the southeastern United States. Closed-circuit television by a sewer repair company captured an alligator in a combined storm sewer on tape.[40]

See also


  1. ^ Metcalf & Eddy, Inc. (1972). Wastewater Engineering. New York: McGraw–Hill. p. 119.
  2. ^ a b c d e f Report to Congress: Impacts and Control of CSOs and SSOs (Report). Washington, D.C.: U.S. Environmental Protection Agency (EPA). August 2004. EPA-833-R-04-001.
  3. ^ "Brief History of Sewers in the Ancient World."
  4. ^ Burrian, Steven J.; et al. (1999). The Historical Development of Wet-Weather Flow Management (PDF) (Report). EPA. EPA/600/JA-99/275.
  5. ^ Lawler, Joseph C. (1969). Design and Construction of Sanitary and Storm Sewers. American Society of Civil Engineers and Water Pollution Control Federation. p. 136.
  6. ^ a b Okun, Daniel A. (1959). Sewage Treatment Plant Design. American Society of Civil Engineers and Water Pollution Control Federation. p. 6.
  7. ^ Lawler, Joseph C. (1969). Design and Construction of Sanitary and Storm Sewers. American Society of Civil Engineers and Water Pollution Control Federation. pp. 112–114.
  8. ^ Fan, Chi-Yuan; Field, Richard; Lai, Fu-hsiung. "Sewer-Sediment Control: Overview of an EPA Wet-Weather Flow Research Program" (PDF). University of California Los Angeles. United States Environmental Protection Agency. Retrieved 12 March 2016.
  9. ^ Report to Congress on Impacts and Control of Combined Sewer Overflows and Sanitary Sewer Overflows; Fact Sheet (Report). EPA. August 2004. EPA 833-R-04-001.
  10. ^ "Combined Sewer Overflow Frequent Questions". National Pollutant Discharge Elimination System. EPA. 2017-12-20.
  11. ^ Dhiman, Gaurav; Burns, Emma N.; Morris, David W. (October 2016). "Using Multiple Antibiotic Resistance Profiles of Coliforms as a Tool to Investigate Combined Sewer Overflow Contamination". Journal of Environmental Health. 79.3: 36–39. ISSN 0022-0892.
  12. ^ EPA (1994-04-19). "Combined Sewer Overflow (CSO) Control Policy." Federal Register, 59 FR 18688.
  13. ^ Perciasepe, Robert (1996-11-18). January 1, 1997, Deadline for Nine Minimum Controls in Combined Sewer Overflow Control Policy (Memorandum) (Report). EPA.
  14. ^ United States. Wet Weather Quality Act of 2000, Section 112 of Division B, Pub.L. 106–554, December 21, 2000. Added section 402(q) to Clean Water Act, 33 U.S.C. § 1342(q).
  15. ^ "Combined Sewer Overflows" (PDF). Stockton-on-Tees, UK: Thompson Research–Project Management Ltd. Archived from the original (PDF) on 2006-11-11.
  16. ^ Canada-wide Strategy for the Management of Municipal Wastewater Effluent (PDF) (Report). Canadian Council of Ministers of the Environment. 2009-02-17.
  17. ^ a b c Investment in Reducing Combined Sewer Overflows Pays Dividends (PDF) (Report). Detroit, MI: Southeast Michigan Council of Governments. September 2008. pp. 1–6.
  18. ^ Combined Sewer Overflow Control Program: Frequently Asked Questions (PDF) (Report). Seattle, WA: Seattle Public Utilities. 2012. Archived from the original (PDF) on 2013-05-15.
  19. ^ Combined Sewer Overflow Management Fact Sheet: Sewer Separation (PDF) (Report). EPA. September 1999. EPA-832-F-99-041.
  20. ^ DC Water Clean Rivers Project: Rock Creek Sewer Separation (PDF) (Report). District of Columbia Water and Sewer Authority (DCWASA). 2010. Archived from the original (PDF) on 2016-08-27.
  21. ^ Long Term Control Plan Consent Decree Status Report: Quarter No. 2 - 2011 (PDF) (Report). DCWASA. July 2011. p. 10. Archived from the original (PDF) on 2016-08-27.
  22. ^ "Clean Rivers Project". DCWASA. Retrieved 2018-03-05.
  23. ^ "DC Water's Anacostia River Tunnel beating all projections for a cleaner Anacostia". DCWASA. 2018-09-21.
  24. ^ Clean Rivers Project News: Combined Sewer Overflow Control Activities (PDF) (Report). DCWASA. October 2011. Biannual Report.
  25. ^ EPA (2002-08-28). "United States and Ohio Reach Clean Water Act Settlement with City of Toledo, Ohio." Press release.
  26. ^ Combined Sewer Overflow Technology Fact Sheet: Screens (PDF) (Report). EPA. September 1999. EPA 832-F-99-040.
  27. ^ L. Montestruque, M. Lemmon (2015). "Globally Coordinated Distributed Storm Water Management System." 1st International Workshop on Cyber Physical Systems for Smart Water Networks, doi:10.1145/2738935.2738948.
  28. ^ "Going Against the Flow: Green Tech, Sensors and Industrial Internet Make Sewer Systems Smart". Txchnologist. General Electric. 2013. Retrieved October 16, 2015.
  29. ^ Roy, Steve; Quigley, Marcus; Raymond, Chuck (2013-10-03). "Rainwater Harvesting–Controls in the Cloud". New England Facilities Development News. Pembroke, MA: High Profile Monthly.
  30. ^ Vezzaro, L. and Grum, M. (2012). "A generalized Dynamic Overflow Risk Assessment (DORA) for urban drainage RTC." Proceedings of the 9th International Conference on Urban Drainage Modelling,
  31. ^ a b c d e Abellán, Javier (2017). "Water supply and sanitation services in modern Europe: developments in 19th-20th centuries". 12th International Congress of the Spanish Association of Economic History: University of Salamanca, Spain.
  32. ^ Steven J. Burian, Stephan J. Nix, Robert E. Pitt, and S. Rocky Durrans (2000). "Urban Wastewater Management in the United States: Past, Present, and Future." Journal of Urban Technology, Vol. 7, No. 3, pp. 33-62. doi:10.1080/713684134.
  33. ^ Cady Staley, George S. Pierson (1899). The Separate System of Sewerage, Its Theory and Construction. (New York: Van Nostrand.)
  34. ^ Benidickson, Jamie (2011). The Culture of Flushing: A Social and Legal History of Sewage. UBC Press. ISBN 9780774841382. Retrieved 2013-02-07.
  35. ^ Goodman, David C. and Chant, Colin (1999) European Cities and Technology (London: Routledge).
  36. ^ Kendall F. Haven (2006). 100 Greatest Science Inventions of All Time. Libraries Unlimited. pp. 148–149.
  37. ^ George Commair, "The Waste Water Network: and underground view of Paris," in Great Rivers History: Proceedings and Invited Papers for the EWRI Congress and History Symposium, May 17–19, 2009, Kansas City, Missouri, ed. Jerry R. Roger, (Reston: American Society of Civil Engineers, 2009), 91-96
  38. ^ United Kingdom. Water Industry Act 1991, c. 56. Section 106, "Right to communicate with public sewers." National Archives, UK. Accessed 2017-06-13.
  39. ^ Subway Art: New York's Underground Treasures : NPR
  40. ^ YouTube – Bad sewer pipes across America

External links

Anacostia River

The Anacostia River is a river in the Mid Atlantic region of the United States. It flows from Prince George's County in Maryland into Washington, D.C., where it joins with the Washington Channel to empty into the Potomac River at Buzzard Point. It is approximately 8.7 miles (14.0 km) long. The name "Anacostia" derives from the area's early history as Nacotchtank, a settlement of Necostan or Anacostan Native Americans on the banks of the Anacostia River.

Heavy pollution in the Anacostia and weak investment and development along its banks have led to it becoming what many have called "D.C.'s forgotten river." In recent years, however, private organizations, local businesses, and the D.C., Maryland and federal governments have made joint efforts to reduce its pollution levels in order to protect the ecologically valuable Anacostia watershed.

Becks Run

Becks Run is a tributary of the Monongahela River. As an urban stream, it is heavily polluted, receiving combined sewer outflow from Carrick (Pittsburgh) and Mount Oliver, Pennsylvania. There is a waterfall on a tributary, just downstream from a slate dump, near the intersection of Wagner Avenue and Mountain Avenue. There were coal mines along the stream, including Becks Run #2, owned by the estate of James H. Hays, served by an incline and the H.B. Hays and Brothers Coal Railroad. Other mines at various times were operated by the Birmingham Coal Company, H.G. Burghman, Jones & Laughlin, and the Monongahela River Consolidated Coal and Coke Company.It is the namesake of the Pittsburgh and Beck's Run Railroad (1877-1880), which ran from the Smithfield Street Bridge to the Jones and Laughlin Iron Works, and was absorbed by the P&LE Railroad. A former town, located where Becks Run enters the Monongahela, was also named Becks Run.

East Side Big Pipe

The East Side Big Pipe is a large sewer line and tunnel in Portland in the U.S. state of Oregon. It is part of a combined sewer system of pipes, sumps, drains, pumps, and other infrastructure that transports sewage and stormwater run-off to the city's Columbia Boulevard Wastewater Treatment Plant. The East Side Big Pipe project, begun in 2006 and finished in 2011, was the largest of a 20-year series of projects designed to nearly eliminate combined sewer overflows (CSO)s into the Willamette River and the Columbia Slough. The combined projects were completed on time, and they reduced CSOs into the river by 94 percent and into the slough by more than 99 percent.

First flush

First flush is the initial surface runoff of a rainstorm. During this phase, water pollution entering storm drains in areas with high proportions of impervious surfaces is typically more concentrated compared to the remainder of the storm. Consequently, these high concentrations of urban runoff result in high levels of pollutants discharged from storm sewers to surface waters.

Marine outfall

A marine outfall is a pipeline or tunnel that discharges municipal or industrial wastewater, stormwater, combined sewer overflows, cooling water, or brine effluents from water desalination plants to the sea. Usually they discharge under the sea's surface (submarine outfall). In the case of municipal wastewater, effluent is often being discharged after having undergone no or only primary treatment, with the intention of using the assimilative capacity of the sea for further treatment. Submarine outfalls are common throughout the world and probably number in the thousands. More than 200 outfalls alone have been listed in a single international database maintained by the Institute for Hydromechanics at Karlsruhe University for the International Association of Hydraulic Engineering and Research (IAHR) / International Water Association (IWA) Committee on Marine Outfall Systems.The world's first marine outfall was built in Santa Monica, United States, in 1910. In Latin America and the Caribbean there were 134 outfalls with more than 500 m length in 2006 for wastewater disposal alone, according to a survey by the Pan American Center for Sanitary Engineering and Environmental Sciences (CEPIS) of PAHO. According to the survey, the largest number of municipal wastewater outfalls in the region exist in Venezuela (39), Chile (39) and Brazil (22). The world's largest marine outfall stems from the Deer Island Waste Water Treatment Plant located in Boston, United States.Currently, Boston has approximately 235 miles of combined sewers and 37 active CSO outfalls. Lots of outfalls are simply known by a public used name, e.g. Boston Outfall.

Milwaukee Metropolitan Sewerage District

The Milwaukee Metropolitan Sewerage District (MMSD) is a state-chartered government agency which provides wastewater services for 28 municipalities within Milwaukee County and also portions of the surrounding counties. It treats, and also releases, the largest amount of water pollution of any agency or company in the State of Wisconsin.With headquarters and a central laboratory along the Menomonee River near downtown Milwaukee, it has two wastewater treatment plants which are located at Jones Island (43°01′23.5″N 87°53′58″W) in Milwaukee and at the South Shore (42°53′16″N 87°50′44″W) in Oak Creek. These facilities were operated by United Water under a 10-year agreement ending March 1, 2008. Veolia Water is the current operator.

"The world’s first large scale wastewater treatment plant was constructed on Jones Island, near the shore of Lake

Michigan." The primary wastewater treatment plant at Jones Island was one of the first of its kind when the original activated sludge plant was constructed in 1925. MMSD was the first to market biosolids created through this process as a fertilizer under the name "Milorganite." The Jones Island Plant was among the first sewage treatment plants in the United States to succeed in using the activated sludge treatment process. "It was the first treatment facility to economically dispose of the recovered sludge by producing an organic fertilizer." In the early 1980s the plant needed extensive reworking, "this does not detract from its historic significance as a pioneering facility in the field of pollution control technology." It had the largest capacity of any plant in the world when constructed. The 1925 plant has been designated as a Historic Civil Engineering Landmark by the American Society of Civil Engineers.

Piney Branch

Piney Branch is a tributary of Rock Creek in Washington, D.C. It is the largest tributary located entirely within the Washington city limits.

River Westbourne

The Westbourne or Kilburn is a mainly re-diverted small River Thames tributary in London, rising in Hampstead and which, notwithstanding one main meander, flows southward through Kilburn and the Bayswater (west end of Paddington) to skirt underneath the east of Hyde Park's Serpentine lake then through central Chelsea under Sloane Square and it passes centrally under the south side of Royal Hospital Chelsea's Ranelagh Gardens before historically discharging into the Inner London Tideway. Since the latter 19th century its narrow basin has been further narrowed by corollary surface water drains and its main flow has been replaced with a combined sewer beneath its route.

Sanitary sewer

A sanitary sewer or foul sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings (but not stormwater) to treatment facilities or disposal. Sanitary sewers are part of an overall system called a sewage system or sewerage.

Sewage may be treated to control water pollution before discharge to surface waters. Sanitary sewers serving industrial areas also carry industrial wastewater.

Separate sanitary sewer systems are designed to transport sewage alone. In municipalities served by sanitary sewers, separate storm drains may convey surface runoff directly to surface waters. Sanitary sewers are distinguished from combined sewers, which combine sewage with stormwater runoff in one pipe. Sanitary sewer systems are beneficial because they avoid combined sewer overflows.

Sanitary sewer overflow

Not to be confused with combined sewer overflow (CSO)

Sanitary sewer overflow (SSO) is a condition in which untreated sewage is discharged from a sanitary sewer into the environment prior to reaching sewage treatment facilities. When caused by rainfall it is also known as wet weather overflow. It is primarily meaningful in developed countries, which have extensive treatment facilities. Frequent causes of SSO spills include:

Blockage of sewer lines

Infiltration/Inflow of excessive stormwater into sewer lines during heavy rainfall

Malfunction of pumping station lifts or electrical power failure

Broken sewer lines.SSOs can cause gastrointestinal illnesses, beach closures and restrictions on fish and shellfish consumption.


Sewage (or domestic wastewater or municipal wastewater) is a type of wastewater that is produced by a community of people. It is characterized by volume or rate of flow, physical condition, chemical and toxic constituents, and its bacteriologic status (which organisms it contains and in what quantities). It consists mostly of greywater (from sinks, tubs, showers, dishwashers, and clothes washers), blackwater (the water used to flush toilets, combined with the human waste that it flushes away); soaps and detergents; and toilet paper (less so in regions where bidets are widely used instead of paper).

Sewage usually travels from a building's plumbing either into a sewer, which will carry it elsewhere, or into an onsite sewage facility (of which there are many kinds). Whether it is combined with surface runoff in the sewer depends on the sewer design (sanitary sewer or combined sewer). The reality is, however, that most wastewater produced globally remains untreated causing widespread water pollution, especially in low-income countries: A global estimate by UNDP and UN-Habitat is that 90% of all wastewater generated is released into the environment untreated. In many developing countries the bulk of domestic and industrial wastewater is discharged without any treatment or after primary treatment only.

The term sewage is nowadays regarded as an older term and is being more and more replaced by "wastewater". In general American English usage, the terms "sewage" and "sewerage" mean the same thing. In common British usage, and in American technical and professional English usage, "sewerage" refers to the infrastructure that conveys sewage.

Sewage regulation and administration

Sewage disposal regulation and administration describes the governance of sewage disposal and treatment.


Sewer may refer to:

Part of sewerage, the infrastructure that conveys sewage

Sanitary sewer, a system of pipes used to transport sewage - several types of sanitary sewers can be distinguished

Storm drain, a collection and transportation system for storm water

Combined sewer

Sewer, one who does sewing

Keeper of sewer, official overseeing service to King Henry VIII's household

Sewers (album)


Sewerage is the infrastructure that conveys sewage or surface runoff (stormwater, meltwater, rainwater) using sewers. It encompasses components such as receiving drains, manholes, pumping stations, storm overflows, and screening chambers of the combined sewer or sanitary sewer. Sewerage ends at the entry to a sewage treatment plant or at the point of discharge into the environment. It is the system of pipes, chambers, manholes, etc. that conveys the sewage or storm water.

It is also an alternate noun for the word sewage.

In American colloquial English, "sewer system" is applied more frequently to the large infrastructure of sewers that British speakers more often refer to as "sewerage".

Storm drain

A storm drain, storm sewer (U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain (Australia and New Zealand) is infrastructure designed to drain excess rain and ground water from impervious surfaces such as paved streets, car parks, parking lots, footpaths, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems.

Drains receive water from street gutters on most motorways, freeways and other busy roads, as well as towns in areas with heavy rainfall that leads to flooding, and coastal towns with regular storms. Even gutters from houses and buildings can connect to the storm drain. Many storm drainage systems are gravity sewers that drain untreated storm water into rivers or streams—so it is unacceptable to pour hazardous substances into the drains.

Storm drains often cannot manage the quantity of rain that falls in heavy rains or storms. Inundated drains can cause basement and street flooding. In many areas require detention tanks inside a property that temporarily hold runoff in heavy rains and restrict outlet flow to the public sewer. This reduces the risk of overwhelming the public sewer. Some storm drains mix stormwater (rainwater) with sewage, either intentionally in the case of combined sewers, or unintentionally.

Tanner Creek

Tanner Creek is a small tributary of the Willamette River in Portland in the U.S. state of Oregon. Named after a tannery owned by one of the city's founders, it begins in what is now the Sylvan–Highlands neighborhood in the Tualatin Mountains (West Hills) west of downtown. In the 19th century the creek flowed on the surface, running northeast across the city, past what later became Providence Park and into a shallow lake (Couch Lake) and wetlands in what became the Pearl District, bordering the river.

Late in the century, the city began re-routing Tanner Creek and other West Hills streams into combined sewers and filling their former channels and basins to make flat land for homes and businesses. In the 21st century, Tanner Creek is nearly invisible, flowing through a conduit (but not a combined sewer) that empties into the Willamette at Outfall 11, near the Broadway Bridge. Structures along the former course of the creek include Vista Bridge and Tanner Springs Park as well as Providence Park.

Urban stream

An urban stream is a formerly natural waterway that flows through a heavily populated area. Urban streams are often polluted by urban runoff and combined sewer outflows. Water scarcity makes flow management in the rehabilitation of urban streams problematic.Governments may alter the flow or course of an urban stream to prevent localized flooding by river engineering: lining stream beds with concrete or other hardscape materials, diverting the stream into culverts and storm sewers, or other means. Some urban streams, such as the subterranean rivers of London, run completely underground. These modifications have often reduced habitat for fish and other species, caused downstream flooding due to alterations of flood plains, and worsened water quality.

Some communities have begun stream restoration projects in an attempt to correct the problems caused by alteration, using techniques such as daylighting and fixing stream bank erosion caused by heavy stormwater runoff. Streamflow augmentation to restore habitat and aesthetics is also an option, and recycled water can be used for this purpose.


Wastewater (or waste water) is any water that has been affected by human use. Wastewater is "used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff or stormwater, and any sewer inflow or sewer infiltration". Therefore, wastewater is a byproduct of domestic, industrial, commercial or agricultural activities. The characteristics of wastewater vary depending on the source. Types of wastewater include: domestic wastewater from households, municipal wastewater from communities (also called sewage) and industrial wastewater from industrial activities. Wastewater can contain physical, chemical and biological pollutants.

Households may produce wastewater from flush toilets, sinks, dishwashers, washing machines, bath tubs, and showers. Households that use dry toilets produce less wastewater than those that use flush toilets.

Wastewater may be conveyed in a sanitary sewer which conveys only sewage. Alternatively, it can be transported in a combined sewer which includes stormwater runoff and industrial wastewater. After treatment at a wastewater treatment plant, the treated wastewater (also called effluent) is discharged to a receiving water body. The terms "wastewater reuse" or "water reclamation" apply if the treated waste is used for another purpose. Wastewater that is discharged to the environment without suitable treatment causes water pollution.

In developing countries and in rural areas with low population densities, wastewater is often treated by various on-site sanitation systems and not conveyed in sewers. These systems include septic tanks connected to drain fields, on-site sewage systems (OSS), vermifilter systems and many more.

West Side CSO Tunnel

The West Side Combined Sewer Overflow Tunnel (also West Side Big Pipe) is a tunnel in Portland, Oregon, United States. It receives and stores overflow from the combined sewer system before it can reach the Willamette River. The main tunnel is 14 feet (4.3 m) in diameter and 3.5 miles (5.6 km) long for a capacity of 2,850,000 cubic feet (81,000 m3) and connects to dozens of smaller sewer overflow interceptors along the west side of the Willamette River.

The tunnel receives flows that might otherwise reach the river. Instead, the CSO tunnel transports them to the Swan Island Pump Station. Portland's 1930s sewer design combined street and surface runoff with sewage in a common system that was overwhelmed during heavy precipitation. The original system handled overflows by sending excess flow into the river.The tunnel is 120 to 160 feet (37 to 49 m) below ground level. It passes under the Willamette River between the NW Nicolai Street shaft (45.54072°N 122.69751°W / 45.54072; -122.69751 (Nicoli shaft)) to the confluent vertical shaft on Swan Island (45.55302°N 122.69565°W / 45.55302; -122.69565 (Swan Island confluent shaft)), which also receives the East Side Big Pipe. From Nicolai, it travels roughly south close to Front Avenue. There are vertical shafts at Upshur (45.53643°N 122.68642°W / 45.53643; -122.68642 (Upshur shaft)), Ankeny (45.52234°N 122.66992°W / 45.52234; -122.66992 (Ankeny shaft)), and Clay streets (45.51163°N 122.67530°W / 45.51163; -122.67530 (Clay Street shaft)). The Clay Street shaft receives the Southwest Parallel Interceptor, a 3-to-6-foot (0.91 to 1.83 m) pipeline which runs along the west Willamette shore for 3 miles (4.8 km) to Virginia Avenue and Taylors Ferry Road 45.47015°N 122.67240°W / 45.47015; -122.67240 (SW Parallel Interceptor south end).The project is a part of the Willamette River combined sewer overflow expansion program. Construction occurred from November 2002 to September 2006, and the project became fully operational in December 2006.A 20-year series of related CSO projects, including the West Side Big Pipe, culminated in late 2011 with completion of the East Side Big Pipe. The combined projects reduced the city's sewer overflows into the Willamette River by 94 percent and into the Columbia Slough by more than 99 percent. The total cost of the projects, about $1.4 billion, is being financed over time through additions to the Portland sewer rates. Almost no financial support for the projects came from state or Federal governments.

Construction materials
Related equipment
Liquids transported
Quality indicators
Treatment options
Disposal options

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