A combined sewer is a type of
gravity sewer
A gravity sewer is a conduit utilizing the energy resulting from a difference in elevation to remove unwanted water. The term ''sewer'' implies removal of sewage or surface runoff rather than water intended for use;''Design and Construction of San ...
with a system of pipes, tunnels, pump stations etc. to transport
sewage
Sewage (or domestic sewage, domestic wastewater, municipal wastewater) is a type of wastewater that is produced by a community of people. It is typically transported through a sewer system. Sewage consists of wastewater discharged from residenc ...
and
urban runoff
Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain , storms and other precipit ...
together to a
sewage treatment
Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding envir ...
plant or disposal site. This means that during rain events, the sewage gets diluted, resulting in higher flowrates at the treatment site. Uncontaminated
stormwater
Stormwater, also spelled storm water, is water that originates from precipitation (storm), including heavy rain and meltwater from hail and snow. Stormwater can soak into the soil ( infiltrate) and become groundwater, be stored on depressed la ...
simply dilutes sewage, but runoff may dissolve or suspend virtually anything it contacts on roofs, streets, and storage yards.
As rainfall travels over roofs and the ground, it may pick up various contaminants including
soil
Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...
particles and other
sediment
Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand an ...
, heavy metals,
organic compounds
In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
, animal waste, and
oil
An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
and
grease. Combined sewers may also receive dry weather drainage from landscape
irrigation
Irrigation (also referred to as watering) is the practice of applying controlled amounts of water to land to help grow Crop, crops, Landscape plant, landscape plants, and Lawn, lawns. Irrigation has been a key aspect of agriculture for over 5,00 ...
, construction
dewatering, and washing
buildings and
sidewalks
A sidewalk (North American English), pavement (British English), footpath in Australia, India, New Zealand and Ireland, or footway, is a path along the side of a street, highway, terminals. Usually constructed of concrete, pavers, brick, stone ...
.
Combined sewers can cause serious
water pollution
Water pollution (or aquatic pollution) is the contamination of water bodies, usually as a result of human activities, so that it negatively affects its uses. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. Water ...
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 as 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.
Mitigation of combined sewer overflows include sewer separation, CSO storage, expanding sewage treatment capacity,
retention basins, screening and disinfection facilities, reducing stormwater flows,
green infrastructure
Green infrastructure or blue-green infrastructure refers to a network that provides the “ingredients” for solving urban and climatic challenges by building with nature.Hiltrud Pötz & Pierre Bleuze (2011). Urban green-blue grids for sustainab ...
and real-time
decision support systems
A decision support system (DSS) is an information system that supports business or organizational decision-making activities. DSSs serve the management, operations and planning levels of an organization (usually mid and higher management) and h ...
.
This type of
gravity sewer
A gravity sewer is a conduit utilizing the energy resulting from a difference in elevation to remove unwanted water. The term ''sewer'' implies removal of sewage or surface runoff rather than water intended for use;''Design and Construction of San ...
design is less often used nowadays when constructing new sewer systems. Modern-day sewer designs exclude surface runoff by building
sanitary sewer
A sanitary sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings (but not stormwater) to a sewage treatment plant or disposal. Sanitary sewers are a type of gravity sewer and are part of an o ...
s instead, but many older cities and towns continue to operate previously constructed combined sewer systems.
Development
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.,
chamber pots, into town and city streets and slaughter animals in open street "
shambles
Shambles is an obsolete term for an open-air slaughterhouse and meat market.
Shambles or The Shambles may also refer to:
*The Shambles, a historic street in York, England
*Shambles Square, Manchester, England
*Shambles Glacier, Adelaide Island, A ...
". The use of draft animals such as horses and herding of livestock through city streets meant that most contained large amounts of excrement. Before the development of
macadam
Macadam is a type of road construction, pioneered by Scottish engineer John Loudon McAdam around 1820, in which crushed stone is placed in shallow, convex layers and compacted thoroughly. A binding layer of stone dust (crushed stone from the o ...
as a paving material in the 19th century, paving systems were mostly porous, so that precipitation could soak away and not run off, and urban rooftop rainwater was often saved in rainwater tanks. 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, while streets and footpaths were increasingly covered with impermeable paving systems. Most sewage collection systems of the 19th and early to mid-20th century used single-pipe systems that collect both sewage and
urban runoff
Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain , storms and other precipit ...
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.
Most cities at that time did not have
sewage treatment
Sewage treatment (or domestic wastewater treatment, municipal wastewater treatment) is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable for discharge to the surrounding envir ...
plants, so there was no perceived public health advantage in constructing a separate "surface water sewerage" (UK terminology) or "
storm sewer
A storm drain, storm sewer (United Kingdom, 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 surface ...
" (US terminology) system.
Moreover, before the automobile era, runoff was likely to be typically highly contaminated with animal waste. Further, until the mid-late 19th century the frequent use of shambles contributed more waste. The widespread replacement of horses with automotive propulsion, paving of city streets and surfaces, construction of municipal slaughterhouses, 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 previously saved rooftop rainwater after combined sewers were already widely adopted.
When constructed, combined sewer systems were typically sized to carry three
to 160 times the average dry weather sewage flows.
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
In hydrology, snowmelt is surface runoff produced from melting snow. It can also be used to describe the period or season during which such runoff is produced. Water produced by snowmelt is an important part of the annual water cycle in many part ...
or
convective precipitation
In meteorology, the different types of precipitation often include the character, formation, or phase of the precipitation which is falling to ground level. There are three distinct ways that precipitation can occur. Convective precipitation is g ...
. 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
Headworks is a civil engineering term for any structure at the head or diversion point of a waterway. It is smaller than a barrage and is used to divert water from a river into a canal or from a large canal into a smaller canal. at Web archive
An ...
.
Combined sewer overflows (CSOs)
![Anacostia combined sewer outflow 2018](https://upload.wikimedia.org/wikipedia/commons/7/78/Anacostia_combined_sewer_outflow_2018.jpg)
These relief structures, called "storm-water regulators" (in
American English
American English, sometimes called United States English or U.S. English, is the set of variety (linguistics), varieties of the English language native to the United States. English is the Languages of the United States, most widely spoken lan ...
- or "combined sewer overflows" in
British English
British English (BrE, en-GB, or BE) is, according to Lexico, Oxford Dictionaries, "English language, English as used in Great Britain, as distinct from that used elsewhere". More narrowly, it can refer specifically to the English language in ...
) are constructed in combined sewer systems to divert flows in excess of the peak design flow of the sewage treatment plant.
[ 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.]
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
An outfall is the discharge point of a waste stream into a body of water; alternatively it may be the outlet of a river, drain or a sewer where it discharges into the sea, a lake or ocean.
In the United States, industrial facilities that discha ...
, 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.
The storm water component contributes pollutants to CSO; but a major faction of pollution is the first foul flush of accumulated biofilm
A biofilm comprises any syntrophic consortium of microorganisms in which cells stick to each other and often also to a surface. These adherent cells become embedded within a slimy extracellular matrix that is composed of extracellular ...
and sanitary solids scoured from the dry weather wetted perimeter
The wetted perimeter is the perimeter of the cross sectional area that is "wet". The length of line of the intersection of channel wetted surface with a cross sectional plane normal to the flow direction. The term wetted perimeter is common in ci ...
of combined sewers during peak flow turbulence
In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...
. 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
A fecal coliform (British: faecal coliform) is a facultatively anaerobic, rod-shaped, gram-negative, non-sporulating bacterium. Coliform bacteria generally originate in the intestines of warm-blooded animals. Fecal coliforms are capable of growth ...
from pet and wildlife waste, and pesticide
Pesticides are substances that are meant to control pests. This includes herbicide, insecticide, nematicide, molluscicide, piscicide, avicide, rodenticide, bactericide, insect repellent, animal repellent, microbicide, fungicide, and lampri ...
s 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.
Comparison to sanitary sewer overflows
CSOs differ from sanitary sewer overflow
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. Cause ...
s in that the latter are caused by sewer system obstructions, damage, or flows in excess of sewer capacity (rather than treatment plant capacity.) 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
About 860 communities in the US have combined sewer systems, serving about 40million people. Pollutants from CSO discharges can include bacteria
Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...
and other pathogen
In biology, a pathogen ( el, πάθος, "suffering", "passion" and , "producer of") in the oldest and broadest sense, is any organism or agent that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ ...
s, toxic
Toxicity is the degree to which a chemical substance or a particular mixture of substances can damage an organism. Toxicity can refer to the effect on a whole organism, such as an animal, bacterium, or plant, as well as the effect on a subst ...
chemicals, and debris. These pollutants have also been linked with antimicrobial resistance
Antimicrobial resistance (AMR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. All classes of microbes can evolve resistance. Fungi evolve antifungal resistance. Viruses evolve antiviral resistance. ...
, posing serious public health concerns. The U.S. Environmental Protection Agency
The Environmental Protection Agency (EPA) is an Independent agencies of the United States government, independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon pro ...
(EPA) issued a policy in 1994 requiring municipalities to make improvements to reduce or eliminate CSO-related pollution problems.[EPA (1994-04-19)]
"Combined Sewer Overflow (CSO) Control Policy."
''Federal Register
The ''Federal Register'' (FR or sometimes Fed. Reg.) is the official journal of the federal government of the United States that contains government agency rules, proposed rules, and public notices. It is published every weekday, except on feder ...
'', 59 FR 18688. 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
A publicly owned treatment works (POTW) is a term used in the United States for a sewage treatment plant owned, and usually operated, by a government agency. In the U.S., POTWs are typically owned by local government agencies, and are usually desig ...
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. In 2000 Congress
A congress is a formal meeting of the representatives of different countries, constituent states, organizations, trade unions, political parties, or other groups. The term originated in Late Middle English to denote an encounter (meeting of a ...
amended the Clean Water Act
The Clean Water Act (CWA) is the primary federal law in the United States governing water pollution. Its objective is to restore and maintain the chemical, physical, and biological integrity of the nation's waters; recognizing the responsibiliti ...
to require the municipalities to comply with the EPA policy.
Mitigation of CSOs
Mitigation of combined sewer overflows include sewer separation, CSO storage, expanding sewage treatment capacity, retention basins, screening and disinfection facilities, reducing stormwater flows, green infrastructure
Green infrastructure or blue-green infrastructure refers to a network that provides the “ingredients” for solving urban and climatic challenges by building with nature.Hiltrud Pötz & Pierre Bleuze (2011). Urban green-blue grids for sustainab ...
and real-time decision support systems
A decision support system (DSS) is an information system that supports business or organizational decision-making activities. DSSs serve the management, operations and planning levels of an organization (usually mid and higher management) and h ...
. For example, cities with combined sewer overflows employ one or more engineering
Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...
approaches to reduce discharges of untreated sewage, including:
* utilizing a green infrastructure
Green infrastructure or blue-green infrastructure refers to a network that provides the “ingredients” for solving urban and climatic challenges by building with nature.Hiltrud Pötz & Pierre Bleuze (2011). Urban green-blue grids for sustainab ...
approach to improve storm water management capacity throughout the system, and reduce the hydraulic
Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...
overloading of the treatment plant
* repair and replacement of leaking and malfunctioning equipment
* increasing overall hydraulic capacity of the sewage collection system (often a very expensive option).
The United Kingdom Environment Agency
The Environment Agency (EA) is a non-departmental public body, established in 1996 and sponsored by the United Kingdom government's Department for Environment, Food and Rural Affairs, with responsibilities relating to the protection and enha ...
identified unsatisfactory intermittent discharges and issued an Urban Wastewater Treatment Directive requiring action to limit pollution from combined sewer overflows. In 2009, the Canadian Council of Ministers of the Environment The Canadian Council of Ministers of the Environment (CCME) is an inter-governmental organization in Canada with members from the federal government, ten provincial governments and three territorial governments. Membership is at the Ministerial lev ...
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 the frequency of combined sewer overflows.
Rehabilitation of combined sewer systems to mitigate CSOs require extensive monitoring networks which are becoming more prevalent with decreasing sensor and communication costs. These monitoring networks can identify bottlenecks causing the main CSO problem, or aid in the calibration of hydrodynamic or hydrological models to enable cost effective CSO mitigation.
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
Michigan () is a state in the Great Lakes region of the upper Midwestern United States. With a population of nearly 10.12 million and an area of nearly , Michigan is the 10th-largest state by population, the 11th-largest by area, and the ...
was estimated at more than 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 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.
Many other areas in the US are undertaking similar projects (see, for example, in the Puget Sound of Washington). Cities like Pittsburgh
Pittsburgh ( ) is a city in the Commonwealth (U.S. state), Commonwealth of Pennsylvania, United States, and the county seat of Allegheny County, Pennsylvania, Allegheny County. It is the most populous city in both Allegheny County and Wester ...
, Seattle
Seattle ( ) is a seaport city on the West Coast of the United States. It is the seat of King County, Washington. With a 2020 population of 737,015, it is the largest city in both the state of Washington and the Pacific Northwest regio ...
, Philadelphia
Philadelphia, often called Philly, is the largest city in the Commonwealth of Pennsylvania, the sixth-largest city in the U.S., the second-largest city in both the Northeast megalopolis and Mid-Atlantic regions after New York City. Sinc ...
, and New York
New York most commonly refers to:
* New York City, the most populous city in the United States, located in the state of New York
* New York (state), a state in the northeastern United States
New York may also refer to:
Film and television
* '' ...
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. In 2011, Washington, D.C.
)
, image_skyline =
, image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
, separated its sewers in four small neighborhoods at a cost of $11 million. (The project cost also included improvements to the drinking water
Drinking water is water that is used in drink or food preparation; potable water is water that is safe to be used as drinking water. The amount of drinking water required to maintain good health varies, and depends on physical activity level, a ...
piping system.)
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. 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
)
, image_skyline =
, image_caption = Clockwise from top left: the Washington Monument and Lincoln Memorial on the National Mall, United States Capitol, Logan Circle, Jefferson Memorial, White House, Adams Morgan, ...
., 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
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. ...
, that will reduce overflows to the river by 98 percent, and 96 percent system-wide. The system will comprise over of tunnels with a storage capacity of . The first segment of the tunnel system, in length, went online in 2018. The remaining segments of the storage system are scheduled for completion in 2023. (The city's overall "Clean Rivers" project, projected to cost $2.6 billion, includes other components, such as reducing stormwater flows.) The South Boston CSO Storage Tunnel is a similar project, completed in 2011.
Indianapolis
Indianapolis (), colloquially known as Indy, is the state capital and most populous city of the U.S. state of Indiana and the seat of Marion County. According to the U.S. Census Bureau, the consolidated population of Indianapolis and Marion ...
, Indiana, is building underground storage capacity in the form of a diameter deep rock tunnel system which will connect the two existing wastewater treatment plants, and provide collection of discharge water from the various CSO sites located along the White River, Eagle Creek, Fall Creek, Pogue's Run
Pogue's Run is an urban creek that starts near the intersection of Elizabeth Street and Lennington Drive on the east side of Indianapolis, Indiana, and empties into the White River south of the Kentucky Avenue bridge over that river. At the strea ...
, and Pleasant Run. Citizens Energy Group is managing the efforts to construct the first phases of the work, which includes a deep Deep Rock Tunnel Connector between the Belmont Wastewater Treatment Plant and the Southport Wastewater Treatment Plant. Additional tunnels will branch under the existing watercourses located in Indianapolis. The planned cost for the project will total $1.9 billion dollars.
Fort Wayne
Fort Wayne is a city in and the county seat of Allen County, Indiana, United States. Located in northeastern Indiana, the city is west of the Ohio border and south of the Michigan border. The city's population was 263,886 as of the 2020 Censu ...
, Indiana, is constructing a , diameter, $180M tunnel under the 3RPORT (Three Rivers Protection and Overflow Reduction Tunnel) to address the myriad CSOs which outfall into the St. Mary's, St. Joseph
Joseph is a common male given name, derived from the Hebrew Yosef (יוֹסֵף). "Joseph" is used, along with "Josef", mostly in English, French and partially German languages. This spelling is also found as a variant in the languages of the mo ...
, and Maumee River
The Maumee River (pronounced ) ( sjw, Hotaawathiipi; mia, Taawaawa siipiiwi) is a river running in the United States Midwest from northeastern Indiana into northwestern Ohio and Lake Erie. It is formed at the confluence of the St. Joseph and ...
s. The 3RPORT is approximately below grade, and is anticipated to enter service in 2023.
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
Toledo ( ) is a city in and the county seat of Lucas County, Ohio, United States. A major Midwestern United States port city, Toledo is the fourth-most populous city in the state of Ohio, after Columbus, Cleveland, and Cincinnati, and according ...
, 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.)
Retention basins
Retention treatment basins or large concrete tanks
A tank is an armoured fighting vehicle intended as a primary offensive weapon in front-line ground combat. Tank designs are a balance of heavy firepower, strong armour, and good battlefield mobility provided by tracks and a powerful engine ...
that store and treat combined sewage are another solution. These underground structures can range in storage and treatment capacity from to 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
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. Conse ...
compartment captures and stores flows with the highest level of pollutants from the first part of a storm. These pollutants include motor oil
Motor oil, engine oil, or engine lubricant is any one of various substances used for the lubrication of internal combustion engines. They typically consist of base oils enhanced with various additives, particularly antiwear additives, deterg ...
, sediment, road salt
Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...
, and lawn
A lawn is an area of soil-covered land planted with grasses and other durable plants such as clover which are maintained at a short height with a lawnmower (or sometimes grazing animals) and used for aesthetic and recreational purposes. L ...
chemicals (pesticides and fertilizer
A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
s) 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
A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than st ...
by injecting sodium hypochlorite
Sodium hypochlorite (commonly known in a dilute solution as bleach) is an Inorganic chemistry, inorganic chemical compound with the chemical formula, formula NaOCl (or NaClO), comprising a sodium cation () and a hypochlorite anion (or ). It may ...
, 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.
The City of Detroit
Detroit ( , ; , ) is the largest city in the U.S. state of Michigan. It is also the largest U.S. city on the United States–Canada border, and the seat of government of Wayne County. The City of Detroit had a population of 639,111 at th ...
, Michigan, utilizes a system of nine CSO retention basins and screening/disinfection facilities that are owned and operated by the Great Lakes Water Authority
The Great Lakes Water Authority (GLWA) is a regional water authority in the U.S. state of Michigan. It provides drinking water treatment, drinking water distribution, wastewater collection, and wastewater treatment services for the Southeast Michi ...
. These basins are located at original combined sewer outfalls located along the Detroit River
The Detroit River flows west and south for from Lake St. Clair to Lake Erie as a strait in the Great Lakes system. The river divides the metropolitan areas of Detroit, Michigan, and Windsor, Ontario, Windsor, Ontario—an area collectively refe ...
and Rouge River within metropolitan Detroit. These facilities are generally designed to contain two inches of stormwater runoff, with the ability to disinfect overflows during extreme wet-weather rainfall events.
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.
Reducing stormwater flows
Communities may implement low impact development techniques to reduce flows of stormwater into the collection system. This includes:
*constructing new and renovated streets, parking lots and sidewalks with interlocking stones, permeable paving
Permeable paving surfaces are made of either a porous material that enables stormwater to flow through it or nonporous blocks spaced so that water can flow between the gaps. Permeable paving can also include a variety of surfacing techniques ...
and pervious concrete
Pervious concrete (also called porous concrete, permeable concrete, no fines concrete and porous pavement) is a special type of concrete with a high porosity used for concrete flatwork applications that allows water from precipitation and other ...
*installing green roof
A green roof or living roof is a roof of a building that is partially or completely covered with vegetation and a growing medium, planted over a waterproofing membrane. It may also include additional layers such as a root barrier and drainage ...
s on buildings
*installing bioretention
Bioretention is the process in which contaminants and sedimentation are removed from stormwater runoff. The main objective of the bioretention cell is to attenuate peak runoff as well as to remove stormwater runoff pollutants.
Construction of ...
systems, also called rain garden
Rain gardens, also called bioretention facilities, are one of a variety of practices designed to increase rain runoff reabsorption by the soil. They can also be used to treat polluted stormwater runoff. Rain gardens are designed landscape sites t ...
s, in landscaped areas
*installing rainwater harvesting
Rainwater harvesting (RWH) is the collection and storage of rain, rather than allowing it to run off. Rainwater is collected from a roof-like surface and redirected to a tank, cistern, deep pit (well, shaft, or borehole), aquifer, or a reservoir w ...
equipment to collect runoff from building roofs during wet weather for irrigating landscapes and gardens during dry weather
*implementing graywater
Greywater (or grey water, sullage, also spelled gray water in the United States) refers to domestic wastewater generated in households or office buildings from streams without fecal contamination, i.e., all streams except for the wastewater fro ...
collection and use on site to reduce sewage discharges at all times
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
Green infrastructure or blue-green infrastructure refers to a network that provides the “ingredients” for solving urban and climatic challenges by building with nature.Hiltrud Pötz & Pierre Bleuze (2011). Urban green-blue grids for sustainab ...
. 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.
Real-time decision support systems
Recent technological advances in sensing and control have enabled the implementation of real-time decision support systems
A decision support system (DSS) is an information system that supports business or organizational decision-making activities. DSSs serve the management, operations and planning levels of an organization (usually mid and higher management) and h ...
(RT-DSS) for CSO mitigation. Through the use of internet of things
The Internet of things (IoT) describes physical objects (or groups of such objects) with sensors, processing ability, software and other technologies that connect and exchange data with other devices and systems over the Internet or other comm ...
technology and cloud computing
Cloud computing is the on-demand availability of computer system resources, especially data storage ( cloud storage) and computing power, without direct active management by the user. Large clouds often have functions distributed over mul ...
, 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
Adaptive traffic control system (ATCS) is a traffic management strategy in which traffic signal timing changes, or adapts, based on actual traffic demand. This is accomplished using an adaptive traffic control system consisting of both hardware ...
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 and Europe.
Real-time control (RTC) can be either heuristic
A heuristic (; ), or heuristic technique, is any approach to problem solving or self-discovery that employs a practical method that is not guaranteed to be optimal, perfect, or rational, but is nevertheless sufficient for reaching an immediate, ...
or model based. Model-based control is theoretically more optimal, but due to the ease of implementation, heuristic control is more commonly applied. Generating sufficient evidence that RTC is a suitable option for CSO mitigation remains problematic, although new performance methods might make this possible.
Regulations
United Kingdom
There is in the UK a legal difference between a storm sewer and a surface water sewer. There is no right of connection to a storm-water overflow sewer under section 106 of the Water Industry Act.
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 there is a right of connection for 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.
History
Combined sewer systems were common when urban sewerage systems were first developed, in the late 19th and early 20th centuries.
Society and culture
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
Resistance movements during World War II occurred in every occupied country by a variety of means, ranging from non-cooperation to propaganda, hiding crashed pilots and even to outright warfare and the recapturing of towns. In many countries, r ...
in World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
. Fighting erupted in the sewers during the Battle of Stalingrad
The Battle of Stalingrad (23 August 19422 February 1943) was a major battle on the Eastern Front of World War II where Nazi Germany and its allies unsuccessfully fought the Soviet Union for control of the city of Stalingrad (later re ...
. The only survivors from the Warsaw Uprising
The Warsaw Uprising ( pl, powstanie warszawskie; german: Warschauer Aufstand) was a major World War II operation by the Polish resistance movement in World War II, Polish underground resistance to liberate Warsaw from German occupation. It occ ...
and Warsaw Ghetto
The Warsaw Ghetto (german: Warschauer Ghetto, officially , "Jewish Residential District in Warsaw"; pl, getto warszawskie) was the largest of the Nazi ghettos during World War II and the Holocaust. It was established in November 1940 by the G ...
made their final escape through city sewers. Some have commented that the engraving
Engraving is the practice of incising a design onto a hard, usually flat surface by cutting grooves into it with a Burin (engraving), burin. The result may be a decorated object in itself, as when silver, gold, steel, or Glass engraving, glass ...
s of imaginary prison
A prison, also known as a jail, gaol (dated, standard English, Australian, and historically in Canada), penitentiary (American English and Canadian English), detention center (or detention centre outside the US), correction center, correc ...
s by Piranesi were inspired by the Cloaca Maxima
The Cloaca Maxima ( lat, Cloāca Maxima, lit. ''Greatest Sewer'') was one of the world's earliest sewage systems. Its name derives from Cloacina, a Roman goddess. Built during either the Roman Kingdom or early Roman Republic, it was constructed ...
, one of the world's earliest sewers.
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
''Teenage Mutant Ninja Turtles'' is an American media franchise created by the comic book artists Kevin Eastman and Peter Laird. It follows Leonardo (Teenage Mutant Ninja Turtles), Leonardo, Michelangelo (Teenage Mutant Ninja Turtles), Miche ...
, Stephen King's '' It'', ''Les Misérables
''Les Misérables'' ( , ) is a French historical novel by Victor Hugo, first published in 1862, that is considered one of the greatest novels of the 19th century.
In the English-speaking world, the novel is usually referred to by its original ...
'', ''The Third Man
''The Third Man'' is a 1949 British film noir directed by Carol Reed, written by Graham Greene and starring Joseph Cotten, Alida Valli, Orson Welles, and Trevor Howard. Set in postwar Vienna, the film centres on American Holly Martins (Cotten), ...
'', '' Ladyhawke,'' ''Mimic
MIMIC, known in capitalized form only, is a former simulation computer language developed 1964 by H. E. Petersen, F. J. Sansom and L. M. Warshawsky of Systems Engineering Group within the Air Force Materiel Command at the Wright-Patterson AFB in ...
'', ''The Phantom of the Opera
''The Phantom of the Opera'' (french: Le Fantôme de l'Opéra) is a novel by French author Gaston Leroux. It was first published as a serial in from 23 September 1909 to 8 January 1910, and was released in volume form in late March 1910 by Pierr ...
'', ''Beauty and the Beast
''Beauty and the Beast'' (french: La Belle et la Bête) is a fairy tale written by French novelist Gabrielle-Suzanne de Villeneuve, Gabrielle-Suzanne Barbot de Villeneuve and published in 1740 in ''La Jeune Américaine et les contes marins'' ( ...
'', and ''Jet Set Radio Future
is a 2002 action game developed by Smilebit and published by Sega for the Xbox; it is a sequel to the Dreamcast game ''Jet Set Radio'' (2000). As a re-imagining of the original game, it features improved gameplay mechanics, updated graphics, larg ...
''. The Todd Strasser novel '' Y2K-9: the Dog Who Saved the World'' is centered on a dog thwarting terroristic threat A terroristic threat is a threat to commit a crime of violence or a threat to cause bodily injury to another person and terrorization as the result of the proscribed conduct. Several U.S. states have enacted state law, statutes which impose criminal ...
s to electronically sabotage American sewage treatment plants.
Sewer alligators
A well-known urban legend
An urban legend (sometimes contemporary legend, modern legend, urban myth, or urban tale) is a genre of folklore comprising stories or fallacious claims circulated as true, especially as having happened to a "friend of a friend" or a family m ...
, the sewer alligator
An alligator is a large reptile in the Crocodilia order in the genus ''Alligator'' of the family Alligatoridae. The two extant species are the American alligator (''A. mississippiensis'') and the Chinese alligator (''A. sinensis''). Additiona ...
, is that of giant alligators or crocodile
Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term crocodile is sometimes used even more loosely to inclu ...
s 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
A manhole (utility hole, maintenance hole, or sewer hole) is an opening to a confined space such as a shaft, utility vault, or large vessel. Manholes are often used as an access point for an underground public utility, allowing inspection, m ...
.
Alligators have been known to get into combined storm sewers in the southeastern United States. Closed-circuit television
Closed-circuit television (CCTV), also known as video surveillance, is the use of video cameras to transmit a signal to a specific place, on a limited set of monitors. It differs from broadcast television in that the signal is not openly t ...
by a sewer repair company captured an alligator in a combined storm sewer on tape.YouTube – Bad sewer pipes across America
/ref>
See also
*Fatberg
A fatberg is a rock-like mass of waste matter in a sewer system formed by the combination of flushed non-biodegradable solids, such as wet wipes, and fat, oil and grease (FOG) deposits. The handling of FOG waste and the buildup of its deposits ...
(sewer obstruction)
*Sanitary sewer overflow
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. Cause ...
*Thames Tideway Scheme
The Thames Tideway Tunnel is a combined sewer under construction running mostly under the tidal section (estuary) of the River Thames across Inner London to capture, store and convey almost all the raw sewage and rainwater that currently ove ...
*Storm drain
A storm drain, storm sewer (United Kingdom, United States, U.S. and Canada), surface water drain/sewer (United Kingdom), or stormwater drain (Australia and New Zealand) is infrastructure designed to Drainage, drain excess rain and ground water ...
References
External links
U.S. EPA – Combined Sewer Overflows
{{DEFAULTSORT:Combined Sewer
Environmental engineering
Hydraulic engineering
Sewerage infrastructure
Water pollution