Gravity sewer

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Gravity sewer installation illustrating the significant excavation depth often required to maintain a favorable gradient. Sansewer.jpg
Gravity sewer installation illustrating the significant excavation depth often required to maintain a favorable gradient.
Gravity sewer flow as seen looking down into an open sanitary manhole. Manhole1.JPG
Gravity sewer flow as seen looking down into an open sanitary manhole.

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; [1] and the term gravity excludes water movement induced through force mains or vacuum sewers. Most sewers are gravity sewers because gravity offers reliable water movement with no energy costs wherever grades are favorable. Gravity sewers may drain to sumps where pumping is required to either force sewage to a distant location or lift sewage to a higher elevation for entry into another gravity sewer, and lift stations are often required to lift sewage into sewage treatment plants. Gravity sewers can be either sanitary sewers, combined sewers, storm sewers or effluent sewers.

Contents

Gravity sewer hydraulics

Gravity sewer systems typically resemble the regional runoff pattern with large trunk sewers in each valley receiving flow from smaller lateral sewers extending up hillsides. Sewer systems within comparatively level terrain require careful planning and construction to minimize energy losses in free falls, sharp bends, or turbulent junctions. Every reach of the sewer should routinely experience the minimum velocity necessary to maintain solids in suspension and avoid blockage from solids deposition in low-velocity areas. Sewers in hilly areas, however, may require energy dissipation features to avoid sewer damage from high fluid velocities and the scouring effects of gritty solids in turbulent flow. Covered sewers are buried below the frost line to avoid freezing, and deep enough to receive gravity flow from anticipated wastewater sources. [2] Long gravity sewers may require significant excavation depths or tunneling to maintain acceptable gradients near the sewer outfall.

Pumping alternatives to gravity

Availability of reliable pumps allows lifting accumulations of water into gravity sewers from collection sumps in excavations like mines or building foundations; but the cost of pumping surface runoff discourages use of lift stations in storm drains or combined sewers. The capitalized cost of operation and maintenance of lift stations and emergency power supplies usually justifies considerable first cost for excavation or tunneling to build a gravity sewer. [3] Sewage treatment is most efficient at centralized locations; and pumping is often required to lift sewage from lower elevations to the sewage treatment plant headworks. Structures called regulators allow overflow into gravity outfall sewers when peak flow in combined sewers exceeds pumping capacity. [4] Sanitary sewers are preferred for cost-effective pumping of sewage when treatment is required. Gravity sewers are preferred where grades are favorable, but lift stations often move sewage to sewage treatment plants.

Vacuum sewers have a permanent negative pressure; due to improvements in technology, they have become more comparable to gravity sewers in operation and maintenance costs, and are cheaper to install. [5]

History

The earliest sewers were ditches to remove standing water from muddy locations where dry ground was preferable for human activity. Early sewers served a function similar to modern storm drains (sometimes called storm sewers.) Combined sewers evolved from the practice of using flow in early drainage ditches to remove other wastes including draft animal feces. [1] Sewers became offensive as waste concentrations increased in communities with high population density. Culverts were installed to cover the offensive liquids. Prosperous communities used masonry and brickwork to cover early sewers. Terracotta pipes were used for low volume sewers. [6] The portion of a sewer discharging into a natural water feature is called an outfall. [7]

Improved sewer materials

Old brick gravity sewers like this one are typically replaced by reinforced concrete. Manhole Brighton.jpg
Old brick gravity sewers like this one are typically replaced by reinforced concrete.

The Industrial Revolution increased population density in manufacturing districts, and produced pipes useful for drain-waste-vent systems from buildings to sewers. Gravity sewers have been assembled from cast iron pipe, vitrified clay pipe, precast concrete pipe, asbestos-cement pipe, and plastic pipe. [8] While some older brickwork sewers remain in use, new sewers of diameters exceeding 1 metre (39 in) typically use reinforced concrete. Corrugated metal pipe may be used for storm drains or wastewaters with similarly low risk of corrosive conditions. [9] Non-circular cross-sections may have advantages for large-diameter sewers. [10]

Related Research Articles

<span class="mw-page-title-main">Sewerage</span> Infrastructure that conveys sewage or surface runoff using sewers

Sewerage is the infrastructure that conveys sewage or surface runoff 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.

<span class="mw-page-title-main">Sanitary sewer</span> Underground pipe for transporting sewage

A sanitary sewer is an underground pipe or tunnel system for transporting sewage from houses and commercial buildings to a sewage treatment plant or disposal. Sanitary sewers are a type of gravity sewer and are part of an overall system called a "sewage system" or sewerage. Sanitary sewers serving industrial areas may also carry industrial wastewater. In municipalities served by sanitary sewers, separate storm drains may convey surface runoff directly to surface waters. An advantage of sanitary sewer systems is that they avoid combined sewer overflows. Sanitary sewers are typically much smaller in diameter than combined sewers which also transport urban runoff. Backups of raw sewage can occur if excessive stormwater inflow or groundwater infiltration occurs due to leaking joints, defective pipes etc. in aging infrastructure.

<span class="mw-page-title-main">Storm drain</span> Infrastructure for draining excess rain and ground water from impervious surfaces

A storm drain, storm sewer, surface water drain/sewer, or stormwater drain 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.

<span class="mw-page-title-main">London sewer system</span> English infrastructure system

The London sewer system is part of the water infrastructure serving London, England. The modern system was developed during the late 19th century, and as London has grown the system has been expanded. It is currently owned and operated by Thames Water and serves almost all of Greater London.

<span class="mw-page-title-main">French drain</span> Sub-surface drainage system

A French drain is a trench filled with gravel or rock, or both, with or without a perforated pipe that redirects surface water and groundwater away from an area. The perforated pipe is called a weeping tile. When the pipe is draining, it "weeps", or exudes liquids. It was named during a time period when drainpipes were made from terracotta tiles.

<span class="mw-page-title-main">Pumping station</span> Facilities including pumps and equipment for pumping fluids from one place to another

Pumping stations, also called pumphouses in situations such as drilled wells and drinking water, are facilities containing pumps and equipment for pumping fluids from one place to another. They are used for a variety of infrastructure systems, such as the supply of water to canals, the drainage of low-lying land, and the removal of sewage to processing sites. A pumping station is an integral part of a pumped-storage hydroelectricity installation.

<span class="mw-page-title-main">Northern Outfall Sewer</span> Sewer in London

The Northern Outfall Sewer (NOS) is a major gravity sewer which runs from Wick Lane in Hackney to the Beckton Sewage Treatment Works in east London. Most of it was designed by Joseph Bazalgette, as a result of an outbreak of cholera in 1853 and the "Great Stink" of 1858.

<span class="mw-page-title-main">Combined sewer</span> Sewage collection system of pipes and tunnels designed to also collect surface runoff

A combined sewer is a type of gravity sewer with a system of pipes, tunnels, pump stations etc. to transport sewage and urban runoff together to a sewage treatment plant or disposal site. This means that during rain events, the sewage gets diluted, resulting in higher flowrates at the treatment site. Uncontaminated stormwater 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 particles and other sediment, heavy metals, organic compounds, animal waste, and oil and grease. Combined sewers may also receive dry weather drainage from landscape irrigation, construction dewatering, and washing buildings and sidewalks.

<span class="mw-page-title-main">First flush</span> Initial surface runoff of a rainstorm

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.

<span class="mw-page-title-main">Sanitary sewer overflow</span> Discharge of untreated sewage

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. Causes of sanitary sewer overflows 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. Prevention of such overflow events involves regular maintenance and timely upgrades of infrastructure.

<span class="mw-page-title-main">Vacuum sewer</span> Method of transporting sewage from its source to a sewage treatment plant

A vacuum sewer or pneumatic sewer system is a method of transporting sewage from its source to a sewage treatment plant. It maintains a partial vacuum, with an air pressure below atmospheric pressure inside the pipe network and vacuum station collection vessel. Valves open and reseal automatically when the system is used, so differential pressure can be maintained without expending much energy pumping. A single central vacuum station can collect the wastewater of several thousand individual homes, depending on terrain and the local situation.

<span class="mw-page-title-main">Ashbridges Bay Wastewater Treatment Plant</span>

The Ashbridges Bay Wastewater Treatment Plant is the city of Toronto's main sewage treatment facility, and the second largest such plant in Canada after Montreal's Jean-R. Marcotte facility. One of four plants that service the city of Toronto, it treats the wastewater produced by some 1.4 million of the city's residents and has a rated capacity of 818,000 cubic metres per day. Until 1999 it was officially known as the Main Treatment Plant. The plant has a 185 m (607 ft) high smokestack which is visible from most parts of the city.

<span class="mw-page-title-main">Harbour Solutions</span> Canadian public infrastructure project

Harbour Solutions is a Canadian public infrastructure project in Halifax, Nova Scotia.

<span class="mw-page-title-main">Sewage</span> Wastewater that is produced by a community of people

Sewage 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 residences and from commercial, institutional and public facilities that exist in the locality. Sub-types of sewage are greywater and blackwater. Sewage also contains soaps and detergents. Food waste may be present from dishwashing, and food quantities may be increased where garbage disposal units are used. In regions where toilet paper is used rather than bidets, that paper is also added to the sewage. Sewage contains macro-pollutants and micro-pollutants, and may also incorporate some municipal solid waste and pollutants from industrial wastewater.

Allegheny County Sanitary Authority is a Municipal Authority in Allegheny County, Pennsylvania that provides wastewater treatment services to 83 communities, including the city of Pittsburgh. Its principal sewage treatment plant is along the Ohio River downstream from Pittsburgh.

<span class="mw-page-title-main">Effluent sewer</span>

Effluent sewer systems, also called septic tank effluent gravity (STEG), solids-free sewer (SFS), or septic tank effluent drainage (STED) systems, have septic tanks that collect sewage from residences and businesses, and the liquid fraction of sewage that comes out of the tank is conveyed to a downstream receiving body such as either a centralized sewage treatment plant or a distributed treatment system for further treatment or disposal away from the community generating the sewage. Most of the solids are removed by the interceptor tanks, so the treatment plant can be much smaller than a typical plant and any pumping for the supernatant can be simpler without grinders.

Infiltration/Inflow is the process of groundwater, or water from sources other than domestic wastewater, entering sanitary sewers. I/I causes dilution in sanitary sewers, which decreases the efficiency of treatment, and may cause sewage volumes to exceed design capacity. Although inflow is technically different from infiltration, it may be difficult to determine which is causing dilution problems in inaccessible sewers. The United States Environmental Protection Agency defines the term infiltration/inflow as combined contributions from both.

The Bondi Ocean Outfall Sewer is a heritage-listed sewerage infrastructure at Blair Street, North Bondi, Sydney, Australia. The sewer line commences at the intersection of Oxford Street and College Street in Darlinghurst and then travels in a more-or-less easterly direction for 6.1 kilometres (3.8 mi) passing through a number of suburbs until it reaches Blair Street in North Bondi. It was designed and built by the Public Works Department between 1880 and 1889. It is also known as BOOS (Bondi Ocean Outfall Sewer) and Main Northern Ocean Outfall Sewer. The property is owned by Sydney Water.

<span class="mw-page-title-main">Sewage Pumping Station 3</span> Historic site in New South Wales, Australia

Sewage Pumping Station 3 is a heritage-listed sewerage pumping station located near 1 Booth Street, Annandale, City of Sydney, New South Wales, Australia. The building is located adjacent to Johnstons Creek. It was built from 1902 to 1904 by the New South Wales Public Works Department. It is also known as SPS 3, SP0003, Booth Street Sewage Pumping Station and Annandale Sewage Pumping Station. The property is owned by Sydney Water. It was added to the New South Wales State Heritage Register on 18 November 1999.

<span class="mw-page-title-main">Sanitary manhole</span> An access point to an underground sanitary sewer system

A sanitary manhole is a manhole that is used as an access point for maintenance and inspection of an underground sanitary sewer system. Sanitary manholes are sometimes used as vents to prevent the buildup of pressurized sewage gas. Additionally, they are used for debris removal, and application of chemicals such as degreaser and insecticide.

References

  1. 1 2 Design and Construction of Sanitary and Storm Sewers(1970)p.1
  2. Design and Construction of Sanitary and Storm Sewers(1970)pp.119-127
  3. Design and Construction of Sanitary and Storm Sewers(1970)pp.287&288
  4. Okun(1959)p.6
  5. "Keeping Ocean Shores Clean: Coastal Town Relies on Vacuum Sewers". informedinfrastructure.com. 18 October 2016.
  6. Steel&McGhee(1979)p.318
  7. Abbett(1956)p.19-02
  8. Steel&McGhee(1979)pp.331-347
  9. Design and Construction of Sanitary and Storm Sewers(1970)p.123
  10. Steel&McGhee(1979)pp.348-351

Sources