Sanitary sewer

This article is about modern sewer system for carrying waste. For early sewers designed to carry both waste and storm water, see combined sewer. For the street runoff drainage system, see storm drain.

PVC Sanitary Sewer Installation. Sanitary sewers are sized to carry the amount of sewage generated by the collection area. Sanitary sewers are much smaller than combined sewers designed to also carry surface runoff.

A sanitary sewer (also called a foul sewer and a covered sewer) is an underground carriage system specifically for transporting sewage from houses and commercial buildings through pipes to treatment or disposal. Sewage ought to be treated to control water pollution before discharge to surface waters. Sanitary sewers serving industrial areas also carry industrial wastewater. The system of sewers is called sewerage.

Sanitary sewers are distinguished from open sewers in that the sanitary system is closed off from its surroundings to limit interaction between the waste and the landscape that it travels through. They are distinguished from combined sewers, which combine sewage with surface runoff. 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. Open sewers are not used in developed countries today, but they remain in use in many developing countries.

Conventional design

Manhole access to sewer; person shows scale

Schematic of a conventional sanitary sewer to convey blackwater and greywater from households to a centralized sewage treatment facility

A manhole cover for a sanitary sewer access point.

View looking down into an open manhole showing two converging sanitary sewer lines. The larger line enters from the right and changes direction within the manhole to exit from the top of the photo. A smaller line enters from the bottom of the photo under the access steps. The concrete floor of the manhole has channels to minimize accumulation of solids.

Interior photo of a large sanitary sewer from an access manhole.

In the developed world, sewers are pipes from buildings to one or more levels of larger underground trunk mains, which transport the sewage to sewage treatment facilities. Vertical pipes, usually made of precast concrete, called manholes, connect the mains to the surface. Depending upon site application and use, these vertical pipes can be cylindrical, eccentric or concentric. The manholes are used for access to the sewer pipes for inspection and maintenance, and as a means to vent sewer gases. They also facilitate vertical and horizontal angles in otherwise straight pipelines. Sewers are generally gravity powered, though pumps may be used if necessary.

Pipes conveying sewage from an individual building to a common gravity sewer line are called laterals. Branch sewers typically run under streets receiving laterals from buildings along that street and discharge by gravity into trunk sewers at manholes. Larger cities may have sewers called interceptors receiving flow from multiple trunk sewers. A lift station is a gravity sewer sump with a pump to lift accumulated sewage to a higher elevation. The pump may discharge to another gravity sewer at that location or may discharge through a pressurized force main to some distant location.

Design and sizing of sewage collection systems considers population served, commercial and industrial flows, flow peaking characteristics and wet weather flows.

Comparison with combined sewers

Combined sewer systems are designed to transport both stormwater runoff and sewage in the same pipe. Besides the projected sewage flow, the size and characteristics of the watershed are the overriding design considerations for combined sewers. Often, combined sewers can not handle the volume of runoff, resulting in combined sewer overflows and causing water pollution problems in nearby water bodies.

Separate sanitary sewer systems are designed to transport sewage alone. In communities served by separate sanitary sewers, another pipe system is constructed to convey stormwater runoff directly to surface waters. Most municipal sewer systems constructed today are separate sewer systems.

Although separate sewer systems are intended to transport only sewage, all sewer systems have some degree of inflow and infiltration of surface water and groundwater, which can lead to sanitary sewer overflows. Inflow and infiltration is highly affected by antecedent moisture conditions, which also represents an important design consideration in these system.

Maintenance

All sewers deteriorate with age, but infiltration/inflow is a problem unique to sanitary sewers, since both combined sewers and storm drains are sized to carry these contributions. Holding infiltration to acceptable levels requires a higher standard of maintenance than necessary for structural integrity considerations of combined sewers. A comprehensive construction inspection program is required to prevent inappropriate connection of cellar, yard, and roof drains to sanitary sewers. The probability of inappropriate connections is higher where combined sewers and sanitary sewers are found in close proximity, because construction personnel may not recognize the difference. Many older cities still use combined sewers while adjacent suburbs were built with separate sanitary sewers.

For decades, when sanitary sewer pipes cracked or experienced other damage, the only option was an expensive excavation, removal and replacement of the damaged pipe, typically requiring street repavement afterwards. In the mid-1950s a unit was invented where two units at each end with a special cement mixture in between was pulled from one manhole cover to the next, coating the pipe with the cement under high pressure which then cured rapidly, sealing all cracks and breaks in the pipe.

History

Main article: Combined sewer

Animal feces were plentiful on city streets while animal-powered transport moved people and goods. Accumulations of animal feces encouraged dumping chamber pots into streets where night soil collection was impractical. The earliest sewers were designed to carry street runoff away from inhabited areas and into surface waterways without treatment. Indoor plumbing was often drained to combined sewers through the 19th century. Sewage treatment facilities built for combined sewers become ineffective during periods of precipitation or snowmelt. Cities were built with sanitary sewers operated separately and independently of storm drains carrying the runoff of rain after wagons and carriages powered by internal combustion engines reduced the advantages of treating street runoff; but many cities built prior to the twentieth century have not replaced combined sewer infrastructure.

Recent variants

Effluent sewer

See also: Effluent sewer

Effluent sewer systems, also called septic tank effluent drainage (STED) or solids-free sewer (SFS) systems, have septic tanks that collect sewage from residences and businesses, and the effluent that comes out of the tank is sent to either a centralized sewage treatment plant or a distributed treatment system for further treatment. Most of the solids are removed by the septic tanks, so the treatment plant can be much smaller than a typical plant. In addition, because of the vast reduction in solid waste, a pumping system can be used to move the wastewater rather than a gravity system. The pipes have small diameters (typically 1.5" to 4"). Because the wastestream is pressurized, they can be laid just below the ground surface along the land's contour.

Simplified sewer

See also: Simplified sewerage

Simplified sanitary sewers consist of small-diameter pipes (typically 100 mm or about 4 inches), often laid at fairly flat gradients (1 in 200). The investment cost for simplified sanitary sewers can be about half the cost of conventional sewers. However, the requirements for operation and maintenance are usually higher. Simplified sewers are most common in Brazil and are also used in a number of other developing countries.

Vacuum sewer

Main article: Vacuum sewer

In low-lying communities, wastewater may be conveyed by vacuum sewer. Pipelines range in size from pipes of six inches (150 mm) in diameter to concrete-lined tunnels of up to thirty feet (10 m) in diameter. A low pressure system uses a small grinder pump located at each point of connection, typically a house or business. Vacuum sewer systems use differential atmospheric pressure to move the liquid to a central vacuum station.

See also

• Blackwater (waste)
• Buchan trap
• Combined sewer
• Sanitary sewer overflow (SSO)
• Sewage treatment
• Stormwater
• Water pollution

External links

• Experiences and Challenges in Sewers: Measurements and Hydrodynamics (2008), International Meeting on Measurements and Hydraulics of Sewers IMMHS'08, Hydraulic Model Report No. CH70/08, University of Queensland, Brisbane, Australia, 114 pages (ISBN 9781864999280)