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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 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 sewerage, sewer system. Sewage consists of wastewater discharged fro ...
or
surface runoff Surface runoff (also known as overland flow or terrestrial runoff) is the unconfined flow of water over the ground surface, in contrast to ''channel runoff'' (or ''stream flow''). It occurs when excess rainwater, stormwater, meltwater, or other ...
rather than water intended for use;''Design and Construction of Sanitary and Storm Sewers''(1970)p.1 and the term ''
gravity In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...
'' 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.


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 In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by Chaos theory, chaotic changes in pressure and flow velocity. It is in contrast to laminar flow, which occurs when a fluid flows in parallel layers with no disrupt ...
. Covered sewers are buried below the frost line to avoid freezing, and deep enough to receive gravity flow from anticipated wastewater sources. 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.
Sewage treatment Sewage treatment is a type of wastewater treatment which aims to remove contaminants from sewage to produce an effluent that is suitable to discharge to the surrounding environment or an intended reuse application, thereby preventing water p ...
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. 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.


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 Feces (also known as faeces American and British English spelling differences#ae and oe, or fæces; : faex) are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the ...
. Sewers became offensive as waste concentrations increased in communities with high population density.
Culvert A culvert is a structure that channels water past an obstacle or to a subterranean waterway. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe (fluid conveyance), pipe, reinforced concrete or other materia ...
s were installed to cover the offensive liquids. Prosperous communities used
masonry Masonry is the craft of building a structure with brick, stone, or similar material, including mortar plastering which are often laid in, bound, and pasted together by mortar (masonry), mortar. The term ''masonry'' can also refer to the buildin ...
and brickwork to cover early sewers. Terracotta pipes were used for low volume sewers. The portion of a sewer discharging into a natural water feature is called an outfall.


Improved sewer materials

The
Industrial Revolution The Industrial Revolution, sometimes divided into the First Industrial Revolution and Second Industrial Revolution, was a transitional period of the global economy toward more widespread, efficient and stable manufacturing processes, succee ...
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 Precast concrete is a construction product produced by casting concrete in a reusable molding (process), mold or "form" which is then cured in a controlled environment, transported to the construction site and maneuvered into place; examples i ...
pipe, asbestos-cement pipe, and plastic pipe. While some older brickwork sewers remain in use, new sewers of diameters exceeding typically use
reinforced concrete Reinforced concrete, also called ferroconcrete or ferro-concrete, is a composite material in which concrete's relatively low tensile strength and ductility are compensated for by the inclusion of reinforcement having higher tensile strength or ...
. Corrugated metal pipe may be used for storm drains or wastewaters with similarly low risk of corrosive conditions. Non-circular cross-sections may have advantages for large-diameter sewers.Steel&McGhee(1979)pp.348-351


References


Sources

* * * * * {{sewerage Hydraulic engineering Sewerage infrastructure