Sewage collection and disposal
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Urban areas require some methods for collection and disposal of sewage.
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[edit] Collection
Wastewater collection systems consist of buried pipelines which convey the wastewater by gravity to the water treatment plant. Where pipeline excavation is difficult because of rock or there is limited topographic relief ie. due to flat terrain, gravity collection systems may not be practical and the sewage must be pumped through the pipeline to the treatment plant. For some low-lying communities wastewater may be conveyed by vacuum. Pipelines may 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.
Sewerage can also be collected by low pressure pumps and vacuum systems. 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. Typically a vacuum sewer station can service approximately 1200 homes before it becomes more cost effective to build another station.
[edit] Design and Analysis
Design and sizing of sewage collection systems considers population served, commercial and industrial flows, flow peaking characteristics and wet weather flows. 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 stormwater runoff, resulting in combined sewer overflows. Separate sanitary sewer systems are designed to transport sewage, with a second pipe constructed for storm water runoff. Most 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. Inflow and infiltration is highly impacted by antecedent moisture conditions, which also represents an important design consideration in separate sewer systems.
[edit] Contaminant source control
Wastewater is collected by sanitary sewer systems (foul sewerage system in the UK) and is typically conveyed to a centralized wastewater treatment facility where it is treated in several stages to reduce the level of some contaminants. Because industrial liquid waste may contain a wide range of chemicals, solvents, and other contaminants that cannot be effectively removed by the centralized wastewater treatment plant, industries are often required to pre-treat their liquid wastes prior to discharging to sewer. Most major municipal jurisdictions in North America with significant industrial liquid waste sources have discharge bylaws that restrict the quantity and maximum level of specific contaminants that may be discharged to sewers. In the UK charges are levied on industries who discharge industrial waste to the sewers. These charges relate to volume, organic strength and their toxic metal content.
[edit] Historical sewage conveyance and disposal
As recently as 100 years ago in major cities of developed countries, and up to the present day in many parts of the world, the primary concern with sewage was the matter of conveying it away from inhabited areas. Aside from its unpleasant odor, even early humans were aware that health problems arose when human waste was allowed to contaminate drinking water supplies[citation needed].
Therefore, the historical focus of sewage treatment was on conveyance of raw sewage to a natural body of water, such as a river or ocean, where it would be satisfactorily diluted and dissipated. Early human habitations were often built next to water sources. Rivers could double as a crude form of natural sewage disposal.
Higher population densities required more complex sewer collection and conveyance systems in order to maintain (somewhat) sanitary conditions in crowded cities. The ancient cities of Harappa and Mohenjo-daro of the Indus Valley civilization constructed complex networks of brick-lined sewage drains from around 2600 BC and also had outdoor flush toilets connected to this network. These were the earliest artificial sewage systems. Ancient Minoan civilization had stone sewers that were periodically flushed with clean water.
Roman towns and garrisons in the United Kingdom between 46 BC and 400 CE had complex sewer networks sometimes constructed out of hollowed out Elm logs which were shaped so that they butted together with the down-stream pipe providing a socket for the upstream pipe.
A significant development was the construction of a network of sewers to collect waste water, which began from the Indus Valley civilization. In some cities, including Rome and Istanbul (Constantinople), networked ancient sewer systems continue to function today as collection systems for those cities' modernized sewer systems. Instead of flowing to a river or the sea, the pipes have been re-routed to modern sewer treatment facilities.
However, many cities had no sewers and relied on nearby rivers or occasional rain to wash away sewage. In some cities, waste water simply ran down the streets, which had stepping stones to keep pedestrians out of the muck, and eventually drained as runoff into the local watershed. This was enough in early cities with few occupants but the growth of cities quickly overpolluted streets and became a constant source of disease. Even as recently as the late 19th century sewerage systems in parts of the highly industrialised United Kingdom were so inadequate that water-borne diseases such as cholera and typhoid were still common. In Merthyr Tydfil, a large town in South Wales, most houses discharged their sewage to individual cess-pits which persistently overflowed causing the pavements to be awash with foul sewage.
[edit] See also
- Select Society of Sanitary Sludge Shovelers
- sewage
- sewage treatment
- William Lindley – pioneering 19th century engineer
- John Todd
