Water supply system
A water supply
system or water supply network is a system of engineered hydrologic
and hydraulic components which provide water supply. A water supply system
typically includes:
1. A
drainage basin (see water purification - sources of drinking water);
2. A
raw (untreated) water collection point (above or below ground) where the water
accumulates, such as a lake, a river, or groundwater from an underground
aquifer. Untreated drinking water (usually water being transferred to the water
purification facilities) may be transferred using uncovered ground-level
aqueducts, covered tunnels or underground water pipes.
3. Water
purification facilities. Treated water is transferred using water pipes
(usually underground).
4. Water
storage facilities such as reservoirs, water tanks, or watertowers. Smaller
water systems may store the water in cisterns or pressure vessels. (Tall
buildings may also need to store water locally in pressure vessels in order for
the water to reach the upper floors.)
5. Additional
water pressurizing components such as pumping stations may need to be situated
at the outlet of underground or above ground reservoirs or cisterns (if gravity
flow is impractical)
6. A
pipe network for distribution of water to the consumers (which may be private
houses or industrial, commercial or institution establishments) and other usage
points (such as fire hydrants)
7. Connections
to the sewers (underground pipes, or aboveground ditches in some developing
countries) are generally found downstream of the water consumers, but the sewer
system is considered to be a separate system, rather than part of the water
supply system
Water abstraction and raw water transfer
Raw water (untreated)
is collected from a surface water source (such as an intake on a lake or a
river) or from a groundwater source (such as a water well drawing from an
underground aquifer) within the watershed that provides the water resource.
Shallow
dams and reservoirs are susceptible to outbreaks of toxic algae, especially if
the water is warmed by a hot sun. The bacteria grow from stormwater runoff
carrying fertilizer into the river where it acts as a nutrient for the algae.
Such outbreaks render the water unfit for human consumption.
The raw water is
transferred to the water purification facilities using uncovered aqueducts,
covered tunnels or underground water pipes.
Water treatment
Virtually all large
systems must treat the water; a fact that is tightly regulated by global, state
and federal agencies, such as the World Health Organization (WHO) or the United
States Environmental Protection Agency (EPA). Water treatment must occur before
the product reaches the consumer and afterwards (when it is discharged again).
Water purification usually occurs close to the final delivery points to reduce
pumping costs and the chances of the water becoming contaminated after
treatment.
Traditional surface
water treatment plants generally consists of three steps: clarification,
filtration and disinfection. Clarification refers to the separation of
particles (dirt, organic matter, etc.) from the water stream. Chemical addition
(i.e. alum, ferric chloride) destabilizes the particle charges and prepares
them for clarification either by settling or floating out of the water stream.
Sand, anthracite or activated carbon filters refine the water stream, removing
smaller particulate matter. While other methods of disinfection exist, the
preferred method is via chlorine addition. Chlorine effectively kills bacteria
and most viruses and maintains a residual to protect the water supply through
the supply network.
Water
distribution network
Pressurizing the water
is required between the small water reserve and the end-user
The product, delivered
to the point of consumption, is called fresh water if it receives little or no
treatment, or drinking water if the treatment achieves the water quality
standards required for human consumption.
Once treated, chlorine
is added to the water and it is distributed by the local supply network. Today,
water supply systems are typically constructed of plastic, ferrous, or concrete
circular pipe. However, other "pipe" shapes and material may be used,
such as square or rectangular concrete boxes, arched brick pipe, or wood. Near
the end point, the network of pipes through which the water is delivered is
often referred to as the water mains.
The energy that the
system needs to deliver the water is called pressure. That energy is
transferred to the water, therefore becoming water pressure, in a number of
ways: by a pump, by gravity feed from a water source (such as a water tower) at
a higher elevation, or by compressed air.
The water is often
transferred from a water reserve such as a large communal reservoir before
being transported to a more pressurised reserve as a watertower.
In small domestic
systems, the water may be pressurised by a pressure vessel or even by an
underground cistern (the latter however does need additional pressurizing).
This eliminates the need of a water-tower or any other heightened water reserve
to supply the water pressure.
These systems are
usually owned and maintained by local governments, such as cities, or other
public entities, but are occasionally operated by a commercial enterprise (see
water privatization). Water supply networks are part of the master planning of
communities, counties, and municipalities. Their planning and design requires
the expertise of city planners and civil engineers, who must consider many
factors, such as location, current demand, future growth, leakage, pressure,
pipe size, pressure loss, fire fighting flows, etc. - using pipe network
analysis and other tools. Constructioncomparable sewage systems, was one of the
great engineering advances that made urbanization possible. Improvement in the
quality of the water has been one of the great advances in public health.
As water passes through
the distribution system, the water quality can degrade by chemical reactions
and biological processes. Corrosion of metal pipe materials in the distribution
system can cause the release of metals into the water with undesirable
aesthetic and health effects. Release of iron from unlined iron pipes can
result in customer reports of "red water" at the tap . Release of
copper from copper pipes can result in customer reports of "blue
water" and/or a metallic taste. Release of lead can occur from the solder
used to join copper pipe together or from brass fixtures. Copper and lead
levels at the consumer's tap are regulated to protect consumer health.
Utilities will often
adjust the chemistry of the water before distribution to minimize its
corrosiveness. The simplest adjustment involves control of pH and alkalinity to
produce a water that tends to passivate corrosion by depositing a layer of
calcium carbonate. Corrosion inhibitors are often added to reduce release of
metals into the water. Common corrosion inhibitors added to the water are
phosphates and silicates.
Maintenance
of a biologically safe drinking water is another goal in water distribution.
Typically, a chlorine based disinfectant, such as sodium hypochlorite or
monochloramine is added to the water as it leaves the treatment plant. Booster
stations can be placed within the distribution system to ensure that all areas
of the distribution system have adequate sustained levels of disinfection.
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