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WRITTEN BY: Jerry A. Nathanson
Encyclopædia Britannica
Water supply system, infrastructure for the collection,
transmission, treatment, storage, and distribution of water for
homes, commercial establishments, industry, and irrigation, as well as for such public needs as firefighting and street flushing.
Of all municipal services, provision of potable water is perhaps the most
vital. People depend on water for drinking, cooking, washing, carrying away
wastes, and other domestic needs. Water supply systems must also meet
requirements for public, commercial, and industrial activities. In all cases,
the water must fulfill both quality and quantity requirements.
Historical Background
Developments in supply systems
Water was an important factor in the location of the earliest
settled communities, and the evolution of public water
supply systems is tied directly to the growth of cities.
In the development of water resources beyond their natural condition in rivers,
lakes, and springs, the digging of shallow wells was probably the earliest innovation. As the need for water increased and
tools were developed, wells were made deeper. Brick-lined wells were built by
city dwellers in the Indus River basin as early as 2500 BCE, and wells almost 500 metres (more
than 1,600 feet) deep are known to have been used in ancient China.
Construction of qanāts,
slightly sloping tunnels driven into hillsides that contained groundwater,
probably originated in ancient Persia about 700 BCE. From the hillsides the water was conveyed by gravity
in open channels to nearby towns or cities. The use of qanāts
became widespread throughout the region, and some are still in existence. Until
1933 the Iranian capital city, Tehrān,
drew its entire water supply from a system of qanāts.
The need to channel water supplies from distant sources was
an outcome of the growth of urban communities. Among the most notable of
ancient water-conveyance systems are the aqueducts built between 312 BCE and 455 CE throughout the Roman
Empire. Some of these impressive works are still in existence. The
writings of Sextus Julius Frontinus (who was appointed
superintendent of Roman aqueducts in 97 CE)
provide information about the design and construction of the 11 major aqueducts that
supplied Rome itself. Extending from a distant spring-fed area, a lake,
or a river, a typical Roman aqueduct included a series of underground
and aboveground channels. The longest was the Aqua Marcia, built in
144 BCE. Its source was
about 37 km (23 miles) from Rome. The aqueduct itself was 92 km (57 miles)
long, however, because it had to meander along land contours in order to maintain a steady flow
of water. For about 80 km (50 miles) the aqueduct was underground in a covered
trench, and only for the last 11 km (7 miles) was it carried aboveground on an
arcade. In fact, most of the combined length of the aqueducts supplying Rome
(about 420 km [260 miles]) was built as covered trenches or tunnels. When
crossing a valley, aqueducts were supported by arcades comprising one or more levels of massive
granite piers and impressive arches.
The aqueducts ended in Rome at distribution reservoirs,
from which the water was conveyed to public baths or fountains. A few very
wealthy or privileged citizens had water piped directly into their homes, but
most of the people carried water in containers from a public fountain. Water
was running constantly, the excess being used to clean the streets and flush
the sewers.
Ancient aqueducts and pipelines were not capable of
withstanding much pressure. Channels were constructed of cut stone,
brick, rubble, or rough concrete. Pipes were typically made of drilled
stone or of hollowed wooden logs, although clay and lead pipes were also used. During the Middle
Ages there was no notable progress in the methods or materials
used to convey and distribute water.
Cast iron pipes with joints capable of
withstanding high pressures were not used very much until the early 19th
century. The steam engine was first applied to water-pumping
operations at about that time, making it possible for all but the smallest
communities to have drinking water supplied directly to individual homes. Asbestos cement, ductile iron, reinforced concrete, and steel came
into use as materials for water supply pipelines in the 20th century.
Developments in water treatment
In addition to quantity of supply, water quality is also of concern. Even the
ancients had an appreciation for the importance of water purity. Sanskrit writings from as early as
2000 BCE tell how to
purify foul water by boiling and filtering. But it was not until the middle of
the 19th century that a direct link between polluted water and disease (cholera)
was proved, and it was not until the end of that same century that the German
bacteriologist Robert Koch proved the germ
theory of disease, establishing a scientific basis for the
treatment and sanitation of drinking water.
Water treatment is the alteration of a water source in
order to achieve a quality that meets specified goals. At the end of the 19th
century and the beginning of the 20th, the main goal was elimination of deadly
waterborne diseases. The treatment of public drinking water to remove
pathogenic, or disease-causing, microorganisms began about that time. Treatment
methods included sand filtration as well as the use of chlorine for
disinfection. The virtual elimination of diseases such as cholera and typhoid in developed countries proved the success of this
water-treatment technology. In developing countries, waterborne
disease is still the principal water quality concern.
In industrialized countries, concern has shifted to the
chronic health effects related to chemical contamination. For example, trace
amounts of certain synthetic organic substances in drinking
water are suspected of causing cancer in humans. Lead in drinking water,
usually leached from corroded lead pipes, can result in gradual lead poisoning and may cause developmental
delays in children. The added goal of reducing such health risks is seen in the
continually increasing number of factors included in drinking-water standards.
Water Sources
Global distribution
Water is present in abundant quantities on
and under Earth’s surface, but less than 1 percent of it is liquid fresh water.
Most of Earth’s estimated 1.4 billion cubic km (326 million cubic miles) of
water is in the oceans or frozen in polar ice caps and glaciers.
Ocean water contains about 35 grams per litre (4.5 ounces per gallon) of
dissolved minerals or salts, making it unfit for drinking and for most
industrial or agricultural uses.
There is ample fresh water—water containing less than 3
grams of salts per litre, or less than one-eighth ounce of salts per gallon—to
satisfy all human needs. It is not always available, though, at the times and
places it is needed, and it is not uniformly distributed over the globe,
sometimes resulting in water
scarcity for susceptible communities. In many locations the
availability of good-quality water is further reduced because of urban
development, industrial growth, and environmental pollution.
Surface water and groundwater
Surface water and groundwater are both important sources
for community water
supply needs. Groundwater is
a common source for single homes and small towns, and rivers and lakes are
the usual sources for large cities. Although approximately 98 percent of liquid
fresh water exists as groundwater, much of it occurs very deep. This makes
pumping very expensive, preventing the full development and use of all
groundwater resources.
The hydrologic
cycle
Water is in constant circulation, powered by the energy
from sunlight and gravity in a natural process called
the hydrologic cycle. Water evaporates from the ocean
and land surfaces, is held temporarily as vapour in the atmosphere,
and falls back to Earth’s surface as precipitation. Surface water is the residue of precipitation
and melted snow, called runoff. Where the average rate of precipitation
exceeds the rate at which runoff seeps into the soil, evaporates, or is
absorbed by vegetation, bodies of surface water such as streams, rivers, and
lakes are formed. Water that infiltrates Earth’s surface becomes groundwater,
slowly seeping downward into extensive layers of porous soil and rock
called aquifers. Under the pull of gravity, groundwater
flows slowly and steadily through the aquifer. In low areas it emerges in springs and
streams. Both surface water and groundwater eventually return to the ocean,
where evaporation replenishes the supply of
atmospheric water vapour. Winds carry the moist air over land, precipitation
occurs, and the hydrologic cycle continues.
The total land area that contributes surface runoff to
a river or lake is
called a watershed, drainage basin, or catchment area. The volume of
water available for municipal supply depends mostly on the amount of rainfall.
It also depends on the size of the watershed, the slope of the ground, the type
of soil and vegetation, and the type of land use.
The flow
rate or discharge of a river varies with time. Higher flow
rates typically occur in the spring, and lower flow rates occur in the winter,
though this is often not the case in areas with monsoon systems.
When the average discharge of a river is not enough for a dependable supply of
water, a conservation reservoir may
be built. The flow of water is blocked by a dam, allowing an artificial lake to be formed.
Conservation reservoirs store water from wet weather periods for use during times
of drought and low streamflow. A water intake structure is built within the
reservoir, with inlet ports and valves at several depths. Since the quality of
water in a reservoir varies seasonally with depth, a multilevel intake allows
water of best quality to be withdrawn. Sometimes it is advisable, for economic
reasons, to provide a multipurpose reservoir. A multipurpose reservoir is
designed to satisfy a combination of community water needs. In addition to
drinking water, the reservoir may also provide flood control, hydroelectric power, and recreation.
The value of an aquifer as
a source of groundwater is a function of the porosity of the geologic stratum,
or layer, of which it is formed. Water is withdrawn from an aquifer by pumping
it out of a well or infiltration gallery. An infiltration gallery
typically includes several horizontal perforated pipes radiating outward from
the bottom of a large-diameter vertical shaft. Wells are constructed in several ways, depending on the
depth and nature of the aquifer. Wells used for public water supplies, usually
more than 30 metres (100 feet) deep and from 10 to 30 cm (4 to 12 inches) in
diameter, must penetrate large aquifers that can provide dependable yields of
good-quality water. They are drilled using impact or rotary techniques and are
usually lined with a metal pipe or casing to prevent contamination. The annular
space around the outside of the upper portion of the casing is filled with
cement grout, and a special sanitary seal is installed at the top to provide
further protection. At the bottom of the casing, a slotted screen is attached
to strain silt and sand out of the groundwater. A submersible pump driven
by an electric motor can be used to raise the
water to the surface. Sometimes a deep well may penetrate a confined artesian aquifer,
in which case natural hydrostatic pressure can
raise the water to the surface.
Water Requirements
Municipal water supply systems include facilities for
storage, transmission, treatment, and distribution. The design of these
facilities depends on the quality of the water, on the particular needs of the
user or consumer, and on the quantities of water that must be processed.
Drinking-water quality
Water has such a strong tendency to dissolve other
substances that it is rarely found in nature in a pure condition. When it falls
as rain, small amounts of gases such as oxygen and carbon dioxide become dissolved in it;
raindrops also carry tiny dust particles and other substances. As it flows over
the ground, water picks up fine soil particles,
microbes, organic material, and soluble minerals. In lakes, bogs,
and swamps, water may gain colour, taste, and odour
from decaying vegetation and other natural organic matter. Groundwater usually
acquires more dissolved minerals than does surface runoff because of its longer
direct contact with soil and rock. It may also absorb gases such as hydrogen sulfide and methane.
In populated areas the quality of surface water as well as groundwater is
directly influenced by land use and by human activities. For example,
stormwater runoff contaminated with agricultural or lawn pesticides and fertilizers,
as well as with road deicing chemicals or motor oil, can
flow into streams and lakes. In addition, effluent from malfunctioning septic tanks and subsurface leaching fields
can seep into groundwater.
Health concerns
Five general types of impurities are of public
health concern. These are organic chemicals, inorganic
chemicals, turbidity, microorganisms, and radioactive substances. Organic
contaminants include various pesticides,
industrial solvents, and trihalomethanes such as chloroform.
Inorganic contaminants of major concern include arsenic, nitrate,
fluoride, and toxic metals such as lead and mercury. All these substances can harm human
health when present above certain concentrations in drinking water.
A low concentration of fluoride, however, has been proved to promote dental
health. Some communities add fluoride to their water for
this purpose.
Turbidity refers to cloudiness caused by very small
particles of silt, clay,
and other substances suspended in water. Even a slight degree of turbidity in
drinking water is objectionable to most people. Turbidity also interferes with
disinfection by creating a possible shield for pathogenic organisms.
Groundwater normally has very low turbidity, because of the natural filtration
that occurs as it percolates through the soil. Surface waters,
though, are often high in turbidity.
The most important microbiological measure of drinking-water quality is a
group of bacteria called coliforms. Coliform bacteria normally are not
pathogenic, but they are always present in the intestinal tract of humans and
are excreted in very large numbers with human waste. Water contaminated with
human waste always contains coliforms, and it is also likely to contain
pathogens excreted by infected individuals in the community. Since it is easier to test for the
presence of coliforms rather than for specific types of pathogens, coliforms
are used as indicator organisms for measuring the biological quality of water.
If coliforms are not found in the water, it can be assumed that the water is
also free of pathogens. The coliform count thus reflects the chance of
pathogens being present; the lower the coliform count, the less likely it is
that pathogens are in the water.
Radioactive materials from natural as well
as industrial sources can be harmful water contaminants. Wastes from uranium mining, nuclear power plants, and medical research are
possible pollutants. Strontium-90 and tritium are
radioactive contaminants that have been found in water as a result of nuclear weapons testing. Naturally occurring
substances such as radium and radon gas
are found in some groundwater sources. The danger from
dissolved radon gas arises not from drinking the water but from breathing the
gas after it is released into the air.
Aesthetic concerns
Colour, taste, and odour are physical
characteristics of drinking water that are important for aesthetic reasons rather than for health
reasons. Colour in water may be caused by decaying leaves or by algae,
giving it a brownish yellow hue. Taste and odour may be caused by naturally
occurring dissolved organics or gases. Some well-water supplies, for example,
have a rotten-egg odour that is caused by hydrogen sulfide gas. Chemical impurities
associated with the aesthetic quality of drinking water include iron, manganese, copper, zinc,
and chloride. Dissolved metals impart a bitter taste to water and may stain
laundry and plumbing fixtures. Excessive chlorides give
the water an objectionable salty taste.
Hardness
Another parameter of water quality is hardness. This
is a term used to describe the effect of dissolved minerals (mostly calcium and magnesium).
Minerals cause deposits of scale in hot water pipes, and they also interfere
with the lathering action of soap.
Hard water does not harm human health, but the economic problems it causes make
it objectionable to most people.
Standards
Water quality standards set limits on the concentrations of
impurities allowed in water. Standards also affect the selection of raw water
sources and the choice of treatment processes. The development of water quality
standards began in the United
States in the early 20th century. Since that time, the total
number of regulated contaminants has increased as toxicological knowledge
and analytical measurement techniques have
improved. Modern testing methods now allow the detection of contaminants in
extremely low concentrations—as low as one part contaminant per one billion
parts water or even, in some cases, per one trillion parts water. Water quality
standards are continually evolving, usually becoming more stringent. As a
result, the number of regulated contaminants increases over time, and their
allowable concentrations in water are lowered.
Drinking-water regulations in the United States include two
types of standards: primary and secondary. Primary standards are designed
to protect public health, whereas secondary standards are based on aesthetic
factors rather than on health effects. Primary standards specify maximum
contaminant levels for many chemical, microbiological, and radiological parameters of water quality. They reflect
the best available scientific and engineering judgment and take into account
exposure from other sources in the environment and from foods. Turbidity is
also included in the primary standards because of its tendency to interfere
with disinfection. Secondary standards are guidelines or suggested maximum
levels of colour, taste, odour, hardness, corrosiveness, and certain other
factors.
Municipal water consumption
Water consumption in a community is characterized by several types
of demand, including domestic, public, commercial, and industrial uses.
Domestic demand includes water for drinking, cooking, washing, laundering, and
other household functions. Public demand includes water for fire protection, street cleaning,
and use in schools and other public buildings. Commercial and industrial
demands include water for stores, offices, hotels, laundries, restaurants, and
most manufacturing plants. There is usually a wide variation in total water
demand among different communities. This variation depends on
population, geographic location, climate, the extent of local commercial and
industrial activity, and the cost of water.
Water use or demand is expressed numerically by average
daily consumption per capita (per person). In the United States the average is
approximately 380 litres (100 gallons) per capita per day for domestic and
public needs. Overall, the average total demand is about 680 litres (180
gallons) per capita per day, when commercial and industrial water uses are
included. (These figures do not include withdrawals from freshwater sources for
such purposes as crop irrigation or cooling operations at electric
power-generating facilities.) Water consumption in some developing countries
may average as little as 15 litres (4 gallons) per capita per day. The world
average is estimated to be approximately 60 litres (16 gallons) per person per
day.
In any community, water demand varies on a seasonal, daily,
and hourly basis. On a hot summer day, for example, it is not unusual for total
water consumption to be as much as 200 percent of the average demand. The peak
demands in residential areas usually occur in the morning and early evening
hours (just before and after the normal workday). Water demands in commercial
and industrial districts, though, are usually uniform during the work day.
Minimum water demands typically occur in the very early or predawn morning
hours. Civil and environmental engineers must carefully study each community’s
water use patterns in order to design efficient pumping and distribution systems.
Water in rivers or lakes is
rarely clean enough for human consumption if it is not first treated or
purified. Groundwater, too, often needs some level of
treatment to render it potable. The primary objective of water treatment is to
protect the health of the community. Potable water must, of course, be free of
harmful microorganisms and chemicals, but public supplies should also be
aesthetically desirable so that consumers will not be tempted to use water from
another, more attractive but unprotected source. The water should be crystal
clear, with almost no turbidity, and it should be free of objectionable colour,
odour, and taste. For domestic supplies, water should not be corrosive, nor
should it deposit troublesome amounts of scale and stains on plumbing fixtures.
Industrial requirements may be even more stringent; many industries provide
special treatment on their own premises.
The type and extent of treatment required to obtain potable
water depends on the quality of the source. The better the quality, the less
treatment is needed. Surface water usually needs more extensive treatment than
does groundwater, because most streams, rivers, and lakes are polluted to some
extent. Even in areas remote from human populations, surface water contains
suspended silt, organic material, decaying vegetation, and
microbes from animal wastes. Groundwater, on the other hand, is usually free of
microbes and suspended solids because of natural filtration as the water moves
through soil, though it often contains relatively high concentrations of
dissolved minerals from its direct contact with soil and rock.
Water is treated in a variety of physical and chemical
methods. Treatment of surface water begins with intake screens to prevent fish and
debris from entering the plant and damaging pumps and other components.
Conventional treatment of water primarily involves clarification and disinfection.
Clarification removes most of the turbidity, making the water crystal clear.
Disinfection, usually the final step in the treatment of drinking water,
destroys pathogenic microbes. Groundwater does
not often need clarification, but it should be disinfected as a precaution to
protect public health. In addition to clarification and
disinfection, the processes of softening, aeration, carbon adsorption, and
fluoridation may be used for certain public water sources. Desalination processes are used in areas
where freshwater supplies are not readily available.
Clarification
Impurities in water are either dissolved or suspended. The
suspended material reduces clarity, and the easiest way to remove it is to rely
on gravity. Under quiescent (still) conditions, suspended
particles that are denser than water gradually settle to the bottom of a basin
or tank. This is called plain sedimentation. Long-term water storage (for
more than one month) in reservoirs reduces the amount of suspended sediment and bacteria.
Nevertheless, additional clarification is usually needed. In a treatment plant,
sedimentation (settling) tanks are built to provide a few hours of storage or
detention time as the water slowly flows from tank inlet to outlet. It is
impractical to keep water in the tanks for longer periods, because of the large
volumes that must be treated.
Sedimentation tanks may be rectangular or
circular in shape and are typically about 3 metres (10 feet) deep. Several
tanks are usually provided and arranged for parallel (side-by-side) operation.
Influent (water flowing in) is uniformly distributed as it enters a tank.
Clarified effluent (water flowing out) is skimmed from the surface as it flows
over special baffles called weirs. The layer of concentrated solids that
collects at the bottom of the tank is called sludge.
Modern sedimentation tanks are equipped with mechanical scrapers that
continuously push the sludge toward a collection hopper, where it is pumped
out.
The efficiency of a sedimentation tank for removing suspended
solids depends more on its surface area than on its depth or volume. A
relatively shallow tank with a large surface area will be more effective than a
very deep tank that holds the same volume but has a smaller surface area. Most
sedimentation tanks, though, are not less than 3 metres (about 10 feet) deep,
in order to provide enough room for a sludge layer and a scraper mechanism.
A technique called shallow-depth sedimentation is
often applied in modern treatment plants. In this method, several prefabricated
units or modules of “tube settlers” are installed near the tops of tanks in
order to increase their effective surface area.
Coagulation and flocculation
Suspended particles cannot be removed completely by plain
settling. Large, heavy particles settle out readily, but smaller and lighter
particles settle very slowly or in some cases do not settle at all. Because of
this, the sedimentation step is usually preceded by a chemical process known
as coagulation. Chemicals (coagulants) are added to
the water to bring the nonsettling particles
together into larger, heavier masses of solids called floc. Aluminum sulfate (alum)
is the most common coagulant used for water purification. Other chemicals, such as
ferric sulfate or sodium aluminate, may also be used.
Coagulation is usually accomplished in two stages: rapid
mixing and slow mixing. Rapid mixing serves to disperse the coagulants evenly
throughout the water and to ensure a complete chemical reaction. Sometimes this is accomplished
by adding the chemicals just before the pumps, allowing the pump impellers
to do the mixing. Usually, though, a small flash-mix tank provides about one
minute of detention time. After the flash mix, a longer period of gentle
agitation is needed to promote particle collisions and enhance the growth of floc. This gentle
agitation, or slow mixing, is called flocculation;
it is accomplished in a tank that provides at least a half hour of detention
time. The flocculation tank has wooden paddle-type mixers that slowly rotate on
a horizontal motor-driven shaft. After flocculation the water flows into the
sedimentation tanks. Some small water-treatment plants combine coagulation and
sedimentation in a single prefabricated steel unit called a solids-contact
tank.
Even after coagulation and flocculation, sedimentation does
not remove enough suspended impurities from water to make it crystal clear. The
remaining nonsettling floc causes noticeable turbidity in the water and can
shield microbes from disinfection. Filtration is a physical process that
removes these impurities from water by percolating it downward through a layer or
bed of porous, granular material such as sand.
Suspended particles become trapped within the pore spaces of the filter media,
which also remove harmful protozoa and
natural colour. Most surface water supplies require filtration after the
coagulation and sedimentation steps. For surface waters with low turbidity and
colour, however, a process of direct filtration, which is not preceded by sedimentation,
may be used.
Two types of sand filters are in use: slow and rapid. Slow filters require
much more surface area than rapid filters and are difficult to clean. Most
modern water-treatment plants now use rapid dual-media filters following
coagulation and sedimentation. A dual-media filter consists of a layer of anthracite coal
above a layer of fine sand. The upper layer of coal traps most of the large
floc, and the finer sand grains in the lower layer trap smaller impurities.
This process is called in-depth filtration, as the impurities are not
simply screened out or removed at the surface of the filter bed, as is the case
in slow sand filters. In order to enhance in-depth filtration,
so-called mixed-media filters are used in some treatment plants. These
have a third layer, consisting of a fine-grained dense mineral called garnet,
at the bottom of the bed.
Rapid filters are housed in boxlike concrete structures,
with multiple boxes arranged on both sides of a piping gallery. A large tank
called a clear well is usually built under the filters to hold the clarified water
temporarily. A layer of coarse gravel usually
supports the filter media. When clogged by particles removed from the water,
the filter bed must be cleaned by backwashing. In the backwash process, the
direction of flow through the filter is reversed. Clean water is forced upward
through the media, expanding the filter bed slightly and carrying away the
impurities in wash troughs. The backwash water is distributed uniformly across
the filter bottom by an underdrain system of perforated pipes or porous tile
blocks.
Because of its reliability, the rapid filter is the most
common type of filter used to treat public water supplies. However, other types
of filters may be used, including pressure filters, diatomaceous earth filters, and
microstrainers. A pressure filter has a granular media bed, but, instead of
being open at the top like a gravity-flow rapid filter, it is enclosed in a
cylindrical steel tank. Water is pumped through the filter under pressure.
In diatomaceous earth filters, a natural
powderlike material composed of the shells of microscopic organisms
called diatoms is used as a filter media. The
powder is supported in a thin layer on a metal screen or fabric, and water is
pumped through the layer. Pressure filters and diatomaceous earth filters are
used most often for industrial applications or for public swimming pools.
Microstrainers consist of a finely woven stainless-steel
wire cloth mounted on a revolving drum that is partially submerged in the
water. Water enters through an open end of the drum and flows out through the
screen, leaving suspended solids behind. Captured solids are washed into a
hopper when they are carried up out of the water by the rotating drum.
Microstrainers are used mainly to remove algae from
surface water supplies before conventional gravity-flow filtration. (They can
also be employed in advanced wastewater treatment.)
Disinfection destroys pathogenic bacteria and
is essential to prevent the spread of waterborne disease.
Typically the final process in drinking-water treatment, it is accomplished by
applying either chlorine or chlorine compounds, ozone, or ultraviolet radiation to clarified water.
The addition of chlorine or
chlorine compounds to drinking water is called
chlorination. Chlorine compounds may be applied in liquid and solid forms—for
instance, liquid sodium hypochlorite or calcium hypochlorite in tablet or
granular form. However, the direct application of gaseous chlorine from pressurized
steel containers is usually the most economical method for disinfecting large
volumes of water.
Taste or odour problems are minimized with proper dosages
of chlorine at the treatment plant, and a residual concentration can be
maintained throughout the distribution system to ensure a safe level at the
points of use. Chlorine can combine with certain naturally occurring organic
compounds in water to produce chloroform and
other potentially harmful by-products (trihalomethanes). The risk of this is
small, however, when chlorine is applied after coagulation, sedimentation, and
filtration.
The use of chlorine compounds called chloramines (chlorine
combined with ammonia) for disinfecting public water supplies
has been increasing since the beginning of the 21st century. This disinfection
method is often called chloramination. The disinfecting effect of chloramines
lasts longer than that of chlorine alone, further protecting water quality
throughout the distribution system. Also, chloramines further reduce taste and
odour problems and produce lower levels of harmful by-products, compared with
the use of chlorine alone.
Ozone gas may be used for disinfection of
drinking water. However, since ozone is unstable, it cannot be stored and must
be produced on-site, making the process more expensive than chlorination. Ozone
has the advantage of not causing taste or odour problems; it leaves no residual
in the disinfected water. The lack of an ozone residual, however, makes it
difficult to monitor its continued effectiveness as water flows through the
distribution system.
Ultraviolet radiation destroys pathogens,
and its use as a disinfecting agent eliminates the need to handle chemicals. It
leaves no residual, and it does not cause taste or odour problems. But the high
cost of its application makes it a poor competitor with either chlorine or ozone
as a disinfectant.
Additional treatment
Clarification and disinfection are the conventional
processes for purifying surface water supplies. Other techniques may be used in
addition, or separately, to remove certain impurities, depending on the quality
of the raw water.
Several types of synthetic semipermeable membranes can be
used to block the flow of particles and molecules while allowing smaller water
molecules to pass through under the effect of hydrostatic pressure.
Pressure-driven membrane filtration systems include microfiltration
(MF), ultrafiltration (UF), and reverse osmosis (RO); they differ basically
in the pressures used and pore sizes of the membranes. RO systems operate at
relatively high pressures and can be used to remove dissolved inorganic
compounds from water. (RO is also used for desalination, described below.) Both MF and UF
systems operate under lower pressures and are typically used for the removal of
particles and microbes. They can provide increased assurances of safe drinking water because
the microbial contaminants (viruses, bacteria,
and protozoa) can be completely removed by a physical
barrier. Low-pressure membrane filtration of public water supplies has
increased significantly since the late 1990s because of improvements in
membrane manufacturing technology and decreases in cost.
Softening is the process of removing the dissolved calcium and magnesium salts
that cause hardness in water. It is achieved either by adding chemicals that
form insoluble precipitates or by ion exchange. Chemicals used for softening
include calcium hydroxide (slaked lime) and sodium carbonate (soda ash).
The lime-soda method of water softening must be followed by sedimentation
and filtration in order to remove the precipitates. Ion exchange is accomplished by passing the
water through columns of a natural or synthetic resin that
trades sodium ions for calcium and magnesium ions. Ion-exchange columns must
eventually be regenerated by washing with a sodium
chloride solution.
Aeration is a physical treatment process used for taste and
odour control and for removal of dissolved iron and manganese.
It consists of spraying water into the air or cascading it downward through
stacks of perforated trays. Dissolved gases that cause tastes and odours are
transferred from the water to the air. Oxygen from
the air, meanwhile, reacts with any iron and manganese in the water, forming a
precipitate that is removed by sedimentation and filtration.
Carbon adsorption
An effective method for removing dissolved organic
substances that cause tastes, odours, or colours is adsorption
by activated carbon. Adsorption is
the capacity of a solid particle to attract molecules to its surface. Powdered
carbon mixed with water can adsorb and hold many different organic impurities.
When the carbon is saturated with impurities, it is cleaned or reactivated by
heating to a high temperature in a special furnace.
Many communities reduce the incidence of tooth
decay in young children by adding sodium fluoride or other
fluorine compounds to filtered water.
The dosage of fluoride must be carefully controlled. Low concentrations
are beneficial and cause no harmful side
effects, but very high concentrations of fluoride may cause discoloration of
tooth enamel.
Desalination, or desalting, is the separation of
fresh water from salt water or brackish water. Major advances
in desalination technology have taken place since the 1950s,
as the need for supplies of fresh water has grown in arid and densely populated
areas of the world. Desalted water is the main source of municipal supply in
areas of the Caribbean, the Middle
East, and North
Africa, and its use is increasing in the southeastern United States.
Although it is relatively expensive to produce, desalted water can be more economical
than the alternative of transporting large quantities
of fresh water over long distances.
There are two basic types of desalting techniques: thermal
processes and membrane processes. Both types consume considerable amounts of
energy. Thermal methods involve heat transfer and a phase change of the water from liquid
into vapour or ice. Membrane methods use very thin sheets of special plastic
that act as selective barriers, allowing pure water to be separated from the
salt.
Thermal processes
Distillation, a thermal process that includes
heating, evaporation, and condensation, is the oldest and most widely used
of desalination technologies. Modern methods for the distillation of large
quantities of salt water rely on the fact that the boiling temperature of water
is lowered as air pressure drops, significantly reducing the
amount of energy needed to vaporize the water. Systems that utilize this
principle include multistage flash distillation, multiple-effect
distillation, and vapour-compression distillation.
Multistage flash distillation plants account for more than
half of the world production of desalted water. The process is carried out in a
series of closed vessels (stages) set at progressively lower internal
pressures. Heat is added to the system from a boiler. When preheated salt water
enters a low-pressure chamber, some of it rapidly boils, or flashes, into water
vapour. The vapour is condensed into fresh water on heat-exchange tubes that
run through each stage. These tubes carry incoming seawater, thereby reducing
the heat required from the boiler. Fresh water collects in trays under the
tubes. The remaining brine flows into the next stage at even lower pressure,
where some of it again flashes into vapour. A multistage flash plant may have
as many as 40 stages, permitting salt water to boil repeatedly without
supplying additional heat.
Multiple-effect distillation also takes
place in a series of low-pressure vessels (effects), but it differs from
multistage distillation in that preheated salt water is sprayed onto evaporator
tubes in order to promote rapid evaporation in each vessel. This process
requires pumping the salt water from one effect to the next.
In the vapour-compression system, heat is provided by
the compression of vapour rather than by direct heat input from a boiler. When
the vapour is rapidly compressed, its temperature rises. Some of the compressed
and heated vapour is then recycled through a series of tubes passing through a
reduced-pressure chamber, where evaporation of salt water occurs. Electricity is
the main source of energy for this process. It is used for small-scale
desalting applications—for example, at coastal resorts.
Two other thermal processes are solar humidification and freezing. In solar
humidification, salt water is collected in shallow basins in a “still,” a
structure similar to a greenhouse.
The water is warmed as sunlight enters through inclined glass or plastic
covers. Water vapour rises, condenses on the cooler covers, and trickles down
to a collecting trough. Thermal energy from the sun is free, but a solar still
is expensive to build, requires a large land area, and needs additional energy
for pumping water to and from the facility. Solar humidification units are
suitable for providing desalted water to individual families or for very small
villages where sunlight is abundant.
The freezing process, also called crystallization, involves cooling salt water to
form crystals of pure ice. The ice crystals are separated from the unfrozen
brine, rinsed to remove residual salt, and then melted to produce fresh water.
Freezing is theoretically more efficient than distillation, and scaling as well
as corrosion problems are lessened at the lower
operating temperatures, but the mechanical difficulties of handling mixtures of
ice and water prevent the construction of large-scale commercial
plants. In hot climates, heat leakage into the facility is also a significant
problem.
Membrane processes
Two commercially important membrane processes used for
desalination are electrodialysis and reverse osmosis. They are used mainly to desalt brackish
or highly mineralized water supplies rather than much saltier seawater. In both
methods, thin plastic sheets act as selective barriers, allowing fresh water
but not salt to flow through.
Most salts dissolved in water exist in the form of
electrically charged particles called ions.
Half are positively charged (e.g., sodium), and half are negatively charged
(e.g., chloride). In electrodialysis an electric voltage is
applied across the saline solution. This causes ions to migrate toward the
electrode that has a charge opposite to that of their own. In a typical
electrodialysis unit, several hundred plastic membranes that are selectively
permeable to either positive ions or negative ions, but not both, are closely
spaced in alternation and bound together with electrodes on the outside.
Incoming salt water flows between the membrane sheets. Under the applied
voltage the ions move in opposite directions through the membranes, but they
are trapped by the next membrane in the stack. This forms alternate cells of
dilute salt water and brine. The more-dilute solution is recycled back through
the stack until it reaches freshwater quality.
When a semipermeable membrane separates two solutions of
different concentrations, there is a natural tendency for the concentrations to
become equalized. Water flows from the dilute side to the concentrated side.
This process is called osmosis. However, a high pressure applied to the
concentrated side can reverse the direction of this flow. In reverse osmosis, salty water is pumped into a
vessel and pressurized against the membrane. Fresh water diffuses through the
membrane, leaving a more concentrated salt solution behind.
Next to multistage flash distillation, reverse osmosis is
the second-ranking desalting process. It will play a greater role in the
desalting of seawater and brackish water as more-durable membranes are
developed. It can also be applied to the advanced treatment of municipal sewage
and industrial wastewater.
Cogeneration and hybrid processes
Desalting costs are reduced by using cogeneration and
hybrid processes. Cogeneration (or dual-purpose) desalination plants are
large-scale facilities that produce both electric power and desalted seawater.
Distillation methods in particular are suitable for cogeneration. The
high-pressure steam that runs electric generators can be recycled in the
distillation unit’s brine heater. This significantly reduces fuel consumption compared with what is required
if separate facilities are built. Cogeneration is very common in the Middle
East and North Africa.
Hybrid systems are units that operate with two or more
different desalting processes (e.g., distillation and reverse osmosis). They
offer further economic benefits when employed in cogeneration plants,
productively combining the operation of each process.
Effluent disposal
Desalination produces fresh water but
also a significant volume of waste effluent, called brine.
Since the primary pollutant in the brine is salt, disposal in the ocean is
generally not a problem for facilities located near a coastline. At
inland desalination facilities, care must be taken
to prevent pollution of groundwater or
surface waters. Methods of brine disposal include dilution, evaporation,
injection into a saline aquifer,
and pipeline transport to a suitable disposal point.
Water Distribution
A water distribution system is a network of pumps,
pipelines, storage tanks, and other appurtenances. It must deliver adequate
quantities of water at pressures sufficient for operating plumbing fixtures
and firefighting equipment, yet it must not deliver water at pressures high enough
to increase the occurrence of leaks and pipeline breaks. Pressure-regulating
valves may be installed to reduce pressure levels
in low-lying service areas. More than half the cost of a municipal water
supply system is for the distribution network.
The pipeline system of a municipal water distribution
network consists of arterial water mains or primary feeders, which convey
water from the treatment plant to areas of major water use in the community, and smaller-diameter pipelines
called secondary feeders, which tie in to the mains. Usually not less than
150 mm (6 inches) in diameter, these pipelines are placed within the public
right-of-way so that service connections can be made for all potential water
users. The pipelines are usually arranged in a gridiron pattern that allows
water to circulate in interconnected loops; this permits any broken sections of
pipe to be isolated for repair without disrupting service to large areas of the
community. “Dead-end” patterns may also be used, but they do not permit
circulation, and the water they provide is more susceptible to taste and odour
problems because of stagnation.
A water distribution pipeline must be able to resist
internal and external forces, as well as corrosion.
Pipes are placed under stress by internal water pressure, by the weight of the
overlying soil, and by vehicles passing above. They may have to withstand
water-hammer forces; these occur when valves are closed too rapidly, causing
pressure waves to surge through the system. In addition, metal pipes may rust internally
if the water supply is corrosive or externally because of corrosive soil
conditions.
Materials
Distribution pipes are made of asbestos cement, cast
iron, ductile iron, plastic, reinforced concrete, or steel.
Although not as strong as iron, asbestos cement, because of its corrosion
resistance and ease of installation, is a desirable material for secondary
feeders up to 41 cm (16 inches) in diameter. Pipe sections are easily joined
with a coupling sleeve and rubber-ring gasket. Cast
iron has an excellent record of service, with many
installations still functioning after 100 years. Ductile iron, a stronger and more elastic type of
cast iron, is used in newer installations. Iron pipes are provided in diameters
up to 122 cm (48 inches) and are usually coated to prevent corrosion.
Underground sections are connected with bell-and-spigot joints, the spigot end
of one pipe section being pushed into the bell end of an adjacent section. A rubber-ring gasket in
the bell end is compressed when the two sections are joined, creating a
watertight, flexible connection. Flanged and bolted joints are used for above ground
installations.
Plastic pipes are available in diameters up
to 61 cm (24 inches). They are lightweight, easily installed, and
corrosion-resistant, and their smoothness provides good hydraulic
characteristics. Plastic pipes are connected either by a bell-and-spigot
compression-type joint or by threaded screw couplings.
Precast reinforced concrete pipe sections up to 366
cm (12 feet) in diameter are used for arterial mains. Reinforced concrete pipes
are strong and durable. They are joined using a bell-and-spigot-type connection
that is sealed with cement mortar. Steel pipe
is sometimes used for arterial mains in aboveground installations. It is very
strong and lighter than concrete pipe, but it must be protected against
corrosion with lining of the interior and with painting and wrapping of the
exterior. Sections of steel pipe are joined by welding or with mechanical
coupling devices.
Fittings
In order to function properly, a water distribution system
requires several types of fittings, including hydrants, shutoff valves, and
other appurtenances. The main purpose of hydrants is to provide water for
firefighting. They also are used for flushing water mains, pressure testing,
water sampling, and washing debris off public streets.
Many types of valves are used to control the quantity and
direction of water flow. Gate valves are usually installed throughout the pipe
network. They allow sections to be shut off and isolated during the repair of
broken mains, pumps, or hydrants. A type of valve commonly used for throttling
and controlling the rate of flow is the butterfly valve. Other valves used in water
distribution systems include pressure-reducing valves, check valves, and
air-release valves.
Installation
Water mains must be placed roughly 1 to 2 metres (3 to 6
feet) below the ground surface in order to protect against traffic loads and to
prevent freezing. Since the water in a distribution system is under pressure,
pipelines can follow the shape of the land, uphill as well as downhill. They
must be installed with proper bedding and backfill. Compaction of soil layers
under the pipe (bedding) as well as above the pipe (backfill) is necessary to
provide proper support. A water main should never be installed in the same
trench with a sewer line. Where the two must cross, the water main should be
placed above the sewer line.
Many kinds of pumps are used in distribution systems. Pumps
that lift surface water and move it to a nearby treatment
plant are called low-lift pumps. These move large volumes of water at
relatively low discharge pressures. Pumps that discharge treated water into
arterial mains are called high-lift pumps. These operate under higher
pressures. Pumps that increase the pressure within
the distribution system or raise water into an elevated storage tank are
called booster pumps. Well pumps lift water from underground and discharge
it directly into a distribution system.
Most water distribution pumps are of the centrifugal type, in which a rapidly
rotating impeller adds energy to the water and raises the pressure inside
the pump casing. The flow rate through a centrifugal pump depends on the pressure
against which it operates. The higher the pressure, the lower the flow or
discharge. Another kind of pump is the positive-displacement type. This pump
delivers a fixed quantity of water with each cycle of a piston or rotor. The
water is literally pushed or displaced from the pump casing. The flow capacity
of a positive-displacement pump is unaffected by the pressure of the system in
which it operates.
Storage tanks
Distribution storage tanks, familiar sights in many communities, serve two basic purposes: equalizing
storage and emergency storage. Equalizing storage is the volume of water needed
to satisfy peak hourly demands in the community. During the late night and very early
morning hours, when water demand is lower, high-lift pumps fill the tank.
During the day, when water demand is higher, water flows out of the tank to
help satisfy the peak hourly water needs. This allows for a uniform flow rate
at the treatment plant and pumping station. Water in a distribution storage
tank may also be needed for fighting fires, cleaning up accidental spills of
hazardous materials, or other community emergencies. The capacity of a
distribution storage tank is designed to be about equal to the average daily
water demand of the community.
Distribution storage tanks are built at ground level on
hilltops higher than the service area. In areas with flat topography, the tanks may be elevated aboveground
on towers in order to provide adequate water pressures, or ground-level storage
tanks with booster pumping may be provided.
Jerry
A. Nathanson
Professor
of Engineering, Union County College, Cranford, New Jersey. Author of Basic
Environmental Technology: Water Supply, Waste Disposal, and Pollution Control.
The primary water reservoir of São Paulo, Braz.
qanāt
Segovia
aqueduct
In the hydrologic cycle, water is transferred
between the land surface, the ocean, and the atmosphere. The numbers on the
arrows indicate relative water fluxes.
Glen Canyon DamConstruction of the Glen Canyon
Dam on the Colorado River formed Lake Powell in Arizona.
water
purification plant
Basic steps in the treatment of municipal water
Schematic diagram of a rapid-filter water
treatment facility.
A water tower in Pecos, Texas, U.S.
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