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Water Demand, Consumption And Treatment
Municipal water consumption
WRITTEN
BY:
The
Editors of Encyclopaedia Britannica
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 Treatment
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
Sedimentation
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 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.
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