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Chapter 20 Study Guide


Ally Harper

20.3

Water Treatment


In the United States and many other developed nations, water is treated in order to
remove impurities.


Safety Checks:
monitor the quality of water to ensure it is safe to drink.


Tap water is not considered safe to drink in
South America, Mexico, China and parts of
Africa.


A
large supply of fresh water

is essential to a nations development.


Areas in the world that don’t have a large supply of water many not be able to support
their
human populations or populations of othe
r organisms.


Some nations use
alternative sources

to increase their supply of fresh water. They may
look for the seas, salt lakes, or oceans for water. Penguins can tolerate the high
concentration of salt in ocean water because they have
special glands

t
hat remove salts.


Desalination


Desalination:
is the process by which salts are removed from water


Desalination can be used to obtain fresh water for drinking, cooking and irrigation. It can
also be used to remove salt from agricultural waste water.


High concentrations of salt in waters of
Colorado River

were killing crops in Mexico. The
salts were leaching into the water as a result of the irrigation of farmland.

-

The solution was the US built a desalination plant near Yuma, Arizona. The plant
produ
ces water with a lower salt content which is returned to the river for use by
Mexican Farmers.


Three Most Common Methods of Desalination


-
Distillation


-
Reverse Osmosis


-
Freezing


They are all very
expensive.


Chapter 20 Study Guide


Ally Harper

The cost of obtaining fresh water through these methods may be as much as
4 times
greater than the cost from traditional sources.


Distillation



Steps

1.

Salt water is heated to boiling

2.

Water is evaporated but the salt remains

3.

The
water vapor

is cooled

4.

The liquid water is collected


Reverse Osmosis


Steps

1.

Salt water is forced through strainer* that traps the salt and lets the fresh
water pass

2.

The
pores

in the
strainer

are large enough for the water to pass through but
too small for the salt.


*A
straine
r
is a thin membrane with tiny pores.


Freezing


Steps

1.

Salt water is frozen

2.

As it freezes it separates forming ice and brine slush

3.

The ice is free of almost all salt and can be melted to obtain fresh water


A desalination plant in
Wrightsville Beach, North Carolina,
obtains almost
1 million L

of
fresh water each day using the freezing method.


Water Purification


The treatment of fresh water for the removal of minerals may sometimes be combined
with the treatment of water for pu
rification.


Water Purification Processes:


-
sedimentation


-
filtration

Chapter 20 Study Guide


Ally Harper


-
aeration


-
sterilization


Sedimentation and Filtration


In many water
-
treatment plants,
screens

are used to trap and remove debris that floats
or is suspended in water.


Steps

1.

Screen

water

2.

Once screened, the water is put into a settling tank where it is allowed to
stand undisturbed

3.

As the water stands, particles
suspended

in the water settle to the bottom
as sediment

4.

After the water is
drained
away, the sediment is removed from
the setting
tank

5.

The water drained from the
settling tank

is then ready for filtering

6.

The water is passed through a
1 m thick
layer of fine sand

7.

The sand filters out many of the particles that were not removed by
sedimentation


Some particles in the water

are so fine they do not settle out but remain in the water.


Often chemicals called
coagulants
are added to the water to aid the settling process


Coagulant


-
it causes the fine particles to clump together


-
the heavier particle masses settle to the bot
tom of the tank


Together the sedimentation and coagulation
remove most suspended particles

like
bacteria from the water.


Aeration and Sterilization


Water that has to be cleaned and filtered may still have undesirable qualities.


-
dissolved gases in wa
ter may give it an unpleasant taste or odor


-
harmful bacteria break down organic matter still present in the water


Taste, odor and bacteria
are treated through aeration and sterilization


Chapter 20 Study Guide


Ally Harper

Aeration
is the exposure of water to air


-
aeration is achieved by

spraying
the water into the air or by allowing it to flow as
a waterfall


In aeration
bacteria

that aid in purification enter the water. These bacteria break down
organic matter still present in the water.


-
At the same time
oxygen*
mixes with the water


*oxygen is a powerful purifying agent


Any
harmful bacteria and microorganisms
introduced by the previous processes are killed
through sterilization


Sterilization


-
can occur through the use of
extreme heat or chemicals.


-
the chemicals most often used

in water sterilization are
chlorine and ozone


Chlorine
is a very powerful purifier


-
it can be produced and stored easily


-
it destroys microorganisms and removes unwanted
odors, colors and tastes.


-
chlorine adds a distinctive
smell
to drinking water



-
the drinking water may smell like the water in swimming pools


Ozone
is also a strong purifying agent and sterilizes water more rapidly than chlorine


-
the use of ozone is
limited
because it is more
expensive
and more difficult to use
than chlorine


-
ozo
ne must be
refrigerated
and can only be stored for a short amount of time


Both Chlorine and Ozone are dangerous


-
very little
chlorine and ozone are needed to purify large volumes of water.


-
some studies show that the use of chlorine may produce other
harmful chemicals.