10.5
Making Water Drinkable
In Canada, we usually get our drinking water from wells, lakes,
or rivers. Sometimes it is pumped from these sources or stored
behind dams in reservoirs. However, in nature, water often contains
impurities and toxins. How is fresh water made safe for drinking?
The Water Treatment Process
Sources of fresh water are seldom pure enough to be used directly
as drinking water because water often contains dissolved substances.
Even water that is clear, colourless, and odourless may contain
substances and organisms that make it unsafe to drink. We want to
remove all potentially dangerous substances before we drink water
from any source. Water treatment is the process of making water
suitable for a desired end-use (such as drinking). Look at Figure 1
carefully to learn how a typical water treatment process works.
water treatment: the process of
removing potentially dangerous
substances from water to make
it more acceptable for a desired
end-use
addition of chemicals
river, lake, or reservoir
intake
sedimentation
filtration
coagulation
and
flocculation
your neighbourhood
disinfection
storage
Figure 1 Follow water through the treatment process to your neighbourhood.
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To obtain clean drinking water, the dangerous substances must be
removed. The water treatment process follows the steps outlined below.
• The first step is to remove any large objects, such as sticks, leaves,
and garbage, from the water. A metal screen covers the end of the
pipe that draws the water from its source. This screen blocks objects
from entering the treatment plant with the water.
• The water enters a large tank where alum (a chemical) is added.
The alum is stirred into the water and forms small, sticky lumps
floc: sticky clumps formed from the
that attract most of the waste solids floating in the water. This
reaction of alum in water, combined
combination of chemical and waste solids is called floc.
with sand and other waste solids
• The water and floc enter a settling tank (Figure 2). The water flows
slowly to allow the floc to settle to the bottom of the tank.
• The partially cleaned water from the top of the
settling tank now moves through a sand and
charcoal filter. This removes any remaining waste
solids and small pieces of floc that did not settle
in the previous step. The water is now clear,
colourless, and odourless.
• The water may still contain micro-organisms,
such as bacteria, that can make people ill.
Chlorine or ozone can be added to the water
to kill these organisms. Chlorine is particularly
effective because it remains active in water.
• The treated water can then be stored in sealed
Figure 2 These huge settling tanks at a water treatment plant
containers, ready for use. This ensures a reliable
allow the floc to sink to the bottom.
flow of water whenever we need it.
Other Treatment Options
Water can be treated in several ways. These methods can be used
instead of, or in addition to, the steps described above.
• Reverse osmosis is a technological process in which mechanical
pressure forces water through a membrane that acts like a fine filter.
The holes in the membrane are so tiny that only water particles can
pass through. All other substances are trapped and removed. The
membrane must be properly maintained to prevent algae and other
organisms from growing on it. Reverse osmosis is sometimes used
to treat well water that supplies individual homes or businesses. It is
rarely used as a municipal water treatment.
• Ultraviolet (UV) radiation effectively destroys most viruses and
micro-organisms, such as bacteria (Figure 3). This method of water
treatment is sometimes used when it is especially important that
the water be free of organisms and viruses, such as the water supply
for a hospital. Micro-organisms can contaminate water stored in
containers before it is consumed, so stored water is continuously
exposed to UV radiation to prevent re-contamination.
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Figure 3 UV radiation is increasingly
being used in water treatment. Many
bottlers of spring water now use UV
disinfection equipment to remove
micro-organisms from water.
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• In nature, water absorbs the minerals calcium and magnesium from
surrounding rock. This makes the water “hard.” Hard water is safe
to drink, but when heated it leaves mineral deposits in water pipes,
hot-water heaters, and kettles (Figure 4). Adding water-softening
chemicals removes the “hardness” from the water.
Figure 4 Kettles and other containers
that hold heated water often have
limescale, which is a deposit of
minerals that are insoluble in water
at high temperatures.
• Boiling is used as an emergency measure if there is a breakdown in
the normal water treatment system. Most disease-causing organisms
can be killed by boiling water for one to three minutes. Boiled water
should be used within a few hours or stored in the refrigerator.
Using Water Sustainably
sustainability: being able to meet
the needs of the present without
compromising the ability of future
generations to meet their own needs
To learn more about using
water sustainably,
Go to Nelson Science
Water treatment processes do not remove all chemicals from water.
There is no process at water treatment plants to remove substances
such as pharmaceuticals (drugs such as antibiotics and artificial
hormones) and oil-based liquids (motor oil and paint) from the
water. It is very important that we keep these substances out of our
water supply.
Only a tiny fraction of the world’s water is suitable for human use.
A rising world population, along with pollution, is putting increasing
pressure on this limited resource. Sustainability means working
and behaving in a way that protects resources to ensure that they are
available to future generations. The sustainability of the world’s water
resources is an urgent global issue. Are we using water in a way that
will make it available to future generations? Using water sustainably is
everyone’s responsibility.
Unit Task
There are several ideas in this section that might be useful as you
consider a water issue for the Unit Task. Make a list of relevant ideas.
CHECK YOUR LEARNING
C
1. List and describe the six main steps that water goes through
in the water treatment process.
3. Describe how the membrane works in the reverse osmosis
process. Use a labelled diagram in your description.
2. Is clear, colourless, and odourless drinking water always safe
to drink? Explain.
4. (a) Why is it important to ensure that treated water remains
safe to drink when it is stored after treatment?
(b) Describe one way to make stored water safe.
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