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 other 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 that 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
produces 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.
2.
3.
4.
Salt water is heated to boiling
Water is evaporated but the salt remains
The water vapor is cooled
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 strainer 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 purification.
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 bottom 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 water 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
-ozone 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.