NCERT book Class 7 - Science - Wastewater Story

 Page 1


SCIENCE 156
Wastewater Story
13
All of us use water in our homes and
make it dirty.
Dirty! Are you surprised?
Rich in lather, mixed with oil, black–
brown water that goes down the drains
from sinks, showers, toilets, laundries
is dirty. It is called wastewater. This
used water should not be wasted. We
must clean it up by removing pollutants.
Have you ever thought where the
wastewater goes and what happens to it?
13.1 WATER, OUR LIFELINE
Clean water is a basic need of human
being. Let us make a mindmap of the
many uses of clean water.
Activity 13.1
(We have given one example of the
use of clean water. You can add many
more.)
Clean water that is fit
for use is unfortunately
not available to all. It has
been reported that more
than one billion of people
have no access to safe
drinking water. This accounts for a large
number of water-related diseases and
even deaths. People even children walk
for several kilometres to collect clean
water. Is it not a serious matter for
human dignity?
The increasing scarcity of fresh-water
due to population growth, pollution,
industrial development,mismanagement
and other factors. Realising the urgency
of the situation on the World Water Day,
on 22 March 2005, the General
Assembly of the United Nations
proclaimed the period 2005– 2015 as the
International Decade for action on
“Water for life”. All efforts made during
this decade aim to reduce by half the
number of people who do not have
access to safe drinking water.
There has been perceptible progress
in the direction of the aim but still there
is a lot to achieve.
Cleaning of water is a process of
removing pollutants before it enters a
water body or is reused. This process of
wastewater treatment is commonly
Clean water
put to use
Drinking
Reprint 2024-25
Page 2


SCIENCE 156
Wastewater Story
13
All of us use water in our homes and
make it dirty.
Dirty! Are you surprised?
Rich in lather, mixed with oil, black–
brown water that goes down the drains
from sinks, showers, toilets, laundries
is dirty. It is called wastewater. This
used water should not be wasted. We
must clean it up by removing pollutants.
Have you ever thought where the
wastewater goes and what happens to it?
13.1 WATER, OUR LIFELINE
Clean water is a basic need of human
being. Let us make a mindmap of the
many uses of clean water.
Activity 13.1
(We have given one example of the
use of clean water. You can add many
more.)
Clean water that is fit
for use is unfortunately
not available to all. It has
been reported that more
than one billion of people
have no access to safe
drinking water. This accounts for a large
number of water-related diseases and
even deaths. People even children walk
for several kilometres to collect clean
water. Is it not a serious matter for
human dignity?
The increasing scarcity of fresh-water
due to population growth, pollution,
industrial development,mismanagement
and other factors. Realising the urgency
of the situation on the World Water Day,
on 22 March 2005, the General
Assembly of the United Nations
proclaimed the period 2005– 2015 as the
International Decade for action on
“Water for life”. All efforts made during
this decade aim to reduce by half the
number of people who do not have
access to safe drinking water.
There has been perceptible progress
in the direction of the aim but still there
is a lot to achieve.
Cleaning of water is a process of
removing pollutants before it enters a
water body or is reused. This process of
wastewater treatment is commonly
Clean water
put to use
Drinking
Reprint 2024-25
WASTEWATER STORY 157
known as “Sewage Treatment”. It takes
place in several stages.
13.2 WHAT IS SEWAGE?
Sewage is wastewater released by homes,
industries, hospitals, offices and other
users. It also includes rainwater that has
run down the street during a storm or
heavy rain. The water that washes off
roads and rooftops carries harmful
substances with it. Sewage is a liquid
waste. Most of it is water, which has
dissolved and suspended impurities.
Activity 13.2
Locate an open drain near your home,
school or on the roadside and inspect
water flowing through it.
Record colour, odour and any other
observation. Discuss with your friends
and your teacher and fill up the
following Table 13.1.
We know that sewage is a complex
mixture containing suspended solids,
organic and inorganic impurities,
nutrients, saprophytes and disease
causing bacteria and other microbes.
These include the following.
Organic impurities   –Human faeces,
animal waste,
oil, urea (urine),
pesticides,
herbicides, fruit
and vegetable
waste, etc.
Inorganic impurities – Nitrates,
Phosphates,
metals.
Nutrients                  – Phosphorus
and Nitrogen.
Bacteria                    – Such as vibrio
cholera which
causes cholera
and salmonella
paratyphi which
causes typhoid.
Other microbes         – Such as
protozones
which cause
dysentery.
13.3 WATER FRESHENS UP — AN
EVENTFUL JOURNEY
In a home or a public building generally
one set of pipes brings clean water and
another set of pipes takes away
wastewater. Imagine that we could see
through the ground. We would see a
network of big and small pipes, called
Table 13.1 Contaminant survey
S. No. Type of sewage Point of origin Contaminants Any other
remark
1. Sullage water Kitchen
2. Foul waste Toilets
3. Trade waste Industrial
and commercial
organisations
Reprint 2024-25
Page 3


SCIENCE 156
Wastewater Story
13
All of us use water in our homes and
make it dirty.
Dirty! Are you surprised?
Rich in lather, mixed with oil, black–
brown water that goes down the drains
from sinks, showers, toilets, laundries
is dirty. It is called wastewater. This
used water should not be wasted. We
must clean it up by removing pollutants.
Have you ever thought where the
wastewater goes and what happens to it?
13.1 WATER, OUR LIFELINE
Clean water is a basic need of human
being. Let us make a mindmap of the
many uses of clean water.
Activity 13.1
(We have given one example of the
use of clean water. You can add many
more.)
Clean water that is fit
for use is unfortunately
not available to all. It has
been reported that more
than one billion of people
have no access to safe
drinking water. This accounts for a large
number of water-related diseases and
even deaths. People even children walk
for several kilometres to collect clean
water. Is it not a serious matter for
human dignity?
The increasing scarcity of fresh-water
due to population growth, pollution,
industrial development,mismanagement
and other factors. Realising the urgency
of the situation on the World Water Day,
on 22 March 2005, the General
Assembly of the United Nations
proclaimed the period 2005– 2015 as the
International Decade for action on
“Water for life”. All efforts made during
this decade aim to reduce by half the
number of people who do not have
access to safe drinking water.
There has been perceptible progress
in the direction of the aim but still there
is a lot to achieve.
Cleaning of water is a process of
removing pollutants before it enters a
water body or is reused. This process of
wastewater treatment is commonly
Clean water
put to use
Drinking
Reprint 2024-25
WASTEWATER STORY 157
known as “Sewage Treatment”. It takes
place in several stages.
13.2 WHAT IS SEWAGE?
Sewage is wastewater released by homes,
industries, hospitals, offices and other
users. It also includes rainwater that has
run down the street during a storm or
heavy rain. The water that washes off
roads and rooftops carries harmful
substances with it. Sewage is a liquid
waste. Most of it is water, which has
dissolved and suspended impurities.
Activity 13.2
Locate an open drain near your home,
school or on the roadside and inspect
water flowing through it.
Record colour, odour and any other
observation. Discuss with your friends
and your teacher and fill up the
following Table 13.1.
We know that sewage is a complex
mixture containing suspended solids,
organic and inorganic impurities,
nutrients, saprophytes and disease
causing bacteria and other microbes.
These include the following.
Organic impurities   –Human faeces,
animal waste,
oil, urea (urine),
pesticides,
herbicides, fruit
and vegetable
waste, etc.
Inorganic impurities – Nitrates,
Phosphates,
metals.
Nutrients                  – Phosphorus
and Nitrogen.
Bacteria                    – Such as vibrio
cholera which
causes cholera
and salmonella
paratyphi which
causes typhoid.
Other microbes         – Such as
protozones
which cause
dysentery.
13.3 WATER FRESHENS UP — AN
EVENTFUL JOURNEY
In a home or a public building generally
one set of pipes brings clean water and
another set of pipes takes away
wastewater. Imagine that we could see
through the ground. We would see a
network of big and small pipes, called
Table 13.1 Contaminant survey
S. No. Type of sewage Point of origin Contaminants Any other
remark
1. Sullage water Kitchen
2. Foul waste Toilets
3. Trade waste Industrial
and commercial
organisations
Reprint 2024-25
SCIENCE 158
sewers, forming the sewerage. It is like
a transport system that carries sewage
from the point of being produced to the
point of disposal, i.e. treatment plant.
Manholes are located at every 50 m
to 60 m in the sewerage, at the junction
of two or more sewers and at points
where there is a change in direction.
Activity 13.3
Study the sewage route in your home/
school/building. Do the following:
? Make a line diagram of the sewage
route.
? Walk down the street or survey the
campus to find the number of
manholes.
? If possible, observe open drain and
record which living organisms are
found in and around it.
In case you do not have a sewerage
system in your locality, find out how
sewage is being disposed off.
Treatment of polluted water
Perform the following activity. It will help
you understand the processes that take
place at the wastewater treatment plant.
Activity 13.4
Divide yourself into groups to perform
the activity. Record observations at each
stage:
? Fill a large glass jar 3/4 full of water.
Add some dirty organic matter such
as grass pieces or orange peels, a
small amount of detergent, and a few
drops of an ink or any colour.
? Cap the jar, shake it well and let the
mixture stand in the sun for two
days.
? After two days, shake the mixture
and pour a small sample into test
tube. Label this test tube “Before
treatment; Sample 1”. How does it
smell?
? Use an aerator from an aquarium to
bubble air through the sample in the
glass jar. Allow several hours for
aeration; leave the aerator attached
overnight. If you do not have an
aerator, use a mechanical stirrer or
a mixer. You may have to stir it
several times.
? The next day when aeration is
complete, pour another sample into
a second test tube. Label it as “After
aeration; Sample 2”.
? Fold a piece of filter paper to form a
cone. Wet the paper with tap water
and then insert the cone in a funnel.
Mount the funnel on a support
(as you have learnt in Class VI).
? Place layers of sand, fine gravel and
finally medium gravel in the funnel
(Fig. 13.2). (An actual filtration plant
does not use filter paper, but the sand
filter is several metres deep).
? Pour the remaining aerated liquid
through the filter into the beakers.
Do not allow the liquid to spill over
the filter. If the filtered liquid is not
clear, filter it a few  times till you get
clear water.
? Pour a sample of the filtered water
into a third test tube labelled
“Filtered; Sample 3”.
Reprint 2024-25
Page 4


SCIENCE 156
Wastewater Story
13
All of us use water in our homes and
make it dirty.
Dirty! Are you surprised?
Rich in lather, mixed with oil, black–
brown water that goes down the drains
from sinks, showers, toilets, laundries
is dirty. It is called wastewater. This
used water should not be wasted. We
must clean it up by removing pollutants.
Have you ever thought where the
wastewater goes and what happens to it?
13.1 WATER, OUR LIFELINE
Clean water is a basic need of human
being. Let us make a mindmap of the
many uses of clean water.
Activity 13.1
(We have given one example of the
use of clean water. You can add many
more.)
Clean water that is fit
for use is unfortunately
not available to all. It has
been reported that more
than one billion of people
have no access to safe
drinking water. This accounts for a large
number of water-related diseases and
even deaths. People even children walk
for several kilometres to collect clean
water. Is it not a serious matter for
human dignity?
The increasing scarcity of fresh-water
due to population growth, pollution,
industrial development,mismanagement
and other factors. Realising the urgency
of the situation on the World Water Day,
on 22 March 2005, the General
Assembly of the United Nations
proclaimed the period 2005– 2015 as the
International Decade for action on
“Water for life”. All efforts made during
this decade aim to reduce by half the
number of people who do not have
access to safe drinking water.
There has been perceptible progress
in the direction of the aim but still there
is a lot to achieve.
Cleaning of water is a process of
removing pollutants before it enters a
water body or is reused. This process of
wastewater treatment is commonly
Clean water
put to use
Drinking
Reprint 2024-25
WASTEWATER STORY 157
known as “Sewage Treatment”. It takes
place in several stages.
13.2 WHAT IS SEWAGE?
Sewage is wastewater released by homes,
industries, hospitals, offices and other
users. It also includes rainwater that has
run down the street during a storm or
heavy rain. The water that washes off
roads and rooftops carries harmful
substances with it. Sewage is a liquid
waste. Most of it is water, which has
dissolved and suspended impurities.
Activity 13.2
Locate an open drain near your home,
school or on the roadside and inspect
water flowing through it.
Record colour, odour and any other
observation. Discuss with your friends
and your teacher and fill up the
following Table 13.1.
We know that sewage is a complex
mixture containing suspended solids,
organic and inorganic impurities,
nutrients, saprophytes and disease
causing bacteria and other microbes.
These include the following.
Organic impurities   –Human faeces,
animal waste,
oil, urea (urine),
pesticides,
herbicides, fruit
and vegetable
waste, etc.
Inorganic impurities – Nitrates,
Phosphates,
metals.
Nutrients                  – Phosphorus
and Nitrogen.
Bacteria                    – Such as vibrio
cholera which
causes cholera
and salmonella
paratyphi which
causes typhoid.
Other microbes         – Such as
protozones
which cause
dysentery.
13.3 WATER FRESHENS UP — AN
EVENTFUL JOURNEY
In a home or a public building generally
one set of pipes brings clean water and
another set of pipes takes away
wastewater. Imagine that we could see
through the ground. We would see a
network of big and small pipes, called
Table 13.1 Contaminant survey
S. No. Type of sewage Point of origin Contaminants Any other
remark
1. Sullage water Kitchen
2. Foul waste Toilets
3. Trade waste Industrial
and commercial
organisations
Reprint 2024-25
SCIENCE 158
sewers, forming the sewerage. It is like
a transport system that carries sewage
from the point of being produced to the
point of disposal, i.e. treatment plant.
Manholes are located at every 50 m
to 60 m in the sewerage, at the junction
of two or more sewers and at points
where there is a change in direction.
Activity 13.3
Study the sewage route in your home/
school/building. Do the following:
? Make a line diagram of the sewage
route.
? Walk down the street or survey the
campus to find the number of
manholes.
? If possible, observe open drain and
record which living organisms are
found in and around it.
In case you do not have a sewerage
system in your locality, find out how
sewage is being disposed off.
Treatment of polluted water
Perform the following activity. It will help
you understand the processes that take
place at the wastewater treatment plant.
Activity 13.4
Divide yourself into groups to perform
the activity. Record observations at each
stage:
? Fill a large glass jar 3/4 full of water.
Add some dirty organic matter such
as grass pieces or orange peels, a
small amount of detergent, and a few
drops of an ink or any colour.
? Cap the jar, shake it well and let the
mixture stand in the sun for two
days.
? After two days, shake the mixture
and pour a small sample into test
tube. Label this test tube “Before
treatment; Sample 1”. How does it
smell?
? Use an aerator from an aquarium to
bubble air through the sample in the
glass jar. Allow several hours for
aeration; leave the aerator attached
overnight. If you do not have an
aerator, use a mechanical stirrer or
a mixer. You may have to stir it
several times.
? The next day when aeration is
complete, pour another sample into
a second test tube. Label it as “After
aeration; Sample 2”.
? Fold a piece of filter paper to form a
cone. Wet the paper with tap water
and then insert the cone in a funnel.
Mount the funnel on a support
(as you have learnt in Class VI).
? Place layers of sand, fine gravel and
finally medium gravel in the funnel
(Fig. 13.2). (An actual filtration plant
does not use filter paper, but the sand
filter is several metres deep).
? Pour the remaining aerated liquid
through the filter into the beakers.
Do not allow the liquid to spill over
the filter. If the filtered liquid is not
clear, filter it a few  times till you get
clear water.
? Pour a sample of the filtered water
into a third test tube labelled
“Filtered; Sample 3”.
Reprint 2024-25
WASTEWATER STORY 159
? Pour another sample of the filtered
water into a fourth test tube. Add a
small piece of a chlorine tablet. Mix
well until the water is clear. Label the
test tube “Chlorinated; Sample 4”.
? Observe carefully the samples in all
the test tubes. Do not taste! Just
smell them!
Now answer the following questions:
(a) What changes did you observe in the
appearance of the liquid after
aeration?
(b) Did aeration change the odour?
(c) What was removed by the sand filter?
(d) Did chlorine remove the colour?
(e) Did chlorine have an odour? Was it
worse than that of the wastewater?
13.4 WASTEWATER TREATMENT PLANT
(WWTP)
Treatment of wastewater involves
physical, chemical, and biological
processes, which remove physical,
chemical and biological matter that
contaminates the wastewater.
1. Wastewater is passed through bar
screens. Large objects like rags,
sticks, cans, plastic packets, napkins
are removed (Fig. 13.3).
Fig. 13.2  Filtration process
Fig. 13.3  Bar screen
Fig. 13.4  Grit and sand removal tank
2. Water then goes to a grit and sand
removal tank. The speed of the
incoming wastewater is decreased to
allow sand, grit and pebbles to settle
down (Fig. 13.4).
3. The water is then allowed to settle in
a large tank which is sloped towards
the middle. Solids like faeces settle
at the bottom and are removed with
Reprint 2024-25
Page 5


SCIENCE 156
Wastewater Story
13
All of us use water in our homes and
make it dirty.
Dirty! Are you surprised?
Rich in lather, mixed with oil, black–
brown water that goes down the drains
from sinks, showers, toilets, laundries
is dirty. It is called wastewater. This
used water should not be wasted. We
must clean it up by removing pollutants.
Have you ever thought where the
wastewater goes and what happens to it?
13.1 WATER, OUR LIFELINE
Clean water is a basic need of human
being. Let us make a mindmap of the
many uses of clean water.
Activity 13.1
(We have given one example of the
use of clean water. You can add many
more.)
Clean water that is fit
for use is unfortunately
not available to all. It has
been reported that more
than one billion of people
have no access to safe
drinking water. This accounts for a large
number of water-related diseases and
even deaths. People even children walk
for several kilometres to collect clean
water. Is it not a serious matter for
human dignity?
The increasing scarcity of fresh-water
due to population growth, pollution,
industrial development,mismanagement
and other factors. Realising the urgency
of the situation on the World Water Day,
on 22 March 2005, the General
Assembly of the United Nations
proclaimed the period 2005– 2015 as the
International Decade for action on
“Water for life”. All efforts made during
this decade aim to reduce by half the
number of people who do not have
access to safe drinking water.
There has been perceptible progress
in the direction of the aim but still there
is a lot to achieve.
Cleaning of water is a process of
removing pollutants before it enters a
water body or is reused. This process of
wastewater treatment is commonly
Clean water
put to use
Drinking
Reprint 2024-25
WASTEWATER STORY 157
known as “Sewage Treatment”. It takes
place in several stages.
13.2 WHAT IS SEWAGE?
Sewage is wastewater released by homes,
industries, hospitals, offices and other
users. It also includes rainwater that has
run down the street during a storm or
heavy rain. The water that washes off
roads and rooftops carries harmful
substances with it. Sewage is a liquid
waste. Most of it is water, which has
dissolved and suspended impurities.
Activity 13.2
Locate an open drain near your home,
school or on the roadside and inspect
water flowing through it.
Record colour, odour and any other
observation. Discuss with your friends
and your teacher and fill up the
following Table 13.1.
We know that sewage is a complex
mixture containing suspended solids,
organic and inorganic impurities,
nutrients, saprophytes and disease
causing bacteria and other microbes.
These include the following.
Organic impurities   –Human faeces,
animal waste,
oil, urea (urine),
pesticides,
herbicides, fruit
and vegetable
waste, etc.
Inorganic impurities – Nitrates,
Phosphates,
metals.
Nutrients                  – Phosphorus
and Nitrogen.
Bacteria                    – Such as vibrio
cholera which
causes cholera
and salmonella
paratyphi which
causes typhoid.
Other microbes         – Such as
protozones
which cause
dysentery.
13.3 WATER FRESHENS UP — AN
EVENTFUL JOURNEY
In a home or a public building generally
one set of pipes brings clean water and
another set of pipes takes away
wastewater. Imagine that we could see
through the ground. We would see a
network of big and small pipes, called
Table 13.1 Contaminant survey
S. No. Type of sewage Point of origin Contaminants Any other
remark
1. Sullage water Kitchen
2. Foul waste Toilets
3. Trade waste Industrial
and commercial
organisations
Reprint 2024-25
SCIENCE 158
sewers, forming the sewerage. It is like
a transport system that carries sewage
from the point of being produced to the
point of disposal, i.e. treatment plant.
Manholes are located at every 50 m
to 60 m in the sewerage, at the junction
of two or more sewers and at points
where there is a change in direction.
Activity 13.3
Study the sewage route in your home/
school/building. Do the following:
? Make a line diagram of the sewage
route.
? Walk down the street or survey the
campus to find the number of
manholes.
? If possible, observe open drain and
record which living organisms are
found in and around it.
In case you do not have a sewerage
system in your locality, find out how
sewage is being disposed off.
Treatment of polluted water
Perform the following activity. It will help
you understand the processes that take
place at the wastewater treatment plant.
Activity 13.4
Divide yourself into groups to perform
the activity. Record observations at each
stage:
? Fill a large glass jar 3/4 full of water.
Add some dirty organic matter such
as grass pieces or orange peels, a
small amount of detergent, and a few
drops of an ink or any colour.
? Cap the jar, shake it well and let the
mixture stand in the sun for two
days.
? After two days, shake the mixture
and pour a small sample into test
tube. Label this test tube “Before
treatment; Sample 1”. How does it
smell?
? Use an aerator from an aquarium to
bubble air through the sample in the
glass jar. Allow several hours for
aeration; leave the aerator attached
overnight. If you do not have an
aerator, use a mechanical stirrer or
a mixer. You may have to stir it
several times.
? The next day when aeration is
complete, pour another sample into
a second test tube. Label it as “After
aeration; Sample 2”.
? Fold a piece of filter paper to form a
cone. Wet the paper with tap water
and then insert the cone in a funnel.
Mount the funnel on a support
(as you have learnt in Class VI).
? Place layers of sand, fine gravel and
finally medium gravel in the funnel
(Fig. 13.2). (An actual filtration plant
does not use filter paper, but the sand
filter is several metres deep).
? Pour the remaining aerated liquid
through the filter into the beakers.
Do not allow the liquid to spill over
the filter. If the filtered liquid is not
clear, filter it a few  times till you get
clear water.
? Pour a sample of the filtered water
into a third test tube labelled
“Filtered; Sample 3”.
Reprint 2024-25
WASTEWATER STORY 159
? Pour another sample of the filtered
water into a fourth test tube. Add a
small piece of a chlorine tablet. Mix
well until the water is clear. Label the
test tube “Chlorinated; Sample 4”.
? Observe carefully the samples in all
the test tubes. Do not taste! Just
smell them!
Now answer the following questions:
(a) What changes did you observe in the
appearance of the liquid after
aeration?
(b) Did aeration change the odour?
(c) What was removed by the sand filter?
(d) Did chlorine remove the colour?
(e) Did chlorine have an odour? Was it
worse than that of the wastewater?
13.4 WASTEWATER TREATMENT PLANT
(WWTP)
Treatment of wastewater involves
physical, chemical, and biological
processes, which remove physical,
chemical and biological matter that
contaminates the wastewater.
1. Wastewater is passed through bar
screens. Large objects like rags,
sticks, cans, plastic packets, napkins
are removed (Fig. 13.3).
Fig. 13.2  Filtration process
Fig. 13.3  Bar screen
Fig. 13.4  Grit and sand removal tank
2. Water then goes to a grit and sand
removal tank. The speed of the
incoming wastewater is decreased to
allow sand, grit and pebbles to settle
down (Fig. 13.4).
3. The water is then allowed to settle in
a large tank which is sloped towards
the middle. Solids like faeces settle
at the bottom and are removed with
Reprint 2024-25
SCIENCE 160
After several hours, the suspended
microbes settle at the bottom of the tank
as activated sludge. The water is then
removed from the top.
The activated sludge is about 97%
water. The water is removed by sand
drying beds or machines. Dried sludge
is used as manure, returning organic
matter and nutrients to the soil.
The treated water has a very low level
of organic material and suspended
matter. It is discharged into a sea, a river
or into the ground. Nature cleans it up
further.  Sometimes it may be necessary
to disinfect water with chemicals like
chlorine and ozone before releasing it
into the distribution system.
Become an active citizen
Waste generation is a natural part of
human activity. But we can limit the
a scraper. This is the sludge. A
skimmer removes the floatable solids
like oil and grease. Water so cleared
is called clarified water (Fig. 13.5).
The sludge is transferred to a
separate tank where it is decomposed
by the anaerobic bacteria. The biogas
produced in the process can be used as
fuel or can be used to produce electricity.
4. Air is pumped into the clarified water
to help aerobic bacteria to grow.
Bacteria consume human waste, food
waste, soaps and other unwanted
matter still remaining in clarified
water (Fig. 13.6).
Fig. 13.6  Aerator
Did you know ?
It has been suggested that we should
plant eucalyptus trees all along
sewage ponds. These trees absorb all
surplus wastewater rapidly and
release pure water vapour into the
atmosphere.
The water in a river is cleaned
naturally by processes that are
similar to those adopted in a
wastewater treatment plant.
Fig. 13.5  Water clarifier
Reprint 2024-25