NCERT Textbook - Wastewater Story Class 7 Notes | EduRev

 Page 1


SCIENCE 220
Wastewater Story
18
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?
18.1 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 18.1
(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
our fellow human beings
have no access to safe drinking water.
This accounts for a large number of
water-related diseases and even deaths.
Women and girls walk for several
kilometres to collect clean water, as you
read in Chapter 16. Is it not a serious
matter for human dignity?
You have studied in Chapter 16
about 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.
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
known as “Sewage Treatment”. It takes
place in several stages.
Clean water
put to use
Drinking
© NCERT
not to be republished
Page 2


SCIENCE 220
Wastewater Story
18
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?
18.1 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 18.1
(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
our fellow human beings
have no access to safe drinking water.
This accounts for a large number of
water-related diseases and even deaths.
Women and girls walk for several
kilometres to collect clean water, as you
read in Chapter 16. Is it not a serious
matter for human dignity?
You have studied in Chapter 16
about 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.
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
known as “Sewage Treatment”. It takes
place in several stages.
Clean water
put to use
Drinking
© NCERT
not to be republished
WASTEWATER STORY 221
18.2 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. These
impurities are called contaminants.
Activity 18.2
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 18.1.
We know that sewage is a complex
mixture containing suspended solids,
organic and inorganic impurities,
nutrients, saprotrophic and disease
causing bacteria and other microbes.
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 which
cause cholera
and typhoid.
Other microbes         – Such as which
cause dysentery.
18.3 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
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
Table 18.1 Contaminant survey
S. No. Type of sewage Point of origin Substances which Any other
 contaminate remark
1. Sullage water Kitchen
2. Foul waste Toilets
3. Trade waste Industrial
and commercial
organisations
© NCERT
not to be republished
Page 3


SCIENCE 220
Wastewater Story
18
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?
18.1 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 18.1
(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
our fellow human beings
have no access to safe drinking water.
This accounts for a large number of
water-related diseases and even deaths.
Women and girls walk for several
kilometres to collect clean water, as you
read in Chapter 16. Is it not a serious
matter for human dignity?
You have studied in Chapter 16
about 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.
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
known as “Sewage Treatment”. It takes
place in several stages.
Clean water
put to use
Drinking
© NCERT
not to be republished
WASTEWATER STORY 221
18.2 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. These
impurities are called contaminants.
Activity 18.2
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 18.1.
We know that sewage is a complex
mixture containing suspended solids,
organic and inorganic impurities,
nutrients, saprotrophic and disease
causing bacteria and other microbes.
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 which
cause cholera
and typhoid.
Other microbes         – Such as which
cause dysentery.
18.3 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
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
Table 18.1 Contaminant survey
S. No. Type of sewage Point of origin Substances which Any other
 contaminate remark
1. Sullage water Kitchen
2. Foul waste Toilets
3. Trade waste Industrial
and commercial
organisations
© NCERT
not to be republished
SCIENCE 222
of two or more sewers and at points
where there is a change in direction.
Activity 18.3
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.
§ Follow an open drain and find out
where it ends and 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 18.4
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. 18.2). (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”.
§ 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”.
© NCERT
not to be republished
Page 4


SCIENCE 220
Wastewater Story
18
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?
18.1 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 18.1
(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
our fellow human beings
have no access to safe drinking water.
This accounts for a large number of
water-related diseases and even deaths.
Women and girls walk for several
kilometres to collect clean water, as you
read in Chapter 16. Is it not a serious
matter for human dignity?
You have studied in Chapter 16
about 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.
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
known as “Sewage Treatment”. It takes
place in several stages.
Clean water
put to use
Drinking
© NCERT
not to be republished
WASTEWATER STORY 221
18.2 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. These
impurities are called contaminants.
Activity 18.2
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 18.1.
We know that sewage is a complex
mixture containing suspended solids,
organic and inorganic impurities,
nutrients, saprotrophic and disease
causing bacteria and other microbes.
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 which
cause cholera
and typhoid.
Other microbes         – Such as which
cause dysentery.
18.3 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
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
Table 18.1 Contaminant survey
S. No. Type of sewage Point of origin Substances which Any other
 contaminate remark
1. Sullage water Kitchen
2. Foul waste Toilets
3. Trade waste Industrial
and commercial
organisations
© NCERT
not to be republished
SCIENCE 222
of two or more sewers and at points
where there is a change in direction.
Activity 18.3
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.
§ Follow an open drain and find out
where it ends and 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 18.4
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. 18.2). (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”.
§ 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”.
© NCERT
not to be republished
WASTEWATER STORY 223
§ 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?
18.4 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. 18.3).
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. 18.4).
Fig. 18.2  Filtration process
Fig. 18.3  Bar screen
Fig. 18.4  Grit and sand removal tank
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
a scraper. This is the sludge. A
skimmer removes the floatable solids
like oil and grease. Water so cleared
is called clarified water (Fig. 18.5).
© NCERT
not to be republished
Page 5


SCIENCE 220
Wastewater Story
18
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?
18.1 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 18.1
(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
our fellow human beings
have no access to safe drinking water.
This accounts for a large number of
water-related diseases and even deaths.
Women and girls walk for several
kilometres to collect clean water, as you
read in Chapter 16. Is it not a serious
matter for human dignity?
You have studied in Chapter 16
about 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.
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
known as “Sewage Treatment”. It takes
place in several stages.
Clean water
put to use
Drinking
© NCERT
not to be republished
WASTEWATER STORY 221
18.2 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. These
impurities are called contaminants.
Activity 18.2
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 18.1.
We know that sewage is a complex
mixture containing suspended solids,
organic and inorganic impurities,
nutrients, saprotrophic and disease
causing bacteria and other microbes.
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 which
cause cholera
and typhoid.
Other microbes         – Such as which
cause dysentery.
18.3 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
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
Table 18.1 Contaminant survey
S. No. Type of sewage Point of origin Substances which Any other
 contaminate remark
1. Sullage water Kitchen
2. Foul waste Toilets
3. Trade waste Industrial
and commercial
organisations
© NCERT
not to be republished
SCIENCE 222
of two or more sewers and at points
where there is a change in direction.
Activity 18.3
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.
§ Follow an open drain and find out
where it ends and 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 18.4
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. 18.2). (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”.
§ 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”.
© NCERT
not to be republished
WASTEWATER STORY 223
§ 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?
18.4 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. 18.3).
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. 18.4).
Fig. 18.2  Filtration process
Fig. 18.3  Bar screen
Fig. 18.4  Grit and sand removal tank
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
a scraper. This is the sludge. A
skimmer removes the floatable solids
like oil and grease. Water so cleared
is called clarified water (Fig. 18.5).
© NCERT
not to be republished
SCIENCE 224
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
type of waste and quantity of waste
produced. Often we have been repelled
by offensive smell. The sight of open
drains is disgusting. The situation
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. 18.6).
After several hours, the suspended
microbes settle at the bottom of the tank
as activated sludge. The water is then
removed from the top.
Fig. 18.6  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. 18.5  Water clarifer
© NCERT
not to be republished