Unit-I PGDEM-03 WATER POLLUTION Rajesh Kumar STRUCTURE 1.0. OBJECTIVES 1.1. INTRODUCTION 1.2. WATER QUALITY STANDARDS 1.2.1 Drinking Water Standards 1.2.2 Stream Standards 1.2.3 Irrigation Standards 1.2.4 Effluent Standards 1.2.5 Minimum National Standards (MINAS) 1.3 SOURCES OF WATER POLLUTION 1.3.1 Municipal and Domestic Wastes 1.3.1.1 Harmful Effects of Domestic Wastes 1.3.2 Industrial Wastes 1.3.2.1 General Effects of Domestic Wastes 1.3.3 Agricultural Wastes 1.3.3.1 General Properties of Pesticides 1.3.3.2 The Effects of Pesticides on Target and Non- target Organisms. 1.3.4 Heat and Radioactive Wastes 1.4 SUMMARY 1.5 KEYWORDS 1.6 SELF ASSESSMENT QUESTIONS 1.7 SUGGESTED BOOKS 1
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Unit-I PGDEM-03 WATER POLLUTION Rajesh Kumar STRUCTURE · 1.3.1 Municipal and Domestic Wastes 1.3.1.1 Harmful Effects of Domestic Wastes 1.3.2 Industrial Wastes 1.3.2.1 General Effects
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Unit-I PGDEM-03
WATER POLLUTION
Rajesh Kumar
STRUCTURE
1.0. OBJECTIVES
1.1. INTRODUCTION
1.2. WATER QUALITY STANDARDS
1.2.1 Drinking Water Standards
1.2.2 Stream Standards
1.2.3 Irrigation Standards
1.2.4 Effluent Standards
1.2.5 Minimum National Standards (MINAS)
1.3 SOURCES OF WATER POLLUTION
1.3.1 Municipal and Domestic Wastes
1.3.1.1 Harmful Effects of Domestic Wastes
1.3.2 Industrial Wastes
1.3.2.1 General Effects of Domestic Wastes
1.3.3 Agricultural Wastes
1.3.3.1 General Properties of Pesticides
1.3.3.2 The Effects of Pesticides on Target and Non-
target Organisms.
1.3.4 Heat and Radioactive Wastes
1.4 SUMMARY
1.5 KEYWORDS
1.6 SELF ASSESSMENT QUESTIONS
1.7 SUGGESTED BOOKS
1
1.0 OBJECTIVE:
After shedding this unit, you will be able to:
• Understand what are the main causes of water pollution.
• Understand why there is a need of establishing different
water quality standards.
• Become familiar to different water quality standards
• Become familiar to different sources of water pollution
and their characteristics and harmful effects.
1.1 INTRODUCTION
Water exists in various forms in various places. Water can exist in
vapour, liquid or solid forms and exists in the atmosphere
(atmospheric water), above the ground surface (surface water), and
below the ground surface (sub-surface water). Both surface and
sub-surface water originate from precipitation, which includes all
forms of moisture from clouds, including rain and snow. A portion
of the precipitated liquid water run off over the land (surface
runoff), infiltrates and flows through sub-surface (sub-surface
flow) and eventually finds its way back to the atmosphere through
evaporation from lakes, rivers, and ocean, transpiration from trees
and plants; or evapo-transpiration from vegetation. This chain is
known as Hydrological Cycle.
2
Fresh water accounts for just 1/10000 of the total water available
on the planet, yet this quantity seems immense when the volume
is expressed as 1,25,000 km3 . On global scale, this amount is
quite constant year to year, being constantly replenished by
precipitation of reviously evaporated from the ocean (3,50,000 km3
) and from land (70,000 km3). Unfortunately, most of the
precipitation fall back into the ocean and only area 1,10,000 km3
falls on the land. More than half of the 40,000 km3 of water that
does not evaporate run off to the ocean in flood events and is not
available for use throughout the year.
Lakes contain almost all of the fresh surface water on the planet.
The water in rivers and streams make up less than one percent of
the volume in lakes. This fact alone suggests that lakes require
special protection from contamination.
Another fact to consider is that it takes many years to replenish
lakes owing to the relatively small amount of precipitation that falls
on the lake and the small amount of stream water that runs
directly into lakes. On average, lake replenishment takes 100
years, whereas the replacement time for water in streams and
rivers is 11 days. Thus, if contaminants are distributed throughout
the average lake, the incoming water cannot restore the lake to its
initial quality for a long time.
Water quality characteristics of aquatic environment arise from a
multitude of physical, chemical and biological interaction. The
water bodies (rivers, lakes and estuaries) are continuously subject
to a dynamic state of change with respect to their geological age
and geochemical characteristics. This is demonstrated by
continuous circulation, transformation and accumulation of energy
and matter through the medium of living things and their
activities.
3
The water stored in reservoir and lakes, together with the water
that flows perennially in stream, is subject to heavy stress;
because it is used for water supplies, agriculture, industries and
recreation, it can be easily misused. Most cities and industries
discharge wastewaters to streams and rivers, rather than to lakes
and reservoirs. Even though wastewaters are treated, large
quantities of contaminants flow down steam on the way to the
ocean as the water is used over and over again.
Pollution is a qualitative term. It describes the situation that
occurs when the level of contaminants is such that intended water
use is impaired. It takes just a small amount of contaminant to
pollute a water body intended for a drinking water supply. But the
same water might not be considered polluted if the water were to
be used, for example for agriculture. Pollution is not restricted to
contaminants. Physical factors of the environment can also
contribute to pollution. For example, heated water discharged from
4
a power plant can change the temperature of an aquatic
environment. It might not be a problem in a lake or a river during
the winter, but it can certainly be a problem in the summer time.
The major sources of surface water contamination are
construction, municipalities, agriculture, and industries. However,
the water delivered to earth in the forms of precipitation in not
necessarily pure to begin with. Near the coast, it may contain
particulate and dissolved sea salts; further inland; it may contain
organic compounds and acids scrubbed from contaminants added
to the atmosphere both by natural processes and by anthropogenic
(human) activities. Gases from plant growth and decay and gases
from geological activities are example of naturally derived
atmospheric contaminants that can be returned to earth via
precipitation. The acid rain problem of the New England states is a
classic example of anthropogenically derived atmospheric
contaminants that contribute to surface water pollution.
The Environmental Pollution Panel of the U.S. Presidents Science
Advisory Committee defines environmental pollution as the
unfavorable alteration of our surrounding, wholly or largely as a
by-product of man’s actions, through direct or indirect effects of
changes in energy patterns, radiation levels, chemical and physical
constitution and abundance of organisms. These changes may
affect man directly or indirectly through his supplied of water and
of agricultural and other biological products, his physical objects
or possessions, or his opportunities for recreation and appreciation
of nature.
The substances which cause pollution are known as pollutants.
Pollutants may be defined as any substance that is released
intentionally or unintentionally by man into the environment in
such concentration that may cause adverse affect on environment
health. The Indian Environment (Protection) Act, 1986 defines 5
pollutant as any solid, liquid or gaseous substance present is such
concentration as may be or tend to be injurious to environment.
However, nature by itself treats, recycles and makes good use of
these pollutants. But as the 20th century comes to a close, the
large volume and increasing poisonous nature of man made
pollutants threats the integrity of nature and cultural development
of man.
The term water pollution is referred to the addition to water of an
excess of material (or heat) that is harmful to humans, animals or
desirable aquatic life, or otherwise causes significant departures
from the normal activities of ‘various living communities in or near
bodies of water. The National Water Commission (1973) stated that
‘water gets polluted if it has been not of sufficiently high quality to
be suitable for highest uses; people wish to make of it at present or
in the future.
1.2 WATER QUALITY STANDARDS
All sort of pollutants are added to the water bodies. We have
limited resources of water and the requirements are numerous, so
there lies the demand of conserving and minimizing the pollution
of water.
Polluted water is hardly of any use for most purposes. It cannot be
utilized for drinking due to health risk. Water with high salt
content is unsuitable for agriculture and industrial purposes. The
quality of water interferes with the aesthetic and economic
pursuits of water bodies by affecting the fish and other aquatic
organisms. However, the water which is not suitable for drinking
may be good for irrigation, or water unsuitable for irrigation may
be quite suitable for industrial cooling or fish growth. Thus it can
be seen that each use of water has its own limits on the degree of
pollution it can accept.
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The achievement for this minimum quality of water for diverse uses
has led to the formation of water quality criteria, water quality
objectives and water quality standards. Water quality criteria can
be considered as specific requirement on which a decision to
support a particular use will be based. The criteria for various uses
are developed based on experimental data, our current knowledge
of health and ecological and economic effects of water quality.
Water quality objectives can be defined as aim or goal with regard
to the water quality which is to be achieved. It is not as rigid and
authoritative as a standard and does not have the enforcement
element of requirement. The term standard applies to any definite
principle or measure established by an authority by limiting
concentration of constituents in water which ensure safe use of
water and safeguard the environment. However, sometime
standards may not be fair due to lack of sound scientific
knowledge. Thus, standard may change with the accumulation of
more scientific knowledge and on other consideration.
To attain desired water quality objectives, the standard can be
applied in two ways. One type called ‘effluent standards’ are
applicable for municipal, agricultural or industrial wastes
discharge into water resources and on land. Second type
concerned with water receiving or affected by the effluents.
1.2.1 Drinking Water Standards
Raw water quality and standards depends upon the end use. The
four main uses are municipal, industrial, agricultural and
recreational (fish and wildlife). As water quality is degraded day by
day, so, it become very important to set the drinking water
standards for the safety of water of our limited resources. Different
agencies have set environment standards for safe drinking water
like Bureau of Indian Standards (BIS), World Health Organization
(WHO), European Economic Community (EEC) etc. 7
Drinking water standards, are regulation that Bureau of Indian
Standards (BIS) set to control the level of contamination in the
drinking water. Bureau of Indian Standard consider the inputs
from many organization i.e. Central, State, Semi Government,
Municipal Corporation, Public Health Organization, etc.
nitrate, PPN; peroxy butyl nitrate, PBN nitrogen dioxide, NO2 and
hydrogen peroxide (H2O2). The desirable ambient air levels of
photochemical oxidants are 240 µg/m3 for 1 hour duration.
13
Hydrocarbons: Hydrocarbons are evolved into the atmosphere from
crankcase of automobile, various refrigerants, decay of several organic
matters and from trees. Methane is the major naturally occurring
hydrocarbon emitted into the atmosphere. Human activities contribute
nearly 20% of the hydrocarbons emitted to the atmosphere every year.
Animals contribute about 80-85 million tonnes of methane in the
atmosphere every year. The hydrocarbons on reacting with nitric oxide
and sunlight form photochemical smog which causes irritation to eye end
decrease in visibility. Formaldehyde and peroxy acetyl nitrate (even at 1
ppm) are eye irritants. PAN also causes plant damage. The oxidation
reactions accompanied by formation of aerosols or haze also result in eye
irritation and plant damage. Hydrocarbons at high concentrations have
carcinogenic effects on lungs. They cause swelling when they enter the
lungs. Aromatic hydrocarbons are more dangerous than a cyclic and
alicyclic hydrocarbons. The inhalation of their vapours cause acute
irritation to the mucous membrane. Excess of hydrocarbon increases
mucous secretion as a result of which respiratory tracks are blocked and
man coughs regularly. Because of continuous cough much pressure is
caused on the trachea of lungs due to which the lining membrane of
alveoli bursts and very small area is left for exchange of oxygen and
carbon dioxide, Benzopyrene, which is present as trace amounts in
tobacco, charcoal, boiler stacks and gasoline exhausts etc. is a
dangerous cancer inducing hydrocarbon pollutant. Methane also is a
severe gas pollutant and occurs in air by volume of 0.0002 percent: Its
higher levels in absence of oxygen create narcotic effects on human
beings. A group of hydrocarbons, especially the carcinogenic
hydrocarbons, cause cancer in man and animal affecting DNA and cell
growth.
14
TABLE-I: HAZARDS OF CARCINOGENIC HYDROCARBONS
Sr. No. Compound Health Hazards
1. Benzene Bladder cancer
2. Naphthylamine Cancer in urinary bladder
3. Bichloromethyl ether Lung cancer
4. Ethylene dichloride Stomach, spleen and lung cancer,
5. Vinyl chloride Liver cancer
6. Ethyleneamine Cancer
7 . Propiolacetone Potential carcinogen
8. Naphthylamine Bladder cancer
9. Nitrophenol Bladder cancer
10. 3-3' dichlorobenzidine Cancer
Particulates : The effect of particulates on human beings depends
mainly on their size and characteristics. Size is one of the most important
physical parameters of particulates. Particle sizes are measured in
micrometers. Particle sizes larger than 50 µm can be seen with unaided
eye. Particulates smaller than 1 µm do not tend to settle out rapidly.
Settling is the major natural self-cleansing process for the removal of
particulates from atmosphere. Particulates can generally be classified as
suspended or settleable. Suspended particulates vary in size from less
than 1 µm to nearly 20 µm. Settleable particles or dust, are larger and
heavier and settle out close to their sources. They are generally greater
than 10 µm in size. Particulates greater in size (over 10 µm) are easily
removed by hairs at the front of nose.
Generally, coarse dusts, fly ash etc. are greater in size and seldom enter
the human system. Particulates with size range in between 2 to 10 µm
like fumes, dusts and smoke particles, are removed as movement of cilia
sweeps mucous upward, carrying particles from wind pipe to mouth
where they are swallowed. If the size of the particulates is less than 2 µm
(like aerosols and fumes) they will enter the lungs easily. Lymphocytes
15
and phagocytes in the lung attack some submicron particles, but all of
them cannot be removed effectively. Similarly, there is a great variation in
the chemical composition of the particulates found in the atmosphere.
Atmospheric particulates contain both organic components like phenols,
organic acids and alcohols and inorganic components like dusts. The
biological particles include protozoa, bacteria, viruses, fungi, spores,
pollens and algae. Their life time is very small due to lack of nutrients
and presence of UV rays from sun. However certain bacteria and fungi
can survive for longer periods.
Effects of Particulates : The success or failure of respiratory defense
system depends, in part upon the size of the particulates inhaled and the
depth of their penetration into the respiratory tract. About 40 percent of
the particles 1-2 µm in size are retained in bronchioles and alveoli
Particles ranging in size from 0.25 to 1 µm show a decrease in retention.
2.2.4.2 Air Pollution Effects on Vegetation
The most obvious damage caused by air pollutants to vegetation occurs
in leaf structure. The surface of leaf is covered by a waxy layer known as
cuticle. Between the waxy layers, epidermis is present, which is a single
layer of cells forming the surface skin of the leaf. The epidermis protects
the inner tissues from excessive moisture loss and prevents the
admission of CO2 and oxygen to these internal tissues. The leaf surface
has a large number of openings called the stomata. Guard cells protect
the stomata and also control the opening and closing of stomata. A
typical plant cell has three components-the cell wall, the protoplasm and
the non-living inclusions within the cell. Because the cell wall is
extremely thin during the formative stage, new growth is very much
susceptible to air pollution damage. The protoplasm is composed of
several chemical compounds, water and the central nucleus which
contains the hereditary and reproductive mechanism. The leaf also
16
contains the chloroplasts, which are the key structures in the
photosynthesis process of food manufacture in the green plant. These
plant inclusions are the store house for food and waste material. A cross
section of a leaf shows four principal layers, the upper epidermal cells,
the palisade parenchyma, the spongy parenchyma and the lower
epidermal cells. The excess oxygen generated in this process escapes
from the plant into the atmosphere and helps to purify the air. Many of
the atmospheric pollutants act as phytotoxicants (plant damaging
substances) and result in various injuries to the plants:
• Bifacial Necrosis: Tissues are killed on both upper and lower
surfaces of the leaf.
• Pigmented Lesions : Dark brown, black, purple or red spots
appear on the leaf surface.
• Epinasty : The rapid growth of the upper side of the leaves,
causing the leaf blade to curl under.
• Acute injury: Results from short term exposure to high
concentrations of pollutants. A severe visible damage to leaf tissues
is often associated with plasmolysis and tissue collapse.
• Chronic Injury: Resulting from long-term exposure, to low levels of
pollutants and often, shows up as a colour change or chlorosis
because of destruction of chlorophyll with no apparent cell damage.
• Chlorosis: The loss of the green plant pigment chlorophyll is called
chlorosis. The loss of chlorophyll results in yellow pattern.
Chlorosis indicates a deficiency in some nutrient required by the
plant.
• Abscission : Leaf abscission is the dropping of leaves. This will
decrease the life of the plant.
17
• Necrosis: It is the killing or collapse of the plant tissue. Tissue
injured by phytotoxicants often has a characteristic colour. For
example, bleaching is associated with SO2, yellowing with
ammonia, browning with fluoride and silvering or bronzing of
under surfaces of some leaves with PAN.
2.2.4.3 Air Pollution Effects on Materials
Air pollution damage to property is a very important economic aspect of
pollution. Air pollution damage to property covers a wide range of
corrosion of metals, soiling and eroding of building surfaces, fading of
dyed materials, rubber cracking etc. The processes responsible for the
effects of air pollution on materials are:
• Abrasion: Solid particles of considerable size travelling at higher
speeds cause abrasive action, Large sharp edged particles
embedded in fabrics can accelerate wear.
• Chemical Action : Some air pollutants react directly and
irreversibly with materials to cause deterioration. SO2 bleaches
marble, hydrogen sulfide tarnishes silver and acidic mists cause
etching of metallic surfaces.
• Absorption: Certain materials absorb some pollutants and get
damaged when the pollutants undergo chemical changes. SO2
absorbed by leather will be converted to sulfuric acid, which
deteriorates leather.
• Corrosion : Action of air pollutants facilitated by the presence of
moisture causes corrosion. The atmospheric deterioration of
ferrous metal is due to corrosion by an electrochemical process.
• Deposition and Removal : Solid and liquid particles deposited on
surface may damage the material by spoiling its appearance.
18
2.2.4.4 Effects of Air Pollution on Buildings
Polluted air containing oxides of sulfur and nitrogen and particulates,
deteriorate building materials and may ultimately result in a loss in
structural integrity. When buildings dating from antiquity and structures
of great artistic and historic values are disfigured the loss is irreparable.
Less than 100 years of exposure to air pollutants in London has done
more damage to the Cleopatra's Needle than what was caused by nature
during 3500 years in the dry atmosphere of Egypt. The miraculous and
historical monuments built by long years of hard labour are losing their
faces. This shows how materialistic man has become, in years where he
is giving importance to industrial production even at the cost of the art
treasures brought up by his ancestors. Disintegration of stone caused
largely by the expansion of iron by corrosion had badly damaged the
houses of parliament in London in 1920. The Parthenon of Athens, the
Coliseum and Arch of Titus in Rome and the San Marco Basilica in
Venice are fast deteriorating. The situation in Florence, Italy has been
described as disastrous. The massive twin aspired cologne cathedral, the
most magnificent church building of German High Gothic era is facing
the threat of corrosion. Similar is the case in Japan wherein most of the
industrial areas, the century-old shrines and temples are facing the
threat. Taj Mahal at Agra, in India, a miracle in marble is facing the grave
danger from pollution caused by existing foundries, power houses,
railway yards and other industrial units. The problem now seems to be
more aggravated because of the commissioning of the Mathura refinery,
within 30 km. range of the priceless monument which is emitting SO2 in
the air and the wind direction is such that Taj at Agra is under direct
corrosion by the acidic fumes. Some alternative solutions must be
considered. One of the methods that may prove successful is to transport
the corrosive gases form the refinery through a set of anticorrosive
pipelines bye passing Agra, purify the gases and release the emission into
atmosphere at a safest place on the down-wind side of Taj Mahal.
19
Plantation of trees around Taj will give a cover which may absorb atleast
a part of the pollution. The renowned temple of Sri Channakeshava at
Belur (Hasan district, Karnataka State) India is threatened with a similar
hazard. A plywood factory located close to the temple emits soot-laden
fumes which get deposited on the sculptures in the temple and discolour
the surface, inside and outside. Jagannath temple, at Puri and the
Konark Sun temple situated on the East Coast of India are badly hit by
particulates present in air. The abrasive action of the sea sprays is
threatening the longevity of these temples. The Statue of Liberty is also
badly affected by air pollution. Sensitive art objects displayed inside
buildings can be placed in hermetically sealed containers. Air
conditioning can also be used as a protective measure. The sides of books
kept in closely packed rows with restricted air circulation remain in good
conditions for a much longer period, than their exposed backs.
Bacteriocides may be used to protect stones as some bacteria convert
atmospheric SO2 to sulfuric acid which they use as a digestive fluid in
attacking the carbonate stone. Thus air pollution can result in serious
health impacts to humans, plants as well as affect and degrade various
types of materials and buildings.
2.3 SUMMARY
Study of dynamics of atmosphere is called meteorology. Atmospheric
conditions prevailing at a time determine air quality. Decrease in
temperature with increasing altitude is called environmental lapse rate
(ELR). Air parcel, if it despoil exchange heat with the surrounding will
experience decrease of 1°C/100 m due to expansion. This is called
adiabatic lapse rate (ALR). ELR and ALR will determine the dispersion of
air pollutants. In relation to prevailing environmental conditions the
plume will experience. Various shaped like looping, coning, fanning,
lofting, fumigation and trapping.
20
Automobiles contribute to urban air pollution to a great extent.
Hydrocarbons, Co, NOx, SOx and particulate matter are the major
pollutants in the vehicular exhaust of these various pollutants CO forms
about 80% of the total exhaust. Petrol driven vehicles contribute more
CO. The concentration of unburned hydrocarbons are influenced by air-
fuel ratio and is lowest near the stiochiometric ratio.
NO is generated first due to oxidation of nitrogen which changes into
NO2. Mixture of oxides of nitrogen is represented as NOx. Higher
emission of NOx results from the air-fuel ratio on the lean side of the
stiochiometric ratio, enquire compression ratio, spark timing and intake
air temperature and humidity. 70% of particulate matter is in the range
of 0.02 – 0.06 µm. It consists of both organic and inorganic compounds
with high molecular weight. Lead used to form a significant part in the
leaded petrol.
Hydrocarbons that are emitted due to evaporation form 10 to 30% of the
hydrocarbons of the vehicular emission, and 1/5 by the crank case blow
by. Air pollutants have adverse effects on the living beings and materials.
CO combines with haemoglobin (210 times more than oxygen) and
causes problem for people suffering from angina pectoris. Chronic effect
of CO may result in heart and respiratory problems. It may increase the
carcinogenic effects of other pollutants. Symptoms associated with CO
exposure are headache, fatigue, drowsiness, coma and respiratory failure
and death.
Oxides of sulfur may cause intense irritation and reduction of visibility.
Exposure irritates mucous membrane of the respiratory tract and chronic
respiratory diseases like bronchitis and emphysema may develop. Oxides of
nitrogen adversely affect health. NO2 is absorbed by haemoglobin much
more than O2. NO2 irritates the alveoli and lungs. High concentration and
occupational exposure may produce pulmonary oedema.
21
Oxidants like O3 may cause coughing, and severe fatigue, severe chest
pain, headache, damage to RBC, loss of co-ordination and difficulty in
articulation. Hydrocarbons result in eye irritation, lung swelling and
plant damage. May have carcinogenic effects at high concentrations. May
result in excess mucous secretion. Particular matter may carry other
pollutants absorbed on them and enter the respiratory system.
Various pollutants affect the plants by causing injuries. Necrosis may of
the tissue may be on both the sides of the leaf, lesions on the leaf
surface, plasmolysis and tissue collapse. Pollutants may destroy
chlorophyll, result in dropping of leaves. Materials are also affected by
pollutants by causing corrosion of metals, erosion of the building
materials, fading of dyed materials, cracking of rubber.
Various buildings like in Egypt, Athens, Rome, Venice, Italy, India etc.
have been affected.
2.4 KEY WORDS
Meteorology : Study of dynamics of atmosphere
ELR : Decrease in temperature with increase in
altitude is called Environmental Lapse Rate
(ELR).
ALR : Decrease in temperature in the air parcel in
upward movement became of expansion. It is
1°C/100 meters.
Plume Dispersion : Movement of stack plume depending upon
vertical temperature and wind profile.
Chronic Bronchitis : Persistent inflammation and damage to the
cell lining the bronchi and bronchides
causing building up of mucus, painful,
coughing and shortness of breath.
Emphysema : Irreversible damage to air sacs alveoli leading
22
to abnormal dilation of air spaces, loss of
lung elasticity and acute shortness of breath.
Epinasty : The rapid growth of the upper side of the
leaves, causing the leaf blade to curl under.
2.5 SELF ASSESSMENT QUESTIONS
1. Define adiabatic and environmental lapse rate.
2. Discuss Sub adiabatic and Superadiabatic conditions.
3. Discuss various types of plumes in relation to environmental
conditions.
4. Write a short note on automobile pollution.
5. Discuss the major effects of atmospheric pollutants on human
health.
6. Discuss the impact of air pollutants on plants.
2.6 SUGGESTED READINGS
Kaushik, A and Kaushik CP (2004) : Perspectives in Environmental
Studies, New Age International Publishers, New Delhi.
Miller & Tyler Jr. 1999. Environmental Science : Working with the Earth,
7th edition. Wedsworth Publishing Company.
Murali Krishna KVSG (1995) : Air Pollution and Control.
Masters, Gilbert M (1994) Introduction to Environmental Engineering and
Science. Prentice-Hall of India- Private Ltd., New Delhi.
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Unit-III PGDEM-03
Noise Pollution
Dr. Krishan Kumar
STRUCTURE
1.0 Objectives
1.1 Wave Motion
1.2 Sound Waves
1.3 Audible, Infrasonic and Ultrasonic sound
1.4 Definition of Noise
1.5 Sound Pressure Level – The Decibel Scale
1.6 Sources of Noise
1.7 Measurement of Noise
1.8 Indices of Noise Pollution
1.9 Standards of Noise Pollution
1.10 Summary
1.11 Key words
1.12 Review Questions
1.13 Suggested Readings
1.0 Objectives
After studying this unit, you will be able to understand:
• What is the nature of sound waves?
• The concept of noise and the sound pressure level
• How do we measure noise?
• What are various parameters/indices of noise pollution in which standards
of noise pollution are often expressed?
1.1 Wave Motion
We are everyday exposed to sounds of different kinds. Our ears are able to characterize a
sound on the basis of its properties. This ability allows us to discriminate between
different kinds of sounds. What makes these sounds so different from each other? To
know this, we first must understand wave motion. In the following section, we shall
introduce our readers to the fundamentals of wave motion.
What is a wave?
Well a wave is a perturbation/disturbance that travels onwards through a medium due to
the periodic motion of its particles from their mean position. A medium must possess
three important properties for the propagation of wave motion through it : (i) elasticity so
that it tries to return to its original position after being disturbed; (ii) inertia so as to be
able to store up energy and (iii) small frictional resistance so that there is very little
damping of the oscillating particles of the medium.
Based upon the manner in which particles oscillate about their mean position, waves can
be classified into two distinct categories – (i) transverse waves and (ii) longitudinal
waves.
Transverse Waves are the one in which particles of the medium oscillate simple
harmonically up and down about their mean position at right angles to the direction of
propagation of wave. This type of wave motion travels in the form of crests and troughs,
e.g. waves generated in a pond of water when a stone/pebble is thrown into it.
Longitudinal Waves are the one in which the particles oscillate simple harmonically to
an fro about their mean position along or parallel to the direction of propagation of wave.
This type of wave travels in the form of compressions (or condensations) and
rarefactions, e.g. waves produced in air by a source of sound.
To further develop our concepts about wave motion, it is relevant here to define certain
terms related to it.
Wavelength (λλλλ) – It is defined as the distance between two nearest particles of the
medium in the same phase, i.e. the distance between centers of two nearest crests or
troughs in case of transverse waves (fig. ) or that between two nearest condensations and
rarefactions in case of longitudinal waves. Alternatively, it may be defined as the distance
traveled by the wave during the time particles of the medium complete one full
oscillation.
Time Period – The time taken by the wave to complete one full oscillation/cycle is
called its time period. It is reciprocal of frequency (νννν) which means the number of
oscillations/cycles occurring per second.
Wave Velocity (v) – This is the distance traveled by the wave in one second. If λ is the
wavelength of the wave and ν its frequency, then the distance traveled by the wave in one
second is equal to νλ. Thus, the wave velocity may be related to the wavelength and
frequency of the wave by the following expression:
v = νλ
Finally, certain clarifications need to be given regarding wave motion to remove any
doubts from reader’s minds.
• A wave is only a disturbance or a condition that travels through the medium. It
does not involve transfer of any part of the medium from one place to the other.
• Each particle of the medium receives the disturbance a little later than its
predecessor, repeats its movements and passes the disturbance on to the next
succeeding particle. This means that there is a definite phase lag between one
particle and the next i.e. two adjacent particles do not reach their mean and
extreme positions at the same time.
• The velocity with which particles of the medium oscillate is entirely different
from the velocity of the wave.
1.2 Sound Waves
Sound waves are longitudinal in nature and thus travel in the form of condensations
(compressions) and rarefactions in the medium. To understand how a sound wave travels
in a medium, let us consider a vibrating tuning fork (fig. 1.1) whose two prongs move to
and fro about their mean position. As the prongs move to the right of their mean position,
they compress the air in their immediate neighborhood, which in turn, compress the
layers next to them due to the tendency of the medium to regain its original volume
because of its volume elasticity. Therefore, a pulse of compression travels onwards. As
the prongs move backwards to the left of their mean position, the air to the right gets
more space and expands thus producing a rarefaction. Again due to the property of
volume elasticity possessed by the medium, the rarefaction produced also travels towards
the right. Thus, as the tuning fork vibrates, it generates an alternating pattern of
compressions and rarefactions, which travels through the medium. This constitutes what
we call as a sound wave.
Fig. 1.1 Sound wave emission from a tuning fork
So, a sound wave is basically a pressure perturbation that travels through a medium
whose particles oscillate in a to and fro motion along the direction in which the
perturbation travels. During compressions, particles of the medium experience a push in
the positive direction (i.e. the direction in which wave travels) and are closer to each
other. For this reason, compressions are regions of higher pressure. On the contrary,
particles of the medium experience a pull in the negative direction (i.e. opposite to the
direction of wave travel) during a rarefaction and hence are farther apart from each other.
As a result, rarefactions are regions of lower pressure. Mathematically, the sound
pressure at any point (or at any instant of time) may be expressed by the following
equation:
Pt = P0 sin(wt-φ) [N/m2
or Pa] ……………….(1.1)
Where,
P0 = amplitude of sound pressure (N/m2)
t = time (s)
w = 2πf, angular frequency (rad/s)
f = frequency of oscillation (Hz)
φ = phase difference (dependent on initial conditions) (rad)
From the above equation, it may be inferred that two sound waves may be different in
terms of their frequency (or wavelength), amplitude and phase angle. In real life
situations, the sound field at a given point is a combination of sound waves which are
different from each other in all the above aspects.
1.3 Audible, Infrasonic and Ultrasonic Sound
Are we able to hear all the sounds that are incident upon a point. Well, not necessarily.
Our ears are able to hear sounds only within a certain frequency interval. This interval
starts from 20Hz and ends at 20,000Hz for a normal healthy human ear. This frequency
interval is called the audible range of sound frequencies. Even within this range, our ear
is not equally sensitive to all the frequencies. It is less sensitive at the extremes and more
sensitive in the middle of the audible range. Sounds having frequencies less than 20Hz
are called the infrasonic sounds while those having frequencies greater than 20,000Hz are
called the ultrasonic sounds. Our ear is not able to detect sounds outside the audible
range. However, many animals are able to hear sounds belonging to a much wider
frequency interval. For instance, bats can hear sounds even up to 100000Hz. In fact, they
locate their prey in the night with the help of their highly developed capability of hearing.
What makes us sense a sound? To understand this, it is necessary for us to understand the
structure and function of the human ear. The human ear basically consists of three parts-
the outer ear, the middle ear and the inner ear (fig.1.2). The outer ear is comprised of the
pinna and the ear canal leading to the ear drum. The pinna is basically a sound collecting
and focusing device for the incident sound energy. Though most animals can move the
pinna in the direction of the sound source, humans have lost this ability and hence they
have to move their head to identify the direction of a sound source. The ear canal is about
¼ inch in diameter and 1 inch in length. This length supports resonance for sound waves
of the order of 1000Hz frequencies and this is the reason why human ear is more
sensitive at middle frequencies of the audible spectrum. The ear canal terminates at the
ear drum which oscillates when the sound energy is incident upon it. The ear drum
consists of a very sensitive and delicate membrane. After this starts the middle ear which
contains a number of bones connected to each other in a manner that they transmit the
vibrations of ear drum to the inner ear. The inner ear is a complex bony cavity called the
cochlea which is filled with a colorless fluid. The cochlea is divided in the middle by
membranes that are partly gelatinous and partly bony. These membranes have fine hair
like cells which move when the cochlear fluid vibrates. This motion is sensed by nerve
cells and processed by the brain to give us a sensation we call as sound. These hair-cells
become stiff in people who are exposed to high noise levels for a long period and are the
major cause of hearing loss in such people.
Fig. 1.2 Structure of human ear
1.4 Definition of Noise
Noise means any irritating sound which affects the physiological and psychological well
being of a person in an adverse manner. It is now established that repeated exposure to
noise may either result in temporary or permanent hearing loss which in extreme cases
may lead to total deafness. Further, noise may interfere with speech communication,
disturb sleep and affect work performance, thus, causing anxiety in a person.
Human responses to a sound may be different for different persons. Also, a person may
respond to same sound differently at different times. Thus, the identification of a sound as
noise becomes a subjective problem, even though there are some sounds that may be
universally regarded as noise. The degree of annoyance and discomfort experienced by a
person depends on the frequency spectrum and intensity of sound, the aural sensitivity of
the listener and upon the time and surrounding environment when the individual is
exposed to noise.
1.5 Sound Pressure Level – The Decibel Scale
Since the range of sound pressures commonly encountered by the human ear is very
wide, it has been condensed into a more manageable logarithmic scale by the acoustical
scientists by devising the concept of sound pressure level ( Lp ), given by
L p pp re= 10 10
2 2log ( / ) [dB] or
L p pp re= 20 10log ( / ) [dB] ..........(1.2)
where
pre = international reference pressure of 2 10 5× − Pa which represents the
average threshold of hearing for the normal healthy human ear.
p = root mean square (rms ) sound pressure (N/m2)
In terms of equation (1.1), the root mean square pressure can be given by
pT
t dtrmsT
T
= −→∞ ∫lim
1
0
p sin ( ) 0
2 2 ω φ ...............(1.3)
Fig. 1.3 The decibel scale
An average normal human ear can respond to sound waves in a frequency range of 20Hz
to 20,000Hz and to pressures ranging from 20µPa (~ 0 dB) which represents the
threshold of hearing to more than 100Pa which corresponds to the threshold of pain. A
scale showing the sound pressure levels (in decibels) of certain common noise
phenomena in relation to sound pressures (in micropascals) is depicted in fig. 1.3.
1.6 Sources of Noise
Noise sources may be classified differently.
(i) Point Source
If the dimensions of a source are small compared with the distance to the
listener, it is called a point source, for example, fans and chimney stacks. The
sound energy spreads out spherically, so that the sound pressure level is the
same for all the points equidistant from the source and decreases by 6dB per
doubling of distance. This holds true until ground and air attenuation
noticeably affect level.
(ii) Line Source
If a noise source is narrow in one direction and long in the other compared to
the distance to the listener, it is called a line source. It can be a single source
such as a pipe carrying a turbulent fluid, or it can be composed of many point
sources operating simultaneously, such as a stream of vehicles on a busy road.
Here, the sound energy spreads out cylindrically, so that the sound pressure
level is the same at all points at the same distance from the line and decreases
by 3 dB per doubling of the distance. This holds true until ground and air
attenuation noticeably affect the level.
Another way to categorize noise is on the basis of type of activity producing the noise.
Thus noise can be classified as traffic noise, industrial noise, commercial noise,
community noise etc.
Noise assessment is generally about evaluating the impact of one specific source, for
example, the noise from a specific production plant. This is not always an easy task. In
reality, a large number of different sources contribute to the ambient noise at a particular
point. Ambient noise is the noise from all sources combined – e.g. factory noise, traffic
noise, birdsong, running water etc. Specific noise is the noise from the source under
investigation. The specific noise is a component of the ambient noise and can be
associated with a specific source. Noise levels emitted by different types of sources are
shown in fig.
1.7 Measurement of Noise
The job of measuring the sound field at a given point is accomplished with the help of a
sound level meter. The principal components of a typical sound level meter are shown in
the schematic diagram of fig. 1.4. The microphone senses a sound pressure signal and
converts it to an analog electrical signal. The preamplifier is used for impedance
matching. Different frequency weighting networks (fig. 1.5) namely, A, B, C are used to
modify the frequency response characteristics of the measuring instrument. This is done
to improve the correlation between sound sensation and instrument reading in accordance
with the sensitivity of human ear in the audible range. The selection of the appropriate
frequency weighting network is dependent upon the type of measurements being made.
For most common steady noises A- weighting network is considered to be most
appropriate. The root mean square detector is the most common detector used in sound
level meters. It provides the running time average of the square of the sound pressure
signal. Finally, display is the component where the results of the measurements are
displayed. The display may be digital or analog in nature.
Fig. 1.4: Schematic Diagram showing the major component of a Sound Level Meter
Fig. 1.5 Different weighting networks used for measuring noise pollution
1.8 Indices of Noise Pollution
Since noise levels in actual field conditions may fluctuate quite wildly, certain
statistically derived indices have been used by acoustical scientists. Few of the most
commonly employed indices in the studies of noise pollution are discussed below:
1. Statistical Percentiles:
The percentile index, Ln , is defined as that level of noise which is exceeded n%
of the time in the total data points obtained for a certain interval of time. L1 is
used as a measure of peak noise levels , L10 , as a representative of levels during
periods of intense noise, L50 , as an indication of the average noise level and
L90 gives an idea of the background noise levels.
2. Traffic Noise Index TNI
In studies related `to traffic noise , another index TNI is also used . This is usually
expressed in terms of L10 and L90 (Magrab 1975 ) as follows :
TNI L L L= − + −4 3010 90 90( )
Where the term L L10 90− indicates the range of "noise climate" and describes the
variability of noise, L90 as mentioned above represents the background noise
level and the third term 30 is introduced to give convenient numbers. It
emphasizes that a significant degree of annoyance arises from the variable
character of noise.
3. Equivalent Continuous Sound Level Leq
One of the most important and widely used index to characterized noise is the
Equivalent Continuous Sound level Leq .This is the level of a theoretical constant
noise equivalent in energy content to the actual fluctuating noise over a given
period of time. Mathematically
LT
p
pdt
Tdteq
T
L
T
=
=
∫ ∫10
110
11010
0
2
0
10
10
0
log log ( / )
where L = sound pressure level
T = time interval of observation.
If the sound levels are measured over discrete time intervals ∆Ti s, then Leq can be
given by
LT
Teq i
L
i
n
i=
=
∑101
1010
10
1
log ( / )∆
1.9 Standards of Noise Pollution in India
Following are the ambient noise pollution standards prescribed by CPCB in India.
Area Code Category of Areas Day Time Leq
Levels
Night Time Leq
Levels
A Industrial Area 75 70
B Commercial Area 65 55
C Residential Area 55 45
D Silence Zone 50 40
Here, day time refers to 6.00a.m. to 9.00p.m. while the night time means 9.00p.m. to
6.00a.m. Silence zone includes the areas upto 100meters around certain premises like
hospitals, educational institutions and courts. Honking of vehicle horns, use of
loudspeakers, bursting of crackers etc. are banned in these zones.
1.10 Summary
A wave is a perturbation/disturbance that travels onwards through a medium. A
wave is characterized by its frequency, wavelength and amplitude. Sound waves
are longitudinal waves that travel in the form of condensations (compressions)
and rarefactions in the medium. A normal healthy human ear can hear sounds in
the frequency interval 20 Hz to 20,000 Hz. Noise means any irritating sound
which affects the physiological and psychological well being of a person in an
adverse manner. Since the range of sound pressures commonly encountered by
the human ear is very wide, it has been condensed into a more manageable
logarithmic scale by the acoustical scientists by devising the concept of sound
pressure level, which is expressed in decibels. Sources from which sound is
emitted may be typically classified as the point source and the line source. The
sound pressure level in a sound field is measured with the help of a sound level
meter. The data collected by a sound level meter is then used to compute various
indices of noise pollution, some of which are utilized to formulate standards of
noise pollution at a given place.
1.11 Key Words
Sound Waves: Longitudinal waves that travel in the form of condensations
(compressions) and rarefactions in the medium.
Noise: Any irritating sound which affects the physiological and psychological
well being of a person in an adverse manner.
The percentile index, Ln : The level of noise which is exceeded n% of the time
in the total data points obtained for a certain interval of time
Equivalent Continuous Sound Level Leq : The level of a theoretical constant
noise equivalent in energy content to the actual fluctuating noise over a given
period of time
1.12 Review Questions
1. Define the following:
(i) Transverse waves
(ii) Longitudinal waves
(iii) Wavelength
(iv) Frequency
(v) Time period
2. What is sound? Explain audible, infra-sonic and ultra-sonic sounds?
3. What is noise? Explain the decibel scale with the concept of sound
pressure level.
4. Differentiate between point source and line source.
5. What are different indices of noise pollution?
6. How is noise pollution measured? What are the standards of noise
pollution in India?
1.13 Suggested readings
1. Bell, L. H. and Bell, D. H. (1994), "Industrial Noise Control", Marcel
Dekker, Inc.
2. Kryter, K. D. (1985), “The Effect of Noise on Man”, New York, Academic
Press.
3. Stephens, R. W. B. (1986),"Noise Pollution Effects and Control", SCOPE
John Wiley and Sons.
4. Singal, S. P. (2005), “ Noise Pollution and Control Strategy”, Narosa
Publishing House.
5. Aggarwal, S. K. (2005), “ Noise Pollution” APH Publishing Corporation.
Unit-IV PGDEM-04 Noise and Air Pollution Control-I
Dr. Krishan Kumar
STRUCTURE
1.0 Objectives
1.1 Introduction
1.2 Particulate Control Devices
1.2.1 Electrostatic Precipitators
1.2.2 Fabric Filters
1.3 Strategies for Noise Pollution Control
1.3.1 Silencers
1.4 Summary
1.5 Key words
1.6 Review Questions
1.7 Suggested readings
1.0 Objectives
To sensitize the students about the following major devices for the control of air
and noise pollution
• Electrostatic precipitators
• Fabric Filters
• Silencers
1.1 Introduction
Air pollutants are of two types: gaseous and particulates. Gaseous pollutants are
the pollutants in gas phase. They have the property of filling any available space
until their concentrations reach equilibrium by diffusion. If the space is too large,
the resulting concentration may be negligible. On the other hand, if space is small,
the resulting concentration may reach significant levels e.g. concentrations of
carbon dioxide due to continuous running of a motor vehicle in a closed garage.
Particulates are finely divided solids and liquids, such as dusts, fumes, smoke, fly
ash, mist and spray.
• Dusts are small particles (1.0 to 1000µm) of solids created from the break up
of larger particles by operations such as crushing, grinding and blasting.
• Fumes are fine solid particles (0.03 to 0.3µm) that condense from vapors of
solid materials.
• Smoke is unburned carbon (0.5 to 1.0 µm) that results from the incomplete
combustion of carbon containing substances.
• Fly ash (1.0 to 1000µm) is the noncombustible particle admixed with
combustion gases in the burning of coal.
• Mists are the particles (0.07 to 10µm) formed from the condensation of liquid
vapors.
• Sprays are particles (10 to 1000µm) formed from the atomization of liquids
through nozzles.
Air pollution control may be defined as the various measures taken to meet certain
emission standards. These measures may include changes in processes/raw
materials or modification of equipment. Another method is the installation of
devices at the end of process equipment to treat the exhaust gas stream. These
devices are called air pollution control equipment. In the coming section, we shall
focus on the equipments that are used for the control of particulate matter.
1.2 Particulate Control Devices
There are three general types of particulate control equipment: force-field settlers,
fabric filters, and scrubbers. Force-field settlers are equipments that use a field of
force for the collection of particulate. There are three types of force fields:
gravitational, centrifugal, and electrical. Equipments that make use of gravitational
field for settling particulates are called gravitational settling chambers. Settlers
that utilize centrifugal force for the collection of particulates are called centrifugal
collectors. Devices, which utilize an electric field of force to collect particulates,
are called electrostatic precipitators (ESPs). Fabric filters are devices that use the
principle of filtration for the removal of particulates. Scrubbers remove
particulates from the exhaust gas stream by using water droplets for capturing
them. Of all the devices mentioned above, electrostatic precipitators (ESPs) and
the fabric filters possess the highest collection efficiencies. Particularly, they are
very effective for the collection of small particulates that can be respired by
human beings. Other devices mentioned above are often used for pretreatment of
the effluent gas before directing it ESPs or fabric filters.
1.2.1 Electrostatic Precipitators
Electrostatic precipitators make use of electric field force for the collection of
particulate matter. This is done by applying a high voltage pulsating direct current
to an electrode system consisting of a small diameter discharge electrode which is
usually negatively charged, and a collection plate electrode which is grounded.
This produces a unidirectional, nonuniform electric field whose magnitude is
highest near the discharge electrode. A corona (a kind of glow) is generated near
the discharge electrode, a condition that is essential for the process of charging.
The electric field near the wire (discharge electrode) accelerates electrons present
in the gas to velocities sufficient to cause ionization of the gas in the region near
the wire. The ions produced as a result of the corona migrate toward the collection
electrode and in the process collide with and become attached to particles
suspended in the gas stream. The attachment of ions results in a build up of
electric charge, the magnitude of which is determined by the number of ions
attached.
The charge on the particles in the presence of an electric field results in a new
force in the direction of the collection electrode. The magnitude of the force is
dependent upon the charge and the field. This force causes particles to be
deposited on the collection electrode where they are held by a combination of
mechanical, electrical and molecular forces.
Once the particles are collected, they can be removed by coalescing and draining,
in case of the liquid aerosols, or by periodic impact or rapping, in case of solid
material. In case of rapping, a sufficiently thick layer of dust must be collected so
that it falls into the hopper in coherent masses to prevent excessive re-entrainment
of the particles in the gas stream.
Fig.1.1 A typical ESP installed in an industrial set up
Fig 1.2 Schematic diagram of an Electrostatic Precipitator
1.2.1.1 Components of Electrostatic Precipitator
An electrostatic precipitator is composed of the following components:
(i) Discharge Electrodes
The discharge electrodes are thin round wires varying from 0.05 to 0.15
inch (0.13 to 0.38 cm.) in diameter. Most common designs use wires
approximately 0.1 inch (0.25 cm) in diameter. The discharge electrodes
consist of vertically hung wires supported at the top and held taut and
plumb by the weight at the bottom. The wires are usually made from
high-carbon steel, or of stainless steel, copper, titanium alloy and
aluminum. The weights are made of cast iron and are generally 11.4 Kg
or more. The weights at the bottom are attached to guide frames to help
maintain wire alignments.
(ii) Collecting Electrodes
Most precipitators use plate collection electrodes. The plates are
generally made of carbon steel, stainless steel, or some kind of alloy,
depending upon the gas stream conditions. The plates range from 0.02
to 0.08 inch (0.05 to 0.2cm) in thickness. Plates are spaced from 4 inch
(10 cm) to 12 inch (30 cm) apart. Normal spacing for high efficiency
units is 20-23 cm. Plates are usually 20 to 50 ft (6 to 15 m ) high.
(iii) Shells
The shell structure encloses the electrodes and supports the precipitator
component in a rigid frame. This is done to maintain proper electrode
alignment and configuration. Providing supporting structures to the
precipitator component is a very important aspect of design. Collecting
plates and discharge electrodes are supported at the top so that elements
hang vertically under the force of gravity. This allows the elements to
expand or contract with temperature changes without binding or
distorting.
(iv) Rappers
Removal of accumulated dust deposit on collection electrode is
accomplished by rapping. Dust deposits are dislodged by mechanical
impulses or vibrations imparted to the electrodes. A rapping system is
designed so that rapping intensity and frequency can be adjusted for
varying operational conditions. Rapping of collection plates can be done
by a number of methods. One of the popular methods of mechanical
rapping uses hammers mounted on a rotating shaft. As the shaft rotates,
hammers drop by gravity and strike anvils attached to the collection
plates. Rapping intensity is governed by the weight of hammers and
length of the hammer mounting arm. The frequency of rapping can be
changed by altering the speed of the rotating shafts.
(v) Transformer-Rectifier Sets
The T-R sets are required to control the strength of electric field
generated between the discharge and collection electrodes. They step up
the normal service voltages from 400 to 480V to approximately
50,000V and convert alternating to direct current.
1.2.1.2 Efficiency Of Electrostatic Precipitator
The efficiency of an electrostatic precipitator is given by the Deutsch-Anderson
equation given below:
)/( 1 QwAeE
−−=
Where E is the collection efficiency of the precipitator, A is the effective
collecting plate area of the precipitator, Q is the gas flow rate of the precipitator
and w is the drift velocity i.e. the velocity with which particles migrate towards the
collecting electrode.
The efficiency of an electrostatic precipitator is greatly affected by the particle
resistivity. Therefore, discussion about the performance of electrostatic
precipitator would remain incomplete if no mention is made about it. Rsistivity
refers to the resistance offered by the collected dust layer to the flow of electric
current. By definition, resistivity is the electrical resistance of a dust sample 1.0
cm2 in cross-sectional area, 1.0 cm thick, and recorded in units of ohm.cm. Dust
resistivity values can be classified roughly into three groups:
1. Between 104 and 10
7 ohm.cm – low resistivity
2. Between 107 and 10
10 ohm.cm – normal resistivity
3. Above 1010
ohm.cm – high resistivity
Particles that have low resistivity are difficult to collect since they are easily
charged and lose their charge upon arrival at collection electrode. This happens
very fast and the particles can take on the charge of collection electrode. Particles
thus bounce off plates and are re-entrained in the gas stream.
Particles that have normal resistivity do not rapidly lose their charge upon arrival
at collection electrode. These particles leak their charge to ground and are retained
on the collection plates by intermolecular adhesive and cohesive forces. This
allows a particulate layer to build up, which is then dislodged into hopper through
rapping. At this range of resistivity (i.e. 107 to 10
10 ohm.cm ), therefore, particles
are collected most efficiently.
Particles that exhibit high resistivity are difficult to charge. Once they are finally
charged, they do not readily give up the acquired negative charge upon arrival at
the collection electrode. As the dust layer builds up on the collection electrode, the
layer and the electrode form a high potential electric field.. This produces a
condition called as back corona which produces small holes or craters in the dust
layer, from which back corona discharges occur. Positive ions are generated
within the dust layer and are accelerated toward the negative (discharge) electrode.
This counteracts the process of ion generation at the discharge electrode and thus
results in the reduction of collection efficiency.
1.2.2 Fabric Filters
Fabric filters remove dust from a stream of gas by means of a porous fabric and a
cake of dust as the filter media. These systems are commonly called as baghouses
since the fabric is usually configured in cylindrical bags installed within a housing.
The basic principle of baghouse operation involves the removal of dust from the
dust laden gas by passing the dirty gas through a filtration medium. The cleaned
gas emerges from one side of the medium while the dust is collected on the other
side. Periodically, the collected gas is removed from the fabric.
The type of filter fabric used depends on the temperature and acidity of the gas
stream, the characteristics of the dust, the gas-to-cloth filtration ratio, and the type
of bag cleaning used.
Because all baghouses impose extra pressure drop on any operating process, a fan,
blower, or compresser of some kind must be used to draw the process gases
through the system. Usually, such devices are installed on the baghouse outlet,
which is the clean side of the filtration process. This location has the advantage
that it does not subject the fan to the dust so that the possibility of dust leakage
into the clean gas coming out of the baghouse is reduced. This becomes
particularly important when the dust is toxic.
There are a number of mechanisms through which the fabric filter traps the dust.
Interception takes place when a particle traveling along a stream line in a gas
stream approaches a fiber in the filter. The path of the particle is such that it strikes
the fiber and gets stuck on it. In case of inertial impaction, a gas stream bends its
direction if it comes across a fiber in its path. However, the dust particle being
heavy, can not change its path (due to the property of inertia) and bangs the fiber
where it gets stuck. This collection mechanism is effective for particles about
10µm or larger. For particles below 10µm, this is not a very effective mechanism.
For smaller particles, there is another mechanism that is effective. This is the
process of diffusion. When the particles are too small, their motion can be affected
by collisions with gas molecules. Frequent collisions with gas molecules make the
path of a small particle erratic or random. The random motion of these small
particles continues until they bump into the fiber and collected. Electrical
entrapment can be another mechanism through which particles are collected in a
fabric filter. Often, fibers and particles, both are charged. If these charges are of
opposite sign, the particles are attracted to the fiber and collected on it. Another
mechanism is sieving in which the particles larger than the pore size of the fabric
cannot pass through the fabric. Sieving is a very important mechanism particularly
after the building of dust cake on the surface of fabric. Without the dust cake, the
efficiency of a fabric filter would be just 60 to 70%. It is the dust cake on the
surface of the fabric, which reduces the pore size and thereby, increases the
efficiency of a fabric filter to 99 percent.
Fig. 1.3 A typical baghouse assembly
1.2.2.1 -Types of Fabric Filters
Fabric filters can be classified into different groups in a number of ways. One such
is to group the fabric filter designs by their cleaning methods. There are three
major cleaning methods: shakers, reverse-air, and pulse jets. Another approach is
to group fabric filters as per their capacity to deal different volumes of exhaust
gases. There are three groupings: low volume, medium volume and high volume
fabric filters. Yet another way is to classify the fabric filters according to the type
of filter media they use i.e. woven or felted. Still another way is to categorize on
the basis of temperature applications i.e. high temperature (>400°F), medium
temperature (200 to 400°F) and low temperature (<200°F) applications group.
1.2.2.2 -Cleaning Methods of Fabric Filters
(i) Shakers
Shakers remove the collected dust from the surface of bag by
mechanically shaking it. This is done manually in small dust collectors.
In large size collectors, this process is motorized. The bag is generally
open at the bottom and close at the top where it is attached to the
shaking mechanism. In this configuration, the dust is collected on the
inner sides of the bags. Shaking is done at a frequency of several cycles
per second with the amplitude of a fraction of an inch to a few inches.
The duration of shaking may be 30s to a few minutes. Common bag
diameters are 5, 8 and 12 inches. The operation of shaking is performed
in the off-stream mode.
(ii) Reverse_Flow Cleaning
Reverse-air cleaning involves the removal of dust from the bags by
backflushing them with a low-pressure reverse flow. In the case of high
temperature applications, the just cleaned hot gas is employed to
backflush rather than the ambient air. Woven filter media are generally
employed in conjunction with reverse-air cleaning. Dust is collected on
the inners side of the bags, which are closed at the bottom and open at
the top. Most often, reverse flow systems are comprised of isolatable
compartments. Normally, cleaning is done one compartment at a time.
Duration of cleaning may vary from 1-2minutes. Cleaning is performed
in the off-stream mode. Common bag diameters are 8, 12 inch.
(iii) Pulse Jet Cleaning
Pulse-jet cleaning employs high pressure compressed air, with or
without a venturi, to backflush the bags vigorously. This method creates
a shock wave that travels down the bag, knocking the dust away from
filter medium. This method is generally employed in conjunction with
felted filter media. The duration of cleaning is lower than that of other
two methods. The pulse/shock wave lasts only for a fraction of a
second. The baghouse is often not subdivided into compartments when
pulse-jet cleaning is employed. The bag is closed at the bottom and open
at the top. Dust is collected on the outside of the bag. Usually, a row of
bags is cleaned simultaneously by introducing compressed air briefly at
the top of each bag.
1.2.2.3 -Baghouse Selection
A baghouse is selected on the basis of certain basic information about the
process, the gas stream, and the dust to be collected. Following are the
factors that go behind the selection of a baghouse for a particular
application:
(i) Description of Application – What is the application? It is
important to know fully the application for which the fabric filter
is required.
(ii) The gas volume - An important aspect is the gas flow rate to be
filtered. Normal gas flow, as well as surges and maximum flows,
must be established in order for a properly sized baghouse.
(iii) The gas temperature – Maximum and minimum temperatures
determine to a large degree the selection of bag fabric and other
materials of construction.
(iv) Chemical properties of the gases – It is important to identify the
corrosive gases, combustible gases, and condensable vapors at
inlet conditions. These inputs can greatly influence the selection
of fabric and materials of construction.
(v) Description of dust – Knowledge of dust concentration (grains
per cubic feet of gas), properties of dust such as particle size
distribution, shape, chemical composition, tendencies to
agglomerate or develop electrostatic charges, abrasive
characteristics, and bulk density are all very important factors in
the selection of baghouse and auxiliary equipment.
(vi) Available space – Availability of space is another important
criterion that determines the size of a baghouse to be installed for
a particular application.
(vii) Other equipment in the dust collection system – The dust
collection system may include other equipment, which may
influence the selection of baghouse.
Selection of filter media is another very important aspect of baghouse selection.
The filter media should be able to withstand temporary heat surges. Depending
upon the specific applications, a particular filter media may be selected. The fiber
must also be able to resist degradation from exposure to acids, alkalies, solvents or
oxidizing agents found in the dust laden gas stream. Dimensional stability of the
filter medium is another important factor. The fiber may shrink or stretch within
the application environment. However, these effects must be controlled to
maintain the dimensional stability of the fiber. Finally, cost of the fiber is a very
important factor in the selection of filter medium. Generally, the least costly
selection that satisfies the above mentioned requirements, is preferred. Table 1.1
presents the characteristics of some of the widely used filtration media.
Table 1.1- Characteristics of some common fabric filter media.
Fabric Max.
Temp.
Acid
Resistance
Fluoride
Resistance
Alkali
Resistance
Abrasion
Resistance
Cotton 180°F Poor Poor Good Very Good
Polypropylene 200°F Excellent Poor Excellent Very Good
Polyester 275°F Good Poor to
Fair
Good Very Good
Nomex 400°F Poor to
Fair
Good Excellent Excellent
Teflon 450°F Excellent Poor to
Fair
Excellent Fair
Fiberglass 500°F Fair to
Good
Poor Fair to
Good
Fair
1.2.2.4 Performance of a Baghouse
Despite several sophisticated formulae that have been developed, there is no
satisfactory set of published equations that allows a designer to calculate the
efficiency of a prospective baghouse. One parameter that helps the baghouse
designers is the Gas-to –Cloth (G/C) ratio. This is a measure of the amount of gas
driven through each square foot of fabric in the baghouse. It is given in terms of
the number of cubic feet of gas per minute passing through one square foot of
cloth. Factors influencing the appropriate G/C ratio for a baghouse include the
cleaning method, filter media, dust size, dust density, dust loading, and other
factors that are unique to each situation. Because of their variability, however, it
has not been possible to satisfactorily quantify each of these factors for
application. One approach to overcome this problem is to collect all empirical data
available for the source in question. If there are no data for the industry at hand,
then go to a similar industry, which is using a baghouse and determine the G/C
range successfully employed in that industry and conservatively apply it to your
case.
1.3 Strategies for Noise Pollution Control
There are four general methods of controlling noise: enclosing the noise source,
enclosing the noise receiver, putting a barrier between the noise source and the
receiver, and controlling the noise generator.
Noise is transmitted by vibration. Hence the property of the enclosure must be
such that it should not vibrate when a sound wave hits its surface; otherwise, the
enclosure itself becomes the source of noise. Since vibration is inversely related to
the mass of the material, in the use of enclosures, the effectiveness of control is,
therefore, a function of the mass of the enclosure. Thus, by the mass law, the ideal
enclosure is the heavy enclosure (materials of high density). Table 1.2 shows
surface densities of some common materials of construction.
Table 1.2 – Densities of some common materials of construction.
Material Surface Density in kg/m2/cm of thickness
Brick 19-23
Concrete Blocks 15
Dense Concrete 23
Wood 4-8
Common glass 29
Lead sheets 125
Gypsum board 10
Steel 108-112
Putting a barrier between the source and the receiver is generally used for
controlling highway noise. The effectiveness of a barrier is dependent on the
geometry of the source, barrier and receiver, and on the ground cover. Studies on
different kind of noise barriers reveal that noise attenuation up to 8-14 dBA may
be achieved using barriers 8ft high and 4 inches thick.
1.3.1 Silencers
Control of noise at points of generation may be done with the help of mufflers or
silencers and isolation of noise source by vibration control. There are three basic
types of silencers:
(i) Absorptive Silencers
In these silencers, a lining of some acoustic material is provided directly
on the interior of the duct. The duct may be straight or may have bends,
or the duct may be expanded into plenum lined with the acoustic
material. The acoustic material absorbs the noise, thus attenuating it.
The absorptive silencer is a type of dissipative muffler since it dissipates
the noise by absorbing it.
(ii) Reactive Silencers
These have no lining of absorptive acoustic materials. In them,
attenuation of noise is achieved by reflecting the sound waves so as to
cancel the waves of incoming noise. This process is called destructive
interference. Reactive silencers are found in trucks and automobiles.
(iii) Diffusers
High velocity mass of air impinging on stationary air or solid objects
produces noise due to the turbulence created. Diffusers attenuate noise
by reducing this velocity. The source flow is diffused out into a
multitude of tiny flows having lower velocities using some appropriate
mechanism. The diffuser is an exhaust muffler, since it attenuates noise
by installing it at the end of a duct or pipe.
1.4 Summary
Due to their obvious adverse effects on the physiological as well as
psychological health of human beings, air and noise pollution control are two of
the major components of any pollution management program. Control of
particulate matter, emitting from an industrial process, is one of the important
objectives of any air pollution control initiative. Two of the most efficient devices
used for this purpose are the Electrostatic Precipitators (ESP’s) and Fabric Filters
or the Baghouses. Whereas, electrostatic precipitators work on the principle of
electrostatic charging and subsequent collection of particles by employing a strong
non-uniform electric field, fabric filters use the simple mechanisms of inertial
impaction, diffusion, and sieving for trapping particulate matter. As far as control
of noise pollution is concerned, two of the main strategies in this regard are (i)
controlling noise at the source itself and (ii) isolating the source from the receiver
using a barrier. Silencers and mufflers are important devices used for controlling
the noise at the source itself. Different types of silencers use different principles
for controlling noise.
1.5 Key Words
Particulates: Finely divided solids and liquids, such as dusts, fumes,
smoke, fly ash, mist and spray.
Electrostatic Precipitator: A device that makes use of a strong non-
uniform electric field for the removal of particulate matter from the effluent
gas.
Baghouse: Systems consisting of assemblies of bags which remove dust by
means of a porous fabric and a cake of dust as the filter media.
Silencers: Devices consisting of ducts designed to reduce the level of sound.
1.6 Review Questions
1. What is the principle on which electrostatic precipitator works?
2. What are different components of an electrostatic precipitator?
Explain their significance.
3. How do you calculate the efficiency of an electrostatic
precipitator?
4. What is resistivity? How does it affect the efficiency of a
precipitator?
5. What are different mechanisms through which a baghouse traps
dust?
6. What are different types of cleaning methods used for the
removal of dust from the fabric in the baghouse filters?
7. What are the factors that must be considered before selecting a
baghouse for a particular application?
8. What are different ways of achieving noise control?
9. What are different types of silencers used for noise control?
1.7 Suggested readings
1. Masters, G. M. (1998), “Introduction to Environmental Engineering and
Science” – Prentice Hall of India
2. Boubel, R.W., Fox, D.L., Turner, B. and Stern, A.C. (2005), “
Fundamentals of Air Pollution” – Academic Press.
3. Bell, L. H. and Bell, D. H. (1994), "Industrial Noise Control", Marcel
Dekker, Inc.
4. Stephens, R. W. B. (1986),"Noise Pollution Effects and Control", SCOPE
John Wiley and Sons.
5. Singal, S. P. (2005), “ Noise Pollution and Control Strategy”, Narosa
Publishing House.
UNIT-IV PGDEM-03
RADIOACTIVITY IN ENVIRONMENT
Written by Dr. Hardeep Rai Sharma, SIM conversion by Prof. Anubha Kaushik
STRUCTURE
1.0 OBJECTIVES
1.1 INTRODUCTION
1.2 RADIO ACTIVITY
1.2.1 Radio nuclides
1.2.1.1 Kinds of Radiations
a) Electromagnetic radiation b) Particulate radiation c) Ionizing radiation d) Non-ionizing radiation
1.2.1.2 Sources of Radioactivity in Environment
a) Natural sources b) Man made sources
1.2.1.3 Fate and Movement of Radioactivity in Environment
- Physical and biological half-lives of radio nuclides
1.2.1.4 Biological Effects of Radiations
1.3 SUMMARY
1.4 KEY WORDS
1.5 SELF ASSESSMENT QUESTIONS
1.6 SUGGESTED READINGS
1.0 OBJECTIVES
After studying this unit you should be able to know :
* About radionuclides
* About various kinds of radiations
* Natural and man made sources and fate and movement of
radioactivity in environment.
* Biological effects of radiations.
1.1 INTRODUCTION
1
The smallest unit of an element (as hydrogen carbon, oxygen) that can
exist while retaining the characteristics of that element is called atom.
Each atom has proton (+), neutron (uncharged) and electron (-). Atom of
each element has characteristic numbers of protons, neutrons and
electrons. Most elements, however, in nature contain atoms that are not
exactly like the predominant form. These atoms have different number of
neutrons. These different forms of the same element are called isotopes.
Some isotopes of common elements are stable under ordinary conditions
while others have various degrees of instability, and some of them
disintegrate with the emission of radiations of one kind or the other.
1.2 RADIOACTIVITY
1.2.1 RADIO NUCLIDES
Radioactive isotopes are isotopes that emit ionizing radiation. Since the
radiations are highly energetic (as x-rays) and these tend to split
substances, including living matter, into ions, they are called ionizing
radiation. The term isotope has been used loosely and the appropriate
general term for a particular kind of atom is nuclide. Natural radioactivity
occurs only in elements whose atoms hold a nuclear charge more than
83. The nuclei of such atom are quite, unstable due to large positive
charges on it and emit α (alpha) and β (beta particles). The atomic
nucleus attains an excited state in this manner and emits X-rays or
γ (gamma) rays in order to relieve this energy state .
When the α rays are given off, a new element is formed whose nuclear
charge is reduced by 2 units and whose nuclear mass is reduced by 4
units. For example, the element radium (Ra) is transformed into the rare
gas radon (Rn). 226 4 226
Ra -- He → Rn + energy 88 2 86
If a nucleus loses beta particles of electrons the element receives an
2
additional positive charge on its nucleus without changing the mass of
the nucleus. For example, the lead (isotope) 214Pb is transformed into the
element bismuth (Bi): 214 214
Pb – e– → Bi + energy 88 83
The ions that form in α and β decay proceed quickly into a neutral state
by giving or receiving electrons. The unstable elements which forms from
α and β decay in turn form new elements by further decay, until a-stable
element is found.
Nuclear rays are high energy rays while α rays have 4-9 million electron
volts (MeV) of energy, β rays have usually 0.5-2 Me.V and γ-rays about
0.1-2 MeV of energy. The unusually high energy of the nuclear rays
decreases progressively as they pass through air, water or other media,
because collisions with other materials occur during such passage and
with every collision the atom is excited or ionized. The electron moves
temporally to a higher energy level as it takes an additional energy, and
then again releases that energy and return to its original level. The
energy released can be harnessed for chemical reactions or one can
harness the light it emits, e.g. in a scintillation counter.
The larger the particles of the nuclear rays are, the more frequently they
will collide with molecules and the more quickly they will lose their
energy. The distance that the ray travels is affected by this. (The photons
of γ-rays travel a greater distance than helium nuclei, though their
ionization strength is less than helium nuclei. β-rays lie between α-rays
and γ-rays both in distance they travel and their ionization strength. In
the air γ-rays travel a distance of several to many metres depending on
their energy content. They penetrate entirely the soft tissues of
organisms, as do free neutrons. β rays can travel about 150-850 cm in
the air; they penetrate at most a few centimeters into the soft tissues of
3
organisms. Helium nuclei travel 2.5-9 cm in the air and penetrate only
fractions of millimeters into soft tissues, α-rays and β rays therefore
release their entire energy during their short passage through the tissue.
That implies that the cells suffer severe damage at the point at which
these rays penetrate them.
As radioactive elements decay more frequently, they become more
hazardous to body tissue; for that reason the number of instances of
decay in a ,given quality of food is a matter of importance. The Becquerel
is the unit of measurement; I Becquerel (Bq) = 1 decay per second. The
number of rays, or the dose, is determined by reference to the ion pairs
generated. Rontgen (R) is the unit of measurement; I R = number of rays
that produces 2.082 billion ion pairs in 1 cm3 of air. The number of rays
that is absorbed by body tissue and that is responsible for the biological
effect is measured in “radiation absorbed dose” (rad) i.e. as the number of
rays that is absorbed by a given mass of material, 1 rad =0.01 J/kg. The
rad is usually replaced by the Gray (Gy) in present day measurements.
The relationship between the two is 1 Gy. 1 J/Kg = 100 rad.
Metabolism of radio nuclides
If a radio nuclide absorbed into the body is an isotope of element
normally present (e.g. Na, K or Cl), it will behave like the stable element.
Also, if it has chemical properties similar to an element normally present,
it will tend to follow the metabolic pathway of the natural metabolite (e.g. 137Cs and K or 90Sr and Ca). For other radio Nuclides, their metabolism
will depend on their affinity for biological ligands and for membrane
transport systems.
Calcium has an important function as a major component of bone,
although bone also act as a reservoir of calcium in the body : in man
about 17% of calcium in the skeleton is recycled each year. About 30% of 45Ca is absorbed from the gut and about 65% of that is deposited in the
4
skeleton. 90Sr follows a similar route, although urinary excretion is
greater.
Plutonium mainly enters the body by inhalation. Its compounds can may
be soluble in water (e.g. plutonium nitrate or chloride), or chemically
inert and insoluble (plutonium dioxide). The soluble component is rapidly
absorbed from the lungs and transported in the blood to be either
excreted through the kidneys or deposited in tissues (bones and liver).
Out of plutonium entering the blood, about 45% is deposited in the liver,
45% in the skeleton, and the remainder either excreted or deposited in
other tissues. Biological half-life of Plutonium in the bone and liver are
about 100 years and 40 years, respectively. From animal studies, it is
apparent that the lungs, the cells of inner surface of bone, the bone
marrow and the liver are at the most at risk from accidental intake of
plutonium.
1.2.1.1 Kinds of Radiation
There are different types of radiation discussed below:
a. Electromagnetic Radiations
This form is similar to light in its physical properties. These include a
broad spectrum of energy. These are : a) Ultraviolet rays ; b) X-rays c)
Gamma rays; d) Infra Red rays; e) Radio waves; f) Visible light rays. All
the different kinds of electromagnetic radiations are nothing more than
light rays of different wave length and frequency.
• Ultraviolet Rays
The wavelength of UV rays extends from 0.1 µm (100 nm) to 0.4 µm (400
nm). Ultraviolet radiation is divided into UV-C (wavelength of 200-280
nanometers), UV-B (280-320 nm), and UV-A (320-400 nm). The most
biologically damaging is UV-C and the least damaging is UV-A, with UV-B
5
having intermediate efficiency of biological action. The solar spectrum at
the earth's surface contains only the UV-B and UV-A radiations.
Stratospheric ozone strongly absorbs UV-C radiation and the shorter
wavelength portion of UV-B radiation, thus providing some biological
protection.
• X-rays
X-rays are also a form of electromagnetic radiation, but differ from
gamma radiation in that they result from extra-nuclear loss of energy of
charged particles, for example electrons, but having shorter wavelengths
than ultraviolet radiation. They may be emitted when an electron of an
atom jumps from one orbit to another orbit of lower energy. This
difference in energy is radiated as electromagnetic radiation. If the energy
is high enough to cause ionization the emission is called X-rays.
• γ-Rays
Gamma radiation is emitted only in conjunction with other types of decay
and belongs to the class known as electromagnetic radiation (like radio
waves and visible light, but of very much shorter wavelength and higher
energy). It is emitted when the nucleus produced following radioactive
decay is in an excited state, and then returns to the ground state by
emitting this radiation to carry away excess energy.
• Radiowaves/Microwaves
These are the waves in or near the extremely high frequency or shorter
wavelength range (3 mm to 200 cm). Microwave energy is too low to
disrupt living tissues by ionization. Instead, the energy gets absorbed as
oscillation energy and is converted to heat. This makes it possible to use
microwaves for cooking.
b) Particulate Radiations
6
They consist of the particles ejected from atoms at high speed and often
with tremendous energy. These have electrons, proton or neutron.
Whether the radiation emitted from nuclear disintegration is
electromagnetic or particulate, the emanations are so energetic and
forceful that they can do great damage to living tissues. The radiation
include β-particles, α-particles, proton, neutron and cosmic rays.
• α-radiation
α radiation has been shown to be composed of helium nuclei, consisting
of two protons and two neutrons bound together very tightly to give a
very stable unit. Consequently each particle possesses a positive charge
of 2 units, and a mass of 4 units. One electron volt (eV) is defined as the
energy gained by an electron passing through an electric potential of 1
volt. One gram of radium emits 3.7 x 1010 α-particles and 2 x 109 cal,
which is 2 X 105 times the calorific value of coal.
• β-Radiation
Normally the term beta particle or radiation refers to the high speed
negative electrons of kinetic energy up to more than 3 MeV originating in
the nucleus. One further type of beta particle, is also known, having
same mass as an electron, but is, positively charged and known as
positron radiation Indicated by β+.
• Neutron
Neutron is very common particle, being a basic constituent of the
nucleus and having a mass almost identical to the proton but carrying no
charge. There are no significant naturally occurring neutrons emitters,
but radio nuclides that emit neutrons can be produced artificially. The
neutrons are of great importance both in nuclear fission reactors and in
the production of radio nuclides not available naturally.
• Proton
7
Protons are 1,835 times heavier than electrons. Beta-particle drives into
a tissue like a tiny particle of sand, while the proton lumbers along like a
rock, knocking off pieces of atoms and molecules as it goes. The proton
does not penetrate as far as an electron of the same energy, but it causes
more disruption in a small area.
• Cosmic Rays
The extremely penetrating radiation falling upon the earth's surface
beyond the atmosphere are called cosmic rays. The cosmic rays which
are just entering on earth's atmosphere from outer space are called
primary cosmic rays. They are almost composed of positively charged
atomic nuclei, mostly proton about 89% and about 9% are α-particles
and rest are heavy nuclei such as carbon, nitrogen, oxygen, iron etc.
They have energies ranging from 109 to 1018 eV.
As the primary cosmic rays enter the earth's atmosphere from outer
space, its constituent charged particles collide with the nuclei of
atmospheric gases and splits into smaller nuclear fragments. These
fragments move with high speed and collide with other nuclei and
produce high speed particles and some elementary particles. When these
short lived elementary particles decay, electrons and highly penetrating
γ-rays are emitted. These protons and other high speed particles that are
produced are called as secondary cosmic rays.
Types of radiation on the basis of ionization
Radiation is energy being propagated from one place to another through
space. There are mainly two types of radiation on the basis of ionization:
• Ionising Radiation.
lonising radiation is sufficiently energetic to cause ionizations. An atom
gets ionized when it gains sufficient energy for one or more of its
electrons to get separated from the atom. Ionisation of a molecule might
8
yield two charged fragments, such as H2O → H+ + OH–. If the fragments
are uncharged, then they are referred to as ‘free radicals' as H20 → H +
OH.
• Non-Ionising Radiation
Radiations of shorter wavelength but having greater energy may be able
to harm the microorganisms but are able to injure only the surface
tissues of higher plants and animals. These radiations includes
ultraviolet radiation, microwaves and extra low frequency (ELF)
electromagnetic radiation.
1.2.1.2 Sources of Radioactivity in Environment
Man is exposed to different sources of radiation. These are :
a) Natural sources
These include: a) cosmic rays; b) environment (rocks, water, air); c) living
organisms (internal).
Radio nuclides of radium, thorium, uranium and isotopes of potassium
(40K) and carbon (14C) are very common in soil, rocks, air and water.
Marine sediments generally have higher concentrations of radio nuclides.
On an average, man receives about 50 m rads/yr from terrestrial
radiations and it may be as high as 2000 m rads/yr in areas where
uranium containing rocks exist as in Kerala.
Radiations from atmosphere are also common. For instance, radioactive
gases like radon are present in air through with low values of roughly 2
m rad/yr.
Man is also exposed to internal radiations from radioactive substances in
the body tissues. For instance, uranium, thorium and isotopes of
potassium, strontium and carbon exist in small amounts in the body.
Internal radiation values vary from 25 to 75 m rads/yr.
9
b) Man made sources
These include :
(i) Use of X-ray machines and laser beam (diagnostic and radio
therapeutic) is one such source.
(ii) Radioactive fall out (nuclear test) : Explosion of nuclear weapons
would generate small particles that would drift in the atmosphere
as an aerosol, and in the course of months and years would settle
on the earth's surface as fallout and is the cause of fairly uniforml
release of radioactivity.
(iii) Nuclear reactor wastes: The use of radioactive substances like 233U, 235U or 239Pu in nuclear power plants is another source of
radioactivity in the environment. The risk of melting down of a
reactor is a major risk in case of atomic power plants. Two very
serious instances have already occurred : one in Harrisburg in
1979 and another in the super reactor in Chernobyl in 1986. An
area of at least 100,000 km2 of the soil has been so intensively
polluted with radioactive material that in future no agriculture will
be possible on it. A nuclear power plant must be stopped after
about 30 years because of the constant contamination it sustains.
The process of dismantling a retired plant and of removing the
contaminated parts is hazardous and is a cause of radioactivity in
environment. The disposal of the tritium contaminated Water in
the reactor is also a problem. If the release of water is uncontrolled
the tritium enters the air and drinking water and eventually
reaches humans through the food chain.
(iv) Industrial and research uses of radioactive materials : Radio active
material are used in R & D activities and from there enter into
environment.
10
(v) Miscellaneous, (fulminous watch dials) : Several radioactive
materials find use in daily life and emit the radiations.
1.2.1.3 Fate and Movement of Radioactivity in Environment:
The radioactive pollutants reaching the freshwater resources or oceans
are rapidly lost to the sediments and may bio-accumulate in the body of
plants and animals directly or through food chains (bio-magnification). In
the body of the organisms, they behave chemically as their stable
counterparts, but are more dangerous because of the radiation which
they emit internally. Algae, macrophytes and fish concentrate the radio
nuclides in greater amounts from ambient water.
Man is the ultimate sufferer who consumes the contaminated food and
water. However dose radioactive waste extremely low. The irrigation by
contaminated water will pollute the soil from where the radio nuclides are
transferred to the crops. Soils also get polluted by a direct release of low
activity waste waters and by radioactive fall out. The radioactivity from
the soil moves through the food chains and reaches man after
consumption of crops, meat, milk, eggs etc. The underground water may,
also receive radio nuclides after leaching from the soil.
The radioactivity released into the atmosphere is rapidly diluted by
atmospheric processes, but it may be of concern to man in certain highly
contaminated areas, for example, in the vicinity of atomic explosions or
in atomic power plants. The atmospheric fall out depositing directly onto
the leaves is efficiently passed into the grazing animals, such as cattle,
and reach to us. Cesium-I37 and Strontium-90 are two most important
radio nuclides found to reach humans in this manner.
Physical and Biological half-lives of Radio nuclides
The amount of a radioactive element that decays in a unit of time is
always proportional to the remaining amount. Every element has a
11
constant decay time, so different elements have different decay times. For
practical purposes time in which· the number of radioactive atom of a
nuclide is reduced by half called half-life is used.
To assess the time during which a radioactive element contaminates the
body after its incorporation, the biological half-life is relevant. The term
biological half-life is the time span during which half of the received
material is eliminated from the body, since radio nuclides do not
decompose in the body. From the biological half-life Tb. and the physical
half-life Tp, the effective half-life (Teff) for the entire organism or for a
specific organ can be calculated; this figure indicates how long the
organism or a specific tissue has been exposed to radiation.
Teff = Tb Tp Tb+Tp
The physical, biological and effective half lives of a few radioactive
elements are given in Table 1.
Table-1: Physical, biological and effective half-life of some radionuclides. For plutonium and half-life refer to bones. In the lung (as a non-water-soluble compound) the value is one year.
Element Half-life Type of ray
Physical Biological Effective
H-3 12.26 Years 19 Days 19 Days β-
C-14 5730 Years 35 Days 35 Days β-
P-32 14.3 Days 10 Years 14.1 Days β-
K-40 1.25 x 109 Years 37 Days 37 Days β-, β+
Ca-45 165 Days 50 Years 163.5 Days β-, γ
Sr-90 28.1 Days 11 Years 7.9 Days β-
1-131 8.07 Days 138 Days 7.6 Days β-, γ
Cs-137 30.23 Years 70 Days 69.6 Days β-, γ
Ba-140 12.8 Days 200 Days 1.2 Days β-, γ
12
Rn-222 3.824 Days α
Ra-226 1600 Years 55 Days 53.2 Years α, γ
U-233 1.62 x 105 Years 300 Days 300 Days α, γ
Pu-239 2.44 x 104 Years 120 Years 120 Years α, γ
Source : The Chemistry of Pollution, Gunter Fellenberg, pp. 164
1.2.1.4 Biological Effects of Radiations
Radioactive substances are among the most toxic substances known.
Radium is 25,000 times more lethal than arsenic. The most tragic early
evidence of the potency of radiation toxicity was the death of Marie Curie,
while working with her husband; Pierre Curie. She died of leukemia due
to radiation exposure.
Ionizing radiations bring about more dangerous effects than other
toxicants. Their effect may continue in subsequent generations. They
bring about following two types of undesirable effects in organisms.
i) Somatic Effects
These are the direct results of action of radiation on the body cells and
tissues. Radiologists, uranium mine workers and painters of radium dials
suffer the most. More evidence of degree and kind of damage from
radiation comes from studies of the Nagasaki and Hiroshima survivors.
The somatic effects may be immediate or delayed.
High radiation exposures have much acute toxicity and can kill animals
quickly. A dose of 400 to 500 roentgen on whole body is fatal in about
50% cases of man, and 600-700 roentgen in practically every case. The
victim declines in vitality and dies from anemia, infection and
hemorrhage. Parts of body differ in sensitivity. The most sensitive tissues
are intestine, lymph nodes, spleen and bone marrow.
The radiations destroy the body's immune response. The effects of low
13
penetrating radiations are less severe than the penetrating ones.
In delayed effects the patient may survive for months or years. Delayed
effects of radiations include eye cataracts, leukemia, malignant tumors,
cardiovascular disorders, premature ageing and reduced life span.
Diagnostic X-rays exposure of pregnant women may increase the risk of
cancer in child.
ii) Genetic Effects
Both, natural and man-made radiations bring about genetic effects.
Studies on Drosophila (fruit fly) have shown that mutation rates go very
high due to radiation exposures. Most genetic effects are brought about
by man-made radiations mostly from medicare and exposure from
nuclear power plants. People in industry, research and medicine using
radio nuclides are exposed more than others. The greatest damage is in
dividing cells, chiefly the gonads. The effects include mutation or lethal
effects on egg or embryo. The intensity of radiation affects the rate of
mutation. Generally higher animals are more susceptible to genetic
damage than lower animals as insects. Genetic effects also occur in
plants.
1.3 SUMMARY
Radioactive isotopes are the ones that emit ionizing radiations, which are
highly energetic and tend to split substances including living matter.
There is emission of alpha (α), beta (β) and gamma (γ) particles, each
having characteristic charge, penetrating power and energy. Radioactive
elements are more hazardous to body tissue as they decay more
frequently and Bequerel (Bq) is the unit of measurement of 1 decay per
second. Radionuclides, when absorbed into the body, behave like a stable
element and follow the metabolic pathway of a natural metabolite e.g. 90Sr behaves like calcium. Radiations can be of various types. The
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electromagnetic radiations include ultra-violet rays (UV), X-rays, gamma
rays, radio waves and infrared rays. UV-B and UV-C rays are very
hazardous for biological systems. We are exposed to radiations from
natural sources like cosmic rays, rock, air, water etc. or from man-made
sources like X-rays, nuclear reactor wastes or nuclear fall-outs. The
radioactivity also moves through the food chain and reaches man’s body.
Half life of the radionucleides is very important to know how long the
radioactive substance will remain in the tissue or in the environment.
Radiations have adverse effects on living organisms causing damage to
body cells and tissues, destruction of immune response, genetic effects,
cancer and even death.
1.4 KEY WORDS
Isotopes : An element having atoms with different
number of neutrons.
Radioactive isotopes : Isotopes that emit radiations
Becquerel : Unit of measurement of decay (1 Bq = 1 decay
per second).
Rontgen (R) : Unit of measurement of X-ray
Rad : Radiation absorbed dose i.e. the number of
rays absorbed by a given mass of material.
UV rays : Ultraviolet rays with wavelength 0.1 µm to 0.4
µm. UV-C and UV-B are very harmful.
Alpha rays : Positively charged particles consisting two
protons and two neutrons
Beta rays : High speed negatively charged electrons.
1.5 SELF ASSESSMENT QUESTIONS
1. What are the different sources of radioactivity in environment?
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2. Explain the somatic effects and genetic effects caused by
radiations.
3. Describe various types of electromagnetic and particulate radiation.
4. Differentiate between ionizing and non-ionizing radiations.