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					Environmental Pollution

                            Environmental Pollution
                         Author: Partha Das Sharma
   (E.mail:, Website:

Pollution can be defined as any undesirable change in the composition of air, water and
soil or any segment of environment against nature. The pollution results in deteriorating
the nature and the quality of the environment. The undesirable or unwanted foreign
species causing pollution are called pollutants.

On the basis of factors causing pollution, pollution can be classified under the following
1. Air pollution
2. Water pollution
3. Soil pollution
4. Solid waste pollution
5. Hazardous waste pollution

A. Air pollution:
It can be defined as the presence of chemicals in the atmosphere in quantities and
duration that are harmful to human health and the environment. It occurs when the
concentration of certain materials become high enough to cause the atmospheric
environment to become toxic.
* Sources of air pollution:
The sources of air pollution can be natural or man made.

i) Natural Sources
· Volcanic eruptions emitting poisonous gases
· Decay of vegetation
· Marsh gases
· Pollen grains
· Forest fires

ii) Man made Sources
· Increase in pollution
· Deforestation
· Fossil fuel combustion
· Vehicular emissions
· Industrialization
· Use of pesticides, insecticides in agriculture
· Explosives used in wars

* Types of Air Pollutants:
The undesirable component in the atmosphere, causing pollution is referred to as
pollutant. Pollutant can be classified on the basis of origin and physical state. On the
basis of origin, pollutants are of two types:

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a) Primary pollutant: it is the product of natural events (like fires, volcanic eruptions)
and human activities added directly to air. Examples include, CO, NO2, SO2 and
b) Secondary pollutant: it is formed by the interaction of primary pollutants with each
other or with normal components of the air. Examples include ozone, photochemical
smog etc.

On the basis of physical state, pollutants are of two types:
i) Gaseous pollutants: these are the gases which mix with the air without settling down.
Gaseous pollutants include CO, SO2, CO2, SO3, NOx and hydrocarbons.
ii) Particulate pollutants: it comprises of finely divided solids or liquids. These include
dust, smoke, smog, lead, mercury, cadmium and asbestos.

* Some Common Air Pollutants:
1. Carbon Monoxide (CO) - It is a colourless, odourless, tasteless gas that is by far the
most abundant of the air pollutants.
a. Source- It is produced due to:
· Incomplete combustion of fuels
· Automobile exhausts
· Industrial operation
CO is also produced through natural processes such as volcanic activity, natural gas and
marsh gas emissions.

b. Sink for CO - Sink is a system which absorbs the pollutant, thereby, nullifying its
harmful effect. A large mass of CO is generated and also there is continuous increase in
CO emissions within recent years. So it is obvious that the amount of CO in the
atmosphere should also increase. But it has been found that the amount of CO in the
atmosphere remains relatively constant suggesting that a sink or scavenging process also
exists in the atmosphere. The micro organisms present in the soil act as a major sink for
CO.A significant amount of CO is converted into CO2 by these microorganisms.

c. Effects of CO - The levels of CO present in the urban air do not affect significantly the
plants and materials. However, these levels adversely affect human health. In urban areas,
the soil available is insufficient to act as a sink thereby increasing the level of CO beyond
permissible limits. CO interferes with the blood’s ability to carry oxygen to different
parts of the body. The oxygen combines with hemoglobin, which is also known as
oxygen carrier, to form oxyhemoglobin. This oxyhemoglobin travels to different parts of
the body cells where it gives oxygen to the cell and takes up CO2 through the lungs. If
CO is inhaled, it readily binds to hemoglobin to form carboxyhemoglobin and blood
carries less oxygen to various parts of the body. Fortunately CO is removed from the
blood stream when clear air is cleaned.

d. Control of CO- Control of pollution caused by CO can be achieved through the
following techniques:

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(i) Modification of engine design: A low fuel-air ratio reduces NOx emissions but
increase CO emissions. So engine design should design should be modified so that right
proportion of oxygen is maintained for complete oxidation of carbon and hydrogen to
CO2 and H20 respectively.
(ii) Fuel modification: The fuels which release lesser amount of CO are recommended to
be used. These fuels include natural gas, methane and blends of light hydrocarbons.
(iii) Treatment of exhaust gases: Two stag catalytic converters are used to lower the
pollution from exhaust gases. In the first stage NOx are reduced to N2 and NH3 in the
presence of catalyst such as Pt, Pd and Ruthenium in the presence of reducing gas such as
CO. in second stage, oxidizing catalysts of noble metals supported on ceramic materials
are used, which ensure oxidation of CO and CO2.

2. Oxides of Nitrogen (NOx)- A number of oxides of nitrogen such as NO, N2O, NO2,
N2O3 and N2O5 are introduced into the atmosphere due to natural as well as human
activity. Out of these, the two oxides NO and NO2 are responsible for pollution and are
considered as the pollutants and are represented by NOx. NO2 is reddish brown in color
having pungent smell and is suffocating, whereas NO is colorless and odorless gas.

a. Source- The sources of NOx include:
(i) Natural: during lightening discharge, N2 and O2 in air combine to form NO
(ii)Man-made sources:
· NO is formed when N2 and O2 at very high temperatures. This temperature is usually
attained during combustion of fossil fuel in air.
· NOx is also produced in chemical industries as by products such as in coal based power
plants, sulphuric acid and nitric acid manufacturing plants

b. Sink for NOx - The NO and NO2 undergo various processes in the atmosphere leading
to the formation of HNO3. Thus NO2, NO, pollutants gets precipitated as nitrates during
rainfall. Ozone plays a significant role in these photochemical reactions. In presence of
volatile organic compounds, NO2 reacts with water in presence of oxygen resulting in
formation of nitric acid. HNO3 comes down from the atmosphere to the surface of earth
in form of acid rain. Here it reacts with bases such as ammonia, lime etc. to form nitrates.

c. Effects - Almost all the NOx emissions are in the form of NO, which has no known
adverse health effects at the concentrations found in atmosphere. However, NO can
oxidize to NO2, which in turn may react with hydrocarbons in the presence of sunlight to
form photochemical smog. The HNO3 formed by NO2 causes acid rain which has
corroding effect on marble and the metallic structures. It decreases pH of the soil,
affecting its fertility.

d. Control of NOx - The control measures include:
(i) Modifying the engine design: Reducing the amount of excess air for combustion in air
helps in controlling NOx emissions. The burners are so modified that the fuel and the air
mix more slowly reducing the intensity and temperature of combustion.
(ii) Scrubbing the flue gases: The flue gases are scrubbed with H2SO4 in a scrubber. The
reaction product thus obtained is then decomposed to nitric acid and NO.
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(iii) Selective catalytic reduction: The selective catalytic reduction can be achieved
through CH4, NH3 and CO. The NOx is added to the exhaust gases and the mixture is
passed over a fixed bed catalyst such as copper oxide.

It is interesting to note that the air pollution control methods also require modification of
combustion methods. But the methods that improve CO emissions tend to make
emissions of NOx problem worse, and vise versa.

* Environmental Effects of Air Pollution:
The air pollutants have a great impact on our environment. They are posing a threat to
earth’s general environment. The major effects are:
1. Photochemical Smog:
‘Smog’ is the combination of two words ‘smoke’ and ‘fog’. Smog is of two types:
(a) London Smog:
The smog from SOx particulates and humidity is known as London Smog. This type of
smog caused death of around 4000 people in London in 1952, hence the name. The
mixture of smoke, SOx and fog is chemically a reducing mixture and hence also termed
as reducing smog.
(b) Los Angeles Fog (Photochemical Smog): it results from the photochemical reactions
of the atmosphere. It is the atmospheric haze that is formed near many large cities and is
due to the action of sunlight on hydrocarbons and the nitrogen oxides. This type of smog
was first observed in Los Angeles in 1950.

2. Acid Rain:
Unpolluted rain water is slightly acidic due to the presence of CO2 in the atmosphere.
CO2 combines with the water to form a weak solution of carbonic acid with a pH of
about 5.6. However in polluted environments, the rain passes through an atmosphere
polluted with oxides of sulphur and nitrogen. The falling rain reacts with these oxides to
form a mixture of sulphuric acid and nitric acid and water. This is known as acid rain.
Acid Rain Formation:
The oxides of sulphur react with moisture to form H2SO4 in steps:
SO2 + O2 → 2SO3
SO3 + H2O → H2SO4
Similarly oxides of nitrogen form HNO3
NO + O3 → NO2 + O
NO2 + O3 → NO3 + O2
NO2 + NO3 → N2O5
N2O5 + H2O → 2HNO3
These acids along with HCL gives rise to the acid rain.

3. Green House Effect:
The green house effect is the rise in temperature that the earth experiences because
certain gases in the atmosphere absorb energy from the sun. Without these gases, heat
would escape back into space and the earth’s average temperature would be about 60
colder. The phenomenon has been named green house effect as it is similar to heat
trapping effect of the glass walls in a horticulture green house.
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a. The Process - The earth receives a tremendous quantity of radiant energy from the sun,
about 30% of which is reflected back into space by the earth’s atmosphere. The
remaining energy passes through the atmosphere to the earth’s surface. Some of the
energy is absorbed by plants to drive photosynthesis and some by the oceans to evaporate
water, but most is absorbed by soil, rock and water to increase the temperature of the
earth’s surface. This energy is in turn radiated from the heated surface mainly as infrared
radiation, often called heat radiation. However, only a small portion of this energy makes
it back into the space. The majority of infrared radiations are absorbed the green house
gases present in the atmosphere. A net amount of thermal energy is retained by the earth
that makes it warmer than it would be without these gases in the atmosphere.

4. Ozone Layer and its Depletion:
Ozone is a very small part of our atmosphere but its presence is nevertheless vital to
human wellbeing. Most ozone resides in the upper part of the atmosphere called
stratosphere extending from 16 km to 40 km. the high concentration of ozone in the
ozone layer shields us from the harmful ultraviolet light from sun.
a. Effect of Ozone Layer - Ozone protects us from harmful UV rays absorbing these
radiations. In absence of ozone layer these rays would reach earth and cause:
(a)Skin caner
(b)Damage to the plants
(c) Sun burns
(f) Cataracts etc.
The absorption of UV radiation by ozone is a source of heat in stratosphere. As a result
ozone plays a key role in maintaining the temperature structure of earth’s atmosphere.
b. Depletion of Ozone - Ozone depletion has been found to occur through the following
types of compounds:
· Chloroflouro carbons (Freon)
· Nitric oxide
· Reactive hydroxyl radicals
· Atomic oxygen
c. Effects of depletion of Ozone:
· The existing ozone layer screens out more than 99% of incoming UV radiations. The
small fractions gets through is known to cause sunburn, skin cancer and various kinds of
damage to animal and plants.
· The genetic material DNA has the capacity to absorb UV radiations which may cause
various disruptive defects.
· 5% depletion in ozone produces 10% increase in radiations reaching the earth according
to an estimate.
· Exposure to UV radiation damages the cornea and lens of the eye causing blindness.

B. Water Pollution:
When toxic substances enter lakes, streams, rivers, oceans and other water bodies, they
get dissolved or lie suspended in water. This results in the pollution of water where by the
quality of water deteriorates. The word polluted water is defined as the deterioration of
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physical, chemical and biological properties of water brought mainly by human activities
and natural resources and which cause harmful effect on human and aquatic life.
* Sources of water pollution - Water pollution is caused by human as well as natural
i) Natural Sources - These include decomposed vegetable, animal and weathered
products which are brought into main water resources.
ii) Human Activities - These include domestic as well as industrial pollution:
a. Domestic: The release of huge quantities of municipal and domestic wastes through
the drains into the rivers and canals is the major cause of pollution. The domestic waste
water contains human faeces, kitchen wastes, organic water that provides nutrition to
fungi and bacteria.
b. Industrial: These include effluents from factories, refineries and a number of chemical
industries. Water gets polluted by acids, alkalis, detergents, copper, zinc, lead, mercury
etc. which are constantly added to water by industrial effluents.
c. Apart from these two, the water is also polluted through agricultural discharge such as
pesticides, insecticides, fertilizers etc. besides these, bacteria, algae, virus also cause
water pollution.

* Classification of Water Pollutants:
The problem of water pollution due to discharge of domestic and industrial waste into
water has already become a serious problem in the country. To aid in the systematic
discussion of water pollutants, they have been classified into nine categories:
1. Oxygen demanding wastes
2. Pathogens
3. Synthetic organic compounds
4. Plant nutrients
5. Inorganic chemicals and minerals
6. Sediments
7. Radioactive substances
8. Thermal discharges
9. Oil
1. Oxygen Demanding Wastes:
Dissolved oxygen (DO) is essential for sustaining the plant and animal life in any aquatic
system. Warm-water fishes require a minimum DO level of 5 mg/L. if DO level drops
below the level necessary to sustain normal life, then water is classified as polluted. The
amount of DO in water is reduced because of oxygen demanding wastes. These are
substances that oxidize in water, reducing the amount of DO. These include organic
substances contained in municipal waste water or in effluents from certain industries. The
oxygen demanding wastes are oxidized by bacteria or micro organisms consuming DO in
water to CO2 and water. These substances produce undesirable odor, tastes and reduce
the acceptability of water as domestic supply.

There are several measures f oxygen demand commonly used:
(a)Biological Oxygen Demand (BOD): BOD is the amount of oxygen required by
microorganisms to biologically degrade the wastes. It is the most important measure of
the strength of organic pollution
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(b)Chemical Oxygen Demand (COD): COD is the amount of oxygen needed to
chemically oxidize the wastes.

1. Pathogens - These are disease causing organisms that grow and multiply within the
host. Water is potential carrier of pathogenic microorganisms. These pathogens are
carried into the water bodies by sewage and wastes from farm and various industries.
Contaminated water caused by poor sanitation can lead to both water borne and water
contact diseases. Water borne diseases are those acquired by ingestion of pathogens not
only in drinking water, but also from the water that makes it to person’s mouth from
washing food, utensils and hands. Examples are cholera, typhoid etc. Water Contact
diseases do not require that the individual ingest the water. Just contact with the water
causes the disease. For example bilharzia is the most common water contact disease in
the world.
2. Synthetic Organic Compounds - These include pesticides, synthetic organic chemicals
and detergents. These compounds are not biodegradable and persist for longer periods.
Most of these are accumulative toxic poisons and ultimately reach objectionable levels in
water. Pesticides cover a range of chemicals that kill organisms that human consider
undesirable. These enter the water bodies from run off from agricultural lands, waste
discharge by pesticides manufacturers and by other means. Because of their world wide
usage, nearly all the rivers and oceans of the world contain pesticides residue. Detergent
means cleansing agent. The basic active ingredient in detergents is surfactant or surface
active agent with contains hydrophobic and hydrophilic groups. Surfactants decrease the
surface tension of water so that they can penetrate the surface and interstices of the object
to be cleaned. Te remainder comprises of polysulphate salts called builders and other
ingredients. Surfactants concentrations as low as 1ppm produce foam in rivers and in
sewage treatment plants. Although these concentrations are non-toxic to humans, their
presence gives off taste of drinking water. The detergent builders pose greater problem
today. The polyphosphates builders are released into water and act as plant nutrients. The
extensive growth of algae consumes most of dissolved oxygen from water.
3. Plant nutrients - Nutrients are chemicals, such as nitrogen, phosphorus, carbon,
sulphur, calcium, potassium, iron etc. that are essential to growth of living things.
However, in terms of water, these nutrients are considered as pollutants; when their
concentrations are sufficient allow excessive growth of aquatic plants, particularly, algae.
When these algae die and decompose they add undesirable odor and objectionable taste
of water. Further, the decaying of organic matter oxidizes and leads to reduced DO
levels. The gradual accumulation of silt and organic matter is known as eutrophication.
High concentration of nutrients poses the problem of eutrophication but also when found
in drinking water it is a serious health hazard.
4. Inorganic Chemicals and Minerals - This include inorganic salts mineral acids and
finely divided metal or metal compounds, trace elements, cyanides, organo-metallic
compounds etc. they are added to the water bodies through municipal and industrial
waste water and mine run off.
a. Acid Mine Drainage - It is a source of increasing acidity in natural water, acid rain
also adds to it. The mining of sulphur bearing ores containing lead, zinc and copper lead
to acid drainage. Coal mines discharge is also a cause of acid mine drainage.

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b. Soluble Salts - The salts or solids passing through water on its way to sea include
cations such as sodium, calcium, potassium, magnesium and anions like chlorides,
sulphates and bicarbonates. The measure of salinity is concentration of total dissolved
solids (TDS).
c. Heavy Metals - It refers to metals with specific gravity greater than 4 or 5. Metals may
be inhaled or digested and have adverse effects on the body. Cadmium, lead and mercury
are nephrotoxic metals.

5. Sediments - They include soil, sand and mineral particles washed into aquatic
environment by storms and flood waters. They are sources of organic and inorganic
matter which reduces the storage capacity. This decreases evolution of oxygen and hence
cannot support aquatic animals.
6. Radioactive Substances - They enter aquatic system through the use of naturally
occurring or artificially produced radioactive materials. These pollutants enter the water
system through:
(a) Mining and processing of ores
(b) Nuclear power plants
(c) Leakage from underground nuclear detonations
(d) Radioisotopes in medicine etc.

7. These substances can enter human body through food and water and cause cancer, eye
cataract etc.
a. Thermal Discharges:
The used coolant water in industries id directly discharged into water bodies which
increases their temperature. This is thermal pollution. Rise in temperature decreases DO
content which affects the aquatic life.
b. Oil:
It is added to water bodies from industries as effluents oil refineries, storage tanks and
automobile waste oil. Oil being insoluble in water, floats over it. It may penetrate the
feathers of birds such that they find difficulty in floating and flying. They may ingest it
and die.
c. Volatile Organic Compounds:
They are volatile solvents used in industries. They are toxic and cause harm when present
in drinking water. Some of the VOCs are:
(a)Vinyl chloride

* Waste Water treatment Process: Available waste water treatment processes can be
classified as:
· Physical
· Chemical
· Biological
The waste water treatment processes are generally grouped according to water quality
they are expected to produce
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1. Primary Treatment - It utilizes physical processes like screening to remove a portion
of pollutants that settle or float.
(a) Pretreatment: It consists of screening and grit removal. Screening removes large
floating objects which are disposed off. Then water passes into grit chamber where
velocity of water is reduced.
(b)Sedimentation: From grit chamber sewage passes into primary settling tank called
sedimentation basin. It clarifies suspended solids and 40% of organic matter.
2. Secondary Treatment - The purpose here is to remove organic matter and is based
upon biological process similar to natural biodegradation by aerobic bacteria. They
(a) Coagulation of colloidal matter
(b) Oxidation of organic matter
(c) Conversion of nitrogeneous matter to ammonia and finally to nitrites and nitrates
(d)Anaerobic digestion of sludge
3. Tertiary Treatment - It improves the quality of effluent further. It includes removal of:
(a)Suspended solids
(c)Dissolved organic solids

Some of the techniques are:
(a)Micro-staining: It removes the solid wastes that get retained on fabric of filter media,
(b)Removal of Dissolved Solids: It can be done by adsorption of soluble organics on
activated carbon, solvent extraction, ion-exchange (To remove hardness of water),
reverse osmosis and chemical precipitation through lime etc.,
(c)Removal of Nutrients: it includes removal of nitrogen (ammonia stripping) and
phosphorous which may be present as orthophosphates by alum.
(d)Removal of bacteria: By retaining effluents in maturation pond for specified period
of times.

C. Soil Pollution:
* Sources: Soil pollution is degradation of soil mainly through human misuse. The
human influences include:
(a)Industrial waste: this is disposed into the soil and poses a detrimental effect.
(b)Disposal of solid wastes: this is accumulation of lead particles from automobile
exhausts, garbage containing plastics etc.
(c)Agricultural practices: this is due to fertilizers and use of pesticides.
(d)Biological agents: this is through excreta of humans, birds and animals.
(e)Soil erosion: nutrients for supporting vegetation are wiped off.

* Effects: Soil pollution adds a number of chemicals to it and these are transferred into
humans through food chains. Metallic contaminants destroy the beneficial micro
organisms in soil and hence effect plant growth. Excessive use of fertilizers makes soil
alkaline or acidic. Use of pesticides makes soil contaminated and is also persistent. The
most dangerous pesticide DDT is now banned as it accumulates in the food chain.
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* Control: They include method to reduce and dispose soil wastes. This can be achieved
through following ways-
(a)Proper dumping of soil wastes
(b)Banning of highly toxic and persistent chemical pesticides
(c) Proper awareness of the masses
(d)Increased crop rotation
(e)Recycling of the waste
(f) Plantation of trees to a larger extent.

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Description: Pollution can be defined as any undesirable change in the composition of air, water and soil or any segment of environment against nature. The pollution results in deteriorating the nature and the quality of the environment. The undesirable or unwanted foreign species causing pollution are called pollutants.