Environmental Pollution and Degradation

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June 27, 2017
Material Science : Chapter 7: Composites
June 29, 2017
  1. Environmental Degradation
  2. Pollution and Pollutants
    1. What is environmental pollution?
    2. Different types of pollution
    3. Different pollutants
  3. Air Pollution
    1. Types of pollutants
    2. Effects of different pollutants
    3. Emission norms
    4. Air Quality Index
    5. National Legislations for Air pollution control
    6. Pollution in cities
    7. Pollution control methods
  4. Water Pollution
    1. Water Pollutants and their Impacts
    2. Ground water pollution
    3. Surface water pollution
    4. Marine pollution and degradation
    5. Waste Water Treatment
    6. National Legislations for Water Pollution Control
  5. Land Pollution and Degradation
    1. Solid waste
    2. Agro chemicals
    3. Soil erosion and degradation
  6. Noise Pollution
    1. Noise Limits
    2. Health Impacts
    3. Noise reduction
  7. Ozone layer Depletion
  8. FAQs
    1. Particulate Matter
    2. What constitues the ‘Fly Ash’?
    3. Persistent Organic Pollutants
    4. Ocean Acidification
    5. Advantages of Activated Sludge Process
  9. Practice Questions

1. Environmental Degradation

Environmental degradation means the deterioration of the environment via various ways. It affects the basic functions of the ecosystem like supporting life forms, provision of resources, protection etc. Environmental degradation includes pollution, depletion, global warming, biodiversity loss etc.


2. Pollution and Pollutants

2.1 What is environmental pollution?

Pollution is any undesirable change in physical, chemical or biological characteristics of the environment.

2.2 Different types of pollution

On the basis of the which sphere of environment is affected by pollutants, pollution is classified into

  • Air pollution
  • Water pollution
  • Land pollution
  • Marine pollution
  • Radiation Pollution
  • Thermal Pollution
  • Noise pollution etc

2.3 Different pollutants

Pollutants are any substance that causes pollution. They can be

  • Gases (ex- SO2, NO2)
  • Particulate matter (ex- PM10, PM2.5) [FAQ 1]
  • Radioactive substances (ex- radium, thorium, uranium). They can be from nuclear reactor accidents, nuclear bomb explosions or from radiations in nuclear reactors, laboratories, hospitals etc
  • Bio wastes (ex- food waste, agricultural residues)
  • Non biodegradable substances (ex-plastics)
  • Persistent chemicals (ex- DDT)
  • Heavy metals (ex – Mercury, Lead )

3. Air Pollution

Air pollution is mainly due to gaseous pollutants like NO2 and particulate matter like dust. The main sources of pollutants are exhaust from combustion engines, fires, dust from dry land and construction activities

3.1 Types of pollutants

Primary pollutants Secondary Pollutants
Directly produced from sources like combustion engines

Gases Particulate matter
  • SO2
  • H2S
  • NO2
  • Hydrocarbons
  • Suspended Solid particles like dust
  • Liquid particles like Aerosols
  • Biological agents like pollen and spores
  • Metals like Pb,Cu,Hg
Are formed because of reaction of primary pollutants


  • Smog
  • Photochemical smog
  • Ozone
  • Acid rain

Particulate matter

  • Particulate matter suspended in air are dust and soot(released from the industrial chimneys)
  • Fly ash[FAQ 2] is a particulate ejected mostly by thermal power plants during coal burning operations
  • Effects of Fly ash pollution
    • Fly ash pollutes air and water and may cause heavy metal pollution in water bodies
    • Fly ash affects vegetation as a result of its direct deposition on leaf surfaces or indirectly through its deposition on soil
  • Fly ash utilisation 
    • Fly ash is now being used for making bricks and as a land fill material
  • Lead and other metals particles Tetraethyl lead (TEL) used as an anti-knock agent in petrol vehicles comes out from the exhaust pipes and is mixed with air.
  • Oxides of iron, aluminium, manganese, magnesium, zinc and other metals are released during mining operations 


  • Are suspensions of solid or liquid particles in a gas (The particulate portion of an aerosol is referred to as Particulate Matter)
  • The aerosols are of following types
    • Solid-Particle Aerosols – Includes dust and fume(Fumes are solid particles formed by condensation from the gaseous state)
    • Liquid-Droplet Aerosols – Includes Mist and fog
    • Solid-Liquid particle Aerosols – Includes smoke(is a collection of tiny solid, liquid and gas particles) and smog(smoke + fog)
  • Main sources of aerosols are half-burnt carbon particles from vehicle exhausts or crop residues. Natural sources are fog and haze.
  • Effect on meteorology – Aerosols shield the solar radiation and depress the land temperatures. This causes variations in the monsoon rain pattern.


  • It is a mixture of smoke, fog and sulphur dioxide.
  • Chemically it is a reducing mixture and so it is also called as reducing smog.
  • It occurs in cool humid climate.

Photochemical Smog

  • It occurs in warm, dry and sunny climate.
  • The main components of the photochemical smog result from the action of sunlight on unsaturated hydrocarbons and nitrogen oxides produced by automobiles and factories. 
  • Ozone and NO2 react with the unburnt hydrocarbons to produce chemicals such as formaldehyde, acrolein and peroxyacetyl nitrate (PAN).
  • Photochemical smog has high concentration of oxidising agents and is called as oxidising smog. 
  • Usually catalytic converters are used in the automobiles, which prevent the release of nitrogen oxide and hydrocarbons to the atmosphere.


  • Ozone in the troposphere is a potential green house gas
  • It is a key component of the photochemical smog

Acid Rain

  • Is a precipitation having a pH less than 5.6
  • Acid can precipitate either as rain or as dry deposition 
  • Contains traces of H2SO4, HCl and HNO3
  • When rain water and oxygen combines with sulphide bearing minerals like pyrite (iron) they form sulphuric acid
  • Effects of acid rain
    • Acid rain affects the buildings and monuments. Marble and various metals are corroded by acid rain
    • It affects both aquatic and terrestrial life
    • It damages tree foliage and weaken the trees
    • It promotes leaching of heavy metals such as aluminum, lead and mercury from soil
    • It reduces soil fertility by leaching away the nutrients
    • The acidic soils are more affected, as acid rain further increases their acidity. (Indian soils are slightly alkaline, so the effect of acid rain is not so prominent as in European countries)
    • Soils have natural capacity to neutralize some inputs of acids. This normal soil-buffering capacity can be depleted by the acid rain. When the soil Ph is below 4.5, most valuable nutrients are rapidly lost and bacterial activities will be greatly reduced. This affects the soil fertility and in turn, affects the vegetation.

3.2 Effects of different pollutants

Pollutants Effects
Silica Chronic cough and chest pain
Asbestos Cancer
Tobacco smoke Cancer
Lead Damage to red blood cells, kidney, liver, gastrointestinal diseases
Particulate matter Respiratory illness
Carbon monoxide Reduces oxygen carrying capacity
Hydrocarbons Cancer
Sulphur dioxide Respiratory illness, acid rain
Nitrogen oxides Heart and lung problems
Ozone Lung diseases, corrosion
Smog Poisonous for human
Radiation damage to internal organs

3.3 Emission norms

  • Major pollutants such as fine particulate matter, sulphur dioxide, nitrogen oxides and carbon monoxide are emitted by automobiles.
  • The BS — or Bharat Stage — emission standards are norms instituted by the government to regulate the output of air pollutants from internal combustion engine equipment
  • India has been following the European (Euro) emission norms, though with a time-lag.
  • From April 2017,  all of India moved to Bharat Stage IV (BS-IV) vehicular emission norms.
Timeline  of Emission Norms
BS III Throughout country since 2010
BS IV For 4 wheelers in 13 megacities since 2010
BS IV Throughout country since 2017
BS V Scheduled by 2019, will be skipped directly to BS VI
BS VI Scheduled on 2020 (originally proposed to come in by 2024)

3.4 Air Quality Index

  • The national air quality index (AQI) is to rate the quality of air in 10 cities in real time

Image from AQI website

  • Monitors eight parameters (PM10, PM2.5, NO2, SO2, CO, O3, NH3, and Pb)
  • There are 6 categories – Good, Satisfactory, Moderately polluted, Poor, Very poor and severe  with associated health impacts

3.5 National Legislations

  • Air (Prevention and Control of Pollution) Act, 1981
  • Central Pollution Control Board (CPCB) is a statutory organisation under the Ministry of Environment and Forests (MoEF)
  • CPCB along with its counterparts State Pollution Control Boards (SPCBs) are responsible for implementation of legislations relating to prevention and control of environmental pollution.
  • Article 21- “No person shall be deprived of his life or personal liberty except according to procedure established by law’’ , encompasses the right to environment also
  • Environment (Protection) Act, 1986

3.6 Pollution in cities

  • India has 13 of the top most 25 polluted cities in the world
  • Delhi leads the country in its levels of air-pollution
  • India has the second highest premature deaths caused by outdoor air pollution
  • In Delhi the levels of PM10 and PM2.5 particles have reached more than 8 times the the safe limits
  • A joint report by IIT-Kanpur, Delhi Pollution Control Committee and Department of Environment, NCT Delhi (2016), the single largest contributor of pollution in Delhi is road dust
  • Of the seven sources of outdoor air pollution, residential energy use is the most important category that causes the most premature deaths
  • The main pollutants in cities are aerosols[FAQ 2] and particulate matter

3.7  Pollution control methods

These pollutants must be separated/ filtered out before releasing the harmless gases into the atmosphere. The various methods are


The pollutants are absorbed in suitable absorbent material. Is used to control the pollutants like SO2, NO2, H2S and hydrocarbons. For ex, Ammonia is used to absorb SO2.


The fuel is completely burned to reduce the pollutants.

Ex 1.Selective Catalytic Reduction (SCR)

Image from dieselforum.org

  • Selective Catalytic Reduction (SCR) injects a liquid-reductant agent into the exhaust stream of a diesel engine.
  • The reductant known as Diesel Exhaust Fluid (DEF) sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2)


Ex 2.Catalytic converters

Image from mastermuffler.net

  • Have expensive metals namely platinum-palladium and rhodium as the catalysts
  • As the exhaust passes through the catalytic  converter, unburnt hydrocarbons are converted into carbon dioxide and water, and carbon monoxide and nitric oxide are changed to carbon dioxide and nitrogen gas, respectively.
  • Motor vehicles equipped with catalytic converter should use unleaded petrol because lead in the petrol inactivates the catalyst.


The polluted gas is passed through a porous solid material (absorbent),to attract and hold the gas molecules. The adsorption can be a physical phenomenon (held by van der Waal’s forces) or chemical adsorption (chemisorption).


There are several ways of removing particulate matter. Some examples are

Ex 1. The Electrostatic Precipitator

Image from NCERT

  • Is the most widely used
  • can remove over 99 per cent particulate matter present in the exhaust from a thermal power plant
  • particulate matter that are very very small cannot be removed by these precipitators

Ex 2. Scrubber

  • can remove gases like sulphur dioxide
  • The exhaust is passed through a spray of water or lime


Alternate fuels

The fuels that burn completely can be a better choice. Example,

Ex 1.Compressed Natural gas

  • Is cleaner than gasoline or diesel fuel. CNG emits significantly fewer pollutants (e.g., carbon dioxide (CO2), unburned hydrocarbons (UHC), carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides (SOx) and PM (particulate matter) than petrol.
  • Natural gas vehicles show an average reduction in ozone-forming emissions of 80% compared to gasoline vehicles.
  • CNG does not contain any lead
  • CNG fuel systems are sealed, preventing fuel losses from spills or evaporation.

Ex 2. Biodiesel

  • Is a clean burning fuel made from natural sources such as vegetable oils

4. Water Pollution

Water pollution is the contamination of water bodies (e.g. lakes, rivers, oceans, aquifers and groundwater). The major pollutants are

(i) Sewage Pollutants (Domestic and Municipal Waste)

(ii) Industrial Pollutants

(iii) Agricultural Pollutants

(iv) Radioactive and Thermal Pollutants

4.1 Water Pollutants and their Impacts

Source : NIOS Environment

4.2 Ground water pollution

When the polluted water seeps into the ground and enters an aquifer it results into ground water pollution. Groundwater gets polluted in a number of ways. The dumping of raw sewage on soil, seepage pits and septic tanks cause pollution of groundwater.

  • Groundwater is a major source of irrigation and drinking water
  • The groundwater resources are fast depleting
  • In India, 19 states have reported fluoride contamination
  • Ground water of at least 10 states in India is contaminated with arsenic
  • Iron is the biggest groundwater chemical contaminant in India

4.3 Surface water pollution

image from NCERT

Surface water contamination receives a lot of attention because of the visible pollution of this water. The composition of wastewater will be as shown in figure.




The effect of sewage discharge

image from NCERT

The following figure shows the effect of sewage discharge on some important characteristics of a river.

1.Reduce dissolved oxygen

  • Domestic sewage primarily contains biodegradable organic matter, which readily decomposes.
  • Microorganisms involved in biodegradation of organic matter in the receiving water body consume a lot of oxygen, and as a result there is a sharp decline in dissolved oxygen
  • This causes mortality of fish and other aquatic creatures.


image from NCERT

  • Is the presence of large amounts of nutrients in waters
  • It causes excessive growth of planktonic (free-floating) algae causing an algal bloom
  • Algal blooms cause deterioration of the water quality and fish mortality
  • Some bloom-forming algae are extremely toxic to human beings and animals

2.4 Marine pollution and degradation

The major marine pollution are

  1. Plastic debris
  2. Hazardous chemicals such as mercury, Lead etc
  3. Persistent Organic Pollutants [FAQ 4] like pesticides and industrial chemicals
  4. Oil spills

The effects of oil spills can be summarised as follows;

Short term effects Long term effects Remediation
  • Physical barrier to transfer of O2
  • Oil slick drenches birds feathers and kills them
  • Oil intake is poisonous to marine organisms
  • The hydrocarbons are passed through food chain causing biomagnification [FAQ 3]
  • Oil tars deposit on coasts
  • Removal using ships called skimmers
  • Dispersal using chemicals
  • Using absorbents like polystyrene
  • Bioremediation using mycobacterial strains which either emulsify or degrades the oil. eg: Oil Zapper

5. Ocean acidification

Ocean acidification[FAQ 5] is caused due to global warming. The excessive CO2 in atmosphere dissolves in ocean water and increase its acidity.

6. Algal blooms

An algal bloom is a rapid increase in the population of photosynthetic algae in an aquatic system. Because some algae produce toxins, they can be harmful to humans, mammals, birds and fish . As blooms grow, they deplete the oxygen in the water and block sunlight from reaching fish and plants.


2.5 Waste Water Treatment

Treatment of wastewater involves physical, chemical, and biological processes, which remove physical, chemical and biological matter that contaminates the wastewater. The different stages are

A.Primary Treatment

  1. Physical Separation – Wastewater is passed through bar screens. Large objects like rags, sticks, cans, plastic packets, napkins are removed
  2. Sedimentation – Water then goes to a grit and sand removal tank. The speed of the incoming wastewater is decreased to allow sand, grit and pebbles to settle down.
  3. Coagulation – Fine particles and colloidal suspension are combined into large particles by a process called coagulation. This step is carried out by the addition of special chemicals called coagulants (flocculants) such as potash alum.
  4. The water is then allowed to settle in a large tank which is sloped towards the middle. Solids like faeces settle at the bottom and are removed with a scraper. This is the sludge. A sludge skimmer removes the floatable solids like oil and grease. Water so cleared is called clarified water

B.Secondary Treatment (Biological treatment)

  1. The sludge is transferred to a separate tank where it is decomposed by the anaerobic bacteria. The biogas produced in the process can be used as fuel or can be used to produce electricity
  2. Aeration – Air is pumped into the clarified water to help aerobic bacteria to grow. Bacteria consume human waste, food waste, soaps and other unwanted matter still remaining in clarified water. This is called activated sludge process[FAQ 6].
  3. After several hours, the suspended microbes settle at the bottom of the tank as activated sludge. The water is then removed from the top.
  4. Dried sludge is used as manure, returning organic matter and nutrients to the soil.

B.Tertiary Treatment

  1. The water after the secondary treatment has a very low level of organic material and suspended matter. It can be discharged into a sea, a river or into the ground. Nature cleans it up further.
  2. Water is disinfected with chemicals like chlorine and ozone before releasing it into the distribution system. Chlorine is the most commonly used disinfectant used for killing bacteria. Other methods of disinfection are ultraviolet radiation, reverse osmosis etc.

4.6 National Legislations

  • Prevention and Control of Water Pollution Act,1974
  • The standards have been prescribed for water pollution under Environment (Protection) Act 1986. There are general and specific standards for water pollutants for discharge of effluents in water bodies on land (inland surface water, public sewers, irrigated land and coastal areas)
  • The state pollution control boards are empowered to grant/renew consent to new/existing water polluting industries. They have power to shut down any industrial unit which fails to meet the prescribed standards under the Prevention and Control of Water Pollution Act Act.
  • The state governments have also been authorized to take punitive measures against defaulting industries.

5. Land Pollution and degradation

Land pollution affects the natural balance in the soil and land which can lead to poor plant growth, loss of habitat, water pollution, soil erosion and desertification. The sources of land pollution are hazardous waste, sewage, industrial activities, deforestation etc.

5.1 Solid waste

Solid waste includes wastes from

  • Domestic sources : plastic bags, kitchen waste, glass bottles, paper etc
  • Industrial sources : chemical residue, fly ash, metallic waste etc
  • Agricultural residues
  • E-waste

Plastic Waste

Plastic items made from low density polyethylene (LDPE), is virtually indestructible, create colossal environmental hazard. Plastic is non biodegradable and burning of plastic in garbage dumps release highly toxic and poisonous gases like carbon monoxide, carbon dioxide, phosgene, dioxine and other poisonous chlorinated compounds.


E-Waste includes any item containing circuitry or electrical components with either power or battery supply. As global consumer demand continues to increase, the e-waste also continues to increase. Most electronics that are discarded contain some form of harmful materials such as beryllium, cadmium, mercury and lead.  Almost all electronic waste contains some form of recyclable material, including plastic, glass and metals. So proper recycling of e-waste can save valuable resources and prevent environment contamination.

5.2 Agro chemicals

  • Fertilizers and pesticides contaminate soil and water.
  • Nutrients like phosphate and nitrate causes eutrophication of water bodies
  • Causes Biomagnification
  • Biomagnification is the process of increasing concentration of a substance, such as a toxic chemical, in the tissues of organisms at successively higher levels in a food chain. The process by which the substance are getting accumulated in the tissues of organisms is called bioaccumulation.

    Image source : NCERT

5.3 Soil erosion and Degradation

  • Soil erosion is the removal of top soil by agents like water (running water, ground water, rain, sea waves) and wind
  • Removal of vegetative cover via over grazing or deforestation is the major cause of soil erosion
  • Soil erosion causes soil degradation, ie is decreased fertility and productivity of soil
  • Soil degradation causes desertification, that is, the land gradually becomes unfit for cultivation or habitation
  • Excessive use of chemical fertilisers and pesticides, causes soil acidification, increased salinity and alkalinity, reduced organic matter, and increased levels of organic pollutants and toxins and heavy metals (like Cadmium, Lead, etc.)
  • Water logging due to over irrigation causes salinity and alkalinity

6. Noise Pollution

Noise pollution is an excessive and displeasing environmental noise that disrupts the activity or balance of human or animal life.

6.1 Noise Limits

The Central Pollution Control Board has recommended noise standards for ambient air, for automobiles, domestic appliances and construction equipments, which are notified in Environment(Protection) Rules,1986 as given below

6.2 Health Impacts

  • Chronic exposure to noise may cause noise-induced hearing loss.
  • High and moderately-high noise levels can contribute to cardiovascular effects
  • Unwanted sound can damage physiological and psychological health.

6.3 Noise Reduction

The different methods are

1. Sound Control

Can be done through various methods at the source itself like,

    1. Maintenance of moving parts to reduce noise in operation
    2. Using Noise reduction techniques such as (source: www.hse.gov.uk)
      1. Damping
      2. Proper Fan installations
      3. Using Pneumatic exhausts 
      4. Appropriate Electric Motors etc

2. Sound Absorption

  1. Tree plantation to absorb noise
  2. Using sound absorbing materials

3. Sound proofing

  1. Sound proofing of buildings
  2. Buffer zone around industrial areas

7. Ozone Layer Depletion

Increase in chlorine and other halides containing compounds such as CFC(Chlorofluorocarbons) in the atmosphere has led to depletion of Ozone in the stratosphere(the second layer of atmosphere) and increase in the ozone in troposphere(lower atmosphere). 

  • CFCS are used in refrigerator, as propellants in aerosol sprays, and in the bubbles in many plastic foams.

As mentioned in this article, ozone in the lower layers of atmosphere is a pollutant. Whereas ozone in stratosphere is important because it acts as protection layer from the harmful UV rays.

  • A small amount of UV-radiation is necessary for well-being of human beings and other organisms, such as UV-B promote synthesis of vitamin-D.
  • UV-radiation also act as a germicide to control microorganisms.
  • However, increased UV dose is highly dangerous to living organisms, especially the UV-C radiations.
  • UV-A wavelength is between 320-400nm, UV-B wave length is lesser than 320 nm, and UV-C wavelength is lesser than 280 nm.

The ozone formation and depletion is a reversible process.

  • UV rays reacts with Oxygen molecules (photo chemical process) as shown below.

Image source: esrl.noaa.gov

  • In the reverse process, O3 absorbs a photon and is broken down into a diatomic oxygen molecule and an oxygen atom (occurs simultaneously to step 2 shown in the fig)

                                       Thus Rate of O3 formation = Rate of O3destruction.

It means ozone is continuously produced and destroyed in the stratosphere.

How the ozone hole forms?

  • The chlorine radicals acts as catalysts and accelerates the depletion process. 
  • Depletion of ozone creates an ‘Ozone hole’ in the stratosphere.
  • It means that the amount of ozone gets reduced and the layer cannot prevent the entry of UV radiation.
  • Ozone hole is detected over south pole during the early spring season and sometimes above the north pole.

Why ozone hole appear only in polar regions?

  • Cyclonic winds called polar vortex exits around the poles
  • These winds isolates air in the polar region
  • The extreme cold temperature inside the vortex creates frozen crystals and forms ‘polar stratospheric clouds(PSCs)’
  • These crystals provide a surface for the reactions that free chlorine from the compounds
  • When the sunlight returns to the South Pole in October, UV light rapidly breaks the bond between the two chlorine atoms, releasing free chlorine radicals
  • Free chlorine takes part in reactions that depletes the ozone faster
  • The ozone hole grows throughout the early spring until temperatures warm and the polar vortex weakens

CFCs are everywhere in the atmosphere and ozone depletion due to CFCs occurs worldwide, but in the Antarctica, because of PSCs, ozone depletion is severe enough to create an ‘ozone hole’.


1. Particulate Matter(PM)

PM is the sum of all solid and liquid particles suspended in air.Particulate matter size ranges from 0.001 to 500 µm in diameter. Particles less than 10µm float and move freely with the air current. Particles which are more than 10µm in diameter settle down. Particles less than 0.02 µm form persistent aerosols.

Based on size, they are divided into two main groups:

  • PM 10 – They contain the larger particles with a size ranging from 2.5 to 10 µm
  • PM 2.5 – They contain the smaller ones with a size up to 2.5 µm

PM10 enter into lungs easily. PM2.5 is of specific concern because it contains a high proportion of various toxic metals and acids, and aerodynamically it can penetrate deeper into the respiratory tract. According to Central Pollution Control Board (CPCB), (PM 2.5) are responsible for causing the greatest harm to human health.

Image source: www.epa.gov

2. What is ‘Fly Ash’? What are its constituents? What are its applications?

Fly ash, also known as “pulverised fuel ash”, is a coal combustion product composed of fine particles that are driven out of the boiler with the flue gases. Ash that falls in the bottom of the boiler is called bottom ash.


  • Fly ash is a pozzolan, a substance containing aluminous and siliceous material that forms cement in the presence of water.


  • It can be mixed with lime and water as it forms a compound similar to Portland cement.
  • So fly ash cement can be used in concrete as a substitute to portland cement for certain applications like PCC pavements and bricks manufacturing.

3. Persistent Organic Pollutants (POPs)

They are organic chemical substances. They possess a particular combination of physical and chemical properties such that, once released into the environment, they

  • remain intact for exceptionally long periods of time (many years)
  • become widely distributed throughout the environment
  • accumulate in the fatty tissue of living organisms including humans (bioaccumulation)
  • are toxic to both humans and wildlife

What is bioaccumulation of POPs?

POPs are not soluble in water. But they are readily absorbed in fatty tissues. There their concentrations can become magnified by up to 70,000 times (biomagnification). Fish, predatory birds, mammals, and humans are high up the food chain and so absorb the greatest concentrations.

What are the harmful effects of POPs?

  • They can cause cancer, allergies and hypersensitivity
  • Cause damage to the central and peripheral nervous systems, reproductive disorders, and disruption of the immune system.
  • Some POPs are endocrine disrupters ,ie they alter the hormonal system

What are the POPs?

Initially, twelve POPs have been recognised(as per the Stockholm convention) that are placed in 3 categories

  • Pesticides: aldrin, chlordane, DDT, dieldrin, endrin, heptachlor, hexachlorobenzene, mirex, toxaphene
  • Industrial chemicals: hexachlorobenzene, polychlorinated biphenyls (PCBs)
  • By-products: hexachlorobenzene; polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/PCDF), and PCBs.

4. Ocean Acidification

  • Ocean acidification is called the “evil twin of global warming”.  
  • The carbon dioxide from the atmosphere dissolves into oceans. To achieve chemical equilibrium, some of it reacts with the water to form carbonic acid(H2CO3 ). Some of these extra carbonic acid molecules react with a water molecule to give a bicarbonate ion(HCO3 – – ) and a hydronium ion( H3O+), thus increasing ocean acidity (H+ ion concentration). 
  • The concentration of carbonate ions(CO3 – – ) decreases.The carbonate ions are essential for the calcification process to build shells. Thus the survival of calcifying organisms becomes difficult.
  • More CaCO3 gets dissolved(from the shells of marine organisms) to maintain the chemical equilibrium.
  • The dissolution of CaCO3 releases more carbon to the geochemical cycle thus accentuating the global warming .
  • Acidification would lead certain marine organisms to emit less of the sulphur compounds which affects formation of clouds.

5. Advantages of Activated Sludge Process

When large number of aerobic bacteria and other organism (activated sludge) is mixed with raw sewage containing sufficient O, the bacteria oxidizes the organic solids in an aerobic process. Also it promotes coagulation and flocculation and helps the dissolved solids to settle down.

Mechanical Aeration

The commonly used methods for aeration  are 

  1. Diffused air aeration
  2. Mechanical aeration
  3. Combine aerator

The advantages include,

  1. Low installation cost
  2. Good quality effluent
  3. Low land requirement
  4. The flow of wastewater is smooth
  5. Produces less odour

But the process can have some disadvantages like, 

  1. Not very flexible because the process cannot be adjusted easily to the sewage flow volume.
  2. Operational costs are high
  3. Sludge disposal is required on large scale
  4. This process cannot be used for certain industrial wastes
  5. Skilled supervision is required 

Hey, Don’t Ignore me!

Hi there.. We know, you just need single reading to grasp the concepts…Please prove us right


  1. AMBUJ KUMAR says:

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  3. SACHIN KUMAR says:

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  9. pankaj says:

    India follow bs 4 norms,, so why ans is B rather than C of question 10

  10. pankaj says:

    India use BS-4 norms ….. So why ans is B instead of C of question 10 , plz clear .

  11. Ankur says:

    Question 6 has wrong answer Both statements 1&2 should be correct.

  12. shovon says:

    Great article. As a student of Geography and Environmental science, it is very helpful to my environmental courses! thank you.

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