Air Pollution

SyllabusAir Pollution:Definition, Composition of Atmospheric Air,Classification and Sources of air pollutants. Effectsof air pollution on human, plant and material, Airpollution control methods, equipment and safety.

Air Pollution

• Definition:- Air pollution is define as

“Excessive concentration of foreign matter in air, whichadversely affects the well being of individuals or causes damageto the property”.

Composition of Air

Ambient Air Quality Standards

Pollutant Time Weighted Average

Concentration in Ambient Air

Industrial Area Residential, Rural and other Sensitive Area

Sulphur Dioxide (SO2) Annual 24 hours

80 µg/m3

120 µg/m360 µg/m3

80 µg/m315 µg/m3

30 µg/m3

Oxides of Nitrogen (NO2)

Annual 24 hours

80 µg/m3

120 µg/m360 µg/m3

80 µg/m315 µg/m3

30 µg/m3

Suspended Particulate Matter (SPM)

Annual24 hours

360 µg/m3

500 µg/m3140 µg/m3

200 µg/m370 µg/m3

100 µg/m3

Respirable ** Particulate Matter (RPM)

Annual

24 hours

120 µg/m3

150 µg/m3

60 µg/m3

100 µg/m350 µg/m3

75 µg/m3

Lead (pb) Annual 24 hours

1.0 µg/m3

1.5 µg/m30.75 µg/m3

1.00 µg/m30.50 µg/m3

0.75 µg/m3

Carbon Monoxide(CO)

8 hours 1 hour

5.0 µg/m3

10.0 µg/m32.0 µg/m3

4.0 µg/m31.0 µg/m3

2.0 µg/m3

* Ministry of Environment and Forests, Government of India notification** Particle size less than 10 µm

Major Sources of Air Pollution

Major Sources of Air Pollution

• Natural Sources:- The natural sources of air pollution are volcaniceruptions, releasing of poisonous gases like SO2 ( Sulfur dioxide),Hydrogen sulphide (H2S), Carbon Monoxide(CO), Forest fires,natural organic and inorganic decays, Pollen grains, wind blowndust, among them pollen grain are important because it causesallergic reactions in individuals.

Major Sources of Air Pollution

Anthropogenic Sources• Rapid Industrialization:- The Industries such as pulp and paper,

chemical, metallurgical plants, refineries are responsible for airpollution, the most common pollutants are CO2, SO2, CO, NO, H2Setc..

• Transportation:- Automobile exhausts release smoke. The smoke isa result of incomplete combustion of carbonaceous matter. Thecommon pollutants are CO(Carbon Monoxide), CO2, andHydrocarbons.

Rapid Industrialization Transportation

• Burning of Fossil fuels and fires:- The conventional sources ofenergy are wood, coal, and fossil fuels. The byproducts of fossilfuels is nothing but poisonous gases such as CO, CH4 (methane),SO2, and NOX.

• Deforestation:- The balance of O2 and CO2 is maintained in thenature by the vegetation. The deforestation by man for his ownneeds has disturbed the balance of CO2 and O2, Concentration.

Burning of Fossil fuels and fires Deforestation

• Increase in Population:- an increase in population leads to globalwarming and emission of green house gases. It also contributes tolosses in forest cover and loss of wild life.

• Agriculture activities:- Various pesticides and insecticides are usedfor agricultural purposes. Thus they cause air pollution.

• Radioactive fallout:- Nuclear reaction, nuclear weapon testingcontributes towards Nuclear pollution. Nuclear Pollutants are veryharmful to man, animals and vegetations.

Anthropogenic sources

Increase in Population Agriculture activities

Radioactive fallout

Classification of Air Pollutants

• Air pollutants are broadly classified as• (a) According to origin• (b) According to State of matter.

According to Origin

• Air Pollutants can be classified as• (a) Primary Pollutants• (b) Secondary Pollutants• Primary Pollutants:- Primary pollutants are those which are directly

emitted from identifiable source• These pollutants are emitted directly to the atmosphere.• e.g.

(a) Particulate matter(b) Oxides of Sulphur(c) Oxides of Nitrogen(d) Carbon Monoxide(e) Radioactive Compounds

• Secondary Air Pollutants:- Secondary air pollutants are thosewhich are produced in the air by the reaction of two or moreprimary pollutants.

• e.g.• Ozone• Formaldehyde• Photochemical smog• Peroxy acetyl nitrate (PAN)

According to State of Matter

Air pollutants according to state of matter are classified as(a) Particulate Matter(b) Gases and Vapours

Particulate matter

These may be liquid or solid. Theparticulate matter are identified asaggregates which are larger than0.002 µ but smaller than 500 µDust (1 To 10,000 µ) : Small solid

particles resulting from break up oflarge masses through processes suchas crushing, grinding or blasting.Smoke ( 0.5 To 1 µ) Consist of finely

divided solid particles produced byincomplete combustion of organicparticles such as coal, wood, ortobacco.

• Mist:- These are liquid particles formed by condensation of vapor• Fog (1 to 40 µ ):- High concentration of mist is called as fog. It is

dispersion of liquid particles in air.• Fumes ( 0.03 to 0.3 µ ):- Fine solid particles formed by the

condensation of vapors of solid material. The fumes are generallyemit from melting operations.

• Ash ( 1 to 1000 µ ):- fine, noncombustible particles are known asfly ash.

• Spray (10 to 1000 µ):- Liquid particles formed by automation

Particulate Matter

Mist Fog

Smoke Ash

Gases And Vapors

• Carbon Monoxide:- (CO) This is an odorless, tasteless and colorlessgas formed due to incomplete combustion of carbonaceous matter.The sources of carbon monoxide are motor vehicles, burningof fossil fuel and industrial processes.

• Oxides of Sulphur:- They Include 4 different gaseous compoundssuch as Sulfur monoxide ( SO), Sulfur dioxide ( SO2), Sulphurtrioxide ( SO3) and sulphur tetra oxide (SO4).

• It is a major air pollutant gas produced by the combustion of fuels likecoal. The main source of electricity production is by burning of fossil fuelsin India and the whole world. The sulphur content of the coal varies from1 to 4% and fortunately the Indian coal is low in sulphur content. SO isalso produced in the metallurgical operations.

Oxides of Nitrogen

Oxides of Nitrogen• Oxides of nitrogen are produced either in the production of

nitric acid or in the automobile exhausts and as the effluent ofpower plants.

• Out of the seven oxides of Nitrogen (N2O, NO, NO2, NO3, N2O)• only nitric oxide and nitrogen dioxide are classified as the

main pollutants.• All the oxides of nitrogen are collectively known as NOX.

Hydrocarbon

• Organic compounds containing only carbon and hydrogen areclassified as Hydrocarbons.

• They are of 2 types.• (a) Aliphatic hydrocarbon• (b) Aromatic hydrocarbon• Hydrocarbon present in the atmosphere in the atmosphere from

both natural and man made sources.• The major sources of hydrocarbon is burning of gasoline and

industrial processes.

Aliphatic hydrocarbon

Other Gases• Hydrogen sulphide: Hydrogen Suphide is an obnoxious (bad smelling) gas. It

is produced mainly by the anaerobic (in absence of air) decomposition oforganic matter. Other air polluting sulfur compounds are methyl mercaptan(CH3 SH) and dimethyle sulphide (CH3 S CH ) etc..

• Hydrogen fluoride: It is an important pollutant even in very lowconcentrations. It is produced in the manufacturing of phosphate fertilizers.

• Chlorine and hydrogen chloride: It is mixed in the air either from theleakages from water treatment plants or other industries where it is producedor used. Hydrogen chloride is also evolved in various industrial chemicalprocesses. The main effect of chlorine is respiratory irritation which may befatal.

• Ozone: It is a desirable gas in the upper layers of atmosphere as it absorbs theUV radiation of sunlight. But near the earth surface it is a poisonous gas. Itmakes poisonous chemicals by photochemical reactions.

Other Gases• Aldehydes: They are produced by the incomplete oxidation of motor fuels

and lubricating oil. They may also be formed because of photochemicalreactions. Formaldehydes are irritating to the eyes.

Other Gases

Hydrogen Sulphide Hydrogen fluoride

Hydrogen ChlorideOzone

Classification Based On Position

• Another way of classification of air pollutants is the source being stationary or mobile.

• This way they can be classified as:-• (i) Point source (large stationary source as power plants)• (ii) Area source (small stationary sources like residential heating)• (iii) Mobile source (line source like highway vehicles or area

source like Aircraft at airports)

Point Source

Area Source

Mobile Source

Air Pollutants And Their Major Sources.

Effects of Some Common Air Pollutants

Particulate Matter• Effects on human:-• The pollutants of size less than 1 µ causes lung damage• Asbestos fibers causes cancer• Lead from automobile exhaust affects brain• It causes several respiratory diseases• Silica , arsenic, dust causes cancer

Effects of Air Pollution on Humans

Effects of Air Pollution on Humans

Effects on Plants• Deposition of particulate matter containing toxic metals affects the

growth of plants• The particulate matter after deposition on plant leaves block

stomata opening of plants and reduce plant growth.• The particulate matter with acid rain, reduces pH of the soil which

makes the infertile.

Effects on Plants

• Effect on Material• The particulate matter causes damage to building• The corrosion is enhanced• It causes cracks and fading of pointed surface• Particulates accumulate on the soil and reduces fertility of soil.

Effect on Material

Harmful Impacts of Air Pollution on Plants

Harmful Impacts of Air Pollution on Human beings

Harmful Impacts of Air Pollution on Human beings

Harmful Effects of Air Pollutants on Human Health

Harmful Effect of Air Pollution on Material

Control of Air Pollution• Air pollution cannot be fully abated but can be

controlled if certain measures are taken.• Controlling measures for air pollution can be mainly

divided into• Preventive measures• Control measures• i.e. Control of gaseous/ particulate pollutants• From industry/ automobiles using equipments

Preventive Measures• These measures are mainly aimed at correction right at

the source so there will be lesser amount of pollutantemitted from the industry/ automobiles.

• Some of the important preventive measures are• Using low sulphur coal in power plants• Using natural gas in place of coal for power generation.• Using LPG/ CNG instead of diesel or petrol in

automobiles.

Preventive Measures• Modifications in industrial processes and/ or

equipments to reduce the emissions• Section of suitable site and zoning for industrial unit:

Zoning means setting aside separate areas forindustries so that they are far from residential areas.This will help in proper dispersion of pollutantsresulting in lower concentration of pollutants in theair of residential area.

• By using proper environment impact studies beforesetting the industry.

Control Measures• The most common method of eliminating air pollution at source

some measures are to be adopted to control the level of pollution• The most common method of eliminating or reducing pollutants

to an acceptable level includes:• Collecting the pollutants by using equipments to prevent its

escape into atmosphere.• Destroying the pollutants by thermal or catalytic combustion• Changing the pollution to less toxic form.• By releasing the pollutants through tall chimneys for greater

dispersion. Tall chimneys will liberate the pollutants beyondthe inversion layer there by reducing the concentration ofpollutants near the source. But the liberated pollutants willsettle somewhere and cause of the air pollutants even far awayfrom the source.

Control Of Air Pollution• Prevention is always better than cure. So one should try to prevent the air

pollution by changing the raw material, the process conditions, procedureand by having the preventive maintenance one can prevent the airpollution. For this purpose some of the examples are as follows:-

• (i) Controlling the spread of coal dust by sprinkling water on it beforehandling in a thermal power plant.

• (ii) Preventive maintenance by repairing leaky valves in advance so as toprevent the leakages of the harmful gases in air.

• (iii) Applying zoning to distribute the impact of air pollution in acommunity.

• (iv) Selection of proper material. For example using low sulfur coalreduces the problem.

Control Of Air Pollution• In addition to the preventive measures one has to have

control technologies as follows.• Depending upon the situation different control technologies

are adopted. For dealing with the particulate matter thefollowing control technologies are used,

• Settling chambers• Cyclone separators• Fabric filters• Electrostatic precipitators• Wet collector (scrubbers)• The first two of the above are used for coarse particulates

where as the other three are used for fine particulates.

Air pollution Control• The atmosphere like a river, do possesses self-cleansing

properties, which continuously clean and remove thepollutants from the atmosphere under natural processes. Solong as the pollutants discharged by man into theenvironment, is lower than the natural cleansing capacity ofthe environment, we live happily without any air pollution.But as when the discharged pollutants exceed the naturalcleansing capacity, our environment becomes polluted,Attempts are then made to reduce the emission of pollutantsfrom the automobiles of factories by adopting mechanicalmeans, or by using high rise chimneys for better dispersionand dilution of pollutants over a longer range ofenvironment. The natural self cleaning processes of theenvironment, and the engineering measures adopted toartificially clean the industrial and vehicular gases, beforethey are emitted into the environment, are discussed in thefollowing articles.

The Natural Self-Cleansing Properties of the Environment

• The Various natural properties, which continuouslyclean the environment, automatically, are:

• Dispersion• Gravitational Settling with or without flocculation of

particles• Absorption including washout and scavenging;• Rainout and• Adsorption• All these processes, naturally occur periodically in the

ambient air, thereby removing or reducing the variouspollutants entering the air. These processes are brieflydiscussed below:

Dispersion• Dispersion of pollutants by winds reduces the

concentration of air pollutants at one place,although in the strict sense, it does not remove themfrom the environment as a whole. This mechanism istherefore a diluting mechanism only.

• The dispersion through long chimneys, dilutes theair pollutants near their source only, and does notreduce their long term undesirable effect on thecommunity as a whole.

Dispersion

Gravitational Settling• Gravitational setting is the most important natural

mechanism, under which large heavy particles fromambient air settles down on buildings, trees andother objects. This generally happens for theparticles which are larger than 50 µm in size.

• This process also helps in removing flocculatedparticles formed by uniting of smaller particles overlarger particles, till a floc particle, large and heavyenough to settle out under gravity, is formed.

Gravitational Settling

Absorption• In the natural absorption process, the gaseous as well

as particulate pollutants from the air get collected inthe rain or mist, and may settle out with that moisture.This phenomenon takes place below the cloud level,when falling raindrops absorb pollutants, and is alsoknown as washouts or scavenging. The process,however, does not help in removing particles smallerthan 1 µm in size.

• The gaseous pollutants are removed in dissolved statewith moisture, either with or without chemicalchanges.

Absorption

Rainout• Rainout is the process involving precipitation above

the cloud level, where submicron particles presentin the atmosphere in the cloud, serve ascondensation nuclei. Around which drops of watermay form, and fall out as raindrop. Thisphenomenon helps in increased rainfall and fogformation in urban areas, containing huge quantityof such particles, raising high above the cloud level.

Adsorption• Adsorption is the phenomenon in which the gaseous,

liquid or solid pollutants present in the ambient air arekept attached, generally electrostatically, by a surface,where they are concentrated and retained. Naturalsurfaces, such as soils, rocks, leaves, blades of grass,buildings, and other objects can adsorb and retainpollutants. The particles may come in contact with suchsurfaces either by gravitational setting or by inertialimpaction, under which the pollutants are carried tosuch surfaces by winds. Impaction is particularlyeffective for particles of size 10 to 15 µm.

Dilution Method for Controlling Air Pollution from Stationary Sources

• As Stated in the earlier paragraph, the emitted smokes can bespread over a large area, through the use of high risechimneys.

• Thereby transporting the pollutants over larger distances, andthus, reducing the pollution near the emission source.

• This method is largely adopted in developing countries,because here the pollution is generally confined over smallerenvironment near cities and industrial towns only. Theneighbouring environment, which is free from emission, isthus made to share some of the pollutants burden, therebycausing somewhat equitable distribution of the pollutants.Nevertheless, this method only reduces the concentration ofpollutants at particular place (s), rather than reducing orremoving the pollution load from the total environment, as awhole.

Controlling Air Pollution from Stationary Sources by Installing Engineering Devices

• In order to reduce the pollution load entering theenvironment from stationary sources, severalmeasures may be taken; out of them,replacement of burning fuel by electricity orsolar energy is by far, the best method, as it willeliminate the vary production of pollutants in thecombustion process. Besides this we can makeuse of better quality of fuels and efficientengines, for reducing pollution loads fromemissions. Say for example, LPG and LNG may beused in industries in place of coal, as they willproduce much less pollutants in emission.

Controlling Air Pollution from Stationary Sources by Installing Engineering Devices

• Replacement of old obsolete processes in industrieswith the new efficient processes may also lead toreduce pollution emissions. Besides such innovationsand precautions, certain mechanical devices may beinstalled in the industrial processes, which may helpin reducing the emission of pollutants.

• Such mechanical devices, are generally divided intotwo categories; i.e.

• (i) those devices which help in reducing particulatematter; and

• (ii) those devices which help in reducing gaseouspollutants.

Control of Particulate Matter in Industry

• The important devices, which are used to controlparticulate matter are;

• (a) Gravitational Settling chambers• (b) Centrifugal collectors including cyclone

collectors and dynamic precipitators.• (c) Wet Scrubber including spray towers, wet

cyclone scrubbers and venturi-scrubbers.• (d) Electronic Precipitators;• (e) Fabric Filters.

Gravitational Settling Chamber• It is used to remove particles with size greater than 50µ m. Velocity

of the flue gas is reduced in large chamber thereby inducingsettling the particles under gravitational force. Settling is governedby stokes law.

• The usual velocity of the flue gas through settling chambers isbetween 0.5 to 2.5 m/s.

• The largest sized particles (d) that can be removed with 100 %efficiency in such chamber of length L and height H is given byequation.

Gravitational Settling Chamber• The above equation applies to quiescent condition

but quiescent condition cannot be maintained in aflow through settling chamber, so a correction factoris applied and eqn becomes.

Gravitational Settling Chamber

Gravitational Settling Chamber

Advantages of Settling Chamber• Low Initial Cost.• Simple to design• Low pressure drop• Low Maintenance cost• Dry and continuous disposal of solid particulates.

Disadvantages of Settling Chamber• The collection efficiency can be increased by

providing the baffles and horizontal baffles/ trayin the chamber to reduce the settling path.

• Settling chambers are used widely for removal oflarge solid particulates from natural draftfurnace, kiln, etc.. They are also, sometimes usedin the process industries, particularly the foodand metallurgical industries, as first step in dustcontrol.

Cyclone Separator• Centrifugal force is utilized to separate the particulate matter from the

gas. As Centrifugal force is much greater than gravitational force smallerparticles can be removed. (10 to 50 µm)

• A cyclone is a specially designed closed chamber, in which the velocity ofthe inlet gas is transformed into spinning vortex, and the particles fromthe gas are thrown out under the centrifugal force. The particles thrownout on the walls of the chamber, slides down to the hopper and thusremoved. Its efficiency depends on the centrifugal force generated, whichin turn, depends on the mass of a particular(Mp) , inlet gas velocity Vi andradius of cyclone (R) and is given by the eqn

• Its collection efficiency is in between 50 to 90 %

Cyclone Separator

Cyclone Separator

Cyclone Separator

Advantages of Cyclone Separators• Low Initial Cost• Requires less floor area• Simple construction and maintenance• Low to moderate pressure loss (2.5 to 20 cm)• Can handle large volume of gas at high temperature.• Dry continuous disposal of collected dust.

Dis-advantages of Cyclone Separators• Requires large head room• Less efficiency for smaller particles (< 10 µ)• It is subjected to severe abrasive deterioration.• Quite Sensitive to variable dust load and flow rates.• Cyclones are widely used in industries producing

larger quantities of gas containing larger sizedparticles like, cement and fertilizer industries,petroleum refineries, grain mills and textile industries.

• Cyclone can be arranged in series and in parallel toincrease the efficiency of collection compare to asingle large sized cyclone.

Electrostatic Precipitators• It works on the principle of electrical charging of

the particulate matter(-ve Charge) and collecting iton a differently charged (+ve charged) collectingsurface. It has a very high efficiency of about 99 %and can remove particles in the size range of 0.1 µmto 1 µm efficiently.

Electrostatic Precipitators

Electrostatic Precipitators

Advantages of Electrostatic Precipitators

• High Collection Efficiency• Particles as small as 0.1 µm can be removed• Particles may be collected dry or wet.• They can be operated at high temperature up to

300- 450 0C• Maintenance is nominal, unless corrosive and

adhesive materials are present in the flue gases.• Few moving parts.• Treatment time is negligible (0.1 – 10 S).

Disadvantages of Electrostatic Precipitators

• High Initial Cost.• They use high voltage, and hence may pose risk to

personal safety of the staff.• Possible explosion hazards during collection of

combustible gases or particulates.• Collection efficiency reduces with time.• Space requirement is more because of the large size

equipments.• They are widely used in thermal power plants, cement

factories, paper and pulp industries, mining andmetallurgical industries, iron and steel industries,chemical industries etc.

Fabric Filters• In this device, fabric filter’s out the particulate matter from

the gas stream and allow clear gas to flow. In such a system,the flue gas is allowed to pass through a woven or feltedfabric, which filters out the particulate matter and allows thegas to pass. Small particles are retained on the fabric, initiallythrough interception and electro-static attraction; and lateron, when a dust mat is formed, the fabric starts collectingparticles more efficiently it can remove particles up to 1 µm.Its efficiency is around 99 %.

Fabric Filters• A bag house filter provided in an ordinary room

of the factory, contains several vertically hangingfabric cylindrical bags (1.8 to 9m long) ; theupper ends of the bags are closed, and lowerends are attached to a hopper, where the inlet ofthe flue gas is also located. The upwards movinggas drops out particulate matter in these bags,which settles down in to the hopper, and cleanergas goes out through the fabric filters. Theframework, housing the hanging bags, isprovided with a automatic shaking device forremoving the collected dust.

Fabric Filters

Advantages of Fabric Filters• High collection efficiencies for all particle sizes,

especially for particles smaller than 10 µm indiameter.

• Performance decrease becomes visible, givingprewarning.

• Nominal power consumption• Dry disposal of collected particles.

Dis-advantages of Fabric Filters• High temperature gases need to be cooled to the

range within which filter are stable.• High maintenance and fabric replacement cost.• Large size equipments.• The fabric is liable to chemical attack.

Fabric Filters

Scrubbers• In scrubbers the particulate matter are incorporated into

liquid droplets and thus are removed from the gas stream.Different types of Scrubbers includes.

• (a) Spray towers• (b) Venturi Scrubbers• (c) Cyclone Scrubber• In these devices the flue gas is made to push up against a

down falling water current. The particulate matter mixes upwith water droplets and thus, falls down and get removed.

• Water solution, when mixed with aqueous chemicalsolutions, like lime, potassium carbonate, slurry of MnO andMgO, etc., do help in removing gaseous pollutants also fromthe flue gases.

Cyclone Scrubber

Spray Towers

Venturi Scrubbers

Advantages of Scrubbers• They can simultaneously remove particulate and

gaseous pollutants.• Hot gases can be cooled down• There is no particles re-entrainment• Corrosive gases can be recovered and neutralize.• The separated gases through contact with aqueous

chemicals may produce useful byproducts, aschemicals and fertilizers.

Disadvantages of Scrubbers• A lot of wastewater needing disposal may be

produced.• Wet outlet gases cannot rise high from the stack.• Poses freezing problems in cold countries.• Maintenance cost is high, when corrosive materials

are collected.

Control of Gaseous Pollutants• For gaseous pollutants the following control

technologies are used.• Absorption• Adsorption• Combustion• For the control of gases such as NO2 and SO

produced in combustion, wet and dry scrubbers areused.

Control of Gaseous Pollutants• The gaseous pollutants can be controlled through the

techniques of combustion, absorption and adsorption.• (a) In combustion process oxidizable gaseous pollutants are

completely burnt at a high temperature. This processes isused to control gaseous pollutants in petro-chemical,fertilizers, paint and varnish industry.

• (b) In absorption techniques effluent gases are passedthrough absorbers containing liquid absorbents that remove,treat or modify one or more gaseous pollutants. Liquidadsorbent may utilize either chemical or physical change toremove pollutants.

• In Adsorption, the air pollutants are adsorbed on a solidsurface. Commonly used adsorbents are activated carbon,activated alumina, silica gel etc..

Control of Gaseous PollutantsCombustion or Incineration• Combustion or Incineration Equipments may be

used to purify polluted gases, when the pollutants inthe gas stream are oxidisable to an inert gas.Pollutants, like hydrocarbons and carbon monoxide(CO) can be easily burnt, oxidized, and removed insuch equipments.

• Both Direct flame combustion by afterburners andcatalytic combustion have been used in commercialapplications.

Control of Gaseous PollutantsAbsorption Units• The absorption units work on the principle of transfer of the

pollutants from the gas phase to the liquid phase. In other words,the pollutants from the dirty gas, get absorbed in the liquid,through which the gas is made to pass, in these units. Suchabsorption takes place by diffusion as well as dissolution.

• The effectiveness of these devices, naturally depends upon thesolvent, through which the gas is made to pass.

• Absorption Units Like Spray towers, plate towers, packed towers,and venturi scrubbers. Out of these devices, spray towers andventuri-scrubbers can simultaneously be used for removingparticulate pollutants. Scrubbers are generally less effective inremoving gaseous pollutants than towers, but then, towers getfrequently clogged by particulate matter.

• Other effective devices for removing gaseous pollutants, i.e. platetowers and packed towers.

Control of Gaseous PollutantsAdsorption Units• Using adsorbents like activated carbon, molecular sieves, activated

alumina, silica gel, etc..• In adsorption units, the dirty gas are made to pass through the beds

of such adsorbent materials, wherein the pollutants are effectivelycaught and removed. Activated carbon beds can very effectivelycatch hydrocarbons, H2S and SO 2

• The above adsorbents, except activated carbon, have the drawbackof preferably capturing water, before catching any of the gaseouspollutants; and hence have to be removed from the gas, before it istreated in such units.

• However, almost all the adsorbents are subjected to destruction atmoderately high temperature (150 0 C to 600 0 C) Hence theyprove to be very inefficient for purifying industrial gases at suchhigh temperature.

Control of Automobile ExhaustAutomobile Exhaust can be controlled by:• (a) The un-burnt hydrocarbon in automobile emission can be

reduced by use of efficient engines (MPFI, CRDI, TDItechnologies).

• (b) Catalytic converter can convert NO to Nitrogen gas andreduce potential hazards.

• By enforcing stringent emission standards for the vehicles likeBharat Stage 4 or Euro 4 right at the manufacturing stage.

• Using lead free petrol.• By using cleaner fuel like CNG, hybrid car, electric car, etc..• By proper maintenance of the vehicles• Developing good mass transportation facilities.

Control of Automobile Exhaust

Control of Automobile Exhaust

Notations

• parts-per-million (ppm) 10–6, parts-per-billion (ppb) 10–9, and parts-per-trillion (ppt) 10–12 level

Case Study: Air Pollution EpisodeBhopal Gas Tragedy

• The Story of Bhopal Gas Tragedy: The Lucky Ones Died ThatNight

• It was five minutes midnight in Bhopal on December 2 1984.The congested city was asleep and the winter air was heavy.Many had gone sleep. It was all quite, but the city was to changeforever.

• Suddenly, 27 tons of lethal gases including methyl Isocyanate(MIC) started leaking from Union Carbide’s Pesticide factory.The cloud of gases rapidly blanked the city.

• Grossly under designed safety systems were eithermalfunctioning, under repair, or had been switched off as partof a cost cutting exercise. The warning siren at the factory hadreportedly been turned off. It was the world’s worst IndustrialDisaster.

Case Study: Air Pollution EpisodeBhopal Gas Tragedy

• Here is a survivor’s account of what happened onthat ill fated night:

• “ The poison cloud was so dense and searing that thepeople were reduced to near blindness. As theygasped for breath its effects grew more suffocating.The gases burned the tissues of their eyes and lungs,and attacked their nervous systems. People lostcontrol of their bodies Those who escaped with theirlives are the unlucky ones; the lucky ones are thosewho died that night.”

Case Study: Air Pollution EpisodeBhopal Gas Tragedy

• 70,000 people were evacuated from the area afterthe accident and 2,00,000 more fled in panic. Theeffect of the gas was felt up to 100 sq km from thefactory. More than 8000 people died in the few daysfollowing the disaster. Within few months death tollraised to 20000 due to exposure related illness.

• The gases affected more than 5,00,000 people,leaving them with the life time of ill health andtrauma.

Bhopal Gas Tragedy

How did the accident occur?

• How did the accident occur?• Most probably water entered the storage tank and

caused a runway chemical reaction that led to anincrease in temperature, which converted the liquidMIC into gas. Investigations reveled that there hadbeen six early accidents in the plant and thatworkers had complained off exposure to dangeroussubstances. Yet proper safety mechanisms wereallegedly not put in place.

Bhopal Gas Plant

Bhopal Gas Plant

Case Study: Air Pollution EpisodeBhopal Gas Tragedy

• Union Carbide accepted only moral responsibility for thedisaster and not any liability. The Government of Indiafiled a case against the company for US $ 3 billion, butstrangely accepted US $470 million as settlement in1989. Nearly 95 % of the survivors had received just Rs25000 for lifelong injury and loss of livelihood. Thatworks out to be 9 US cents a day for more than 20 yearsof Unimaginable suffering.

• A criminal case was filed in the court of Bhopal againstUnion Carbide, and its then chairmen Warren Andersonfor negligence,. They did not attend the courtproceedings and Anderson has been proclaimed afugitive from justice by the court.

Bhopal Gas Tragedy

What does the story of the Bhopal Tragedy Tell us ?

• The tragedy shows that poor communities aredisproportionately affected by toxic materialsdischarged into air, land and water. When a crisisoccurs, or an accident occurs these people cannotget justice from the polluters, or the governments.

Source: Basics of Environmental Studies By R. Rajgopalan

Exam Questions

• What is Air Pollution? Explain in detail its effects on propertyand materials.

• Explain primary and secondary air pollutions. State the effectsof air pollution on plant and material.

• Explain the effect of air pollution on men, material andanimals.

• Describe different primary and secondary air pollutants.• Write a S.N. on Electrostatic precipitator.• Describe the devices used for control of particulate matter.

References

Sewage Disposal and Air Pollution Engineering : By Prof S.K. GargKhanna Publishers

Environmental Engineering : By Prof B.R.Shah Prof A M Malek

Mahajan Publishing House

Internet Websites