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Unit II

Pollution engineering:

Types of pollution,Air pollution:

sources,

effects,technology to combat air pollution and

air quality standards.

Technology to combat soil pollution: bioremediation,

organic farming. 02 Hrs

Pollution

Definition: Any andesirable change in any of the property (physical, chemical, or biological

characteristics) of any of the components of environment which leads to instability, disorder, harm

or discomfort to the ecosystem is called pollution. The contaminat whcich brings about suchchanges is called polluatant.

Although there are some natural pollutants such as volcanoes, pollution generally occurs because

of human activity.  Biodegradable pollutants, like sewage cause no permanent damage if they are

adequately dispersed, but non-biodegradable pollutants, such as lead, may be concentrated as theymove up the food chain. Air pollution, associated with basic industries such as oil refining,

chemicals, and iron and steel, as well as with the internal combustion engine, is probably the

 principal offender, followed by water and land pollution. Present-day problems of pollutioninclude acid rain and the burning of fossil fuels to produce excessive carbon dioxide.

Different types of pollution based on the part of environment which is polluted:

1. Air Pollution/atmospheric pollution

2. Noise or sound pollution3. Water pollution/Hydrospheric pollution

4. Soil pollution/Lithospheric pollution

5. Radioactive pollution

6. Visual pollution

7. Thermal pollution

Air pollution: The air or atmosphere is said to be polluted if the concentration of any of the

constituents increases or decreases drastically to cause harmful effect on human health or hisenvironment.

Noise or sound pollution: Noise excessive enough to cause psychological or physical damage is

considered noise pollution.

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Water pollution: Addition of undesirable substances to water that make it harmful to man, animal

and aquatic life causing significant changes in the normal activity is referred to as water pollution.

Soil pollution: Natural and synthetic materials that can adversely affect the physical, chemical and biological properties of soil thereby affecting the productivity is called as soil pollution.

Radioactive pollution: Pollution caused   by the radioactive  substances  mainly in water  is

radioactive pollution.

Visual pollution: Thus litter, billboards and auto junkyards are said to constitute visual pollution.

Thermal pollution: Waste heat that alters local climate or affects fish populations in rivers isdesignated thermal pollution.

Air pollution

Definition: The presence of natural and artificial substances in the atmospheric environment thataffect human health or well-being, or the well-being of any other specific organism.

Pragmatically, air pollution also applies to situations where contaminants impact structures

and artifacts or  esthetic sensibilities (such as visibility or smell). Most artificial impurities are

injected into the atmosphere at or near the Earth's surface. The lower atmosphere (troposphere)cleanses itself of some of these pollutants in a few hours or days as the larger particles settle to the

surface and soluble gases and particles encounter  precipitation or are removed through contact with

surface objects. Unfortunately, removal of some pollutants (eg.,  sulfates and nitrates) by precipitation and dry deposition results in acid deposition, which may cause serious environmental

damage. Also, mixing of the pollutants into the upper atmosphere may dilute the concentrations

near the Earth's surface, but can cause long-term changes in the chemistry of the upper atmosphere,including the ozone layer . 

Sources of air pollution

I) Sources may be classified in a number of ways. Firstly,

1.  Natural sources. Eg. Volcanic eruptions, Forest fires.

2. Anthropogenic/ Manmade sources: Eg. Industrial, Automobile and Fireworks exhausts.

II) Another frequent classification is in terms of 

1. Stationar  y sources (power plants, incinerators, industrial operations, and space heating)

2. Moving sources (motor vehicles, ships, aircraft, and rockets).

Types of air pollutants: 

1. Primary pollutants - products of natural events (like fires and volcanic eruptions) and human

activities added directly to the air  

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2. Secondary pollutants - formed by interaction of primary pollutants with each other or with

normal components of the air  

Major Classes of Air Pollutants: 

• Carbon oxides (CO & CO2) o sources = incomplete/complete combustion of fossil fuels 

transportation, industry, & home heating 

o CO2 is an important greenhouse gas, CO2 causes nausea and headache.

o CO (carbon monoxide)

the most abundant pollutant know to affect human health

combines with hemoglobin to form carboxyhaemoglobin & may create problems for infants, the elderly, & those with heart or respiratory diseases

• Sulfur oxides (Sulphur dioxide (SO2), Sulphur trioxide (SO3) & Hydrogen sulphide,

mainly SO2,) 

o source = combustion of coal & oil (esp. coal), metallurgical process and hydrogensulphide gas from natural sources.

o can react with gases in atmosphere to form sulfuric acid ('acid rain'). SO2 and SO3

react with water to form sulphurous acid (H2SO3) and sulphuric acid (H2SO4)respectively. These acids cause irritation of eyes, nose and throat; they also

adversely affect aquatic life. SO2 also damages leather because of the formation of 

H2SO4 on the leather surface which causes hydrolysis of leather proteins.

o High concentrations of SO2 cause chlorosis, plasmolysis, membrane damage,

reduced growth, yield and death.

o Exposure to SO2 can cause impairment of respiratory function, aggravation of 

existing respiratory disease (especially bronchitis), and a decrease in the ability of 

the lungs to clear foreign particles. It can also lead to increased mortality, especiallyif elevated levels of particulate matter (PM) are also present. Groups that appear most sensitive to the effects of SO2 include asthmatics and other individuals with

hyperactive airways, and individuals with chronic obstructive lung or 

cardiovascular disease. Elderly people and children are also likely to be more

sensitive to SO2.o They play an important role in the formation of secondary pollutants called

 photochemical smog (originally called as London smog). When they combine with

another air pollutant Hydrocarbons, they react with one another in the presence of sunlight and form a mixture of compounds including which appears as a blackish

haze called photochemical smog.

(smoke+fog=smog) (smog formed under the influence of sunlight is photochemical smog)

To form, it requires SOx, Hydrocarbons, water vapours, sunlight &

atmospheric inversion It is oxidative

• Nitrogen oxides - NO (nitric oxide) & NO2 (nitrogen dioxide) are major pollutants.

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o source = motor vehicles, industry (burning fossil fuels) and volcanic eruption.

 Nitrogen dioxide is also formed from the atmospheric nitrogen by electrical energy

of lighting and by natural ionizing radiation. The oxides of Nitrogen (NO 2) formabout 10% of air pollutants.

o can react with other gases in atmosphere to from nitric acid (HNO3) ('acid rain')

These acids cause irritation of eyes, nose and throat; they also adversely affectaquatic life.

o  Nitrogen monoxide like Carbon monoxide lowers the oxygen carrying capacity of 

the blood.o Being a strong reducing agent, NO2 react with metals, textiles and other 

materials.

o  Nitrogen oxides damage plants causing defoliation and necrosis.

o  Nitrogen oxides play an important role in the formation of secondary pollutants

called photochemical smog (originally called as Los Angeles smog). When

 Nitrogen oxides combine with another air pollutant Hydrocarbons, they react withone another in the presence of sunlight and forms a compound called PAN (Peroxy

Acetyl Nitrate) and ozone which appears as a yellowish brown haze calledphotochemical smog. PAN cause eye irritation, soar throat, respiratory irritationand so on. After about 12 hours of exposure cardiopulmonary (heart) activities are

impaired.

(smoke+fog=smog)

(smog formed under the influence of sunlight is photochemical smog)

To form, it requires Nox, Hydrocarbons, water vapours, sunlight &

atmospheric inversion

It is reductive

• Volatile organic compounds (hydrocarbons) - methane, benzene, propane, &

chlorofluorocarbons (CFC's)o source = motor vehicles (evaporation from gas tanks), industry, & various

household products 

o Concentrations of many VOCs are consistently higher in indoors than outdoors.

o Eye and respiratory tract irritation, headaches, dizziness, visual disorders, and

memory impairment are among the immediate symptoms that some people have

experienced soon after exposure to some organics. At present, not much is known

about what health effects occur from the levels of organics usually found in homes.

Many organic compounds are known to cause cancer in animals; some aresuspected of causing, or are known to cause, cancer in humans.

o They are also part of photochemical smog.

o They cause blood disorder like Methemoglobinemia

• Suspended particulate matter 

o solid particles (e.g., dust, soot, & asbestos) & liquid droplets (e.g., pesticides)

o sources = power plants, iron/steel mills, land clearing, highway construction,

mining, & other activities that disturb or disrupt the earth's surface

o act as respiratory irritants; some are known carcinogens (e.g., asbestos)

o can aggravate heart/respiratory diseases

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• Toxic compounds 

o trace amounts of at least 600 toxic substances (such as lead and mercury) produced

 by human activities sources of mercury = burning coal and waste (such as medical wastes)

Mercury is an element that occurs naturally in the earth’s crust. Most people

and wildlife can generally tolerate the extremely low levels of this naturallyoccuring substance. When mercury enters the body it becomes concentrated

in tissue, an effect known as bioaccumulation. Because this element is toxic

at very low concentrations, even slight increases in the minuteconcentrations naturally present in the environment can have serious effects

on humans and wildlife. Once mercury enters the water it can be converted

to its most toxic form, methyl mercury, by bacteria or chemical reactions.Methyl mercury is absorbed by tiny aquatic organisms, which are then eaten

 by small fish. The chemical is stored in the fish tissue and is passed on at

increasing concentrations to larger predator fish. People and wildlife at the

top of the food chain are consequently exposed to elevated amounts of 

methyl mercury through the contaminated fish they consume.

SUMMARY of Effects of Air Pollutants

The major concern with air pollution relates to its effects on humans. Since most people spendmost of their time indoors, there has been increased interest in air-pollution concentrations in

homes, workplaces, and shopping areas. Much of the early information on health effects came

from occupational health studies completed prior to the implementation of general air-qualitystandards.

Air pollution principally injures the respiratory system, and health effects can be studied through

three approaches, clinical, epidemiological, and toxicological. Clinical studies use human subjects

in controlled laboratory conditions, epidemiological studies assess human subjects (health records)in real-world conditions, and toxicological studies are conducted on animals or simple cellular 

systems. Of course, epidemiological studies are the most closely related to actual conditions, but

they are the most difficult to interpret because of the lack of control and the subsequent problemswith statistical analysis. Another difficulty arises because of differences in response among

different people. For example, elderly asthmatics are likely to be more strongly affected by sulfur  

dioxide than the teenage members of a hiking club.

Damage to vegetation by air pollution is of many kinds. Sulfur dioxide may damage field cropssuch as alfalfa  and trees such as pines, especially during the growing season. Both hydrogen

fluoride (HF) and nitrogen dioxide (NO2) in high concentrations have been shown to be harmful to

citrus trees and ornamental plants, which are of economic, importance in central Florida. Ozoneand ethylene are other contaminants that cause damage to certain kinds of vegetation.

Air pollution can affect the dynamics of the atmosphere through changes in longwave and

shortwave radiation processes. Particles can absorb or reflect incoming short-wave solar radiation,

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keeping it from the Earth's surface during the day. Greenhouse gases can absorb long-wave

radiation emitted by the Earth's surface and atmosphere.

Carbon dioxide, methane, fluorocarbons, nitrous oxides, ozone, and water vapor are importantgreenhouse gases. These represent a class of gases that selectively absorb long-wave radiation.

This effect warms the temperature of the Earth's atmosphere and surface higher than would befound in the absence of an atmosphere (the greenhouse effect). Because the amount of greenhouse

gases in the atmosphere is rising, there is a possibility that the temperature of the atmosphere willgradually rise, possibly resulting in a general warming of the global climate over a time period of 

several generations.

Researchers are also concerned with pollution of the stratosphere (10–50 km or 6–30 mi above theEarth's surface) by aircraft and by broad surface sources. The stratosphere is important, because it

contains the ozone layer, which absorbs part of the Sun's short-wave radiation and keeps it from

reaching the surface. If the ozone layer is significantly depleted, an increase in skin cancer in

humans is expected. Each 1% loss of ozone is estimated to increase the skin cancer rate 3–6%.

Visibility is reduced as concentrations of aerosols or particles increase. The particles do not just

affect visibility by themselves but also act as condensation nuclei for cloud or haze formation. In

each of the three serious air-pollution episodes discussed above, smog  (smoke and fog) were

 present with greatly reduced visibility.

Air pollution control measures:

There are 5 methods of controlling air pollution. They are

1. Proper planning of industrial area like zoning.2. Dilution of source discharge by use of tall stalks.

3. Using source correction methods4. Vegetation and

5. Use of controlling equipments

1. Proper planning of industrial area like zoning

Air pollution can be effectively controlled by adopting the zoning system at the planning stage

itself.

Cumulative zoning in the past has resulted in less availability of land for industries.

Permissible system: Modified system of Cumulative zoning, though this system has also resulted

in crowding of industrial zones with other uses besides industry.

Exclusive zoning system: which provides for compatible uses for each zone, excluding other uses.

In this system a separate zone or area is set aside for industries (known as industrial area or 

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industrial zone), thereby reducing the ill effects of air pollution on urban dwellers. By proper 

zoning, the planning of the city should be done such that residential areas and heavy industries are

not located too close to teach other. This is achieved by providing a green belt between the

industries and the township.

In India, zoning system varies from city to city. For example, in the industrial estate of Bangalore, only three zones are provided for light, medium and large industries. Bombay plan has

listed four zones, 1. Small repairing and light manufacturing units, permissible in residential areas,

2. Service industrial zone, 3. Special industrial zone and 4. General industrial zone.

2. Dilution of source discharge by use of tall stacks

The atmosphere, like natural stream, possesses self-cleansing properties, which continuously clean

and remove the pollutants from the atmosphere under natural conditions, provided the pollutants

are discharged in the atmosphere judiciously so that effective dispersion takes place. If the

 pollutants are carried away to some distance or taken to high altitudes, they are reduced in

concentration by diffusion and dilution. The pollutants are taken to high altitudes by means of tall

stacks. The height of the stack should be such that the maximum ground level concentration, which

varies inversely with the square of the stack height, is within the permissible limits. Tall stacks

 penetrate the inversion layer and disperse the contaminants easily so that ground level

concentration is less harmful. Dilution of pollutants in air depends on atmospheric temperature and

speed and direction of the wind.

Disadvantage: It is a short term contact measure, which in reality brings about highly undesirable

long range effects. This is so because dilution only dilutes the contaminants to levels at which their 

harmful effects are less noticeable near their original source whereas at a considerable distance

from the source, there contaminants eventually come down in some from or another.

3. Using source correction methods: This method is known as air pollution prevention at source.

This can be achieved through:

a. Change in raw materials b. Process changes and

c. Equipment or replacement.d. Maintenance of equipment

a. Change in raw material: If one type of the raw material, currently in use results in air pollution problem while a substitute material, which may be of purer grade, does not, the substitution will be

more desirable. The raw material in current use may contain certain ingredient which is not

essential for the process by which contributes to pollution, the non-essential ingredient should be

removed through prior processing so that pollution can be minimized. (Low sulphur fuel which has

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less pollution potential can be used as an alternative to high sulphur fuels.).Comparatively more

refined liquid petroleum gas (LPG) or liquefied natural gas (LNG) can be used instead of 

traditional high contaminant fuels such as coal.

b. Process modification: Atmospheric pollutants emissions can sometimes be reduced by

adopting modified or new processes.

1. If coal is washed before pulverized, then fly ash emissions are considerably reduced.

2. If air intake of boiler furnace is adjusted, then excess fly ash emissions at power plantscan be reduced.

3. Use of exhaust hoods and ducts over several types of industrial ovens have not only

reduced pollutants but also have resulted in the recovery of valuable solvents that couldhave become air pollutants.

4. Similarly volatile substances can be recovered by condensation and the non

condensable gases can be recycled for additional reactions.

c. Equipment modification or replacement: Old equipment, which contributes to greater degreeof air pollution, can be modified or completely replaced.

1. Smoke, carbon monoxide and fumes can be reduced if open hearth furnaces are

replaced with controlled basic oxygen furnaces or electric furnaces.

2. In petroleum refineries, loss of hydrocarbon vapours from storage tanks due toevaporation, temperature changes or displacement during filling, etc., can be reduced

 by designing the storage tanks with floating roof covers.

3. Pressuring the storage tanks in the above case can also give similar results.

d. Maintenance of equipment

An appreciable amount of pollution is caused due to poor maintenance of the equipment,which includes the leakage around ducts, pipes, valves and pumps etc. emissions of pollutants due

to negligence can be minimized by a routine check-up of the seals and gaskets.

4. Vegetation

Plants contribute towards controlling air pollution by utilizing carbon dioxide and releasingoxygen in the process of photosynthesis. This purifies the air removal of gaseous pollutants (CO2)

for the respiration of men and animals. Gaseous pollutants like carbon monoxide are fixed by some

 plants, namely Coleus blumeri, Ficus variegate and Phascolus vulgaris. Species of Pinus, Quercus,

Pyrus, Juniperus and Vitis depollute the air by metabolizing nitrogen oxides. Plenty of trees should be planted, especially around those areas which are declared as high risk areas of pollution.

5. Control at source by equipment

The most effective means of dealing with the problem of air pollution is to prevent emission at the

source itself. In the case of industrial pollutants, it is often achieved by investigating variousapproaches at an early stage of the process. Design, development and selecting of those methods

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should be emphasized which do not contribute to air pollution. Pollution control equipments are

generally classified into two types.

The particulate matter from gas stream can be removed at source using various types of control

equipment. Various methods are available, but to select the required equipment certain basic data

must be available. The required data are:

1. Quality of gas to be treated and its variation with time.2. Nature and concentration of matter to be removed.

3. Temperature and pressure of gas stream.

4. Nature of the gas phase.

Particulates Control:Gravity settling chambers

A typical horizontal flow gravity settling chamber is constructed in the form of a longhorizontal box with inlet, outlet, and dust collection hoppers. The dust-laden air stream enters the

unit at the inlet, then enters the expansion section, which causes the air velocity to be reduced and particles to settle by gravity. Gravitation force may be employed to remove particulatesin settling chambers when the settling velocity is greater than about 0.12 m/s. Thechambers are provided with enlarged areas to minimize horizontal velocities andallow time for the vertical velocity to carry the particle to the floor. The chamberusually operates with velocity between 0.5 and 2.8 m/s, although for best operatingresults the gas flow should be uniformly maintained at less than 0.3 m/s.

Control devices for particulate

contaminants

Gravitational settlingCyclone separatorsFabric filtersElectrostatic precipitatorsWet collectors (scrubbers)

Dry

absorption

Wet

absorption

Control devices for gaseous

contaminants

Cyclonic scrubberSpray towerVenturi

scrubber

Pollution control

equipments (PCE)

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Settling chamber design based on laminar flow requires either a very large sizeor large number of trays with an awkward shape of chamber. The laminar chamberhas the advantage of giving theoretically perfect collection efficiency for particles of the designated size but is of little practical value since the efficiency drops off rapidlywith smaller particles. The most practical flow in the settling chamber will probably beturbulent rather than laminar. Thus, turbulent chamber offers a more practical design

concept. The chamber should be reasonably designed for the removal of 99% of theparticles. Here an improved type of settling chamber is the baffle chamber beeing used; the

 baffles cause sudden changes of the direction of the air stream thus enhancing particle separation

and collection. The motion thus induced is superimposed on the motion due to gravity. Thus,

 particle collection is accomplished by a combination of gravity and an inertial effect. Particles as

small as 10 to 20 µm can be collected. The settling chamber with baffles is more compact and

requires less space then the simple gravity settling chamber. Settling chambers are the simplest and crudest PCE and are  best used upstream of 

more efficient collectors. They can then reduce the load, improve the performance and extend the

life of the more efficient and more expensive device, and the dust can be recovered more easily.

Settling chambers have been used in many industries, including metal refining, foodstuffs, and

 power plants.The advantages of settling chambers include: low cost of construction and maintenance;

few maintenance problems; relatively low pressure drops; temperature and pressure limitationsimposed only by the materials of construction used; dry disposal of solid particulates. The

disadvantages include large space requirements and relatively low overall collection efficiency.

 

Cyclone separators

The most widely used mechanical collector is the cyclone. Instead of gravitational force,

centrifugal force is utilized by cyclone separators to separate the particulate matter from the

 polluted gas. Centrifugal force several times greater than gravitational force can be generated by

spinning gas stream and this quality makes cyclone separators more effective in removing much

smaller particulates than can possibly be removed by gravitational settling chambers. A simple

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cyclone separator consists of a cylinder with a conical base. A tangential inlet discharging near the

top and an outlet for discharging the particulates is present at the base of the cone.

Mechanism of action: The mechanism depends on centrifugal force instead of gravity to separate particles from gas stream. The generated centrifugal force can be several times greater than the

gravitational force. Therefore, particles that can be removed in centrifugal collection are much

smaller than those that can be removed in gravity settling chambers. Reverse flow cyclone

consists of a cylindrical shell, a conical base dust hopper and an inlet where the dust laden gas

enters tangentially. The dust laden gas enters tangentially, receives a rotating motion and

generates a centrifugal force due to which the particulates are thrown to the cyclone walls as the

gas spirals upwards inside the cone (i.e., flow reverses to form an inner vortex which leaves flow

through the outlet). Particles slide down the walls of the cone and into the hopper. The operating

or separating efficiency of cyclone depends on the magnitude of the centrifugal force exerted on

the particles. The greater the centrifugal force, the greater the separating efficiency.

Like settling chambers, cyclones are frequently used as pre-treatment units to precede other,higher-efficiency air cleaning devices.

The advantages of settling chambers apply also to cyclones, and in addition cyclones can be usedover a broad range of pressures and temperatures, from below ambient to above 1000oC. Their 

 performance is insensitive to inlet dust concentration at the inlet; efficiency can increase with

increasing particle concentration; and they can be used effectively for the removal of liquiddroplets from gases, as in the discharge from absorption columns. The disadvantage is the low

efficiency for airborne particles finer than 5 µm.

Fabric filters

Filtration is one of the oldest and most widely used methods for separation of particulates from acarrier gas. A filter is a porous structure composed of a granular or fibrous material, which retains

the particulates and allows the gas to pass through the voids of the filter. Small particles are

initially retained on the fabric by direct interception, inertial impaction, diffusion and gravitational

settling. The filter is constructed of any material compatible with the carrier gas and particulates

and may be arranged in fabric, cloth filters or deep-bed filters.

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Fibrous and deep-bed filters have large void spaces amounting from 97 to 99 % of the total

volume. Fabric filters are made in the form of tubular bags or cloth envelopes and are suitable for a

dust loading of the order of 1 g/m3. They are capable of removing dust particles as small as 0.5

microns and will remove substantial quantities of particles of which are closed and lower ends are

attached to an inlet manifold. The hopper at the bottom serves as a collector for the dust. The gas

entering through the inlet pipe strikes a baffle plate, which causes the larger particles to fall into

the hopper due to gravity. The carrier gas then flows upward into the tubes and then outward

through the fabric, leaving the particulate matter as a cake on the inside of the bags. The filter 

efficiency during pre-coat formation is low but increases as the pre-coat formed, until a final

efficiency of over 99% is achieved. The pre-coat acts as a part of the filter medium, which further 

helps in the removal of particulates. Many such bags are hung in a baghouse. For efficient filtration

and a longer life, the filter bags must be cleaned occasionally by a mechanical shaker to prevent

too many particulate layers from building up on the inside surfaces of the bag.

The advantages of fabric filters include high efficiency for fine particles, ease of operation and

maintenance, dry disposal of solid particles. The disadvantages include relatively high installationand operating costs, limitations for use in high temperatures and in handling sticky materials.

Electrostatic precipitators (ESPs)

Electrostatic precipitators are the most popular methods for efficient removal of fine solids and

liquids from gas streams: they can have collecting efficiencies of over 99%. A high potentialelectric field is established between discharge and collecting electrodes of opposite polarity. The

discharge electrode is of small cross sectional area, such as a wire or piece of flat stock, and the

collection electrode is large in surface area, such as a plate. The dust-laden gas to be cleaned passes

through the field. At a critical voltage, the gas molecules are ionized at or near the surface of thedischarge electrode. Ions of the same polarity as the discharge electrode attach themselves to

neutral dust particles, which are then attracted to the collecting plate. On contact with the

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collecting surface, dust particles lose their charge and can then be easily removed by vibration,

washing or by gravity.

The advantages of ESPs include: high collection efficiency for small particles; low operating pressure drops; temperature and pressure limitations imposed only by the construction materials

used; dry disposal of solid particles. The disadvantages include large space requirements, high

cost, and need for skilled operation and maintenance. The success of a precipitator depends notonly on the quality of the system but also on adequate operation and maintenance; therefore, well-

trained operators are required.

Scrubbers

Scrubber is a device used to entrap a targeted object using a scrubbing medium. The scrubbingmedium can be selected based on the properties of the pollutant and the carrier gas in the exhaust.

In a scrubber, the polluted gas stream is brought into contact with the suitable scrubbing medium,

 by spraying, by forcing through, by sucking out or by some other contact method. The scrubbing

media entraps the targeted pollutant by physical phenomenon like simple capture, adsorption, etc.or by chemical phenomenon like absorption, dissolution, ion exchange, etc.

Based on the type of scrubbing media used, the scrubbers are of two types, dry scrubbers and wet

scrubbers. In dry scrubbers solid scrubbing media like activated granular charcoal, activated

alumina, zeolt, etc. been used

Scrubbers can be designed to collect particulate matter and/or gaseous pollutants. Wet scrubbersremove dust particles by capturing them in liquid droplets. Wet scrubbers remove pollutant gases

 by dissolving  or  absorbing  them into the liquid. Wet scrubbers are widely used in cleaning

contaminated gas streams because of their ability to remove effectively both particulate and

gaseous pollutants. They are designed to incorporate small dust particles into larger water droplets,which can then be removed by simple mechanisms such as gravity, impaction on baffles, or by

centrifugal collectors. The droplets are produced, for example, by spray nozzles, by the shearing aliquid film with the gas stream, or by the motion of a mechanically driven rotor, and principles

used to incorporate the dust into droplets include inertial impaction, direct interception, diffusion,

Advantages and disadvantages

For particulate control, wet scrubbers (also referred to as wet collectors) are evaluated against

fabric filters and electrostatic precipitators (ESPs). Some advantages of wet scrubbers over thesedevices are as follows:

• Wet scrubbers have the ability to handle high temperatures and moisture.

• In wet scrubbers, the inlet gases are cooled, resulting in smaller overall size of equipment.

• Wet scrubbers can remove both gases and particulate matter.• Wet scrubbers can neutralize corrosive gases.

Some disadvantages of wet scrubbers include corrosion, the need for entrainment separation or 

mist removal to obtain high efficiencies and the need for treatment or reuse of spent liquid.

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Table 1 summarizes these advantages and disadvantages. Wet scrubbers have been used in a

variety of industries such as acid plants, fertilizer  plants, steel mills,  asphalt plants, and large

 power plants.

Table 1. Relative advantages and disadvantages of wet scrubbers compared toother control devices

Advantages Disadvantages

Small space requirements

Scrubbers reduce the temperature andvolume of the unsaturated exhaust stream.

Therefore, vessel sizes, including fans and

ducts downstream, are smaller than thoseof other control devices. Smaller sizes

result in lower capital costs and more

flexibility in site location of the scrubber.

No secondary dust sources

Once particulate matter is collected, it

cannot escape from hoppers or during

transport.

Handles high-temperature, high-

humidity gas streams

 No temperature limits or condensation problems can occur as in baghouses or 

ESPs.Minimal fire and explosion hazards

Various dry dusts are flammable. Using

water eliminates the possibility of 

explosions.

Ability to collect both gases and

particulate matter

Corrosion problems

Water and dissolved pollutants can formhighly corrosive acid solutions. Proper 

construction materials are very important.

Also, wet-dry interface areas can result incorrosion.

High power requirements

High collection efficiencies for particulate

matter are attainable only at high pressuredrops, resulting in high operating costs.

Water-disposal problems

Settling ponds or sludge clarifiers may beneeded to meet waste-water regulations.

Difficult product recovery

Dewatering and drying of scrubber sludgemake recovery of any dust for reuse very

expensive and difficult.Meteorological problems

The saturated exhaust gases can produce a

wet, visible steam plume. Fog and

 precipitation from the plume may cause

local meteorological problems.

Gaseous pollutants Control:

Control of gas and vapor pollutants can be carried out in several ways. Elimination of the source

of pollution should be the first consideration. Unfortunately, there are relatively few economic

alternatives to soil vapor extraction processes (SVE) when only the soil above the groundwater 

table (vadose zone) requires treatment. In some instances the contamination extends through the

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vadose zone to the groundwater, and a treatment technology used to treat the groundwater may be

combined to treat the vadose zone soil. Additional such technologies are described under 

alternatives to SVE. More typically, SVE is a favorable economic remedy, and toxic vapors will

require treatment to prevent their release to the atmosphere.

 

Commonly applied technologies for treating gases and vapor include the following: absorption,

adsorption, condensation, thermal oxidation (catalytic and non-catalytic) and biofiltration. Each of 

these treatment techniques are typically economic over a certain range of concentrations, and are

 better suited to treat some pollutants than others. Because pollutant concentrations from SVE

operations often decrease over the period of treatment from a moderate (about 1000 ppm) to low

(about 10 ppm or less) level, condensation and absorption have rarely been used for SVE.1[1] Those

two processes also do not destroy the pollutant, but they can be combined with processes that do

destroy the compounds. Those two technologies will not be discussed further here.

Adsorption

Adsorption is an interfacial phenomenon, molecules adhere at the surface of the solid adsorbent. A

large surface area is required to collect the pollutant molecules resulting in a larger physical size

and container. Adsorption from dilute gas streams is complicated by the occurrence of high

humidity and the presence of other compounds whose removal is not desired, but which also

occupy adsorption sites. Competition for adsorption sites reduces the capacity of the adsorbent for 

the target molecules. At high relative humidity, a phenomenon known as "capillary condensation"

results in further loss of adsorption capacity further increasing the need for adsorbent and the size

of the unit. Although hydrophobic adsorbents are now available, used in industrial applications,

and perform better under high humidity conditions than granular activated carbon GAC, they aretoo expensive for use with non-regenerable systems. Adsorption systems used for SVE operation

have traditionally been non-regenerable systems because of the low concentrations. Since

 pollutants are only trapped, and not destroyed by adsorption, disposal of the spent adsorbent must

 be taken into consideration, since it may need to be treated as a hazardous waste.

 Absorption

The removal of one or more selected components from a gas mixture by absorption is probably the

most important operation in the control of gaseous pollutant emissions. Absorption is a process in

which a gaseous pollutant is dissolved in a liquid. Water is the most commonly used absorbent

liquid. As the gas stream passes through the liquid, the liquid absorbs the gas, in much the sameway that sugar is absorbed in a glass of water when stirred. Absorption is commonly used to

recover products or to purify gas streams that have high concentrations of organic compounds.

Absorption equipment is designed to get as much mixing between the gas and liquid as possible.

1

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Absorbers are often referred to as scrubbers, and there are various types of absorption equipment.

The principal types of gas absorption equipment include

spray towers, packed columns, spray chambers, and venture

scrubbers. The packed column is by far the most commonly

used for the absorption of gaseous pollutants. The packed

column absorber has a column filled with an inert (non-

reactive) substance, such as plastic or ceramic, which

increases the liquid surface area for the liquid/gas interface.

The inert material helps to maximize the absorption

capability of the column. In addition, the introduction of the

gas and liquid at opposite ends of the column causes mixing

to be more efficient because of the counter-current flow

through the column. In general, absorbers can achieve

removal efficiencies grater than 95 percent. One potential problem with absorption is the

generation of waste-water, which converts an air pollution problem to a water pollution problem.

Incineration

Incineration, also known as combustion, is most used to control the emissions of organic

compounds from process industries. This control technique refers to the rapid oxidation of a

substance through the combination of oxygen with a combustible material in the presence of heat.

When combustion is complete, the gaseous stream is converted to carbon dioxide and water vapor.

Incomplete combustion will result in some pollutants being released into the atmosphere. Smoke is

one indication of incomplete combustion. Equipment used to control waste gases by combustion

can be divided in three categories: direct combustion or flaring, thermal incineration and catalytic

incineration. Choosing the proper device depends on many factors, including type of hazardouscontaminants in the waste stream, concentration of combustibles in the stream, process flow rate,

control requirements, and an economic evaluation.

A direct combustor or flare is a device in which air and all the combustible waste gases react at

the burner. Complete combustion must occur instantaneously since there is no residence chamber.

Flares are commonly used for disposal of waste gases during process upsets, such as those that

take place when a process is started or shut down. A flare can be used to control almost any

emission stream containing volatile organic compounds. Studies conducted by EPA have shown

that the destruction efficiency of a flare is about 98 percent.

In thermal incinerators the

combustible waste gases pass over or 

around a burner flame into a residence

chamber where oxidation of the waste

gases is completed. For thermal

incineration, it is important that the

 Typical packed column

diagram

 Thermal incinerator: General

case

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vapor stream directed to the thermal incinerator have a constant combustible gas concentration and

flow rate. These devices are not well-suited to vapor streams that fluctuate, because the efficiency

of the combustion process depends on the proper mixing of vapors and a specific residence time in

the combustion chamber. Residence time is the amount of time the fuel mixture remains in the

combustion chamber. Often, supplementary fuel is added to a thermal incinerator to supplement

the quantity of pollutant gases being burned by the incinerator. Energy and heat produced by the

incineration process can be recovered and put to beneficial uses at a facility. Thermal incinerators

can destroy gaseous pollutants at efficiencies of greater than 99 percent when operated correctly.

Catalytic incinerators are very similar to thermal incinerators. The main difference is that after 

 passing through the flame area, the gases pass over a catalyst bed. A catalyst is a substance that

enhances a chemical reaction without being changed or consumed by the reaction. A catalyst

 promotes oxidation at lower temperatures, thereby reducing fuel costs. Destruction efficiencies

greater than 95 percent are possible using a catalytic incinerator. Higher efficiencies are possible if 

larger catalyst volumes or higher temperatures are used. Catalytic incinerators are best suited for emission streams with low VOC content.

Condensation

Condensation is the process of converting a gas or vapor to liquid. Any gas can be reduced to a

liquid by lowering its temperature and/or increasing its pressure. The most common approach is to

reduce the temperature of the gas stream, since increasing the pressure of a gas can be expensive.

A simple example of the condensation process is droplets of water forming on the outside of a

glass of cold water. The cold temperature of the glass causes water 

vapor from the surrounding air to pass into the liquid state on the

surface of the glass.

Condensers are widely used to recover valuable products in a

waste stream. Condensers are simple, relatively inexpensive

devices that normally use water or air to cool and condense a

vapor stream. Condensers are typically used as pretreatment

devices. They can be used ahead of adsorbers, absorbers, and

incinerators to reduce the total gas volume to be treated by more

expensive control equipment. Condensers used for pollutioncontrol are contact condensers and surface condensers. In a

contact condenser, the gas comes into contact with cold liquid. In

a surface condenser, the gas contacts a cooled surface in which

cooled liquid or gas is circulated, such as the outside of the tube.

Contact condenser

Surface condenser

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Removal efficiencies of condensers typically range from 50 percent to more than 95 percent,

depending on design and applications

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Air Quality Standards

The National Ambient Air Quality Standards (NAAQS) are standards established by the United States Environmental Protection Agency that apply for outdoor air  throughout the country. Primary

standards are designed to protect human health, including sensitive populations such as children,

the elderly, and individuals suffering from respiratory disease. Secondary standards are designed to protect public welfare (e.g. building facades, visibility, crops, and domestic animals).

 NAAQS requires the EPA to set standards on eight criteria  pollutants:

1. Ozone (O3)

2. Particulate Matter 

o PM10, course particles: 2.5 micrometers (μm) to 10 μm in size (although current

implementation includes all particles 10 μg or less in the standard)

o PM2.5, fine particles: 2.5 μm in size or less

3. Carbon monoxide (CO)

4. Sulfur dioxide (SO2)5.  Nitrogen oxides (NOx)6. Lead (Pb)

Standards

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Review questions

1. Discuss the sources of air pollution.2. Mention the effects of air pollution on human health, vegetation and materials.

3. Explain the control methods of air pollution.

4. Discuss on the control of gaseous pollution.5. Explain the control strategies of automobile emission.

6. Write a note on the equipments used for control of gaseous emissions.