AIR
POLLUTION
Definition.
Chemical Composition of Dry Atmospheric Air.
Types of Air Pollutants.
Effects of Air Pollutants.
Green House Effect
Ozone Layer Depletion.
Sources of Air Pollution.
Sources Pie Charts
Ambient Air Quality Levels in Greater Mumbai.
Levels of Air Pollution Exposure in Mumbai
National Ambient Air Quality Standards : India.
One method of defining an air pollutant is first to specify the composition of "clean" or "normal" dry atmospheric air and then to classify all other materials or increased amounts of those materials given in the composition of atmospheric air as pollutants. Table 1 lists the chemical composition of dry atmospheric air typically found in rural areas or over oceans away from land. At the present time neither carbon dioxide nor uncombined water vapor is considered to be a pollutant. This condition could change since the discharge of either substances could lead to phenomenon like green house effect which may result in significant change in the global atmospheric temperature.
Table 1 : Chemical Composition of Dry Atmospheric Air
|
Substance |
Volume (percent) |
Concentration (ppm)a |
|
Nitrogen Oxygen Argon Carbon dioxide Neon Helium Methane Krypton Hydrogen Xenon Nitrogen dioxide Ozone |
78.084± 0.004 20.946 ± 0.002 0.934 ± 0.001 0.033 ± 0.001 |
780,900 209,400 9,300 315 18 5.2 1.2 0.5 0.5 0.08 0.02 0.01-0.04 |
Source: Handbook of Air Pollution, PHS Publication AP-44 (PB 190-247), 1968 [40]
A general list of primary air pollutants is presented in Table 2.
Table 2 : General Primary Pollutants
Particulate matter
Respirable Particulate Matter (PM10) : Less than 10 m
Sulfur oxides
Oxides of nitrogen
Carbon compounds
Halogen compounds
Radioactive compounds
Note: 1 m = 10-4 cm
PARTICULATE MATTER.
SULFUR DIOXIDE.
OXIDES OF NITROGEN.
CARBON MONOXIDE.
HYDROCARBONS.
PHOTOCHEMICAL OXIDANTS.
ASBESTOS AND METALS.
B Effect
on Materials, Vegetation and Animals :
Particles will soil painted surfaces, clothing, and curtains merely by settling on them. The cost of repainting interior and exterior surfaces of homes and buildings and of cleaning or replacing textiles is estimated at hundreds of millions of dollars annually.
Particulate matter can cause direct chemical damage by corrosion.
Little is known of the effects of particulate matter in general on vegetation. However, several specific substances have been observed to cause some damage. Cement-kiln dust in combination with mist or light rain has been observed to form a crust on the leaves of plants, resulting in plant damage. Particles containing fluorides appear to cause some plant damage, and magnesium oxide falling on agricultural soils has resulted in poor plant growth.
An animal's health may suffer when the animal feeds on plants covered by toxic particulate. Fluorosis in animals has been attributed to their ingestion of vegetation covered with a fluoride-containing particulate matter. Cattle and sheep that have ingested vegetation on which arsenic containing particles have settled have been victims of arsenic poisoning.
C Effects of Atmospheric Particulate Matter on Human Health :
It is extremely difficult to obtain a direct relationship between exposure to various concentrations of particulate matter and the resulting effects on human health. However, statistical analysis of data indicate a relationship between increases in particulate concentration and rises in the number of clinic and hospital visits for upper respiratory infections, cardiac diseases, bronchitis, asthma, pneumonia, emphysema, and the like. Deaths of elderly persons afflicted with respiratory diseases and cardiac conditions also show an increase during periods when the concentration of particulate matter is unusually high for several days. A growing volume of evidence indicates that much of the particulate matter in the atmosphere is carcinogenic in nature.
In some cases it has been observed that exposure to particulate in combination with other pollutants such as SO2 produces more severe health deterioration than does exposure to each pollutant separately.
A. Effects of Sulfur dioxide and sulfur compounds on Visibility and Materials:
Aerosols reduce visibility. Since aerosols of sulfuric acid and other sulfates make up from 5 to 20 percent of the total suspended particulate in urban air, they contribute significantly to the reduction in visibility.
Sulfur compounds are responsible for the major damage to materials. Observations indicate that concentrations of 1 to 2 ppm of SO2 in the atmosphere cause an increase of 50 to 100 percent in the drying time of a paint film, and higher concentrations- 7 to 10 ppm - increase the drying time to 2 to 3 days.
The most detrimental pollutant that contributes to metal corrosion generally is sulfur dioxide. Sulfur oxides generally accelerate metal corrosion by forming sulfuric acid.
Textiles made of nylon, especially nylon hose, are also susceptible to SO2.
Various animal species, including man, respond to sulfur dioxide by bronchoconstriction, which may be assessed in terms of slight increase in airway resistance, and lung disease. Sulfuric acid is a much more potent irritant to man than is sulfur dioxide; therefore, most studies deal with combined sulfurous materials rather than with SO2 alone. Sulfur oxide in combination with particulate and moisture is a potentially serious health hazard and results in increased mortality.
Effects of Oxides of Nitrogen on
materials and health:
Of the six or seven oxides of nitrogen, nitric oxide (NO) and nitrogen dioxide (NO2) are important air pollutants. Neither NO nor NO2 causes direct damage to materials; however, NO2 can react with moisture present in the atmosphere to form nitric acid, which can cause considerable corrosion of metal surfaces. Nitrogen dioxide absorbs visible light and at a concentration of 0.25 ppm will cause appreciable reduction in visibility. It also suppresses the growth of plants resulting into lower yield.
Nitrogen dioxide acts as an acute irritant and in equal concentration is more injurious than NO. However, at concentrations found in the atmosphere, NO2 is only potentially irritating and potentially related to chronic pulmonary fibrosis. Some increase in bronchitis in children ( 2 to 3 years old ) has been observed at concentrations below 0.01 ppm. In combination with unburned hydrocarbons, the oxides of nitrogen react in the presence of sunlight to form photochemical smog. It is because of this chemical activity that the primary air quality standard for the oxides of nitrogen has been set as 80 m g/m3 annual average. The components of photochemical smog which are the most damaging to plants and detrimental to human health are the photochemical oxidants.
Effects of Carbon Monoxide on Materials and Plants:
Carbon monoxide appears to have no detrimental effects on material surfaces and plant life.
Effects of Carbon Monoxide on Health:
There are many studies which show that high concentrations of carbon monoxide can cause physiological and pathological changes and ultimately death (> 750 ppm). Carbon monoxide is a poisonous inhalent that deprives the body tissue of necessary oxygen. The combination of carbon monoxide with hemoglobin leads to carboxyhemoglobin, COHb; the combination of oxygen with hemoglobin leads to oxyhemoglobin, O2Hb. Hemoglobin has an affinity for CO that is approximately 210 times its affinity for oxygen. Fortunately the formation of COHb in the bloodstream is a reversible process. The direct effect of COHb is to reduce the oxygen carrying capacity of the blood. However, a secondary effect is also present. COHb interferes with the release of the oxygen carried by the remaining hemoglobin.
Blood samples from 29,000 blood donors in U.S. cities, suburbs, and rural areas revealed high concentrations of COHb. Forty-five percent of nonsmokers had blood containing 1.5 percent COHb. The blood of most smokers averaged above 5 percent COHb. Nonsmokers in Chicago averaged 2 percent COHb. Table 3 lists the health effects attributed to certain CO levels in the environment and COHb levels in the blood.
Table 3
Health Effects of Carbon Monoxide and COHb
|
Carbon Monoxide |
|
|
Environmental Condition |
Effects |
|
9 ppm 50 ppm 6-wk exposure 50 ppm 50 min. exposure
50 ppm 8 to 12 hr exposure, nonsmokers |
Ambient air quality standard Structural changes in heart and brain of animals Changes in relative brightness threshold and visual acuity Impaired performance on psychomotor tests |
|
Carboxyhemoglobin |
|
|
COHb Level (percent) |
Effects |
|
< 1.0
> 5.0 10.0 - 80.0 |
No apparent effect Some evidence of effect on behavioral performance Central nervous system effects, Impairment of time interval discrimination, visual acuity, brightness discrimination, and certain other psychomotor functions Cardiac and pulmonary functional changes Headaches, fatigue, drowsiness, coma, respiratory failure, death |
Effects of Hydrocarbons on
materials and health:
Hydrocarbons do not appear to cause any appreciable corrosive damage to materials. Particulate, or soot made up of unburned hydrocarbons and carbon, soils surfaces. Of all the hydrocarbons, only ethylene has adverse effects on plants at known ambient concentrations. The principal effect of ethylene is to inhibit plant growth.
Hydrocarbons do not cause any adverse effect on human health. Some cancers appear to be caused by exposure to aromatic hydrocarbons found in soots and tars. Identifiable airborne carcinogens are mostly polynuclear aromatic hydrocarbons. Unburned hydrocarbons in combination with the oxides of nitrogen in the presence of sunlight form photochemical smog that do have adverse effects on human health and on plants.
Effects of photochemical oxidants on
materials and health:
Oxidizing agents, ozone (O3), peroxyacetyl nitrate (PAN), peroxybenzoyl nitrate (PBN), and other trace substances which can oxidize the iodide ion of potassium iodide are termed as photochemical oxidants. Ozone and PAN are present in the highest concentrations. The aerosols formed due to chemical reactions in the smog cause a marked reduction in visibility and give the atmosphere a brownish cast. Ozone attacks synthetic rubbers, thereby reducing the life of tires, rubber insulation, and so on. Ozone also attacks the cellulose in textiles, reducing the strength of such items. All of the oxidants cause some fading of the fabrics. Oxidants, primarily PAN and PBN, cause eye irritation, and in combination with ozone they irritate the nose and throat, cause chest constriction and at high concentration (3900 m g/m3) produce severe coughing and inability to concentrate.
Effects of asbestos and metals on
materials and health:
Many studies have shown a higher than expected incidence of bronchial cancer among people whose occupations expose them to asbestos. In addition, asbestos has been identified as a causal factor in the development of cancers of the membranes lining the chest and abdomen. Beryllium has also proved dangerous to health, producing both acute and chronic lethal inhalation effects as well as causing damage to the skin and eyes. Most of the cases studied concern occupational exposure. Exposure to metallic mercury vapors may cause injury to central nervous system eventually causing brain damage.
Because of these proven hazards, emission standards were established for asbestos, beryllium, and mercury. The standard for asbestos stipulates how asbestos material is to be applied as well as the precautions to be taken in its manufacture and the demolition of buildings.
GLOBAL EFFECTS OF AIR POLLUTION
GREEN HOUSE EFFECT:
The incoming solar radiation towards earth is in the visible wavelength. However, the earth reradiates in the form of long wavelength infrared radiation. Much of this radiation is absorbed by the water vapor and carbon dioxide in the atmosphere near the earth's surface. Since both water vapor and carbon dioxide are transparent to most of solar radiation but absorb the long wave radiation from the earth's surface, the net effect causes a warming of the atmosphere depending upon the quantity of H2O and CO2 present. This effect has been termed as the "greenhouse effect". Some researchers have indicated that a 100 percent increase in H2O and CO2 content in the atmosphere could result in an increase of between 20 and 40 F in the atmospheric temperature.
On the other hand, one result of industrialization throughout the world is the significant increase in particulates emitted. Particulates in the atmosphere tend to block passage of solar radiation toward the earth's surface. This blocking effect is responsible for the drop in the average atmospheric temperature. It is difficult to predict which of these two factors will have the overriding influence on atmospheric temperatures over the decades ahead. From 1890 to 1945 the average temperature rose by nearly 0.90 F, but from 1945 to 1970 the average ground temperature dropped by roughly 0.60 F. It should be noted that the earth's average temperature during the great ice age was only about 70 lower than during the warmest periods.
A layer of ozone exists in the upper atmosphere. This layer absorbs and limits the ultraviolet radiation reaching the earth. Life on earth was possible due to reduction of UV radiation by ozone layer. However, due to chloroflurocarbons which are emitted from refrigeration and air-conditioning applications, aerosol sprays the ozone layer is getting depleted. Some other ozone layer depleting substances are also emitted from industrial activities. Due to this, the UV radiation reaching earth has increased and this is resulting into severe skin problems.
Transportation, fuel combustion and industrial processes are the major sources of air pollution. A large number of studies for the source apportionment of air pollution have been conducted in the US. In Table 4 we see emissions estimates for major air pollutants for the US in 1991.
Table 4 National Emissions Estimates
for 1991 in US
(Values in millions of metric tons /
yr )
|
Source Category |
PM10 |
SOx |
CO |
NOX |
VOCs |
Total |
|
Transportation Fuel Combustion Industrial Process Solid Waste Disposal Miscellaneous |
1.51 1.10 1.84 0.26 0.73 |
0.99 16.55 3.16 0.02 0.01 |
43.49 4.67 4.69 2.06 7.18 |
7.26 10.59 0.60 0.10 0.21 |
5.08 0.67 7.86 0.69 2.59 |
58.33 33.58 18.15 3.13 10.72 |
|
Total |
5.44 |
20.73 |
62.09 |
18.76 |
16.89 |
123.91 |
PM10 = Particulate matter, 10 m or smaller
VOCs = Volatile Organic Compounds (Hydrocarbons)
Miscellaneous mostly includes forest fires
Table 5 Sources of Air
Pollutants in Mumbai
(Values in metric tons / yr)
|
Source Category |
TSP |
PM10 |
SO2 |
NOx |
|
Vehicular Exhaust |
3673 |
3673 |
3490 |
19520 |
|
Resuspension from Roads |
10200 |
2550 |
-- |
-- |
|
Power Plant |
1500 |
1500 |
16000 |
11200 |
|
Industrial Combustion |
1838 |
1496 |
38710 |
4085 |
|
Domestic Combustion |
4432 |
2235 |
1688 |
1344 |
|
Marine (Dock) Combustion |
560 |
469 |
9350 |
1245 |
|
Refuse Burning |
4108 |
4108 |
26 |
153 |
|
Stone Crushers |
6053 |
-- |
-- |
-- |
To view pie charts click here
FOUR BASIC ASSUMPTIONS:
The rational control of air pollution rests on four basic assumptions:
AIR ACT
(PREVENTION AND CONTROL OF POLLUTION) 1981:
Air act is passed by the Parliament of India for the control of air pollution. Unlike Water Act which is applicable throughout the country, this act is applicable in certain sensitive areas like Mumbai Metropolitan Region, Pune industrial region etc. Under this act, every industry/organization has to take consent letter from the State/Central Pollution Control Board before emitting air pollutants in the atmosphere.
The problem of Air Pollution Control can be tackled in three steps:
1. Collection of polluting air through hoods and ducts
2. Installing air pollution control device
3. Exhausting the clean air through ventilation system
HOOD AND DUCT DESIGN:
In various industrial enterprises, hazardous or potentially hazardous conditions exist which can affect the health and safety of those who work there. Fumes and vapours are given off from storage tanks, processing vats, and other types of processing equipment. Dusts are given off from grinders, pulverizers, hammer-mills, conveying equipment, mixers and many other types of equipment.
In general, most air pollution control equipment is more efficient when handling higher concentration of contaminants, all else being equal. Therefore, the gas handling system should be designed to concentrate contaminants in the smallest possible volume of air. This is important since, exclusive of the blower, the cost of control equipment is principally based upon the volume of gas to be handled and not on the quantity of particulate to be removed.
In designing local exhaust hoods an attempt is made to create a controlled air velocity that will prevent escape of contaminants from the controlled area to the general environment. The air velocity that will just overcome the dispersive motions of the contaminant, plus a suitable safety factor, is termed the “control velocity”. The control velocity is adjusted to obtain the least air flow rate that gives satisfactory control results for minimum gas volume and maximum contaminant loading.
AIR POLLUTION CONTROL DEVICES:
Depending on the type of air pollutant, air pollution control devices are divided into two categories as follows:
|
Particulate Control Devices |
Gaseous Control Devices |
|
1. Settling Chambers 2. Cyclones 3. Scrubbers 4. Fabric filters 5. Electrostatic Precipitators |
1. Adsorption Devices 2. Absorption towers 3. Incinerators |
PARTICULATE COLLECTION MECHANISMS:
The six available mechanisms for collecting particles may
be classified as gravitational settling, centrifugal impaction, inertial
impaction, direct interception, diffusion, and electrostatic effects. One or
more of these mechanisms are responsible for the removal of particulates in any
of the industrial collection devices discussed in the following sections.
PARTICULATE CONTROL DEVICES:
Settling Chamber: Gravitational force may be employed to remove particulate in settling chambers when the settling velocity is greater than 13 cm/s. In general, this applies to particles larger than 50 µ if the material density is low, down to 10 µ if the material is reasonably dense. Smaller particles than this would require excessive horizontal flow distances, which would lead to excessive chamber volumes.
Cyclones: Cyclone separators are gas cleaning devices which employ a centrifugal force generated by a spinning gas stream to separate the particulate matter (solid or liquid) from the carrier gas. The separator unit may be a single large chamber, a number of small tubular chambers in parallel or series. Units in parallel provide increased volumetric capacity, while units in series provide increased removal efficiency. In the simple cyclone separator, the circular motion is attained by a tangential gas inlet. The operation depends upon the tendency (inertia) of particles to move in a straight line when the direction of gas stream is changed.
The cyclone separator is usually employed for removing particles 10 µ in size and larger. However, conventional cyclones seldom remove particles with an efficiency greater than 90 percent unless the particle size is 25 µ or larger. High efficiency cyclones which are effective down to 5 µ, are available.
Scrubbers: In a scrubber, a liquid, usually water, is used to capture particulate dust or to increase the size of aerosols. In either case the resulting increased size facilitates the removal of contaminants from the gas stream. One of the primary aims of the scrubbers should be adequate dispersion of liquid phase in order to achieve good contact between the particulate (or aerosol) phase and the liquid phase. Following are the major types of scrubbers:
1. Spray chamber scrubbers
2. Cyclonic scrubbers
3. Venturi scrubbers
Packed tower could be added to the list, but they are used primarily for gas adsorption.
1. Spray Chamber Scrubbers: One of the simplest devices for wet collection is the circular or rectangular spray tower. The polluted gas flows upward and the particles collide with liquid droplets produced by suitable nozzles located across the flow passage. If the gas flow rate is relatively slow, the contaminated liquid droplets produced will settle by gravity to the bottom of the tower. A mist eliminator is usually placed at the top of the tower to remove both excess clean water droplets and dirty droplets which are very small and thus are carried upward by the gas flow.
2. Cyclone scrubbers: The simplest type of cyclonic scrubber is achieved by inserting banks of nozzles in ring fashion inside a conventional dry cyclone. The spray acts on the particles in the outer vortex, and the dust-loaded liquid particles are thrown outward against the wet inner wall of the cyclone. The dust-laden solution flows down the walls of the cyclone to the bottom, where it is removed. The water spray may also be located in the cyclone inlet. A moisture eliminator is required at the outlet.
3.
Venturi Scrubber: A venturi is
a rectangular or circular flow channel which converges to a narrow throat
section and then diverges back to its original cross-sectional area. In the
converging section flow work associated with the fluid is converted into
kinetic energy, with a concomitant decrease in static pressure and rise in
velocity. The area ratio between the inlet and the throat typically is 4:1 in a
venturi scrubber. The scrubbing action occurs through the introduction of water
either in the throat region or at the beginning of the convergent section.
Fabric Filters: Filtration is one of the oldest and most widely used methods of separating particles from a carrier gas. A filter generally is any porous structure composed of granular or fibrous material which tends to retain the particulate as the carrier gas passes through the voids of the filter. The filter is constructed of any material compatible with the carrier gas and particulate, and may be arranged in deep beds, mats, or fabric. Mat and deep bed filters have large void space amounting to from 97 to 99 percent of the total volume. They are used for very light dust loads and are designed to remain in service for long periods. In general, they are cleaned in place periodically at relatively short intervals. Fabric filters are usually formed into cylindrically tubes and hung in multiple rows to provide large surface areas for gas passage. Fabric filters have efficiencies of 99 percent or better when collecting 0.5 µ particles and can remove substantial quantities of 0.01 µ particles. Typical dust loadings handled are from 0.23 to 23 g/m3. Fabric filters are designed on the basis of air to cloth ratio and pressure drop.
Electrostatic Precipitators: Particulate and aerosol collection by electrostatic precipitation is based on the mutual attraction between particles of one electrical charge and a collecting electrode of opposite polarity. Its advantages are a capacity to handle large gas volumes, high collection efficiencies even for submicron-size particles, low energy consumption and draft loss, and ability to operate with relatively high temperature gases. Electrostatic precipitators have been built for volumetric rates from 100 to 4,000,000 ft3/min, and they are used to remove particles from 0.05 to 200 µ. For most applications, the collection efficiency runs from 98 to 99 percent. Gas temperatures up to 12000 F (920 0 K) and pressures up to 150 psi (10 bars) can be accommodated. Desirable characteristics such as these account for the wide use of electrostatic precipitators by industry, especially in the electrical power generating field. In addition, it is important to note that the energy expended in separating particulate from a waste gas stream by means of an electrostatic precipitator acts solely on the particulate, and not on the gas stream. This is unique for collection equipment in the air pollution field, since other devices based on different principles of separation require that energy be expended on the entire gas stream in order to accomplish the desired effect.
GASEOUS POLLUTANTS CONTROL DEVICES :
By far the major portion of the recognized air pollutants are gases such as carbon monoxide, the oxides of nitrogen, the oxides of sulfur, and unburned hydrocarbons. Data indicate that on a mass basis approximately 91 percent of the air pollutants are gases, with carbon monoxide contributing 56 percent. Major control devices for gases are:
1. Adsorption devices
2. Absorption devices
3. Incinerators
1. ADSORPTION DEVICES: Adsorption is a separation process based on
the ability of certain solids to remove gaseous (or liquid) components
preferentially from a flow stream. The pollutant gas or vapour molecules
present in a waste stream collect on the surface of the solid material. The solid adsorbing medium is frequently
termed the adsorbent, while the gas vapour adsorbed is called the adsorbate. In
addition to dehumidifying air and other gases, adsorption is useful in removing
objectionable odours and pollutants from industrial gases as well as recovering
valuable solvent vapours from air or other gases. Adsorption is particularly
useful technique when:
i) The pollutant gas is noncombustible or difficult to burn.
ii)
The pollutant is sufficiently
valuable to warrant recovery
iii)
The pollutant is in a very dilute
concentration in the exhaust system.