ENVIS Centre on Control of Pollution Water, Air and Noise

INTRODUCTION

Air pollution is the introduction of particulates, biological molecules, or other harmful materials into Earth's atmosphere, causing diseases, death to humans, damage to other living organisms such as animals and food crops, or the natural or built environment.

 

According to The Air (Prevention and Control of Pollution) Act, 1981, “Air pollution is the presence of any solid, liquid, or gaseous substances in the atmosphere in such concentration as may be or tend to be injurious to human beings or other living creatures or plants or property or environment”.

SOURCES OF AIR POLLUTION

Basic sources of air pollution are : Natural and Anthropogenic

Natural sources of pollution are those that are caused due to natural phenomena. Ex: Volcanic eruptions, Forest fires, Biological decay, Pollen grains, Marshes, Radioactive materials.

Anthropogenic (Man-made) sources of pollution are those which are created by human activities. Ex: Thermal power plants, Industrial emissions, Vehicular emissions, Fossil fuel burning, Agricultural activities etc.

 

AIR POLLUTANTS

The substances which are responsible for causing air pollution are called air pollutants. Air pollutants can be categorized by various means:

A. Based on source of origin
B. Based on method of origin
C. Based on chemical composition
D. Based on state of matter

 

A. Based on source of origin

1. Natural air pollutants: Natural air pollutants are emitted from natural sources such as volcanic activity, dust, sea-salt, forest fires, lightening, soil outgassing etc.

 

2. Anthropogenic air pollutants: These pollutants include the emissions from stationary point sources (e.g. emission from industries), mobile sources (e.g. vehicular emission, marine vessels, airplanes etc.), waste disposal landfills, controlled burning etc.

 

B. Based on method of origin

1. Primary air pollutants: Those pollutants which are emitted directly from any emission source in the atmosphere are termed as Primary air pollutants. E.g. Sulphur dioxide (SO₂), Carbon monoxide (CO), Lead (Pb), Ammonia (NH₃) etc.

 

2. Secondary air pollutants: Secondary pollutants are formed by the reactions between primary air pollutants and normal atmospheric constituents. In some of the cases, these pollutants are formed by utilizing the solar energy. E.g. Ozone, Peroxyacetylnitrate (PAN), Nitrogen dioxide (NO₂), Smog etc.

 

C. Based on chemical composition

1. Organic air pollutants: Examples are hydrocarbons, aldehydes, ketones, amines, and alcohols etc.

 

2. Inorganic air pollutants: Examples are carbon compounds (CO and carbonates), nitrogen compounds (NO_X and NH₃), sulphur compounds (H₂S, SO₂, SO₃, H₂SO₄), halogen compounds (HF, HCl etc.), flyash, silica etc.

 

D. Based on state of matter

1. Gaseous air pollutants: Pollutants which are in the form of gas are termed as gaseous air pollutants. E.g. SO₂, NO_X, O₃, CO etc.

 

2. Particulate air pollutants: Particulate air pollutants or particulate matter (PM) can be defined as the microscopic solid or liquid matter suspended in the earth’s atmosphere.
There are various subtypes of particulate matter:

a. Total suspended particulate matter (TSPM): The concentration of particulate matter which is obtained when a high volume bulk sampling is done on a filter substrate. It includes particles of all sizes.

b. PM₁₀: These are the particles less than 10 µm in diameter.

c. PM_2.5: These are the particles less than 2.5 µm in diameter.

d. PM_1.0: These are the particles less than 1 µm in diameter.

Particles which lie between 10 µm to 2.5 µm are termed as Coarse particles whereas particles with diameter less than 2.5 µm are called Fine particles. Fine particles also include ultra-fine particles of size less than 0.1 µm (PM_0.1).

 

EFFECTS OF AIR POLLUTANTS

(in terms of health aspects and environmental aspects)

The effects of various pollutants can be understood in terms of health aspects and environmental aspects. The effects are summarized in the following table:

 

Pollutants	Major effects
	Health effects	Environmental effects
Sulfur oxides (SOx)	Respiratory problems, Heart and lung disorders, Visual impairment	Acid rain
Nitrogen oxides (NOx)	Pulmonary disorders, increased susceptibility to respiratory infections	Precursor of ozone formation in troposphere, Aerosol formation
Particulate matter (PM)	Respiratory problems, liver fibrosis, lung/liver cancer, Heart stroke, Bone problems	Visibility reduction
Carbon monoxide (CO)	Anoxemia leading to various cardiovascular problems. Infants, pregnant women, and elderly people are at higher risk	-
Ozone (O3)	Respiratory problems, Asthma, bronchitis etc.	O3 in upper troposphere causes green house effects, Harmful effects on plants as it interferes in photosynthesis and results in death of plant tissues since it assists in the formation of Peroxyacetylnitate (PAN)
Lead (Pb)	Serious effects on central nervous system since it is absorbed rapidly in blood stream, Anemia, toxic for soft tissues and bones	-
Ammonia (NH3)	Immediate effects lead to burning of eyes, nose, throat, and respiratory tract. Prolonged effects result in blindness, lung damage, or death.	-

Health Effects	Environmental effects

CONTROLLING MEASURES OF AIR POLLUTION

The atmosphere has several in-built cleaning processes such as dispersion, flocculation, absorption, rain-washout, etc. to cleanse the atmosphere. However, control of contaminants at their source level is a desirable and effective method through preventive or control technologies.

Some measures which can be adopted in this direction are as follows:

1. Use of unleaded petrol

2. Using fuels with low sulphur and ash content

3. Promotion of use of public transport

4. Sensitive locations (hospitals, schools, playgrounds etc.) should not be located along the busy streets

5. Vegetation cover should be increased along the roadside, busy traffic intersection points, and on the road dividers.

6. Industries and waste disposal sites should preferably be situated in outskirts of the city.

CONTROL MEASURES IN INDUSTRIAL ESTABLISHMENTS

There are various controlling measures for air pollutants; however for the ease of understanding, it can be divided into two categories:

I. Control of gaseous air pollutants
II. Control of particulate pollutants

I. Control of gaseous air pollutants

There are essentially two classes of techniques by which gaseous pollutants may be removed from a gas stream:

a. Sorption of pollutant, for example, through absorption in a liquid or adsorption on a solid surface, and

b. Chemical alteration of the pollutant, usually, through combustion or catalytic treatment.

A. Absorption by liquids

Absorption is one of the most frequently used techniques for controlling the concentration of gaseous pollutants before they are discharged into the atmosphere. In this process, effluent gases are passed through absorbers (scrubbers) which contain liquid absorbants that remove one or more of the pollutants in the gas stream.

Suitable solvents for some gaseous pollutants are given below:
a. Sulphur dioxide (SO₂) – Sodium hydroxide (NaOH), Magnesium oxide (MgO), Calcium carbonate (CaCO₃), Calcium oxide (CaO), Calcium hydroxide (Ca(OH)₂) solutions.
b. Nitrogen oxides (NO_x) – Ammonium bicarbonate (NH₄HCO₃), ammonium bisulphite (NH₄HSO₄), Calcium hydroxide (Ca(OH)₂), Magnesium hydroxide (Mg(OH)₂), and Sodium hydroxide (NaOH) solution.
c. Hydrogen sulphide (H₂S) – Sodium hydroxide (NaOH), Potassium hydroxide (KOH)
d. Ammonia (NH₃) – Sulphuric acid (H₂SO₄), Nitric acid (HNO₃)

Various equipments used for gas absorption are Packed tower, plate or tray tower, spray tower, venturi scrubber etc.

B. Adsorption by solids

Adsorption is a surface phenomenon by which gas or liquid molecules are captured and allowed to adhere on the surface of solid adsorbent. Adsorption equipments are generally the packed bed of some adsorbing material.

Uses of some common adsorbents are given below:
a. Activated carbon – Removal of odours and trace impurities from gases, purification of industrial gases and hydrocarbons etc.
b. Activated alumina – Dehydration of gases and liquids.
c. Silica gel – Dehydration and purification of gases
d. Molecular sieves – Selective adsorption of carbon dioxide (CO₂), ammonia (NH₃), acetylene (C₂H₂), hydrogen sulphide (H₂S), sulphur dioxide (SO₂)

Installation costs for adsorbers are high but maintenance and operation costs are not excessive.

C. Combustion

Combustion processes can be utilized to greatest advantage when the gases or vapours to be controlled are organic in nature. If the waste gas contains sufficient combustible material then incineration may be the simplest route to air pollution control.

There are 3 methods of combustion in common use today:
a. Direct combustion (Flaring) – Highly combustible streams with high heating values can be eliminated by direct flaring. However, flaring is not a satisfactory solution when the gas streams contain excessive amounts of inorganic pollutants like S, Cl, and F.
b. Thermal incineration (after burning or flame combustion) – In thermal incinerator, the waste gas is preheated often over an auxillary fuel fired burner and passed into a combustion chamber where a temperature of 500 – 800 °C is maintained. The gas stream is kept at this elevated temperature long enough to allow complete oxidation.
c. Catalytic oxidation – When the concentration of the combustible portion of gas stream is below flammable range and when lower operating temperatures are desired, catalytic combustion processes are used.

II. Control of particulate pollutants

The basic instruments for removing particulate matter from gas streams may be classified as:

A. Gravitational settling chamber
The gravitational settling chamber is the simplest type of equipment used for the collection of solid particulates. This technique is generally used to remove large, abrasive particles (usually > 50 µm) from gas streams. This device works on the principle of gravitational force. In this device, there is a chamber in which the carrier gas velocity is reduced so as to allow the particulates to settle out of the moving stream under the action of gravity. The solid particulates having higher density than the surrounding gas, settle under the influence of gravity on the base of the chamber, from where they are removed through the hoppers.

Advantages:
1. Low initial cost
2. Simple construction
3. Low maintenance cost
4. Dry and continuous disposal of solid particulates

Disadvantages:
1. Large space requirements
2. Only comparatively large particles can be collected.

B. Cyclone separators
A cyclone separator can be defined as a structure, without moving parts in which the velocity of an inlet gas stream is transformed into a confined vortex from which centrifugal forces tend to drive the suspended particles to the wall of the cyclone body. Cyclone separators utilize a centrifugal force generated by a spinning gas stream to separate the particulate matter from the carrier gas. In operation, the particle laden gas upon entering the cyclone cylinder receives a rotaing motion. The vortex so formed, develops a centrifugal force, which acts to throw the particles radially towards the wall.Cyclones are used widely for the control of gas borne particulates in such industrial operations as cement manufacture, feed and grain processing, food and beverage processing, mineral processing, paper and textile industries, and wood working industries etc.

Advantages:
1. Low initial cost
2. Simple construction and operation
3. Low maintenance requirements
4. Continuous disposal of solid particulates

Disadvantages:
1. Low collection efficiency for particles below 5-10 µm in diameter.
2. Equipment is subject to severe abrasive deterioration.

C. Fabric filters
Filtration is the oldest and generally one of the most versatile and efficient methods for removing particulate matter from industrial gases. Filters can be classified as either a packed filter or a fabric filter, depending on the way in which the fibers are held in place. In a packed filter, the fibers are loosely packed inside an enclosure where the dust laden gas takes a long path on its way through the filter. In a fabric filter, fibers are woven into a thin layer of fabric (bag filters), usually made from natural, synthetic metal or glass fibers.

Fabric filters are the most commonly used particle collectors in industry where as packed filters find wide use in air conditioning and other applications where the dust loading is relatively small.

In principle, the dust laden gas passes through the filter in which the particulates are trapped on to the fibres by the mechanisms of inertial impaction, direct interception, and diffusion.

Fabric filters find extensive applications in the industries like metallargical industries, foundries, cement industries, chalk and lime paints, brick works, ceramic industries, flour mills etc.

Advantages:
1. High particle collection efficiencies for all particle sizes, esp. for particles < 10 µm in diameter.
2. Simple construction and operation
3. Dry disposal of collected material
4. Retention of finest particles.

Disadvantages:
1. Their application is possible only where process temperature is generally < 285°C.
2. High construction, maintenance, and fabric replacement costs.
3. Large size of equipment.

D. Electrostatic Precipitators (ESP)
The electrostatic precipitator is one of the most widely used devices for controlling particulate emissions at industrial installations. ESPs are particulate collection devices that utilize electrical energy directly to assist in the removal of the particulate matter. Particles as small as 1/10th of a micron can be removed. The principle on which this equipment operates is that, when a gas containing aerosols is passed between two electrodes that are electrically insulated from each other and between which there is a considerable difference in electric potential, aerosol particles precipitate on the low potential electrode. Actually, electrostatic precipitation is a physical process by which particles suspended in gas stream are charged electrically and, under the influence of the electrical field, separated from the gas stream.

ESPs find applications in industries like cement industries, pulp and paper mills, steel plants, non-ferrous metal industries, petroleum industries, carbon black industries etc.

Advantages:
1. High collection efficiency
2. Particles as small as 0.1 µm can be removed.
3. Low maintenance and operating costs
4. Treatment time is negligible
5. There is no limit to solid, liquid, or corrosive chemical usage.

Disadvantages:
1. High initial costs
2. Space requirement is more
3. Possible explosion hazards during collection of combustible gases or particulates
4. Precautions are necessary to maintain safety during operation, as operating voltage is high.

E. Wet Collectors or Scrubbers
These are devices which utilize a liquid to assist in the removal of particulates from the carrier gas stream. Generally, water is used as the scrubbing liquid. In a wet collector, the dust is agglomerated with water and then separated from the gas together with the water. The mechanism of particulates removal involves inertial impaction, direct interception, diffusion, and condensation.

Wet scrubbers are particularly useful,

• when, the hot gas must be cooled for some reason
• when, addition of a liquid to the gas is harmless or even beneficial
• when, the particulate is combustible or any flammable gas is present, even in trace amounts, in the bulk gas phase
• when, a waste water treatment system is available on site with adequate reverse capacity to handle the liquid effluent
  

Advantages:
1. Low initial cost
2. Moderately high collection efficiency for small particles
3. Applicable for high temperature installations
4. These can simultaneously remove particulates and gases
5. Effective performance over a wide loading range
6. Small space requirement

Disadvantages:
1. High power consumption for higher efficiency
2. Wet disposal of collected material
3. Very small particles (sub-micron size) may not be captured.

RECENT DEVELOPMENTS

This Section emphasizes the recent developments in the field of "Air Pollution Control".

1. Respirable Dust Sampler (RDS)

2. ‘Air-Shed’ approach for air pollution mitigation

1. Respirable Dust Sampler (RDS)
The respirable dust fraction is dust that enters the ‘deep lung’ and is considered to be < 10 µm. It causes serious respiratory problems on continuous inhalation. Respirable dust is sampled using a cyclone dust sampler designed to sample for a specific fraction of dust at a set flow rate. This particular instrument uses an improved cyclone with sharper cutoff (D₅₀ at 10 µm) to separate the coarser particulates from the air stream before filtering it on the glass microfibre filter. The cyclone sampler is designed such that smaller particles are carried onto the filter paper inside the cassette and larger unwanted particles drop into the pot. By using this instrument, measurement of respirable particulate matter can be done accurately and total suspended particulate matter (TSPM) can also be assessed by collection of dust retained in the cyclone cup.

Important specifications of RDS are as follows:

Flow rate	0.9 – 1.4 m3/min free flow
Particle size	Particles of ≤ 10 µm collected on filter paper holder. SPM > 10 µm are collected in a separate sampling bottle under the cyclone
Sampling time	28 hours (maximum)
Sampling time record	0 to 9999.99 hrs. recorded on a time totalizer
Automatic sampling	24 hrs programmable timer to automatically shut off the system after preset time interval
Power requirement	Nominal 220 V, single phase, 50 Hz AC mains supply

Special Features
1. Brushless blower reduces equipment downtime and maintenance effort.
2. Significantly reduced noise.
3. Electromagnetic interference (EMI) to TVs totally eliminated.
4. Provision of light for ‘flow and time reading’ during night.
5. Toolbox within the instrument.
6. Lockable casters for field convenience.
7. Lockable top cover and gaseous attachment.
8. Improved cabinet design which is more sturdy and durable.

2. 'Air-Shed’ approach for air pollution mitigation
Air-shed or air quality region is that geographical area which encompasses both the area’s air pollution sources and its receptors. A regional air-shed can be defined as a government covering an entire air pollution area, so that ideally within such an airshed:

• There is a discrete and common air mass, i.e., all air pollution remains within such as air basin and does not pass out of its boundaries,
• All pollution sources and pollution-producing activities are included, and
• All the affected population is covered.

However, practically, it is impossible to define such an air drainage area, solely by meteorological criteria. A regional air-shed is interjurisdictional, often interstate. Rather, a regional air-shed is best related to a metropolitan area including the central city, its suburbs, and surrounding rural areas. All political jurisdictions within an air quality region would have the same air quality standards. In general, air-shed would not be contiguous but would be separated by non-urban hinterland. If two separated regions were to expand in area in the future until they became contiguous, they would presumably then be merged into one larger air-shed. Since each air-shed would adopt its own air quality standards, they could differ among the several regions.

Among the most advantageous and appropriate functions for a regional airshed to undertake would be:
1. An environmental center for research and training
2. A monitoring system on an area-wide basis
3. An emergency alert procedure or a quickly triggered early warning system.
4. The allocation of scarce, low sulfur fuels, during emergencies, or continuously, as a uniform, regional market.

Special Feature
Air pollution does not respect political boundary lines. Existing local and state agencies deal with air pollution on a limited, piecemeal and uncoordinated basis, therefore, an areawide agency covering the entire problem-shed would be the most effective unit to deal with the problem.