Air pollution is defined as limited to the situation in which the outdoor ambient atmosphere contains materials in such concentrations which are harmful to man and its surrounding environment. In general air pollution may be defined as the imbalance in quality in the air due to the introduction of foreign material from natural or artificial sources to the air so as to cause adverse effects biological community.
Source of air pollution
The principal source of air pollution can either be natural or anthropogenic.
Natural sources of air pollution :
- Forest fire
- Soil erosion
- Volcanic eruption
- Evaporation of volatile organic matter
- Bacterial decomposition product
Anthropogenic sources of air pollution
- Automobile exhaust:-Due to incomplete combustion of petrol ,CO, NOX ,ethane ,benzo pyrene are produced.
- Industrial exhaust:-The industrial exhaust contains CO ,CO2 ,NO ,NO2 , N2O ,Cl2 ,NH2 , F2&particulate matter.
- Release of organic substance:-The process like biological decomposition or organic matter , seepage from natural gas and oil field &volatile emission from plant, sewage etc. ,organic substances CH4 ,C2H2 ,C6H5NH2 ,C2H4
- Release of chloroflouro carbon (CFC):-Release from A/C.
- Petrochemical oxidants:-These are OZONE & peroxy acetyl nitrate (PAN) formed by petrochemical reaction.
- Particulate pollution:-Solid & liquid aerosols suspended in atmosphere ae referred to as particulate matter. This arises from grinding of ores, spraying &soil erosion. Aerosols are chemicals, which are release in to air in vapour form. For example- Leaded petrol when coming out as fume contains lead
- Agricultural chemicals:-Agric chemicals like pesticides, insecticides, fungicides are released in to air as pollutants.
- Petrochemical smog:-when different gases like SO2, NO, N2O ,NO2&Hydro carbons are released into atmosphere, these form petro chemical smog.
- Release from fertilizer plant:-Release of NH3 gas from ammonium fertilizer plant .
Types of air pollutant
Usually, they are divided into two categories
1) Primary pollutant
2) Secondary pollutant
A primary pollutant is those that are emitted directly from sources .typical pollutants included in this category are particulate matter such as ash, smoke, dust, fumes, mist, inorganic gases such as SO2, H2S, Nitric oxide, NO, NH3, CO, CO2, aromatic hydrocarbons.
Secondary pollutants are those that are formed in the atmosphere by chemical interaction among primary pollutants and atmospheric constituents. Pollutants such as SO3, NO2, PAN (peroxyacetyl nitrate), OZONE, various sulfates & nitrate salts.
Effect on eco system & health
- Atmospheric particles due to fuel combustion can scatter and absorb sunlight &reduce visibility.
- Increased level of CO2 due to greenhouse effect.
- CFC & NOx cause OZONE layer depletion & hole
- Toxic effects of particulate matter on animals & human being
- Intrinsic toxicity due to chemical or physical properties .Ex-CO can combine with hemoglobin and reduce its oxygen carrying capacity
- HbO2+CO =HbCO+O2
- Interference with clearance mechanism in respiratory tract
- Toxicity due to absorbed toxic substances lead from vehicle exhaust in higher dose can kill out right but in lower dose shorter life span.
- Oxides of Sulphur & nitrogen combine with vapour of atmosphere to cause acid rain.
- Benzopyrene can play important role in higher cancer rate.
Basic of air pollution control equipment
Emission control equipment may be classified into two general types
- Particulate control type
- Gas & odour control type
Particulate control type
Particulate matter emitted in a gaseous stream consists of discrete and minute suspended particles, particular size range generally 1oo μm to 0.1 μm. The choice of collection device depends upon a number of factors. The physical and chemical characteristics of particle, size, and concentration in the gas, volume to be handled, temperature & humidity of gaseous medium. The basic mechanism of removing particulate matter from gas stream may be classified as:
- Gravitational settling – Gravitational settling chamber
- Centrifugal impaction –Cyclone separator
- Inertial impaction -Fabric filter
- Direct impaction – Fabric filter
- Diffusion -Fabric filter
- Electrostatics precipitator (ESP)
Electrostatics precipitator (ESP)
ESP is one of the most widely used devices for controlling particulate emission at industrial installations.
Electrostatics precipitator is a physical process by which the particles suspended in a gas stream are charged electrically and under an electrical field separated from the gas. The precipitation system consists of a positively charged collecting surface and a high volt discharge electrode wire suspended from an insulator at the top and held in position by weight at the bottom. At a very high DC voltage of the order of 50KV, a corona discharge occurs close to the negative electrode, setting up an electric field between the emitter and a grounded surface. Gas enters near the bottom and flows upward. The gas close to the negative electrode, thus ionized upon passing through the corona as the negative ions and electrons migrate towards the +ve charged grounded surface. They in turn charge passing particles. Periodically the collected particle must be removed from the collecting surface. This is done by rapping. Usually, a large number is placed in parallel in a single housing.
The fabric filter system typically consists of a tubular bag suspended in such a manner that the collected particles fall into a hopper when dis-loaded from the fabric. The structure in which the bags hang is known as a baghouse. The particle-laden gas enters the bag at the bottom and passes through the fabric while particles are deposited on the inside surface of the bag.
A centrifugal force generator by spinning gas stream to separate particulate matter from the carrier gas. The centrifugal force on particles in a spinning gas is much greater than gravity, therefore cyclones are effective in removing much smaller particles than gravity settling chambers.
In operation, the particle’s ladder gas upon entering the cyclone receives a rotating motion. The vertex so form develops a centrifugal force, which acts to throw the particle radially towards the wall. The gas spirals downwards to the bottom of the cone and at the bottom, the gas flow reverses to form an inner vertex that leaves through the outer pipe.
The basic function of a wet scrubber is to provide contact between the scrubbing liquid (usually water) &the particulate to be collected; there are many scrubbers’ designs available where the contact between the scrubbing liquid s particles is achieved in various ways. They are :
- Spray tower
- Centrifugal scrubber
- Packed bed
- Ventury scrubber
Dust suppression system
There are two types of techniques by which gaseous pollutants may be removed from an effluent stream.
- Absorption of pollutant (i.e. absorption or adsorption)
- Chemical alteration of pollutant by catalytic treatment
Absorption by liquid
Absorption is one of the most frequently used techniques which for controlling the concentration of gaseous pollutant before they are discharged into the atmosphere. It involves the transfer of pollutants from the gas phase to the liquid phase across the interface in response to a concentration gradient, with concentration decreasing in the direction of mass transfer.
Gas absorption equipment
- Packed tower
- These are very efficient absorption devices, involving a continuous contact of 2 phases. They use variety of packing of material like ceramic packing to crushed rock. The liquid is distributed over the packing there by increasing surface area of contact .The liquid and gas flow opposite to each other .
- Tray tower
- Venturi scrubber
Adsorption by solid
An alternative to absorption by liquid is the adsorption of pollutants on solids. Adsorption is a surface phenomenon by which gas or liquid molecules are captured & adhere to the surface of the solid absorbent. An adsorption technique is widely used in the field of odor control for removing small quantities of pollutants.
Monitoring techniques of air pollution :
There are three kinds of air quality monitoring
a) Stack emission monitoring kits
b) Fugitive emission monitoring
c) Ambient air quality monitoring
Stack emission monitoring equipment
THIS EQUIPMENT has two basic components
- Particulate matter collection equipment assembly
- Gaseous sampling train–During gas sampling, samples were collected from Isokinetic location with help of sampling dust, particulates were measured by gravimetric method, which gaseous pollutants measured by respective spectro photometric method.
Fugitive emission technique
When pollutants enter the ambient air without the aid of a primary emission system like stack or any other air pollutant generating system, then such emissions are called fugitive emissions. This requires innovative methods like the roof monitoring method, upwind/downwind sampling method. In general upwind/downwind sampling methods are most commonly used. The sampling is done by a high-volume sampler of both particulate & gaseous pollutants.
Ambient air quality monitoring
Air quality sampling was made at pie selected sites with help of a high-volume sampler. 8 pm is collected by filter paper placed against the suction pump. The clean gaseous samples are then collected in gas impinge for absorption of pollutants using respective solutions.