Air pollution can be minimized by the following methods:
- Siting of industries after proper Environment Impact Assessment studies.
- By dilution of emissions. This can be done by increasing the stack height (though up to permissible height) beyond inversion layer. Wind current will disperse the pollutants. But this results in interstate dispute and is not considered to be solution for air pollution problem.
- Minimize activities which cause pollution like transportation and energy production.
- Modification of process and/or equipment.
- Use of appropriate material.
- Using low sulphur coal in industries.
- Removing sulphur from coal (by washing or with the help of bacteria).
- Removing Nox during the combustion process and controlling the flow of air and fuel in industrial boilers.
- Vehicular pollution can be checked by regular tune-up of engine; replacement of more polluting old vehicle; installing catalytic converters; by engine modification to have fuel efficient(lean) mixture to reduce CO and hydrocarbons emissions; and slow and cooler burning of fuel to reduce NOx emission (Honda technology ).
- Using mass transport system, bicycles etc.
- Shifting to less polluting (clean) fuels (hydrogen gas).
- Using non-conventional sources energy.
- Using biological filters and bio-scrubbers.
- Planting more trees.
- Reduction of pollution at source.
Reduction of air pollution at source
Gaseous pollutants: Gaseous pollutants can be reduced by physical adsorption on porous solid materials like activated charcoal, silica gel, Fuller’s earth, etc. Effluent gases can be absorbed in liquid absorbent, e.g. SO2 absorbed in ammonia solution. They can be removed by condensation which is carried out by cooling medium in tubes where the gases in contact condense and can be collected thereafter. Combustion can be used to reduce pollution by burning the pollutants in combustion equipment at optimal condition of oxygen and temperature.
Particulate matters: Many devices are available nowadays, choice of which depends on characteristics of particulate, flow rate, collection efficiency, costs, etc.
Cyclones separator: it consists of a cylinder with an inverted cone attached at the bottom. The gas with particles in it enters tangentially at the top of the cylinders and spins forming a vortex. Due to centrifugal force, the particles strike the wall of the cylinder. The particles then fall in the hopper due to gravity from where they are removed. The spinning gas forms an inner vortex and leaves from the top. The cyclone is very efficient for removal of larger particles. However, smaller particles which pose human health problems are not removed efficiently. Therefore, cyclones are employed before the use of other costly devices.
Bag house filters: A bag hose filter contains a large number of filter bags made of fabric. They are hung upside down in several compartments of bag hose filter. Dirty gas is passed through the filter bags which leaves the bags through their pores. The dust particles get deposited on the inner surface of the bag filters and many from a cake which can be removed by shaking. The device is efficient for removal of very small particles and is preferred in various types of industries. The bag house filters are expensive and cannot be operated for moist gases. Corrosive gases may damage the material of the bags. Various types of materials, depending on the nature of the flue gases to be cleaned, are used for making the filter bags.
Wet scrubbers: Dirty gases are passed through water in the chamber or water is sprayed on the gas. Particles are made wet and are removed from the gas stream which leaves from the top of the scrubber. Wet scrubbers are very efficient for removing the particulates. The scrubbers are very useful for removal of toxic and acidic gases also.
Electrostatic precipitators: the electrostatic precipitators may be plate type or cylinder type. Vertical wires are placed between the parallel plates or wire is hung along the axis of the cylinder. High negative voltage is applied to the wire. Dust particles while passing from the lower end get negatively charged (ionized) and are collected on the positively charged surface (plates/cylindrical body) while the clean gas leaves from the top.
The deposited dust particles fall down in the dust collector or are removed by scrapping or by liquids. Electrostatic precipitators utilize electric and can efficiently remove even sub microscopic particles.
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