The control of air quality implies limiting the emission of pollutant. The conventional approach is usually called ‘end of the pipe approach’ which takes for granted the emissions coming out from the source and a suitable equipment is then designed and installed to meet the pollutant emission standards.
However, a more rational approach would be: to prevent generation of pollutants first and then control whatever is technically or economically not preventable. The control systems for industrial emission depend on the type of pollutants. The most common types of control systems for suspended particulate matter (SPM) are:
- Electrostatic Precipitators
- Bag Filters
These are the most conventional types of control system. These work on the principle of dust separation by centrifugal force. The dirty gas enters tangentially in the cyclone and swirls in it. The dust particles get thrown on the periphery and clean gas escapes from the centers of the cyclone. Cyclone usually works at a dust separation efficiency of 70-75 percent. The efficiency drops down sharply as the particle size goes below 10 microns. Use of cyclone is not suggestible for moisture-laden gas.
ii) Electrostatic Precipitator (ESP)
The ESPs are now widely used for separations of dust in large industries like thermal power stations, cement industries, etc. They work on the principle of charging the dust by application of high voltage electricity to settle the particles down. The ESPs are simple in operation and most cost effective in larger sizes. They work with efficiency in the range of 99 percent or even higher.
iii) Bag Filters
Of late, tremendous developments have taken place in the field of bag filters. As the name implies these, consist of filter bags through which dust laden gas is passed. The dust gets filtered and clean air escapes. Bags are periodically shaken to remove the dust collected. Different types of filter bag materials are available to suit various types of dust. The common bag materials are cotton, polyester, glass wool, Teflon, polyamide and of late, ceramic fiber. Therefore, these filters are used by ceramic, cotton, insulation and textile industries.
Another method of separation of dust is by using scrubbers, which also are the only equipment for separation of gaseous pollutants. A scrubber is an apparatus used in air sampling or cleaning, in which air is passed through a space containing wet packing materials or spray. The gaseous pollutants are removed principally by scrubbing. The process consists of absorbing the gaseous pollutant in appropriate liquids to bring the pollutants from gaseous phase to liquid/solid phase. The liquid is used to remove dust particles and soluble gases from the air system.
Various types of scrubbers are available. To name a few, spray type scrubbers, ventury scrubbers, impingement scrubbers etc. Scrubbers can also be used for removal of particulate matter, their efficiency ranges from 80 to 98 percent. The principal disadvantages are that the gaseous pollutant gets converted into liquid/solid phase and have to be further treated before disposal. Scrubber operation is generally a costly affair as it consumes a large quantity of water and power.