Describe briefly various methods to control particulate matter in air pollution.

Explanation

(i) Mechanical Devices.
(a) Gravity. In this process, the particles settle down by gravitational force.
(b) Sudden change in direction of the gas flow. This causes the particles to separate oụt due to greater momentum.
(ii) Fabric Filters. The gases containing dust are passed through a porous medium. These porous media may be woven or filled fabrics. The particles present in the gas are trapped and collected in the filters. The gases forced from the particles are discharged.
(iii) Electrostatic Precipitator. It has electrode wires which are maintained at very volts, produce a corona that releases electrons. When a gas or an air stream containing aerosols in the form of dust or fumes is passed between two electrodes, then the aerosol particles get precipitated on the electrodes, giving them a net negative charge. The collecting plates are grounded and attract the charged dust particles. The velocity of air between the plates must be low erough to allow the dust fall.
(iv) Scrubbers. There are used in chemical, mining and metallurgical industries to remove gases like `SO_2, NH_3` and metal fumes etc. In a scrubber, the exhaust is passed through a spray of water or lime However, very very small particulates are not removed by these scrubbers.