It’s the need of the hour to take measures for air pollution control and prevention since millions of Indians are constantly exposed to polluted air. For instance, they breathe up to 25 micrograms/cubic metre of the lethal, microscopic pollutant PM 2.5 on a 24-hour average. This is well above the World Health Organization’s (WHO) limit of 10 micrograms/cubic metre.

To begin with, let’s first look at the causes of air pollution.

Air Pollution Causes

Industrialisation and urbanisation have technologically upgraded our lives. But, they had some negative byproducts like degradation of the environment, air pollution etc. The air quality index in most of the metropolitan cities is alarmingly high. Let’s have a quick glimpse at some of the major causes of air pollution:

• Emissions from Industries and Power Plants
• Construction and Demolition
• Mining
• Vehicular Emissions
• Burning of waste and stubble

If you wish to dig deeper into the causes of air pollution, please check our blog Air Pollution Causes – A Comprehensive Guide

 If we can prevent the release of toxic gases by removing them from the flue gas stream or converting it into harmless compounds, we can control air pollution to a great extent. Similarly, if we can collect the dust and dispose of it properly, we can avoid particulate pollution. 

Also read: Air Pollution Effects and Causes – A complete overview

Air Pollution Control Measures and Devices

Air pollution control equipment refers to devices and facilities used in industries to control and prevent the emission of particulate matter and toxic gases. Fans or blowers direct industrial emissions and pollutants into air pollution control equipment and systems. Subsequently, they eliminate or reduce air pollutants using one or more of the following procedures:

• Combustion i.e., destroying the pollutant.
• Conversion i.e., chemical conversion of the pollutant to a less harmful compound.
• Collection i.e., removal of the pollutant from stack gas before releasing into the atmosphere.

Having understood the basic mechanism of pollution control devices, let’s have a closer look at each of the devices.

Scrubbers

• Scrubbers are the most widely used air pollution control devices in production and manufacturing facilities.
• They use a physical process called scrubbing to remove particulates and gases from industrial emissions before releasing them into the atmosphere.
• Scrubbers are of two types: dry scrubbers and wet scrubbers.

Dry Scrubbers

• Dry scrubbers inject dry, neutralising chemical agents such as sodium bicarbonate into the exhaust stream.
• Subsequently, the gaseous pollutants undergo a chemical reaction that either neutralises or transforms the pollutants into harmless compounds.
• When the chemical reaction is finished, the expended agents are collected and removed from the cleansed emission gas by filters within the scrubber chamber.
• Dry scrubbers are typically used to neutralise acid gas in oil refineries, wastewater treatment plants and metallurgical plants

Wet Scrubbers

• Also known as wet adsorption scrubbers or wet collectors.
• Wet scrubbers capture and remove water-soluble gas and particulate emissions from industrial emissions using liquid solutions—typically water.
• A gas stream is passed through a liquid solution or a liquid solution is injected into a gas stream in the wet scrubbing process.
• The solution on coming in contact with the gas stream absorbs the pollutant.
• This process eliminates the pollutants from the gas and clean gas is released into the atmosphere.
• The types of wet scrubbers include venturi, packed bed and bubbling scrubbers.
• Flue gas desulphurisation employs wet scrubbing with a slurry of alkaline sorbent, usually limestone or lime.

Air Filters

• Air filters are air pollution control systems that use a certain type of filtration media such as fabric, sintered metal, ceramic, etc.
• They capture and remove dry particles and contaminants from air passing through them, such as dust, pollen, microorganisms, chemicals, and so on.
• These devices remove pollutants from exhaust air and enhance the air quality in residential, commercial, and industrial buildings.
• There are various types of air filters available for industrial purposes, including HEPA filters, fabric filters, and cartridge dust collectors.

Bag Filters

• Also known as baghouses or fabric filters.
• The bag filter uses cylindrical fabric bags to trap and remove dust and other pollutants in the air.
• Particulates aggregate on the filter’s surface as the polluted air travels through a baghouse.
• This particle buildup improves the filter’s efficiency by reducing the surface area of openings.
• This allows even smaller particles to be collected.
• Fabric filters usually offer collection efficiencies exceeding 99.9%.
• These filters find wide applications in industrial processes, such as power plants, metal processing centres, and foundries.

Periodic cleaning is crucial due to continuous dust accumulation and the associated pressure differential. Baghouses use a variety of methods to remove the accumulation from the filter bags, including:

• Shaking the filter bags.
• Increasing the air pressure on the bag such that the bag collapses or deforms and dislodges the accumulated dust. 

Particulates fall from the filter cloth to the bottom of the baghouse enclosure into a collection hopper for processing and disposal.

HEPA Filters

• Also known as high-efficiency particulate air filters.
• These filters use fibreglass filter mats to physically remove airborne particulates like pollen, smoke, dust, and bio-contaminants from the workspace.
• Fibres in fibreglass filter mats typically range in size from 0.5 to 2 metres.
• According to the US Department of Energy (DOE), a filtering system maintaining a 99.97% efficiency for collecting particulates more than or equal to 0.3 m in diameter can be designated as a HEPA filter.
• Widely used in pharmaceuticals, computer and electronics manufacturing, aerospace applications and nuclear power plants.

Cyclones – Air pollution control measures

• Cyclones, also known as cyclone dust collectors, are air pollution control equipment that collects and remove particulates using centrifugal force.
• When gas streams enter a cyclone, they spiral around the cylindrical chamber.
• The centrifugal force experienced by the spinning gas stream is considerably higher than gravity.
• Hence the centrifugal force throws the larger particles against the chamber wall, slowing their inertia and forcing them to fall into the collection hopper below.
• The treated gas streams proceed upward and out of the cyclone.

The separation factor of a cyclone is defined as the ratio of centrifugal force to gravitational force. The higher the separation factor, the better is the cyclone performance. 

Electrostatic Precipitators

• Electrostatic precipitators (ESPs), like air filters and cyclones, collect and remove particulate matter, such as dust, from industrial emissions and exhaust.
• ESPs establish a large static electrical potential difference between charging electrodes and collecting plates, using transformers.
• At very high DC voltages of the order of 50 kV a corona discharge adjacent to the negative electrode.
• This creates an electric field between the positively charged collecting surface and the emitter.
• Consequently, the electric field ionises the dust particles as the particle-laden gas flows upwards.
• The electrostatic force directs ionised particles towards the grounded plates.
• Particulate Matter deposit is periodically removed from the collecting plates and dumped in a collection hopper below.
• Wet ESPs uses water to rinse off the dust particles.
• ESPs’ efficiency reaches 99% since they have several collection plates.
• The Deutsch equation gives the collection efficiency of an ESP.

Mist Collectors

• Also known as mist or moisture eliminator filters.
• These air pollution control devices remove moisture and vapour from gas streams, such as smoke, oil, mist, etc.
• Fine mesh-like filters separate liquid droplets from gas and collect them in a separate chamber.
• Finds wide applications in food and chemical processing, desalination plants, paper and pulp mills etc.
• For submicron liquid particles, mist collectors have exceptional filtering efficiencies, with some collectors offering 99.9% efficiency for particles 0.3 μm in diameter.

Shall we wrap up?

Conclusion – Air pollution control Measures

In this blog, we saw some air pollution control measures and For the effective control of air pollution, the National Green Tribunal and the pollution control boards should strictly monitor and ensure the usage of these devices in the industries. We are still in need of green technologies like solar cells for power generation instead of coal-fired power stations, clean coal technologies, electric vehicles etc. Together, it is possible to reduce and control air pollution for a green future.