Much media attention has been placed on how the environment becomes polluted but not much emphasis has been placed on how these pollutants are cleaned up.

When it comes to cleaning up pollutants, most people would be familiar with the word "bioremediation" which strictly speaking, applies to the use of microorganisms which remove pollutants through their metabolism. But not all pollutants can be removed through bioremediation. In this week's article I will to focus on the methods used to clean up the pollutants in our air and their advantages and disadvantages. Next week I will focus on how water pollutants are removed.

When it comes to removing particles from the air that contribute to air pollution, scrubber systems are employed. The term scrubber is used to describe a system where liquid is used to wash pollutants from a stream of gas.

But it is also applied to describe systems where dry reagents or slurries are used to wash acid gases. Scrubbers are used in conjunction with combustion.

This may sound strange as combustion is one of the major contributors to air pollution, but it can also be used for exhaust gas cleaning, where the pollutant particles or gases are in the combustion. Scrubbers can be of two types, wet and dry.

Wet scrubbers are used to clean air or flue gases or other gases from dust particles and other pollutants. This type of scrubber works by saturating the gas stream with moisture by spraying a solution which will make contact with the particulate matter or with specific compounds in the air or gas to be cleaned. For instance, to remove dust particles the solution used would be water. Other water soluble gases and/or corrosive gases that can be removed by this method include ammonia and hydrogen chloride.

Wet scrubbers are also used to remove toxic pollutants. Heavy metals like mercury, commonly found in coal and municipal waste, but only if halogen gas is added. This method can also be used to harness the heat from the gases that are being cleaned. The hot gas enters the bottom of the scrubber and as the vapour condenses it cools the gas and the water adds to the amount or circulating water for the cleaning substances, however the excess water must be removed.

The dry scrubbing technique on the other hand is quite different in that either no moisture is added or what little moisture is added will evaporate without condensing. This technique is used to remove acid gases like sulphur dioxide, which is the cause for acid rain and hydrogen chloride and for the removal of corrosive or odorous gases from wastewater treatment plants.

Dry scrubbers use two main types of devices, where the first introduces the gas absorbent into the gas stream, usually an alkaline material like soda ash or hydrated lime. Then an acid meets an alkaline the acid becomes neutralised and in this case forms a salt which is then removed. This method is used in medical waste incinerators and some municipal waste combustors. However, that amount of sulphur dioxide and hydrogen chloride removed is limited.

The second type of dry scrubber device is a spray dryer absorber where using an absorbing tower is this case the drying the gas makes contact with alkaline slurry. Again the reaction produces a solid salt which is easily removed. This method has a higher success of cleansing the gas and is used in municipal waste incinerators and industrial boilers. Another type of dry scrubber is simply through the use of absorbent material, so that the gas to be cleaned is passed through a cartridge filled with one or more absorbent materials.

Therefore it can be seen that pollutants found in the air can be cleaned through the use of systems called scrubbers, which use moisture (wet scrubbers) and dry absorbent material (dry scrubbers).

This method of cleaning the air from pollutants can also harness heat which can be used as an alternative energy source. However, these cleaning systems need to be monitored and maintained as they are prone to disease-carrying bacteria which can be spread to humans.

Next week I will be looking at some ways of removing pollutants from water.