Bioremediation is often used in "clean-up" operations and is a term widely used to describe how pollutants are removed from our environment but it is often used in the wrong context.
Last week I looked at how water is cleaned from pollutants. This week I will focus on why dust emissions are an environmental problem and some of the methods employed to remove excess amounts of dust from the air.
Dust particles enter our environment from activities such as quarrying, mining, driving on unpaved roads, or through the use of specialised cleaning techniques like abrasive blasting or sand blasting as it is more commonly called. Human activities are not the only source of dust, large quantities can be generated from natural sources, for instance, pollen, from volcanic eruptions or from wind blowing across dry riverbeds.
Studies have shown that some of the effects experienced by plants where the leaves are coated with dust include; increased growth of bacteria and fungi, decreased photosynthesis, higher temperatures due to increased absorption of infra-red radiation and increased water loss, however, the amount of water lost is dependent on the particle size of the dust. In particular decreased photosynthesis is detrimental as it impedes the growth of the plant, that is, to produce more leaves which mean a greater surface area to process more carbon dioxide. Also the amount of carbon dioxide removed by the existing foliage will be decreased. Plants are also at the bottom of the food chain, termed producers, that is they capture energy from sunlight which is passed along when it consumed to other animals, however, being dust-coated will make it unsuitable for consumption. Therefore when the plant dies its nutrients will be released where it stood and supply other plants and animals that feed on decaying matter, rather than being passed higher up the food chain. Huge amounts of dust in our atmosphere can also lead to serious health problems in humans and animals.
Air pollutants are removed though the use of devices referred to as dust collectors. A dust collector consists of four main parts; a blower, dust filter, filter cleaning system and a receptacle to collect and remove the dust gathered. There are five types of dust collectors; inertial separators, fabric filters, wet scrubbers, electrostatic precipitators and unit collectors. The first type, inertial separators use a variety of forces to remove the dust from the air. For instance, settling chambers, which are a type of inertial separator, are large structures made out of a variety of materials, these structures suddenly expand which causes the air speed in the duct to decrease thus allowing for heavier dust particles to settle and fall out of the air flow.
Baffle chambers are another type of inertial separator where the presence of a plate causes the air flow to suddenly change direction, so that the larger, heavier particles continue on their original path into an empty space where they settle and are removed. The final type of separator is a centrifugal separator. In this type, the air enters from an angle and rapidly spins as in a cyclone or hurricane. This action has the effect of throwing the dust particles on the side of the collector where it falls into a collector bin. This principle is used in some vacuum cleaners. However, the first two collectors are unfortunately not very efficient as collectors and are more used as pre-treaters, that is, they remove the larger dust particles from the air stream.
Another type of collector is fabric collectors, commonly called baghouses and has a collection efficiency of over 99%. The dust laden gas passes through fabric bags which can be made of fibre glass, woven cotton or synthetic material. The dust collects on the surface of these bags as the air passes through. However, baghouses must be cleaned periodically. The next type of dust collectors are wet scrubbers. I will not go through wet scrubbers in detail as they were dealt with in part one, last week. The term scrubber is used to describe a system where liquid is used to wash pollutants from a stream of gas. Wet scrubbers are used to clean air as well as flue gases from dust particles and other pollutants. In order to remove dust particles, the air stream is saturated with water which makes contact with the particles.
Then there are electrostatic precipitators or ESP for short, which basically consist of collecting electrodes sandwiched between discharge electrodes. A high voltage direct current is passed through the discharge electrodes when the exhaust gas passes through the collector, it forms a negative charge and is attracted to the collecting electrodes and cling to it. The dust is removed from the system by vibrating the electrode which can be done during the collection process.
The final type of collector is unit collectors. These are small, self-contained units consisting of a fan and one of the four types of dust collectors mentioned. This is not used in general dust collecting but rather to get dust from its source, for example, from a silo, when grain or aggregates are on a conveyor belt or being transferred. Unit collecting can be of two types; fabric or cyclonic. The former is used to collect fine dust while the latter is used for larger dust particles, like metal grindings and for wood working.
So it can be seen that dust pollution not only affects our health but our environment at a very basic level in our food web. However, if dust filled waste from industries are treated before being released into our environment then all organisms on the planet can enjoy a healthier and more productive lifestyle.