After reading this article you will learn about Bio Monitoring of Air Pollution:- 1. Monitoring Techniques of Air Pollution 2. Bio Monitoring of Air Pollution 3. Control Devices.
Bio Monitoring of Air Pollution:
The concept of monitoring of air quality by plants is a well-established fact. The plants used for this purpose are termed indicator plants. It is known that some plants are very sensitive to air pollutants; they are thus used as indicator species for bio monitoring of air quality. In recent years, increasing efforts are being made to use plants for detection of air quality, particularly the presence of SO2, NOX, H2S, O3 and HF.
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The use of lichens and moss for air pollution level mapping in urban and industrial area of Europe and also in North America are the finest examples of plant Bio monitoring of air quality.
The use of higher plants for monitoring of air pollution is, however, a recent development. A number of plant parameters either simply or in combination may be used for evaluating the pollution stress.
Table 10.19 shows the list of plants species used for bio monitoring of different air pollutants.
In India, considerable work has been done on the effects of air pollution on plants at BHU (Varanasi), JNU (New Delhi), NBRT (Luck-now), CIJ and KU (West Bengal) and AU (Vishakhapatnam) in recent years.
This method now appears to be a sensitive and low cost technique, only appropriate standardisation of techniques has to be made.
Identically, in Europe, similar type of plant species selection was made for air quality bio monitoring (Table 10.20).
The use of lichen indicators for air pollution monitoring was attempted in urban environment during recent decades by Santra and Mitra (1990).
By examining the pattern of lichen patch occurrence, an index of air pollution was determined in different sites of Kolkata city (WB). Thus an air pollution zone map was prepared (Fig. 10.17). Similar attempt was also made in Haldia industrial zone. The details are given in Box subsequently.
Air Pollution Control Devices:
In order to reduce the pollution load entering the environment from industries (stationary sources), several measures may be taken.
These measures include replacement of burning fuel by electricity or solar energy or by improvement of fuel quality and fuel burning processes. Besides such innovations there are a number of mechanical devices that might be helpful for reduction of pollutant emission level from stationary sources.
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There are two categories of devices that are often used by the polluting industries, viz.:
(a) Those devices which help in reducing particulate matters; and
(b) Those devices which help in reducing gaseous pollutants.
Control of particulate pollutants:
The important devices which are used to control particulate matters are:
(i) Gravity settling chamber,
(ii) Centrifugal collectors, viz., cyclone collectors and dynamic precipitators;
(iii) Wet scrubbers, viz., spray towers and venturi scrubbers;
(iv) Electrostatic precipitators, and
(v) Fabric filters.
i. Gravity Settling Chambers:
These are the oldest and simple type of particulate collector (Fig. 10.19). Particles of size between 40-100 µm in diameter are readily collected by this technique. Their efficiency is very poor ort fine dust and decreases as the load increases. They have a very large physical size, but, on the other hand, they are inexpensive and reliable. The cost of equipment is very low and very easy to maintain.
ii. Cyclone Collector:
It operates on the principle of centrifugal force. As the carriages enters the cylindrical shaped collector, it takes a helical path and the inertia of the particles carries them to the walls from where they drop into a hopper at the bottom.
The clean gas rises near the centre and is exhausted through the top. The cyclone collector is quite simple and reliable with low initial cost, easy maintenance and high temperature capabilities. Cyclone collectors tend not to be efficient on particles smaller than 10 nm and are best for 15-50 µm particles (Fig. 10.20).
iii. Dynamic Precipitators:
Dynamic precipitators (Fig. 10.21) also operate on the principle of centrifugal force. They are really a combination of a specially designed centrifugal form and a dust collector. The centrifugal force generated by the rotating blades probes the particles in the air stream to the tips of the blades, from where they are drawn off in a concentrated stream.
It is fairly efficient, even on particles 5-20 µm in diameter. However, this type of collector is unsuitable for sticky or fibrous materials, for the solids tend to build up on the blades.
iv. Spray Towers:
There are two types of spray towers: open spray towers and packed towers. Open spray towers (Fig. 10.22) are best for coarse particulate (> 10 µm in diameter) such as attained in the iron pyrite roasting step of steel production. They tend to be good for heavy particulate loads.
While, a packed tower consists of a series of contact beds, through which the gases and liquids flow either counter current or cross flow. A typical application would be for the absorption of fluorine compounds.
v. Venturi Scrubbers:
In the venturi scrubbers, the dirty gas and scrubbing liquid are moving at very high velocities (100 – 450 mph), generating much turbulence and mixing, and thus exhibiting good collection efficiency.
There are two types of venturi scrubbers:
Dry and wet type.
The dry types (Fig. 10.23), are not really dry, where the dirty gas enters, concentrates, and speeds up due to the narrowing passageway, is dry, but as the gas passes through the constricted area, it is inundated by a transverse water spray.
The wet types, on the other hand, require that both collector sections be wet. The water spray contacts the dust as soon as it enters the collector and the two mix immediately. One advantage of this immediate mixing is to allow collection of hotter gases.
Venturi scrubbers are very efficient (better than 99%) on particles as small as 0.5 µm. As such, they are typically used in the steel industry for fine particulates, for acid mists and for lime and coal dust (whose size is often 1 µm or less).
Electrostatic precipitators: The electrostatic precipitator is a very versatile and efficient type of collector, which can operate at over 1,000 °F (540 °C) and can be used for dry particulates or fumes, as well as mists. Electrostatic precipitators (Fig. 10.24) operate on the basis of electrostatic attraction.
The dirty gas is channelled between two electrodes, a high voltage (between 40,000 v and 50,000 v) discharge electrode and a grounded collecting electrode. Usually a precipitator will have more than one set of electrodes, thus the dust can be collected in different “compartments“. Precipitators allow for the dry collection of very fine particles. They are highly efficient, often approaching 99.9% efficiency.
