Environment of Earth
August 11, 2008
PLANTS AS AMELIORATORS (MITIGATORS) OF POLLUTION
Many
types of higher and lower plants and microorganisms have the capability
to tolerate and absorb large amounts of gaseous, liquid and solid
pollutants from the environment and decompose them into harmless
substances. This ability of such organisms can be usefully exploited in
the control of environmental pollution.
Plants
cover about one third of the land area as forests and grasslands and
another one third as agricultural crops. Large surface area of ponds,
lakes, rivers and sea coasts is also covered with aquatic flora. All
this vegetation provides huge surface area that can be gainfully used to
trap pollutants from the environment. In this large vegetational cover,
those plant species that can absorb large quantities of pollutants and
accumulate them in their tissues without damage (tolerant-accumulator species)
provide natural storehouses or sinks of various pollutants without any
cost. Such plants are very important resource materials in environmental
pollution control strategies. Generally, woody plants absorb more
pollutants than herbaceous plants. Actively growing tissue of wood
absorbs larger amounts of pollutants than dormant tissues. Therefore,
trees are comparatively more useful than herbaceous plants in pollution
control.
Use of tolerant-accumulator plats in the control of air and soil pollution involves:
- Identification of pollutant(s) present in the area,
- Identification of tolerant-accumulator plant species suitable for the climate and the pollution problem of the area and then
- Plantation of identified plant species in the area on available bare lands, fallow lands, community lands, waste lands, along roads, railway lines, canal banks as well as green belts, shelter belts, wind breaks, city forests, parks etc.
Trees
reduce the velocity of air passing through them thus facilitating the
absorption of pollutants by the foliage. Establishment of
tolerant-accumulator epiphytic mosses and lichens on the trees may also
help in absorption of pollutants. Many microorganisms that can degrade
pollutants to harmless substances are also established on the leaves as
phyllosphere. Many plants absorb pollutants from the soil and thus check
their runoff into water bodies or their leaching into the groundwater.
Use
of plants in the control of water pollution mostly involves treatment
of sewage an industrial effluents before their release into lakes or
rivers and treatment of polluted water bodies. Introduction and
maintenance of accumulator aquatic plants like Eichhornia, Azolla, Cladophora, Fontinalis squamosa
etc. in the ponds is very effective in cleaning polluted water and
keeping them free of organic, chemical and metal pollution. Most
important sewage and effluent treatment systems using plants are:
Peat moss treatment systems: Many types of systems have been designed to treat urban sewage and industrial effluents using Sphagnum
moss. These can treat upto 91,000 litres of effluent per day. Effluents
are passed through columns containing the moss, which absorbs metals
and the used up moss is then destroyed. Pre-treatment of moss with CaCO3 increases the metal absorption.
Eichhornia treatment systems: Diluted sewage or industrial effluent is passed through a zig-zag system of ponds and canals in which Eichhornia plants are grown. These plants asorb pollutants and retain them in their rhizomes. Plants from these hyacinth lagoons
are regularly harvested and can be used to produce biogas in suitable
digesters or after extraction of absorbed metals, can be used in making
paper, boards etc. In the lagoons, the growth and death rates of plants
are continuously monitored and managed by regulating the dissolved
oxygen, biological oxygen demand and chemical oxygen demand, pH,
turbidity and conductivity of the water at different stages of cleaning.
Microbial treatment/biogas systems:
Treatment systems for organic sewage use suitable aerobic/anaerobic
decomposer bacteria, ammnifying and sulphur bacteria in anaerobic
lagoons. The organic matter in he polluted water is decomposed producing
methane and carbon dioxide. In such systems, combination of decomposer
bacteria and algae like Spirulina, Scendesmus, Clorella etc.
can also be used. The decomposed organic matter is used up by algal
growth. These protein-rich algal cultures can be regularly harvested and
used as cattle feed. In the biogas systems, anaerobic bacterial
decomposition of organic matter produces combustible gas. The freed
metals and other pollutants are removed and clean effluent is used as
rich fertilizer.
Many bacteria can absorb metals (e.g. Thiobacillus ferrooxidans and T. thiooxidans absorb copper), degrade petroleum oil (Pseudomonas sp.)
or other chemicals. Genetic engineering is being applied to create new
and more efficient strains of bacteria for use in mining and industries
to control metal pollution, to clean oil spills, to degrade herbicides
and pesticides in the soil and to absorb SO2 from chimney gases before their release into the atmosphere. Pseodomonas, Staphylococcus and a cornybacterium that accumulate metal bearing particles like silver, Aspergillus and Penicilium that concentrate uranium and phosphate are examples of such useful bacteria. Sulphur bacteria like Chlorobium, Chromatium and Thiospirillum may be helpful in control of SO2 pollution of air.
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