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.