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Bioaugmentation - Many low molecular weight compounds are not metabolized at significant rates by the natural microorganisms.
These chemicals are collectively termed as recalcitrant chemicals.
They include solvents, refrigerants, propellents, pesticides and herbicides. Usually they, persist in the environment.
The presence of large quantities of herbicides, pesticides and other recalcitrant chemicals for a longer period in the environment exerts a selective pressure on the microorganisms.
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As a result, microorganisms capable of metabolizing a few of these compounds have emerged naturally.
However, metabolism of these recalcitrants is taking place at a very low rate.
Hence, much research activities have been focussed on the growth and activity of these microorganisms to speed up degradation. This involves a number of processes.
Adding excess nutrients to a contaminated site is one of the methods. Screening processes have been developed to identify microorganisms that can effectively degrade the recalcitrants.
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Modified laboratory techniques have also been employed to improve the efficiency of biodegradation by microorganisms.
In one such technique, a microorganism, which can metabolize a naturally occurring compound that is molecularly related to the non-biodegradable synthetic material, is grown in continuous culture gradually substituting compounds with increasing similarity to the target substance.
Finally the microorganism develops the ability to degrade the non-biodegradable synthetic material. Other techniques employ genetic engineering to develop a bacterium with degrading ability.
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Bacteria can be genetically modified so that it can convert the recalcitrant chemical into useful products. Genes coding for the biodegradation characters from different bacteria can be put into one bacterium, enabling it to degrade a wide range of recalcitrants.
The introduction of microbes into a contaminated environment to bring out detoxification is termed bioaugmentation. It has a wide range of applications in environmental pollution.
Addition of bacterial formulations to wastewater treatment plants, which are not performing well due to their biological conditions, is a well known example of bioaugmentation. Problems due to bulking, deflocculation, poor BOD removal of specific compounds such as phenols can be rectified using bioaugmentation.
The bacterial formulation consists of freeze dried bacterial suspensions generated from pure cultures.
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These suspensions are blended to produce a mixture of different species together with wetting agents, nutrients, enzyme preparations and other ingredients. Usually large doses of preparations will be used. But frequently smaller follow-up doses are required because the organisms are gradually lost from the system.
Bioaugmentation is also finding use in the detoxification of halogenated compounds such as the highly carcinogenic polychlorinated biphenyls (PCBs). They are the major contaminants of the groundwater. Microorganisms act on these compounds freeing the elements for recycling and preventing the accumulation of toxic intermediates.
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There are hundreds of such compounds and the problem is that an organism degrading one compound cannot act on another compound.
Moreover, in certain cases, a number of microorganisms is needed to completely breakdown the toxic compound. Controlling the activity in microbial mixtures is another problem.
For instance, under aerobic conditions certain bacteria degrade the toxic trichloroethylene (TCE) into its inorganic components.
Under anaerobic conditions methanogens convert TCE into an even more harmful product vinyl chloride.
Due to the increasing incidence of oil pollution in .aquatic environments, hydrocarbon degradation by microorganisms is gaining importance nowadays.
Only small volumes of oil are degraded by oil decomposing bacteria and fungi.
Numerous types of organisms are involved in hydrocarbon degradation but the most common forms are pseudomonads and Alcaligens (Hughes & Stafford, 1993).
Other isolates include Micrococcus, Corynebacterium, Mycobacterium, Nocardia, Candida and Penicillium.
These hydrocarbon-digesting bacteria are used to degrade oil in industrial wastes, in oil spills in oceans and in oil-soaked soil near refineries.
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