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Status of Bioremediation
Approaches and its future Prospectus
ENVIS Centre, Department of Zoology
University of Madras, Chennai – 600 025.

Microbial diversity and its diverse metabolic activities are of critical importance to the sustainability of life on our planet, including recycling elements on which primary productivity depends, producing and consuming gases important for maintaining our climate, and destroying the wastes of human civilization. Besides that, microbes often play key roles in conservation of higher organisms and in restoration of degraded ecosystems. Hence, microbial diversity goes hand in hand with goals for maintenance of higher organism diversity and ecosystem management.

Bioremediation refers to cleaning contaminated environments with the aid of microorganisms that transform contaminants to harmless or less harmful compounds. Bioremediation offers three main approaches for treating contaminants, i.e. natural attenuation, biostimulation, and bioaugmentation. In natural attenuation, the naturally occurring, i.e.indigenous microorganisms degrade the contaminants without any additions and the rate as well as level of degradation is carefully monitored. In biostimulation, indigenous microorganisms are stimulated by the addition of nutrients and electron acceptors. In bioaugmentation, contaminant degradation is enhanced by introducing microorganisms into the sites via inoculation. There is a better understanding of the benefits of bioremediation than of other approaches, especially about in-situ bioremediation of contaminated soils. Technologies related to the application of microorganisms to the soil, release of nutrients into the soil, and enhancement of microbial decontamination are being tested through various additives such as surfactants, ion exchange resins, limestone, or dolomite. New equipments have been developed for crushing and mixing or inject ing and sparging the microorganisms, as have new reactor technologies (e.g., rotating aerator reactors, biometal sludge reactors, and special mobile containers for simultaneous storage, transportation, and biodegradation of contaminated soil).

Bioremediation Approaches

As various pollution problems are addressed worldwide, the scope and diversity of bioremediation in general, especially in-situ bioremediation technology continues to grow. In Europe, particularly in Germany, The Netherlands and Denmark huge money is being spent for soil remediation and funding for bioremediation research and development.

In-situ bioremediation is based on stimulating the natural breakdown of petroleum hydrocarbons within the subsurface by enhancing environmental conditions. Groundwater is extracted and treated in a surface mounted bioreactor. The effluent from the reactor, rich in microorganisms, nutrients and oxygen, is then reinjected into the aquifer upgradient of the extraction point. The treated groundwater can also be recirculated through the soil and allowed to percolate to the groundwater to promote in-situ biodegradation within the soil in addition to the groundwater. Besides that, bioremediationtechnology, especially when it can be carried out in-situ, is a cost-effective means of removing many chemical pollutants that adversely spoil human health or environmental quality.

Further, when the bioremediation technology is applied for any ecosystem restoration, the end products such as water and carbon dioxide are non-toxic and are harmless to the environment and living organisms.

Development and application of bioremediation technology in various countries;

Germany is one of the most developed nation and it gives due importance for both industrialization and environment management, and has spent more time and money compared to other countries in identifying environmental problems. Therefore, more companies are involved on bioremediation. Most of these companies are located in the region formerly known as East Germany. The list of contaminated sites and needed remedial actions had been dramatically increased after German reunification. Risk sites include vehicle workshops, airports, traffic and parking areas, waste dumps, fuel storage and transfer points, and munitions sites. According to the Federal Ministry of Research and Technology (BMFT) in Germany, 28 bioremediation techniques have been developed there. BMFT has sponsored 16 projects with a total funding of 20 million DM ($1 2.5 million U.S.). The German Research Association has also conducted projects in enzymatic dehalogenation of contaminants using Pseudomonas Streptomyces and thermophilic microorganisms, and in biodegradation for "dioxin-like" substances.

A number of companies conduct polyaromatic hydrocarbon ( PA H ) decontamination using microbes. Researchers De Ruiter Milieutechnoiogie, Halfweg, conducted a demonstration project involving aliphatic or aromatic hydrocarbons to study the influence ofpH and nutrient addition (potassium, nitrate, and others) , and inoculation of adapted microorganisms. The German bioremediation firm, Argus Umweltbiotechnoiogie GmbH, uses infiltration of air and addition of nutrients to degrade hydrocarbons in situ. Research at the Department of Chemical Microbiology of the Fraunhofer Institute of Interface Technology and Biotechnology is focused on microbial and engineering aspects of bioremoval of xenobiotic compounds from wastewaters and exhausted air. In particular, they have demonstrated that PAH biodegradation can be achieved in airlift bioreactors and accelerated using water-soluble solvents as lipophilic mediators to facilitate mass transfer. The biological process in airlift reactors is carried out in an organic-aqueous mixed phase. Wilhelm University of Muenster and the Technical University of Munich studied the application of specially developed, immobilized microorganisms to xenobiotically degrade soil contaminatio n. These immobilized microorganisms have better resistance to soil microflora, because they are affixed to a microporous support that provides a habitat promoting reproduction of microbial cells yet allowing release of cells from the support.

Work is under way in Germany to introduce nutrients into the soil using explosive cartridges. Soil-mixing machines expedite mixing the soil with ion exchange resins, dolomite or limestone (to adjust pH), and nutrients. Microorganisms and enzymes are
immobilized on wood chips, granular clay, anthracite, and synthetic polymers to assist their establishment in the soil matrix.

The Netherlands

The Netherland and Denmark are leaders in establishing nation wide programs for decontaminating thousands of sites through the processes of bioremediation. A number of wellestablished companies are located in the Netherlands, and a significant number of sites have been cleaned up since 1982. Soil pollution is an environmental problem of the highest prioritybecause of the limited land area and proximity to sea level. In-situ bioreclamation is one of several methods available for treating oily wastes and PAH in sediments. A petroleum contaminated site at Asten was used to evaluate the feasibility of in situ bioremediation and showed good prospects for remediation of the petroleum spill if hydrogen peroxide was added as a chemical alternative to oxygen.

A biological method for water treatment is available that uses controlled biological oxidation in sulfide reactors. The process treats the groundwater, highly polluted with sulfate and heavy metals, underneath the property. Sulfur compounds are reduced to hydrogen sulfide using anaerobic sulfate-reducing bacteria, and heavy metals are precipitated as metal sulfides. The remaining sulfide is oxidized to elemental sulfur using aerobic sulfide-oxidizing bacteria, and elemental sulfur is then separated from the water.

Other Regions

Other European countries working to address contamination issues include Italy, France, the Spanish province of Catalonia, Switzerland, and the United Kingdom. These countries have made efforts to identify contaminated sites (the United Kingdom reportedly has 50,000 to 100,000 contaminated waste sites) but have not yet defined nationwide decontamination measures, selected technical approaches, or planned large decontamination projects. Meanwhile, Spain, Portugal, Greece, and Ireland are just beginning to assess contamination problems and sites to be remediated.

An interesting development in France is the use of algal cultures in aqueous solutions to stabilize cesium and strontium in the soil. These cultures are used primarily for shallow surface contamination, but adaptations may be possible to extend the technology to groundwater and subsurface contamination. Experimental programs are being conducted in collaborationwith the former USSR.

The French DVM (Decontaminating Vegetal Network) process is a biomechanical method for removing soil contamination using
plants that create a dense root network to trap the contaminated soil particles. Removing the turf then removes the contaminated soil. Biosurfactant-producing microorganisms have been used to increase the removal of contaminants using soil washing.

In Eastern Europe, several Czech companies offer reasonably advanced bioremediation services. A microbial mixed population is being studied by the University of Prague to treat surface contamination in an abandoned site polluted with petroleum hydrocarbons.

Although, bioremediation offers several advantages over physical and chemical processes used to treat contaminated water and soil, the cleanup costs using bioremediation is cheaper. It is also a simple technology when compared to other remediation technologies. Besides that, in-situ bioremediation can be carried out with minimal space and with less health risk.

Future Prospects

The application of microorganisms to enhance the fertility of soil conditions and removing the soil contaminations through bioremediation technology is extensively used in Europe and USA. In India, progress has been made in applying microorganisms to the restoration of polluted soil through bioremediation processes. However, the application of bioremediation technology in the restoration of ecosystem and soil management is used less compared to Europe and USA. Hence, we need extensive research programs to increase the capabilities of bioremediation to deep, extensive, subsurface contamination due to chlorinated hydrocarbons and complex mixed wastes, including soils and groundwater. Besides that, The American Academy ofMicrobiology (AAM) has concluded that enough knowledge is now available for field trials of bioremediation technology for organic compounds and further they emphasized that research is needed for the following classes of environmental pollutants: metals, metalloids, radionuclides and complex polycyclic hydrocarbons. The on-going microbial genomics studies will deliver more robust technologies for the bioremediation of metal – contaminated waters and land. Exciting developments in the use of microorganisms for the recycling of metal waste, with the formation of novel biominerals with unique properties are also predicted in the near future.

Further reading:

Bioremediation Principles, Eweis, Ergas, Chang .Schroeder., (1998) Mc Graw Hill Inc.

Bio Remediation Field Experience, Paul E.Flathman, Douglas E.Jerger & Jurgen H.Exner., (1993) Lewis Publishers, Inc.

Hydrocarbon Bioremediation, Robert E.Hinchee,Bruce C.Alleman, Ron E. Hoeppel & Ross N. Miller (1994) Lewis Publishers, Inc.

Bioremediation (Applied Microbial Solutions for Real - World Environmental Cleanup) Ronald M.Atlas and Jim Philp., (2005) American society for Microbiology (Asm Press).

ENVIS CENTRE Newsletter Vol.6,No 1 March 2008 Back 
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