It is the technique in which the microorganisms such as bacteria, fungi, and actinomycetes are used to transform hazardous contaminants into harmless products. While using chemicals for their own metabolism these microorganisms destroy organic contaminants. For e.g. Desulphovibrio desulfuricans are involved in the breakdown of mercury to methylmercury and Pseudomonas species are involved in converting methylmercury to mercuric acid, P. putida is involved in the bioremediation of toluene, naphthalene. Mycobacterium, Corynebacterium, Aeromonas, Rhodococcus, and Bacillus species are involved in the biodegradation of aromatic hydrocarbons.

The following are certain facts regarding bioremediation

• Pseudomonas putida F1 bacterial strain is involved in the degradation of aromatic hydrocarbon.
• The bacterium Deinococcus radiodurans consume and digest toluene and ionic mercury from highly radioactive nuclear waste.
• Biodegradable plastics are made using polyhydroxy alkanoates (PHAs) such as polyhydroxybutyrate (PHB).
• Microbial leaching involves the process of dissolution of metals from ore breaking rocks using microorganisms. Acidithiobacillus ferroxidans bacteria help in leaching copper from its ore.

There are various bioremediation technologies, which are used such as composting, bioventing, biopiles and land treatment.

  • Composting- is a process by which organic wastes are degraded by microorganisms, typically at much higher temperatures at around 55° to 65°C, This increase in temperature is due to heat produced by microorganisms during the degradation of the organic material in the waste. Composting has many advantages such as it helps in reducing mass of waste, leads to pathogen reduction, converts wastes into useful products etc.
  • Biopile- is a kind of bioremediation technique in which contaminated soils are heaped into piles and stimulation of microbial activity of aerobic microorganism is done by the addition of minerals, nutrients, moisture and aeration within the soil heap. It is also known as biocells or bioheaps. In this treatment, contaminants are mostly reduced to carbon dioxide and water. Biopiles are somewhat similar to landfarms as they are aerated but in a different way that is by forcing air into the biopiles by injection or by piping system rather than tilling and ploughing as in case of landfarms.
  • Bioventing – It is an in situ bioremediation technology that uses microorganisms to biodegrade organic constituents adsorbed on soils in the unsaturated zone. In this process, air or oxygen is supplied to the unsaturated zone directly by air injection to enhance the micro-organisms activity degrading hydrocarbons and other contaminants. This technique has applications like remediation of soils contaminated with pesticides, organic chemicals, non-chlorinated solvents and other substances.
  • Biostimulation- It is the release of nutrients, oxidants or electron donor into the environment to stimulate naturally occurring microorganisms to degrade a contaminant.
  • Landfarming – turning contaminated soil for aeration and sifting to remove contaminants, or deliberately depleting a soil of nitrogen to remove nitrogen based organisms.
  • Bioreactor – the use of specially designed containers to hold the waste while bioremediation occurs
  • Bioaugmentation – adding microbes and organisms to strengthen the same in waste to allow them to take over and decontaminate the area
  • Rhizofiltration – the use of plants to remove metals in water.
  • Mycoremediation-is a type of bioremediation in which fungi degrade the contaminants present in the environment. Mycelium reduces toxins by stimulating microbial and enzyme activity in in-situ conditions. Some fungi are hyperaccumulators and are able to absorb and concentrate heavy metals in the mushroom fruit bodies.
  • Chemoremediation -Chemoremediation refers to the reduction of contaminants in the environment using chemical methods. The basic concept of chemoremediation is the retention of polymer metal complexes, according to their molecular size, in aqueous solution by using membrane filtration. Chemical treatment of contaminated land and water involves the addition of an oxidizer to enhance the rate of degradation of the pollutants. Hydrogen peroxide can be used as an oxygen source in a bioremediation process of a crude oil polluted site. Oxygen is also provided by aerating the water and sediments.
  • Phytoremediation- includes a group of technologies in which plants are used for remediating soils, sediments and contaminated water. It uses plants through natural, biological, chemical or physical activities and processes to remove detoxify or immobilize environmental contaminants. It is highly cost-effective and involves the ability of plants to concentrate elements and compounds from the environment and to metabolize various molecules in their tissues.

Strategy for phytoremediation includes phytodegradation, continuous removal through hyperaccumulators and chelate mediated extraction of pollutants. Plants that are used to decontaminate soils must

o take up contaminants from soil particles into roots,
o bind the contaminant into their root tissue,
o transport the contaminant from their roots to shoots,
o inhibit the contaminant from leaching out of the soil.

Advantages of Bioremediation
• It is a natural process and is very cost effective.
• Bioremediation completely removes and destroys toxic chemicals from the environment.
• Bioremediation can be carried out on site itself i.e., it is an in situ technology.
• Bioremediation produces harmless products which can be disposed of easily and do not cause any threat to the environment.

Disadvantages of Bioremediation
• Time-consuming.
• Not able to remove heavy metals completely from the environment.
• High permeability of soil is necessary for in situ bioremediation.
• For microorganisms to work in these processes there number should be sufficient which has to be maintained by providing them optimal conditions and these conditions vastly vary among different sites.
• There is a difficulty in extrapolating bioremediation from small scale to full-scale operations.