The use of microbes such as bacteria and fungi for soil rejuvenation is a form of environmental remediation. The objective of microbial remediation is to remove soil contaminants and pollutants.
Although the industrial use of microbes for removing contaminants goes back only three decades, microbes — aerobes, anaerobes and facultative anaerobes — have been contributing to soil improvement for billions of years. They help with nitrogen fixation, limiting growth of plant pathogens, and decomposition of heavy metals, pesticides and hydrocarbons in the soil.
The microbial flora feeds on the contaminants, degrading them for energy and reproduction. Microbial remediation can be divided into three grades:
- Natural attenuation: The process takes place naturally with indigenous soil microorganisms.
- Biostimulation: The natural process receives external help in the form of nutrients, moisture and an ideal pH for the microorganisms.
- Bioaugmentation: This involves the use of externally introduced microorganisms, which is the case in situations such as oil spills where the naturally occurring microbes may die out because of the intensity of the contamination.
Genetically engineered microorganisms (GEMs) are playing an increasingly important role in tackling soil contamination. Naturally-occurring chemicals can be broken down by microbes that have evolved for the purpose. However, man-made chemicals found in cosmetics, pesticides, insecticides, cleaners and paints cannot be degraded into less toxic products by naturally occurring microbes.
Many synthetic chemicals such as dioxin and DDT have been declared persistent organic pollutants (POPs). They are termed persistent because they are difficult to break down and tend to accumulate within organisms, including human beings, leading to cancer and defects of the reproductive system.
One microorganism that has attracted the attention of geneticists is Deinococcus radiodurans, a soil bacterium capable of surviving radiation levels several times higher than what would kill humans. The bacterium has demonstrated capabilities to break down radioactive mercury and toluene. After being engineered to work in radioactive waste environments, it may be used to decompose radioactive waste generated in the fields of medicine, energy and military.
Microbial remediation is practiced through two approaches — in situ and ex situ. The chief difference between the two approaches is the site where remediation takes place.
When remediation occurs at the site of contamination without removing the polluted soil from the substrate, the process is termed in situ. The advantage with this system is that it is easily put in place and can be implemented with minimum disturbance to an area's flora and fauna. There is no risk of contamination spreading from one point to another.
Since there is no handling of contaminated soil involved, humans are saved from exposure to harmful materials. The process of in situ remediation is a slow one and preferred when contamination levels are low and widespread. The drawback with this method is that the final result of the decontamination process may not be uniform over the area.
With ex situ microbial remediation, the soil is removed from the substrate and taken to a treatment facility. The cleaning takes place under optimized conditions that are easier to regulate and monitor. This makes off-site treatment of contaminated soil a quicker alternative. This approach is suited for scenarios of localized medium to high contamination.
The field of bioremediation is a continuously evolving one. Contaminants once considered impossible to treat are now readily broken down. On the other hand, bioremediation still has no answer for pollution caused by inorganic acids, asphalt and bitumen.
