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Biological treatment of contaminated soil

Contamination of soils occurs from environmental pollution by toxic chemicals such as

oil and heavy metals. Technology is continuously being developed in order to treatcontaminated soil and bioremediation is among the most promising due to its low cost

and effectiveness in destroying pollutants. Bioremediation is a process that uses

microorganisms, green plants, fungi or their enzymes to restore a contaminated site to its

original condition. Bioremediation can be applied either in-situ or ex-situ, dependent on

the technique. Limitations to this technology include heavy metals being not readily

absorbed by organisms.

Some examples of this technology are landfarming, biostimulation, bioaugmentation, and

phytoremediation.

Landfarming is the process of incorporating contaminated cells, sludge or sediments into

the soil surface while ploughing in order to allow air into the mixture. This technique is

useful in the management and disposal of oily sludge and other petroleum based refinery

wastes (Hejazi et al., 2003). Hydrocarbons and pesticides are treated via the use of in-situ

systems for near surface soil contamination.

Limitations to the technique is that it requires large space for its in-situ application,

inorganic materials are not biodegraded, potentially present metal ions could be toxic to

microbes and large amounts of particulate matter could potentially be released via use of

landfarming (Hejazi et al., 2003).

Biostimulation is the process of modifying the environment with the goal of stimulatingexisting bacteria capable of bioremediation. Various forms of rate limiting nutrients and

electron receptors i.e. phosphorus, nitrogen, oxygen or carbon is added in order to make

biostimulation possible (Stucki et al., 2007). Additives are added to the subsurface via

injection wells. Removal of contaminated material is possible, yet expensive.

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The main advantage with this process is the use of well distributed spatially, indigenous

microbes rather than the introduction of exogenous microbes potentially not as well

suited to the subsurface environment. A limitation posed by local geography i.e. clay can

hinder even distribution of nutrients to microbes (Stucki et al ., 2007).

Bioaugmentation is the introduction of natural or genetically engineered microbes in

order to achieve bioremediation. Generally, indigenous microbes are studied in order to

determine if biostimulation is possible. Should the indigenous microbes be incapable of

performing the remediation process, other microbes with sufficient metabolic pathways

are introduced (Fantroussi et al, 2005).

Phytoremediation is the treatment of environmental problems through the use of plants.

This technique involves depolluting contaminated soils with plants capable of absorbing,

degrading or eliminating solvents, metals, crude oil, pesticides, etc. from the surrounding

environment (Cunningham et al., 1995). Advantages are that it is clean, inexpensive,

efficient and is an in-situ technique thus being environmentally undisruptive.

Phytoremediation is limited to the surface area and depth occupied by roots as well as

slow growth and low biomass means this technique requires a long term commitment as

well as the survival of the plant affected by the toxicity of the soil (Cunningham et al.,1995).

The use of biological means to treat environmental problems is not a new one. However,

it has been shown to be of great use. Bioremediation is not limited to the examples above.

Other examples include composting and bioventing. Techniques range from being

expensive to inexpensive and differ also in how intrusive they are to the surrounding

environment.

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