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WEEK 8 · Environmental Biotechnology · Thieman & Palladino, 'Introduction to Biotechnology' sch. 9.3, 9.4
CLEANUP SITES AND STRATEGIES
Some of the following questions must be considered
before starting the cleanup process:
•Do the chemicals pose a fire or explosive
•Do the chemicals pose a threat to human
health including the health of cleanup workers?
•Was the chemical released into the
environment through a single incident, or was
there long-term leakage from a storage
•Where did the contamination occur?
•Is the contaminated area at the surface of the
soil? Below the ground? Does it affect water?
•How large is the contaminated area?
Treatment strategies for soil and water usually involve
ex situ remediation, removing chemicals from the
contaminated site to another location for treatment, or
in situ remediation, cleaning up at the contaminated
site without excavation or removal. In situ approaches
rely on stimulating microorganisms in the contaminated
soil or water. In situ remediation is often the preferred
method of bioremediation for two reasons:
It is usually less expensive than ex situ
Because the soil or water doesn't have
to be excavated or pumped out of the
site, larger areas of contaminated soil
or water can be treated at a time.
However, in situ bioremediation is not always
the best solution.
This approach is most effective in
sandy soils, which are less compact
and allow microorganisms and
fertilising materials to spread rapidly.
Solid clay and dense rocky soils are
not typically good sites for in situ
Contamination with chemicals that
persist for long periods of time can take
years to clean up using this approach.
In situ approaches that require aerobic degradation
methods often involve bioventing, or pumping either
air or hydrogen peroxide (H2O2) into the contaminated
Hydrogen peroxide is frequently used because
it is easily degraded into water and oxygen to
provide microbes with a source of oxygen.
Fertilisers may also be added to the soil
through bioventing to stimulate the growth and
degrading activities of indigenous bacteria.
One common ex situ technique is called slurry-phase
bioremediation. This approach involves moving
contaminated soil to another site and then mixing the
soil with water, fertilisers, and oxygen in large
bioreactors, where the conditions of biodegradation by
microorganisms can be carefully monitored and
Slurry-phase bioremediation is a rapid process
that works fairly well when small amounts of
soil need to be cleaned and the composition of
pollutants is well-known.
For many other soil cleanup strategies, solid-phase
bioremediation techniques are required.
Solid-phase processes are more time-
consuming than slurry-phase approaches.
Solid-phase processes typically require large
amounts of space.
Three solid-phase techniques are widely used.
Composting can be used to degrade household
wastes such as food scraps and grass clippings; similar
approaches are used to degrade chemical pollutants in
contaminated soil. In a compost pile, hay, straw, or
other materials are added to the soil to provide bacteria
with nutrients that help them degrade chemicals.
Landfarming strategies involve spreading
contaminated soils on a pad so that water and
leachates can leak out of the soil. A primary goal of this
approach is to collect leachate so thatpolluted water
cannot further contaminate the environment.
Because the polluted soil is spread out in a
thinner layer than it would be if it were
below the ground, landfarming also allows
chemicals to vaporise from the soil and
aerates the soil so that microbes can better
Soil biopiles are used particularly when the chemicals
in the soil are known to evaporate easily and microbes
in the soil are rapidly degradng the pollutants. In this
approach, contaminated soil is piled up several metres
high. Biopiles differ from compost piles in that relatively
few bulking agents are added to the soil, and fans and
piping systems are used to pump air into or over the
pile. As chemicals in the pile evaporate, the vacuum
airflow pulls the chemical vapours away from the pile
and either releases them into the atmosphere or traps
them in filters for disposal, depending on the type of