Environmental Engineering Reference
In-Depth Information
place avoiding excavation and transport of
contaminants. In situ treatment is certainly lim-
ited by the depth of the soil that can be effectively
treated. The most important in situ land treat-
ments are bioventing, biosparging, and bioaug-
mentation (Jadia and Fulekar
2009
). Bioventing
is the most common in situ treatment that involves
supplying air and nutrients through wells to con-
taminated soil to stimulate the indigenous bacte-
ria. In situ biodegradation involves supplying
oxygen and nutrients by circulating aqueous
solutions through contaminated soils to stimulate
naturally occurring bacteria to degrade organic
contaminants. Biosparging involves the injection
of air under pressure below the water table to
increase groundwater oxygen concentrations and
enhance the rate of biological degradation of
contaminants by naturally occurring bacteria.
Bioaugmentation involves the addition of micro-
organisms indigenous or exogenous to the con-
taminated sites to enhance the degradation of the
contaminants.
1
Introduction
Industrial pollution is the most solemn problem to
the environment that required main concern. A
wide variety of chemicals and toxic wastes such
as heavy metals and persistent organic pollutants,
including pesticides, have been detected in differ-
ent biota such as soil, water, and air (Turgut
2003
).
Among the pollutants, heavy metals pose a criti-
cal concern to human health and the environment
due to their high occurrence as a contaminant, low
solubility in biota, and the classifi cation of several
heavy metals as carcinogenic and mutagenic
(Alloway
1995
). Moreover, the metals cannot be
degraded to harmless products and hence persist
in the environment indefi nitely. As a result, many
different remediation methods have been tried to
address the rising number of heavy metal-contam-
inated sites. Conventional cleanup technologies
are expensive and feasible only for small but
heavily polluted sites where rapid and complete
decontamination is required. In addition, some
methods, such as soil washing, electrokinetic
remediation, and vitrifi cation, can pose an adverse
effect on biological activity, soil structure and fer-
tility and acquire signifi cant engineering costs
(Pulford and Watson
2003
). Therefore, sustain-
able on-site techniques for remediation of heavy
metal-contaminated sites need to be developed.
2.2
Ex Situ Bioremediation
The ex situ techniques involve the excavation or
removal of contaminated soil from the ground.
The most important ex situ treatments are land-
farming, composting, biopiles, and bioreactors.
Landfarming is a simple technique in which con-
taminated soil is excavated and spread over a pre-
pared bed and periodically tilled until pollutants
are degraded. The main purpose is to stimulate
indigenous biodegradative microorganisms and
facilitate the aerobic degradation of contami-
nants. Biopiles are a hybrid of landfarming and
composting, typically used for treatment of sur-
face contamination with petroleum hydrocarbons
(Fahnestock et al.
1998
). It is a refi ned version of
landfarming that tends to control physical losses
of the contaminants by leaching and volatiliza-
tion. Biopiles provide a favorable environment
for indigenous aerobic and anaerobic microor-
ganisms. Bioreactors, slurry reactors, or aqueous
reactors are used for ex situ treatment of contami-
nated soil and water pumped up from a contami-
nated plume. Bioremediation in reactors involves
2
Bioremediation Process
Various bioremediation techniques are employed
for remediation of heavy metal-contaminated sites
depending on the degree of saturation and aeration
of an area. In situ techniques are defi ned as those
that are applied to soil and groundwater at the site
with minimal disturbance, whereas ex situ tech-
niques are applied to the site which has been
removed via soil excavation or water pumping.
2.1
In Situ Bioremediation
In general, in situ techniques are the most desir-
able options due to lower cost and fewer distur-
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