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Remediation Technologies
Site cleanup technologies
reduce the contaminant levels or the levels of risks on used sites to levels
pre-specified, by regional standards or site specific studies, as protective
of humans and ecology. Below is a list and brief descriptions of tested
remediation technologies. The list will be added to regularly:
Excavation and Disposal ("Dig and Dump")
Earlier practices of
excavation and disposal
involved digging out contaminated soil from the location of contamination
and dumping it off in a landfill or other locations (e.g. a depression)
where the contaminated soil is not considered a hazard to human and
ecological health. No treatment might have been carried out on the
contaminated soil prior to dumping at the target site; hence the approach is
nick-named “Dig and Dump”. Regulatory
restrictions on land disposals have brought
some
changes to this practice. In
North America and most countries in Europe, it is now illegal to co-dispose
hazardous and non-hazardous wastes in landfills. There are only designated
or licenced landfill sites that accept PHC contaminated soils. Furthermore,
disposal in landfill sites without pre-treatment is now outlawed in several
jurisdictions.
Wastes for
disposal in landfills should be characterised and the parameters must meet
certain pre-specified waste acceptance criteria.
Dig and
Dump approach is very simple to implement. However, it's main disadvantages
are cost of transporting the contaminated soil to its final destination and
the contamination (if soil is dumped untreated) of the final destination (in
case of future re-use).
Soil Vapour Extraction (SVE)
Soil Vapour Extraction (SVE) is an
in-situ remediation approach used for reducing the concentration of
contaminants, particularly the volatile Organic Compound (VOC) and the semi
volatile organic compound (SVOC) constituents of petroleum hydrocarbons to
levels pre-specified as safe for human health and the environment. The VOC
and the SVOC constituents usually resulting from lighter petroleum products
such as, gasoline are removed mainly through vapourization. Extraction
wells are constructed all over the contaminated site following a design
considered effective to produce desired reduction of the contaminants. A
vaccum is applied, by blowers, through the wells, creating a negative
pressure that pulls the vapours from the contaminant out of the ground
through the wells. The extracted vapours are then treated and discharged to
the atmosphere or re-injected into the subsurface if permitted. This approach is effective for use in the
upper soil layer or the unsaturated/vadoze soil zone.
Bioventing
Bioventing (BV) is an in-situ remediation
technology that removes PHC constituents through biodegradation of the
constituents by microorganisms. Similar to SVE, BV involves the use of
extraction wells or injection wells to introduce air into the subsurface
aiding the mirobial activities responsible for the breakdown of the PHC
constituents. Though some volatilization occur in BV, the predominant
process for contaminant reduction is biodegradation. To minimize
volatilization, extraction or injection wells are operated at lower air flow
rates. Some nutrients can be added to aid the microbial activities and
accelerate biodegradation. BV, similar to SVE, is also applicable for soils
within the topmost soil layer or the unsaturated/vadoze zone of the soil. BV
has been used successfully to reduce concentrations of mid-range PHC
constituents from petroleum products such as diesel and jet fuels.
Air Sparging (AS)
Air Sparging (AS) is an in situ remedial method that is used to
reduce lighter molecule PHC constituents such as benzene, ethylbenzene,
toluene, and xylene (BTEX), from petroleum products like gasoline from the
saturated soil zone. Both the PHC constituents adsorbed to the soil and in
the groundwater turns to vapours and are removed during AS. Main clean up
process is volatilization using extraction wells. AS is often combined with
SVE for effectiveness.
Bio-Sparging (BS)
BS is similar to bioventing. It is used to
reduce middle range PHC constituents from the saturated zone through
biodegradation. volatilization is minimized by ensuring lower rate of air
flows. Nutrients may be added to speed up biodegradation.
SVE/AS and BV/BS combinations are commonly
adopted for effective clean up. AS and BS can also be used to remove heavy
PHC molecules from petroleum products such as lubricating oils.
Land Farming (or Land Treatment)
Land Farming (LF) is a remediation strategy
that reduces PHC constituents in soils by biodegradation. The process is
aided by aeration (tilling) and addition of nutrients and/or moisture (as
required). LF can be carried out in-situ (no soil excavation required), when
contamination is limited to the top 1 m. For contaminations to depth 1.5m
and more, soils should be excavated and transported into Land Farm Cells or
Land Treatment Units (LTU) for LF treatment, where they are spread thinly,
tilled regularly to encourage microbial activities.
Biopiles
Similar to LF, Biopiles also treat
contaminated soils through biodegradation using microorganisms to break down
PHC constituents with or without the addition of nutrients. Contaminated
soils are excavated and heaped into piles. The piles are aerated, to aid
biodegradation, through a system of extraction/injection wells connected to
horizontal perforated pipes running through the piles. Biopiles has proved
successful in biodegrading all categories of PHC constituents (lighter to
heavy molecules).
Thermal Desorption
Thermal Desorption uses HEAT to separate
contaminants (e.g. PHC) from contaminated soils. Contaminated soil is
excavated and transported to a Thermal Desorber where it is heated to
temperatures high enough to volatize and desorb or physically separate the
contaminant's constituents from the the soil. Larger particles (of diameters
> 50 mm) are first screened out before introducing the soil into the
desorber. The larger particles are introduced after been shredded into
smaller pieces.
Partial decocompostion do take place inside
the desorbers, although desorbers are not made to break down contaminant's
constituents. The vapour escaping from the desorbers are treated inside the
Afterburner/Oxidizer.
Condensers and carbon adsorption units trap
organic compounds.
Treated soils are heaped into
piles. Confirmatory sampling will be carried out on them to ensure that the
contaminants have been reduced to the required standards.
The
soil can be returned to the original location they were taken from (if
permitted).
Chemical Oxidation
Involves the use of chemicals for the
destruction of organic contamiant constituents. The Chemical oxidants most
commonly employed to date include peroxide, ozone, and permanganate.
Phytoremediation
Phytoremediation is the use of plants to
remove contamiants from soil. This process can take several forms (source:
Wikipedia):
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Phytoextraction - uptake and
concentration of substances from the environment into the plant biomass.
-
Phytostabilization - reducing the
mobility of substances in the environment, for example by limiting the
leaching of substances from the soil.
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Phytotransformation - chemical
modification of environmental substances as a direct result of plant
metabolism, often resulting in their inactivation, degradation (phytodegradation)
or immobilization (phytostabilization).
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Phytostimulation - enhancement of
soil microbial activity for the degradation of contaminants, typically by
organisms that associate with roots. This process is also known as
rhizosphere degradation.
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Phytovolatilization - removal of
substances from soil or water with release into the air, sometimes as a
result of phytotransformation to more volatile and / or less polluting
substances.
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Rhizofiltration - filtering water
through a mass of roots to remove toxic substances or excess nutrients.
The pollutants remain absorbed in or adsorbed to the roots.
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