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The following points highlight the top three techniques used for the treatment of polluted soil. The techniques are: 1. In Situ Bioremediation 2. Land Farming 3. Slurry-Phase Bioreactors.
Technique # 1. In Situ Bioremediation:
In situ bioremediation relies on biological clean-up without excavation. This technique of treatment of polluted soil is normally applied in situations where commination is deep in the sub-surface or under buildings, roadways, etc. In situ bio-restoration of pollutants is becoming quite popular since it avoids excavation costs and produces no toxic by-products as is the case with ex situ physico-chemical treatment.
Water is cycled through the sub-surface using a series of recovery and recharge trenches or wells. Water may be oxygenated by sparging with air or via addition of H2O2. Microbial clean-up by enhancement of anaerobic degradative activity in situ awaits more study. The obvious drawback of in situ bioremediation is that it is difficult to stimulate microbial activity throughout the polluted soil volume.
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It is so because the injected water carrying the necessary nutrients and microorganisms tends to flow through larger soil interstices, leaving substantial amounts of residual contaminant within more impermeable layers. Such contaminants may take years to diffuse to the bio-active zones where their biodegradation takes place.
One specific type of in situ soil bioremediation, called bioventing, has emerged recently as one of the most cost-effective and efficient technologies available for the remediation of the vadose zone (unsaturated zone above the groundwater table) of petroleum-contaminated sites.
Bioventing consists of stimulating aerobic biodegradation by circulating air through the sub-surface. High removal efficiencies (>97%) can be obtained for soluble paraffins (<C16) and poly aromatic hydrocarbons (PAHs) after several years operations. Bioventing is however limited to homogeneous sub-surface formations since heterogeneities would cause the air to move through the most permeable areas causing treatment to occur only in limited areas.
Phytoremediation is another success story of in situ soil bioremediation. In it, the specific plants are cultivated that accumulate heavy metals in the above-ground plant tissue or stimulate organic break-down in their rhizosphere (the zone immediately adjacent to the roots).
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While phytoremediation is elegant, its main drawbacks are that only the surface layer of soil (0-50 cm) can be treated and that the treatment takes several years and leaves substantial residual levels of contaminants in the soil. Phytoremediation is however undergoing full development at present.
Technique # 2. Land Farming:
Removal of oil slicks by so-called land farming is an established method based on microbial degradation (Fig. 30.6). Given half-lives of the order of one year, it generally takes about 7 years of treatment to remove 6.4 g hydrocarbon per kg soil down to the clean-up goal of 50 mg kg-1.
This technology can be somewhat upgraded by mixing the soil with fresh organic residues (compost). Elevated temperatures and increased microbial diversity and activity increase reaction rates. Moreover specific co- substrates favour co-metabolism.
Land farming systems can be upgraded by including anaerobic pretreatment. For example, anaerobic tunnels are used to reduce compounds such as tri-nitrotoluene by adding nutrients and co- substrates for the indigenous bacteria.
In a second aerobic stage, the reduced metabolites are either completely mineralised or polymerised and irreversibly immobilised in the soil matrix. This approach has also been used successfully to decontaminate soils polluted with chloroethene and BTX aromatics (mixtures of benzene, toluene and xylene).
Technique # 3. Slurry-Phase Bioreactors:
Slurry-phase bioreactors may achieve the same clean-up levels in considerably less time in comparison to land farming. In this method, excavated polluted soil is treated under controlled optimal conditions, ensuring effective contact between contaminant and microorganisms.
The latter are, in most cases, specific cultures of adapted microorganisms. With overall degradation rates in the range 0.2-2 g oil per kg soil per day, solid residence times of 30 days, in place of several years, are sufficient to meet the cleanup levels.
