Page 198 - Soil Degradation, Conservation and Remediation
P. 198

6.2   Sources of Soil Pollutants                                187

               In Situ Bioremediation

              These techniques are generally the most desirable options because of lower cost and
            less disturbance of the soil. It does not need excavation and transport soil materials
            to a temporary treatment shed and spreading contaminants (USEPA  1996 ). It is,
            however, restricted by the depth of the soil. In many soils, effective oxygen diffusion
            for desirable rates of bioremediation extends to a range of only a few centimeters
            to about 30 cm into the soil. However, depths greater than 60 cm have been effec-
            tively treated in some cases. The most important in situ bioremediation methods are
            described in brief here.

                 Bioventing : This is the most common method of in situ treatment of organic
              contaminants. It involves the supplying of air and nutrients through wells to
                contaminated soil for stimulating the indigenous bacteria. Bioventing employs

              low air flow rates and provides only the amount of oxygen necessary for the
              biodegradation while minimizing volatilization and release of contaminants to
              the atmosphere. It works for simple hydrocarbons and can be used where the
              contamination is deep under the surface.
                In situ biodegradation : It involves supplying oxygen and nutrients by circulating
              aqueous solutions through contaminated soils to stimulate naturally occurring
              bacteria to degrade organic contaminants. It can be used for soil and groundwater.

              Generally, this technique includes conditions such as the infiltration of water-
              containing nutrients and oxygen or other electron acceptors for groundwater
              treatment.
                Biosparging : 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.
              Biosparging increases the mixing in the saturated zone and thereby increases the
              contact between soil and groundwater.  The ease and low cost of installing

              small- diameter air injection points allows considerable flexibility in the design
              and construction of the system.
                Bioaugmentation : Bioremediation frequently involves the addition of microorganisms
              indigenous or exogenous to the contaminated sites. Most soils with long- term
              exposure to biodegradable waste have indigenous microorganisms that are effective
              degraders if the land treatment unit is well managed. Some soils need the import
              of exogenous microorganisms, but they often suffer from competition of the
              native microbial community and fail to develop effective colonization.



               Ex Situ Bioremediation
             These techniques involve the excavation or removal of contaminated soil from ground
            and transporting the soil materials to a temporary treatment shed. These methods
            offer the risk of spreading pollutants off-site and are not suitable for large- scale
            remediation. The types of ex situ remediation methods are briefl y discussed.
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