Technologies involved in bioremediation

In terms of technologies utilised within the wider remit of biotechnology, a number of specific terms are used to describe the activity of micro-organisms and the way they are used (Ball, 2007). This section discusses the main ones.

Monitored natural attenuation (intrinsic bioremediation) is one method of applying in situ bioremediation. One component of natural attenuation is the use of indigenous micro-organisms to degrade the contaminants of concern without human intervention (such as supplementing the available nutrients). Site characterisation and long-term monitoring comprise the activities required to implement natural attenuation. Long-term monitoring is used to assess the fate and transport of the contaminants compared against the predictions. The reactive transport model can then be refined to obtain better predictions. Natural attenuation processes typically occur at all sites, but to varying degrees of effectiveness depending on the types and concentrations of contaminants

III. 8. THE INTENTIONAL RELEASE OF MICRO-ORGANISMS INTO THE ENVIRONMENT: CHALLENGES TO COMMERCIAL USE - 119

Figure 8.3. Range and weighting of industries that utilise bioremediation

present and the physical, chemical and biological characteristics of the soil and groundwater. As they rely on naturally available micro-organisms in each site in combination with abiotic processes, natural attenuation processes may reduce the potential risk posed by site contaminants in three ways:

  • • the contaminant may be converted to a less toxic form through destructive processes, such as biodegradation or abiotic transformations
  • • potential exposure levels may be reduced by lowering of concentration levels (through destructive processes or by dilution or dispersion)
  • • contaminant mobility and bioavailability may be reduced by sorption to the soil or rock matrix.

In situ bioremediation (ISB) is the use of micro-organisms to degrade contaminants in place with the goal of obtaining harmless chemicals as end products. Most often, in situ bioremediation is applied to the degradation of contaminants in saturated soils, although bioremediation in the unsaturated zone can occur. ISB has the potential to provide advantages such as complete destruction of the contaminant(s), lower risk to site workers and lower equipment/operating costs. ISB can be categorised by metabolism or by the degree of human intervention. At a high level, the two categories of metabolism are aerobic and anaerobic. The target metabolism for an ISB system will depend on the contaminants of concern. Some contaminants (e.g. fuel hydrocarbons) are degraded via an aerobic pathway, some anaerobically (e.g. carbon tetrachloride) and some contaminants can be biodegraded under either aerobic or anaerobic conditions (e.g. trichloroethene).

Accelerated in situ bioremediation is where substrate or nutrients (termed biostimulation) are added to an aquifer to stimulate the growth of a target consortium of bacteria. Usually the target bacteria are indigenous; however, enriched cultures of bacteria (from other sites) that are highly efficient at degrading a particular contaminant can be introduced into the aquifer (termed bioaugmentation). Accelerated ISB is used where it is desired to increase the rate of contaminant biotransformation, which may be limited by lack of required nutrients, electron donor or electron acceptor. The type of amendment required depends on the target metabolism for the contaminant of interest. Aerobic ISB may only require the addition of oxygen, while anaerobic ISB often requires the addition of both an electron donor (e.g. lactate, benzoate) as well as an electron acceptor (e.g. nitrate, sulfate). Chlorinated solvents, in particular, often require the addition of a carbon substrate to stimulate reductive dechlorination. The goal of accelerated ISB is to increase the biomass throughout the contaminated volume of aquifer, thereby achieving effective biodegradation of dissolved and sorbed contaminant.

The addition of either nutrients or micro-organisms generally bring about an increase in the rate of bioremediation, but the increased cost of utilising this approach ensures that their application is based around the particular requirements of the remediation. For example, if the site is to be built upon shortly, enhancing the natural rate of remediation through the addition of biostimulation and/or bioaugmentation may be necessary and cost effective. In contrast, if the site is to be left for some time (i.e. years) then monitored, natural attenuation will generally be employed as it is the most cost-effective bioremediation.

 
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