Page 8 - OSMERT-2 Module 6 (inside pages) final
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system than  in situ  or in solid-phase systems because the contained environment is more
               manageable and hence more controllable and predictable. Despite the advantages of reactor
               systems, there are some disadvantages. The contaminated soil  requires  pretreatment  (e.g.
               excavation) or alternatively, the contaminant can be stripped from the soil via soil washing or
               physical extraction (e.g. vacuum extraction) before being placed in a bioreactor.

               Advantages of bioremediation
                     Bioremediation is a natural process and is therefore perceived by the public as  an
                       acceptable waste treatment process for contaminated material such  as  soil. Microbes
                       are able to degrade the contaminant increase in numbers when  the contaminant is
                       present; when the contaminant is degraded, the biodegradative population declines. The
                       residues for the treatment are usually harmless products which include carbon dioxide,
                       water, and cell biomass.
                     Theoretically, bioremediation is useful for the complete destruction of a wide variety of
                       contaminants. Many compounds that are legally considered to be hazardous can be
                       transformed to harmless products. This eliminates the chance of future liability
                       associated with treatment and disposal of contaminated material.
                     Instead of transferring contaminants from one environmental medium to another, for
                       example, from land to water or air, the complete destruction of target pollutants is
                       possible.
                     Bioremediation can often be carried out  on  site, often without causing a major
                       disruption of normal activities. This also eliminates the need to transport quantities of
                       waste off site and the potential threats to human health and the environment that can
                       arise during transportation.
                     Bioremediation can prove less expensive than  other technologies that are used for
                       clean-up of hazardous waste.

               Disadvantages of bioremediation
                     Bioremediation is limited to those compounds that  are biodegradable. Not all
                       compounds are susceptible to rapid and complete degradation.
                     There are some concerns that the products of biodegradation may be more persistent
                       or toxic than the parent compound.
                     Biological processes are often highly specific. Important site factors required for success
                       include the presence of metabolically capable microbial populations, suitable
                       environmental growth conditions, and appropriate levels of nutrients and contaminants.
                     It is difficult to extrapolate  from bench and pilot-scale studies to  full-scale field
                       operations.
                     Research is needed to develop and engineer bioremediation  technologies that are
                       appropriate for sites with complex  mixtures  of contaminants that are not evenly
                       dispersed in the environment. Contaminants may be present as solids, liquids, and gases.
                     Bioremediation often takes longer than other treatment options, such as excavation and
                       removal of soil or incineration.
                     Regulatory uncertainty  remains regarding acceptable performance criteria  for
                       bioremediation. There is no accepted definition of “clean”, evaluating performance of
                       bioremediation is difficult, and there are no acceptable endpoints  for bioremediation
                       treatments.









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