Biomedical Engineering Reference
In-Depth Information
Thus, the type and concentration of the contamination, its scale and extent, the
level of risk it poses to human health or the environment, the intended eventual
site use, the time available for remediation, available space and resources and
any site specific issues all influence this decision. Many of the key issues have
already been discussed and the earlier Figure 5.1 set out the factors governing
technology transition between the in situ and ex situ techniques.
Essential Features of Biological Treatment Systems
All biotechnology treatments have certain central similarities, irrespective of
the specific details of the technique. The majority of applications make use
of indigenous, resident microbes, though in some cases the addition of specialised
organisms may be warranted. Thus, the functional biology may be described as a
process of bioenhancement or bioaugmentation, or occasionally a mixture of both.
Bioenhancement concentrates solely on the existing micro-fauna, stimulating
their activity by the manipulation of local environmental conditions. Bioaugmen-
tation, by contrast, requires the deliberate introduction of selected microbes to
bring about the required clean-up. These additions may be unmodified 'wild-
type' organisms, a culture selectively acclimatised to the particular conditions to
be encountered, or genetically engineered to suit the requirements. Enzyme or
other living system extracts may also be used to further facilitate their activity.
Some land remediation methods simultaneously bioenhance resident bacteria and
bioaugment the process with the addition of fungi to the soil under treatment.
In the final analysis, all biological approaches are expressly designed to opti-
mise the activities of the various micro-organisms (either native to the particular
soil or artificially introduced) to bring about the desired remediation. This gen-
erally means letting them do what they would naturally do, but enhancing their
performance to achieve it more rapidly and/or more efficiently. Effectively it is
little different from accelerated natural attenuation and typically involves man-
agement of aeration, nutrients and soil moisture, by means of their addition,
manipulation or monitoring, dependent on circumstance. However simple this
appears, the practical implications should not be underestimated and careful
understanding of many inter-related factors is essential to achieve this goal. For
example, successful aerobic biodegradation requires an oxygen level of at least
2 mg/l; by contrast, when the major bioremediation mechanism is anaerobic, the
presence of any oxygen can be toxic. The presence of certain organic chemicals,
heavy metals or cyanides may inhibit biological activity; conversely, under certain
circumstances microbial action may itself give rise to undesirable side effects like
iron precipitation, or the increased mobilisation of heavy metals within the soil.
In situ techniques
The fundamental basis of in situ engineered bioremediation involves introducing
oxygen and nutrients to the contaminated area by various methods, all of which
Search WWH ::




Custom Search