Contaminated Land and Bio-Remediation - Environmental Biotechnology: Theory and Application - page 94

the contaminants are not actually destroyed and the soil structure is irrevocably
damaged. Moreover, significant amounts of reagents are required and it is
generally not suitable for organic contaminants.
Thermal
Contaminants are destroyed by heat treatment, using incineration, gasification,
pyrolysis or volatisation processes. Clearly, the principal advantage of this
approach is that the contaminants are most effectively destroyed. On the
negative side, however, this is achieved at typically very high energy cost, and
the approach is unsuitable for most toxic elements, not least because of the
strong potential for the generation of new pollutants. In addition, soil organic
matter, and, thus, at least some of the soil structure itself, is destroyed.
In situ and Ex situ Techniques
A common way in which all forms of remediation are often characterised is
as in situ or ex situ approaches. These represent largely artificial classes, based
on no more than where the treatment takes place - on the site or off it - but
since the techniques within each do share certain fundamental operational sim-
ilarities, the classification has some merit. Accordingly, and since the division
is widely understood within the industry, these terms will be used within the
present discussion.
In situ
The major benefit of approaches which leave the soil where it is for treatment,
is the low site disturbance that this represents, which, in many cases, enables
existing buildings and features to remain undisturbed. They also avoid many
of the potential delays with methods requiring excavation and removal, while
additionally reducing the risk of spreading contamination and the likelihood of
exposing workers to volatiles. Generally speaking, in situ methods are suited to
instances where the contamination is widespread throughout, and often at some
depth within, a site, and of low to medium concentration. Additionally, since
they are relatively slow to act, they are of most use when the available time for
treatment is not restricted.
These methods are not, however, without their disadvantages and chief amongst
them is the stringent requirement for thorough site investigation and survey,
almost invariably demanding a high level of resources by way of both desktop and
intrusive methods. In addition, since reaction conditions are not readily controlled,
the supposed process 'optimisation' may, in practice, be less than optimum and
the true end point may be difficult to determine. Finally, it is inescapable that all
site monitoring has an in-built time-lag and is heavily protocol dependent.