Contaminated Land and Bioremediation - Environmental Biotechnology: Theory and Application - page 93

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 enables existing build-
ings and features to remain undisturbed, in many cases. 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.
Ex situ
The main characteristic of ex situ methods is that the soil is removed from where
it originally lay, for treatment. Strictly speaking this description applies whether
the material is taken to another venue for clean-up, or simply to another part
of the same site. The main benefits are that the conditions are more readily
optimised, process control is easier to maintain and monitoring is more accurate
and simpler to achieve. In addition, the introduction of specialist organisms, on
those occasions when they may be required, is easier and/or safer and generally
these approaches tend to be faster than corresponding in situ techniques. They
are best suited to instances of relatively localised pollution within a site, typically
in 'hot-spots' of medium to relatively high concentration which are fairly near
to the surface.
Amongst the main disadvantages are the additional transport costs and the
inevitably increased likelihood of spillage, or potential secondary pollution, rep-
resented by such movement. Obviously these approaches require a supplementary
area of land for treatment and hence they are typically more expensive options.
As Figure 5.1 illustrates, the decision to use in situ or ex situ techniques is a
comparatively straightforward 'black-or-white' issue at the extremes for either