Environmental Engineering Reference
In-Depth Information
3.5.3 Macroscale phenomena .................................................................. 46
3.6 Relationship between scales of observation............................................ 46
3.7 Conclusions ................................................................................................... 47
3.8 Summary ....................................................................................................... 48
References............................................................................................................... 49
3.1 Introduction
Biotreatment can be broken into two basic categories:
in situ
and
ex situ
.
“With
in-situ
techniques, soil and associated groundwater are treated in place
without excavation” (Blackburn and Hafter, 1993). Examples of
in situ
tech-
niques include pump and treat, percolation (flooding), bioventing, and air
sparging. In each of these systems, the contaminated media are treated
without excavation. With
ex situ
techniques, soil and groundwater are
removed from their original locations for treatment. Examples of
ex situ
techniques include land farming, irrigation, soil treatment units, composting,
engineered biopiles, and bioreactors. Both
in situ
and
ex situ
techniques are
capable of saturated and unsaturated zone remediation, although restriction
exists depending on the exact system used. However, many factors can
influence the effectiveness of each technique. Blackburn and Hafter (1993)
(Table 3.1) evaluated the influence of these techniques for operating states
of microbiological processes (i.e., bioactivity).
In general, this study by Blackburn and Hafter showed that
ex situ
techniques allow more opportunities to control or engineer conditions for
remediation; however, this is not to suggest that
ex situ
treatment is the
preferred technique. Although some sites may be more easily controlled and
maintained with
ex situ
configurations, others are more effective with
in situ
treatment. For example, many sites are located in industrial and commercial
areas, and these sites normally consist of numerous structures interconnected
by concrete and asphalt. These physical barriers would make excavation
extremely difficult, and if the contamination is deep in the subsurface, exca-
vation becomes too expensive. As a result of these physical barriers, the
required excavation efforts may make
ex situ
biotreatment impracticable.
Other factors could also have an impact on the type of treatment. At a typical
site, the contamination is basically trapped below the surface. Exposing the
contamination to the open environment through excavation can result in
potential health and safety risks. In addition, the public's perception of the
excavation of contaminants could be negative, depending on the situation.
All of these conditions clearly favor
in situ
biotreatment. Nonetheless, the
key is to carefully consider the parameters involved with each site before
evaluating which technique to use.
