Environmental Engineering Reference
In-Depth Information
does not work on more complex contaminants, such as five- and six-ring
compounds (Potter et al., 1999). Currently, it is also a very inexact method
for remediation.
3.5 Transitioning from bench-level to full-scale design
Systems at the bench level are relatively simple by design, whereas systems
naturally occurring in the field can be complex. The bench level focuses on
specific aspects of a process (i.e., ascertaining rates of degradation). Individ-
ual parameters (i.e., concentration and type of contaminant, species of micro-
organism, etc.) are controlled and varied so that their relative impacts on the
overall process can be measured. In many instances, a homogeneous soil
and single type of contaminant are used for these experiments. These bench
experiments are run in order to gather information on the degradation pro-
cesses so that the information can be applied to full-scale site remediation
designs. However, many field sites consist of heterogeneous soil and a mix-
ture of contaminants, which may alter the degradation rates. Therefore,
difficulty exists in scaling up processes from the bench level to the field level
due to competing and inhibiting factors. In order to assist with this transition,
three scales of observation will be outlined so that limiting processes at each
level can be identified.
3.5.1 Microscale phenomena
The microscale is the scale at which chemical and microbiological species
and reactions can be identified independent of any transport phenomena.
The dimension of this scale is analogous to the size of the microbial cell,
which is approximately 10
-6
to 10
-5
m. This is the level at which most bench
experiments focus (Figure 3.1). When assessing the limiting factors at this
scale, the underlying question is:
Can the bugs eat the contaminant?
Other associated issues that pertain to this scale involve the following:
determining the concentration of degrading microorganisms at the site (i.e.,
the technique used to count the microbes), factors limiting biotransformation
and whether it can achieve acceptable regulatory levels for the contaminant,
and the relationship between laboratory kinetics and field kinetics.
3.5.2 Mesoscale phenomena
The mesoscale is the scale at which transport phenomena and system geom-
etry are apparent at the pore channel, soil particle, or microbial aggregate
level (Figure 3.2). The approximate dimension of this scale is 10
-5
to 10
-2
m.
When assessing the limiting factors at this scale, the underlying question is:
Can the bugs get to the contaminant?
Other associated issues that pertain to this scale include the extent to which
sorption and interphase transport influence the rate of bioremediation.
