Environmental Engineering Reference
In-Depth Information
7.7.4 Application of Chemotaxis for Contaminant
Containment - A Variable Length Biocurtain
Preventing off-site migration of contaminants with ground-water flow thereby
containing the source for biodegradation is known as contaminant contain-
ment. In practice, barriers such as slurry walls and reactive barriers in conjunc-
tion with continuous or intermittent pumping are used for containment of
contaminated sites. A biocurtain is a biologically active zone around a con-
taminated region designed to contain and remediate the site. A biocurtain
containing chemotactic bacteria can be advantageous by increasing the length
of the reactive zone. Changes in ground-water flow conditions may occur due to
pumping failures or extreme precipitation events. Chemotaxis enables bacteria
to adjust their position with respect to changes in the physical and chemical
characteristics of the contaminant plume, thereby reducing the risk of off-site
migration. In addition, since chemotactic bacteria can sustain higher flow
velocities [47, 53] pumping costs associated with contaminant containment
can be reduced by decreasing pumping rates and frequency.
7.8 Conclusion
Chemotaxis is traditionally studied in medicine and other areas of microbiolo-
gical research. It has recently attracted interest in the environmental research
community due to its potential for improving in situ bioremediation. Research
to understand the impact of chemotaxis on bacterial transport in the subsurface
and its role in enhancing bioremediation is ongoing. However, there are several
key areas, described below, which would lead to a better understanding of the
role chemotaxis may play in improving remediation of contaminated aquifers.
Only a portion of bacteria in a uniformly distributed contaminant plume
forms a chemotactic band [41]. Identification of the critical bacterial con-
centration that maximizes chemotaxis would help engineers minimize bio-
clogging and optimize bacterial and nutrient injection requirements.
High contaminant concentrations can be toxic to degrading bacteria [10, 56].
Negative chemotaxis enables bacteria to swim toward lower concentrations
of contaminants, however, this process and the ultimate distribution of
bacteria surrounding high contaminant loads are not well understood.
Chemotaxis studies related to bioremediation have all been reported under
aerobic conditions. However, anaerobic conditions prevail in many subsur-
face environments. Chemotaxis in anaerobic conditions should be consid-
ered for groundwater remediation processes.
Evidence from recent laboratory column studies and numerical simulations
suggests that chemotaxis can potentially improve in situ bioremediation of
contaminated sites [5, 7, 8, 21, 41, 42, 43, 46, 47, 49, 53]. However, to the best
