Environmental Engineering Reference
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(a) Non-chemotactic degradative bacteria
(b) Chemotactic degradative bacteria
Low permeability region with trapped contaminant
Dissolving contaminant
GW Flow
GW Flow
Chemotactic bacterial accumulation
Dispersed nonchemotactic bacteria
Fig. 7.2 Removal of contaminant trapped in low-permeability lenses. Non-chemotactic
strains (a) remain dispersed and wash away with ground-water flow. Chemotactic bacteria
(b) sense and respond to the chemical gradient formed due to contaminant diffusion and
accumulate around the contaminated site
7.7.2 Application of Chemotaxis to Bioaugmentation
Introduction of beneficial microorganisms into contaminated aquifers for the
purpose of enhancing biodegradation is referred to as bioaugmentation.
Bioaugmentation is highly site-specific and dependent on the microbial ecology
and physiology of the site [54], however, it is feasible for combined chemotaxis
and genetically improved degradation capabilities to significantly improve the
remediation rate of an aquifer.
Figure 7.3 shows a schematic of a bioaugmentation strategy that takes
advantage of chemotaxis for improved contaminant removal from an aquifer
contaminated due to leakage from an underground storage tank. Ground-
water flow past the contaminated region will slowly dissolve the contaminant
and a contaminant plume will form that may contaminate the entire aquifer
system. Figure 7.3a represents the initial step of a bioaugmentation scheme.
Injected non-chemotactic bacterial strains would consume the contaminant
plume in the vicinity of an injection point or would rely on contaminant
advection away from the injection point for further consumption (Fig. 7.3b).
Formation and movement of chemotactic bacterial bands in a uniformly
distributed contaminated region (such as a contaminant plume) has been
demonstrated [47]. A moving bacterial band can migrate upgradient and/or
downgradient against contaminant gradients that are moving with ground
water to overcome mass transfer limitations (Fig. 7.3c). Moving bacterial
bands capitalize on naturally available nutrient resources (such as electron
acceptors) within the aquifer while non-chemotactic strains depend solely on
the addition of external nutrients to stimulate the biodegradation process. Since
chemotactic bacteria have the ability to swim against typical ground-water flow
velocities [47, 53], a given bacterial washout rate may be slower for chemotactic
strains compared with non-chemotactic strains, which would ultimately result
in less frequent bacterial and nutrient injections. Bioaugmentation using
