BIOAUGMENTATION FOR MTBE REMEDIATION - Bioaugmentation for Groundwater Remediation - page 301

7. Large numbers of bacteria may be needed for effective treatment : Several of the
pilot- and large-scale demonstrations of this technology have emphasized inoculation
with relatively high concentrations of biomass, because MTBE-degrading organisms
tend to be relatively slow growing and have low cell yields when grown on MTBE as a
sole carbon source (Salanitro et al ., 1994 ). It might be possible to introduce MTBE-
degraders into the subsurface in more dilute solutions and then grow them to higher
cell densities in situ on alternate carbon sources (Smith et al., 2003 ; Steffan et al., 1997 );
however, it should be recognized that bacteria from dilute solutions tend to be filtered
out within a short distance of an injection well, and the non-native MTBE degraders
introduced have to compete with indigenous organisms for the alternate carbon source.
Another point of concern associated with high density cell cultures (2.5 g-total sus-
pended solids/L) injected into the subsurface is that these TSS concentrations are high
enough to plug most conventional well screens, so delivery to the aquifer through
conventional wells is not feasible. In addition, even if the well screens did not filter the
suspended solids, the formation would filter the bacterial flocs over a short distance if
the infiltration rate was slow (Streger et al., 2002 ). For more insight into inoculum
distribution, the reader is referred to the two-dimensional laboratory scale visualization
studies conducted by Braunschneider ( 2000 ), where the relationship between aquifer
characteristics and distributions of bacteria resulting from different delivery methods
were examined. Braunschneider's work includes photos of bacteria distributions with
time during injection into a number of idealized geologies. In brief, that work shows
that: (1) distributions are roughly spherical and localized in coarse-grained sediments
(sands and gravels); (2) the injection causes fracturing and distribution of culture in the
fractures for fine-grained silts and clays; and (3) the culture will travel through
fractures in fine-grained sediments to the more permeable layers in layered settings.
10.7 CURRENT STATUS
Bioaugmentation is rarely used to enhance MTBE degradation. The key physical limitations
include the difficulties in distributing microorganisms (and oxygen) effectively, especially in
lower-permeability formations and the difficulty (and therefore expense) of seeding aquifers
more than 25 ft (7.6 m) deep. These limitations have largely restricted the application of
bioaugmentation for MTBE remediation to shallow contamination in sandy to silty-sand
aquifers. In addition, the high cell densities needed to bioaugment a site effectively have
resulted in relatively high costs for the culture volumes needed, a practical limitation to routine
use of bioaugmentation for MTBE remediation.
Ultimately, the major limitation to bioaugmentation for MTBE and TBA remediation is
that it has not been demonstrated to be necessary, since bacteria have proven capable of
adapting to these contaminants under most conditions. In situ bioremediation of MTBE and
TBA has proven effective, most commonly in aerobic biobarriers, without the need for
augmentation. A few field tests show that bioaugmentation can drastically minimize the lag
time required to achieve measurable or optimal MTBE degradation rates, but biodegradation
does occur in time after biostimulation by only oxygen addition. Several side-by-side field tests
have resulted in essentially equivalent MTBE reduction in plots treated by biostimulation only
or by biostimulation with bioaugmentation.
Although several cultures have been developed for aerobic bioaugmentation, few are
commercially available currently. For example, the mixed and single isolate cultures used in
the Environmental Security Technology Certification Program (ESTCP)-sponsored large-scale
biobarrier demonstration (Miller et al., 2003 ) were supplied by Shell Global Solutions, but these
are no longer commercially available. A survey of vendors and consulting firms found only one