BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 164

be affected by several factors, including: (1) the concentration of contaminant present;
(2) hydrogeochemical conditions at the site; (3) competition by indigenous organisms; (4) the
number and relative activity of dehalogenating organisms (i.e., Dhc ) in the added consortium;
(5) in situ growth or death of the added organisms; and (6) filtration, attachment and
detachment of the added culture.
Although several hundred bioaugmentation projects have now been performed around the
world, most have relied on simple assumptions, simple models, cost barriers, a sense of feel, or
wild guesses to determine the amount of culture to add. Because relatively few of these
applications have been described in published literature, and because the published studies
often do not describe how many organisms were used or how the amount of culture was
selected, the task of selecting the “correct” amount of culture for a given application remains
challenging.
Some guidance on the amount of culture needed for successful remediation was provided
by Lu et al. ( 2006 ) who evaluated eight sites to determine the amount of Dhc needed to achieve
reasonable rates of remediation at field scale. They concluded that sites with a “generally
useful” rate of dechlorination of cis- DCE and VC (rate constant 0.3/year) had Dhc densities

greater than 10 7 cells/L of groundwater. Although this data set was small, the results are
consistent with field-scale results where successful bioaugmentation was associated with Dhc
numbers > 10 7 /L (Hood et al., 2008 ; Ellis et al., 2000 ; Lendvay et al., 2003 ; Major et al., 2002 ;
Ritalahti et al., 2005 ).
However, R¨ling ( 2007 ) analyzed the data provided by Lu et al . by using “metabolic
control analysis” (MCA) and concluded that the flux reported by Lu et al. was not regulated
by population size, but rather it was regulated at the cellular level (e.g., the specific activity of
the cells). The key point is that effective bioaugmentation requires not only an adequate
number, but the organisms also must be in an appropriate physiological condition. A recent
study by Schaefer et al. ( 2009 ) provides support for this conclusion, and suggests that in situ
treatment of VOCs can select for Dhc populations with faster dechlorination rates. Unfortu-
nately, these findings complicate the challenge of predicting the amount of Dhc organisms
that must be added to a target aquifer to achieve timely and cost effective remediation.

Laboratory Studies

Several laboratory microcosm, column and model aquifer studies have been performed to
evaluate bioaugmentation for chlorinated solvent remediation. Although these studies are
useful for evaluating the efficacy of a bioaugmentation remedy for a chosen site, they rarely
compare the effect of different cell dosages on remedial efficiency. In fact, the amount of Dhc
cells added to microcosms is often not reported. In one case, 2 milliliters (mL) of a culture
presumably containing 10 6 Dhc /mL was added to microcosms containing 60 g of soil and
150 mL of groundwater and a TCE concentration of approximately 800 m g/L TCE (Major et al.,
2002 ). This inoculum density is equal to approximately 1 10 7 Dhc /L. TCE degradation began

after about 30 days of incubation and all of the TCE was converted to cis- DCE by about day 42.
When a 100-fold higher concentration of TCE was added, TCE degradation began at about day
42 and was complete by about day 90.
In another laboratory study (Sleep et al., 2006 ), 30 mL of a Dhc culture containing

10 7 Dhc /mL was added to a bench-scale flow cell with a pore volume of approximately
2 L of water and a PCE DNAPL. This inoculum density represented a Dhc concentration of
~3

2

10 8 Dhc /L. cis- DCE was detected in the flow cell effluent 13 days after bioaugmenta-
tion, and the Dhc concentrations in the effluent increased corresponding to increased
concentrations of cis- DCE in the effluent. Over the course of the study, approximately