Biomedical Engineering Reference
In-Depth Information
hydrogen (H
2
) and acetate. Hydrogen serves as an electron donor for dechlorinating bacteria,
and a chlorinated ethene (usually PCE or TCE) is added as the electron acceptor to allow
growth. The substrate fermentation product, acetate, may serve as a carbon source and in some
cases also serves as an electron donor.
3.1.2 Why High Density Microbial Cultures Are Important
One of the significant challenges of performing bioaugmentation at a commercial scale is
the large size of contaminant plumes and the large amount of culture that often is needed to
facilitate timely and successful remediation. The issue of scale can best be illustrated by a
simple hypothetical example. One acre of land (0.4 hectare [ha], equal to 43,560 square feet [ft
2
]
or 4,047 square meters [m
2
]) is slightly smaller than the size of an American football field
including the end zones (57,600 ft
2
; 5,353 m
2
). If we assume that a groundwater plume extends
throughout this 1 acre area (300 ft
44 m) within a 10 ft (3 m) saturated
thickness, the total volume of the contaminated media would be ~435,000 cubic feet (ft
3
)
(123,000 cubic meters [m
3
]). If the aquifer has an effective porosity of 25%, the total volume
of contaminated water in the plume would be 109,000 ft
3
145 ft; 91 m
10
6
liters [L]).
To achieve a final
Dhc
concentration of 10
7
Dhc
/L of groundwater to effectively remediate the
site (Lu et al.,
2006
), 3
(3,087 m
3
;~3
10
13
Dhc
cells would be required. If the culture growing process
produced 10
9
Dhc
/L (Major et al.,
2002
), ~30,000 L of
Dhc
culture would be required. At a cost
of $150-$300/L, the culture cost for this moderately-sized plume would be $4.5-$9 million.
Using a culture with 10
11
Dhc
/L would reduce the cost by a factor of 100.
Of course, several factors come into play in actual remediation scenarios (Lee et al.,
1998
).
For example, it may be unrealistic to expect even distribution of the
Dhc
across a contaminated
aquifer, so we would expect higher concentrations of culture, and dechlorination activity, near
injection points. Practitioners also may consider constructing a series of
in situ
flow-through
Dhc
-seeded barriers or recirculation systems, depending on the remedial goals, to reduce the
amount of culture needed. In addition, if conditions are favorable, some growth of the culture
can be expected
in situ
. Nonetheless, it is apparent that large volumes of culture may be needed
to treat some plumes, and production of high cell density cultures can greatly reduce the
volume of culture needed for, and the cost of, bioaugmentation treatment.
If only a small amount of organisms is added and
in situ
growth is anticipated, however,
the actual cost of growing these organisms
in situ
under sub-optimal growth conditions also
should be considered. For example, typical
in situ
temperatures (12-15 degrees Celsius [
C])
would likely promote significantly slower and less efficient growth of
Dhc
than the optimum
temperatures (25-35
C) maintained in reactor systems. Similarly, although substrate feed rates
can be carefully controlled in a reactor system,
in situ
substrate feeding, especially electron
acceptor feeding, is determined by groundwater flow rates and is likely to result in inefficient
growth of
Dhc
and inefficient electron donor utilization. Poor growth and substrate utilization
can ultimately result in greater overall treatment costs than adding additional high density
culture at the beginning of treatment.
3.2 GROWING INOCULA
3.2.1 Microbial Growth Options: Batch Versus Continuous
Growth of bacterial inocula is a mature science, but in practice it is often as much art as
science. Ljungdahl and Wiegel (
1986
) have provided excellent general guidance for growing
anaerobic bacteria. The production of consistent bioaugmentation cultures for chlorinated
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