ECONOMICS AND VALUATION OF BIOAUGMENTATION - Bioaugmentation for Groundwater Remediation - page 312

including: (1) an adequate supply of electron donor; (2) an adequate supply of electron acceptor
(i.e., chlorinated ethenes for Dhc ) (Cupples et al., 2004 ); (3) appropriate geochemical conditions
(AFCEE, 2004 ); (4) the absence of inhibitory compounds in the groundwater (Grostern et al.,
2010 ); and (5) an appropriate groundwater temperature (Friis et al., 2007 ).
A well-designed EISB program will provide for an adequate supply of electron donor to
ensure reducing conditions are maintained and to promote rapid growth of native or bioaug-
mented microorganisms. Sufficient electron donor is required for effective treatment, but
adding too much electron donor should be avoided because it can cause excess production of
undesirable byproducts such as methane, and it represents a nonbeneficial cost to the project.
In EISB systems where there are adequate amounts of donor supplied, it is possible that
concentrations of chlorinated ethenes may be too low to support Dhc growth. Dhc populations
require chlorinated ethene concentrations greater than about 0.05 milligrams per liter (mg/L) to
grow at significant rates (Cupples et al., 2004 ; Schaefer et al., 2009 ), since these compounds are
required for respiration. If the concentrations of the chlorinated ethenes are too low, growth
rates and subsequent spread of the microorganisms may be slow, requiring considerably more
culture initially and tighter spacing of inoculation points to achieve the required distribution of
cells and reasonable degradation rates. At sites with chlorinated ethene concentrations at or
near these lower limits, higher volumes of culture and repeated injections may be required to
maintain biodegradation activity.
Adverse geochemical conditions in groundwater in the vicinity of the injection point also
can inhibit active growth of bioaugmented microorganisms. In situations where the pH of
the groundwater is outside the neutral range (6.8-7.8) optimal for dechlorinating bacteria
(Middledorp et al., 1999 ) there is likely to be a benefit to adjusting the pH of the groundwater.
Non-reduced groundwater also can be problematic. It may be necessary to delay bioaugmenta-
tion until after the electron donor has been added in order for anaerobic conditions to develop
in the aquifer. An oxidation reduction potential (ORP) of -100 millivolts (mV) or less is
considered ideal for Dhc growth and survival (Dennis, 2005 ).
Finally, the presence of certain inhibitory compounds (such as chloroform) can slow or
prevent growth of bioaugmentation cultures (Grostern et al., 2010 ). If the concentrations of
inhibitory compounds are high, consideration may be given to implementing pretreatment of
the groundwater prior to bioaugmentation to reduce the concentrations of inhibitory com-
pounds, or to bioaugmentation with other cultures that target these inhibitory compounds
(Grostern and Edwards, 2006 ).
The effort required to create an environment that is favorable for the growth and activity
of bioaugmented or even indigenous microorganisms can be substantial. For example, highly
acidic and buffered aquifers may require large amounts of alkaline amendments before Dhc
can grow in situ . In such cases, EISB with bioaugmentation may not be economically attractive.
11.3 COSTS, VALUE AND BENEFITS OF BIOAUGMENTATION
11.3.1 Costs for Bioaugmentation Culture and Injection
Bioaugmentation cultures containing Dhc range in price from approximately $100 to
several hundred dollars per liter. However, there can be a wide range in the Dhc cell density,
with concentrations in the range of 10 11 up to 10 12 cells/L (Vainberg et al., 2009 ). Given the
impact of cell density on the time to achieve results after injection, a purchasing decision should
not solely be based on the cost per liter of culture. The cell density and ability of a bioaugmen-
tation culture to degrade different contaminants concurrently also must be considered. Bioaug-
mentation cultures containing Dhc and Dehalobacter ( Dhb ) are currently available that can
degrade chlorinated ethenes and chlorinated ethanes (Grostern and Edwards, 2006 ).