ECONOMICS AND VALUATION OF BIOAUGMENTATION - Bioaugmentation for Groundwater Remediation - page 310

11.2.2.1 Volume of Active Culture
The volume of active culture required for effective inoculation is a function of the
population density of viable cells in the culture shipped to the site, and the number of microbial
cells that remain viable in the subsurface following injection of the culture. In the case of Dhc ,
research suggests that “generally useful” rates of biodegradation will occur when the density of
viable Dhc cells has increased to a concentration of 10 7 cells per liter (cells/L) or greater (Lu et al.,
2006 ). This target can be used to estimate the volume of culture needed for a specific site.
For example, assuming that a bioaugmentation culture contains approximately 10 11 cells/L
(typical for Dhc inocula), and that it is uniformly delivered within a target volume of the aquifer,
then 1 L of culture could be used to inoculate approximately 10,000 L of groundwater. The onset
of degradation should be observed within that volume of groundwater almost immediately
provided geochemical conditions are appropriate to support microbial activity. Obviously, if less
culture is added then more time will be required before degradation activity is observed (the time
required will be a function of the growth rate of the introduced microorganisms in the field).
Similarly, faster degradation rates will be observed if more culture is initially added.
From a practical perspective, a minimum of 1 L of culture is typically added to a single
injection location or well, although smaller amounts have been used (Philip Dennis, SiREM
Laboratories, personal communication, 2010). In many cases, a typical application rate for
culture is 1 L of culture for every 35,000 L of groundwater. This inoculation rate is based on
significant field experience and results in an initial cell density slightly less than 10 7 cells/L. This
cell density is adequate because of the impact of the growth of Dhc cells following injection.
The inoculation concentration may be increased in situations where there is a potential that
geochemical or other factors may result in slower than typical growth rates following injection
or where a faster response time is required.
Given the impact of the initial cell density of a bioaugmentation culture on the time before
the observed onset of degradation, the number of viable cells that can be delivered to the
subsurface is a key parameter. The supplier must provide cultures containing appropriate cell
densities, and equipment and procedures for culture transport and injection. Factors such as
exposure to oxygen during transport, storage times and injection procedures affect the viable
number of Dhc cells that are actually injected at a site (Vainberg et al., 2009 ). Containers used
for transport and storage must be carefully designed to prevent exposure to air and to allow the
culture to be added to the subsurface under anaerobic conditions. If these conditions are not
met, cell death may occur and additional culture or additional time will be required to meet
project objectives, both of which involve additional project cost.
11.2.2.2 Distribution of Introduced Organisms
The distribution of the introduced culture is a function of: (1) the distance between injection
points; (2) how the culture is injected (e.g., the volume of anaerobic water used to chase or carry
the culture from the injection point into the formation); (3) the transport properties or
“stickiness” of the microorganisms; and (4) site-specific factors such as the geology and
geochemistry of the groundwater. Effective distribution of the culture is one of the key
considerations in a bioaugmentation plan.
Injection of the culture may be performed several days or weeks following injection of the
electron donor (typical for an active or semi-passive approach to adding electron donors) or
may be conducted at the same time (typical for a passive approach of electron donor addition).
Injection is accomplished by displacing the culture from the shipping containers into the
injection well using an inert gas while maintaining the culture under anaerobic conditions.