BIOAUGMENTATION CONSIDERATIONS - Bioaugmentation for Groundwater Remediation - page 146

Buffer capacity testing of the groundwater and soil or determination of the acid demand of
the aquifer material and acidity of the groundwater by Standard Method 2310 should be
conducted to determine the appropriate dose of the selected buffer (in the absence of biological
reactions). Design tools can be employed to estimate buffer dose (e.g., EOS Remediation
design spreadsheet for AquaBupH TM or spreadsheet tool and equations for sodium bicarbon-
ate [Robinson and Barry, 2009 ; Robinson et al., 2009 ]). Geochemical models, such as
PHREEQC and MINTEQ, and bench-scale biotreatability tests also can be employed to
estimate buffer requirements.
5.3.3 Culture Requirements
Even though bioaugmentation for chlorinated VOC remediation is widely used, no rigorous
model has been generated to estimate the amount of inoculum needed for a given site. Most
practitioners appear to rely on the guidance of Lu et al. ( 2006 ) and attempt to achieve a
minimum in situ concentration in the range of 10 7 Dhc /L, where complete degradation of TCE
to ethene is often observed.
The simplest approach to estimating the amount of culture required is to estimate the pore
volume of water within a targeted treatment zone and then multiply the treatment volume by
10 7 Dhc /L. The volume of culture needed to achieve 10 7 Dhc /L is dependent on the Dhc
concentration in the bioaugmentation culture, which varies from vendor to vendor. Also, it
generally is preferable to include a lag time to achieve the desired culture density, not only to
reduce the culture costs but also because it is difficult and costly to distribute the cells
throughout the target treatment zone without relying on growth and migration in situ . In any
case, some amount of time (weeks to months) will be required for the culture to grow and
spread throughout the treatment area.
The use of this simple approach does not account for the effects of potentially important
factors, notably the VOC concentrations in the target aquifer and the actual hydrogeology of
the site. Both factors can have a significant effect on the distribution and growth of the added
culture. In cases with relatively high concentrations of VOCs, the model may overestimate the
amount of culture needed, provided other geochemical conditions are appropriate for Dhc
growth and transport. The model may underestimate the amount of culture needed in aquifers
where VOC concentrations are low or where other geochemical factors may limit growth or
transport of Dhc cells in situ . Fine tuning the model may be difficult in many cases, and it will
likely require extensive laboratory microcosm and column studies and the application of more
complex models like those provided by Schaefer et al. ( 2009 ).
Ultimately, the decision may be made by comparing the cost or risk of adding too much or
too little culture with the cost of performing extensive laboratory testing. To aid in evaluating
the amount of culture to apply, Table 5.1 provides data from several pilot and field-scale
bioaugmentation projects to treat a variety of aquifers with varying chemical and hydrogeolo-
gical characteristics. Further information on the relationship between inoculum density and
degradation rates can be found in Appendix A .
5.3.4 Injection Techniques
Prior to adding the Dhc bioaugmentation culture, the well or drive point is purged with
certified 100% argon or nitrogen to remove any oxygen from the well casing and to maintain an
inert gas blanket in the well headspace. All of the required tubing also is purged to remove
oxygen. Following purging, the compressed gas is used to pressurize the culture vessel and push
the culture out of the vessel and down the pre-purged tubing positioned within the well screen at