Biomedical Engineering Reference
In-Depth Information
square meters (m 2 ) in area (roughly 0.06 acre), over a 3-m (10-foot [ft]) deep interval.
This additional cost estimate includes the costs for both the culture as delivered to the site
(assuming a requirement for 20 L of culture) and injection into existing wells. The added cost of
bioaugmenting amounted to $10.67/m 3 , or $8.20 per cubic yard (cy), which represented 11%
of the capital cost for the treatment system, and 2.5% of the total Net Present Value (NPV)
cost of the overall remedy.
This template site is relatively small, and it is likely that some unit costs will be lower at larger
sites or under differing conditions. Further, the amount of culture that has to be injected can
differ significantly. A separate cost evaluation was completed for this chapter, assuming a larger
model site and lower volumes of culture per unit treated aquifer volume, based on the detailed
cost analyses developed by Steffan et al. ( 2008 ). This cost analysis was based on a review of
actual costs for 40 sites that had been bioaugmented by Shaw Environmental, Inc. The average
cost of the bioaugmentation culture for the 40 sites was $0.30/m 3 , or $0.23/cy. The model site
used for this analysis was 0.4 hectare (ha) (1 acre) in area, with a 3-m (10-ft) thick target treatment
interval. The analysis assumed a maximum culture cost of $300/L, a requirement for roughly
50 L of culture and approximately $3,000 for the labor required to inject the culture in existing
wells. The resulting upper estimate of the total additional cost to bioaugment the model site
(12,000 m 3 , or roughly 15,700 cy) was $12,000 (an additional cost of $1.00/m 3 , or $0.76/cy).
As these separate cost evaluations demonstrate, the costs at specific sites can vary greatly
depending on several factors. Because the costs are so variable, any economic assessment will
require a preliminary site-specific design. These preliminary designs should consider the wide
variety of bioaugmentation options that are available to optimize the technology for a given set
of objectives and site conditions (see Chapter 5, Methods of Bioaugmentation). Also, it can be
important to consider some potential economic impacts that are often overlooked. For example,
the evaluations done in Chapter 11 do not take into account some expenses that proactive
bioaugmentation may prevent, such as additional monitoring events or project meetings.
The most critical site characteristics that will impact the decision to bioaugment are those
that control the volume of inoculum needed, the spacing needed between injection points and the
need for active recirculation to improve distribution within the treatment zone. These character-
istics include the depth and thickness of the contaminated zone, the hydraulic conductivity, the
degree of heterogeneity and the spatial distribution of the contamination within the subsurface.
The type of bioremediation strategy used also can impact the cost comparison. For
example, source zone bioremediation often is an active remediation process, involving frequent
or even continuous injections of electron donors. In this case, the costs of waiting for optimal
performance can be much higher than the cost of bioaugmentation. On the other hand,
bioaugmentation may be less attractive when using more passive, long-term bioremediation
approaches (e.g., use of longer lasting donors, such as edible oils, to treat sources or plumes, or
to establish biobarriers).
Economic assumptions and requirements can affect bioaugmentation decisions in many
ways. In essence, bioaugmentation becomes more attractive as the long-term life cycle costs
become a more important consideration, and it becomes less attractive as minimizing the initial
capital costs becomes more crucial. For example, the discount rate used in assessing the NPV of
expenses incurred often is much higher for private industries than for the public sector (typical
discount rates may be 7% for the private sector, as opposed to approximately 3% for
government agencies). The result is that in the private sector it may be more attractive to
avoid expenses early in a project (e.g., bioaugmenting at the start of treatment), especially if
there is a reasonable chance that the expense may not be needed later.
Finally, there can be some less obvious economic drivers. Some responsible parties may
have financial constraints that make deferring expenditures more attractive, even if the life
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