Environmental Engineering Reference
In-Depth Information
system, such as pump-and-treat, be installed as a contin-
gency to the phytoremediation system.
All remedial strategies, including phytoremediation, have
associated costs. Some costs are borne up front when the
project is started. Other costs include those future costs to be
incurred over the life cycle of the project. The conventional
approach used to account for such future costs includes
experience, but mostly a life-cycle approach is used. A
life-cycle approach sums up the total cost of the remedial
strategy, from installation to projected costs of operation,
maintenance, and monitoring for the anticipated duration of
the treatment, with annual adjustments for inflation, salary
costs, etc. This approach is one way to compare the cost
effectiveness of one remedial strategy relative to an alterna-
tive remedial strategy.
To examine this comparison of cost effectiveness and
phytoremediation more closely, the costs to treat and hydro-
logically control contaminated groundwater at a generic site
will be compared using an example given by Tsao (2005). In
that study, the cost to remediate a site by hydrologic control
using an extraction system was compared to the cost of using
plants to create a hydrologic barrier. The site was
characterized by groundwater at 5-13 ft (1.5-3.9 m) below
ground surface in an aquifer of low hydraulic conductivity,
near 5
tend to decrease the cost savings realized between the
mechanical and plant-based hydrologic control systems.
To account for this devaluation of money over time, an
approach based on a Net Present Value (NPV) cost compar-
ison can be made. The NPV is the difference between the
sum of the discounted cash flows, or net benefits. The NPV is
used widely in the financial industry to assess the likelihood
of an investment reaping a return above initial costs. The
NPV approach also can be used for remediation projects
such as phytoremediation to determine if the money spent
will bring in a return on that investment or simply result in a
cost savings as shown in the previous example.
To determine the NPV for a particular project being
proposed for hydrologic control, or to decide which one of
multiple projects should receive funding, the expected cash
flow per year from the investment also is calculated. From
this amount is subtracted the cost of capital to perform the
project, such as all the costs needed to make a project
happen, often done using an interest rate. From this amount
is subtracted the initial investment costs, the balance being
the NPV. A positive value for NPV indicates that a particular
project would be economically advantageous. Moreover, the
selected payback period has to be met, which is the time
needed for the project costs to be recouped. If the research
and development (R&D) has already occurred and the costs
previously recovered, the payback period becomes moot.
Another factor that affects whether or not
phytoremediation might be used at a site to control or con-
tain contaminated groundwater is that the money that would
go to set up a phytoremediation system, the capital invest-
ment, has to guarantee a rate of return on that investment.
This is important for large companies that have multiple
sites of contamination, because much of their expenditures
are for activities that generate revenue. Although the use of
phytoremediation at a site often does result in measurable
cost savings, phytoremediation may not be selected because
the rate of return is too low (Tsao 2005).
In the example presented by Tsao (2005), the NPV was
determined using a 2.5% rate, as recalculated and shown in
Table
10.2
. The cost savings comparison still selects
phytoremediation over pump-and-treat, but
10
6
cm/s, and adjacent to a former petroleum
refinery. A plume of gasoline that contained benzene,
toluene, ethylbenzene, and xylenes (BTEX) had extended
out beyond the refinery property boundaries
into a
neighborhood.
The initial cost comparison offered by Tsao (2005) used
the life-cycle approach and is summarized in Table
10.1
.
In this example, implementation of the phytoremediation
system would result in a cost savings of $1.29 million
dollars, as the more expensive pump-and-treat alternative
would not be deployed. Such a life-cycle approach is rela-
tively straightforward but does not include the time value of
money or the reduced power of a dollar due to inflation over
the life of the project. Inclusion of this economic fact would
Table 10.1
Life-cycle costs for pump-and-treat using horizontal
extraction compared to phytoremediation.
the savings
realized is slightly lower.
There are other factors to consider when comparing
which of the two technologies, pump-and-treat or phytore-
mediation, should be used to achieve hydrologic control.
Regarding the above example, Tables
10.1
and
10.2
include
tangible costs. Less tangible and less quantifiable factors
also affect the costs of pump-and-treat relative to
phytoremediation. These semi- to non-quantifiable costs
include risk assessment and reporting costs, regulatory
acceptance, and the favorability of the local community to
each remedy. All of these factors can affect the NPV and,
therefore, total cost of the remediation.
Horizontal
Plant-based
Extraction
Hydrologic barrier
Item
Cost
Item
Cost
R&D
$0
R&D
$110,000
Installation
$1,000,000
Installation
$200,000
$750,000
a
Operation and
Maintenance
Monitoring
Operation and
Maintenance
Monitoring
$45,000 year 1
$25,000 year 2
$80,000 year 3-8
Total Life-Cycle
Cost
$1,750,000
$460,000
Cost Savings
(
$460,000)
$1,290,000
a
@$150,000/year for 5 years
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