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Constructed wetlands, both surface-flow and subsurface-flow,
are being used to treat petroleum-contaminated waters. As
compliance managers shift their attention to the ”end”
game of remediation, they are more and more frequently considering
constructed wetland systems as a cost-effective, long-term
solution. Recent advances in constructed wetland design have
opened up new opportunities for the application of engineered
wetlands for remediating petroleum-contaminated sites.
Why Wetlands
Work
Petroleum wastes naturally degrade in wetland environments.
This is because the microbial community associated with wetland
environments supports the breakdown of many volatile organic
compounds. Engineered wetland systems offer the benefits of
natural wetlands, but can be tailored to meet the treatment
and construction needs of each individual site.
Wetland bioremediation systems require much less operation
and maintenance than conventional mechanical treatment systems.
Their visual impact is minimal, allowing them to be easily
integrated into site re-use opportunities, such as brownfields
redevelopment, in contrast with obvious mechanical treatment
systems. Assuming that there is enough space for a treatment
wetland and the economics are favorable, acceptance of wetland
treatment by the general public and neighboring landowners
is often quite high. With proper attention to hydrology and
plant selection, they can be designed as visually attractive
“amenities” that enhance the value of surrounding
areas. Because of their low visual impact, wetland treatment
systems are ideally suited for integration into parks, golf
courses, prairies, and other open spaces.
Surface or Subsurface?
Two types of constructed wetlands are currently used for remediation
applications: surface-flow systems and subsurface-flow systems.
Surface-Flow Wetlands
A surface-flow system is an engineered wetland with areas
of open water similar to that of a natural marsh. These systems
are typically designed to support the growth of emergent wetland
plants, such as cattails and bulrushes, although deeper, pond-like
areas may be incorporated in the design. Surface-flow systems
are less expensive to construct than their subsurface-flow
counterparts. They also typically create more waterfowl and
wildlife habitat than can subsurface-flow systems.
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| One of the features
of the project was the construction of surface-flow wetlands,
which were designed to handle up to 11,000 cubic meters
per day of gasoline-contaminated groundwater. |
Subsurface-Flow
Wetlands
In subsurface-flow wetlands, the water level is kept below
the surface of the bed and is not exposed during the treatment
process. Water flows horizontally through the bed, which is
planted with emergent wetland plants. Due to the higher surface
area present in a gravel bed, subsurface-flow wetlands can
provide better treatment per square foot than surface-flow
systems can.
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Removal of BTEX (benzene, toluene, ethylbenzene, xylene)
compounds happens through volatilization and aerobic biodegradation.
A recent development by North American Wetland Engineering
in wetland remediation systems has been to add Forced Bed
Aeration to subsurface-flow wetlands. Aerated subsurface-flow
wetlands have been demonstrated to be more effective than
non-aerated systems in removing these hydrocarbon compounds
from contaminated groundwaters, a critical factor in the growing
popularity of this application. Due to their simplicity of
operation, low maintenance needs and high treatment efficiencies,
aerated wetland systems are gaining in popularity.
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Biology Versus Mechanics
Wetland systems provide biological complexity instead of mechanical
complexity. Advances in wetland design (such as aerated wetlands)
are now blurring the line between “passive” and
“active” treatment systems. Economics are also playing
a role in this decision process, because plants and bacteria
work for free; people and machines do not.
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| Radial-flow wetlands designed to address subsurface-flow
distribution. |
The economics of wetland treatment is most favorable for
site managers who can trade space for mechanical complexity,
and who must operate a treatment system over a long period
of time. A larger site with ample open space is more favorably
suited for a wetland system than a tightly constrained site
in the middle of a major metropolitan area.
Depending on the type of contaminant, capital costs for wetland
systems can be comparable to their mechanical counterparts.
The cost of wetland systems is highly dependent on the cost
of local labor and materials (earthwork, gravel, plants, etc.).
Areas with low costs for land, labor, and local materials
will be better candidates than areas with high material costs.
In addition, if material from the jobsite can be recycled
for use in the wetland, such as was done by British Petroleum
(BP) in Wyoming (see project profile), capital costs dramatically
drop.
Project Profile: Wetland Remediation in Wyoming
A wetland system implemented by BP in Casper, WY, is the largest
constructed remediation wetland in the US. The site was one
of the oldest and largest Amoco Oil Co. refineries in the
West, which began operation in 1908. It was the largest refinery
in North America during the 1920s and continued operation
until 1991. As a result of common operating practices during
the first 50 years of operation, much of the site is underlain
with residual hydrocarbons. Since 1981, over 34,000 cubic
meters of light non-aqueous phase liquids have been removed
from the groundwater.
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| The patented wetland aeration system, seen here being
tested during construction, accelerates the treatment
of BTEX and MTBE compounds in the treatment cells. |
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| The center-feed radial-flow design
of the subsurface-flow wetlands cells had never been used
in North America. The system, which went online in May
2003, has been hydraulically loaded at roughly 2,600 cubic
meters per day. |
Faced with the rising cost of environmental cleanup, Amoco
decided to close the refinery in 1993. Due to the residual
hydrocarbons on the site, BP was faced with a potentially
$350 million liability due to court injunctions and citizen
lawsuits. Rather than embark on the typical path of lengthy
litigation with a timeframe of 10–15 years, the company
decided to engage in an innovative remediation strategy, accelerating
the entire remedy process into a six-year window. With the
sobering realization that the remediation of this site will
continue for the next 50–100 years, the traditional solutions
of a mechanical system with high operation and maintenance
costs was not an attractive option. The cost and benefits
of eco-technology were apparent. In the case of the Casper
project, construction of an engineered wetland system saved
BP over $12.5 million compared to a conventional mechanical
plant. Over the first 50 years of site remediation, the lower
operating costs associated with constructed wetlands is anticipated
to save an additional $15.7 million.
In 1998, the Wyoming Department of Environmental Quality
finalized a consent decree establishing the framework for
site remediation. In order to clean up the site, BP negotiated
an innovative agreement with the City of Casper. BP would
demolish the old refinery structures and convert the property
into an 18-hole premier golf course—designed by Robert
Trent Jones II LLC), complete with an office park, riverfront
trails, and a whitewater kayak course—designed by Recreation
Engineering and Planning of Boulder, CO.
A constructed wetland was identified as a low-maintenance
alternative to conventional options that was also aesthetically
compatible with the golf course on the property. The design
team was challenged to create a remediation treatment system
that could handle up to 11,000 cubic meters per day of gasoline-contaminated
groundwater, blend into the middle of a premier golf course,
and have a lifetime operation of more than 100 years.
Redevelopment of the 300-acre refinery site was a vast project.
At the peak of construction, over 50 engineering teams and
over 250 full-time construction personnel were involved in
the project. Over 200 miles of underground pipes were excavated
and recycled. Over 300,000 tons of concrete were recovered
from tank foundations, crushed onsite, and re-used as aggregate
for the wetland treatment system. Construction of the golf
course required over 1 million cubic yards of grading, and
over 60 dual-phase recovery wells feed the treatment system.
All design and construction elements were completed in a three-year
period.
North American Wetland Engineering (NAWE), of White Bear
Lake, MN, was selected to design the wetland treatment system.
NAWE had pioneered the development of insulated, cold-climate
wetlands, and they were comfortable designing a wetland that
could operate in Casper’s –35°F winter temperatures.
They could also incorporate their patented wetland aeration
process (US Patents 6,200,469 and 6,406,627) to accelerate
the treatment of BTEX and MTBE compounds in the wetland treatment
cells.
The wetland treatment system design was based on the results
of a pilot system operated at the project site. In order to
meet site objectives, the potential iron fouling of the wetland
media—identified during the pilot operation—needed
to be addressed. To solve this problem in the full-scale design,
NAWE designed the initial stages of the treatment process
to include a cascade aeration system (for iron oxidation)
and a surface-flow wetland (for iron precipitation). Treatment
of BTEX compounds is completed in subsurface-flow wetland
cells.
To address flow distribution in the subsurface-flow cells,
an innovative radial-flow wetland configuration was adopted.
Because the gravel media for the wetland was made of recycled
crushed concrete from structures on the site, it was possible
to tailor the media to provide an appropriate hydraulic conductivity.
Since the pilot testing had demonstrated that aerated wetlands
were more efficient in removing constituents of interest,
a patented Forced Bed aeration system designed specifically
for the wetlands by NAWE was implemented.
The project has substantially advanced the field of engineered
wetland treatment systems. In the pilot-testing phase, the
project was one of the first in North America to determine
degradation rate constants for BTEX under aerated and non-aerated
conditions. Scaling up from the pilot system required a 1,200-fold
increase in reactor volume. As a result the full-scale subsurface-flow
wetlands were designed using an innovative center-feed radial-flow
design to optimize hydraulic efficiency. This radial-feed
design had never been used in North America before this project.
The full-scale system was put online in May 2003. Since startup,
the system has been hydraulically loaded at approximately
2,600 cubic meters per day and system performance to date
has exceeded expectations.
The sustainable wetland treatment systems were integral to
the project and reflect BP’s environmental stewardship
commitments. Perhaps more importantly, the Casper project
has transformed a site that once seemed destined to remain
an unused property near downtown Casper into a landmark helping
to redefine the community. Recreational facilities created
by this project will benefit the citizens of Natrona County
and Casper for years to come, and in a powerful symbol of
support, the Wyoming Oil and Gas Conservation Commission—the
state agency regulating oil and gas development—became
the anchor tenant on the property by opening their new building
in March 2004.
Implications for Compliance Managers
Compliance managers at many industrial sites are now settling
in for the long haul. The sobering realization is that remediation
systems at many sites will have to be actively operated and
maintained for the next 50 to 100 years. Even if in situ strategies
such as bioremediation or phytoremediation are pursued, the
need to maintain gradient control at sites often results in
the continued generation of a contaminated groundwater source
requiring treatment. In this context, the mechanical systems
installed in the 1980s and early 1990s begin to look much
less attractive due to their high operation and maintenance
(O&M) costs. In many instances, the high levels of contamination
these mechanical systems were designed to treat have come
and gone. The remediation challenge faced by compliance managers
is the need to deal with large quantities of groundwater with
low levels of contamination over a very long period of time.
The low-maintenance, environmentally compatible option of
constructed wetlands solves this dilemma.
Conclusion
New challenges in land development (and redevelopment) are
forcing innovative approaches to infrastructure service. By
using new, “green” technology, as embodied by engineered
wetlands, environmental professionals can continue to solve
their client’s problems in innovative, creative ways.
Wetland treatment offers the most hope to compliance managers
facing the challenge of remediating their sites. Ongoing advances
in wetland technology will continue to offer new remediation
options to compliance managers in the twenty-first century.
Because they are a land-based technology, wetlands cannot
be used everywhere. However, on sites with adequate space,
the benefits of wetlands can result in substantial cost savings,
especially for systems that have to operate over long periods
of time. Wetland systems, through their complex assemblages
of plants and bacteria, can provide treatment of recalcitrant,
difficult-to-degrade compounds.
Since wetlands rely on plants and bacteria instead of people
and machines, their O&M costs are much less than mechanical
treatment systems. On sites where compliance managers can
trade space for mechanical complexity, wetland systems can
offer cost-effective, long-term solutions to site remediation
challenges.
SCOTT WALLACE, P.E., is a founding partner
and executive vice president of North American Wetland Engineering
LLC.
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- November/December 2005
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