Remediation Marathon Style

Constructed wetlands are an economical way of cleaning up petroleum-contaminated sites that require treatment over long periods of time

• By Scott D. Wallace
• Jun 01, 2004

In-situ biological treatment (bioremediation) systems have now gained widespread acceptance for dealing with sites impacted by petroleum hydrocarbons. However, at many remediation sites, the need to pump groundwater to maintain gradient control still generates a stream of contaminated water requiring treatment, even if in-situ technologies are being employed.

Wetlands, through their complex assemblages of plants and bacteria, are ideally suited for these applications. Petroleum wastes have been documented to naturally degrade in natural wetland environments because the microbial community associated with the plants creates an environment conducive to the degradation of many volatile organic compounds. Engineered wetland systems (constructed wetlands) offer the benefits of natural wetlands, but can be "tailor made" to meet the treatment and construction needs of each individual site.

Advances in wetland design have opened up new applications for remediation wetlands at many contaminated sites. Both surface flow wetlands and subsurface flow wetlands are currently being used to treat petroleum-contaminated water sources. As compliance mangers shift their attention to the "end game" of remediation, constructed wetland systems can be used as a cost-effective, long-term solution.

Surface or Subsurface?
Two types of constructed wetlands are currently used for remediation applications: surface flow systems and subsurface flow systems. Both systems rely primarily on microbial communities growing as biofilms on plant detritus or in the plant root zone (rhizosphere).

Surface Flow Wetlands
Surface flow systems are engineered open water bodies similar to natural marshes. These systems are typically designed to support the growth of emergent wetland plants, such as cattails or bulrushes, although deeper, pond-like areas may be incorporated in the design. Surface flow systems are more tolerant of high-suspended solids than gravel-bed (subsurface flow systems), and they typically create much greater habitat for waterfowl and wildlife than subsurface flow systems. In the United States, surface flow constructed wetlands have been used to treat petroleum wastewaters since the early 1970s.

Subsurface Flow Wetlands
Subsurface flow wetlands use a bed of granular media (typically sand or gravel). Water flows horizontally through the bed, which is planted with emergent wetland plants, typically Common Reed (Phragmites australis) in Europe, or different species of bulrushes (Scirpus spp.) in North America. The water level is kept below the surface of the bed, so water is not exposed during the treatment process. Due to the higher surface area present in a gravel bed, subsurface flow wetlands can provide increased treatment per square foot, as compared to surface flow systems. Initial work on the use of subsurface flow wetlands to treat industrial organic compounds was completed in the early 1960s in Germany.

A recent development adding to the effectiveness of wetland remediation systems is the addition of aeration to subsurface flow wetlands. Removal of benzene, toluene, ethylbenzene, xylene (BTEX) compounds occurs through volatilization and aerobic biodegradation. In this application, aerated subsurface flow wetlands have been demonstrated to be more effective than non-aerated systems in removing BTEX compounds from contaminated groundwater. One successful example is the Williams Pipeline, which has successfully operated an aerated wetland system since 1998.

These wetland bioremediation systems require much less operation and maintenance than conventional mechanical treatment systems. Because their visual impact is minimal, they can be easily integrated into site reuse opportunities, such as brownfields redevelopment.

Wetland Remediation in Wyoming
A wetland system implemented by British Petroleum (BP) in Casper, Wyo., is the largest and most recent remediation wetland in the United States. This remediation treatment system needed to handle up to 3,000,000 gallons per day of gasoline-contaminated groundwater, blend into the middle of a premier golf course and operate for more than 100 years. Although this may sound impossible, it was the challenge presented to the design team.

The site was one of the oldest and largest Amoco Oil Company refineries in the West, which started 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 9 million gallons of light non-aqueous phase liquids (LNAPL) have been removed from the groundwater.

Faced with the rising cost of environmental cleanup, Amoco decided to close the refinery in 1993. In 1998, the Wyoming Department of Environmental Quality finalized a consent decree establishing the framework for site remediation. Under the consent decree, BP and the City of Casper agreed to convert the former refinery site into a golf course and office park with a trail system along the North Platte River.

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), complete with an office park, river front trails, and a whitewater kayak course (designed by Recreation Engineering and Planning).

The problem was the presence of a large amount of contaminants below the water table. Maintaining gradient control requires the pumping of groundwater for decades before the contaminants can be adequately removed. Because of the time required for remediation (50 to 100 years), BP became very interested in biological treatment processes due to potential cost savings. Knowing that organic chemicals such as benzene, xylene and toluene are biodegradable, BP turned toward wetland treatment technology. A constructed wetland was identified as a low-maintenance system compatible with the golf course use of the property.

A pilot plant was built in 2001 to establish site-specific degradation parameters. Full-scale design started in 2002 after extensive permit negotiations with the Wyoming Department of Environmental Quality.

The wetland treatment system designed was based on the results of the pilot system operated at the project site. In order to meet site objectives, the full-scale wetland had to be capable of operating at 6,000 cubic meter per day (m³/day) and the potential fouling of the wetland media, identified during the pilot operation, needed addressing. To solve this problem in the full-scale design, a cascade aeration system (for iron oxidation) and a surface-flow wetland (for iron precipitation) was added to the system.

North American Wetland Engineering (NAWE) in Forest Lake, Minn., early developers of insulated, cold-climate wetlands, designed the system. The surface flow wetland was designed to allow the precipitation of iron that had oxidized in the cascade. Without iron removal in these wetlands, precipitation would occur in the aerated subsurface flow wetlands, which could conceivably create a fouling problem, given the 100-year operating life of the system.

NAWE had also developed a wetland aeration process (US Patents 6,200,469 and 6,406,627) that could accelerate the treatment of BTEX and methyl tertiary butyl ether (MTBE) compounds in the wetland treatment cells, making them particularly qualified to handle this project.

To address flow distribution, an innovative radial-flow wetland configuration was adopted. Because the gravel media for the wetland was made of crushed concrete recycled from tank foundations, it was possible to tailor the media to provide the needed hydraulic conductivity. Due to the large size of the cells, a patented aeration system designed specifically for wetlands by NAWE was implemented.

The full-scale system was put on line in May 2003. Since startup, the system has been hydraulically loaded at approximately 700,000 gallons per day. System performance to date with compounds measured in milligrams per liter (mg/L) is summarized in Table 1.

Compound	Influent	Cascade Effluent	Wetland Effluent
Benzene, mg/L	0.33	0.15	Non Detect
BTEX, mg/L	0.53	0.26	Non Detect
DRO, mg/L	11.41		Non Detect
GRO, mg/L	2.46	2.02	Non Detect

Table 1. System's Performance from May 2003 - February 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 operations 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 end game 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.

Biology vs. Mechanics
Wetland systems provide biological complexity instead of mechanical complexity. Rather than relying extensively on external energy inputs (chemicals, heat, aeration, etc.), wetlands are often designed to maximize the use of natural energy inputs (solar energy, wind energy, atmospheric diffusion etc.). Site managers are often faced with a choice between a large, relatively passive system (i.e. a wetland) and a smaller, more complex mechanical system: although advances in wetland design (such as aerated wetlands) are now blurring the line between "passive" and "active" treatment systems.

Wetland Economics
The cost benefits of wetland treatment technology can be summed up in a simple phrase: "Plants and bacteria work for free; people and machines don't."

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 long periods of time. A large site away from population centers is more favorably suited for a wetland system than, say, 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 on a par with their mechanical counterparts. The cost of wetland systems is highly dependent on the cost of local labor and materials (earthwork, gravel, plants, etc.) than mechanical systems, which can often be skid-mounted as a package and shipped to the job site. Consequently, the cost for wetland treatment systems varies widely; areas with low costs for land, labor and local materials will be better candidates than areas with high material costs.

However, if material from the job site can be recycled for the wetland, capital costs dramatically drop. For the Casper, Wyo., project, BP was faced with the need and the cost to crush and dispose of concrete tank foundations. They were also faced with the need to purchase sand and gravel for the wetlands. Recycling 22,000 tons of concrete, by crushing and screening it to the size gradation needed for the wetlands, resulted in substantial cost savings. The wetland system was constructed for $3,400,000; expansion and upgrade of the existing mechanical system (air stripping with catalytic oxidation) was projected to cost $15,900,000.

Public Acceptance
Assuming that the space is available for wetland treatment and the economics are favorable, acceptance of wetland treatment by the general public and neighboring landowners is often quite high. Wetland treatment offers non-monetary benefits through the preservation of open space. 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.

The effective use of wetlands technology for bioremediation opens up land use opportunities for brownfields redevelopment. For instance, redevelopment of a site into an office park may be hindered by the presence of a very obvious mechanical treatment system, serving as a constant reminder and negative perception with prospective tenants that the site is contaminated. Utilizing wetland treatment, the treatment modules can be integrated into open space surrounding the office buildings. Tenants look out of their windows to see meadows, fields and ponds. In each case, the site is actively being remediated; however, the wetland eliminates the perception problem and promotes public acceptance for re-use of the site.

Conclusion
Wetland treatment offers the most hope to compliance managers facing the "end game" of remediating their sites. New wetland systems, such as those using aeration, are blurring the line between "passive" systems (those relying exclusively on natural energy inputs) and "active" systems (those relying exclusively on human-made energy inputs). These new "hybrid" systems offer higher levels of treatment (or the same level of treatment in a smaller footprint area). Ongoing advances in wetland technology will continue to offer new remediation options to compliance managers in the 21st century.

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, as illustrated by the BP system in Casper, Wyo.

References
1. American Petroleum Institute. The Use of Treatment Wetlands for Petroleum Industry Effluents. Knight R.L., Kadlec R.H. and Ohlendorf H.M. PI Publication Number 4672. 1998. API Publishing Services. Washington, DC. Reference Type: Report.

2. Litchfield D.K. and Schatz D.D. "Constructed Wetlands for Wastewater Treatment at Amoco Oil Company's Mandan, North Dakota Refinery." Hammer D.A. (Editor). 1989. Constructed Wetlands for Wastewater Treatment: Municipal, Industrial, and Agricultural. Lewis Publishers. Chelsea, Mich. Pages 233-237.

3. Moore B.J., Ross S.D., Gibson D. and Callow L. "Constructed Wetlands for Treatment of Dissolved Phase Hydrocarbons in Cold Climates." Means J.L. and Hinchee R.E. 333-340. Wetlands & Remediation: An International Conference. 2000. Battelle Press. Columbus, Ohio. Reference Type: Conference Proceeding.

4. Pardue J.H., Kassenga G., and Shin W.S. Design. "Approaches for Chlorinated VOC Treatment Wetland." Means J.L. and Hinchee R.E. 301-308. Wetlands & Remediation: An International Conference. 2000. Battelle Press. Columbus, Ohio. Reference Type: Conference Proceeding.

5. Pruden A., Sedran M., Suidan M. and Venosa A."Biodegradation of MTBE and BTEX in an Aerobic Fluidized Bed Reactor." 2003. Water Science and Technology, 47(9). Pages 123-128.

6. Schnoor J., Litch L.A., McCutcheon, S.C., Wolfe N.L. and Carreira L.H. "Phytoremediation of Organic and Nutrient Contaminants." 1995. Environmental Science and Technology. 318A-323A.

7. Seidel K. System for Purification of Polluted Water.3,770,623. 1973. Washington, DC. Reference Type: Patent.

8. United States Environmental Protection Agency. Design Manual: Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment. EPA/625/1-88/022.1988. Cincinnati. Reference Type: Report.

9. Wallace S. "Onsite Remediation of Petroleum Contact Wastes Using Subsurface Flow Wetlands." Proceedings of Wetlands and Remediation: The Second International Conference. 2001a. Battelle Institute. Columbus, Ohio. Reference Type: Conference Proceeding.

10. Wallace S. System for Removing Pollutants from Water. 6,200,469 B1. 2001b. United States. Reference Type: Patent.

11. Wemple C. and Hendricks L. "Documenting the Recovery of Hydrocarbon-Impacted Wetlands -- A Multi-Disciplinary Approach." Means J.L. and Hinchee R.E. 73-78. Wetlands & Remediation: An International Conference. 2000. Battelle Press. Columbus, Ohio. Reference Type: Conference Proceeding.

This article originally appeared in the 06/01/2004 issue of Environmental Protection.