Engineered wetland design for onsite bioremediation of hydrocarbon-contaminated groundwater and wastewater

September 2010 » Web Exclusive » PROJECT CASE STUDY
Scott Wallace, P.E.

Petroleum producers are regularly faced with the challenge of remediating contaminated sites to return them to viable use. Engineered wetlands have emerged as a leading technology for the onsite bioremediation of petroleum hydrocarbon-contaminated water. The engineering optimization of this natural treatment process provides a viable alternative to mechanical remediation systems.

What is an engineered wetland?
Unlike a typical surface water wetland, an engineered wetland often incorporates a subsurface-flow gravel bed reactor, lined with an impermeable liner and equipped with a Forced Bed Aeration system to enhance oxygen delivery to the aerobic microorganisms in the engineered wetland. Design parameters include biodegradation rate coefficients for compounds such as benzene, toluene, ethylbenzene, and xylenes (BTEX), and flowrate, hydraulic residence time, influent, and required effluent concentrations.

While employing many of the same biological processes found in natural wetlands, engineered wetlands are man-made systems that have been designed to emphasize specific characteristics of wetland ecosystems for improved treatment capacity. Leveraging proven engineering principles, the biological complexity and efficiency of natural systems can be exploited to treat complex industrial waste streams. In addition, engineered wetlands can be integrated into parks, golf courses, nature centers, trail networks, natural habitats, and other green spaces where they become a central amenity and an asset to the community. These workhorses are capable of superior performance, delivering significant long-term operation and maintenance savings through passive treatment, and creating public amenities and natural assets for the larger community.

Both surface-flow wetlands and subsurface-flow wetlands are currently being used to treat petroleum-contaminated waters. As compliance mangers shift their attention to the "end game" of remediation, they are more frequently considering engineered wetland treatment systems as a robust, cost-effective long-term solution.

Why engineered wetlands work
Petroleum compounds naturally degrade in wetland environments because the microbial community associated with wetland plants and sediments is adept at breaking down hydrocarbons. Engineered wetland treatment systems can be tailored to meet the treatment and construction needs of each individual site.

Wetlands 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 brownfield redevelopment. For instance, the redevelopment potential of a remediation site into an office park may be hindered by the presence of a large mechanical treatment system. Utilizing engineered wetlands, the treatment modules can be integrated into the 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, unlike mechanical treatment systems, an engineered wetland can actually enhance the aesthetic quality of the site and promote public acceptance of the site redevelopment plan.

Field applications
Such a system is in operation at the former Sinclair Refinery in Wellsville, N.Y. The former site of an oil refinery from 1901 to 1958, currently home to a campus of Alfred State College, is located next to the Genesee River. The long-term closure plan for the site includes a barrier wall to prevent migration of contaminated groundwater to the river. Goals for the remediation included that the systems would operate for decades with minimal maintenance requirements and benefit and blend in with the Wellsville community.

Groundwater extraction pumps deliver contaminated water to a treatment wetland constructed onsite. The system consists of a cascade aerator, sedimentation pond, surface-flow wetland, and vertical-flow wetland, and provides treatment for 650 cubic meters per day of groundwater. The influent has elevated levels of iron, manganese, and organic compounds (including aniline and nitrobenzene).

The cascade aerator provides passive aeration of the influent flow, permitting the iron and manganese to be oxidized. The oxidized metals generate precipitates that are allowed to fall out in the downstream sedimentation pond. After the sedimentation pond, the flow enters a series of surface-flow wetlands, which are lined and operate at water depths between 0.3 meters and 1.0 meter. There are three beds in parallel (each 0.3 hectares) designed to expedite the biodegradation of petroleum hydrocarbons in the water. Flow is then introduced into a series of vertical-flow wetlands comprised of limestone aggregate. The limestone beds are used to adjust for the pH depression related to the upstream iron precipitation.

The design incorporates an upgrade of the adjacent rails-to-trails path, restoration of the adjacent river swale, and a 20-acre comprehensive site plan using native plants including trees, shrubs, wildflowers, grasses, and wetland plants.

This engineered wetland treatment system provides an ecologically-based, low-maintenance, long-term solution to treat the contaminated groundwater. The treatment system requires minimal energy input and is an integral part of the landscape, turning a challenging problem into a community amenity. (For more information, visit www.arwellsville.com)

Conclusion
New challenges in land development and redevelopment are forcing innovative approaches to infrastructure service. Engineered wetland treatment offers real advantages to compliance managers facing the challenge of remediating their sites. At sites with adequate area, the benefits of wetlands can result in substantial life cycle cost savings, especially for treatment systems that must operate for long periods of time. Engineered 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 operations and maintenance costs are much less than mechanical treatment systems. On sites where compliance managers can trade space for mechanical complexity, engineered wetland systems can offer cost-effective, long-term solutions to site remediation challenges.

Engineered wetlands are being used to treat petroleum contaminated groundwater at the site of a former Sinclair refinery in Wellsville, N.Y.
In an engineered wetland treatment system near Wellsville, N.Y., water travels by gravity through a series of wetland cells where native plants naturally break down hydrocarbon contamination and clean the water to meet state and federal water quality standards before it is released into the Genesee River.

Scott Wallace, P.E., is president of Stillwater, Minn.-based Naturally Wallace Consulting LLC. Wallace currently holds one Canadian and four U.S. patents for wastewater treatment processes including Forced Bed Aeration. He is the co-author of “Treatment Wetlands 2nd Edition,” a textbook on wetland treatment systems. He can be reached at scott.wallace@naturallywallace.com.


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