Wind-driven SVE system extracts VOCs from landfill

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An innovative soil vapor extraction (SVE) system has operated at the former Ferdula Landfill site in Frankfort, NY, for the past 10 years. The system relies on wind-driven vacuum processes rather than electrically powered air blowers to extract volatile organic compounds (VOCs) from the landfill. After a two-year trial beginning in 1998, the New York State Department of Environmental Conservation (NYSDEC) approved long-term use of the SVE system to address VOC-contaminated soil serving as a contaminant source within the landfill.

Soil-gas monitoring indicates that concentrations of contaminants of concern, primarily toluene and trichloroethene (TCE), have decreased 91-94% since implementing system upgrades in late 2001. The landfill received industrial waste such as chlorinated and non-chlorinated solvents generated during gun manufacturing at the nearby Remington Arms Company. Wastes from the wood finishing and metal cleaning processes were disposed at the landfill from approximately 1967 to 1975. Subsurface soil consists of 2-10 feet of a lacustrine sand underlain by a 75-foot-thick lacustrine silt and clay unit with fine seams of sand. Ground water is encountered at a depth of 5-6 feet. In 1996, the landfill was closed and covered with a conventionally engineered cap constructed of clay and a synthetic membrane.

As a second containment remedy, an upgradient diversion system comprising a bentonite slurry wall and underdrain was installed to divert clean ground water away from the site, thereby reducing the amount of leachate requiring treatment. Following downgradient collection, leachate is discharged to the county wastewater treatment system. The SVE system was designed to remove VOCs from the unsaturated portion of the 1.8-acre landfill. The system employs a single windmill to create a vacuum that extracts the VOCs for aboveground carbon treatment (Figure 1). The windmill’s 12-ft blades reciprocate a single 10in air cylinder fitted with check valves that enable each intake stroke to draw a vacuum from the landfill vents. In turn, the cylinder intake piping is attached to a network of nine gas vents on the landfill cap. Check valves on both ends of the cylinder allow a vacuum to be generated during each intake cycle (both up and down strokes of the windmill), producing about 85 ft3/hr/mph of vacuum for each mile of wind speed per hour. Each cylinder exhaust stroke pushes the extracted air through carbon canisters before emission at the top of a 40-ft tower.

The windmill operates at all times with winds of 3-20 mph. During winds exceeding 20 mph, a safety feature automatically furls the mill away from the wind and applies a brake to suspend operations. In contrast to continuous operation of air blowers used in conventional SVE systems, wind intermittency inherently provides the pulsed effect typically found to be more effective in venting operations. System upgrades were implemented after the two-year trial operation to increase the rate of VOC extraction. The area of venting influence was expanded by installing six additional vents to supplement the original three vents. In addition, windmill performance during low wind (< 5 mph) was improved by replacing the original steel blades with aluminum blades as well as the original four bronze bearings with steel roller bearings.

This reduction in rotating mass and bearing friction increased operational intermittency of the system 50-90% while maintaining a pulsed effect and increasing the rate of VOC extraction more than 25%. Remote data collection systems also were installed to allow for continuous monitoring. Soil-gas sampling showed that total VOC concentrations decreased from over 2,000 mg/m3 in 2002 to approximately 175 mg/m3 in 2007. Toluene concentrations in soil gas decreased from an estimated 1,400 mg/m3 in 2002 to 90 mg/m3 in 2007, and TCE concentrations decreased from 200 mg/m3 to 56 mg/m3 in the same period. Since operations began, approximately 1,500 pounds of total VOC mass have been removed. Treatment emissions have consistently met New York state standards for air quality. The SVE system and auxiliary equipment require no electricity and are not tied to the utility grid. Avoidance of electricity consumption provided a one-year payback for the $14,000 cost of windmill equipment and installation. Project construction costs totaled approximately $40,000, including $23,000 for the building housing the data collection and treatment systems. In contrast, capital cost for a conventional blower-driven SVE system large enough to achieve the comparable rate of VOC removal was estimated at nearly $500,000.

Differences in operation and maintenance (O&M) costs also are significant. Annual O&M for the wind-driven extraction system, primarily for oil changes and parts replacement associated with the windmill, average below $500. Thirty-year O&M costs for a conventional SVE system using a 25-hp air blower were estimated at $75,000 each year, including $15,000 for 164 MWh of electricity. Community impacts from the windmill-powered system are negligible compared to a traditional system, which would have required a larger building and generated continuous noise impacting residences within 50 feet of the site. Situated on the highest point of the property, the windmill and small treatment building blend well with the local farming community. The SVE system will operate for at least five more years, depending on continually high rates of VOC extraction, or possibly up to 30 years in support of long-term contaminant containment provided by the landfill and ground-water diversion system. Based on these results, NYSDEC is showcasing the project as an effective, efficient, and sustainable strategy for source removal when combined with long-term containment actions.

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