ONE OF THE MANY CHALLENGES faced by landfill managers is reducing landfill emissions. Short of a magic wand, the Sullivan County Landfill, Monticello, N.Y., instead used ingenuity and elbow grease to reduce landfill emissions and stabilize waste.
In 2001, three adjoining areas of a new cell at the Sullivan County Landfill were used to evaluate the feasibility of vacuum-induced, semi-aerobic (VSA) stabilization of municipal solid waste (MSW). Although the process sounds strange, what's notable is that it reduced landfill gas (LFG) emissions by up to 99 percent.
The VSA method involves installing 12-inch-diameter landfill pipes in horizontal trenches below the waste surface. As the waste is placed and compacted, leachate followed by a synthetic daily cover are both spray-applied. Then, a small vacuum applied through perforated pipes creates vacuum pressure, which is measurable from 137 feet away. This process accelerates the biostabilization of organic material and reduces LFG emissions.
To achieve semi-aerobic biostabilization, the waste in upper layers is aerated for a period of 30 to 90 days, after which it reverts to normal anaerobic landfill conditions as more waste is placed above. Unlike conventional composting, which involves aeration throughout the decomposition period, this process only aerates the waste's upper zone for a short time. Lower layers generate normal LFG containing methane. Overall, LFG emissions are significantly reduced when vacuum gas collection is operated concurrently with biostabilization activities.
The VSA process reduced methane gas by an average of 90 percent up to 55 feet away. Directly above the pipes, gas flux was reduced by more than 99 percent when compared to a conventional cell. Under nonvacuum conditions, gas flux was approximately 30 times greater than the test cell. Vacuum areas also produced approximately 90 percent fewer nonmethane organic compounds than nonvacuum areas.
“By burying these pipes and applying this small vacuum, we reversed the direction of flow at the surface and drew a small amount of air inward,” explains David Hansen, president of Landfill Service Corp., Apalachin, N.Y. “In a conventional landfill, [operators] fill the cell and drill wells later when [waste is] capped, but in some cases, up to half the total gas has already escaped.”
Waste temperatures also were lower than expected in the semi-aerobic areas, with a maximum of approximately 145 F. Typical temperatures in the nonaerated areas ranged between 100 F and 120 F. Hansen attributes the moderate temperatures to the wet waste and minimal air being drawn in.
“These relatively moderate temperatures in the semi-aerobic area indicate a low potential risk for internal fire ignition,” Hansen says.
Added leachate increased the effective density of waste by 4 percent to 8 percent more than waste that was not wetted. The spray-applied cover increased the landfill effective density by 20 percent, compared with use of typical soil and ash daily covers.
The VSA method is financially comparable to conventional methods of controlling LFG, according to Hansen. For example, pipes used at the Sullivan Landfill added an additional 20 cents per ton of waste, but reduced expenses associated with vertical wells. And many landfills are starting to use horizontal gas collectors, Hansen adds. The total costs for adding leachate and the daily cover totaled approximately $520 per day. Spraying leachate requires a full-time operator and dedicated equipment, but the equipment also can be used to spray the daily cover.
The avoided costs for leachate disposal and cover soil typically defray much of the equipment costs while increasing cell capacity by 20 percent. For example, the added airspace potentially creates an extra revenue of $5,000 per day if a landfill receives 1,000 tons per day of waste at a tipping fee of $25 per ton.
“Installing these pipes does cost money. The surprising and encouraging thing was that we could install a conduit in the garbage near the surface and there was a little bit of a vacuum created at a considerable horizontal distance,” Hansen says. “We used the affordable spacing between pipes that many landfills are doing anyway, so it doesn't create an unusual cost burden.”
Other benefits of the Sullivan County Landfill project are reduced environmental emissions fee costs, reduced costs to handle public odor complaints and improved public relations for future landfill permitting activities.
“Vacuum-induced, semi-aerobic stabilization appears to be a cost-effective, workable procedure to increase density and accelerate biostabilization of MSW, while simultaneously reducing gaseous atmospheric emissions,” Hansen says.
The project was sponsored by the Albany-based New York State Energy Research and Development Authority (NYSERDA) and conducted by Landfill Service Corp., EMCON/OWT engineers in Mahwah, N.J., and the Sullivan County Solid Waste Division, also located in Monticello. NYSERDA invested $200,000 in the project and other entities provided $67,000.
At press time, NYSERDA's final project report was due in late July 2003. For more information or a copy of the report, e-mail David Hansen at [email protected].