MORE THAN A DECADE AGO, treated leachate from the New Hanover County, N.C., landfill was so toxic it killed at least one fat-head minnow during a state toxicity test. Now, nature is treating the leachate instead of being harmed by it, thanks to the state's first leachate-treatment wetlands.
The New Hanover County Department of Environmental Management (DEM), Wilmington, N.C., began investigating leachate-treatment options during the early 1990s, following a state test that revealed that treated leachate from the county's landfill was too toxic to continue discharging into the Northeast Cape Fear River. Engineers recommended that the department revamp its leachate-treatment plant by installing a costly mechanical ammonia treatment system. The system would require constant maintenance for the next 50 years.
However, Ray Church, director of environmental management for the department, believed he could find a less costly and more natural solution. “My degree is in biology,” he explains, “so I like to ask, ‘Can nature do it?’ A friend of mine had been doing research on wetlands and suggested we could use them to treat the leachate.”
The wetlands alternative also was attractive because it would provide leachate treatment for 25 years while the landfill was in operation and for an additional 30 years or longer after closure. Maintenance costs could be significantly reduced through a natural rather than mechanical system.
The county decided to build the wetlands, but the department needed to determine which method would be most effective at treating the leachate: surface-flow or subsurface-flow. In a surface-flow wetland, microscopic organisms that live in the roots of bullrush and cattail plants break down ammonia and nitrogen, thus removing contaminants from the water. A subsurface-flow system works mostly the same way, but the water moves through the root zone and is not seen on the surface.
The county decided to conduct a two-year pilot project to compare the two options. The results determined that the surface-flow wetlands worked better than the subsurface-flow wetlands, Church says. The pilot also proved that wetlands could not only treat the leachate but also could reduce the volume through “evapotranspiration,” which is when a growing plant takes up water and releases it through its leaves.
Using the positive results from the pilot, the county designed a full-scale system and plan to end leachate discharge into the Northeast Cape Fear River. The DEM secured a grant for $785,000 from the North Carolina Clean Water Management Trust Fund (NCCWM) to build the wetlands to treat landfill runoff. The NCCWM was attracted to the project by the aspect of the plan that called for installing a spray-irrigation system that would use treated leachate to water and fertilize closed landfill cells. That would eliminate the need to discharge effluent into the river and would create fertile ground for the landfill's ultimate purpose: to become a wildlife park. The grant then prompted the county's Board of Commissioners to contribute $243,500 to the project, bringing the total budget to $1.03 million. Landfill staff completed nearly $250,000 of clearing, grubbing and rough grading to keep the project under budget.
Cambridge, Mass.-based Camp, Dresser and McKee helped to obtain permits and design the wetlands, which cover a 5-acre plot between a closed cell of the landfill and nearby natural wetlands. Wilmington, N.C.-based Thompson and Co. constructed the project.
North Carolina's first leachate-treatment wetlands began operating in June 2003. During the first full year of operation in 2004, the wetlands treated 3 million gallons of leachate and reduced the county's discharge into the Cape Fear River by 16.5 percent. The landfill's operating budget also was reduced by more than $30,000.
The wetlands now can treat up to 60,000 gallons of leachate per day by passing it through two parallel cells and discharging the treated water into a 0.6-acre effluent-retention pond. By 2020, the county expects to divert 100 percent of the treated leachate flow. Last year, ammonia-nitrogen in the leachate was reduced by 98.3 percent and total nitrogen was reduced by 96.9 percent.
Operating and maintenance costs for the wetlands and spray irrigation systems are fairly low — about $8,000 per year — because no additional staff are needed to operate the system. No full-time personnel will be needed after the landfill closes. This, combined with the other benefits that the relatively low-tech leachate treatment option has provided — including reduced surface water discharge and providing irrigation water — has garnered national and international attention. New Hanover County hopes its wetlands project will be replicated in other part of the country.