The New Hanover County landfill in Wilmington, N.C., is the only one in the state with a wastewater treatment plant, and it requires high maintenance.
Knowing landfills produce leachate for years after closure made officials think about ways to eliminate the treatment plant. They approached CDM, an engineering firm in Raleigh, N.C., along with North Carolina State University and the Water Environment Research Federation (WERF). CDM suggested a constructed wetland leachate treatment system.
The university conducted a pilot project to see which type of wetland — free surface (water is visible) or subsurface — would work best with the leachate. The free surface system treated it to irrigation quality. The wetlands came online in 2003, funded by the county and WERF grants.
“Every landfill must grow grass on closed cells, and we use the treated water to irrigate two of them,” says Shannon Culpepper, environmental programs assistant for the New Hanover County Department of Environmental Management. “Our course will enable us eventually to recycle all the treated leachate instead of discharging it to the Lower East Cape Fear River.”
Site conditions
The landfill occupies 416 acres, but most of the land serves as a buffer between the facility and river. The landfill receives 160,000 tons of trash per year and has a total disposal capacity of 8.9 million cubic yards.
System components
The wastewater treatment plant is permitted to discharge 50,000 gpd. Its major components are:
• 3-million-gallon, 2-acre lagoon
• 90,000-gallon aeration basin
• 18,000-gallon clarifier
• 50,000 gpd sand filter
CDM designed the constructed wetlands leachate treatment system to handle 60,000 gpd. Its major components are:
• 1-million-gallon, 2.47-acre Pond A
• 1.4-million-gallon, 2.63-acre Pond B
• 480,000-gallon, 0.56-acre retention pond
System operation
Pumps send leachate from the entire landfill and two closed cells through 6-inch HDPE pipes to the lagoon. It receives 95,000 gpd during the wet season and 30,000 gpd during the dry season. Four 10-hp Tornado aerators (Aeromix System Inc.) circulate and push the influent through the lagoon. The average retention time is 48 hours.
A pump in the lagoon draws influent into the treatment plant. It flows to the aeration basin, then to the clarifier, through the sand filter, and discharges to the river. A day before the grass on the 10-acre closed cells requires irrigation, staff members activate a pump in the lagoon that sends influent through a 2.5-inch PVC pipe to the wetlands.
“It takes a minute for influent to reach the head of the wetlands, and then the flow splits to both ponds,” says Culpepper. “Liquid gravity-flows over 24 hours to the retention pond. As soon as it’s full, we irrigate using Greenfield mini center-pivot irrigation systems.” Each unit sprays 10,000 gallons per hour.
During summer, most influent is directed to irrigation. In winter, the bulrushes and cattails in the wetlands go dormant. “We don’t get as much treatment, but we can still irrigate,” says Culpepper.
Installation
The greatest installation challenge was the summer heat. After excavating 8-foot-deep depressions for the wetlands and pond, CDM workers used electric heat welders to join the bottom layer of 20-foot-wide 60-mil HDPE geomembrane sheets. The surface of the liner reached 140 degrees.
“The process was incredibly slow,” says Culpepper. “The men would do one weld, then an inspector checked the seam for bubbles with a special device. The tiniest leak could cause contamination.” A geonet drainage geocomposite with pipes formed the middle layer. “We hook a pump to the pipes and if the vacuum draws no water, the top layer isn’t leaking,” says Culpepper. Another 60-mil geo-membrane liner topped the geonet.
Each wetland has areas of open water surrounded by bulrushes and cattails buried in soil. Workers enclosed the roots of perimeter plants in heavy paper bags to prevent them from spreading. “We must keep the open areas clear of vegetation, enabling incoming flows to circulate the water and provide uniform treatment,” says Culpepper.
Access pipes were installed along the influent line to facilitate cleaning. “Our first closed cell taught us a lot about irrigation and how to plan around weather,” says Culpepper. “Consequently, the second cell is running more effectively. We remain aware of expected rainfalls that may keep us from irrigating.”
Maintenance
The wetland system requires low maintenance and operation. The geonet liner is pumped occasionally to check for leaks, and the influent pipe is cleaned. “If we can’t irrigate after heavy rains, the pressure of all that water in the retention pond may cause some leaks,” says Culpepper. “We just pump out the water between the liners and return it to the wetlands system.”
New Hanover County landfill has taken the first step to attaining its goal of replacing its wastewater treatment plant with constructed wetlands.
