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February 1, 2007
Landfill leachate is a notoriously complex substance to deal with, primarily because of its ever-changing composition. In recent years, engineered wetland systems have proven that they can play a viable role in leachate treatment. Advanced technologies — including non-traditional biological processes and the innovative use of wastewater equipment — have proven to be a powerful force in solving the leachate challenge. This is important because reliable onsite treatment paves the way for onsite disposal, a welcome alternative to the cost and effort of capturing the leachate and hauling it away.
To most, wetlands are a wet piece of land with plants. To an engineer, a wetland is a complex system that facilitates numerous chemical and biological reactions. These reactions can be exploited to remove pollutants. For remote facilities, like landfills, engineered wetlands can provide a valuable, low-maintenance onsite treatment system. Using state-of-the-art techniques, wetland engineers are able to create systems that can effectively clean up the dirtiest of wastewaters — even landfill leachate.
Considerable research has gone into evaluating the use of wetlands for leachate treatment. Research and pilot studies are clarifying the role that bacterial communities play in pollutant removal. Key to this process is the agronomic loading rate, which represents the rate at which plants in these wetland systems can process nitrogen in the leachate. This will determine the amount of leachate that can be disposed of onsite. Current research is focused on the bacterial pathways responsible for ammonia removal or nitrification (the biological oxidization of ammonia into nitrate, which is more easily processed by plants). At high ammonia loading rates, plants will not use all of the nitrogen applied. At low rates, plant growth will be limited. Either scenario hampers the effectiveness of a wetland leachate treatment system.
Much of this research is being conducted onsite at landfills. Pilot programs in Iowa and Delaware have yielded excellent results, and new studies are underway in Florida and Ontario. In Jones County, Iowa, a horizontal, subsurface flow wetland (an engineered wetland where water flows horizontally through a gravel bed planted with reeds) that was fitted with a Forced Bed Aeration system demonstrated consistent cold weather nitrification of leachate. An important aspect of this pilot study was the use of mulch insulation to provide a thermal blanket for the wetland bed. The mulch layer moderated the effects of the Iowa winter and reduced heat loss from the leachate, while the aeration system provided the oxygen necessary for ammonia oxidation.
Other pilot work has focused on the use of vertical, subsurface flow wetlands (a more compact wetland system in which water flows from the top of the plant down through the root zone) and has demonstrated the effective use of multi-cell configurations with aerobic, anoxic and anaerobic zones. One key advantage to a vertical flow wetland is that the operation of recirculation pumps can be adjusted to control performance of the system. As in the Iowa project, the pilot demonstrated consistent cold weather performance.
A full-scale wetland system is in use at a landfill in New Hanover County, N.C. The design incorporates some pilot testing innovations, including distribution trenches, which ensure the even flow of leachate along the length and width of the wetland. This prevents leachate from leaving the system before it has been optimally treated.
“Since installation in April of 2002, the level of treatment provided by the wetland system has exceeded all our expectations,” says Sam Hawes, an environmental specialist for Hanover County. “Our constructed wetlands treatment system receives raw leachate with an average total nitrogen concentration of 150.0 mg/L and reduces that by 90 to 95 percent.”
Meanwhile, a landfill in Kent County, Del., recently began construction of a full-scale vertical flow wetland system. The onsite treatment of leachate will enable irrigation of the effluent to the landfill's innovative phytocap system. This is a natural system of grasses, bushes, trees and shrubs, chosen for their high tolerance to salt and other pollutants, which are integrated into the cap of a closed landfill. Treated leachate can be sprayed on top of landfills that use a phytocap system, which provides more area for leachate disposal.
In Portland, Maine, a full-scale five-cell wetland system is under operation treating underdrain water and leachate from a landfill site. The use of multiple cells allows for more flexibility by allowing operators to adjust flows to different cells in order to optimize treatment performance. The system has been running successfully since 1999.
Controlling nitrogen is key to onsite irrigation, and engineers are showing that use of aerobic zones in wetlands by recirculation or forced air is an effective means to remove ammonia. The use of subsurface wetlands also is showing promise, particularly with respect to introducing elements of operator control, limiting the impact of foul weather and promoting the growth of resident bacteria.
Designers are applying the findings from recent wetland research and pilot studies to refine the performance and reliability of onsite treatment systems. Those advances will help landfill operators find safer, cleaner and more efficient methods of onsite leachate treatment and disposal.
— Mark Liner, P.E.
Senior Engineer, North American Wetland Engineering, LLC
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