In the U.S., between 1978 and 1988, more than 14,000 landfills were closed (Mulamoottil, McBean and Rovers, 1999). It is estimated that there are approximately 20,000 closed landfills in the U.S. (Nivala et. Al., 2007). While closed, these landfills continue to require leachate collection and management. As noted in the article, “Green leachate treatment process cuts costs while enhancing the environment” by Brad Granley, published in Waste Age Magazine this last May, the annual costs to dispose of leachate at an offsite facility can be significant. Granley notes, “The old method of “load, haul and dump” via the local sanitary sewer was costing Republic $100,000 to $125,000 annually, with expenditures expected to increase over time.”
Onsite leachate management options are available that can decrease yearly operational requirements, reduce leachate production and potentially create an additional income from biomass production or carbon sequestration. Combining constructed wetlands (treatment) and an irrigated tree landfill cover (disposal) can produce such effects. Constructed wetlands are currently being used to treat landfill leachate and tree caps (ET Caps) are being used for leachate disposal; combining the two technologies can increase the effectiveness of each and minimize post-closure leachate management costs.
Constructed Wetlands for Leachate Treatment
Constructed wetlands are currently treating landfill leachate around the U.S., ranging in location from Minnesota to Florida. While landfill leachate can contain a wide variety of pollutants primarily including: (1) volatile organic compounds (VOCs), (2) nutrients, (3) heavy metals, and (4) priority toxic organic compounds (Kadlec, 1999), wetlands have been proven to effectively remove many of these common contaminants. In partnership with ET caps, wetlands can provide pre-treatment of leachate to reduce loadings on the soil and provide storage during periods when irrigation is not feasible.
Constructed wetlands remove contaminants in water through a number of natural processes. VOCs are air-stripped and degraded by microbes in the wetland, ammonium volatilizes and undergoes nitrification and denitrification and metals are sequestered in tissues of growing plants while many other organic contaminants are biodegraded (Kadlec, 1999). While other treatment systems can remove similar contaminants, wetlands can be designed to be passive and require limited energy input and subsequently result in lower operating costs. Design considerations for landfill leachate wetland treatment systems may include: (1) wetland system location, (2) daily flows and loadings, (3) required pretreatment, and (4) irrigation season duration and size of storage pond (if required).
Et Caps for ON-Cap Leachate Irrigation
Old ‘conventional’ landfill covers designed with only a grassed clay-rich soil can be modified by planting poplar trees and irrigating with onsite leachate without increased percolation. Most pre-Subtitle D landfills were covered with a layer of compacted soil and local clay, but have cracked within several years by desiccation, freeze and thaw cycles, and animal burrows; these soil cover cracks do not repair and will leak for the remainder of the landfill’s life. When the surface is covered by a grass cover that is mowed several times each growing season, the potential evapotranspiration is not enough to remove all water so percolation continues. The cracks are also the major leak point for the anaerobic landfill gases, resulting in odor and greenhouse gas release.
Alternative ETCaps use techniques different from the current ‘raincoat’ concept to protect human health and the environment. Evapotranspirative (ET) caps develop a ‘sponge and pump’ mechanism to reduce through-cap water percolation. The ‘sponge’ consists of a water-holding top layer of soil and amendments. The ‘pump’ action is achieved by plant transpiration and surface evaporation.
Factors that determine ETCap success when used as the final application of the leachate:
1. Design, installation, and operation plan – the steps used for ETCap implementation are not new but are based on agronomic technology with less reliance on civil engineering. Thus, the regulations conventionally used to permit landfill covers are difficult to fit directly to a cover that is not designed to be a ‘rain coat’ but rather like a rooted sponge.
2. Site topography – steep side slopes are more difficult than large areas of shallow sloped top. When leachate is irrigated on tree rows planted on the landfill surface contour, the root zone and soil cover are hydrated and cracking is reduced.
3. Soil quality and depth – relates directly to the ‘sponge’ volume that supports the plant while holding water and essential nutrients.
4. Plant growth rates (both trees and understory) – relates directly to the plant density and time needed to achieve full leaf canopy over the site.
5. Rooting surface area – the ultimate limit to water and nutrient uptake is root health and extent that fills the entire soil cover volume. Soils, rootable waste, gas flux, plant genetics, planting depth, and water will influence the root depth.
6. Growing season – a long frost-free period with hot and windy conditions evapotranspire the most water, but there is no control over this factor.
7. Leachate quality and quantity – pre-treatment of the leachate is important to keep the irrigation system operating as well as toxicity control.
8. Irrigation mechanical system – piping, pumping, valving, and application equipment will determine cost of installation and operation. The application options are principally spray and drip irrigation equipment.
9. System control – the ability to control the dosing rate and monitor performance is determined by the meters, computers and communications equipment. Scheduling is determined by the soil wetness and weather.
The first ETCap landfill cover for final closure based on poplar tree physiology was planted in 1990 at Lakeside Landfill, Beaverton, Ore. (Oregon Department of Environmental Quality Permit # 214). At the Riverbend Landfill, McMinneville, Ore. MSW leachate has been irrigated on poplar trees since 1992 (Licht and Isebrandes, 2004). Since 1990, many ETCaps and perimeter EBuffers have been installed on, and around, 20 pre-Subtitle D landfills in 12 states. In 2007 and 2008, the first leachate-irrigated ETCap was installed over the pre-Subtitle D Jeffco Landfill in Arnold, Mo.
ET caps and constructed wetlands can be part of future landfill design to achieve stability while maximizing landfill volume, reducing leachate volume, reducing methane emissions, and encouraging a mixed species ecosystem that can enhance a community. RCRA has now been modified by publication of the Research and Development Demonstration Rule (RD&D).
With the RD&D in place, an ‘approved’ state that currently manages its own solid waste program can permit alternative liquid control in the landfill and alternative covers. Under RD&D, a landfill closure and post-closure care program can achieve three different outcomes that were originally allowed by RCRA.
- Wetter cells with water added after formal closure will operate as a biocell. Landfill solids stabilization can be hastened by adding water which provides the reactant essential to anaerobically convert carbon-containing waste to biogas.
- Biogas evolution rates increase, and landfill biogas collection economics can improve. The alternative cover soils will better oxidize methane using methanotrophic microbes. The methane removal performance improves with a less compact, fertile, and porous cover soil.
- Alternative covers that do not achieve the equivalence to the bottom liner permeability are allowed. Specifically, plant and soil covers designed using phytoremediation principles are allowed. Such covers can allow a variable biocell water input by design.
Landfill leachate can be managed onsite using a constructed wetland and ET cap system. This system, or combination of systems, has the potential to reduce yearly management costs, decrease water percolation into the cap and integrate a landfill cap into the surrounding environment. Wetland systems can be designed to function in a passive method and are an alternative to mechanical treatment system, while the ET cap provides a location for disposal of the treated leachate. Landfills planted with poplar trees have the potential to reduce rainfall percolation through the cap even with the application of additional treated leachate through irrigation.
- Congressional Federal Register, http://www.epa.gov/EPA-WASTE/2004/March/Day-22/f6310.htm , 2004.
- Kadlec, R.H. (1999). Constructed Wetlands for Treating Landfill Leachate. Published in Constructed Wetlands for the Treatment of Landfill Leachates. Boca Raton, Florida: Lewis Publishers; 1999.
- • Licht, L.A., and Isebrands, J.G. Linking Phytoremediated Pollutant Removal to Biomass Economic Opportunities. Biomass and Bioenergy, Elsevier Publishing, New York, New York, 2004.
- Licht, L.A. (1993). Ecolotree Cap – Densely Rooted Trees for Water Management on Landfill Covers, Paper #A1549. Air and Waste Management Association, Denver CO.
- Licht, L.A., E. Aitchison, W. Schnabel, M. English, M. Kaempf (2001). Landfill Capping with Woodland Ecosystems. Practice Periodical of Hazardous, Toxic and Radioactive Waste Management, Vol. 5, No. 4.
- Mulamoottil, G., McBean, E.A., and Rovers, F. Constructed Wetlands for the Treatment of Landfill Leachates. Boca Raton, Florida: Lewis Publishers; 1999.
- Nivala, J. et. al. (2007). Treatment of landfill leachate using an aerated, horizontal subsurface-flow constructed wetland. Science of the Total Environment 380: 19-17.