Jeffco Landfill, a Republic Services-owned facility near St. Louis, is using an innovative and environmentally friendly approach to leachate treatment that employs phytoremediation. Literally a âgreenâ technology, phytoremediation harnesses the unique ability of specially selected plants to treat contaminated material. In this way, leachate is utilized as a resource, rather than disposed of as waste.
In October 2007, Leggette, Brashears & Graham (LBG), a groundwater and environmental engineering firm based in St. Paul, Minn., was tasked with developing an economical and environmentally sensitive way to dispose of three million gallons of leachate at the closed St. Louis facility. The old method of âload, haul and dumpâ via the local sanitary sewer was costing Republic $100,000 to $125,000 annually, with expenditures expected to increase over time.
LBG, with support from Ecolotree, Inc., recommended an automatically-adjusting oxidation pre-treatment system followed by phytoremediation. In this instance, hybrid poplar trees would be used to mitigate a range of environmental problems. Over 2,100 hybrid poplar trees were installed on the landfill over a six-acre area in mid-December 2007. For the Jeffco site, a further application of the technology was developed in which leachate is distributed to the phytoremediation system year-round, even during the dormant winter season, the first time this had been tried in a cold-weather climate.
The phytoremediation process at this landfill generally consists of pumping leachate from the landfill sump to a pre-treatment oxidation system for conditioning prior to distribution to the grove of hybrid poplar trees. During the growing season, leachate is distributed through a surface drip irrigation system to maximize evapo-transpiration. This takes advantage of the huge water demand of the poplar trees and simple evaporation. Over 12,600 feet of surface drip tubing were installed across six discrete zones.
In addition to summer distribution, one of the primary project goals was to limit off-site hauling of leachate year-round. This presents the obvious problem of what to do with leachate produced during the winter when trees are dormant and water demand is zero. Surface irrigation would result in direct run off to a nearby creek bed. Freezing conditions combined with intermittent flow would result in breakage of piping and surface drip tubing.
Overcoming this challenge was critical, as nearly 45 percent of annual leachate production occurs between November and March. In response, a subsurface drip irrigation system was designed and installed, consisting of 12,000 feet of drip tubing buried beneath the frost line. This allowed leachate distribution on a year-round basis.
The key to successful winter distribution comes from the huge water demands of the 2,100 hybrid poplars. Over the summer and fall, the trees use so much water that the soil in the upper portion of the landfill is dried out, creating a sponge effect. So in the winter, the dry soils accept leachate. If the shallow field capacity of the landfill is exceeded, excess leachate is captured by strategically placed tiles that drain to the sump, or by the existing internal landfill drainage process, ensuring a closed-loop system.
Another challenge was ensuring ease of operation for landfill personnel. Based on feedback from the local site operator, that goal has been achieved. A single submersible pump controls the entire hydraulic process from the sump to the field. The chemical pre-treatment system automatically adjusts to accommodate fluctuations in leachate composition and flow rate. The floc filter automatically backflushes. A programmable logic control panel and Web-based telemetry package provide flexibility in system operation. In the winter and spring, switching between the surface drip and subsurface drip irrigation systems is as simple as turning a couple of ball valves.
The system passed a critical test in the spring and summer of 2008, when historic precipitation records were broken. No leachate was hauled off site, other than during a prescribed shut down for installation of the sub-surface drip irrigation system. Tree growth was substantial, with trees growing from between four and six feet high in April 2008 to between 12 and 15 feet high by the end of the first growing season.
This system eliminated the need to load, haul and dump an average of 2.5 to 3 million gallons of leachate per year. Direct benefits realized included substantial long-term savings of $2.5 million, easy maintenance, a new alternative for engineers to consider for leachate disposal, advancement of regulatory acceptance, lessening the impact of landfills on the surrounding environment, aesthetic enhancements for surrounding neighbors, habitat for wildlife and carbon-footprint reductions.
Elimination of the hauling process also resulted in numerous ancillary benefits. Conservatively, over 30 years it will eliminate more than 13,000 semi-truck trips and 10,000 hours of semi-truck occupancy on local roads. Approximately 2,250 tons of CO2 will be sequestered. It also will help minimize the burden placed upon an already over-extended wastewater treatment plant.
The project was recently honored with two awards as part of the American Council of Engineering Companies' (ACEC) Engineering Excellence Competition. A grand award was received at the state level along with an honorary award at the national competition, making it one of the top 25 engineering projects in the nation recognized by ACEC in 2008.
The success of this technology has sparked great interest with regional and corporate managers, and a number of additional landfills are slated for evaluation in order to maximize the potential long-term savings across the Republic organization.