Innovative “Green” Leachate Treatment Process
5/2/2009
By Brad Granley, P.E., Senior Environmental Engineer,
St. Paul, Minnesota
A new, award-winning treatment process that is truly ‘green’ in nature will reduce long-term financial liabilities and accruals for leachate disposal by over $2.5 million at a Republic Services, Inc. (Republic) landfill near St. Louis, Missouri. This innovative and environmentally friendly approach employs phytoremediation to utilize leachate as a resource on a year-round basis rather than disposing as a waste.
In October 2007, Leggette, Brashears & Graham, Inc. (LBG) was tasked with developing a more financially favorable and environmentally friendly technique to solve the ongoing and expensive problem of disposing three million gallons of leachate at this non-revenue producing, closed facility. The old method of “load, haul and dump" to the local sanitary sewer method cost 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, which is a novel technology that harnesses the ability of specific hybrid poplar trees to mitigate a range of environmental problems. For this site, an even more innovative application of the technology was developed where leachate would be distributed year round to the phytoremediation system even during the dormant, non-growing winter season, an industry first for a cold-weather climate.
The project was fast-tracked and within 60 days of receiving Client approval, LBG conducted a preliminary site visit, completed a preliminary design including a fatal flaw analysis, performed a regulatory evaluation, and completed a final design. Over 2,100 hybrid poplar trees were installed on the landfill over a six-acre area in mid-December 2007.
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 the evapo-transpiration potential by taking advantage of the huge water demand of the poplar trees and simple evaporation. Over 12,600 feet of surface drip tubing were installed over 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 an 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 intermittent creek. Freezing conditions combined with intermittent flow would result in breakage of piping and surface drip tubing.
Overcoming this challenge was critical as nearly 45% of annual leachate production occurs between November and March. In response, a subsurface drip irrigation system was designed and installed that consisted of 12,000 feet of drip tubing buried beneath the frost line, which would allow leachate distribution on a year-round basis.
The key to successful winter distribution comes from the huge water demand of the 2,100 hybrid poplars. Over the summer and fall the trees utilize so much water that the soil in the upper portion of the landfill is dried out creating a “sponge.” In the dormant winter season, the dry soils accept leachate. In the case that shallow field capacity of the landfill is exceeded, excess leachate can be captured by strategically placed tile that drains to the sump, or by the existing internal landfill drainage process, establishing a closed-loop system.
Another challenge was to ensure ease of operation for Republic personnel. Based upon 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 changing leachate composition and flow-rate fluctuations. The floc filter automatically backflushes. A programmable logic controller panel and web-based telemetry package provide great 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. In all, a lot of technology was made simple and local labor efforts are minimized.
The system passed a critical test in the spring and summer of 2008 when all-time 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 four to six feet in April 2008 to 12 to15 feet in height in only the first growing season.
A novel technology that is literally “green,” phytoremediation harnesses the unique ability of specially selected trees to mitigate environmental problems. In this case, a very robust and self-adjusting system was installed in a compressed timeframe overcoming numerous complex issues. The technology was adapted to treat landfill leachate on site in lieu of loading, hauling and dumping an average of 2.5 to 3 million gallons of leachate per year. In the process many direct benefits were realized, including substantial long-term financial 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. Being conservative, in 30 years over 13,000 semi-truck trips and 10,000 hours of semi-truck occupancy on local roads will be eliminated, 2,250 tons of CO2 will be sequestered, and loading to an already over-extended wastewater treatment plant was avoided.
The project was recently honored with two awards from the American Council of Engineering Companies (ACEC) “Engineering Excellence Competition.” A Grand Award was received at the state level and an honor award was received at the national competition, making it one of the top 25 engineering projects in the nation in 2008 as recognized by ACEC.
The success of this technology has sparked great interest within Republic 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.
