LBG Receives ACEC National Honor Award
ST. PAUL, MN – Leggette, Brashears & Graham, Inc. (LBG), a professional groundwater and environmental engineering services firm, has received a National Honor Award from the American Council of Engineering Companies, as part of the organization’s 2009 Engineering Excellence Awards competition, which recognizes the top 25 engineering projects in the nation. The firm was honored for its utilization of an innovative landfill leachate treatment process that employed phytoremediation, a truly “green” technology. LBG previously received the Grand Award, the highest possible, from the Minnesota Chapter of the ACEC.
LBG was retained by Republic Services, Inc. to solve an ongoing and expensive problem. A landfill located outside of St. Louis, Missouri generates between 2.5 to 3 million gallons of leachate with an annual disposal cost of $100,000 to $125,000. The old disposal method was to load tanker trucks, haul to another facility, and discharge to the local sanitary sewer. Not only was this expensive, but costs were expected to increase over time.
Tasked with developing a more financially favorable and environmentally friendly method of leachate disposal, LBG recommended an automatically-adjusting oxidation pre-treatment system followed by a novel approach using phytoremediation, a truly green technology that harnesses the ability of specific hybrid poplar trees to mitigate a range of environmental problems. In this case, an even newer application for the technology was implemented 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, with support from Ecolotree, Inc. conducted a preliminary site visit, completed a pre-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. Installation activities were complicated by an uncommon 8-inch snowfall in the St. Louis area resulting in wet and difficult conditions.
Year round leachate distribution was a critical component of the project as nearly 45% of leachate production occurs between November and March. During the growing season, a surface drip irrigation system is used to maximize the evapo-transpiration potential by taking advantage of the huge water demand of the poplar trees, grasses, and simple evaporation. However, the dormant, non-growing season presents an obvious problem – the trees use no water. The key to successful winter distribution comes from the water demand of the 2,100 hybrid poplars.
Over the summer and fall, the trees use so much water that the topsoil and refuse in the upper portion of the landfill are dried out, creating a “sponge”. In the dormant winter season the ‘sponge’ that was created (dry soil) accepts leachate. When spring arrives a water source is available to fuel early season tree growth. 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 toe sump, or by the existing internal landfill drainage network. Leachate is recaptured and a closed-loop system is established.
The system was designed to automatically handle both high flow and no-flow conditions, to pre-treat leachate of continually varying conditions, and to be simple to use for site operations personnel. A single submersible pump controls the entire hydraulic process transferring leachate from the landfill toe sump, through the pre-treatment building and out through a force main to a remote valve box that automatically distributes leachate to six different tree zones over a 6-acre area of the landfill. The chemical feed system automatically adjusts dosage based on real-time leachate parameters to oxidize iron and other compounds that could otherwise cause system failure. Floc filters are automatically back-flushed based on in-line differential pressure sensors, and the system can be accessed and controlled via remote telemetry using a simple web page.
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.
In all, a very robust and self-adjusting system was installed in a compressed timeframe overcoming numerous complex issues. In the process many benefits were realized, including substantial long-term financial savings of nearly $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.