Project Profile: Landfill Equilibrium
Modern landfills operate under many regulations. Under EPA Title V, landfills must prevent the escape of methane gas emissions through the installation of a gas collection and control system (GCCS). In addition, EPA Subtitle D requires landfill operators to perform temporary, or intermediate, closure once individual landfill cells are full, and then install a “final cap” once a landfill has reached its maximum permitted capacity and cannot be expanded.
In December 2010, Progressive Waste Solutions (formerly IESI-BFI Canada) installed a new synthetic turf closure system on 9 acres of its Timber Ridge Landfill in Richwoods, MO. With the new system, Progressive was able to combine two distinct components of landfill operations-closure and GCCS-into one streamlined, cost-effective approach that significantly reduces environmental impact.
The new system consists of three primary components: (1) two layers of woven geotextiles with tufted UV-resistant polyethylene grass that is laid over (2) a 50-mil LLDPE structured drainage geomembrane and infilled with (3) sand. The geomembrane layer serves as the containment liner atop the landfill’s intermediate soil cover. Integral 3.6-mm studs on the top surface facilitate drainage, while integral 4.4-mm spikes on the undersurface provide friction. The turf’s grass blades are interlocked with three-quarters of an inch of sand ballast that, combined with the liner’s surface studs, provides sufficient interface friction to provide stability without the need for anchoring. The system is anchored for termination purposes only at the toe or on the outside of a perimeter swale, depending on the site design.
Traditional landfill closures are highly reliant on trucking soils, heavy civil construction, and on-going maintenance and repairs in order to maintain their integrity. Even with diligent maintenance, many fall subject to excessive erosion, gas pressure buildup, and sedimentation issues. Traditional closures are expensive, costing between $115,000 to $200,000 per acre to install and maintain over the EPA-mandated 30-year period. Additionally, they exact a high environmental price. The initial construction and reconstruction destroys land to borrow soil, creates sedimentation issues, consumes significant fuel, and produces significant emissions from trucking and operation of heavy equipment.
For several years, engineers have encouraged the use of exposed geomembrane cover systems (EGCS) for their ability to remediate the construction, operation, and environmental impacts of traditional closure. In time however, EGCS has presented its own disadvantages such as lack of accessibility, lack of membrane protection, wind uplift issues, and aesthetics. EGCS installation requires numerous anchor trenches to resist wind uplift and the need for access during post-closure care operations can be damaging to the membrane.
In contrast, Timber Ridge’s new system requires minimal anchoring with ballast, provides a drainage system that can handle intense weather, and components that work together to allow for traffic access during post-closure care. It installs quickly (a few weeks versus traditional closure’s several months) allowing the closure of three to five acres at a time. The result is better odor control and enhanced compliance with Title V air quality rules.
The product has stabilized slopes once subject to repeated failure. Since rainfall penetrates through the sand and into the high-transmissivity drain liner below (which can handle rainfall of more than 6 inches per hour), erosion energy resides in the structured geomembrane and not in the sand surface, preventing erosion and siltation. A trial conducted at the LaSalle/Grant Landfill, a site notorious for side slope failure, exceeded expectations. “The grass looks great. We had over 150 inches of rain and very severe winds with no maintenance required afterward. We probably would have re-graded and re-vegetated the cap several times by now if it was soil cover,” says Progressive Waste Solution engineer Mike Friesen.
The new system can be laid directly on the EPA required intermediate soil cover, eliminating the need for the final soil and grass cap and creating an additional 2 feet of “fillable” airspace. Progressive has realized a collected savings of $70,000 per acre in installation/maintenance costs and additional airspace revenue.
The new approach has removed several thousand truckloads of dirt from the road, eliminated the need for mowing and vegetation maintenance such as fertilizer and replacement soil, and has quickly contained landfill emissions resulting in significantly less greenhouse gasses being released into the atmosphere. Compared to traditional closure methods, the new approach at Timber Ridge has reduced its total carbon footprint from 652,400 Kg of CO2 per hectare to 132,200 Kg of CO2 per hectare.
With traditional closure methods, the GCCS (composed of an elaborate array of pipes and vertical wells) and the final cap are installed separately. As distinct components, both are time and labor intensive, vulnerable to damage, and environmentally inefficient. According to the EPA there is up to a 30% loss of collection efficiency when no final cap is in place. This results in, per acre, an estimated 4 to 10 standard cubic feet per minute (scfm) of surface emissions escaping into the atmosphere, even with the previous installation of a quality GCCS.
Since it can be installed sooner and quicker, the system at Timber Ridge has allowed landfill operators to “close as they go” resulting in earlier control of surface emissions and a reduction of greenhouse gasses. It is no longer necessary to wait until 20 to 50 acres could be closed in order to obtain economies of scale. The system also provides a new approach in gas collection. Termed “surficial gas collection.” Emissions passively rise to the surface and are collected underneath the system’s impermeable membrane where they are pulled by an applied vacuum to the flare system.
During the system’s installation as a final cap, geocomposite transmissivity strips were placed every 50 feet vertically along the closed area. A total of nine collection points were installed along the transmissivity strips, which were then connected to a header system for gas conveyance to a flare system. A 500-scfm flare was installed for emission destruction and as a vacuum source.
The new approach has surpassed expectations. At the time of submission, the system had exceeded the 300 scfm predicted to be collected. The system’s vacuum pressure had to be turned down on June 1 due to exceeding the designed 500 scfm capacity of the flare. Timber Ridge is currently awaiting a permit modification and flare upgrade that will allow it to resume testing of the system.
This more efficient system has presented an important safety advantage. Under traditional GCCS systems, constant monitoring and adjustments are needed to ensure that oxygen is not pulled into the landfill, leading to potential fire hazards deep within the waste mass. With no vertical wells serving as a pathway for oxygen, that is no longer a possibility at Timber Ridge. Maintenance costs have been dramatically reduced.
The integrated GCCS has allowed Timber Ridge to harvest greater amounts of high-quality methane that can serve as fuel for future green energy generation. Unlike gas collected through vertical wells, the emissions at Timber Ridge produce very little condensate since the gas cools to ambient temperatures as it slowly rises towards the surface. The absence of vertical wells removes the potential for oxygen contamination and thereby increases the potency of the methane.
Progressive Waste Solutions estimates that it has saved approximately $53,000 per acre (over a 30-year period) on GCCS alone. Additional value will be realized in the sale of carbon credits and the development of landfill gas projects. Notes Friesen, “Since the cap acts as our gas system, we basically have a landfill gas system free of charge.”
This project demonstrates that new geosynthetic applications can improve the reliability and performance of landfill closures. In particular, adding a specialized synthetic turf component to an exposed geomembrane cap can significantly improve membrane protection, accessibility, wind resistance, and aesthetics.
Additionally, the turf provides considerable economic and environmental savings. Construction and post-closure maintenance costs are greatly reduced and the new system results in substantial reductions in CO2 emissions from the construction and the earlier capture of methane gas.