From the Geosynthetics Research Institute’s 24th conference, 2011.The articles in this series encompass all types of geosynthetics and their applications viewed from the context of sustainability. Traditional solutions are compared with geosynthetic solutions from both cost and carbon footprint perspectives. (from the Geosynthetic Research Institute’s 24th conference, 2011)
In light of the new emphasis on our carbon footprint, this article revisits the sustainable landfill concept envisioned more than a decade ago.
The sustainable landfill idea was conceived before its time and became lost in the myriad solid waste management options proposed in lieu of landfills. While the original concept considered sustainability in terms of a landfill that could be “reused,” it can also be viewed in terms of today’s focus on sustainability and how it relates to greenhouse gas emissions.
This article takes a fresh look at how landfills can achieve goals for managing waste in an environmentally friendly and cost-effective manner. It will also explore the greenhouse gas emissions associated with the four stages of landfill operations, how geosynthetics have already improved landfill performance, and how they are the key to eventually fulfilling the promise of a sustainable landfill.
It has been less than two decades since the United States moved from garbage dumps to sanitary landfills with the promulgation of federal “Subtitle D” regulations.
Since that time, great strides have been made in landfill technology, particularly the use of geosynthetics as effective replacements of natural materials such as clay and gravel. Even so, the current trend is away from landfills and toward a wide array of more eco-friendly alternatives including recycling, waste-to-energy, and composting.
Traditional landfills, however, remain the most cost-effective short-term disposal option in most cases. The choice is seemingly not a new one—cost vs. environment impact.
Perhaps there is another solution. The promise of the sustainable landfill is to provide a cost-effective and environmentally sound waste management solution by combining green energy production, recycling, and eventual disposal in one operation. The sustainable landfill can effectively use and preserve valuable resources and minimize impacts associated with greenhouse gases.
As this article demonstrates, geosynthetics play instrumental roles throughout the process.
The sustainable landfill, as envisioned by Environmental Control Systems Inc. (ECS), is composed of four stages as shown in Figure 1:
- Cell construction
- Filling cell and bioreactor construction
- Landfill mining
By enhancing decomposition and eventually mining the waste to reclaim airspace, the original concept envisioned a landfill that could be effectively reused. Such a landfill would avoid the costs of future land acquisition, cell construction, and infrastructure, as well as the issues with siting a new landfill.
More importantly, the revisited sustainable landfill will focus more on material recovery and power generation than disposal. The use of geosynthetic materials has already allowed the reduction of the carbon footprint in terms of landfill construction, but the ultimate potential is to use geosynthetics in early collection and control of landfill gases.
The stages of the landfill and the key role played by geosynthetics are discussed below.
1. Cell construction
The sustainable landfill incorporates many of the same advances of the current modern landfill. Figure 2 compares the typical system used in the 1990s and the system most often used in Florida today.Using layers of geosynthetics results in considerable savings both in construction costs and in landfill capacity since geosynthetics are much thinner than the earthen layers they replace.
The geomembrane/GCL composite is now used more than the prescriptive geomembrane/compacted clay layer and has been shown to have advantages over compacted clay. Advances in high-transmissivity geocomposites allow for shallower bottom grades or increased spacing between collection pipes.
High transmissivity geocomposites also reduce the head of the liners below that which can be achieved with gravel and are very important when recirculating leachate in the bioreactor process. Geogrids are used to stabilize foundations and sideslopes or in mechanical stabilized earthen berms around the landfill perimeter to further increase capacity.
Beyond the cell construction, geosynthetics can be used in other landfill infrastructure—from lining leachate ponds to roadway reinforcement and drainage. Manufactured materials also allow for more reliable and easier construction vs. the variability of natural materials.
The total landfill emission factors reported by the U.S. Environmental Protection Agency (EPA) are made up of the following components:
- CH4 (methane) emissions from anaerobic decomposition of biogenic carbon compounds.
- transportation CO2 (carbon dioxide) emissions from landfilling equipment.
- biogenic carbon stored in the landfill.
- CO2 emissions avoided through landfill gas-to-energy projects.
Although the move to geosynthetics has been largely motivated by financial benefits, the carbon footprint can be greatly reduced when considering the greenhouse emissions resulting from manufacturing, transporting, and installing the various landfill components. For example, we compared the greenhouse gas impacts from the construction of a typical 10-hectare (25-acre) landfill cell for a prescriptive landfill cell (circa 1993) to that of a landfill of today in Florida. We were particularly interested in the greenhouse gas impacts on a per-ton-of-waste basis to get a better idea of the cost of disposal.
The 10-hectare cell could contain about 1,750,000m3 of waste or 3,364,000 metric tons, depending on waste density. The greenhouse gas emissions for the various components are taken from “Emission Facts” (EPA, 2005) with results summarized for our simplified 10-hectare site in Table 1.
More detailed information on the various components can be found in other papers presented at “GRI 24—Optimizing Sustainability Using Geosynthetics.”
The original sanitary landfill design (Figure 2) has more layers comprised of soil, which do not have to be manufactured and are available locally but still have significant transportation costs because of the sheer volume of material. For our landfill, we assumed a haul distance of 30km (18.5mi) for clay and gravel, and 10km (6.2mi) for soil cover, but this is certainly optimistic for much of Florida.
This “soil-rich” base liner and leachate collection system construction emits approximately 14,200,000kg CO2 for the 10 hectares or 4.22kg CO2 per metric ton of disposed waste.
Conversely the “geosynthetic-rich” alternative emits only approximately 4,560,000kg CO2 or 1.36kg CO2 per ton, including manufacturing, transportation, and installation. In fact, more of the emissions results from the protective soil layer than all of the geosynthetic components combined.
Although haul distances are typically much greater for the geosynthetics, much less material needs to be hauled. In our example, we have not counted any increase in airspace as a result of the geosynthetic option, but reducing the thickness of the liner and leachate collection system by a few feet can increase airspace by as much as 5%.
Actual emission values can vary significantly depending mainly on location and the availability of suitable soils, but the conclusion is apparent—although driven primarily by financial savings and increases in capacity, the switch to using geosynthetics in today’s alternative landfill has resulted in a significantly smaller carbon footprint for the same size landfill footprint.
See the Appendix for computational details.
2. Filling cell and bioreactor construction
A vital component of the sustainable landfill is to operate it as a bioreactor.
Unlike the “dry tomb” landfill designed to keep water out, the bioreactor recirculates leachate and adds water as necessary to enhance decomposition. This natural process creates additional landfill capacity, increases the production of methane that is collected and used as fuel, and decreases operational expenses and energy associated with mechanical or manual separation.
A bioreactor also stores leachate and may reduce long-term maintenance and monitoring over that of traditional landfills as the decomposition process is condensed and the waste and leachate become more benign. Because distribution of moisture and collection of additional gas are critical in a bioreactor, geosynthetics have additional applications in this process.
Drainage composites and pipes can be used throughout the landfill as waste is being placed. Trenches can be constructed in the waste-to-place pipes, geocomposites, or other drainage materials (Figure 3) or a planar system can be placed and covered.
Vertical wells can also be constructed after waste is placed to serve as moisture distributors and/or gas collectors.
In re-examining sustainability in terms of impacts to the environment, gas collection and control during the waste-filling process are paramount. Current federal Title V regulations do not require gas collection until five years after waste is first placed. Through decomposition, tons of greenhouse gases are emitted during that period—orders of magnitude more than the greenhouse gases associated with the liner construction.
pproximately half of the landfill gas is methane, which has more than 20 times the impact on the environment than CO2. The sustainable landfill provides gas collection and control from the beginning. A gas collection system can be constructed directly below the first lift of waste, and gas can be collected through the leachate collection system.
Horizontal gas collectors, pipes, or even geocomposites can be placed over waste lifts as the landfill is being filled. Tarps, which are basically thin reinforced geomembranes, can be placed over inactive filling areas or even placed overnight to help contain gases. While great advances have been made in reducing the impacts of construction, more focus must be placed on containing landfill gases during the filling operation in a truly sustainable landfill.
Although waste decomposes during filling, most decomposition occurs after the cell is filled. As Figure 4 shows, decomposition (measured by gas production) can take decades in a “dry tomb” landfill.
In our 10-hectare cell, peak gas production is five times that of a traditional landfill and a bioreactor can accomplish the majority of decomposition within years. The previous emphasis of the sustainability landfill was to enhance decomposition to reclaim the airspace. Today’s focus is on containing the gas to reduce emissions and generate green power.
An average landfill emits 1150kg CO2 e/metric ton—three orders of magnitude greater than the construction impact of our geosynthetic liner system. However, landfills that incorporate a good cover, have a gas recovery system, and generate electricity can have zero net emissions (EPA, 2006).
A geomembrane cover with horizontal gas collection can collect 99% of the methane that would have escaped the landfill (Van Kolken Banister, 2010). The single greatest way to optimize sustainability in terms of reducing the carbon footprint is to decrease emissions from decomposition.
At this stage, an exposed geomembrane cover (EGC) is an ideal cover system (Figure 5).
In addition to containing the methane gas, an EGC contains potential seeps of leachate and controls odors and vectors. At half the cost of traditional covers (Koerner, 2011), an EGC also eliminates concerns regarding the stability and erosion of soil covers and provides more operational flexibility such as adding wells and moving pipes since the geomembrane is not covered with soil.
Also, much like the cost for the liner construction, the emissions from the construction of an EGC can be much lower than a traditional cover. The addition of solar panels on the EGC can be another source of green energy.
4. Landfill mining
After bioreacting, the landfill is mined to recover the usable materials—de facto recycling.
The organic component of the waste has been greatly reduced, which facilitates easier separation and sorting. The waste can be excavated and screened (Figure 6), and the larger particles (“overs”) can processed to recover valuable recyclables.
The smaller particles (“unders”) from screening can be placed in the new cell as daily or intermediate cover for further decomposition, possibly processed further into a compost or another use in the future. Delaying the processing of the harder-to-recycle materials such as plastics until the landfill mining stage may allow the recycled markets time to develop methods to provide greater revenues per ton and allow a wider variety of materials to be effectively recycled than can be done currently.
Future technologies may provide more options for the economic and environmentally sound use of the residuals and recycled materials. Valuable resources are not discarded in the sustainable landfill but are simply stored to use in the future. Geosynthetics can be used during this stage as temporary cover for stormwater and odor control during the mining process.
Overall impacts of the sustainable landfill
We have examined the sustainability for each stage of the operation, particularly in regard to greenhouse gas emissions, but there are also some overall benefits.
Going back to the original concept, the lined cells can be reused after mining, so impacts of future landfill construction are dramatically reduced. By offering the benefits of waste-to-energy, composting, and recycling in one facility, multiple trips to different facilities and the associated emissions can be eliminated.
Significant barriers to other technologies, such as large capital costs for waste-to-energy plants or poor markets for some recyclable materials, are eliminated in the sustainable landfill. Traditional landfills are usually the most cost-effective means of disposal, but the sustainable landfill can be just as cost-effective while also being carbon neutral.
Summary and conclusions
The application of geosynthetics in landfill designs has resulted in significant savings in construction costs and capacity. Although not the primary intention, the use of geosynthetics has significantly reduced the carbon footprint. Impacts can vary dramatically and are certainly site specific, but greenhouse gas emissions can easily be cut in half and perhaps even more.
At this stage, we have done what we can to reduce the construction footprint, but the key to overall emissions lies in the cover. In a broader view, using geosynthetics during landfill filling to enhance gas collection and constructing a geomembrane cover to contain gases after filling are the critical elements to reduce emissions.
The sustainable landfill, with a geosynthetic cover and operating with landfill gas recovery and electricity generation, can result in a carbon-neutral landfill.
Don Hullings and Hal Boudreau are with the Jones Edmunds office in Gainesville, Fla.
The articles in this series encompass all types of geosynthetics and their applications viewed from the context of sustainability. Traditional solutions are compared with geosynthetic solutions from both cost and carbon footprint perspectives. (from the Geosynthetic Research Institute’s 24th conference, 2011)