by Hilary Haskell
Nonrenewable resource recovery and greenhouse gas emissions reductions make landfill mining both an economically and environmentally attractive landfill remediation option. Per Frändegård et al. (2013) studied the economic feasibility and environmental impact of landfill mining for Swedish municipal waste under mobile and advanced stationary separation plant scenarios. The authors used Monte Carlo sensitivity analysis to assess the effects of uncertain parameters on landfill composition and greenhouse gas mitigation potential. Of Sweden’s thousands of landfills holding more than 350 million tons of material, many are aging and require remediation. The study concludes that landfill mining could prevent emission of 50 million tons of carbon-dioxide equivalents, provide over five years of energy to Sweden’s district heating system, and supply Sweden with 7 million tons of ferrous and 2 million tons of nonferrous metal materials.
Landfill mining involves the excavation and treatment of waste from landfills. This process recovers potentially valuable resources and provides pollution mitigation. As landfill mining becomes more popular world-wide, current research seeks to determine greenhouse gas mitigation and resource recovery potential by determining factors such as landfill composition, separation technology efficiency, processing resource inputs, and emissions from excavated waste transport.
Economic justifications for landfill mining include extraction of valuable resources, landfill cover material, and waste-to-energy fuel. In addition, environmental benefits from landfill mining include land reclamation, conservation of landfill space, and landfill remediation. Until recently, most landfill mining has included separation of recyclable materials and prevention of hazardous material release during excavation. Now, a shift towards more advanced technology has allowed for a diversity of resource recovery possibilities and energy recovery.
Sweden could implement large-scale landfill mining because less than 1% of its municipal solid waste is currently landfilled, while more than half is incinerated for energy recovery. Therefore, many of the nation’s 4,000 landfills have been closed and are rapidly aging. These landfills require pollution prevention and control to begin their remediation process.
The authors focused on two scenarios; using stationary and mobile separation technology. Both of these scenarios result in parameter and scenario uncertainties. Scenario uncertainties result from differing assumptions and inputs in the scenario. Parameter uncertainties arise from variation in parameters. Through a Monte Carlo sensitivity analysis, Per Frandegard et al. analyzed the effects of these uncertainties on the outcome of the scenarios, which were then aggregated into a probability distribution. The authors ran 50,000 simulations with random samples for all input parameters, which included material composition, separation efficiencies, emission factors, and transport distances. The resulting distribution represents each scenario’s outcome likelihood for net greenhouse gas emissions and total extractable landfill material.
The mobile plant scenario advantage is its rapid set-up and simple operation. According to a panel of recycling experts entitled Sten Metall AB, mobil screening involves a number of steps. First, material is extracted and dumped on a coarse screen to separate non-recyclable, bulky, and hazardous material. Then, a star screen separates out highly degraded material called “fines” that may be used for landfill cover. Next, the air classifier removes combustibles. The remaining waste is filtered through eddy current separation (ECS), using a magnet to remove ferrous metals. With the mobile plant scenario, the separated wastes must be hauled to a number of different processing sites at varying distances from the landfill. The authors estimated distances of processing facilities from each landfill site for each type of material and its respective processing requirements.
For the stationary plant, the authors considered state-of-the-art technology. In this scenario, half of all excavated landfill material is sent to the stationary plant for processing. Upon initial excavation, as in the mobile plant scenario, excavated waste filters through both the coarse and star screens to minimize unnecessary transport of non-recyclable waste to the stationary plant. After filtration through these screens, the remaining five usable material categories— construction material, combustibles, non-ferrous, ferrous and plastics, and residual material—are sent to the stationary plant for further sorting. At the stationary plant, all materials except combustibles are sent to recycling plants, and combustibles are used for energy recovery at a waste incineration plant. For this scenario, the authors also utilized approximate ranges for transportation distances depending on the type of material.
Per Frandergard et al. estimated the potential greenhouse gas emission (carbon-dioxide equivalent) mitigation potential based off of the amount of different materials in the country’s landfills. The authors used both historical surveys and statistics to determine each landfill’s composition. Thirteen landfill mining case studies from developed countries provided ten deposited waste material categories. For each of these categories, the authors calculated the normalized mean and standard deviation for the material categories.
The most economically viable landfill mining sites in Sweden do not pose the greatest environmental threat, nor do the most potentially polluting landfill sites offer as much commercial promise for a landfill mining initiative. Sweden’s older landfills lack pollution prevention and control; however, they are smaller (80% contain fewer than 50,000 tons of material), and therefore offer proportionally less valuable material extraction potential. However, newer landfills already use pollution prevention technology, although 40% have more than 1 million tons of potentially valuable material.
The Methodology for Inventorying and Risk Classification of Contaminated (MIFO) determined the environmental conditions and risk potential posed by each landfill site on a scale from 1 through 4, with class 4 posing a small environmental risk and class 1 representing a very high environmental risk. This classification depends on level of contamination, level of hazardousness of contaminants, risk of contamination spreading, and the area’s sensitivity of conservation value. The authors subsequently determined landfill mining potential for both economic and environmental rationales by categorizing the landfills as total, large, remediation needed, and remediation required landfills. The landfills in the remediation required category include those with a MIFO class of 1. Landfills classified as remediation needed have an MIFO of 1 or 2. These landfills have the greatest environmental potential. Large landfills consisted of those with over 1 million tons of material, which Stena Metall AB consider as the threshold for economically viable landfill mining projects.
The authors estimated material and energy inputs for resource processing with uncertainty distributions from the Ecoinvent life cycle assessment database. Stena Metall AB also provided specific resource use data from material separation processes. The relative efficiency of material recovery is largely dependent on the type of material separation process utilized. With the ten types of deposited materials, the expert panel determined mass balances of each material. This finding described the distribution in weight percentages of materials deposited in the landfill and their resulting recovery efficiency.
Although landfill mining has the potential to eliminate greenhouse gas emissions from landfill gases, the landfill mining process uses resources which also contribute to greenhouse gas emissions. To determine the net greenhouse gas emissions for both scenarios, the authors used emission factors from the Ecoinvent database in an uncertainty distribution. The authors calculated landfill gas emissions from deposited organic material by measuring carbon content and material degradability rates from the Ecoinvent database. The avoided burden approach to assessing greenhouse gas emission was used because energy recovery and materials recycling replace material extraction and conventional energy generation. This approach indicates whether the avoided emissions are greater than the added emissions, thus resulting in a net decrease in greenhouse gas emissions from landfill mining.
To compare avoided landfill mining greenhouse gas emissions with conventional energy generation, the authors used a medium voltage electricity mix from the Ecoinvent database. Sweden relies on renewable energy (hydroelectric) and nuclear power for about two-thirds of its energy portfolio. Only one-third of its energy mix comes from fossil fuels, making Sweden’s energy generation relatively less carbon intensive than that of most nations. Furthermore, Sweden banned landfilling of combustible materials in 2002 to ensure that these materials could be incinerated in combined heat and power plants to generate electricity. The authors used the gross calorific value of combustibles to calculate heat and electricity production potential from burning extracted waste.
The authors used the Ecoinvent database to calculate landfill gas emissions based on carbon content and material composition rates. Most Swedish landfills are old, and therefore most deposited waste has already decomposed, releasing 50–100% of potential landfill gas emissions. Avoided emissions were based on the leakage that would occur if a landfill gas collection system was not in place versus a scenario with one in place. Landfill gas was determined inadequate for commercial use, and was assumed to be flared.
As of 2012, the authors found that there are about 365 million tons of materials in Swedish landfills. With the stationary scenario, 80% of this amount can be extracted with a 20% redeposition rate. For the mobile plant, 60% can be extracted with a 40% re-deposition rate. About two-thirds of landfill material is heavily degraded or inorganic materials that could be used for landfill cover. The remaining one-third is mainly combustible material, recyclable plastics, ferrous, and non-ferrous materials. Hazardous wastes account for less than 1% of landfill composition.
Separation efficiency for combustible materials is greater for stationary plants (80%) than mobile plants (30%), because mobile plant separation equipment is less efficient at separating out combustible materials versus heavily degraded material. In addition, whether or not plastics are separated from combustibles contributes to the overall percentage of combustible extraction. About 70% (2 million tons) of nonferrous metals could be separated and re-marketed using stationary separation. This supply would provide Sweden with scrap metal for up to eight years. Ferrous metals can be extracted and recycled with 20% efficiency, providing 6 to 7 million tons of ferrous scrap metal to meet Swedish demand for several years. Landfill mining time frame, size, and site-specific factors determine ratios between extracted amounts and scrap usage, and thus equate scrap metal supply and demand.
Sweden may face waste fuel shortages for its district heating system, making landfill mining an especially attractive option. Extracted combustible materials could offer approximately 80 million tons of fuel to generate over 300 terawatt-hours of district heating, which could help provide for Sweden’s heating needs for over five years.
Aside from recoverable materials, processing of extracted materials also results in residues for landfill cover and some hazardous wastes requiring special treatment. About 20% of extracted material from both stationary and mobile plants will be residues, according to the study’s Monte Carlo analysis. Residual material may include soil, inert materials, plastics, ferrous materials, and combustibles that were not extracted for further processing.
The Monte Carlo analysis demonstrates that net greenhouse gas emissions would most likely be lower for stationary plants in comparison to mobile plants. Both scenarios indicate with near certainty that net greenhouse gas emissions would be lower with landfill mining than without. If every landfill in Sweden used landfill mining with state-of-the art stationary plant technology, avoided greenhouse gas emissions would equate to 50 million tons. With mobile plant separation, 30 million tons of greenhouse gases could be avoided.
However, it is unlikely that every landfill would be mined in Sweden. Many landfills are small and would have high mining start-up costs, making resource extraction unlikely for purely commercial motives. With stationary plant technology, landfill mining could prevent 30 million tons of greenhouse gas emissions at 432 remediation needed landfills. There are 33 landfills with a combined 20 million tons of material that require remediation. Although their greenhouse gas mitigation potential is not as large as other scenarios, there are still environmental benefits due to remediation prospects. For commercial motives, about 49 landfills have over 1 million tons of material, which is the economically viable cut-off point for landfill mining efforts. In addition, landfill composition is also an important aspect when evaluating the economic potential of a mining project.
Most landfill mining greenhouse gas emissions result from combustible energy recovery and transportation of extracted materials to separation plants. However, mining activities and processing result in relatively low greenhouse gas emissions. Emissions from incineration are 50% lower than conventional energy generation. Installation of landfill gas collection systems and excavation of organic materials largely contribute to avoided greenhouse gas emissions. Because Sweden’s energy mix relies heavily on non-carbon intensive and renewable energy, its energy system results in a lower baseline comparison for avoided greenhouse gas emissions. If fossil-fuel intensive energy portfolios are compared to avoided greenhouse gas emissions from landfill mining, emission mitigation projections tend to be higher.
Plastic recyclability heavily influences overall greenhouse gas emissions, because plastics contribute all of the added greenhouse gas emissions from previously landfilled combustibles. If no plastics were extracted for recycling, added emissions from incineration of combustibles would be much greater, and the added emissions savings from replacing virgin plastic production with landfill mined plastic would not be realized. Although plastics result in a higher energy output from incineration, only 25 million tons of greenhouse gases would be avoided versus 50 million tons if they were not included in the combustible fuel mix. There would also be a 10% risk rather than a 0% risk that a net increase in greenhouse gas emissions could occur from landfill mining if plastics were combusted. In nations other than Sweden with a more carbon-intensive energy portfolio, plastics recycling may not have as large of an impact.
Per Frandergard et al. conclude that landfill mining may present an increasingly attractive option for remediating landfills, generating economic value, and creating environmental benefits. Stationary plants appear to be the most efficient and result in the greatest greenhouse gas emission avoidance. Sweden could implement large-scale landfill mining operations due to its district heating system and large number of landfills requiring remediation.
Frändegård, P., Krook, J., Svensson, N., Eklund, M., 2013. Resource and Climate Implications of Landfill Mining. Journal of Industrial Ecology 17, 742-755.