Article pubs.acs.org/est
Life-Cycle Inventory and Impact Evaluation of Mining Municipal Solid Waste Landfills Pradeep Jain,† Jon T. Powell,† Justin L. Smith,† Timothy G. Townsend,‡ and Thabet Tolaymat*,§ †
Innovative Waste Consulting Services, LLC, 6628 Northwest 9th Boulevard, Suite 3, Gainesville, Florida 32605, United States Department of Environmental Engineering Sciences, University of Florida, Box 116450, Gainesville, Florida 32611-6450, United States § National Risk Management Research Laboratory, United States Environmental Protection Agency (U.S. EPA), 26 West Martin Luther King Street, Cincinnati, Ohio 45268, United States ‡
S Supporting Information *
ABSTRACT: Recent research and policy directives have emerged with a focus on sustainable management of waste materials, and the mining of old landfills represents an opportunity to meet sustainability goals by reducing the release of liquid- and gas-phase contaminants into the environment, recovering land for more productive use, and recovering energy from the landfilled materials. The emissions associated with the landfill mining process (waste excavation, screening, and on-site transportation) were inventoried on the basis of diesel fuel consumption data from two full-scale mining projects (1.3−1.5 L/in-place m3 of landfill space mined) and unit emissions (mass per liter of diesel consumption) from heavy equipment typically deployed for mining landfills. An analytical framework was developed and used in an assessment of the life-cycle environmental impacts of a few end-use management options for materials deposited and mined from an unlined landfill. The results showed that substantial greenhouse gas emission reductions can be realized in both the waste relocation and materials and energy recovery scenarios compared to a “do nothing” case. The recovery of metal components from landfilled waste was found to have the greatest benefit across nearly all impact categories evaluated, while emissions associated with heavy equipment to mine the waste itself were found to be negligible compared to the benefits that mining provided.
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INTRODUCTION There are hundreds of thousands of active, closed, and abandoned landfills worldwide, with estimates of nearly 100 000 in the U.S. and more than 150 000 in Europe.1,2 Investigators have examined the concept of landfill mining or mining (also referred to as landfill reclamation) at closed sites because of various potential benefits, including reducing environmental impacts from uncontrolled gas-phase and/or aqueous-phase contaminant migration, recovering the land for other uses, and using the extracted material for a variety of purposes, including recycling, combustion with energy recovery, or other beneficial uses, such as land application of reclaimed soil.3−7 Previous investigators reported that historically conducted municipal solid waste (MSW) landfill mining projects have been executed to reclaim previously filled space and reduce environmental impacts by relocating waste from unlined cells to landfill cells with engineered protective components, such as low-permeability liners, liquid collection systems, and active biogas collection systems.7,8 Generally, the recycling of recovered components has been found to be a secondary goal, owing largely to the poor quality of mined materials, © 2014 American Chemical Society
which may make them unsuitable in typical remanufacturing processes for traditional recyclables. Oversized materials with high calorific values that do not readily degrade in an anaerobic landfill environment (e.g., plastic, paper with a high lignin content, textiles, and certain vegetative wastes) have been beneficially used through combustion with energy recovery.4 Although mining an old unlined cell with no active landfill gas (LFG) control system and recycling and/or proper containment of the recovered waste potentially would result in the reduction of uncontrolled migration of LFG and leachate, the process itself (using heavy equipment that consume fuel) results in environmental emissions. While the environmental benefits of mining old landfill cells are often qualitatively recognized, the primary driver in the decisionmaking process for landfill mining projects has been economics typically associated with reducing groundwater impacts by removing the contamination source, more efficiently using Received: Revised: Accepted: Published: 2920
October 1, 2013 February 4, 2014 February 10, 2014 February 10, 2014 dx.doi.org/10.1021/es404382s | Environ. Sci. Technol. 2014, 48, 2920−2927
Environmental Science & Technology
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Figure 1. Generalized landfill mining process flow diagram.
areas that were filled without environmental controls, such as low-permeability liner systems and liquid and gas capture systems, the principles and procedures described herein may apply to unlined landfills and modern sanitary landfills. The first step in the landfill mining process is the excavation of any final cover soil to access the underlying waste. The amount of recoverable soil present will depend upon how the closure system was constructed and the chemical quality of the soil. Typically, final cover soil chemical quality is acceptable for reuse (from the perspective of trace constituents of concern) because the majority of the soil is not in physical contact with the underlying waste. The soil may be transported for off-site use or stockpiled for later on-site or off-site use. Following soil removal, most mining projects have proceeded with relocation of the waste from the original location to a new location with modern environmental controls.7 The excavated MSW may be screened to separate out desired components; the composition of the excavated materials varies depending upon the type of waste, age of waste, degree of degradation, and other factors. Table S1 of the Supporting Information summarizes the composition of excavated waste based on previously published efforts related to landfill mining. Generally, the data show that a substantial amount of the excavated material consists of soil-like material and degraded waste material (often referred to as reclaimed soil), although bulkier material fractions, sometimes referred to as recovered waste, that may have recyclable or calorific value are also present.4,7,13,14 Significant landfill airspace can be recovered through the recovery and beneficial use of the reclaimed soil within or beyond the footprint of the landfill. The recovered waste fraction mainly consists of paper, plastic, yard waste, textiles, metals, glass, and ceramics.13,15 The recovered waste can potentially be processed at a material recovery facility to recover traditional recyclables (e.g., plastic,
available disposal space, or mitigating LFG migration problems.6,7,9 Given the growing interest in the international environmental community on the examination of decision making in the context of sustainability, namely, understanding the environmental, social, and economic impacts of decisions, the examination of landfill mining from a life-cycle analysis (LCA) perspective is warranted so that more accurate accounting of environmental impacts and trade-offs can be understood by decision-makers. Previous studies documenting landfill mining LCA assumed emissions from the mining process because of the lack of emission data from full-scale mining data.10−12 To address these questions, a life-cycle inventory was developed for the landfill mining process based on fossil fuel consumption data from previous full-scale projects and documented emissions from heavy equipment commonly used for mining landfills. Following the presentation of the life-cycle inventory, the environmental impacts of landfill mining under a variety of goals and operational scenarios are examined. The results provide decision-makers with tools, boundaries, and inventory data that can be used to tailor a similar analysis to a site-specific case, so that decisions to pursue landfill mining in the future can be made and environmental impacts can be more effectively analyzed in a quantitative manner.
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LANDFILL MINING PROCESS Figure 1 presents a generalized process flow diagram of a landfill mining process along with the materials and energy inputs and outputs and environmental emissions that should be considered when conducting an LCA; dotted lines represent materials, energy, and emissions input and output. Although the type of landfill normally targeted in a mining project consists of 2921
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Table 1. Air Emissions from Diesel-Powered Construction Equipment emission CO2 fossil CO fossil CH4 fossil N2O NOx SO2 SOx HCl NMVOC dioxins and furans VOCs PM10 PM10−PM2.5
EASETECH (g/L of diesel combusted) 2601−2627 5.28−14.85 0.135−0.151 0.0645−0.0741 11.02−39.61 0.0168−0.0170
WRATE (g/L of diesel combusted) 2647−2787 13.64−18.56
22.521−25.12
2787 18.56 0.15 0.074 39.61 0.0170
LCIs (g/MT of waste mined) 4180 27.84 0.23 0.111 59.42 0.0255
1.685−1.69 1.035−6.885 4.433−5.065 2.81 × 10−8−2.83 × 10−8 7.327 1.442−2.89 0.267−0.313
6.89 5.06 2.83 × 10−8 7.33 0.313
10.34 7.59 4.24 × 10−8 11 0.470
(Frey Farm Landfill) involved excavation of waste and transport to an off-site incineration facility. The diesel consumption data for reclaimed soil (instead of the diesel usage for off-site recovered waste transport) were used to estimate on-site waste transport and overall project diesel usage for the Frey Farm Landfill project. Two sets of volumes and diesel usage data were provided for the Perdido Landfill mining project: one corresponding to the entire project and the other corresponding to the portion of the project after the routine operations phase was reached. The fossil fuel use data were available for only two of the four large-scale mining projects identified in this study. Mass-based emission factors are typically used in LCA models. The data collected in the Frey Farm Landfill mining project tracked the mass of waste excavated and the volume of diesel consumed; thus, these numbers were used directly in establishing one of the boundaries of the mass-based emission factors [liters of diesel consumed per million metric tons (MT) of waste excavated]. For the Perdido Landfill mining project, waste excavation volumes were tracked; therefore, a density conversion factor (1.0 MT/m3, which was based on previously reported in-place density estimates4) was used to establish a mass-based emission factor from this project. Landfill Mining Process LCIs. A LCI of vehicular emissions can be developed on the basis of direct emission measurements, which is the preferred approach, or by applying an emission factor tied to fuel consumption (e.g., grams of contaminant per liter of fuel used). The latter approach was used because the full-scale mining projects analyzed did not track actual vehicular emissions. Emission factors of the equipment listed in two different waste management-specific LCA models, EASETECH and WRATE,17,18 were used. Table 1 presents the ranges of emission factors for heavy equipment included in these two models. The maximum of the emission factors for each chemical constituent was used for a conservative analysis. Table 1 also presents the emission factors per unit mass of waste excavated, which was calculated on the basis of the maximum emission factor and the unit fuel usage rate (liters per MT of waste mined) presented earlier. LCA Model and Inputs Used. The LCA software tool EASETECH was used for assessing the LCIs and life-cycle environmental impact of the landfill mining process over a 100 year time horizon. EASETECH traces contaminants on a mass flow (input-specific) basis and a process-specific basis and accounts for emissions to air, soil, and water environmental
paper, glass, and metals) for subsequent use in remanufacturing processes. However, because of quality issues, for example, the presence of soil particles and moisture along with the recyclable components, and thus potentially high processing cost, the potential to recycle a substantial fraction of these materials is generally limited. As presented in Table S1 of the Supporting Information, approximately 35−85% of recovered waste is composed of combustible constituents that can potentially be processed using mobile on-site equipment (e.g., magnets and air classifiers) to produce refuse-derived fuel for energy recovery at a MSW or co-combustion facility (e.g., coal power plant); the degree of processing needed to produce refuse-derived fuel (RDF) would depend upon the presence of non-combustible materials in the mined waste. The reclaimed landfill footprint and airspace can be used to construct new landfill space with modern environmental controls or potentially redeveloped for other productive uses (e.g., recreational park and commercial use). The productive use employed depends upon the amount of environmental impact on the area, which can be examined using traditional site assessment means, such as groundwater and soil sampling, following full excavation of the area.
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used in the model (g/L of diesel combusted)
MATERIALS AND METHODS
Materials and Energy Input for the Landfill Mining Process. The primary materials and energy inputs for the mining process include mobile excavation and hauling equipment and diesel fuel use. The equipment and fuel usage data for four large-scale landfill mining projects (each with more than 200 000 in-place m3 mined) were compiled and analyzed to develop life-cycle inventories (LCIs). The LCIs are representative of data from mining projects that are in routine operations and do not reflect startup and shutdown conditions (overall, the majority of a mining project will be in the routine operations phase; thus, these conditions are broadly applicable). Table S2 of the Supporting Information presents a list of equipment used, diesel usage, and waste volume and mass processed along with project durations for the four large-scale projects. In some cases (e.g., for the Perdido Landfill), fuel consumption was estimated on the basis of the fuel expense receipts of the contractor and prevailing diesel prices in the lower Atlantic region published by the U.S. Energy Information Administration.16 Three of the four projects analyzed involved excavation and transport of mined waste from an unlined landfill to an on-site lined landfill, whereas one of the projects 2922
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compartments over the modeled time horizon for all scenario processes. The model does not account for the emissions associated with the construction or manufacturing of waste handling equipment or processing facilities. The model uses the International Reference Life Cycle Database (ILCD) for process LCIs and offers multiple databases for life-cycle impact analysis (LCIA), including EDIP 97 and ReCipe 2008. Default ILCD LCIs were used for all of the pertinent processes. The landfill mining process was modeled by creating a subprocess, which simulated a piece of heavy equipment with a fuel usage of 1.5 L/MT of waste mined and with the emission factors listed in Table 1. The model default LCIs for diesel production were used. Default EDIP 97 characterization factors were used for the LCIA. The model only accounts for emissions associated with energy usage by heavy equipment and does not account for the energy associated with equipment manufacturing (e.g., steel used in the manufacturing of the heavy equipment). EASETECH determines the volumetric leachate generation rate using a user-specified infiltration rate, time horizon of inventory, waste layer height, and waste bulk density. Model default values were used for these parameters. The default parameters (34 different chemical constituents) and concentrations were used for all modeled scenarios. The model uses first-order decay rate formulation to estimate methane (CH4) and carbon dioxide (CO2) generation rates based on the elemental composition of waste (i.e., hydrogen, oxygen, nitrogen, and anaerobically degradable carbon) and decay rates of each material fraction to determine the CH4/ CO2 ratio and the respective volume of each of these gases that is produced. The model also simulates generation of an additional 21 minor LFG constituents, which are independent of waste composition. The default LFG generation inputs were used for all scenarios. LCAs of Mined Materials Management Alternatives. The following three scenarios were modeled to assess the environmental impacts of mining of a 30-year-old unlined cell containing 1 million MT of MSW for mined materials management alternatives. This value was selected based on the range of the mined volumes reported by the other full-scale mining projects evaluated previously and is expected to represent a typical profile of an old unlined landfill that may be considered for a mining project. Baseline Scenario (Scenario 1). In the first scenario, the LCA of a 30-year-old unlined cell (with no active LFG control) was conducted to assess the environmental impact if the site was left as is (i.e., no mining). In this scenario, the 30-year-old MSW would continue to decompose in the unlined landfill cell and continue to release leachate and LFG into the environment. The impacts of fugitive LFG emissions and leachate migration from the 31st year to the 100th year were modeled. The model requires inputs, such as waste composition and LFG management options. Because the candidate landfill for mining is typically an old unlined cell, the composition of waste landfilled in the year 1980 was chosen for this set of model runs, as presented in Figure 2.19 Waste constituent categories were subdivided as necessary to achieve the desired waste composition from Figure 2. In the EASETECH model, all waste handling operations require a waste mass input for emissions accounting; therefore, each of the three scenarios modeled included a waste generation process with waste fractions, as detailed in Figure 2. LFG collection efficiency was assumed to be 0% in scenario 1 (no LFG collection), and the effects of the first 30 years of
Figure 2. Composition of waste originally placed in the landfill, consistent with the composition of landfilled waste in the year 1980.17
fugitive LFG emissions were excluded from the analysis. Similarly, the environmental effects of leachate emissions to the environment for the first 30 years were excluded from the analysis. The oxidation of fugitive CH4 emissions through the cover was assumed to be 10%.20 Waste Relocation Scenario (Scenario 2). In this scenario, the waste was excavated, transported, and deposited in an onsite lined cell, which represents the case of the majority of the full-scale landfill mining projects undertaken in the U.S. A process using equipment with a fuel consumption rate of 1.5 L of diesel/MT of waste mined with the emissions listed in Table 1 was created to simulate the impact of the landfill mining process because the model does not have a built-in landfill mining process module. Furthermore, the model does not allow for processes to occur downstream of the landfill process; therefore, mining was modeled separately from the landfill portion of the scenario and the resulting emissions/impacts were added for the overall estimation. It was assumed that the LFG collection from the waste relocated into the lined cell will begin 5 years after the start of the relocation activity. LFG from the lined cell was collected from the 35th year to the 100th year. A collection efficiency of 80% was used for modeling.21,22 The collected LFG was beneficially used for electricity generation using the model default combustion and treatment energy generation process, with the exception that waste heat recovery from the power generation process for district heating was removed. The use of LFG for electricity generation was modeled to displace natural gas by selecting a natural gas-fired boiler (
