Article pubs.acs.org/est
Waste Informatics: Establishing Characteristics of Contemporary U.S. Landfill Quantities and Practices Jon T. Powell,*,†,‡ José C. Pons,‡ and Marian Chertow*,‡ †
Yale University, Department of Chemical and Environmental Engineering, 195 Prospect St., New Haven, Connecticut 06511, United States ‡ Yale University, Center for Industrial Ecology, 195 Prospect St., New Haven, Connecticut 06511, United States S Supporting Information *
ABSTRACT: Waste generation is expected to increase in most countries for many decades with landfill disposal still the dominant solid waste management method1−3. Yet, operational characteristics of landfills are often poorly understood with comparative statistics substantially lacking. Here, we call for a more formal waste informatics to organize and standardize waste management knowledge at multiple spatial scales through analysis of recently reported data from 1232 U.S. landfills and other high resolution data sets. We create the first known estimate of available U.S. municipal waste stocks (8.5 billion tonnes) and go on to resolve these stocks at the county level, reflecting prospective urban mining opportunities. Our analysis of disposal rates and landfill capacities reveals that more than half of U.S. states have more than 25 years of life remaining. We also estimate the gross energy potential of landfill gas in the U.S. (338 billion MJ/yr) by examining 922 operational methane collection systems and demonstrate that the greatest energy recovery opportunities lie at landfills with existing collection systems and energy conversion infrastructure. Finally, we found that the number of landfills reaching the federally defined 30-year postclosure care period will more than triple in the coming two decades, with 264 sites expected by the year 2044, highlighting the need to develop and standardize metrics carefully to define and standardize when it is appropriate to end or scale back long-term landfill monitoring.
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INTRODUCTION Landfilling is the dominant method of handling municipal wastes in the U.S. Recent data suggests this trend has continued despite policies and practices designed to reduce or divert waste from disposal through recycling, composting and energy conversion.1 Although sanitary landfilling is common in the U.S., this practice is emerging in many other countries.2,3 Sound waste management is a cross-cutting environmental issue, intersecting with many of the new UN Sustainable Development goals, in particular those related to sustainable production and consumption patterns.4 In light of rapid urbanization and attendant increases in urban waste production globally, a more standardized and replicable data collection program is called for to protect human health and the environment, particularly in cases where sanitary landfilling cannot be avoided. Systemized development of waste informatics is needed to address many of the analytic difficulties in understanding wastes, landfills, and their impacts by leveraging high-quality, measured data to characterize the state of disposal in the U.S., which, in turn, can inform and enhance the scientific community’s understanding of landfill management and serve as a guide to opportunities for enhanced resource recovery in the U.S. Further, archiving and analysis via waste informatics can serve as a valuable repository of information for other nations experiencing both rapid and © XXXX American Chemical Society
moderate growth to glean lessons from decades of regulated sanitary landfilling practice in the U.S. and to prepare for a future of beneficial reuse. Landfill Characteristics and Capacity. Disposal facilities represent only one dimension of waste management, but satisfy a critical need by providing repositories for discarded materials not captured by reuse and recovery efforts. U.S. federal regulations have dictated management of municipal waste for nearly 40 years, and the regulations have evolved over time, progressing from requirements to manage hazardous and nonhazardous waste separately to design and operational requirements for municipal solid waste landfills. Examining U.S. landfills in aggregate, however, to assess their size, capacity, operating characteristics, and other parameters, has not been done consistently despite the decades-long history of modern sanitary landfilling. Local and state rules, along with variable market conditions and waste generation rates, are all factors influencing landfill characteristics.5 Received: June 7, 2016 Revised: September 2, 2016 Accepted: September 21, 2016
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steadily in the past couple of decades,19 substantial additional energy recovery opportunities are expected in light of the evident trend of continued landfilling in the U.S. Furthermore, LFGTE projects are attractive relative to other renewable energy sources because of their continuous energy output, which can help to offset baseload demand. Although the design and implementation of a LFGTE project is by nature a site-specific challenge requiring alignment of end uses (or users) and the producing facility, broader regional- and national-scale estimates of energy production potential are useful for resource planning, renewable energy policy development and implementation, and project facilitation. Previous LFGTE potential estimates have been made, but by necessity relied upon gas production modeling20,21 which can significantly over- or underestimate recoverable gas. These estimates were regional in scale,22 relied upon operating data from a relatively small number of sites,23 or did not utilize information such as site-specific gas recovery or collection infrastructure.24 Because the viability of recovering energy from landfill gas strongly depends on the amount of recoverable methane, national-, regional-, and local-scale energy recovery estimates would benefit from using facility-level recovery data in lieu of modeled or extrapolated figures. Furthermore, tying in available infrastructure (e.g., the presence of energy conversion equipment such as engines, or the presence of gas collection and control systems) can further improve LFGTE potential estimates by indicating energy recovery opportunities in short and long-term time horizons. Finally, site-specific gas recovery information and actual (or potential) energy recovery will help to improve GHG emission inventories and models.25,26 This information could spur innovation to enhance LFG recovery,1 and such enhanced LFG recovery efforts will directly benefit short-term GHG mitigation strategies.27
Historically, even basic information such as how much waste is going to landfill has been difficult to obtain, and past estimates have relied on methods incorporating large-scale material flows, expected product lifetimes, and other information that is hypothetical in nature and quite distant from actual on-site waste management.6 Recent work has demonstrated the value of high-quality measured data collected at disposal facilities, which is important not only for developing baselines of waste management practices at multiple spatial scales, but is also critical when considering estimates of other emissions from waste handling such as short-lived climate pollutants like black carbon from waste burning4 and methane,1 or plastic waste inputs into bodies of water.7 Furthermore, using more accurate waste disposal quantities can better inform climate models. For example, in climate models that use a CO2 emissions budget to meet a defined temperature metric, waste sector emissions (particularly in future scenarios) have been found to have a strong impact on available CO2 budgets.8 Additionally, at the city scale, the use of accurate disposal quantities helps to construct more refined footprint estimates of carbon emissions from urban activities.9 Closure of Landfills and Landfill Mining. The deposition of municipal waste into landfills invariably includes a mixture of materials, some of which have value, that were not recovered for a multitude of reasons, ranging from lack of a collection system to economic or physical inefficiencies associated with the collection or processing of the material. Recapturing the value of these materials has driven recent research examining the potential to mine municipal landfills,10−12 although landfill mining has also been explored to mitigate greenhouse gas emissions and reclaim space for urban development. Although mining landfills could entail a variety of unknowns that may affect its widespread viability (e.g., potential risk from encountering hazardous materials or lack of available markets for mined materials), research has shown that the availability of large material quantities is a prerequisite to achieve scale economies.13 A first step in identifying these resource recovery opportunities−a step not conducted previously−involves estimating the available municipal waste “stocks” in landfills, defined as the total quantity of material accumulated at a particular point in time. Landfill Long-Term Care. In 1993, newly promulgated U.S. federal rules established requirements for long-term care of nonhazardous waste landfills after cessation of waste acceptance, which is referred to as postclosure care (PCC), to ensure appropriate monitoring and control of sites to mitigate environmental impacts.14 These rules specify a PCC period of 30 years, and although U.S. states can modify their regulations to differ from federal rules, most states mirror the federal requirements.15 Pinpointing the conditions that enable a landfill owner or operator to cease the PCC activities (not identified in federal rules) is critical because of the resources needed for the PCC period and the risk of environmental damage if the PCC period ends too early. Although researchers have developed different frameworks to judge whether or not PCC can terminate,16−18 no consensus on this approach has yet been established. Estimating the magnitude of expected landfill closures in the U.S. in the coming years is needed to potentially spur action in policy development and establish clear research directions. Energy Recovery from Landfill Gas. The capture and conversion of landfill gas-to-energy (LFGTE) represents a substantial renewable energy opportunity in the waste sector. Although the number of LFGTE projects in the U.S. has grown
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MATERIALS AND METHODS Landfill size and operational data were extracted from the U.S. EPA’s Greenhouse Gas Reporting Program (GHGRP) database using a procedure described elsewhere.1 Briefly, multiple queries were made for each year from 2010 through 2013 and the outputs were merged by matching unique landfill facility IDs to create detailed profiles of each site. The changes in new landfill size were calculated by grouping together landfills that accepted waste across multiple decades (1960−1969, 1970−1979, 1980− 1989, 1990−1999, 2000−2009, 2010−2014) and creating boxplots of the mass-based waste acceptance capacity for each site in Excel. Landfill disposal surface areas for each state were aggregated and grouped by U.S. Census region and plotted in the statistical and data analysis software package R. Summary statistics (median, upper and lower quartiles) for state-by-state landfill capacity were calculated and plotted in R by examining the number of years of disposal life remaining for every site in each state. The PCC analysis incorporated the progressive closure year of every site (where available) projected out for the next 25 years, which included a closure year estimate for 264 sites. The mass-based landfilled stocks of MSW were estimated by developing a historical disposal model on a site-by-site basis and summing available disposal data. The historical disposal estimates reflect a combination of site-reported estimates and measured disposal data produced by each site’s reporter to the GHGRP. Generally, historical disposal data reflects estimates by site operators and owners that rely on knowledge of site B
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Figure 1. Decadal mass-based total landfill capacity for newly-built landfills in the U.S. The line represents the median, the box edges represent the upper and lower quartile, and the whiskers represent the local minimum (bottom whisker) and local maximum (top whisker), exclusive of outliers. Boxes are grouped by the decade in which waste was first accepted, n refers to the number of landfills that first accepted waste in each decade.
Figure 2. Map of landfill surface area. (a) Growth in total municipal landfill surface area containing waste in the U.S. as reported in the GHGRP from 2010 to 2013 and (b) U.S. map delineating the census regions presented in panel a.
dimensions and operating records, but contemporary disposal amounts almost exclusively reflect high-quality mass measure-
the county level using U.S. Census boundary data, and mapped the deposited stocks using R. Multiple dimensions of LFG recovery and energy potential were calculated using several approaches, with all data utilizing the most recent year (2013) of data available. First, the energy potential of LFG already being recovered at landfills was
ments using scales. For example, 98% of the disposal amount in the year 2010 reflects MSW disposal measured with scales. Here, we aggregated each site’s reported disposal amount resolved at C
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Figure 3. Remaining Disposal Capacity in Each U.S. State as of 2013. The black line represents the median number of years of capacity remaining for all open (those receiving new waste) landfills in the state, the box edges are the upper and lower quartiles, and the whiskers reflect the local maximum and minimum values. Outliers are shown as dots and reflect values greater than 1.5 times the upper quartile.
as an indicator of size, and Figure 1 shows a comparison of new landfills built during each decade beginning with the 1960s. As the figure shows, the median total disposal capacity for new landfills increased in each decade since 1960, and the quartile data show that the largest landfills built in the past 20 years are far larger than those constructed in the 1960s and 1970s. The median capacity of landfills built between 2010 and 2013 was 17.0 million tonnes, approximately 2.5 times greater than the median capacity of landfills built in the 1960s (6.9 million tonnes). In addition to total disposal capacity, landfills were characterized by the total land area occupied by disposed waste. A regional summary of space occupied by landfill in the U.S. across four years (2010−2013), subdivided on the basis of regions defined by the U.S. Census, is presented in Figure 2. Temporal analysis shows limited changes in areal distribution across regions on an annual basis, with the south Atlantic region of the U.S. (Florida, Georgia, South Carolina, North Carolina, Virginia, West Virginia, Maryland, and Delaware) exhibiting the largest cumulative area occupied by waste. In total, we estimate approximately 5.1 × 108 m2 are occupied by municipal landfill in the U.S. as of 2013. Disposal area data at operating landfills from 2010 to 2013 were also analyzed to estimate the growth rate in area, which is a reflection of the landfill’s expansion occurring either through permitting and construction of new disposal space or filling in new areas that were already permitted to accept new waste. A total of 653 landfills exhibited a growth in disposal area from 2010 to 2013, with an average increase of 1.25 ha per annum per site. U.S. Landfill Capacity Estimate and Projections of Sites Reaching the End of the Post-Closure Care Period. Researchers have suggested that concerns over disposal capacity scarcity were mitigated in the U.S. in the late 1990s but predicted that the issue would re-emerge in the 2010s.31 Recently, an analysis of measured disposal amounts in the U.S. found that not only are there several decades of capacity remaining, but also that available data suggested an increase in aggregate capacity from 2010 to 20131. Waste management in general and disposal in particular, however, is by nature a local or regional concern, so the utility of a singular nation-wide figure is somewhat limited. The estimates of landfill capacity on a smaller geographic scale (such as at the state level) can help provide a clearer portrayal of available capacity in addition to informing systems models for longer-term integrated solid waste management planning.32 Figure 3 presents an estimate of state-by-state U.S. disposal capacity. The data reflect summary statistics for all operating
calculated by computing the sum of all recovered LFG (on a volumetric flow basis) and multiplying the LFG flow by the recovered methane content (%v/v basis) and making the necessary conversion to energy content (MJ) for the year 2013. In a similar fashion, the total modeled LFG generation, computed using historical disposal amounts and first-order decay kinetic factors per the GHGRP’s Subpart HH protocol, was computed for all sites in the GHGRP database that have no active LFG collection system infrastructure. Next, the U.S. EPA’s Landfill Methane Outreach Program (LMOP) database28 was used to match sites in the processed GHGRP database with available characteristics of LFGTE infrastructure. Then, each site’s LFG collection efficiency was used to estimate the quantity of unrecovered LFG, representing additional LFG recovery potential, with a separate analysis for landfills with and without LFGTE infrastructure. The computed gross energy potential at landfills in each of the four categories of interest (LFG already collected at landfills, LFG generated at landfills with no gas collection, uncaptured LFG at landfills with no LFGTE infrastructure, and uncaptured LFG at landfills with LFGTE infrastructure) were grouped and plotted in Excel. More details regarding computation of LFG collection efficiency and other dimensions of LFG collection system performance are provided elsewhere.1
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RESULTS AND DISCUSSION
Disposal Facility Characteristics in the U.S. Federal landfill rules in the U.S. incorporated stringent design and construction requirements to enhance environmental protection significantly compared to previous practices of open dumping and uncontrolled burning. These new rules suggested that a market response was necessary to create new capacity after the closure of many smaller landfills that did not meet the new requirements.5 Understanding the actual market response with respect to new site characteristics, the quantity of capacity, and the timing of landfill openings and closings, however, has not been closely studied. We assessed a range of operational information in a database of 1232 operating and closed municipal landfills in the U.S. that was developed in response to the Greenhouse Gas Reporting Rule promulgated in 2009.29 Although it is generally acknowledged that landfills built today are larger than those in the past,30 data confirming this assertion have been scarce. Figure 1 summarizes, on a mass basis, the total capacity of landfills built as a function of the year in which the landfill first accepted waste. Here we use landfill disposal capacity D
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Figure 4. Histogram of number of landfills that will reach 30 years post-closure in the U.S. by 2044.
Figure 5. Quantity of waste disposed of in U.S. municipal landfills from 1960 to 2013, resolved at the county level, with a maximum range of 200−402 million tonnes and a minimum range of less than 2.5 million tonnes.
with recent national estimates of cumulative disposal capacity in the U.S.1 in that many years of capacity are available in most states. High-resolution data presented here can be used in coordination with knowledge of waste flows between states to closely track capacity dynamics at multiple scales. Landfill operational data were also examined to identify facility closures compared to the federally mandated 30-year PCC period. Figure 4 summarizes the number of landfills that will reach 30 years postclosure through the year 2044. The figure
landfills in the state that were analyzed in our database (see Supporting Information for additional information about the database and further state-level analyses). The estimated remaining capacity is a function of site-specific information such as remaining disposal space, current waste acceptance rates, and anticipated future waste acceptance rates. The data show that 27 U.S. states have more than 25 years of disposal capacity remaining, and nearly all (46) states have at least 10 years of disposal capacity remaining. These data are broadly consistent E
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Figure 6. U.S.-wide estimate of energy potential of landfill gas, including sites with active gas collection and without active gas collection, year 2013.
called for to help understand the benefits and drawbacks of landfill mining as a broader resource recovery approach at city scales and larger areas.38 A key component of this knowledge base includes material and substance stock and flow data resolved at small spatial scales.38 To help address this knowledge gap, we estimated total municipal waste stocks in the U.S. and resolved the stock estimates at the county level. We calculated a total municipal waste stock of 8.5 billion tonnes using disposal data from 1960 to 2013 for 1232 landfills in our database. Figure 5 presents the resolution of waste stocks at the county level. Municipal landfill stocks represent a potential target for future resource recovery because the materials include minimal amounts of hazardous waste by definition (a key inhibitor to landfill mining viability10). Additionally, potentially valuable materials such as metals and plastics are effectively concentrated in the landfill as biodegradable components decompose and transform into the gas phase. In reality, the remaining mass of stock at landfills in our database is certainly less than 8.5 billion tonnes because our estimate examined as-received mass of waste, and the decomposition of degradable components expectedly decreases the total mass over time. Thus, the analysis presented here represents a first step toward characterizing the potential for landfill mining in the U.S. Analyzing site-specific waste degradation profiles along with modeling deposited material characteristics would likely facilitate an assessment of mining potential at the necessary resolution. Additionally, our database does not include smaller landfills that operated in recent years or dumpsites that closed following the promulgation of U.S. federal solid waste rules. It is anticipated, however, that the size and unknown nature of historically disposed materials precludes the likelihood of this population of sites from being viable candidates for future landfill mining. Coupling the waste stock data with field-reported resource recovery composition and quality will enable decision-making at local, regional, and national scales regarding the proliferation of landfill mining, particularly because environmental assessment tools such as life-cycle models can utilize these data to identify optimal environmental and economic outcomes.39 In light of projected continued disposal in the U.S., stocks of municipal waste may become increasingly important sources for valorization.40 Another consideration that may further enhance viability of landfill mining is operation of the site as a bioreactor
demonstrates the substantial growth in the number of sites that will reach the end of the PCC period starting in the year 2020 and continuing through at least 2044. Despite the two-plus decades that landfills in the U.S. have operated with the existing PCC requirement, numerous technical questions remain regarding several facets of evaluating the stability of landfills at the end of the PCC period. State regulatory groups in the U.S. have called for more definitive guidance on metrics to employ when considering whether a landfill can scale back or cease the monitoring that comprises a sizable portion of landfill PCC.15 Our data show that a substantial number of landfills will reach the end of the federally defined PCC period in the next decade, which further underscores the need for consensus metrics to evaluate PCC data, the development of new PCC metrics, and establishment of definitive criteria to enable the scaling-back or cessation of PCC at the end of the defined care period. Defining these metrics is critical for near-term and long-term landfill planning, particularly in light of the environmental importance of proper long-term care, the cost of long-term care,18 and the methods by which currently operating sites accumulate funds to pay for landfill care following closure. Comparative analyses that examine differential operational practices, such as landfills that recirculate leachate to enhance waste decomposition, may provide insight as to how and to what degree these practices influence the stabilization of the landfilled waste mass compared to traditional operational practices (see the Supporting Information for an analysis of leachate recirculation practices for 215 landfills from the GHGRP database). Municipal Waste Stocks in the U.S. Landfilled stocks of municipal waste represent a component of society’s manufactured capital along with buildings, transport, energy, water, industrial production facilities, and durable production and consumer goods.33 Resource recovery from landfillsfor instance, various types of metal, soils and degraded materials, and energy-rich materials like plasticshas been recognized as an emerging materials management approach by reusing secondary materials in lieu of primary materials. To leverage the resource recovery opportunity enabled by landfill mining, in addition to a host of other potential benefits (e.g., GHG emission reductions, mitigation of long-term leachate emissions, and reclamation of previously used disposal space for more efficient waste placement10,12,34−37), a new knowledge base has been F
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when considering local and national carbon footprints and emission reduction targets. This observation is consistent with previous analysis that principally identified open landfills that are actively receiving waste as the most logical targets for substantial near-term GHG emission reductions in the waste sector.1 The Future of Waste Informatics. We have demonstrated that accurate, spatially resolved waste data can provide new, important archival information (landfill size and open/closure information), inform policy and planning directions (time to closure, postclosure care data), unearth and inform new resource recovery opportunities (mining), and identify areas for further improvements to recover renewable energy through LFG collection. Cataloguing data in a centralized, harmonized database as EPA has done represents a good model that could be replicated in other countries. This comes at a particularly important time as nations recently developed new Nationally Determined Contributions (NDCs) as part of the COP21 climate change agreement in Paris. Approximately 80% of the NDCs identify the need for major actions related to waste management, including enhanced data collection and use in decision-making.45 The combination of the data management system−one that contains geographic, dimensional, and operational data−coupled with additional applications of informatics (e.g., remote sensors) − could produce a robust waste informatics beyond the U.S. that answers the important call required in the global push to sustainably develop and mitigate environmental impacts from society’s activities. Refs 2 and 3.
to enhance waste stabilization beyond typical degradation conditions by cycling liquids through the waste mass,41 which has also been demonstrated to reduce the stored ecotoxicity of landfill leachate.42 Our analysis demonstrates that landfills across a range of sizes have substantially practiced leachate recirculation, and rainfall distribution data suggest that leachate recirculation was likely practiced in a controlled manner for reasons beyond simple leachate disposal (see Supporting Information). Infrastructure-Resolved LFGTE Potential in the U.S. We examined the operational data for 922 active LFG collection systems and combined these data with known information about existing LFGTE projects to characterize the energy potential of all municipal landfills in our database, which represent about 95% of all municipal waste disposal in the U.S. Figure 6 shows the gross energy potential from LFG using operational data from 2013. The estimate presents both measured and calculated quantities of gross energy from LFG, inclusive of sites with LFG collection and with or without LFGTE infrastructure and sites with no active LFG collection. As shown in Figure 6, the largest portion of the energy recovery potential corresponds to the LFG already collected at landfills (338 billion MJ/yr). Although this energy recovery potential is accurate, because the figure reflects site-measured LFG flow and methane content at all 922 sites throughout 2013, data on actual energy recovery at these landfills are not as accurate because this information is not tracked with a similar resolution and available data reflect net energy output rather than gross energy. The U.S. Energy Information Administration (EIA) reported that approximately 38 billion MJ (net) were produced from all LFG sources in the U.S. in 2013,43 with an historical average annual growth rate of about 9%. This figure contrasts slightly with the reported 58 billion MJ (net) produced from LFGTE projects in the U.S. by the U.S. EPA,19 but the U.S. EPA data tracks additional forms of energy production from LFG (e.g., upconversion to natural gas), so the U.S. EPA-reported quantity likely reflects a more accurate estimate. In either case, if all LFGTE projects were assumed to generate electricity and possessed a production efficiency of 30% (which is at the lower range of typical LFG engine efficiencies20), then a conservatively high estimate of the fraction of collected LFG used to produce energy is 57%. This demonstrates that a substantial amount of LFG already collected is not currently converted to energy, even at sites where LFGTE infrastructure exists. This likely stems from the phasing in and phasing out of infrastructure as a landfill’s gas production and capture increases or decreases over time, respectively. The other portions of the LFGTE production potential shown in Figure 6namely, potential energy from LFG not captured at sites with and without active gas collectionare small relative to the energy content of LFG already captured at landfills. As we demonstrated, a substantial amount of the total potential energy in LFG is not being utilized at sites that have LFGTE infrastructure, so the data suggest that the largest gains in further energy recovery at landfills exist at sites that already have infrastructure. Recently, investigators found that the presence of specific policies (e.g., renewable portfolio standards and tax credits) provide a meaningful nudge to implement LFGTE projects,44 and our data suggest that enhancing the LFG collection efficiency at landfills represents a limited additional benefit from an energy recovery perspective. Effectively, enhancements in LFG collection efficiency at sites that already have active collection systems likely represent a benefit in terms of life-cycle GHG emission reductions, which are important
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.6b02848. Discussion of additional methods, data and analysis on leachate recirculation practices, rates of landfill openings and closures since the early 1920s, and state-level waste stocks and remaining disposal capacity (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*(J.T.P.) Phone: +1 352-682-4007; e-mail:
[email protected]. *(M.C.) Phone: +1 203-432-6197; e-mail: marian.chertow@ yale.edu. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1122492, and the National Science Foundation PIRE Grant No. OISE 1243535. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the NSF or the U.S. Government. We thank Drs. Thomas Graedel and Menachem Elimelech for their guidance and feedback during the manuscript’s development.
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REFERENCES
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DOI: 10.1021/acs.est.6b02848 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
