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Cite This: Environ. Sci. Technol. 2019, 53, 7964−7973

Re-evaluating the TCLP’s Role as the Regulatory Driver in the Management of Municipal Solid Waste Incinerator Ash Kyle A. Clavier, Yalan Liu, Vicharana Intrakamhaeng, and Timothy G. Townsend* Department of Environmental Engineering Sciences, University of Florida, P.O. Box 116450, Gainesville, Florida. 32611-6450, United States

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S Supporting Information *

ABSTRACT: Up to 30% of the municipal solid waste (MSW) that is incinerated for energy recovery ends up as MSW incinerator (MSWI) ash. In light of the large volume of MSWI ash and the expenses and regulatory burden if this ash were managed as a hazardous waste, U.S. MSWI facilities place great emphasis on ensuring MSWI ashes pass the toxicity characteristic leaching procedure (TCLP). The focus on passing the TCLP has the unintended consequence of making recycling more difficult and arguably making the ash less benign. This policy analysis examines current U.S. MSWI ash management practices in relation to the TCLP, and discusses the role of the TCLP as a regulatory driver in the management of MSWI ashes. A review of existing information, example data, and common MSWI ash management practices provide insight into potential issues with the current approach and opportunities for alternative directions.

1. INTRODUCTION

These differences in ash management strategy (commingling and disposal in the U.S. vs ash separation and recycling abroad) are largely driven by the mechanism through which U.S. wastes are classified as hazardous or nonhazardous in accordance with the Resource Conservation and Recovery Act (RCRA). When U.S. hazardous waste definitions were developed in response to the passage of RCRA in 1976,5 a simple procedure was required to identify which solid wastes should be managed as hazardous because of chemical leaching. Today, the Toxicity Characteristic Leaching Procedure (TCLP) (an update to the original RCRA leaching protocol) serves this need and provides one means to classify a waste as hazardous or nonhazardous. Because costs associated with storage, treatment, and disposal of hazardous waste are large compared to nonhazardous waste, MSWI facility operators go to great lengths to “pass” the TCLP and ensure their ash is not classified as hazardous. Minimal changes to U.S. hazardous waste characterization protocols have occurred since the TCLP was established, and as described in this policy analysis, MSWI ash management is still largely governed by a regulatory policy that calls for the use of what many consider a dated test method6−8 that does not accurately portray environmental risk. We make the case here that MSWI operators have become so singularly focused

In the U.S., roughly 235 million metric tons of municipal solid waste (MSW) is generated on an annual basis, with approximately 13% percent managed via MSW incineration (MSWI).1 MSWI involves the combustion of household and commercial garbage for energy production, resulting in 7 million MWh of electricity production yearly in the U.S.2 The process results in about 75% mass reduction, with the remainder being approximately 80% bottom ash (BA) and 20% fly ash (FA). BA consists primarily of unburned materials remaining on the grates after combustion (concrete, brick, glass, ceramic, metals). Solids captured by the air pollution control equipment comprise the FA, including both fine particulates retained in the baghouse and solid byproducts from the scrubber. The different ash sources are conceptually illustrated in Supporting Information (SI) Figure S1. In many countries outside the U.S., BA and FA are managed separately, with FA managed as a hazardous or special waste; concentrations of many elements are notably greater in the FA relative to the BA (e.g., As, Cd). Other elements (e.g., Pb) tend to be of similar magnitude in FA and BA. In the U.S., BA and FA are typically mixed to create a combined ash (CA) product that is disposed of in an engineered landfill or ash monofill (a landfill composed entirely of MSWI ash). Other countries separate the two ash streams, managing some in landfills, but often recycling some ash for applications such as construction aggregate, structural fill material, or cement kiln feed.3,4 © 2019 American Chemical Society

Received: Revised: Accepted: Published: 7964

March 4, 2019 June 11, 2019 June 18, 2019 June 18, 2019 DOI: 10.1021/acs.est.9b01370 Environ. Sci. Technol. 2019, 53, 7964−7973

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Environmental Science & Technology

issues;15,16 the agency was given broad authority to develop a test because of the unknowns at the time. This “deference doctrine”, which gives power to a federal agency like the USEPA to interpret its own statutes in the face of uncertainty, was later formally established in the 1997 Supreme Court decision for Auer v. Robbins.17 When RCRA was again amended in 1984 with the Hazardous and Solid Waste Amendments, the EPTOX was reevaluated and the TCLP was published in 1990.5,18−20 Many of the same assumptions for the EPTOX were included in the decision-making process for the TCLP, with major changes including the expansion of TC chemicals and the introduction of two different extraction fluids based upon waste alkalinity (SI Table S1). In its current form, the TCLP is a batch leaching procedure that uses an acetic acid−based extraction. While the EPTOX mandated one reagent fluid, a distilled deionized water maintained at a pH of 5.0 through continuous addition of a dilute acetic acid solution, the TCLP identifies two separate fluids; the required TCLP fluid is determined based on waste alkalinity. Fluid no. 1, a buffered solution of acetic acid and sodium hydroxide (pH 4.93), is required for less alkaline wastes, whereas fluid no. 2, an unbuffered acetic acid solution without any sodium hydroxide addition (pH 2.88), is required for more alkaline wastes. Both fluids possess the same acetic acid concentration, but differ in their buffering capacity, as one includes sodium hydroxide and one does not. Wastes are leached end-over-end in the required fluid for 18 h, filtered, and the resulting leachate concentrations are compared to the regulated TC limits for eight metals (arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver) and 32 organic compounds. If the TCLP leachate concentration from a waste exceeds the TC limit for any of the prescribed chemicals, the waste is a TC hazardous waste and must be managed accordingly. 1.2. Identified Shortcomings of the TCLP. Shortcomings associated with the TCLP have been previously highlighted by USEPA’s Science Advisory Board (SAB),20,21 the scientific literature6−8,22 and as part of several legal challenges.19,20 The goal of this policy analysis is not to rehash all these issues, but to provide context to TCLP’s history and to establish precedence regarding how well TCLP characterizes various disposal scenarios. The year after the TCLP was promulgated (1991), the Leachability Subcommittee of USEPA’s SAB published a memo recommending the development of multiple leaching tests for different scenarios (to be complemented by advanced modeling) rather than focusing on one disposal scenario.21 In 1993, in response to a challenge by industry regarding TCLP’s applicability to mineral processing waste, the USEPA conceded that their statutory mandate to develop the TCLP did not call for an “approach that would tailor toxicity tests to the specific conditions”. In this case, mineral processors argued that the TCLP did not apply to their waste because the procedure was developed assuming a mismanagement scenario involving landfill codisposal with decomposing waste, and that mineral processing wastes were not disposed of in MSW landfills.19 In 1999 a second SAB memo further highlighted the shortcomings of the TCLP and its overapplication, providing a strong recommendation for improvement of the USEPA leaching framework with field-validated leaching protocols.20 In 2000, the USEPA proposed that the TCLP requirements for hazardous waste K088, spent potliners from primary aluminum reduction, be removed or the test altered (such as replacing the

on passing the TCLP that they generate a waste product which is both more difficult to recycle and often of greater potential environmental risk. MSWI industry professionals through years of study have fine-tuned their ash management practices to pass the TCLP and, as a result of the TCLP requirement, MSWI facility operators have lost incentive to innovate for purposes of resource extraction and achieving more desired environmental outcomes. In this policy analysis, we review the history of the TCLP and its link to the MSWI industry and illustrate using TCLP data on MSWI ash how the singular focus on passing TCLP impedes recycling and disincentivizes more sustainable practices at MSWI facilities. Additionally, we highlight alternatives to the current approach and how recent U.S. Environmental Protection Agency (USEPA) initiatives readily support this evolution. The USEPA’s backing of sustainable materials management (SMM) thinking,9 along with the promulgation of a more refined suite of leaching procedures that include laboratory-to-field evaluations more analogous to disposal conditions,10−14 falls in line with an approach whereby MSWI facility ash generation and handling practices are focused on maximizing resource conservation and minimizing environmental harm, the true intent of RCRA. 1.1. TCLP and Municipal Solid Waste Incineration Ash. The extraction procedure toxicity test method (EPTOX), the leaching protocol preceding the TCLP, was developed in response to the passage of RCRA in 1976. RCRA required the identification of wastes that could pose a hazard to human health and the environment in the event they are mismanaged. The USEPA defined lists of known solid wastes requiring management as hazardous waste, as well as four characteristics that could be measured on unidentified or new solid wastes (ignitability, corrosivity, reactivity, toxicity). The toxicity characteristic (TC) served as the characteristic to identify potential risk from toxic chemicals leaching from wastes upon disposal. The USEPA developed EPTOX as the method to determine if a solid waste required management as a TC hazardous waste. A procedure that could assess plausible worst-case leaching upon disposal, as well as one that could be performed relatively rapidly, was desired. EPTOX was designed around the assumption that landfill disposal would pose the greatest potential for mismanagement and groundwater exposure risk. At the time of its development, science regarding chemical leaching under diverse waste disposal scenarios was limited, thus design of EPTOX required many assumptions; important EPTOX assumptions are summarized in SI Table S1.15 Perhaps most notable are the assumptions surrounding the determination that the leaching media should be an acetic acid−based solution at a pH of 5; codisposal of a waste with MSW in a landfill under biologically active anaerobic conditions would result in exposure to organic acids. At the time, a pH of 5 was thought to represent a realistic worst-case MSW landfill leachate pH. Other assumptions, such as those regarding required particle size and contact time, were effectively a best guess or reasonable compromise among those developing the test.15 The USEPA was mandated by congress to develop a leaching protocol in a timely manner, and this mandate undoubtedly influenced some of the broad assumptions and decisions. The USEPA defended many of its assumptions at the time citing judicial precedence that ruled in favor of agency decisions made in the face of extreme uncertainty regarding newly determined environmental 7965

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ruling of the Seventh Circuit, claiming the generation of MSWI ash was not referenced in the exclusion statute, a decision that became effective in May 1994.35 From this point forth, MSWI ash generation was subject to hazardous waste regulations and the TCLP characterization requirements. In response to the court’s decision, U.S. MSWI facility operators have adopted the practice of comingling BA and FA prior to disposal. This CA stream is disposed of in an engineered landfill equipped with a liner and leachate collection system, either as a monofill consisting only of ash, or codisposed with unburned MSW. This mixing is performed in large part to avoid an outcome where the FA fails TCLP and requires management as a TC hazardous waste; the operators deliberately conduct this mixing within the “four walls” of the MSWI plant so the point of regulatory hazardous waste determination occurs only after the two ash streams have been commingled.36,37 Mixing BA and FA acts to dilute some of the more elevated elements in the FA; but given that hazardous waste status is based on the leachability of TC elements, not simply their total magnitude, mixing these waste streams serves an even more complex role. U.S. MSWI facility operators have learned through years of testing that only two of the eight inorganic TC elements have a realistic chance to exceed TC limits: Cd and Pb. Mixing BA and FA does reduce total concentrations (particularly for Cd), but of equal or greater importance is the resulting leaching environment that occurs when CA is tested using TCLP. It is well demonstrated that inorganic element leaching from a waste is controlled by several factors, but perhaps none more than solution pH.38−41,11 Having introduced the background of the TCLP up to this point, we will now describe several tests methods we performed in order illustrate the discussion herein.

TCLP leaching media with deionized water) because the TCLP under-predicted actual metal concentrations in more alkaline landfill disposal conditions containing this waste.23 Also in 2000, as part of the case Association of Battery Recyclers v. EPA, the courts ruled that the TCLP may not be used for determining whether manufactured gas plant waste is hazardous, as these wastes were not codisposed of with MSW.24 In the years during and since the SAB recommendations and TCLP-related litigation, science related to leaching methodologies and their application to a variety of different disposal environments has expanded greatly.6,7,25−27 As frequently highlighted, candidate hazardous wastes are often disposed of separately from putrescible waste, thus the use of acetic-acid solutions deviates from the likely disposal environment. Multiple studies have found that even when wastes are codisposed with MSW, expected leaching will differ from that measured using TCLP because pH conditions tend to be higher in MSW landfills than those predicted using TCLP,6,8 TCLP does not reflect the behavior of oxoanion-forming elements in MSW leachates,8 and TCLP fails to reproduce the organic matter or redox conditions within actual landfills.28 In fairness to the developers of EPTOX and TCLP, the objective of these procedures was not to simulate expected leachate concentrations likely to occur under different disposal conditions, but rather to simulate a plausible worst-case scenario for leaching from an improperly managed waste.19,25,29 Thus, even if the TCLP fails to account for all the complex interactions occurring within individual disposal scenarios, it still serves its intended purpose if it provides a conservative estimate of leaching. In practice, however, TCLP may not be the most conservative estimate of element release under certain waste disposal conditions. In fact, there are many scenarios where actual landfill leaching is higher than predicted by the test; the TCLP often underestimates As leaching from wastes disposed of in a landfill,6 and may underestimate As leaching from carbonated materials in general.26 Also, the TCLP may not appropriately characterize worst-case leaching of wastes from mining and metallurgical operations,7 and is known to inaccurately characterize leaching of mercury following solidification and stabilization of contaminated soils.30 1.3. Strategies for Managing MSWI Ash as a NonHazardous Solid Waste. After its promulgation, the applicability of RCRA to the management of MSWI ash was a hotly contested issue surrounded by significant legal debate.31 Initially, a provision deemed that the household waste exclusion exempted MSWI ash from regulation under RCRA Subtitle C (should the facility not incinerate hazardous waste), but this provision was challenged in multiple court cases and finally ruled on in the case of Environmental Defense Fund Inc. v. Wheelabrator Technologies Inc. (Wheelabrator). The core of the Environmental Defense Fund (EDF) argument against Wheelabrator (a MSWI company) was that generation of MSWI ash was not exempt from hazardous waste regulation. The District Court dismissed the EDF argument and ruled that MSWI was exempt from Subtitle C hazardous waste regulation.31,32 However, in a separate case in the Seventh Circuit, the EDF won a decision against the City of Chicago; courts ruled that generation of ash was not included under the exclusion language, 33 but a September, 1992 USEPA memorandum reaffirmed the EPA perspective that MSWI ash was exempt.34 The Supreme Court ordered the case remanded and, despite the USEPA memo, later affirmed the

2. MATERIALS AND METHODS 2.1. MSWI Ash Collection. To highlight and supplement discussion of some of the TCLP shortcomings and their associated implications, several leaching tests were performed on multiple different MSWI ash samples. BA, FA, and CA samples were collected from four full-scale MSWI facilities (facility A, facility B, facility C, and facility D) in Florida during normal operating conditions. Sampling from facility A and facility B followed USEPA guidance for sampling MSWI ash to determine the toxicity characteristic,42 during a week-long sampling event whereby ash samples were collected directly from the facility conveyors or ash discharge areas in two 8 h shifts per day so as to represent the temporal variation in the ash streams; a total of 42 ash samples were collected from each facility (14 ash samples for each stream) in 19 L HDPE buckets. Separate sampling events were performed for facility C and facility D, whereby multiple grab samples of MSWI bottom ash were collected from large stockpiles and homogenized in a large mixing area. 2.2. pH Dependent Leaching Characterization. Samples of each ash stream from facility A and facility B were thoroughly mixed to create a composite that was subjected to a modified version of the pH-dependent leaching protocol EPA Method 1313.43 The data from the modified Method 1313 leaching protocol differ from commonly presented pH-dependent leaching results in that they were all produced in the presence of a constant mass of acetic acid, the same as required for TCLP. Some concentrations represent those resulting from fluid no. 1 and fluid no. 2. The remaining concentrations were generated by manually adjusting pH (with 7966

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Environmental Science & Technology HNO3 and NaOH) in the presence of an acetic acid concentration equal to that of the TCLP (the methodology is described in detail in the Supporting Information). These data thus provide an understanding of how TCLP results might change for each element if the alkalinity of the sample were altered (the relevance of which is discussed below). Separately, the composite sample from facility C was leached following standard EPA Method 1313 protocol as well as the modified Method 1313 described above. Comparing data generated from the regular Method 1313 and the modified Method 1313 can provide insights into how the acetate ion may influence TCLP leaching chemistry compared to leaching in a solution that does not contain acetic acid; any leaching differences between the different solutions at the same pH would be attributed to the presence of the acetic acid. 2.3. Standard Leaching and Fluid Determination Characterization. Six random BA samples from facility A and facility B were selected and subjected to the standard EPA Method 1311 TCLP protocol.44 Two other BA samples from these two facilities were randomly selected and subjected to the TCLP fluid determination step (FDS). The FDS involves adding 96.5 mL of water to 5 g of waste (size reduced to less than 1 mm), stirring, and recording the pH. If the pH at this step is less than 5, fluid no. 1 is required. If the pH is greater than 5, 3.5 mL of HCl is added to the mixture which is heated to 50 Celsius for 10 min. If the pH after this step is less than 5, fluid no. 1 is the required fluid, if it is greater than 5, fluid no. 2 is required.44 Variable size reduction and particle selection was applied in order to examine the effects of these variables on FDS outcomes; these variables are displayed alongside SI Figures S4 and S5. In addition, select composite ash streams from all four facilities were subjected to both the TCLP and USEPA Method 1312, the Synthetic Precipitation Leaching Procedure (SPLP), which utilizes a synthetic rainwater leaching extract.45 Samples were selected to illustrate leaching differences between the TCLP and SPLP fluids, described indepth below. Relevant samples were measured for pH and Pb and Cd leaching via inductively coupled plasma-atomic emission spectroscopy after digestion via USEPA Method 3010A.45 Results of these tests are discussed below in the context of this policy discussion on the applicability of the TCLP to U.S. MSWI ash management practices.

Figure 1. Lead leaching as a function of pH in the presence of acetic acid concentration equal to that of the TCLP (modified Method 1313) plot for bottom, fly, and combined ash samples. Samples were collected from two separate MSWI facilities and extractions were performed in duplicate. Data points corresponding to fluid no. 1 and fluid no. 2 extractions for all ash samples are circled and labeled with their corresponding final extract pH. The toxicity characteristic limit is displayed as a horizontal dashed line at 5.0 mg/L.

Figure 2. Cadmium leaching as a function of pH in the presence of acetic acid concentration equal to that of the TCLP (modified Method 1313) plot for bottom, fly, and combined ash samples. Samples were collected from two separate MSWI facilities and extractions were performed in duplicate. Data points corresponding to fluid no. 1 and fluid no. 2 extractions for all ash samples are circled and labeled with their corresponding final extract pH. The toxicity characteristic limit is displayed as a horizontal dashed line at 1.0 mg/ L.

3. RESULTS AND DISCUSSION 3.1. The Influence of pH Manipulation on MSWI Ash Management. Figures 1 and 2 were generated via the modified Method 1313 described in Section 2.2, and they illustrate typical pH-dependent leaching patterns for Pb and Cd, respectively, for ash from two modern U.S. MSWI facilities (facility A and facility B); results for FA, BA, and CA are included. Several important points emerge from examination of Figures 1 and 2. TCLP Pb leaching is minimized in the pH range of 9−10, whereas TCLP Cd leaching is minimized at pH values greater than 7; these pH-dependent leaching patterns are well established in the literature,38−41 though again the results here also include the effects of the TCLP’s acetic acid. The differences in alkalinity among the three ash types are reflected in the measured pH of the TCLP leachates at their respective fluid no. 1 and fluid no. 2 extractions. FA exhibits the highest pH, BA exhibits the lowest pH, and CA falls in between.

The benefits of commingling FA and BA become apparent upon scrutiny of the Cd and Pb TCLP trends in Figure 1 and Figure 2. FA approaches the Pb TC limit because of its high alkalinity and resulting TCLP pH, and mixing with BA produces a CA well below the Pb TC limit. The lower pH of the CA mixture does, however, create the potential that the Cd and Pb concentrations may approach TC limits if the TCLP pH is too low, primarily if fluid no. 2 is required. The strategy that U.S. MSWI facility operators follow to pass TCLP for their entire ash stream becomes clear: commingle the FA and BA so that the FA does not exceed the Pb TC limit, and hope that the CA TCLP pH is sufficiently high to keep CA from 7967

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Environmental Science & Technology failing TCLP for Pb and Cd under fluid no. 2 (or that fluid no. 2 is not required). The implications of failing TCLP and requiring ash management as hazardous are too severe to rely on “hope” alone. MSWI ash characteristics do differ among facilities and can change to some extent depending on the combusted waste stream. Through their knowledge of TCLP and leaching patterns such as those in Figures 1 and 2, facility operators take other steps to ensure they do not exceed TC limits. Some facilities rely on addition of a chemical agent (e.g., the addition of soluble phosphate, lime, and water known as the WES-PHix process46) to reduce TCLP leaching and ensure that the ash consistently passes TCLP. More commonly, facilities rely on simply modifying the alkalinity of the ash to produce a TCLP pH range that ensures TC limits are not exceeded. MSWI facilities in the U.S. must meet permitted acid gas emission limits, and they achieve this through the addition of lime in dry or wet scrubbing systems. As shown in SI Figure S1, the scrubber byproduct is mixed with the baghouse particulates to produce FA, and the amount of unreacted lime in the FA dictates the alkalinity of the CA.47 With knowledge of their facility’s ash leaching characteristics in mind, operators will increase lime addition into the scrubber beyond the amount required to mitigate acid gas emissions to their permitted level to buffer their FA to pass TCLP; lime addition rates are often set solely for the purpose of creating a CA that does not fail TCLP. Some facilities take this a step further by adding dolomitic lime (CaMg(CO3)2) directly onto the ash conveyor; in this case, the lime addition serves no air pollution control purpose at all. Through careful study of their ash streams, most especially the TCLP pH-dependent leaching relationship of Cd and Pb, facility operators introduce additional alkalinity to their ash through their air pollution control system or through direct chemical addition to reach a desired ash pH, thus maximizing the likelihood of passing TCLP. Such additions are allowable by USEPA without a hazardous waste treatment permit as long as the treatment occurs within the “four walls” of their facility, as the point of waste generation is defined as the point that the ash exits the facility.48 3.2. The Fluid Determination Step as Related to MSWI Ash Management. The discussion so far documents how MSWI facility operators tailor their operations so their ash passes TCLP. We are not implying that any of these actions reflect illicit behavior; facility operators are simply responding to a regulatory requirement and doing so within allowable constraints, all to meet the understandable objective of minimizing ash management costs. Before delving into possible negative ramifications of this approach, we first discuss an additional TCLP issue that critically relates to MSWI ash: the fluid determination step. As described already, one of two acetic acid−based solutions must be used as part of TCLP; the required fluid is determined through the method’s FDS. When TCLP was developed and adopted to address several other EPTOX issues, the approach of using one of two fluids depending on the alkalinity of the waste was incorporated. Described in detail in the methods and materials section, the FDS as envisioned would result in most wastes possessing a TCLP leachate pH in proximity to 5 (a more alkaline waste neutralizing an acidic fluid no. 2, and a less alkaline waste maintaining a pH in close proximity to the original fluid no. 1 pH). As illustrated in Figures 1 and 2, however, TCLP extract pH can range dramatically, from just

under 6 to over 12, a result of the very alkaline nature of MSWI ash. A discussion of the TCLP FDS with respect to MSWI ash is warranted for several reasons. First, also as illustrated in Figures 1 and 2, the selection of required fluid can have dramatic repercussions with respect to the potential for hazardous waste characterization. The CA Cd results in Figure 2 are below detection limit when leached with fluid no. 1, but when fluid no. 2 is employed, some samples begin to approach the TC limit; further examples of fluid dependent leaching for Pb and Cd are given in SI Figure S2 and S3, respectively. This adds an additional consideration to the facility operator’s lime addition strategy; an ideal outcome is to add sufficient lime to maintain the TCLP pH in the range well below the TC limit, but not enough to cause fluid no. 2 to be required. The consequences of being characterized hazardous are so extreme, however, that operators will often simply add enough lime that even when fluid no. 2 is reached, it falls within the desired pH range. The FDS step is an arguably arbitrary step in a TCLP methodology that prompts facility operators to adjust their operations only to pass TCLP. The pH of leachate from landfills containing ash, even when codisposed with MSW, is greater than 5,49 thus FDS holds no reality with respect to any likely disposal scenario. Ash sources of essentially the same composition might require completely different management strategies simply because one falls just above a pH of 5 in the FDS step and the other falls just below. Complicating this further is the fact that FDS is vulnerable to manipulation or operator error.50 Only 5 g of sample is required for FDS testing, and the approach used to select this sample can affect fluid determination outcome. MSWI ash can be comprised of both large particles of disparate hardness (and ability to crush) and very fine particles. Preferential selection of larger, more visible ash components for FDS testing results in less alkalinity and a more likely fluid no. 1 outcome, while selection of only finer materials (such as those already less than 1 mm) results in greater alkalinity. FDS specifies a particle size of less than 1 mm; an analyst size reducing to achieve a sample just under 1 mm (but no further) is more likely to achieve a fluid no. 1 outcome than an analyst that size reduces the ash sample to well below 1 mm. Fluid determination outcomes for some samples can also be impacted by the heating and cooling step; if the samples are cooled from 50 °C rapidly, they are less likely to reach a pH of 5.50 As the method does not specify a cooling rate or technique, the analyst has some ability to influence results through this step. Some ash samples will result in fluid no. 1 or fluid no. 2 regardless of method differences because of their existing alkalinities, but as we provide examples for in SI Figure S4 and S5 (for which the FDS was performed on samples of multiple different particle sizes), deviations in the method can result in different fluid outcomes for some samples. Smaller particle sizes resulted in fluid no. 2 determinations. This supports reports from MSWI facility operators that different laboratories testing the same MSWI ash sample will at times result in different fluid determinations, which (as displayed in Figures 1 and 2) may have considerable consequence for trace element leaching. Divergent FDS outcomes are just one of the numerous negative consequences that result from the reliance on the TCLP for characterizing MSWI ash. 3.3. Negative Consequences of TCLP as the Regulatory Driver for MSWI Ash. The information presented 7968

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Acetate is well-known to have a high affinity for Pb, causing a lead-acetate complex at low pH values51 that can increase leaching. These results support that the acetate ion (and thus the TCLP) can contribute to a potential overestimation of Pb leaching, as such organic acid concentrations are usually much less in MSWI ash leachate (monofill scenario). Though a similar effect is not observed for Cd leaching, a graph of standard and modified Method 1313 Cd leaching has also been provided in SI Figure S6. Again, since the purpose of TCLP is to identify solid wastes requiring more protective management as hazardous waste (not a tool to predict leaching in different disposal environments), if the TCLP provides a conservative leaching estimate, it arguably serves its intended purpose. But in the case of MSWI ash, leached concentrations measured using the TCLP often do not represent the most conservative estimate of element release, especially when MSWI ash is disposed of via monofill.22 In an ash monofill, leachate will result from contact of rainwater and associated stormwater with the MSWI ash. The pH of this leachate will be greater than that resulting from TCLP. For elements such as Pb (Figure 1), leached concentrations increase at very high pH. In a study comparing element leachability from different ash sources when leached using both TCLP fluids and other leaching protocols that use either synthetic rainwater or deionized water, the required TCLP fluid was in many cases not the most conservative.22 SI Figure S7 presents examples of similar observations for Pb, whereby Pb leaching for select MSWI ash samples (facilities A, B,C,D) is illustrated to be in many cases lower for TCLP extractions than for those extracted with a synthetic rainwater solution (SPLP solution). Based on discussions above, TCLP may not be the most appropriate choice for characterizing the leaching hazard of landfilled MSWI ash. But beyond this, an MSWI facility’s goal of producing an ash that passes TCLP may exacerbate potential leaching risk. Recall that additional lime is added either through the scrubber or directly on the ash for the purpose of buffering the TCLP to achieve a pH in the “safe” range. This additional lime does not act to buffer against acetic acid in an ash monofill (because the leachate composition in an ash monofill does not contain acetic acid), thus the pH should be greater and may thus result in enhanced leachability of elements such as Pb. The push to create an ash that passes TCLP may inadvertently result in greater landfill leaching. 3.3.2. Impediments to Recycling. Using additional lime to pass TCLP may indeed enhance ash leachability in landfills, but in the U.S., these facilities are designed and constructed to prevent pollutant escape to the environment. Thus, while this practice may be less favorable from a long-term sustainability perspective, additional risk should be minimized, at least in the short term. The TCLP-centered mindset of ash management has other more immediate sustainability ramifications. U.S. MSWI facility operators conventionally commingle BA and FA in large part to avoid characterization of FA as hazardous waste. The resulting CA product makes resource recovery more troublesome. The ferrous and nonferrous metal content of MSWI ash is in most cases valuable enough to warrant extraction and recovery. Primary metals recovery occurs at most facilities within the MSWI plant but a considerable remaining metal content (especially the more valuable nonferrous fraction) is frequently targeted in secondary advanced metals recovery operations at a separate facility or location. FA particles stick together and agglomerate

thus far recounts long-stated concerns raised regarding the TCLP by those such as the USEPA’s SAB and highlights how the U.S. MSWI industry has tailored practices to ensure their ash passes TCLP and does not require management as a hazardous waste. The goal of this policy analysis was not simply to add to the chorus of TCLP criticism, however. Calls for changes to TCLP have thus far not resulted in major regulatory change, and we believe this is in large part because little evidence of overtly negative outcomes of the current system have been documented and discussed. We now present examples of how the singular focus of passing TCLP discourages more sustainable practices at MSWI facilities. 3.3.1. Inappropriate Ash Disposal Characterization. EPTOX and TCLP were designed to represent mismanagement of a waste disposed of in a landfill with actively decomposing putrescible refuse. When waste components such as food scraps and paper products undergo anaerobic biological decomposition in a landfill, organic acids are produced, thus the TC leaching protocols utilize acetic acid at concentrations selected to result in a final extraction solution pH of around 5. MSWI ash disposed of in an ash monofill will not be exposed to a similar environment, thus the same arguments presented for other waste streams (e.g., mineral processing waste19) apply here as well. Even when MSWI ash is codisposed along with MSW, leachate pH environments are typically neutral.49 Thus far, the pH-dependent leaching of Pb in the presence of acetate concentration equal to the TCLP has been used to supplement discussion of some of the TCLP shortcomings. Another important consideration for understanding how the TCLP may inappropriately characterize MSWI ash in its final disposal condition is the influence of acetate complexation on lead leaching. To illustrate this phenomenon, Figure 3 compares lead leaching concentrations from MSWI BA (facility C) as a function of pH for both Method 1313 (no acetate) and the modified Method 1313 (with acetate). In the pH range of 5−10, the solution containing acetic acid results in higher leached Pb concentrations compared to water alone.

Figure 3. Impact of solution complexation by acetate on lead leaching. USEPA Method 1313 (graphically represented by solid circles) was performed for lead leaching as a function of pH (using nitric acid/sodium hydroxide to adjust pH), and a modified Method 1313 was also performed for leaching as a function of pH in the presence of an acetic acid concentration equal to that of the TCLP (graphically represented by open circles). 7969

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but leaching protocols with synthetic rainwater or deionized water), and like the landfill disposal scenario, the impact of excessive lime addition may enhance leachability of some elements. Some MSWI facilities are beginning to separate BA from FA, but looming over all these decisions is the concern that one of their ash streams might fail TCLP. Following the same example as above, assume a more science-based risk assessment approach were implemented that resulted in the recycling of 100 000 t of ash-derived aggregate from the hypothetical facility. Using WARM emission factors associated with virgin aggregate production and a 1:1 offset of process and transportation energy (conservatively assuming recycled bottom ash is immediately ready for use), up to 26 500 million BTU energy usage could be offset.61 The potential aggregate recovery for this one facility should be considered in the context of national MSWI ash generation; 13% of over 235 million metric tons of MSW is managed via MSWI, generating a considerable amount of ash with recycling potential.1 3.3.3. Poor Resource Usage. The U.S. MSWI industry provides one the largest resource recovery efforts in the nation, transforming close to 30 million metric tons of MSW into 7 million MWh of electricity.2,62 Because of MSWI facility practices fixated on the negative consequences of failing a TCLP test, other potential resource recovery opportunities have not received the same attention. Potentially recoverable metals and aggregate products are being landfilled, missing an opportunity to reduce consumption of natural resources and the associated benefits of recycling (energy savings, greenhouse gas emission reductions).63,64 Additionally, the practice of increased chemical addition beyond stoichiometric need for air pollution control poses a further demand for virgin resources. It also means increased cost for facility operators. The operator of a facility combusting 450 000 t per year estimated that the cost associated with additional lime (beyond that needed to mitigate their sulfur dioxide emissions) was over $200,000 per year, and this facility did not add additional dolomitic lime.

other FA and BA particles, clogging mechanical screening equipment and reducing the efficiency of metals recovery equipment (e.g., eddy current separators for nonferrous). In countries that do not rely on the TCLP, significant amounts of both ferrous and nonferrous metals are recovered from MSWI ash as a result of managing BA and FA separately. For instance, a MSWI facility in Austria has reported a typical recovery rate of approximately 76% of the metals in the input waste stream (not accounting for the metals also bound in recovered slag for use in road construction); generally the European MSWI metal recovery rate is around 80%.1 Such processing has lagged in the U.S.; international vendors attempting to tap into the U.S. MSWI ash metals recovery market have been challenged by the U.S. CA stream. Secondary metals recovery initiatives in the U.S. can be profitable, but considerable extra effort is required because of the CA. The deliberate introduction of additional lime to create CA that will pass TCLP further creates an ash even harder to process and with less efficient metals removal. The efficiency of metals recovery from MSWI ash, whether within the facility or at a secondary advanced metals recovery operation, depends on multiple factors, including waste characteristics, types of ash conveying and processing equipment, loading rate, and ash stream characteristics such as lime addition rate and moisture content. To our knowledge no systematic study on the effects of lime addition rate on metals recovery has been reported, but based on anecdotal evidence of those involved in the MSWI and ash processing industry, we construct the following example to highlight potential sustainability benefits associated with a reduction in unnecessary lime addition. Consider a MSWI facility that combusts 500 000 t of MSW per year, from which 125 000 t of residual solid byproduct is produced as a single CA stream prior to any metals removal. We assume (based upon insights provided by MSWI facility experts) that approximately 10% of the unburned material is recovered as ferrous (9%, 11 250 tons) and nonferrous (1%, 1250 tons) metals. If a reduction in lime addition were to result in a 10% increase in recovery of both metal fractions, an additional 1125 and 125 tons of ferrous and nonferrous metals, respectively, could be recovered and recycled. Applying life cycle emission factors for recycled materials from the USEPA’s Waste Reduction Model (WARM), the additional recycled metals from this single MSWI facility would account for an offset of approximately 40 450 million BTU of energy and 3235 t of CO2 equivalents (MTCO2E),52 which would in turn equate to the greenhouse gas emissions associated with yearly energy usage for nearly 400 U.S. homes or nearly 700 passenger vehicles taken off the road for a year.53 The extraction of aggregate products offers another recovery opportunity from MSWI ash. A strong track record of using MSWI BA for applications such as road base, pavement aggregate, and cement kiln feed has been established, especially in Europe.54−58 Denmark, for example, recovers 99% of their bottom ash for use in infrastructural applications.1 A limited number of pilot-scale recycling efforts have been undertaken in the U.S.,59,60 but wider interest in this approach has been hampered by the fact that BA and FA are combined. As indicated already, CA is more difficult to process, especially when high lime addition rates are employed. Aggregate products produced from CA will also contain greater concentrations of some elements, possibly limiting reuse options. As part of a beneficial use demonstration, the ash will require testing for leachability concerns (not using TCLP,

4. ALTERNATIVE PATHS FORWARD MSWI plays a vital role in meeting U.S. solid waste management needs, as it does in many other parts of the world; combustion of MSW for energy will only continue to expand in the future. Ash management at MSWI facilities represents a large operational expense, and understandably facility operators go to great lengths to ensure their ash does not require management as hazardous waste. We describe here that the hazardous waste characterization protocol first developed under RCRA in the late 1970s has resulted in unintended consequences regarding MSWI ash management, and because of the focused emphasis on passing TCLP, opportunities to better and more safely manage MSWI ash have been neglected. A reassessment of current regulation and policy with respect to MSWI ash management falls in line with USEPA’s embrace of changing the way waste is thought of in the U.S., the adoption of sustainable materials management thinking. SMM emphasizes resource efficient actions throughout a material’s life-cycle stages, beginning from extraction and extending into processing, manufacturing, transportation, usage, and end of life management.65−68 SMM-based policies and actions endeavor to decrease environmental burdens, utilize resources more efficiently, ensure social welfare, and minimize costs. When applied to MSWI ash management, SMM concepts would attempt to capture and reuse as many resources from 7970

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Environmental Science & Technology the MSW (and ash) as safely possible, minimize unnecessary use of material inputs, and dispose of nonrecoverable byproducts in a safe and protective manner, all with due consideration of cost and societal impact. Enhanced recycling efforts resulting in the beneficial use of MSWI bottom ash would still leave the fly ash stream to be appropriately managed. The total mass of fly ash disposed of would not change (in fact it might decrease if less lime were used) and this material would likely continue to be disposed of in an engineered landfill. The implementation of a more risk-based approach for characterizing environmental hazards and assigning necessary management strategies should result in safer disposal of this material as the outcomes would be based on more realistic environmental risk, and not on the arbitrary performance in an acetic-based disposal scenario. An appropriate framework for characterizing human health and environmental hazard posed by MSWI ash management (both when disposed of and recycled) is essential. In response to past criticisms of TCLP by the USEPA’s SAB and others, the USEPA now provides a suite of additional leaching methodologies, the LEAF framework.11−14 These methods allow testing and evaluation beyond that provided by the TCLP and can be used to assess leaching as a function of pH and liquid-to-solid ratio, and can be used to more accurately reflect the manner in which MSWI ash might produce a leachate when disposed of in a landfill or recycled in a construction project. Characterization data from this expanded set of leaching protocols have been used in support of recent U.S. rulemaking,69−74,27 but have yet to be integrated into the RCRA regulations. Coupled with fate-and-transport tools such as USEPA’s Industrial Waste Management Evaluation Model (IWEM),75 the MSWI industry and the associated regulatory community can use science-driven data to make more appropriate decisions regarding MSWI ash management. Integration of tools such as LEAF and IWEM into a practical regulatory framework will require time and debate, but MSWI ash provides an ideal waste stream to begin such conversation and implementation. Reliance on MSWI will only grow as society continues to evolve from a disposal mindset to one of resource recovery. Characterization of chemical hazards as a result of MSWI ash management is not well represented by TCLP, and the acute focus on passing TCLP produces an ash that is more difficult to recycle and arguably less environmentally benign. We do not suggest that characterization of MSWI ash using more appropriate methodologies eliminates concerns regarding hazardous waste, but the application of alternative characterization protocols will allow decisions regarding reuse and disposal to be based on more realistic science. This will help promote innovation to more efficiently recover resources and to more protectively dispose of nonrecoverable materials.

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for six bottom ash samples leached with TCLP fluid no. 1 and TCLP fluid no. 2. Figure S3: Cadmium leaching results for six bottom ash samples leached with TCLP fluid no. 1 and TCLP fluid no. 2. Figure S4: Impact of sample selection and size reduction on fluid determination step (FDS) outcomes. Figure S5: Impact of cooling rate on fluid determination step (FDS) outcomes. Figure S6: Cadmium leaching for standard USEPA Method 1313 and a modified Method 1313. Figure S7: Lead leaching with the fluid required by the TCLP fluid determination step outcome vs SPLP lead leaching (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: 352-392-0846; fax: 352-392-3076; e-mail: ttown@ufl. edu. ORCID

Timothy G. Townsend: 0000-0002-1222-0954 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the Hinkley Center for Solid and Hazardous Waste Management. The authors thank the MSWI facility owners and operators that participated in this study by providing materials as well as critical insights regarding MSWI ash management practices and associated costs.

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REFERENCES

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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.9b01370. Material supplied as Supporting Information includes Outline of modified EPA Method 1313 used to derive Figure 1, Figure 2, and Figure S6. Table S1: Assumptions made during the development of EPTOX leaching procedure. Figure S1: Process diagram for a generic MSWI facility. Figure S2: Lead leaching results 7971

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DOI: 10.1021/acs.est.9b01370 Environ. Sci. Technol. 2019, 53, 7964−7973