Effect of waste-derived soil amendments on mitigating leaching impacts from municipal solid waste incineration (MSWI) ash.

This study explores modifying a sandy soil with a low solid to liquid partitioning coefficient (Kd) by adding amendments including iron-rich industrial slag byproducts and biochars, which contain sorption sites for trace metals present in MSWI ash leachate (notably Sb, cited as a concern for reuse applications). Kd values for Sb were determined for the sandy soil to be as low as 1.6 ± 0.1 L/kg. With amendments, Kd values varied from 1.4 ± 0.2 L/kg for combined ash leachate exposed to a blend of sandy soil and 20% iron slag, to 990 L/kg for combined ash leachate exposed to a blend of sandy soil and 20% magnetic solids. A blend of 20% magnetic solids showed orders of magnitude increase beyond 100% sandy soil. The biochars showed limited capacity to reduce leached Sb in the ash-derived leachate, which is likely due to negative surface charges of the biochars and Sb at basic pH. A risk assessment (US EPA IWEM) performed using experimental Kd for each blend suggests that using soil amendments could reduce leached concentrations at points of concern, which could open additional avenues for ash reuse.

Changes in trace metal concentrations throughout the phosphogypsum lifecycle.

Phosphogypsum (PG) samples from four distinct sources in the Southeastern US were analyzed to explore the variation in total metal content between newly generated (fresh) PG and PG disposed of in phosphogypsum stacks for different lengths of time (stack). Fresh PG exhibited greater total metal concentrations relative to stack PG, including those identified in the literature as important from a risk assessment perspective (As, Cd, Co, Cr, Cu, Pb, and Zn). The pH varied between fresh and stack PG, with some stack samples exhibiting lower pH than fresh samples, however the relationship between pH and age of sample was not linear. Stack samples with pH values similar to fresh samples possessed lower concentrations of total inorganic metals than fresh samples suggesting that process water drainage and stack location play an important role in the reusability of PG as they can affect the pH of stack PG and total inorganic metal concentrations. Overall observations show that stacking PG for three or more years prior to beneficial reuse provides a construction material with lower total metal concentrations than fresh PG.

Municipal solid waste incineration (MSWI) ash co-disposal: Influence on per- and polyfluoroalkyl substances (PFAS) concentration in landfill leachate.

Municipal solid waste incineration (MSWI) ash is often managed through co-disposal with unburned wastes in landfills, a practice previously reported to result in enhanced leaching of pollutants (e.g., heavy metals) in landfill leachate. The objective of this study was to evaluate the effect of co-disposed unburned wastes on per- and polyfluoroalkyl substances (PFAS) in MSWI ash landfill leachate. Leachate was collected from four landfills containing MSWI ash, either as a sole waste stream or co-disposed of with sewage sludge and MSW screenings. Samples of ash and unburned materials were collected and assessed separately for leachable PFAS in the laboratory. All samples were analyzed for 26 PFAS. Results showed that greater ash content was associated with lower leachate PFAS concentrations. The pure ash monofill exhibited the lowest PFAS in landfill leachate (290 ng L-1) while the landfill contained a large amount of unburned waste had the highest PFAS (11,000 ng L-1). For laboratory leaching tests, average ∑26PFAS concentration in lab ash leachate (310 ng L-1) was 10 and 24 times lower than observed in lab sewage sludge leachate (3,200 ng L-1) and lab MSW screenings leachate (7,500 ng L-1), respectively. Leachate from the ash-only landfill had ∑26PFAS concentration similar to what was measured in the ash itself. On the contrary, ∑26PFAS concentration in co-disposal landfill leachates were similar to those in PFAS-rich unburned waste itself, regardless of the percentages of landfilled unburned wastes. We hypothesize that leachate generated in co-disposal scenarios preferentially flows through PFAS-rich unburned materials and that biotransformation of precursors enhanced by unburned waste degradation further contributes to higher concentrations of terminal PFAS in ash co-disposal sites. Landfill operators should expect PFAS in leachates to be higher when PFAS-rich unburned wastes are disposed of alongside MSWI ash, even if the unburned fraction is small.

Comparison of trace element mobility from MSWI ash before and after plasma vitrification.

A common perception of plasma arc treatment systems for municipal solid waste incineration ash is that the resulting vitrified slag is inert from an environmental perspective. Research was conducted to examine this hypothesis and to assess whether reduced pollutant release results from pollutant depletion during the process of the ash with plasma, or encapsulation in the glassy vitrified matrix. The concentrations of four discrete municipal solid waste incineration ash samples before and after plasma arc vitrification in a bench-scale unit were compared. Slag and untreated ash samples were leached using several standardized approaches and mobility among the four metals of interest (e.g. As, Cd, Pb and Sb) varied across samples, but was generally high (as high as 100% for Cd). Comparison across methods did not indicate substantial encapsulation in the vitrified slag, which suggests that reduced pollutant release from plasma arc vitrified slag is due to pollutant depletion by volatilization, not encapsulation. This has significant implications for the management of air pollution control residues from waste-to-energy facilities using plasma arc vitrification.

Trace element release from combustion ash co-disposed with municipal solid waste.

Ash products from coal and municipal solid waste combustion constitute a waste stream with characteristics that, unless recycled, require specific disposal practices. Although traditional disposal involves ash placement in a cell dedicated solely for the ash (monofill), new regulations for the management of coal combustion residues in the US might lead to more co-disposal of these residues with unburned municipal solid waste (MSW) that has not been combusted or otherwise processed. Both monofill and co-disposal practices are currently utilized for MSW incineration ash in the US. Column tests were performed using landfill leachate as a leaching solution to simulate co-disposal conditions of ash with MSW, while DI water was used to simulate monofilling. Mobility of As, B and V from coal fly ash was enhanced in the presence of landfill leachate in both batch and column tests, and a similar trend was observed for Cd and Mo release from MSW incineration ash. For several elements, release was greater with the column procedure relative to the batch procedure. The results suggest that long-term implications of co-disposal should be factored into decisions regarding which disposal scenario to pursue.

Hazardous waste characterization implications of updating the toxicity characteristic list.

In the US, the toxicity characteristic leaching procedure (TCLP) determines if a waste is toxicity characteristic (TC) hazardous based on leached concentrations of specific chemicals. The TC limits were originally derived from drinking water standards (DWS) adjusted by a dilution attenuation factor of 100. The TC limits have not been updated along with DWS revisions. This research examines potential implications of updating the TC limits to account for new DWS thresholds and elements, as well as tap-water risk thresholds; this allows a further expanded evaluation of elements that might be regulated as drinking water standards in the future. Fossil fuel combustion residues, batteries, electronic wastes, municipal solid waste incineration (MSWI) ashes, and treated wood were examined with TCLP and the leached metal concentrations were compared to revised TC thresholds. The two wastes most affected by updated TC limits would be batteries and MSWI ashes. Thallium and antimony, which were not included on the original TC list, exceeded the TC thresholds for batteries and MSWI ash, respectively. Copper, a chemical used in current preserved wood formulations, did not cause currently marketed treated wood to be hazardous waste, but arsenic did for older wood products.

Antimony mobility from E-waste plastic in simulated municipal solid waste landfills.

The fate of antimony (Sb) leached from electronic and electrical equipment plastic when disposed of in a municipal solid waste (MSW) landfill was assessed using simulated anaerobic landfill lysimeters and three different batch leaching tests: toxicity characteristic leaching procedure (TCLP), EPA method 1313, and MSW leachate extractions. Plastic from cathode ray tube televisions sets was noted to have the highest Sb concentrations, and was thus the focus of the study. Sb leachability from EPA 1313 stat and TCLP were similar at approximately 0.1% by weight at the same pH (4.93), while MSW landfill leachates extracted less Sb at approximately 0.02% by weight. Solution pH was not the controlling factor, and other conditions resulting from the landfill leachate resulted in lower concentrations of leached Sb. In simulated landfill experiments, Sb leached at approximately 0.01% by weight after a liquid-to-solid ratio of 3. Sb behaves differently in the landfill environment than arsenic leaching from a similar study, most likely from the reducing conditions brought on by the decomposing waste.

Assessment of the total content and leaching behavior of blends of incinerator bottom ash and natural aggregates in view of their utilization as road base construction material.

The total and leachable metal content from mixtures of weathered municipal solid waste incinerator bottom ash (MSWI BA) and conventional natural or recycled aggregates was investigated with a focus on utilization of MSWI BA as a partial component in a road base. Two weathered bottom ashes were combined with various aggregates in multiple replacement percentages of up to 85% traditional aggregate, with the goal of mitigating leaching and direct human exposure risk. Al leaching was found to decrease proportionally to the mass of bottom ash included in the blended products, with over 90% reduction in blends with 85% recycled concrete aggregate (RCA). Release of Sb from the bottom ashes was predominantly controlled by solubility. Sb concentrations were reduced from 0.043 and 0.037 mg/L to 0.006 and 0.007 mg/L for facility A and B respectively blended with the highest tested proportion of RCA, near compliance drinking water standards of 0.006 mg/L. The high pH and presence of calcium-bearing minerals in recycled concrete appeared to facilitate significant immobilization of Sb in comparison to other aggregates. Similar results were observed for several other elements and material blends. Results indicate that blending MSWI BA with conventional aggregates is a feasible recycling application. Blending effectively mitigates environmental risk associated with the un-encapsulated use of MSWI BA in road construction.

Initiatives to reduce lead from electronic devices: evidence of success from the toxicity characteristic leaching procedure.

Discarded electronic devices (E-waste) have historically been found to exceed US Toxicity Characteristic hazardous waste thresholds for lead. Research was conducted to assess whether global and national lead reduction initiatives in the past decade translate to reduced toxicity characteristic leaching procedure (TCLP) lead leaching from E-waste. Nine categories of devices were subjected to TCLP and in all devices except one (smoke detectors), mean TCLP lead concentration results decreased by an order of magnitude or more (to levels below regulation thresholds). Mean TCLP lead concentrations decreased from 29.1 mg/L (2000-2005) to 0.224 mg/L (2008+) for cell phones and 1.26 mg/L (2000-2005) to 0.060 mg/L (2008+) for PCs. Most recently manufactured electronic devices (of those types tested here) comply with the definition of non-hazardous waste under US regulations. Implications: Discarded electronic devices (E-waste) have often been tested as hazardous waste in the US because of lead leaching. Toxicity characteristic leaching procedure (TCLP) testing on more recently manufactured devices reveals that global lead reduction efforts have resulted in newer devices complying with US non-hazardous waste definitions. While these results highlight the success of lead reduction efforts, they raise policy questions regarding how best to incentivize E-waste recycling going forward.

Material- and Site-Specific Partition Coefficients for Beneficial Use Assessments.

Partition coefficient (Kd) values available in the literature are often used in fate and transport modeling conducted as part of beneficial use risk assessments for industrial byproducts. Because element partitioning depends on soil properties as well as characteristics of the byproduct leachate, site-specific Kd values may lead to more accurate risk assessment. In this study, contamination risk to groundwater of beneficially reused byproducts was assessed using batch leaching tests on waste to energy bottom ash and coal combustion fly ash. Leachates were equilibrated with eight different soils to obtain the waste-soil-specific Kd,exp values for the metals of interest. The Kd,exp values were used as inputs in the Industrial Waste Management Evaluation Model to demonstrate the degree to which Kd estimates affect risk assessment outcomes. Measured Kd,exp values for the most part fell within the large range of Kd values reported in the literature, but IWEM results using default Kd values for some types of soils resulted in overestimated risk compared to those derived from Kd,exp values. Modeled concentration at the receptor location was much lower for some elements for those soils with high concentrations of iron and aluminum.

Re-evaluating the TCLP's Role as the Regulatory Driver in the Management of Municipal Solid Waste Incinerator Ash.

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.

Limitations of the TCLP fluid determination step for hazardous waste characterization of US municipal waste incineration ash.

The Toxicity Characteristic Leaching Procedure (TCLP) requires the use of one of two acid-based extraction fluids: fluid #1 or fluid #2, determined by the fluid determination step (FDS). Fluid #1 is a buffered solution of acetic acid and sodium hydroxide while fluid #2 is an unbuffered acetic acid solution. Motivated by divergent fluid determination results among different laboratories, the limitations of the FDS in the TCLP used for characterizing municipal solid waste incineration (MSWI) ash were investigated by varying three test parameters: solution cooling rate, degree of particle size reduction, and sub-sample selection. A faster cooling rate decreases the needed cooling time and has the potential to alter the fluid outcome to fluid #1. Excessive size reduction of particles forces all the fluid outcomes to fluid #2, possibly due to increased surface area and reaction rate of MSWI particles with the FDS media. Selecting larger particles increases the likelihood of obtaining fluid #1 while selecting smaller particles increases the possibility of requiring fluid #2, possibly due to the lower alkalinity of larger particles and higher alkalinity of smaller particles. These findings suggest that FDS can be conducted using different allowable approaches, based on the interpretation of the users who have the potential to alter the fluid outcomes to achieve a favorable extraction fluid. These limitations may allow MSWI ash to consistently "pass" the TCLP. These phenomena highlight a major flaw in the existing hazardous waste testing protocols for MSWI ash.

The efficacy of pH-dependent leaching tests to provide a reasonable estimate of post-carbonation leaching.

pH is a vital factor related to the heavy metal leaching from wastes. Over time, waste materials may be naturally weathered in the presence of water and carbon dioxide, reducing their pH and altering their mineralogy. Here we evaluate whether conducting a pH-dependent leaching test on wastes expected to carbonate sufficiently reflects the leaching of these wastes upon carbonation. Certain elements, such as Al and Sb, exhibited different leaching trends for carbonated and un-carbonated samples of two different waste materials. XRD results observed different mineral phases as a result of carbonation in incineration bottom ash. The application of pH-dependent leaching tests on fresh waste samples (at neutral pH values) were found to potentially mischaracterize leaching from carbonated waste samples at similar pH values for some elements and waste materials.

Limitations of the toxicity characteristic leaching procedure for providing a conservative estimate of landfilled municipal solid waste incineration ash leaching.

Limitations of the toxicity characteristic leaching procedure (TCLP) for simulating pollutant leaching from wastes disposed of in full-scale landfills are well understood in the waste management profession; the TCLP solution has a lower pH and greater organic acid content than typical landfill leachate. The TCLP serves its intended regulatory objective, however, as long as a conservative estimate of leaching is provided. Here, we examine TCLP's ability to represent worst-case leaching conditions for monofilled municipal solid waste incineration (MSWI) ash. A critical examination of TCLP's applicability to MSWI ash is especially relevant, as ash management at MSWI facilities often centers on passing TCLP, regardless of environmental risk posed by the ash or its recyclability. Multiple batch leaching tests were conducted on different MSWI ash streams: mixed ash, fly ash, and different size fractions of bottom ash. Batch-test results were compared with leachate simulating MSWI ash monofills. The TCLP did not consistently provide the most conservative estimate of leaching, supporting the need to consider alternative methodologies in future regulatory development.Implications: This paper analyzes the existing hazardous waste regulatory testing requirement for municipal solid waste incinerator (MSWI) ash management to evaluate whether the TCLP serves its intended purpose in providing the most conservative estimate of landfilled MSWI ash. The results will serve as guidance and motivation for policy makers and the regulatory community to reevaluate the TCLP's application for characterizing MSWI ash leaching in certain disposal scenarios and could promote consideration of alternative testing procedures based upon results of this study. This study serves to promote representative and accurate quantification of leaching risk from MSWI ash.