Release of salts from municipal solid waste combustion residues

Residues from fluidized bed combustion of municipal solid waste were investigated with respect to their leaching behavior and possible extraction of salts. The total water extractable amounts of Na, K, Ca, Cl(-), Br(-), F(-) and SO(4)(2-) along with the total dissolved solids of bottom, hopper, cyclone and bag house filter ashes were determined. A simple multistage washing process (using water as the extraction medium) was tested in lab scale experiments. The effect of variations in parameters, such as water to ash weight ratio, contact time, temperature and number of extraction steps was investigated. The leaching behavior of untreated and washed cyclone and bag house filter ashes was evaluated by a two-step batch-leaching test, i.e. the CEN test. The ashes investigated in this study can be arranged according to their decreasing water extractable contents and total dissolved solids as follows: filter ash > cyclone ash > hopper ash > bottom ash. A triple extraction with water at liquid to solid ratio 2 and extraction time 5 min gave the best results for the extraction of Ca, Na, K, Cl(-) and SO(4)(2-) from the cyclone as well as from the filter ashes. The leached amounts of salts in the CEN test performed on the washed cyclone ash were considerably lower than the corresponding amounts released from the unwashed ash. Thus, the washed cyclone ash was made more stable with respect to salt leachability. On the other hand, large amounts of salts were leached from the washed filter ashes as well as from unwashed filter ashes. Therefore, it can be concluded that three stage water extraction is not a suitable stabilization method for this type of filter ashes.     

Leachability of municipal solid waste ashes in simulated landfill conditions

In Japan the volume of municipal solid waste is reduced by incineration, with fly ash and bottom ash disposed in controlled landfills. The leachability of anions and heavy metal cations, Zn, Cu and Pb, from MSW fly ash and bottom ash at different pHs was examined using batch- and column-leaching tests. The MSW ashes had a high capacity for neutralizing acids. Behaviour during leaching depended on the pH of the solution. For the volumes applied, the leachabilities of MSW fly ash were very similar at pHs from 3 to 6. Due to its amphoteric nature, Pb is leachable at pHs of approximately 10 or more, with leachate concentrations of about 3 and 3-10mg/L for the fly ash and bottom ash, respectively, much higher than for Zn and Cu. Pb concentrations for most leaching solutions were 1 and 3mg/L for the fly ash and bottom ash, respectively. Zn, and Cu leached at low concentrations for solutions of pH 3-6. Na and K ions leached at high concentrations of approximately 5000 mg/L in the first batch leaching test, decreasing to 10mg/L by the fourth leach. Ca and Mg ions leached more gradually than Na and K. Cl(-) and SO(4)(2+) ions were the major anions in the MSW ash. The high pH and cation leaching are expected to have negative impacts on the performance of clay liners.     

Leaching behavior of polychlorinated dibenzo-p-dioxins and furans from the fly ash and bottom ash of a municipal solid waste incinerator

The leaching behavior of dioxins from landfill containing bottom ash and fly ash from municipal solid waste incineration has been investigated by leaching tests with pure water, non-ionic surfactant solutions, ethanol solutions, or acetic acid solutions as elution solvents for a large-scale cylindrical column packed with ash. Larger amounts of dioxins were eluted from both bottom ash and fly ash with ethanol solution and acetic acid solution than with pure water. Large quantities of dioxins were leached from fly ash but not bottom ash by non-ionic surfactant solutions. The patterns of distribution of the dioxin congeners in the leachates were very similar to those in the bottom ash or fly ash from which they were derived.     

Treatment process for MSW combustion fly ash laboratory and pilot plant experiments

Fly ash from combustion of municipal solid waste sometimes contains large amounts of soluble salts, such as NaCl, even though the content of soluble toxic metal compounds is relatively low. Removal of the salts by washing with water has been suggested as a method to increase the stability of this type of ash. In the work presented here, a simple washing process was studied and evaluated. The process includes three steps: leaching with water, filtration and displacement washing. Basic data were obtained in laboratory experiments and used in the construction of pilot plant equipment at a full size fluidized bed boiler, where a side-stream of the cyclone ash was treated. The process was designed to minimize the water consumption while obtaining an effective removal of salts and a stable ash residue. In order to achieve this, recirculation of leaching liquor was used and the displacement washing was adjusted to become close to ideal. The results showed that an ash/water slurry with a liquid to solid ratio as low as 3 could be handled without difficulty in the mixing, pumping and filtration units. Washing of the filter cake at a liquid to solid ratio of 0.5 removed the major part of the remaining dissolved salts in the pore liquid. About 90% of the chloride content was removed from the ash, whereas the contents of Na, K, Ca, Cd, Pb and a number of other minor elements were removed by 10-30%. Before treatment, the results of ash leaching tests were sometimes too high for chloride (2003/33/EC), but the treatment reduced the amount of soluble chlorides to far below the limit values. The leachability of most metals was reduced or unaffected by the ash treatment. For Na, K and Cl, it was less than 10% of the value for the untreated ash. However, the results showed that some ash components may be mobilized by the washing. Antimony is the most important due to its toxicity.     

Life cycle assessment of disposal of residues from municipal solid waste incineration: recycling of bottom ash in road construction or landfilling in Denmark evaluated in the ROAD-RES model

Two disposal methods for MSWI bottom ash were assessed in a new life cycle assessment (LCA) model for road construction and disposal of residues. The two scenarios evaluated in the model were: (i) landfilling of bottom ash in a coastal landfill in Denmark and (ii) recycling of bottom ash as subbase layer in an asphalted secondary road. The LCA included resource and energy consumption, and emissions associated with upgrading of bottom ash, transport, landfilling processes, incorporation of bottom ash in road, substitution of natural gravel as road construction material and leaching of heavy metals and salts from bottom ash in road as well as in landfill. Environmental impacts associated with emissions to air, fresh surface water, marine surface water, groundwater and soil were aggregated into 12 environmental impact categories: Global Warming, Photochemical Ozone Formation, Nutrient Enrichment, Acidification, Stratospheric Ozone Depletion, Human Toxicity via air/water/soil, Ecotoxicity in water/soil, and a new impact category, Stored Ecotoxicity to water/soil that accounts for the presence of heavy metals and very persistent organic compounds that in the long-term might leach. Leaching of heavy metals and salts from bottom ash was estimated from a series of laboratory leaching tests. For both scenarios, Ecotoxicity(water) was, when evaluated for the first 100 yr, the most important among the twelve impact categories involved in the assessment. Human Toxicity(soil) was also important, especially for the Road scenario. When the long-term leaching of heavy metals from bottom ash was evaluated, based on the total content of heavy metals in bottom ash, all impact categories became negligible compared to the potential Stored Ecotoxicity, which was two orders of magnitudes greater than Ecotoxicity(water). Copper was the constituent that gave the strongest contributions to the ecotoxicities. The most important resources consumed were clay as liner in landfill and the groundwater resource which was potentially spoiled due to leaching of salts from bottom ash in road. The difference in environmental impacts between landfilling and utilization of bottom ash in road was marginal when these alternatives were assessed in a life cycle perspective.     

Chloride extraction for quality improvement of municipal solid waste incinerator ash for the concrete industry.

Coal ash from power stations has long been used successfully in the cement industry as binders in several Portland formulations. This is not the case for municipal solid waste (MSW) ash as chloride concentrations, ranging from 10 to 200 g kg(-1) dry weight in the bottom and fly ash, respectively, exceed the maximum allowable concentration in most cement mixtures. To reduce chloride content in MSW bottom ash, a laboratory investigation was carried out based on the exhaustive washing in tap water. The influence of operative parameters such as temperature, granulometric properties and solid/liquid ratio of extraction was evaluated. In addition to optimization of the mentioned operative parameters for full-scale application, the paper gives preliminary indications on mechanistic aspects of the washing operation.     

Leachability of Cr(VI) and other metals from asphalt composites with addition of filter dust

The potential use of filter dust in asphalt composites for road construction was investigated. Filter dust contains high concentrations of metals, of which Cr(VI) and Pb are leached with water. Compact and ground asphalt composites with addition of 2% of filter dust by mass were studied. In order to evaluate their environmental impact, leachability tests were performed using water and salt water as leaching agents. The concentrations of Cr(VI) and Pb were determined in leachates over a time period of 182 days. The results indicated that Pb was not leached with leaching agents from asphalt composites. Cr(VI) was also not leached with leaching agents from compact asphalt composites. However, in ground asphalt composites, Cr(VI) was leached with water in concentrations up to 220 microg L(-1) and in salt water up to 150 microg L(-1). From the physico-mechanical and environmental aspects, filter dust can be used as a component in asphalt mixtures.     

Life cycle based risk assessment of recycled materials in roadway construction

This paper uses a life-cycle assessment (LCA) framework to characterize comparative environmental impacts from the use of virgin aggregate and recycled materials in roadway construction. To evaluate site-specific human toxicity potential (HTP) in a more robust manner, metals release data from a demonstration site were combined with an unsaturated contaminant transport model to predict long-term impacts to groundwater. The LCA determined that there were reduced energy and water consumption, air emissions, Pb, Hg and hazardous waste generation and non-cancer HTP when bottom ash was used in lieu of virgin crushed rock. Conversely, using bottom ash instead of virgin crushed rock increased the cancer HTP risk due to potential leachate generation by the bottom ash. At this scale of analysis, the trade-offs are clearly between the cancer HTP (higher for bottom ash) and all of the other impacts listed above (lower for bottom ash). The site-specific analysis predicted that the contaminants (Cd, Cr, Se and Ag for this study) transported from the bottom ash to the groundwater resulted in very low unsaturated zone contaminant concentrations over a 200 year period due to retardation in the vadose zone. The level of contaminants predicted to reach the groundwater after 200 years was significantly less than groundwater maximum contaminant levels (MCL) set by the US Environmental Protection Agency for drinking water. Results of the site-specific contaminant release estimates vary depending on numerous site and material specific factors. However, the combination of the LCA and the site specific analysis can provide an appropriate context for decision making. Trade-offs are inherent in making decisions about recycled versus virgin material use, and regulatory frameworks should recognize and explicitly acknowledge these trade-offs in decision processes.     

Evaluation of heavy metal leaching from spent household batteries disposed in municipal solid waste

Batch leaching tests and simulated landfill lysimeter tests were performed to evaluate the contents of heavy metals leached from spent batteries in the municipal solid waste. The toxicity characteristic leaching procedure was utilized to perform the batch leaching tests of 36 spent batteries. Four lysimeters were prepared with battery contents ranging from 0% to 100% by weight for column tests, and the experiments were performed at ambient temperature. The age of all the batteries used in the study ranged from freshly disposed up to approximately 3 years old. The results from the batch tests showed that the type of battery influenced the heavy metal concentrations in the leached solutions. The lysimeter experiment results illustrated that at lower pH levels more metals are leached than at higher pH levels. The increasing amount of batteries disposed in landfills can contribute to the leaching of more metals, especially Mn and Zn, into the environment. These results indicate that the direct disposal of spent household batteries into a MSW landfill can increase the heavy metal contents in the landfill leachate.     

Evaluation of leaching and extraction procedures for soil and waste

Laboratory leaching tests may be used for source term determination as a basis for risk assessment for soil-groundwater pathways on contaminated sites. In order to evaluate different leaching procedures, batch extraction tests and percolation tests were performed using three reference materials produced from contaminated soil, demolition waste and municipal solid waste incinerator bottom ash. Emphasis was placed on the investigation of the leachability of the heavy metals copper and chromium, polycyclic aromatic hydrocarbons (PAHs) and the anions chloride and sulfate. Significant discrepancies between column experiments and batch/extraction tests were found for the release of PAHs and to a lesser extent for the heavy metals Cu and Cr. Additionally interlaboratory comparisons were conducted based on different leaching tests with the reference materials and evaluated using the criteria of comparability and reproducibility. The best reproducibility was achieved for all investigated substances in column tests. The reproducibility of batch tests was acceptable except for PAHs. The results from the experimental work will help establish standardized and feasible laboratory procedures as fundamental for substance specific risk assessment of contaminated sites.     

Evaluation of concrete incorporating bottom ash as a natural aggregates replacement

A study on the incorporation of coal bottom ash from thermoelectric power stations as a substitute material for natural sand in the production of concrete is here presented. The normally coarse, fused, glassy texture of bottom ash makes it an ideal substitute for natural aggregates. The use of bottom ash in concrete presents several technical challenges: the physical and mineralogical characteristics of the bottom ash; the effect on water demand and the participation on cements hydratation. In the production of the concrete, substitutions in volume were used. Two different ways to employ bottom ash were used to make up the mix proportions: one considering the natural humidity present in the porous particles and the other not considering it, seeking to maintain the same strength. These considerations are fundamental given that the process of bottom ash extraction is carried out through moisture. Mechanical tests by compressive strength were performed and the elastic modulus was determined. An analysis of the influence of bottom ash in the formation of pores was carried out through tests for the water loss by air drying and water uptake by capillary absorption. The results show that the higher the bottom ash contents in the concrete, the worse the performance regarding moisture transport. However, for one bottom ash concrete type, the mechanical properties were maintained.     

Comparison of leaching characteristics of heavy metals from bottom and fly ashes in Korea and Japan

The objective of this research was to compare the leaching characteristics of heavy metals such as cadmium, chromium, copper, nickel, lead, etc., in Korean and Japanese municipal solid waste incineration (MSWI) ash. The rate of leaching of heavy metal was measured by KSLT and JTL-13, and the amount of heavy metals leached was compared with the metal content in each waste component. Finally, bio-availability testing was performed to assess the risks associated with heavy metals leached from bottom ash and fly ash. From the results, the value of neutralization ability in Japanese fly ash was four times higher than that in Korean fly ash. The reason was the difference in the content of Ca(OH)(2) in fly ash. The amount of lead leached exceeded the regulatory level in both Japanese and Korean fly ash. The rate of leaching was relatively low in ash with a pH in the range of 6-10. The bio-availability test in fly ash demonstrated that the amount of heavy metals leached was Pb>Cd>Cr, but the order was changed to Pb>Cr>Cd in the bottom ash. The leaching concentration of lead exceeded the Japanese risk level in all fly ashes from the two countries, but the leaching concentration of cadmium exceeded the regulatory level in Korean fly ash only.     

Chemical behaviour of municipal solid waste incinerator bottom ash in monofills

A method is presented to predict the short-term (months to years) and long-term (> 30 years) chemical behaviour of bottom ash from municipal solid waste incinerators in monofills. It is based on bottom ash composition and laboratory kinetic studies of bottom ash with water. Bottom ash after the quench tank is a reactive mixture in which slow and fast acid/base reactions occur. These intrinsic acid/base reactions continue for at least several months, and the end point is not yet known. The heavy metal concentrations observed in the aqueous extracts reflect primarily the advance of these reactions. Consequently leaching tests based solely on short-term (hours to months) extraction procedures cannot predict the chemical behaviour of bottom ash in monofills. However, laboratory experiments with samples of bottom ash, in which the intrinsic acid/base reactions have proceeded differently, provide useful information on the nature of chemical reactions significant in the short- and long-term.     

Removal of insoluble chloride from bottom ash for recycling

In order to recycle bottom ash and use it as raw material for cement production, the removal of insoluble chloride was investigated by testing various washing techniques. The present work is also focused on investigating the properties of insoluble chlorides and determining the conditions for dissolving these compounds in order to reduce the chlorine content to the required level, i.e., less than 0.1 wt%. Within this framework, the effect of washing with water and CO2 bubbling was investigated, because the main insoluble chloride found in bottom ash, i.e., Friedel's salt, can be dissolved by CO2. Then, in order to better understand the removal of Cl, Friedel's salt was artificially synthesized by hydration and then the effect of CO2 bubbling was investigated. If all chlorides in the ash are converted into Friedel's salt by hydration, all chlorides can then be dissolved by CO2 bubbling. In addition, the effect of pH on removing the remaining insoluble chlorides was investigated by washing the ash with sulfuric acid solution. It was found that the most effective technique to reduce the Cl content to less than 1000 ppm was washing with sulfuric acid solution, while keeping the pH value at less than 4. By using this method, Friedel's salt and other insoluble chlorides were dissolved.     

Leaching behavior of heavy metals and PAHs from MSWI bottom ash in a long-term static immersing experiment

Bottom ash is the main solid residue (in weight) which is produced by municipal solid waste incineration (MSWI) facilities. This material is composed of a mineral matrix and may be used as secondary raw material for construction purpose. However, for this specific application the leaching behavior of the environmentally relevant elements under field conditions is different from the predicted behavior based on results obtained from the standardized leaching test. Therefore, a 70-day simulative experiment has been carried out in this study to investigate the release of major heavy metals (Cu and Pb) and polycyclic aromatic hydrocarbons (PAHs) from several particle fractions of bottom ash under a static leaching condition, where bottom ash was immersed in water at different initial pH values. Results showed that: (1) the leaching behavior of Cu and Pb was much similar with that depicted by the standardized leaching tests, and fit well with the solubility-controlling mechanism; (2) the sorption mechanism on the neoformed phases may control the solubility of Pb, whereas the dissolved organic carbon (DOC) may play an important role in the solubility of Cu; and (3) the leached PAHs were degraded during the later period of leaching process.     

Numerical model for a watering plan to wash out organic matter from the municipal solid waste incinerator bottom ash layer in closed system disposal facilities

Bottom ash from municipal solid waste incineration (MSWI) is a main type of waste that is landfilled in Japan. The long-term elution of organic matter from the MSWI bottom ash layers is a concern because maintenance and operational costs of leachate treatment facilities are high. In closed system disposal facilities (CSDFs), which have a roof to prevent rainfall from infiltrating into the waste layers, water must be supplied artificially and its quantity can be controlled. However, the quantity of water needed and how to apply it (the intensity, period and frequency) have not been clearly defined. In order to discuss an effective watering plan, this study proposes a new washout model to clarify a fundamental mechanism of total organic carbon (TOC) elution behavior from MSWI bottom ash layers. The washout model considers three phases: solid, immobile water and mobile water. The parameters, including two mass transfer coefficients of the solid-immobile water phases and immobile-mobile water phases, were determined by one-dimensional column experiments for about 2 years. The intensity, period and frequency of watering and other factors were discussed based on a numerical analysis using the above parameters. As a result, our washout model explained adequately the elution behavior of TOC from the MSWI bottom ash layer before carbonation occurred (pH approximately 8.3). The determined parameters and numerical analysis suggested that there is a possibility that the minimum amount of water needed for washing out TOC per unit weight of MSWI bottom ash layer could be determined, which depends on the two mass transfer coefficients and the depth of the MSWI bottom ash layer. Knowledge about the fundamental mechanism of the elution behavior of TOC from the MSWI bottom ash layer before carbonation occurs, clarified by this study, will help an effective watering plan in CSDFs.     

Wastewater remediation using coal ash

 Small-scale domestic septic tanks discharge excess nutrients such as phosphorus and nitrogen, as well as pathogens, which can degrade local water supplies. Unfortunately, traditional chemical and physical treatments are not practicable for single-home dwellings. This work reports on a potentially attractive solution to protect local water supplies by using a low-cost industrial waste, coal ash, for contaminant removal. Coal ash is produced as a consequence of electric power generation. The majority of the ash is disposed of in landfills and surface impoundments, or stored on- or off-site, producing large hills or leveling valleys. Only a small portion of the ash is ever utilized, mainly by cement industries and road construction. For example, in Canada less than 25% is used. Therefore, if useful applications can be found, an opportunity exists to make better use of this waste material. Bench-scale laboratory experiments and full-scale field tests show that coal ash has the capacity to remove phosphorus from domestic waste water. The experimental and field data demonstrate that phosphate levels and calcium levels can be correlated, although not in a simple manner. In addition, the ash in packed beds removed total suspended solid (TSS), biological oxygen demand (BOD), ammonia nitrogen (NH₃—N), total Kjeldahl nitrogen (TKN), and E. coli. The removal of E. coli was close 100% in the cases studied. Received: May 20, 2002 / Accepted: October 5, 2002     

Pretreatment and utilization of waste incineration bottom ashes: Danish experiences

Within recent years, researchers and authorities have had increasing focus on leaching properties from waste incineration bottom ashes. Researchers have investigated processes such as those related to carbonation, weathering, metal complexation, and leaching control. Most of these investigations, however, have had a strong emphasis on lab experiments with little focus on full-scale bottom ash upgrading methods. The introduction of regulatory limit values restricting leaching from utilized bottom ashes, has created a need for a better understanding of how lab-scale experiences can be utilized in full-scale bottom ash upgrading facilities, and the possibilities for complying with the regulatory limit values. A range of Danish research and development projects have, during 1997-2005, investigated important techniques for bottom ash upgrading. The primary focus has been placed on curing/aging, washing with and without additives, organic matter, sampling techniques, utilization options, and assessment tools. This paper provides an overview of these projects. The main results and experiences are discussed and evaluated with respect to bottom ash upgrading and utilization. Based on this discussion, development needs and potential management strategies are identified.     

Hydrothermal treatment of MSWI bottom ash forming acid-resistant material

To recycle municipal solid waste incinerator (MSWI) bottom ash, synthesis of hydrothermal minerals from bottom ash was performed to stabilize heavy metals. MSWI bottom ash was mixed with SiO(2), Al(OH)(3), and Mg(OH)(2) so its chemical composition was similar to that of hydrothermal clay minerals. These solid specimens were mixed with water at a liquid/solid ratio of 5. The reaction temperature was 200 degrees C, and reactions were performed for 24-240h. Generation of kaolinite/smectite mixed-layer clay mineral was found in the samples after the reaction of the mixture of bottom ash, SiO(2), and Mg(OH)(2). Calcium silicate hydrate minerals such as tobermorite and xonotlite were also generated. X-ray powder diffraction suggested the presence of amorphous materials. Leaching tests at various pHs revealed that the concentration of heavy metals in the leachates from MSWI bottom ash hydrothermally treated with SiO(2) and Mg(OH)(2) was lower than that in leachates from non-treated bottom ash, especially under acid conditions. Hydrothermal treatment with modification of chemical composition may have potential for the recycling of MSWI bottom ash.     

Use of co-combustion bottom ash to design an acoustic absorbing material for highway noise barriers

The present study aims to determine and evaluate the applicability of a new product consisting of coal bottom ash mixed with Portland cement in the application of highway noise barriers. In order to effectively recycle the bottom ash, the influence of the grain particle size of bottom ash, the thickness of the panel and the combination of different layers with various particle sizes have been studied, as well as some environmental properties including leachability (EN-12457-4, NEN-7345) and radioactivity tests. Based on the obtained results, the acoustic properties of the final composite material were similar or even better than those found in porous concrete used for the same application. According to this study, the material produced presented no environmental risk.