Biodegradable organic matter in municipal solid waste incineration bottom ash

For investigation of the behavior of municipal solid waste incineration bottom ash in landfill, we have analysed bottom ash samples taken after the quench tank as well as after five months of storage in the laboratory for elements and organic constituents. Water extractable organic carbon, particulate organic carbon, amino acids, hexosamines and carbohydrates considerably decreased during the five months of storage and their spectra revealed microbial reworking. This shows that the organic matter present in the bottom ash after incineration can provide a substrate for microbial activity. The resulting changes of the physico-chemical environment may effect the short-term behavior of the bottom ash in landfill.     

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.     

Lysimeter washing of MSW incinerator bottom ash

Stockpiled municipal waste incinerator bottom ash is frequently considered for utilization as a construction material. Two 360 kg lysimeter experiments were conducted to study percolation washing of contaminants from stockpiled MSW bottom ash. One lysimeter was leached with a concentrated sodium hydroxide solution, as a possible pre-treatment for improvement of the bottom ash characteristics prior to utilization, while the other was leached using distilled water. The lysimeter leachate was analysed, and at the end of the 2-year leaching period, the bottom ash from each lysimeter was subjected to several laboratory tests to assess the effect of the treatments. The laboratory tests showed that distilled water leachability of both treated ashes was an order of magnitude lower than that of fresh ash, but long-term contaminant leachability under acidic conditions had not changed. Although alkaline washing clearly resulted in greater contaminant removal than did distilled water washing, the chemical properties of the alkaline-leached bottom ash were not significantly different from those of the water-leached ash.     

Weathering of a MSW bottom ash heap: a modelling approach

Establishing plausible predictive scenarios represents a challenge for the long-term evolution of waste such as municipal solid waste bottom ash. These systems are characterized by complex and sometimes poorly understood physico-chemical mechanisms. The long term prediction of the evolution of such systems must be based on a dynamic approach involving their study in space and time. A preliminary outline of a model integrating chemistry and mass transfer is currently being tested by BRGM on the results obtained from a 16-month monitoring survey of a pilot bottom ash heap subjected to meteoric weathering. The model is based on a simplified coupled chemistry-transport approach using mass action laws and Kinetics chemical model (the Networks of Chemical Reactors approach). This modelling approach is used to monitor the evolution in chemical composition of a column of meteoric water percolating through the pilot bottom ash heap. The system is divided into representative elementary volumes (chemical reactors) on the basis of the major chemical and mineralogical zonations identified in the system.     

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.     

The neutralization behavior of MSWI bottom ash on different time scales and in different reaction systems

The neutralization behavior of municipal solid waste incineration (MSWI) bottom ash is characterized according to reaction time and reaction system. Batch and flow-through pH titrations are used to determine the acid neutralizing capacity of the MSWI bottom ash in different reaction systems. A reaction path model and a reactive transport model evaluate the detailed mechanisms and titration kinetics of the experiments. The results indicate that both the neutralization kinetics and titration kinetics of the test methods are important. They determine the relation between the time scale of the experiments and that of an actual leaching environment. Current pH titration experiments account mainly for the relatively fast neutralization reactions, so that they cannot assess the long-term neutralization processes of MSWI bottom ash. It is also found that the neutralization characteristics of MSWI bottom ash in a flow-through system are very different from those in a batch system. The movement of a pH front and the washing-out of soluble buffering components have large influences on the neutralization ability of MSWI bottom ash in a reaction system with flow-through of leachant.     

Technical and environmental long-term properties of industrial residues--summary of field and laboratory investigations

In Sweden, use of industrial residues is still hindered by concern for their long-term properties. A three-year research project was therefore initiated aiming to (1) identify the crucial processes of ageing related to the usefulness of residues in roads; (2) investigate the consequences of these processes for technical and environmental properties of the residues, and (3) propose a method for accelerated ageing to predict the long-term properties. This paper gives an overview of the project methodology, a summary of the test results and references to papers where further details are given.The project, running through 2006-2008, compared naturally aged samples of two residues used as sub-bases in existing asphalt paved roads with samples of fresh residues from producers’ piles. Steel slag of electric arc furnace (EAF) type and municipal solid waste incinerator (MSWI) bottom ash were chosen. The samples were thoroughly characterised in order to identify which ageing processes had been crucial.The results showed that:

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Bottom ash from the pavement edge was more aged than bottom ash from the road centre. However, no difference in pH was found, instead the differences were caused by differences in water exposure.

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Steel slag from the pavement edge showed traces of carbonation and leaching processes, whereas slag from the road centre was identical to fresh slag.

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Water exposure to the subbase materials after ten years in an asphalt paved road was calculated to less than 0.1-0.5 litres per kg.

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Ageing reactions in steel slag and MSWI bottom ash, ready for use, were too small to be verified by laboratory measurement of deformation properties under loaded conditions.

An accelerated ageing test for steel slag was set up to achieve the carbonation (decrease in pH) and leaching that was observed in the pavement edge material.An accelerated ageing test for bottom ash was set up to achieve the pozzolan reactions that were observed in SEM analyses of in situ specimens.It is recommended to use uncrushed particles when properties of aged material are studied, in order to preserve the original particle surfaces.     

Understanding the chemical and mineralogical properties of the inorganic portion of MSWI bottom ash

This paper investigates the changes of mineralogical composition of bottom ash in the environment. The chemical and mineralogical bulk composition was determined by X-ray fluorescence (XRF) and X-ray powder diffraction (XRPD) Rietveld method. Single bottom ash particles were investigated by optical microscopy, scanning electron microscopy with quantitative energy-dispersive X-ray microanalysis (SEM/EDX) and electron probe micro analysis (EPMA). SEM/EDX and EPMA are valuable complement to bulk analysis and provide means for rapid and sensitive multi-elemental analysis of ash particles. The fresh bottom ash consists of amorphous (>30 wt.%) and major crystalline phases (>1 wt.%) such as silicates, oxides and carbonates. The mineral assemblage of the fresh bottom ash is clearly unstable and an aging process occurs by reaction towards an equilibrium mineral phase composition in the environmental conditions. The significant decrease of anhydrite and amorphous contents was observed in the aged bottom ash, leading to the formation of ettringite, hydrocalumite and rosenhahnite under atmospheric conditions. In the water-treated sample, the calcite contents increased significantly, but ettringite was altered by the dissolution and precipitation processes in part, to produce gypsum, while the remaining part reacted with chloride to form hydrocalumite. Gypsum and other Ca based minerals may take up substantial amounts of heavy metals and subsequently control leaching behaviour of bottom ash.     

Long-term leaching test of incinerator bottom ash: evaluation of Cu partition

Two types of leaching tests were performed on the bottom ash from municipal solid waste incinerators. A short-term batch test specified by the America Nuclear Society (ANS) and long-term column tests with acetic acid (pH 5.2) as leaching solution were used to evaluate copper leachability. The Cu leaching after the 5-d ANS test is about 1% of the original Cu content of 5300 mg/kg. Upon addition of a stabilizing agent, the Cu leaching quantity is reduced; the extent of reduction depends on the type of chemical used (phosphate, carbonate and sulfide). The 1.6% Na(2)S addition showed negligible Cu leaching, and Na(2)S was, therefore, used in subsequent column tests. The 30-d column test indicates a steady increase of Cu leaching amount with time and reaches about 1.5% of the original Cu content after 30 d. A 180-d column test further increased the Cu leaching to about 5.1% of the original Cu content, whereas no appreciable Cu leaching was found with the addition of 1.6% Na(2)S. A sequential extraction was conducted on the raw ash, ash with the addition of Na(2)S and the residue ash after 30 d of operation to characterize Cu affinity for different solid fractions. The data were used to evaluate the fate of Cu through these interactions.     

Monotonic aspects of the mechanical behaviour of bottom ash from municipal solid waste incineration and its potential use for road construction

Municipal solid waste incineration (MSWI) bottom ash is an atypical granular material because it may include industrial by-products that result from the incineration of domestic waste. The prospects for the beneficial use of this particular material mainly lie in the field of road construction, as a substitute for the traditional natural aggregates. However, its mechanical properties are still little known, particularly in term of stiffness and deformability, characteristics that are essential to the construction of a durable roadway. The purpose of this paper is to describe better the mechanical behaviour of this recycled material. In order to reach this objective, a large experimental campaign is presented. The first part of this paper presents and comments in detail on the results obtained from static monotonic tests. Oedometric and triaxial shear tests were performed on MSWI bottom ash both before and after treatment with a specific hydraulic binder. These tests allow specification of the mechanical characteristics of the MSWI bottom ash, such as the initial Young's modulus, Poisson's ratio, the compressibility index, the friction angle, and the contracting or dilating behaviour of the material. The results reveal a mechanical behaviour similar to that of initially dense standard materials (sands, unbound granular materials) and a dependence on the applied average pressure, characteristic of the mechanical behaviour of granular media. More laboratory data on other samples of MSWI bottom ash are required to ensure that this comparison is statistically valid.     

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.     

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.     

Influence of waste input and combustion technology on MSWI bottom ash quality

The waste input and the process technology of waste incineration plants appear to have a great influence on bottom ash quality. To better understand how these parameters can affect the characteristics of residues, bottom ash from six plants were tested and compared in this study. Bottom ash physico-chemical characteristics were investigated by chemical analyses, and leaching tests. In order to understand their long-term behavior, accelerated ageing experiments and biodegradation tests were also performed. The whole analyses gave complementary information. It was shown that the six samples do have different properties. Waste inputs have a great influence on Cl and S content in bottom ash, as well as on the Ca/Si ratio. The importance of this ratio on the carbonation process has been demonstrated. Combustion parameters have an influence on the quantity and mobility of the residual organic matter.     

An ecotoxicological evaluation of aged bottom ash for use in constructions

Municipal and Industrial Solid Waste Incineration (MISWI) bottom ash is mainly deposited in landfills, but natural resources and energy could be saved if these ash materials would be used in geotechnical constructions. To enable such usage, knowledge is needed on their potential environmental impact. The aim of this study was to evaluate the ecotoxicity of leachates from MISWI bottom ash, aged for five years, in an environmental relevant way using a sequential batch leaching method at the Liquid/Solid-ratio interval 1–3, and to test the leachates in a (sub)chronic ecotoxicity test. Also, the leachates were characterized chemically and with the technique of diffusive gradients in thin films (DGTs). By comparing established ecotoxicity data for each element with chemically analysed and labile concentrations in the leachates, potentially problematic elements were identified by calculating Hazard Quotients (HQ). Overall, our results show that the ecotoxicity was in general low and decreased with increased leaching. A strong correspondence between calculated HQs and observed toxicity over the full L/S range was observed for K. However, K will likely not be problematic from a long-term environmental perspective when using the ash, since it is a naturally occurring essential macro element which is not classified as ecotoxic in the chemical legislation. Although Cu was measured in total concentrations close to where a toxic response is expected, even at L/S 3, the DGT-analysis showed that less than 50% was present in a labile fraction, indicating that Cu is complexed by organic ligands which reduce its bioavailability.     

Leaching behaviour of elements and evaluation of pre-treatment methods for municipal solid waste incinerator residues in column leaching tests.

Two new pre-treatment methods (water-washing/carbonation and carbonation/phosphate stabilization) of municipal solid waste (MSW) incinerator residues were evaluated by column leaching tests under aerobic conditions and anaerobic conditions (which were changed to aerobic conditions after 10 months). A mixture of bottom ash and fly ash (5:1 ratio) was pre-treated using each method. Shredded incombustible residues (SIR) were added to each ash preparation in proportions similar to the ratios present in landfills. For comparison, landfill wastes typical of Japan, namely, a mixture of bottom ash, chelating-pre-treated fly ash, and SIR, were also examined. Leachate samples were collected periodically and analysed over a 15-month period. When compared with chelating pretreatment, both water-washing/carbonation and carbonation/ phosphate stabilization reduced the leaching of Pb, Al, and Cu by about one to two orders of magnitude. Moreover, the initial concentrations of Ca and Pb in leachates from column of water-washing/carbonation were 56-57% and 84-96% less than those from the column of carbonation/phosphate stabilization. Therefore, water-washing/carbonation was considered to be a promising approach to obtain early waste stabilization and to reduce the release of heavy metals to near-negligible levels. The leaching behaviour of elements was also discussed.     

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.     

Chemical changes in different types of coal ash during prolonged,large scale,contact with seawater

In this study, we followed the chemical changes occurring in coal ash exposed to prolonged (300 days), large scale, contact with running seawater. Four major components (Al, Ca, Mg, Fe) and seven minor and trace elements (Cd, Cr, Cu, Mn, Zn, Pb, Hg) were measured in four coal ash types: fly and bottom ash freshly obtained from coal-fired power plant, and old ash (crushed and blocks) recovered from the sea after 3-5 years contact with seawater. Changes occurred in the chemical composition of the coal ash along the experiment: Fe increased in fresh ash, Al increased in old ash and Ca increased in all ash types except old ash blocks. Cu and Hg decreased in fresh fly ash while Cr increased, Cd decreased in all ash types except bottom ash, and Mn decreased in bottom ash. Most of the changes occurred in the fresh fly ash, and not in the old ash, indicating equilibrium after prior exposure to seawater. In addition, more changes occurred in fresh fly ash than in bottom ash, emphasizing the differences between the two ash types. While the changes in the concentrations of the major elements may be an indication of the integrity of the ash matrix, the only elements of environmental significance released to the environment were Hg and Cd. However, calculated seawater concentrations were much lower than seawater quality criteria and therefore the coal ash was considered suitable for marine applications concerning seawater quality.     

Alteration of municipal solid waste incineration bottom ash focusing on the evolution of iron-rich constituents

Municipal solid waste incineration (MSWI) bottom ash contains a considerable amount of Fe-rich constituents. The behaviors of these constituents, such as dissolution and precipitation, are quite important as they regulate the distribution of a series of ions between the liquid (percolated fluid) and solid (ash deposit) phases. This paper studied both fresh and weathered MSWI bottom ash from the mineralogical and geochemical viewpoint by utilizing optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), and powder X-ray diffraction. The analysis results revealed that for the fresh bottom ash, iron preferentially existed in the chemical forms of spinel group (mainly Fe(3)O(4), and a series of Al- or Ti- substituted varieties), metallic inclusions (including Fe-P, Fe-S, Fe-Cu-Pb), hematite (Fe(2)O(3)) and unburned iron pieces. In the 1-20 years weathered bottom ash collected from a landfill site, interconversions among these Fe-rich constituents were identified. Consequently, numerous secondary products were developed, including goethite (α-FeOOH), lepidocrocite (γ-FeOOH), hematite, magnetite, wustite (FeO), Fe-Si-rich gel phase. Of all these transformation products, hydrous iron oxides were the most common secondary minerals. Quantitative chemical analysis of these secondary products by SEM/EDX disclosed a strong association between the newly formed hydrous iron oxides and heavy metals (e.g. Pb, Zn, Ni, and Cu). The results of this study suggest that the processes of natural weathering and secondary mineralization contribute to reduction of the potential risks of heavy metals to the surrounding environments.     

Controlled combustion tests and bottom ash analysis using household waste with varying composition

The influence of the co-combustion of household waste with either sewage sludge, shredder fluff, electronic and electrical waste (WEEE) or PVC on the bottom ash quality and content was investigated under controlled laboratory conditions using a pot furnace. This laboratory approach avoids the interpretation problems related to large variations in input waste composition and combustion conditions that are observed in large scale MSW incinerators. The data for metals content, transfer coefficients and leaching values are presented relative to data for a base household waste composition that did not contain any of the added special wastes. The small WEEE invited direct measurement of precious metals content in the ashes, where measurement accuracy is facilitated by using only mobile phone scrap for small WEEE. The analyses were carried out for different particle size ranges that are of relevance to the recyclability of metals and minerals in the ashes. Positive correlations were found between elements content of the input waste and the bottom ashes, and also between increased levels of Cl, Mo and Cu in the input waste and their leaching in the bottom ashes. These correlations indicate that addition of PVC, small WEEE and shredder fluff in input waste can have a negative influence on the quality of the bottom ashes. Enrichment of Au and Ag occurred in the fractions between 0.15 and 6 mm. The precious metals content represents an economically interesting intrinsic value, even when the observed peak values are properly averaged over a larger volume of ashes. Overall, it has been shown that changes in quality and content of bottom ashes may be traced back to the varied input waste composition.     

Chemical changes and leachate mass balance of municipal solid waste bottom ash submitted to weathering

A study on the chemical stability of municipal solid waste (MSW) bottom ash submitted to weathering was carried out in order to identify and quantify the physico-chemical maturation mechanisms in a large heap (375 tonnes) over a period of about 18 months. The mineralogy and chemical composition of MSW bottom ash were analysed on fresh and maturated material. Calcite is the predominant newly formed mineral during bottom ash maturation, combined with aluminium hydroxides and various sulphates. Lead and zinc are trapped primarily by newly formed carbonates. Monitoring of the pore water and the outlet leachates revealed a marked contrast in the physico-chemical conditions within the heap and at the outlet. The salinity of the fluids peaked at around 16 g/l within the first few weeks and then progressively decreased to fluctuate between 5 and 8 g/l. Due to the high pH of the pore water, the average concentrations of heavy metals in the heap are high: 42.7, 9.6 and 0.8 mg/l for Cu, Pb and Zn. At the heap outlet, however, the leachates are buffered by carbonate precipitation due to equilibration with atmospheric CO2. Copper complexed as a chloride at the outlet remains at a relatively high concentration (10.2 mg/l), whereas Pb and Zn concentrations are below the limit of detection (<25 microg/l). A mass balance carried out over the 18 months of monitoring indicated that 86% of remobilized material within the heap is evacuated from the system. Within the heap, carbonation trapped 43 and 54% of the calcium and bicarbonate flux. The copper, lead and zinc flux at the heap outlet represent only 34, 18 and 19% of the actual remobilized mass of heavy metals.