Quantification of the effects of environmental leaching factors on emissions from bottom ash in road construction

Municipal solid waste incineration residues recycled for typical construction materials-a review

Focusing on the great potential of municipal solid waste incineration (MSWI) residues in the construction sector, the applications of recycling MSWI residues in construction materials are discussed in this review. Incineration is a promising method for managing the great quantity of municipal solid waste (MSW). Careful handling of incineration residues including fly ash, air pollution control (APC) residues, and bottom ash is required for this approach. The yield of these residues is large, and they contain many toxic and harmful substances. On the other hand, these residues contain valuable components such as SiO₂, CaO, Al₂O₃, MgO, which are important components of building materials. Therefore, MSWI residues present huge opportunities for potential recycling and reuse in the construction and building industry. This paper summarized and discussed the application of MSWI residues in four typical building materials including cast stone, glass-ceramic, cement, and concrete. Before utilization, three types of pretreatment methods can be used to reduce the toxicity of the residues and improve the performance of the products. In addition, the current issues and the prospects of this field, and the environmental impacts of this application were discussed. It was concluded that MSWI residues can be used to prepare building materials after proper treatment which can improve the mechanical and chemical properties of the residues. The recycling can gain significant economic and environmental benefits at the same time. However, further researches on treatment methods for fine particles are needed.

Simulating the impact of heavy rain on leaching behavior of municipal solid waste incineration bottom ash (MSWI BA) in semi-aerobic landfill

In Japan, approximately 64% of municipal solid waste incineration bottom ash (MSWI BA) is landfilled. Because landfills in Japan are operated without capping, the landfill body is directly exposed to climatic events. Increased frequency of heavy rain is predicted to affect the chemical stabilization of bottom ash (BA) landfill, as rainwater seeps into and interacts with landfill components. This study examined the effect of normal rainfall (15 mm/h) and heavy rainfall (25, 50, and 100 mm/h) events on the leaching behavior of ions (Cl^-, Na⁺, K⁺, and Ca²⁺) and total organic carbon (TOC) in BA (<10 mm particle size) using a percolation column test. The results showed the decreased leaching of leachate components after heavy rainfall and increased leaching after normal rainfall. In addition, the pH fluctuated around 11-12 after heavy rainfall but decreased to 7-9 after normal rainfall. The carbonation of the leachate and BA layers appears to be the main factor in lowering the pH value. Changes in the TOC and ion concentrations can be explained by dissolution, dilution, and the contact time of water molecules and BA particles. The data showed that the cumulative TOC and ion release rates were not affected by heavy rain intensities. The release rate of leachate components during normal rainfall was higher than that in heavy rainfall in all the scenarios. Significant correlations were found between the leachate components (TOC, Cl^-, Na⁺, K⁺, and Ca²⁺ concentrations) and rainfall variation.

Geotextile clogging at different stages of municipal solid waste landfills co-disposed with bottom ash

Co-disposal of bottom ash (BA) with municipal solid waste (MSW) in landfills is a common way for BA management. However, BA co-disposal in MSW landfills may accelerate geotextile clogging and reduce the performance of leachate collection system. This study compared geotextile clogging in a simulated MSW landfill leachate (MSWL) and a BA co-disposed landfill leachate (BAL) at different landfill stages. Geotextile clogging test was conducted using the MSWL and BAL taken from the simulated landfills on the 10th, 80th, 140th and 200th day, respectively. The results demonstrated that geotextile clogging varied with landfill age, due to the change of leachate characteristics. The mass of clogging material in geotextiles with BAL increased from 0.45 g to 2.74 g, which was 43.87%-63.73% greater than those with MSWL. The formation of biofilm was the main contributor for the geotextile clogging. At the same stage, the amount of biofilm formed on geotextile in different leachate was comparable. However, the amounts of CaCO₃ precipitation on geotextile in BAL were 3.85-10.44 times of those in MSW leachate. The pH of leachate played a critical role in CaCO₃ precipitation. The microbial analysis revealed that the co-disposal of the BA greatly influenced the microbial community diversity and structure.

Construction of an interim storage field using recovered municipal solid waste incineration bottom ash: Field performance study

The leaching of hazardous substances from municipal solid waste incineration (MSWI) bottom ash (BA) has been studied in many different scales for several years. Less attention has been given to the mechanical performance of MSWI BA in actual civil engineering structures. The durability of structures built with this waste derived material can have major influence on the functional properties of such structures and also the potential leaching of hazardous substances in the long term. Hence, it is necessary to properly evaluate in which type of structures MSWI BA can be safely used in a similar way as natural and crushed rock aggregates. In the current study, MSWI BA treated with ADR (Advance Dry Recovery) technology was used in the structural layers of an interim storage field built within a waste treatment centre. During and half a year after the construction, the development of technical and mechanical properties of BA materials and the built structures were investigated. The aim was to compare these results with the findings of laboratory studies in which the same material was previously investigated. The field results showed that the mechanical performance of recovered BA corresponds to the performance of natural aggregates in the lower structural layers of field structures. Conversely, the recovered MSWI BA cannot be recommended to be used in the base layers as such, even though its stiffness properties increased over time due to material aging and changes in moisture content. The main reason for this is that BA particles are prone for crushing and therefore inadequate to resist the higher stresses occurring in the upper parts of road and field structures. These results were in accordance with the previous laboratory findings. It can thus be concluded that the recovered MSWI BA is durable to be used as a replacement of natural aggregates especially in the lower structural layers of road and field structures, whereas if used in the base layers, an additional base layer of natural aggregate or a thicker asphalt pavement is recommended.

Assessment of mobility and bioavailability of contaminants in MSW incineration ash with aquatic and terrestrial bioassays

Incineration of municipal solid waste (MSW) is a waste treatment method which can be sustainable in terms of waste volume reduction as well as a source of renewable energy. In the process fly and bottom ash is generated as a waste material. The ash residue may vary greatly in composition depending on the type of waste incinerated and it can contain elevated levels of harmful contaminants such as heavy metals. In this study, the ecotoxicity of a weathered, untreated incineration bottom ash was characterized as defined by the H14 criterion of the EU Waste Framework Directive by means of an elemental analysis, leaching tests followed by a chemical analysis and a combination of aquatic and solid-phase bioassays. The experiments were conducted to assess the mobility and bioavailability of ash contaminants. A combination of aquatic and terrestrial bioassays was used to determine potentially adverse acute effects of exposure to the solid ash and aqueous ash leachates. The results from the study showed that the bottom ash from a municipal waste incineration plant in mid-Sweden contained levels of metals such as Cu, Pb and Zn, which exceeded the Swedish EPA limit values for inert wastes. The chemical analysis of the ash leachates showed high concentrations of particularly Cr. The leachate concentration of Cr exceeded the limit value for L/S 10 leaching for inert wastes. Filtration of leachates prior to analysis may have underestimated the leachability of complex-forming metals such as Cu and Pb. The germination test of solid ash and ash leachates using T. repens showed a higher inhibition of seedling emergence of seeds exposed to the solid ash than the seeds exposed to ash leachates. This indicated a relatively low mobility of toxicants from the solid ash into the leachates, although some metals exceeded the L/S 10 leaching limit values for inert wastes. The Microtox® toxicity test showed only a very low toxic response to the ash leachate exposure, while the D. magna immobility test showed a moderately high toxic effect of the ash leachates. Overall, the results from this study showed an ecotoxic effect of the solid MSW bottom ash and the corresponding ash leachates. The material may therefore pose an environmental risk if used in construction applications. However, as the testing of the solid ash was rather limited and the ash leachate showed an unusually high leaching of Cr, further assessments are required in order to conclusively characterize the bottom ash studied herein as hazardous according to the H14 criterion.

Characterisation of major component leaching and buffering capacity of RDF incineration and gasification bottom ash in relation to reuse or disposal scenarios

Thermal treatment of refuse derived fuel (RDF) in waste-to-energy (WtE) plants is considered a promising solution to reduce waste volumes for disposal, while improving material and energy recovery from waste. Incineration is commonly applied for the energetic valorisation of RDF, although RDF gasification has also gained acceptance in recent years. In this study we focused on the environmental properties of bottom ash (BA) from an RDF incineration (RDF-I, operating temperature 850-1000°C) and a RDF gasification plant (RDF-G, operating temperature 1200-1400°C), by evaluating the total composition, mineralogy, buffering capacity, leaching behaviour (both at the material's own pH and as a function of pH) of both types of slag. In addition, buffering capacity results and pH-dependence leaching concentrations of major components obtained for both types of BA were analysed by geochemical modelling. Experimental results showed that the total content of major components for the two types of BA was fairly similar and possibly related to the characteristics of the RDF feedstock. However, significant differences in the contents of trace metals and salts were observed for the two BA samples as a result of the different operating conditions (i.e. temperature) adopted by the two RDF thermal treatment plants. Mineralogy analysis showed in fact that the RDF-I slag consisted of an assemblage of several crystalline phases while the RDF-G slag was mainly made up by amorphous glassy phases. The leached concentrations of major components (e.g. Ca, Si) at the natural pH of each type of slag did not reflect their total contents as a result of the partial solubility of the minerals in which these components were chemically bound. In addition, comparison of total contents with leached concentrations of minor elements (e.g. Pb, Cu) showed no obvious relationship for the two types of BA. According to the compliance leaching test results, the RDF-G BA would meet the limits of the Italian legislation for reuse and the European acceptance criteria for inert waste landfilling. RDF-I BA instead would meet the European acceptance criteria for non hazardous waste landfilling. A new geochemical modelling approach was followed in order to predict the leaching behaviour of major components and the pH buffering capacity of the two types of slags on the basis of independent mineralogical information obtained by XRD analysis and the bulk composition of the slag. It was found that the combined use of data regarding the mineralogical characterization and the buffering capacity of the slag material can provide an independent estimate of both the identity and the amount of minerals that contribute to the leaching process. This new modelling approach suggests that only a limited amount of the mineral phases that control the pH, buffering capacity and major component leaching from the solid samples is available for leaching, at least on the time scale of the applied standard leaching tests. As such, the presented approach can contribute to gain insights for the identification of the types and amounts of minerals that control the leaching properties and pH buffering capacity of solid residues such as RDF incineration and gasification bottom ash.

Leaching properties of Mn-slag from the pyrometallurgical recycling of alkaline batteries: standardized leaching tests and influence of operational parameters

In the present study, different complementary leaching tests were applied to a Mn-rich slag sample that was not thoroughly studied until now, in order to understand the potential mobility of the elements (Mn, Si, Fe, Ca, K, Mg, Al) within it. Several standard leaching tests (EN 12457-1 and -2, and TCLP) were carried out in order to compare results to regulatory values: the Mn-rich slag studied could be considered as 'inert' waste with respect to the release of the main trace metallic elements (Cu, Ni and Zn). In order to have a better understanding of the environmental behaviour of the Mn-rich slag constituents, other tests were performed. These experiments revealed that the dissolution of the Mn-rich slag was favoured for pH < or = 6. The mobility of the major elements was governed by the solubility of the primary solid phases and the precipitation of secondary minerals. The Visual MINTEQ modelling allowed the leaching results to be completed. From an environmental point of view, the reuse of this slag in acidic conditions has to be rejected due to the dissolution of the main solid phase.

Effect of weathering treatment on the fractionation and leaching behavior of copper in municipal solid waste incinerator bottom ash

This work describes the effect of weathering of fresh quenched municipal solid waste incinerator (MSWI) bottom ash on the fractionation and leaching behavior of Cu. A sequential extraction procedure was used to characterize the fractionation of Cu in the fresh and weathered MSWI bottom ash samples. It showed that the organic matter bound fraction of Cu decreased drastically from 69% to 37% during the weathering treatment, while the residual fraction, Fe-Mn oxides bound fraction, carbonate bound fraction and exchangeable fraction increased from 24% to 54%, 3% to 4%, 2% to 3% and 2% to 3%, respectively. Furthermore, two standard leaching procedures, synthetic precipitation leaching procedure (SPLP) and toxicity characteristic leaching procedure (TCLP), were carried out on the fresh and weathered samples. The leaching of Cu was attenuated with the process of weathering treatment in the SPLP procedure, but was enhanced in the TCLP procedure. The results suggested that the weathering treatment could change the fractionation and leaching behavior of Cu in MSWI bottom ash. Further research is required to correlate weathering of ash and mobility of Cu.

Wet extraction of heavy metals and chloride from MSWI and straw combustion fly ashes

Fly ash residues from combustion often do not meet the criteria neither for reuse as construction materials nor landfilling as non-hazardous waste, mainly because of the high concentration of heavy metals and chlorides. This work aimed to technically evaluate an innovative wet treatment process for the extraction of chloride (Cl(-)), cadmium (Cd), copper (Cu), lead (Pb) and zinc (Zn) from fly ashes from a municipal solid waste incineration (MSWI) plant and from a straw combustion (SC) facility. Factors investigated were liquid/solid (L/S) ratio, full carbonation (CO(2) treatment), influence of pH and leaching time, using a two-level full factorial design. The most significant factor for all responses was low pH, followed by L/S ratio. Multiple linear regression models describing the variation in extraction data had R(2) values ranging from 58% to 98%. An optimization of the element extraction models was performed and a set of treatment conditions is suggested.

The application of pH(stat) leaching tests to assess the pH-dependent release of trace metals from soils, sediments and waste materials

pH is one of the key parameters that determines heavy metal mobility in soils, sediments and waste materials. In many respects leaching behaviour as reflected by the pH(stat) leaching tests provide a better means of assessing environmental impact than analysis of total elemental composition. This paper discusses the use of pH(stat) leaching tests as a tool to assess the potential mobilisation of trace metals from soils, sediments and waste materials. The possibilities of pH(stat) leaching tests are illustrated by means of different examples. The mathematical fitting of metal leaching behaviour from soils and sediments enabled a distinction between 5 groups of elements with a different leaching behaviour, which could be related to 'pools' with different reactivity. Contrary to single and sequential extractions, where pH is difficult to control, the reactivity and mobility of metals at a user-defined pH can be investigated. Moreover, the potential buffering capacity of the sample and its sensitivity to pH changes as a result of external stresses (e.g. soil acidification, liming) can be estimated. A multidisciplinary approach combining mineralogical analysis (X-ray diffraction) with chemical analysis, pH(stat) leaching tests and geochemical modelling (MINTEQA2) can provide information on the solid-phase speciation and reactivity of heavy metals in soils, sediments and waste materials. Besides the influence of pH on heavy metal leaching behaviour, additional information on heavy metal leachability and retention by the solid matrix was obtained from the kinetics of metal release during a pH(stat) test.

Toxicity of leachate from bottom ash in a road construction

A test road constructed with municipal solid waste incineration (MSWI) bottom ash was monitored over a period of 36 months. Using chemical and toxicological characterisation, the environmental impact of leachates from bottom ash was evaluated and compared with leachates from gravel used as reference. Initial leaching of Cl, Cu, K, Na, NH4-N and TOC from bottom ash was of major concern. However, the quality of the bottom ash leachate approached that of the gravel leachate with time. Leachates from the two materials were compared regarding the concentration of pollutants using multivariate data analyses (MVDA). A standardized luminescent bacteria assay using Vibrio fischeri did not show any toxicity, most likely because saline contamination can mask the toxic response and stimulate luminescence in these marine bacteria. A mung bean assay using Phaseolus aureus revealed that the toxicity of bottom ash leachate collected at the very beginning of the experimental period (October 2001 and May 2002) might be attributed to the following components and their respective concentrations in mg l(-1): Al (34.2-39.2), Cl (2914-16,446), Cu (0.48-1.92), K (197-847), Na (766-4180), NH4-N (1.80-8.47), total-N (12.0-18.5), and TOC (34.0-99.0). The P. aureus assay was judged as a promising environmental tool in assessing the toxicity of bottom ash leachate.

Evaluation of leachate emissions from crushed rock and municipal solid waste incineration bottom ash used in road construction

Three years of leachate emissions from municipal solid waste incineration bottom ash and crushed rock in a full-scale test road were evaluated. The impact of time, construction design, and climate on the emissions was studied, and the predicted release from standard leaching tests was compared with the measured release from the road. The main pollutants and their respective concentrations in leachate from the roadside slope were Al (12.8-85.3 mg l(-1)), Cr (2-125 microg l(-1)), and Cu (0.15-1.9 mg l(-1)) in ash leachate and Zn (1-780 microg l(-1)) in crushed rock leachate. From the ash, the initial Cl(-) release was high ( approximately 20 g l(-1)). After three years, the amount of Cu and Cl(-) was in the same range in both leachates, while that of Al and Cr still was more than one order of magnitude higher in ash leachate. Generally, the release was faster from material in the uncovered slopes than below the pavement. Whether the road was asphalted or not, however, had minor impacts on the leachate quality. During rain events, diluted leachates with respect to, e.g., salts were observed. The leaching tests failed to simulate field leaching from the crushed rock, whereas better agreement was observed for the ash. Comparisons of constituent release from bottom ash and conventional materials solely based on such tests should be avoided.

Demobilisation of critical contaminants in four typical waste-to-energy ashes by carbonation

Two bottom ashes, one air pollution control (APC) residue and one fly ash from three different Swedish municipal solid waste incineration (MSWI) plants were characterised regarding the leaching of environmentally relevant components. Characterisation was performed using a diffusion tank leaching test. The impact of carbonation on the release of eight critical components, i.e., Cl(-), Cr, Cu, Mo, Pb, Sb, Se, SO(4)(2-) and Zn, was assessed at a lab-scale and showed carbonation to have a more pronounced demobilising effect on critical components in bottom ashes than in APC residue and fly ash. From grate type incinerator bottom ash, the release of Cr decreased by 97%, by 63% for Cu and by 45% for Sb. In the investigated APC residue, the releases of Cr, Se and Pb were defined as critical, although they either remained unaffected or increased after carbonation. Cl(-) and SO(4)(2-) remained mobile after carbonation in all investigated residues.