Heavy metal leaching from aerobic and anaerobic landfill bioreactors of co-disposed municipal solid waste incineration bottom ash and shredded low-organic residues

In this study, heavy metal leaching from aerobic and anaerobic landfill bioreactor test cells for co-disposed municipal solid waste incineration (MSWI) bottom ash and shredded low-organic residues has been investigated. Test cells were operated for 1 year. Heavy metals which were comparatively higher in leachate of aerobic cell were copper (Cu), lead (Pb), boron (B), zinc (Zn), manganese (Mn) and iron (Fe), and those apparently lower were aluminum (Al), arsenic (As), molybdenum (Mo), and vanadium (V). However, no significant release of heavy metals under aerobic conditions was observed compared to anaerobic and control cells. Furthermore, there was no meaningful correlation between oxidation-reduction potential (ORP) and heavy metal concentrations in the leachates although some researchers speculate that aeration may result in excessive heavy metal leaching. No meaningful correlation between dissolved organic carbon (DOC) and leaching of Cu and Pb was another interesting observation. The only heavy metal that exceeded the state discharge limits (10mg/l, to be enforced after April 2005) in the aerobic cell leachate samples was boron and there was no correlation between boron leaching and ORP. Higher B levels in aerobic cell should be due to comparatively lower pH values in this cell. However, it is anticipated that this slightly increased concentrations of B (maximum 25mg/l) will not create a risk for bioreactor operation; rather it should be beneficial for long-term stability of the landfill through faster washout. It was concluded that aerobization of landfills of heavy metal rich MSWI bottom ash and shredded residues is possible with no dramatic increase in heavy metals in the leachate.     

A study on the chemical and mineralogical characterization of MSWI fly ash using a sequential extraction procedure

The presence of heavy metals in municipal solid waste incineration (MSWI) fly ash is of environmental concern due to their leaching potential in landfill environments. Sequential chemical extraction was performed on fly ash samples from a large-scale municipal solid waste incineration plant in East China. The transformation of the mineralogical species of fly ash during the sequential extraction was studied using X-ray fluorescence (XRF) and X-ray powder diffraction (XRD). The leaching behavior of heavy metals such as zinc, lead, cadmium and copper in MSWI fly ash was considered to have a dependency relationship with the components of calcium, such as aphthitalite, calcite, anhydrite and calcium aluminate or calcium aluminosilicate.     

In situ heavy metal attenuation in landfills under methanogenic conditions

The purpose of this research was to determine the fate and behavior of heavy metals co-disposed with municipal waste under methanogenic conditions. Two landfill simulating reactors, one with leachate recirculation and the other without, were operated in a constant room temperature at 32 degrees C. These reactors were filled with shredded and compacted municipal solid waste having a typical solid waste composition of Istanbul region. After the onset of the methanogenic conditions, the selected heavy metals including iron, copper, nickel, cadmium and zinc were added according to the amounts suggested for co-disposal under the directives of the Turkish Hazardous Waste Control Regulations. The results of the experiments indicated that about 90% of all heavy metals were precipitated from the reactors within the first 10 days due to the establishment of highly reducing environment and the formation of sulfide from sulfate reduction which provided heavy metal precipitation. No inhibition to the biological stabilization was observed.     

Leaching characteristics of slag from the melting treatment of municipal solid waste incinerator ash

This study investigated the composition and leaching characteristics of municipal solid waste incinerator (MSWI) ash and slag. The modified slags were characterized after the melting of MSWI ash mixtures at 1400 degrees C for 30 min. The ash mixtures were composed of different types of MSWI ash, including cyclone ash, scrubber ash and bottom ash, in various proportions. The results indicate that the Cd leaching concentration of the cyclone ash and the Pb leaching concentration of the scrubber ash reached 1.82 and 8.7 mg/L, respectively, which exceeds the ROC EPA's current regulatory thresholds, and can thus be classified as hazardous. The results of the analysis of the metal content and the leaching behavior of heavy metals, showed high concentrations of Cu and Zn, but a low leaching ratio of these metals. Concerning the characteristics of the modified slags, the X-ray diffraction patterns of the MSWI fly ash slag showed that it contained large amounts of glass. The toxicity characteristic leaching procedure (TCLP) leaching concentrations of the target metals of all the slags, met the ROC EPA's regulatory thresholds. The leaching concentrations of heavy metals in the F- and B1-slag were lower than those in the cyclone and the bottom ash, because there was a high amount of SiO(2), which formed a net-like structure in the bottom ash.     

Temperature development in a modern municipal solid waste incineration (MSWI) bottom ash landfill with regard to sustainable waste management

Municipal solid waste is treated in incineration plants to reduce the volume, the toxicity and the reactivity of the waste. The final product, municipal solid waste incineration (MSWI) bottom ash, was considered as a material with a low reactivity, which can safely be deposited in a MSWI bottom ash landfill, or which can be used, e.g. in road construction after further treatment. However, temperature measurements in MSWI bottom ash landfills showed temperatures up to 90 degrees C, caused by exothermic reactions within the landfill. Such high temperatures may affect the stability of the flexible polymer membrane liner (FML) and may also lead to an accelerated desiccation of the clay barrier. At the beginning of this study it was uncertain whether those reported results would be applicable to modern landfills, because the treatment techniques in MSWI and landfills have changed, bottom and fly ash are stored separately, and the composition of the incinerated waste has changed significantly since the publication of those results.The aim of this study was to gain detailed knowledge of temperature development under standard disposal conditions in relation to the rate of ash disposal, the variation of layer thickness, and the environmental conditions in a modern landfill.Temperatures were measured at nine levels within the body of a landfill for a period of nearly 3 years. Within 7 months of the start of the disposal, a temperature increase of up to 70 degrees C within the vertical centre of the disposal was observed. In the upper and central part of the landfill this initial temperature increase was succeeded by a decrease in temperature. The maximum temperature at the time of writing (May 2000) is about 55 degrees C in the central part of the landfill. The maximum temperature (45.9 degrees C) at the FML was reached 17 months after the start of the deposition. Since then the temperatures decreased at a rate of 0.6 degrees C per month.Temperature variation within each individual layer corresponds to the temperature of the underlying layer and the overall surface-to-volume ratio of the landfill. The temperatures in the uppermost layer are significantly influenced by the ambient temperatures.     

生物质与生活垃圾共气化过程重金属的迁移转化规律

为了研究流化床生物质气化协同处置生活垃圾衍生燃料过程中重金属的迁移转化规律,在湖北某循环流化床气化炉耦合燃煤发电厂进行掺烧试验。研究表明重金属主要赋存于飞灰和炉渣中。空白工况下86.2%的Cr赋存于飞灰中,13.3%于炉渣中;75.5%的Pb在飞灰中,23.8%在炉渣中;79.5%的As迁移至飞灰中,11.7%在炉渣中。RDF工况下75.8%的Cr迁移至飞灰,20.7%迁移至炉渣;44.6%的Pb存在于飞灰中,52%存在于炉渣。协同处置后,重金属在飞灰和炉渣中的分配比例明显发生了变化,飞灰中重金属含量减少12.5%~31.3%,炉渣中重金属增加7.33%~20.1%,气化气的引入改变了炉内重金属的分配情况。协同处置可以有效处理生活垃圾,对固废中的热量进行资源化利用,且出炉物料中重金属含量均低于我国现行标准限值。     

The metal-leaching and acid-neutralizing capacity of MSW incinerator ash co-disposed with MSW in landfill sites

Municipal solid waste (MSW) incinerator (MSWI) bottom ash and fly ash were used as landfill cover or were co-disposed with MSW to measure their potential metal-releasing and acid-neutralizing capacity (ANC) in landfill sites. Five lysimeters (height 1.2m, diameter 0.2m), simulating landfill conditions, were used in the experiment. Four contained either bottom ash (BA) or fly ash (FA) with BA:MSW ratios of 100 and 200 g L(-1) and FA:MSW ratios of 10 and 20 g L(-1), and the fifth was the control, which contained no ash. The lysimeters were arranged so as to contain four layers, with BA or FA placed on top of MSW within each layer. Each lysimeter was recirculated with 100mL leachate using peristaltic pumps, and 100mL of the leachate was collected weekly to measure the soluble metal concentrations. The results showed that the concentrations of soluble alkali metals measured in the leachate were in the order Ca>K>Na>Mg. In addition, the concentrations of soluble alkali metals of Ca and K collected from the lysimeters containing FA were found to be higher than the concentrations from the lysimeters containing BA. The concentrations of heavy metals (Cd, Cr, Cu, Ni, and Zn) were found to be <1 mg L(-1) except for Pb, which reached 2 mg L(-1). These results suggest that for alkali metals there might be an ANC consistent with the results of an acid titration curve, which would provide suitable conditions for anaerobic digestion of the MSW in the landfill. Furthermore, heavy metals and trace metals were found in concentrations, which were too low to exert inhibitory effects on anaerobic digestion, and thus they could serve as micronutrients to exert beneficial rather than detrimental effects on landfill biostabilization.     

Numerical modelling of the generation and transport of heat in a bottom ash monofill

Municipal solid waste is incinerated to reduce its volume, toxicity and reactivity. Several studies have shown that the resulting bottom ash has a high exothermic capacity. Temperature measurements in municipal solid waste incineration (MSWI) bottom ash landfills have found temperatures up to 90 °C. Such high temperatures may affect the stability of the landfill’s flexible polymer membrane liner (FML) and may also lead to an accelerated desiccation of the clay barrier. The purpose of this study was to gain detailed knowledge of temperature development under several disposal conditions in relation to the rate of ash disposal, the variation of layer thickness, and the environmental conditions in a modern landfill. Based on this knowledge, a simulation was developed to predict temperature development. Temperature development was simulated using several storage periods prior to the deposition and several modes of emplacement. Both the storage time and the mode of emplacement have a significant influence on the temperature development at the sensitive base of the landfill. Without a preliminary storage of the fresh quenched bottom ash, high temperatures at the bottom of a landfill cannot be avoided.     

Pb stabilization in fresh fly ash from municipal solid waste incinerator using accelerated carbonation technology

Carbonation technology with CO(2) absorption was used to enhance the stabilization of heavy metals in fresh fly ash from a municipal solid waste incinerator (MSWI). The influence of fundamental parameters affecting the stabilization of heavy metals, especially Pb and diffusivity and reactivity of CO(2), was evaluated. The results indicated that the addition of 10% or more of water could remarkably accelerate the absorption of CO(2) and could also accelerate the stabilization of MSWI fly ash. The stabilization of MSWI fly ash is not distinct within 1d in the air atmosphere for low content of CO(2) (0.03%). The result of the XRD analysis indicated that CO(2) could combine with Ca(OH)(2) to form CaCO(3) and CO(2) could also combine with heavy metal oxide to form heavy metal carbonate in the adsorption of CO(2). The TGA analysis showed that MSWI fly ash has the sequestration capability of 3% (w/w) CO(2). The sequestration of CO(2) has a large impact on Pb, and the exchangeable Pb can be converted into carbonated form in rich CO(2) condition to be stabilized.     

Determination of solid waste sorption capacity for selected heavy metals in landfills

The adsorption process is largely a surface-action phenomenon. In this study, sorption capacities for heavy metals on a solid waste matrix were investigated. Five heavy metals (iron, copper, zinc, nickel and cadmium) were chosen because of their availability in any landfill site. The conditions during all the experimental runs were pH 7.0, temperature 32 degrees C and suppressed microbial degradation. For adsorption isotherm (Freundlich and Langmuir) calculations, fixed quantities of heavy metal ions were mixed with variable quantities of solid waste. The ratio of mass of adsorbate per unit mass of adsorbent was changed five times, by changing only the adsorbent amount. The results showed that the time required to reach equilibrium varied from metal to metal but all reached equilibrium within the first 32 h. The relative potential of sorption of the individual metals and mixed metals on the solid waste matrix is Fe > Zn > Cu > Ni > Cd. The sorption capacity of domestic solid waste matrix for heavy metals is quite significant and this property might prove helpful for the in situ removal of heavy metals in landfill operation.     

Comparison of the characteristics of bottom and fly ashes generated from various incineration processes

This study analyzed and compared the characteristics of bottom and fly ashes from three municipal solid waste incinerators (MSWIs) in Taiwan. Different incineration furnaces were investigated, including: (1) fluidized bed, (2) mass-burning, and (3) mass-burning linked rotary kiln. The particle size distribution, morphology, mineralogical and chemical composition, and leaching behavior of heavy metals of ash samples were evaluated. The results revealed that three types of incineration processes have different characteristic for ashes due to transportation and mixing system inside furnace. Particle size distribution indicated that 28.5% of MSWI-B bottom ash has lower than 180 microm and 61.2% of MSWI-A fly ash has larger than the 250 microm. The leaching concentration of Pb exceeded the regulatory level set by the Taiwan EPA in fly ashes from MSWI-B and MSWI-C, and thus must be considered hazardous wastes. Specifically, the leaching concentration of heavy metals of fly ashes from MSWI-A (fluidized bed incinerator) was lower than that of the others, and was corresponded to the regulatory levels. Therefore, a fluidized bed incineration process appears a potential of handling heavy metals for ashes. The result was also provided the valuable information for incinerator design and operation.     

Characterisation and management of incinerator wastes

Management of municipal and hospital wastes by means of incineration processes generates solid residues, such as bottom and fly ashes and air pollution control residues with high content of heavy metals, inorganic salts and other organic compounds. Characterisation of 24 ash samples, collected from four municipal solid waste incinerators (MSWI) and six hospital medical waste incinerators (HMWI) located in the Basque Country Region (Northern Spain), were carried out at the request of Spanish Regulations and European Economic Community guidelines.The ecotoxicity values, EC(50), of the TCLP leachates show a high variability ranging from 12,967 to 1,000,000mgl(-1) in MSWI samples and from 2917 to 333,150mgl(-1) in HMWI samples. Results from chemical characterisation of DIN 38414-S4 leachates show a high concentration of lead, sulphate and chloride in MSWI samples and chromium in HMWI samples.     

Melting of municipal solid waste incinerator fly ash by waste-derived thermite reaction

This work describes a novel approach for melting municipal solid waste incinerator (MSWI) fly ash, based on self-propagating reactions, by using energy-efficient simulated waste-derived thermite. The self-propagating characteristics, the properties of the recycled alloy and slag and the partitioning of heavy metals during the process are also studied. Experimental results demonstrate that the mix ratio of fly ash to the starting mixture of less than 30% supports the development of the self-propagating reaction with a melting temperature of 1350-2200 degrees C. Furthermore, metallic iron (or alloy) and the slag were retrieved after activation of the thermite reactions among the starting mixtures. It was noted that more than 91wt.% of iron was retrieved as alloy and the rest of non-reductive oxides as slag. During the thermite reactions, the partition of heavy metals to the SFA and flue gas varied with the characteristics of the target metals: Cd was mainly partitioned to flue gas (75-82%), and partition slightly increased with the increasing fly ash ratio; Pb and Zn, were mainly partitioned to the SFA, and the partition increased with increasing fly ash ratio; Cu was partitioned to the SFA (18-31%) and was not found in the flue gas; and moreover stable Cr and Ni were not identified in both the SFA and flue gas. On the other hand, the determined TCLP leaching concentrations were all well within the current regulatory thresholds, despite the various FA ratios. This suggests that the vitrified fly ash samples were environmental safe in heavy metal leaching. The results of this study suggested that melting of municipal solid waste incinerator fly ash by waste-derived thermite reactions was a feasible approach not only energy-beneficial but also environmental-safe.     

Influence of flue gas SO2 on the toxicity of heavy metals in municipal solid waste incinerator fly ash after accelerated carbonation stabilization

The influence of CO₂ content and SO₂ presence on the leaching toxicity of heavy metals in municipal solid waste incinerator (MSWI) fly ash was studied by examining the carbonation reaction of MSWI fly ash with different combinations of simulated incineration flue gases. Compared with raw ash, the leaching solution pH of carbonated ash decreased by almost 1 unit and the leaching concentrations of heavy metals were generally lower, with that of Pb decreasing from 19.45 mg/L (raw ash) to 4.08 mg/L (1# carbonated ash). The presence of SO₂ in the incineration flue gas increased the leaching concentrations of heavy metals from the fly ash to different extents after the carbonation stabilization reaction. The pH of the leaching solution was the main factor influencing the leaching concentrations of heavy metals. The increase in buffer capacity with the pH of carbonated ash caused an increase in heavy metal stability after the carbonation reaction. Accelerated carbonation stabilization of MSWI fly ash could reduce its long-term leaching concentrations (toxicity) of Cu, Pb, Se, and Zn. The leaching concentrations of heavy metals from carbonated ash also likely had better long-term stability than those from raw ash. The presence of SO₂ in the incineration flue gas increased the proportion of exchangeable state species of heavy metals; slightly increased the long-term leaching toxicity of Cu, Pb, Se, and Zn; and reduced the long-term stability of these metals in the fly ash after the carbonation reaction.     

Heavy metal stabilization in municipal solid waste incineration flyash using heavy metal chelating agents

Heavy metal chemical stabilization with synthesized heavy metal chelating agent was assessed for flyash from municipal solid waste incinerator. Flyash can contain heavy metals (e.g. Pb, Cd) which can leach. A new kind of heavy metal chelating agent showed more attractive competition than inorganic chemicals in stabilizing flyash. The synthesizing method of this kind of heavy metal chelating agent was explained in this paper, and the technology process and treatment efficiency of the chelating agent in treating flyash were experimentally studied, which was compared with the results of inorganic chemical agents such as sodium sulfide and lime. The heavy metals in flyash were stabilized more effectively by using heavy metal chelating agents than by using sodium sulfide and lime, furthermore, the stabilized products using the chelating agents can meet the landfill disposal controlling standards for heavy metal waste. pH-dependent leaching experiment showed the stabilized flyash by treatment with heavy metal chelating agent could keep long-term stabilization within a broad range of pH value. Thus, the risk of secondary pollution for the stabilized products was reduced dramatically when the environment condition changes during its disposal period.     

Utilization of MSWI fly ash for stabilization/solidification of industrial waste sludge

This work investigated the potential for utilization of MSWI incineration fly ash as solidification binder to treat heavy metals-bearing industrial waste sludge. In the study, Municipal Solid Waste Incineration (MSWI) fly ash was used along with ordinary Portland cement to immobilize three different types of industrial sludge while MSWI incineration fly ash was stabilized at the same time. The results showed that the matrixes with heavy metals-bearing sludge and MSWI fly ash have a strong fixing capacity for heavy metals: Zn, Pb, Cu, Ni and Mn. Specimens with only 5-15% cement content was observed to be sufficient to achieve the target compressive strength of 0.3 MPa required for landfill disposal. An optimum mix comprising 45% fly ash, 5% cement and 50% of the industrial sludge could provide the required solidification and stabilization. Addition of MSWI can improve the strength of matrix. Meanwhile, the main hydration products of new S/S matrix are ettringite AFt, Friedel's salt and C-S-H. These hydration products play an important role in the fixing of heavy metals. The co-disposal of MSWI fly ash with heavy metals-bearing sludge can minimize the enlargement of the landfill volume and stabilize the heavy metals effectively.     

Fate of heavy metals during municipal solid waste incineration in Shanghai

The transfer behavior of heavy metals during municipal solid waste (MSW) incineration was investigated based on 2-year field measurements in two large-scale incinerators in Shanghai. Great temporal and spatial diversification was observed. Most of Hg and Cd were evaporated and then removed by air pollution control (APC) system through condensation and adsorption processes, thus being enriched in the fine APC residues particles. Cr, Cu, and Ni were transferred into the APC residues mainly by entrainment, and distributed uniformly in the two residues flows, as well as in the ash particles with different sizes. Pb and Zn in the APC residues were from both entrainment and evaporation, resulting in the higher concentrations (two to four times) compared with the bottom ash. Arsenic was transported into the flue gas mainly by evaporation, however, its transfer coefficient was lower. Though the heavy metals contents in the APC residues were higher than that in bottom ash, more than 80% of As, Cr, Cu, and Ni, 74-94% of Zn, as well as 46-79% of Pb remained in the bottom ash, due to its high mass ratio (85-93%) in the residues. While 47-73% of Cd and 60-100% of Hg were transferred into the APC residues, respectively.     

Evaluation of assisting agents for electrodialytic removal of Cd,Pb, Zn,Cu and Cr from MSWI fly ash

Different assisting agents (0.25 M ammonium citrate/1.25% NH(3), 0.25 M Na-citrate, 2.5% NH(3), DI water) have been used for aiding the removal of heavy metals during electrodialytic treatment of municipal solid waste incineration (MSWI) fly ash. In this study, the effectiveness of the different agents was evaluated. The heavy metal speciation in solution was discussed and simulated at the different conditions using the geochemical equilibrium model Visual MINTEQ. The heavy metals examined were Cd, Pb, Zn, Cu and Cr.The 2.5% NH(3) solution was the best assisting agent for removal of Cd, probably due to formation of stable tetraammine complexes; whereas the best Pb removal was obtained with 0.25 M Na-citrate (Pb forms very stable chelates with citrate). The best compromise for removal of all five metals was obtained with the 0.25 M ammonium citrate/1.25% NH(3) solution.     

Electrodialytic removal of heavy metals from municipal solid waste incineration fly ash using ammonium citrate as assisting agent

Electrodialytic remediation, an electrochemically assisted separation method, has previously shown potential for removal of heavy metals from municipal solid waste incineration (MSWI) fly ashes. In this work electrodialytic remediation of MSWI fly ash using ammonium citrate as assisting agent was studied, and the results were compared with traditional batch extraction experiments. The application of electric current was found to increase the heavy metal release significantly compared to batch extraction experiments at comparable conditions (same liquid-to-solid ratio, same assisting agent, and same extraction time). Up to 86% Cd, 20% Pb, 62% Zn, 81% Cu and 44% Cr was removed from 75 g of MSWI fly ash in electrodialytic remediation experiments using ammonium citrate as assisting agent. The time range for the experiments varied between 5 and 70 days.     

Effect of short-term natural weathering on MSWI and wood waste bottom ash leaching behaviour

Short term natural weathering was applied on municipal solid waste (MSW) and wood waste incinerator bottom ash (BA). The materials were analysed at different steps of treatment and characterized for chemical and mineralogical composition. Both short and long term leaching behaviour of main elements and heavy metals were investigated as well. Lead, zinc and copper were the main heavy metals to be released. After 12 weeks of treatment the concentration of leached zinc decreased. Lead concentration was not found to be influenced by pH and decreased only for the biomass samples. Weathering did not have beneficial effects on copper leaching, which was well described by complexation processes with DOC. The findings from the experimental campaign indicated that weathering reactions improved the mineral stability of the analysed materials but, in contrast with previous works, the treatment was not sufficient to guarantee pH stability and to comply with leaching law limits.