Vitrification of fly ash from municipal solid waste incinerator

Fly ash from municipal solid wastes (MSW) incinerators in Korea contains a large amount of toxic materials and requires pertinent treatments. However, since fly ash in Korea has a high chlorine concentration, it is difficult to apply cementation and chemical treatment techniques. In this study, we report the vitrification of fly ash along with the properties of the glasses and leaching characteristics of heavy metal ions.Fly ash can be vitrified by melting at 1500 degrees C for 30 min with the addition of >5 wt.% of SiO2. Glasses showed Vickers hardness of 4000-5000 MPa, bending strength of 60-90 MPa and indentation fracture toughness of approximately 0.9 MPa m(1/2). Glasses also showed the excellent resistance against leaching of heavy metal ions with Cd2+ <0.04 ppm, Cr3+ <0.02 ppm, Cu2+ <0.04 ppm and Pb2+ <0.2 ppm. These results indicate that the vitrification technique is effective for the stabilization and recycling of toxic incinerator fly ash.     

Polychlorinated dibenzo-p-dioxins/dibenzofuran mass distribution in both start-up and normal condition in the whole municipal solid waste incinerator

Although many researches focused on the polychlorinated dibenzo-p-dioxins/dibenzofuran (PCDD/F) emissions from stack, in the bottom ash and in the surrounding environment, researches focused on PCDD/F mass distributions in the whole incineration plant have seldom been addressed. This study determined PCDD/F emissions in the whole plant. A high-resolution gas chromatograph/high-resolution mass spectrometer was utilized for analyzing 17 PCDD/F species. Experimental results displayed that PCDD/Fs were formed during fly ash from super heater (SH), economizer (EC), semi-dryer absorber (SDA) and fabric filter (FF) was transferred to fly ash pit. Mass distribution ratios of PCDD/Fs in g I-TEQ (Toxicity Equivalency Quantity) per week from stack, SH, EC, SDA, FF, generation and bottom residue (BR) in start-up operations were 14.6%, 0.1%, 8.3%, 1.0%, 41.7%, 33.4% and 0.9%, respectively. Above results indicated that main PCDD/F source in the MSWI was from fly ash. However, the fly ash is easily controlled and PCDD/F emitted from stack flue gases will be difficult to be handled. Therefore, we should pay more attention on PCDD/F emission from flue gases especially from start-up procedure. Besides, fly ash should be controlled by sodium hypophosphite before being landfilled. MSWI did require further detoxification treatments for the solid residues and flue gases.     

Thermal treatment and vitrification of boiler ash from a municipal solid waste incinerator

Boiler ash generated from municipal solid waste (MSW) incinerators is usually classified as hazardous materials and requires special disposal. In the present study, the boiler ash was characterized for the chemical compositions, morphology and microstructure. The thermal chemical behavior during ash heating was investigated with thermal balance. Vitrification of the ash was conducted at a temperature of 1400 degrees C in order to generate a stable silicate slag, and the formed slag was examined with chemical and mineralogical analyses. The effect of vitrification on the leaching characteristics of various elements in the ash was evaluated with acid leaching. The study shows that the boiler ash as a heterogeneous fine powder contains mainly silicate, carbonate, sulfates, chlorides, and residues of organic materials and heavy metal compounds. At elevated temperatures, the boiler ash goes through the initial moisture removal, volatilization, decomposition, sintering, melting, and slag formation. At 1400 degrees C a thin layer of salt melt and a homogeneous glassy slag was formed. The experimental results indicate that leaching values of the vitrified slag are significantly reduced compared to the original boiler ash, and the vitrification could be an interesting alternative for a safer disposal of the boiler ash. Ash compacting, e.g., pelletizing can reduce volatilization and weight loss by about 50%, and would be a good option for the feed preparation before vitrification.     

Extraction of metals from municipal solid waste incinerator fly ash by hydrothermal process

This work examined the extraction properties of metallic elements from municipal incinerator fly ash under hydrothermal conditions. The ash was firstly pre-washed by distilled water, then subjected to hydrothermal treatments. The pre-washing process was effective for Na, K, Ca extraction with extraction percentages of 67%, 76% and 48%, respectively. The optimum contact time was 30 min for the pre-washing process. Five types of acids were tested for the extraction experiments and hydrochloric acid was found to be most effective for metal extraction from the ash. Compared to room condition, hydrothermal treatment accelerated the dissolution of the ash, thus promoted the reaction of acid with hazardous metals such as Cr, Cd, Pb, and furthermore, the consumption speed of acid was slowed down under hydrothermal condition. The acid simultaneously reacted with all the metal in the ash under hydrothermal condition but preferentially reacted with Ca at room condition. The optimum hydrothermal treatment temperature, time and liquid/solid ratio were 150 degrees C, 5h and 10:1 (ml:g), respectively.     

Dose-mortality assessment on municipal solid waste incinerator (MSWI) ash

This study provides a novel attempt to put forward, in general toxicological terms, quantitative series of toxicity of various ashes of municipal solid waste incinerator (MSWI) for reusability in various applications. Previous study disclosed that growth inhibition of Escherichia coli DH5alpha occurred at concentrations above 0.156, 0.625 and 0.0195 gL(-1) for bottom ash (BA), cyclone ash (CA), scrubber ash (SA), respectively, suggesting the toxicity series of SA>BA>CA. However, the severity of such a toxicity series was not clearly revealed, thus whether ashes were still feasible for reuse in further applications was still remained uncertain. Compared to NaNO3, CrCl2 and CdCl2, the existing toxicities of ashes were apparently significant even these ashes were all satisfied by the TCLP guidelines for EPA regulations. Dose-response analysis based upon loss of cell viability (e.g., EC50) stated a toxicity series of SA>CrCl2>BA>CdCl2>CA>NaNO3. The ranking of Hill slope B in BA>SA>CA>NaNO3>CrCl2>CdCl2 clearly suggested the smallest tolerance (e.g., ranges from EC20 to EC50) for ashes very likely due to synergistic toxicity of multiple species present in ashes. The findings showed that toxicity attenuation of ashes should be the first-ranking task prior to practical reuse and recycle in applications.     

Monobromopolychlorodibenzo-p-dioxins and dibenzofurans in municipal waste incinerator flyash.

Capillary column gas chromatography/high-resolution mass spectrometry was used in two different selected-ion monitoring modes to analyze trace levels of monobromopolychlorodibenzo-p-dioxins and dibenzofurans (BPCDD/Fs) in municipal waste incinerator (MWI) flyash. The mass profile monitoring mode is well-suited for identification of unknown compounds in uncharacterized matrices because it has superior diagnostic capability. Owing to its high sensitivity, conventional peak top monitoring was used to quantify, on the basis of polychlorodibenzo-p-dioxin and dibenzofuran (PCDD/F) standards, the BPCDD/Fs in the sample. The results were compared with those obtained by using two commercial BPCDDs as standards, and the latter results are 4 times greater, indicating the need for appropriate standards. The high certainty and sensitivity obtained from these two mass spectrometric techniques combined with the resolving power of capillary gas chromatography enabled us to compare for the first time the isomer distribution patterns between BPCDD/Fs and their PCDD/F analogues at a high confidence level. The comparison indicates BPCDD/Fs and PCDD/Fs found in MWI flyash are closely related and that many BPCDD/Fs with a 2,3,7,8-substitution configuration may be present in MWI flyash.     

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.     

Feasibility study of using brick made from municipal solid waste incinerator fly ash slag

This study deals with the effect of MSWI slag on fired clay bricks. Brick samples were heated to temperatures which varied from 800 to 1,000 degrees C for 6h, with a heating rate of 10 degrees C/min. The material properties of the resultant material then determined, including speciation variation, loss on ignition, shrinkage, bulk density, 24-h absorption rate and compressive strength. Toxicity Characteristic Leaching Procedure tests were also conducted. The results indicate that the heavy metal concentrations in the leachates met the current regulatory thresholds. Increasing the amount of MSWI slag resulted in a decrease in the water absorption rate and an increase in the compressive strength of the MSWI-slag bricks. The 24-h absorption rate and compressive strength of the MSWI-slag brick made from samples containing slag sintered at 1,000 degrees C all met the Chinese National Standard (CNS) building requirements for second-class brick. The addition of MSWI slag to the mixture reduced the degree of firing shrinkage. This indicates that MSWI slag is indeed suitable for the partial replacement of clay in bricks.     

Bioremediation of PCDD/Fs-contaminated municipal solid waste incinerator fly ash by a potent microbial biocatalyst

Removal of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) from fly ash poses a serious problem. In the study presented here, we used a microbial biocatalyst which is a mixture of 4 bacterial and 5 fungal dioxin-degrading strains. The ability of this biocatalyst to bioremediate PCDD/Fs from contaminated municipal solid waste incinerator (MSWI) fly ash was examined by solid-state fermentation under laboratory conditions. Treatment of MSWI fly ash with the microbial biocatalyst for 21 days resulted in a 68.7% reduction in total toxic PCDD/Fs. Further analyses revealed that the microbial biocatalyst also removed 66.8% of the 2,3,7,8-substituted congeners from the fly ash. During the treatment period, the presence of the individual strains composing the microbial biocatalyst was monitored by the amplification of strain-specific DNA sequences followed by denaturing gradient gel electrophoresis (DGGE). This analysis showed that all of the bacterial and fungal strains composing this dioxin-degrading microbial mixture maintained under the dioxin treatment conditions. These results demonstrate that this microbial biocatalyst could potentially be used in the bioremediation of PCDD/Fs from contaminated fly ash.     

Thermal treatment of the fly ash from municipal solid waste incinerator with rotary kiln

Reuse of the fly ash from the municipal solid waste incinerator (MSWI) is a policy of Taiwan EPA. However, the fly ash is often classified as a hazardous waste and cannot be reused directly because the concentrations of heavy metals exceed the TCLP regulations. The main objective of this study is to investigate the continuous sintering behavior of fly ash with a rotary kiln and seek a solution to reduce the concentrations of heavy metal to an acceptable value. The partitions of the heavy metals in the process are also considered. The results of TCLP showed that among the metals of Cr, Cd, Cu and Pb, only the concentrations of Pb in raw fly ash exceeded the regulation. At sintering temperatures of 700, 800 and 900 degrees C, the concentration of Pb decreased in sintering products, however, the concentration of Pb still exceeded the limitation at 700 and 800 degrees C. Additionally, the water-washing was used to pre-treat the fly ash before sintering process. The washing treatment effectively reduced the leaching concentrations of Pb to agree the regulations. Therefore, water-washing followed by a sintering treatment is an available process for detoxifying the fly ash of MSWI.     

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.     

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.     

Carbon dioxide sequestration in municipal solid waste incinerator (MSWI) bottom ash

During bottom ash weathering, carbonation under atmospheric conditions induces physico-chemical evolutions leading to the pacification of the material. Fresh bottom ash samples were subjected to an accelerated carbonation using pure CO2. The aim of this work was to quantify the volume of CO2 that could be sequestrated with a view to reduce greenhouse gas emissions and investigate the possibility of upgrading some specific properties of the material with accelerated carbonation. Carbonation was performed by putting 4mm-sieved samples in a CO2 chamber. The CO2 pressure and the humidity of the samples were varied to optimize the reaction parameters. Unsieved material was also tested. Calcite formation resulting from accelerated carbonation was investigated by thermogravimetry and differential scanning calorimetry (TG/DSC) and metal leaching tests were performed. The volume of sequestrated CO2 was on average 12.5L/kg dry matter (DM) for unsieved material and 24 L/kg DM for 4mm-sieved samples. An ash humidity of 15% appeared to give the best results. The reaction was drastically accelerated at high pressure but it did not increase the volume of sequestrated CO2. Accelerated carbonation, like the natural phenomenon, reduces the dangerous nature of the material. It decreases the pH from 11.8 to 8.2 and causes Pb, Cr and Cd leaching to decrease. This process could reduce incinerator CO2 emissions by 0.5-1%.     

Biotoxicity assessment on reusability of municipal solid waste incinerator (MSWI) ash

This study provides a first attempt of dose-response analysis and margin of safety using Escherichia coli DH5alpha, Bacillus subtilis as indicator microorganisms to put forward, in general terms and explanations, the toxicity rankings of various ashes of municipal solid waste incinerator (MSWI) for feasibility in further applications. Since the MSWI ash often contains cations of Si, Ca, Al and Fe, it is frequently considered to be recycled for construction building-materials. Growth inhibition of E. coli DH5alpha occurred at concentrations over 0.156, 0.625 and 0.0195 g/L for bottom ash (BA), cyclone ash (CA), scrubber ash (SA), respectively, suggesting the toxicity ranking of SA>BA>CA. In contrast, except for SA (ca. 0.313 g/L), almost same inhibitory levels of ashes to cell growth were also observed in Bacillus subtilis. Evidently, biotoxicity responses were strongly dependent upon the characteristics of indicator microorganism. Based on DH5alpha, the margins of safety (MOS) were thus 0.195, 1.56 and 6.25 mg/L for SA, BA and CA, respectively. Nearly identical levels of MOS were also suggested by B. subtilis, except for SA (3.13 mg/L). Although MSWI residual ashes qualified EPA's standard test of Toxicity Characteristic Leaching Procedure (TCLP), they might still contain other toxic residues (e.g., chloride ions and/or anions) to cause existing toxicity as indicated in this toxicity study.     

Understanding biotoxicity for reusability of municipal solid waste incinerator (MSWI) ash

This feasibility study using Escherichia coli DH5alpha as a reporter microorganism tended to disclose toxicity ranking of various ashes of municipal solid waste incinerator (MSWI) in comparison with typical toxic chemicals for reusability in further applications. Previous study indicated that growth inhibition to bacterial cells occurred at concentrations above 0.156, 0.625 and 0.0195g/L for bottom ash (BA), cyclone ash (CA), scrubber ash (SA), respectively, suggesting the toxicity ranking of SA>BA>CA. This follow-up study clearly stated that compared to cadmium(II) and chromium(II) SA seemed to be the most toxic species to DH5alpha. Large amounts of supplemented lime (CaO) were used for neutralization of acid gas in incinerator, SA was thus contained high-levels of sulfate, chloride and nitrate salts. Therefore, compared to other ashes a marked increase in toxicity was observed in SA. Regarding soluble cations and anions in ashes, nitrite ion seemed to stimulate instead of repress cell growth. In contrast, nitrate ion showed so-called "sufficient challenge" characteristics for growth enhancement and inhibition at low and high concentration, respectively. Low solubility of metallic ions (e.g., Pb(II) and Cu(II)) in ashes likely resulted in low mobility in the environment and low risk to humans. The findings showed that toxicity attenuation of SA will be inevitably required as SA is even more toxic than Cr(II) and Cd(II).     

The hydration properties of pastes containing municipal solid waste incinerator fly ash slag

This study investigated the hydration properties of Type I, Type III and Type V cements, mixed with municipal solid waste incinerator fly ash, to produce slag-blended cement pastes. The setting time of slag-blended cement pastes that contained 40% slag showed significantly retardation the setting time compared to those with a 10% or even a 20% slag replacement. The compressive strength of slag-blended cement paste samples containing 10 and 20% of slag, varied from 95 to 110% that developed by the plain cement pastes at later stages. An increased blend ratio, due to the filling of pores by C-S-H formed during pozzolanic reaction tended to become more pronounced with time. This resulting densification and enhanced later strength was caused by the shifting of the gel pores. It was found that the degree of hydration was slow in early stages, but it increased with increasing curing time. The results indicated that it is feasible to use MSWI fly ash slag to replace up to 20% of the material with three types of ordinary Portland cement.     

Atmospheric dry deposition of polychlorinated dibenzo-p-dioxins and dibenzofurans in the vicinity of municipal solid waste incinerators

This study focuses on the atmospheric dry deposition flux of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the vicinity of the two municipal solid waste incinerators (MSWIs) located in southern Taiwan. PCDD/Fs in ambient air were taken and analyzed for seventeen 2,3,7,8-substituted PCDD/Fs during November 2004 and July 2005. Results show that the mean concentrations of PCDD/Fs in the ambient air near MSWI-GS and MSWI-RW were 0.090 and 0.097pg I-TEQ/Nm(3), respectively. Dry deposition fluxes of total PCDD/Fs were 18.0 and 23.5pg I-TEQ/(m(2)d) in the ambient air near MSWI-GS and MSWI-RW, respectively, which were considerably higher than that measured in Guangzhou, China. Annual dry deposition fluxes of total PCDD/Fs in the ambient air near MSWI-GS and MSWI-RW were 189 and 217ng/(m(2)year), respectively, which were also much higher than dry deposition of total PCDD/Fs to the Atlantic Ocean. The results of the present study strongly suggest that exposure to PCDD/Fs in this area should be reduced. In addition, parametric sensitivity shows that dry deposition flux of PCDD/Fs is most sensitive to dry deposition velocity of the particle-phase, followed by air temperature and concentration of total suspended particulate but least sensitive to dry deposition velocity of the gas-phase.     

Polychlorinated biphenyls removal from weathered municipal solid waste incineration fly ash by collector-assisted column flotation

Polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) are highly toxic micropollutants emitted from municipal solid waste incineration (MSWI), in particular, concentrated in the unburned carbon (UC) of MSWI fly ash. Because of concerns over their adverse health effects, a number of countries have classified MSWI fly ash as hazardous material and required further treatment before its final disposal in landfills. The technologies for removing the toxic chlorinated micropollutants in the MSWI fly ash have been studied, however, until now no mature technique has been obtained in this purpose. In this research, we used a technique of collector-assisted column flotation to remove PCBs-enriched UC from MSWI fly ash. We found that 36.9% PCBs could be removed from fresh MSWI fly ash with 61.7% UC removal efficiency, whereas only 21.7% PCBs could be removed from weathered MSWI fly ash with a low UC removal efficiency of 33.7%. By adding a mixture of two kinds of surfactants: sorbitan mono-oleate and polyoxyethylene (20) sorbitan mono-oleate to the weathered fly ash slurry as the collector assistant, 39.3% PCBs was removed at the hydrophile–lipophile balance (HLB) value of 13.5, while the UC removal efficiency increased to 49.0%. The results showed that the collector assistant could enhance PCBs and UC removal efficiencies during the column flotation process, and the mechanism has been discussed in detail. Higher PCBs and UC removal efficiencies could be expected by further optimizing the conditions of collector-assisted column flotation.     

Effects of water-washing pretreatment on bioleaching of heavy metals from municipal solid waste incinerator fly ash

Previous studies demonstrated that the bioleaching of municipal solid waste incinerator fly ash by Aspergillus niger was an efficient "green technology" for heavy metals removal, however, it demanded a long operational period. In this study, water-washing was used as a fly ash pretreatment before the bioleaching process (one-step and two-step). This pretreatment extracted 50.6% of K, 41.1% of Na, 5.2% of Ca and 1% of Cr from the fly ash. Due to the dissolution of alkali chlorides which hold particles together, fly ash particles were smashed into smaller granules by the hydraulic flushing action caused by vibration. After the pretreatment, the lag phase and bioleaching period were reduced by 45 and 30%, respectively, in one-step bioleaching of 1% (w/v) fly ash. Meanwhile, the metals extraction yield both in one-step and two-step bioleaching was increased markedly, e.g. in two-step bioleaching, 96% Cd, 91% Mn, 73% Pb, 68% Zn, 35% Cr and 30% Fe was extracted from 1% water-washed fly ash, respectively. The reduction of the bioleaching period and improvement of metals extraction yield will likely allow the practical application of the bioleaching technology for heavy metals removal from fly ash.