Chemical speciation, distribution and leaching behavior of chlorides from municipal solid waste incineration bottom ash

Highly efficient carbonation and dechlorination using flue gas micro-nano bubble for municipal solid waste incineration fly ash pretreatment and its applicability to sulfoaluminate cementitious materials

Cement production is a primary source of global carbon emissions. As a hazardous waste, municipal solid waste incineration fly ash (MSWI-FA) can be pretreated as a cementitious and effective carbon capture material. This study proposes an efficient carbonation dechlorination pretreatment and resource recovery strategy using flue gas micro-nano bubble (MNB) to wash MSWI-FA. The results showed that the flue gas MNB water washing reaction solution inhibited CaCO3 boundary layer blocking and adsorption on NaCl and KCl leaching. Under low water-to-solid ratio and CO2 concentration conditions, two-step washing reduced the MSWI-FA chlorine content to <1%, improving the dechlorination effect by 19.72% compared to conventional carbonation. The flue gas MNB water accelerated the precipitation of Ca2+ and Ca(ClO)2 in the form of calcite. The higher the CO2 concentration in the flue gas MNB, the better the fragmentation and purification of the MSWI-FA shell, leading to improved dechlorination and CO2 fixation. Under optimized conditions, the mean particle size of MSWI-FA decreased by 47.82%, and the CO2 fixation rate reached 73.80%, with a 58.35% increase in the washing carbonation rate. MSWI-FA pretreated by flue gas MNB washing was used as both the raw material and supplementary cementitious material for sulfoaluminate cementitious (SAC) material, exhibiting excellent compressive strength and heavy metal stabilization. The maximum compressive strength of the MSWI-FA-based SAC material cured for 28 d reached 130 MPa. Cr leaching was inhibited with increased hydration time, and the leaching concentration was far below the standard limit.

Evaluation of a battery of biotests to improve waste ecotoxicity assessment (HP 14), using incineration bottom ash as a case study

The assessment of waste ecotoxicity (hazardous property HP14 in the European Union) is fundamental for proper waste classification and safe application/disposal. Biotests are relevant for evaluating waste complex matrices, but their efficiency is crucial to encourage their adoption at the industrial level. This work aims at evaluating possibilities of improving the efficiency of a biotest battery previously suggested in the literature, regarding test selection, duration, and/or laboratory resources optimization. Fresh incineration bottom ash (IBA) was the case study. The test battery analysed included standard aquatic (bacteria, microalgae, macrophytes, daphnids, rotifers, fairy shrimp) and terrestrial (bacteria, plants, earthworms, collembolans) organisms. The assessment followed an Extended Limit Test design (three dilutions of eluate or solid IBA) and the Lowest Ineffective Dilution (LID-approach) for ecotoxicity classification. The results emphasize the importance of testing different species. It was also evidenced that tests with daphnids and earthworms may be shortened to 24 h; the miniaturization of tests is suitable as e.g. differential sensitivity of microalgae and macrophytes was captured with low variability; alternative testing kits can be used when methodological difficulties are found. Microalgae were more sensitive than macrophytes. Similar results were found for the Thamnotoxkit and daphnids test for eluates with natural pH, so the former may be used as an alternative. B. rapa was the most sensitive organism, suggesting that it may be tested as the only terrestrial plant species and that minimum test duration is appropriate. F. candida does not appear to add information to the battery. The differences in sensitivity of A. fischeri and E. fetida compared to the remaining species were not significant enough to exclude them from the battery. Thus, this work suggests a biotest battery to test IBA comprising aquatic tests - Aliivibrio fischeri, Raphidocelis subcapitata (miniaturised test), and Daphnia magna (24 h when clear deleterious effects are observed) or Thamnocephalus platyurus (toxkit) - and terrestrial tests - Arthrobacter globiformis, Brassica rapa (14 d), and Eisenia fetida (24 h). Testing waste with natural pH is also recommended. The Extended Limit Test design considering the LID-approach seems useful in waste testing, particularly for the industry, involving low effort, test material requirements, and few laboratory resources. The LID-approach allowed for differentiating ecotoxic from non-ecotoxic effects and captured different sensitivities between species. Ecotoxicological assessment of other waste may benefit from these recommendations, but caution should be taken given the properties of each waste type.

Multi-objective optimization and parametric study of a hybrid waste gasification system integrated with reverse osmosis desalination

The increasing population has created two fundamental issues on the islands (in this case, Kish Island): an expansion in waste production and handling and a rise in the need for freshwater for daily consumption. Waste gasification can be used to reduce waste and generate energy. Sea salt water can be converted into drinkable freshwater using energy. This article describes a computational code using a combination of EES (Engineering Equation Solver) and MATLAB software for a hybrid cycle that includes waste gasification and reverse osmosis to generate freshwater and power. Kish Island waste data was carefully collected onsite. An exergy evaluation has been conducted to verify the cycle's irreversibility. Lastly, optimization has been performed to identify the best operation mode. In terms of irreversibility, a gasifier is far more irreversible than any other type of plant equipment. Based on the parametric analysis, salt water and waste flow significantly impact freshwater production. Approximately 20,000 m3/day are consumed on Kish Island each day, so if the entire production power of the steam turbine is used in the reverse osmosis high-pressure pump, 2860 m3/day can be obtained each day (14% of the required amount).

A comprehensive investigation of toxicity and pollution potential of municipal solid waste landfill leachate

It is evident from the literature that research on the treatment of leachate generated from municipal solid waste (MSW) landfills has been a focus area of environmental management. However, the available information is discrete because most studies have reported only one or a couple of aspects of either closed or active MSW landfill leachate treatment. Hence, this investigation has focused on comprehensive attributes of both closed landfill leachate (CLL) and active landfill leachate (ALL), including generation, characterisation, and toxicity assessment to quantify and establish their pollution potential. The results indicated that CLL generation is higher (188.59 m³/d) than ALL (49.53 m³/d). The concentrations of principal physical, chemical, and biological constituents and concomitant leachate pollution index were higher in CLL (33.20) than in ALL (26.65). Furthermore, the germination indices of CLL (57.48) and ALL (79.14) and tail DNA damage of CLL (56.49%) and ALL (23.8%) ratified greater phytotoxicity and genotoxicity potential, respectively of CLL over ALL. The reasons for the variations in the generation, characteristics, and toxicity of CLL and ALL were discussed in detail. Evaluation of the commonly used landfill leachate treatment methods through the analytical hierarchy process confirmed that the activated sludge process and Fenton oxidation process are the most and least preferred treatment methods. The comprehensive investigation of CLL and ALL have established their pollution potential and the inevitable necessity for their treatment. The findings of this investigation will serve as a ready reference for researchers from academia and industry who work on the monitoring, treatment, and management of landfill leachate.

Partial Replacement of Municipal Incinerated Bottom Ash and PET Pellets as Fine Aggregate in Cement Mortars

The objective of this study was to examine the optimal mixing ratio of municipal incinerated bottom ash (MIBA) and PET pellets used as a partial replacement of fine aggregates in the manufacture of cement mortars. As a partial replacement for sand, 15 mortar specimens were prepared by mixing 0%, 10%, 20%, 30%, and 40% municipal incinerated bottom ash (MIBA) (A) and 0%, 10%, and 20% PET pellets (P) in 5 cm × 5 cm × 5 cm cube molds. The cement/aggregate ratio was 1:3, and the water/cement ratio was 0.5 for all specimens. The results showed that the compressive strength of cement mortars decreased when increasing the amount of MIBA and PET pellets. The mortar specimens with 10% PET pellets achieved the highest compressive strength (49.53 MPa), whereas the mortar specimens with 40% MIBA and 20% PET pellets achieved the lowest compressive strength (24.44 MPa). Based on this finding, replacing 10% and 20% sand in cement mortar with only MIBA or only PET pellets could result in compressive strengths ranging from 46.00 MPa to 49.53 MPa.

Stabilization of heavy metals in municipal solid waste incineration fly ash via hydrothermal treatment with coal fly ash

The environmental risk of heavy metals in hazardous municipal solid waste incineration fly ash (FA) is one of the most important concerns for its safely treating and disposing. This study investigated the stabilization behavior of heavy metals in FA using coal fly ash (CFA) as an additive via hydrothermal treatment. The effects of water washing pre-treatment and FA/CFA ratio on leaching behavior, speciation evolution, and risk assessment of heavy metals were studied. The results showed that 96.6-98.0 % of Cl can be effectively removed by water washing pre-treatment and hydrothermal treatment. Most heavy metals (Cr, Cu, Ni, Pb and Zn) (>91.5 %) were stabilized in the hydrothermal product, rather than transferred to liquid phase. Tobermorite can be synthesized by adjusting Ca/Si ratio with the addition of CFA. The heavy metals were transferred into more stable residue fractions with increasing CFA addition, which resulted in the significant reduction of leaching concentrations and risk assessment code (RAC) of heavy metals. Among, the product with 30% CFA exhibited the most superior performance with the lowest leaching concentrations of heavy metals and RAC was at no risk level (<1). In addition, the economic performance of hydrothermal treatment exhibited a potential advantage by comparing with FA-to-cement, FA-to-glass slags and FA-to-chelating agent & cement solidification/stabilization. Therefore, the hydrothermal treatment coupled with water washing pre-treatment would be a promising method for the detoxification of FA, as well as synergistic treatment of FA and CFA.

The application of multiple advanced chloride removal methods to synthesized Friedel's salt and municipal solid waste incineration bottom ash

To recycle municipal solid waste incineration bottom ash as a cement raw material, it is important to reduce the Cl concentration in the ash. However, the reduction of chlorides by washing only has limited success due to the presence of insoluble Friedel's salt (FS) in the ash. Although some studies on the decomposition of FS and the application of advanced chloride removal methods to bottom ash have been reported, few studies have compared the effects of different removal methods. Moreover, due to the complex ash composition, it is also necessary to compare the effects on different ashes and pure FS. Therefore, in this study, we applied five advanced chloride removal methods to synthesized FS and two types of bottom ash (FS-High and FS-Low), and compared the effects. For both FS and bottom ash, all methods promoted chloride dissolution more than washing only. For FS, aging was the most effective method, with a Cl removal ratio of 73%. In contrast, for ash FS-High, aging increased the Cl removal ratio to 80% and decreased the Cl content to 2800 mg/kg. The FS-derived peak observed in an X-ray diffraction analysis disappeared following aging. For ash FS-Low, acid washing increased the Cl removal ratio to 64% and decreased the Cl content to 1800 mg/kg. The treatment with the highest removal ratio in each ash had the lowest pH. There was a significant correlation between pH and the Cl removal effect. The Cl remaining after the application of the methods was likely associated with Na.

Experimental Investigation of Vitrification Process for the Disposal of Hazardous Solid Waste Containing Chlorides

Vitrification has attracted much attention as an efficient method for solidifying heavy metals in hazardous solid wastes, but its effect is limited when hazardous solid wastes contain chlorides. Aiming at fly ash, a normal chlorine-containing solid waste, a novel process of chlorination melting and glass curing was developed to completely realize the harmlessness of heavy metals. Melting temperature, time, and auxiliary agent were adequate to realize the harmlessness, and their influence on the migration and transformation of Cl, Na, Pb, and Zn and the leaching of slag were studied. The results showed that the majority of Cl, Na, Pb, and Zn in the fly ash had been transferred to the soot, and the residual part in the slag had been solidified in glass by controlling the process conditions. Under the optimized conditions (12 wt.% CaO and 5 wt.% SiO2 was added, the N2 flow ratio was at 1 L/min, and the melting temperature was 1300 °C for 2 h), the leaching index was determined, including the acid dissolution ratio, the Pb and Zn content of the water leaching solution, and the acid leaching solution, which all met the requirements of the relevant standards. Furthermore, the novel process provided a simple and efficient approach for the disposal of other similar solid wastes containing chlorides and heavy metals.

Ceramisation of hazardous elements: Benefits and pitfalls of the inertisation through silicate ceramics

The addition of wastes to silicate ceramics can considerably expand the compositional spectrum of raw materials with a possible inclusion of hazardous components. The present work quantitatively examines relevant literature to determine whether the benefits of incorporating hazardous elements (HEs) into silicate ceramics outweigh the pitfalls. The mobility of various HEs (Ba, Zn, Cu, Cr, Mo, As, Pb, Ni, and Cd) has been parameterised by three descriptors (immobilisation efficiency, mobilised fraction, and hazard quotient) using leaching data. HEs can be incorporated into both crystalline and glassy phases, depending on the ceramic body type. Moreover, silicate ceramics exhibit a remarkably high immobilisation efficiency (often exceeding 99.9%), as accomplished for Ba, Cd, Ni, and Zn elements. The pitfalls of the inertization process include an insufficient stabilisation of incorporated HEs, as indicated by the high hazard quotients (beyond the permissible limits established for inert materials) obtained in some cases for Mo, As, Cr, Pb, and Cu elements. Such behaviour is related to oxy-anionic complexes (Mo, As, Cr) that can form their own phases or are not linked to the tetrahedral framework of aluminosilicate glass. Pb and Cu elements are preferentially partitioned to glass with a low coordination number, while As and especially Mo are not always stabilised in silicate ceramics. These drawbacks necessitate conducting additional studies to develop appropriate inertisation strategies for these elements.

A Novel Dry Treatment for Municipal Solid Waste Incineration Bottom Ash for the Reduction of Salts and Potential Toxic Elements

The main obstacle to bottom ash (BA) being used as a recycling aggregate is the content of salts and potential toxic elements (PTEs), concentrated in a layer that coats BA particles. This work presents a dry treatment for the removal of salts and PTEs from BA particles. Two pilot-scale abrasion units (with/without the removal of the fine particles) were fed with different BA samples. The performance of the abrasion tests was assessed through the analyses of particle size and moisture, and that of the column leaching tests at solid-to-liquid ratios between 0.3 and 4. The results were: the particle-size distribution of the treated materials was homogeneous (25 wt % had dimensions <6.3 mm) and their moisture halved, as well as the electrical conductivity of the leachates. A significant decrease was observed in the leachates of the treated BA for sulphates (44%), chlorides (26%), and PTEs (53% Cr, 60% Cu and 8% Mo). The statistical analysis revealed good correlations between chloride and sulphate concentrations in the leachates with Ba, Cu, Mo, and Sr, illustrating the consistent behavior of the major and minor components of the layer surrounding BA particles. In conclusion, the tested process could be considered as promising for the improvement of BA valorization.

Co-disposal of municipal solid waste incineration fly ash and bottom slag: A novel method of low temperature melting treatment

Conventional melting for disposing municipal solid waste incineration (MSWI) fly ash or bottom slag needed high temperature and consumed high energy. High calcium content in fly ash and high silicon content in bottom slag brought them high melting point, respectively. Based on the analysis of chemical composition and phase diagram, suitable contents, namely 30%-40% CaO, 45%-60% SiO₂ and 10%-15% Al₂O₃, were proposed to obtain a lower-melting-point mixture system. When the mass ratio of fly ash to bottom slag was 1:5, lowest melting point can be obtained. It was 1,190 ℃, lower than that of fly ash (1,448 ℃) and bottom slag (1,310 ℃). The toxicity characteristic leaching procedure of slags obtained from low melting treatment met the leaching toxicity of Chinese standard GB 5085.3-2007, and the slags containing about 25 wt% CaO, 10 wt% Al₂O₃ and 45 wt% SiO₂ can be used for preparing CaO-Al₂O₃-SiO₂ glass ceramics. The co-process of fly ash and bottom slag realized the low temperature melting treatment with low energy consumption.

Physiochemical characterization and systematic investigation of metals extraction from fly and bottom ashes produced from municipal solid waste

Incineration has emerged as one of the acceptable ways to treat municipal solid waste (MSW) due to its potential in reducing the mass and volume of the waste. However, it produces two major by-product residues, namely MSW-bottom ash (MSW-BA) and MSW-fly ash (MSW-FA). These residues have gained great attention to their hazardous nature and potential to be reused and recycled. In this paper, the physicochemical characterizations of the MSW-BA and the MSW-FA were performed, followed by a systematic investigation of metals extraction from MSW-BA and MSW-FA. Various extracting agents were used to investigate the possibility to extract 21 metals including cadmium (Cd), vanadium (V), chromium (Cr), and lead (Pb). It was revealed that some metals were present in a high amount in the MSW-BA while other metals were higher in the MSW-FA. Moreover, the energy-dispersive X-ray spectroscopy results revealed that the MSW-BA was dominated by oxygen (O) 55.4 ±0.6 wt%, silicon (Si) 22.5 ±0.3 wt%, and calcium (Ca) 18.5 ±0.2 wt%. On the other hand, the MSW-FA was enriched with Ca 45.2 ±0.5 wt%, and O 40.3 ±0.4 wt%. From the scanning electron microscopy, the MSW-BA was observed as flaky with an irregular surface that consisted of large pores, while, the MSW-FA was present as agglomerated particles and had a bimodal distribution. Moreover, Fourier transform infrared spectroscopy revealed that Al-Fe-OH, Al-Al-OH, Si-O, C-O, and C-H were some of the major functional groups present in the ashes. The F-tests concluded that the metal extraction from the MSW-BA and MSW-FA were significantly affected by the acid type. it is concluded that nitric acid and phosphoric acid were the best-suited acid for the MSW-BA while sulfuric acid and phosphoric acid for the MSW-FA. More than 11 wt% of Cd and 9 wt% of Cu were extracted from MSW-BA while 6 wt% of Pb and 4.5 wt% of V were extracted from the MSW-FA. The present methodology is an interesting development in metal extraction from the MSW-BA and the MSW-FA, which can develop in a cost-effective and sustainable option to utilize MSW.