Copper leaching of MSWI bottom ash co-disposed with refuse: effect of short-term accelerated weathering

Environmental and health impacts assessment of long-term naturally-weathered municipal solid waste incineration ashes deposited in soil-old burden in Bratislava city, Slovakia

Municipal solid waste incineration (MSWI) is an effective method for reducing the volume/mass of waste. However, MSWI ashes contain high concentrations of many substances, including trace metal (loid)s, that could be released into the environment and contaminate soils and groundwater. In this study, attention was focused on the site near the municipal solid waste incinerator where MSWI ashes are deposited on the surface without any control. Here, combined results (chemical and mineralogical analyses, leaching tests, speciation modelling, groundwater chemistry and human health risk assessment) are presented to assess the impact of MSWI ash on the surrounding environment. The mineralogy of ∼forty years old MSWI ash was diverse, and quartz, calcite, mullite, apatite, hematite, goethite, amorphous glasses and several Cu-bearing minerals (e.g. malachite, brochantite) were commonly detected. In general, the total concentrations of metal (loid)s in MSWI ashes were high, following the order: Zn (6731 mg/kg) > Ba (1969 mg/kg) ≈ Mn (1824 mg/kg) > Cu (1697 mg/kg) > Pb (1453 mg/kg) > Cr (247 mg/kg) > Ni (132 mg/kg) > Sb (59.4 mg/kg) > As (22.9 mg/kg) ≈ Cd (20.6 mg/kg). Cadmium, Cr, Cu, Pb, Sb and Zn exceeded the indication or even intervention criteria for industrial soils defined by the Slovak legislation. Batch leaching experiments with diluted citric and oxalic acids that simulate the leaching of chemical elements under rhizosphere conditions documented low dissolved fractions of metals (0.00-2.48%) in MSWI ash samples, showing their high geochemical stability. Non-carcinogenic and carcinogenic risks were below the threshold values of 1.0 and 1 × 10^-6, respectively, with soil ingestion being the most important exposure route for workers. The groundwater chemistry was unaffected by deposited MSWI ashes. This study may be useful in determining the environmental risks of trace metal (loid)s in weathered MSWI ashes that are loosely deposited on the soil surface.

Introduction of acid-neutralizing layer to facilitate the stabilization of municipal solid waste landfill

Rapid stabilization is important for landfill operation and beneficial for treatment capacity recovery, biogas production, and pollution control. Acidification of municipal solid waste (MSW) landfill hinders the degradation of MSW. In this study, a leachate-recirculated landfill bioreactor with acid-neutralizing layer (reactor BL) and a control landfill bioreactor without the acid-neutralizing layer (CL) were operated for 509 days. The pH of the landfill was increased by the acid-neutralizing layer. The landfill gas production volume increased by 18.3 % in reactor BL compared with CL during the study period, and the CH4 concentration was also increased. A greater MSW mass reduction was observed in reactor BL than in CL. Microbial community analysis demonstrated that the presence of the acid-neutralizing layer promoted the abundance of methanogens. Based on these observations, it is believed that application of the acid-neutralizing layer accelerated the stabilization by mitigating the acidification of landfill, which promote the abundance of methanogens and enhance the MSW degradation. These results help to understand the influencing mechanism of acid-neutralizing layer on the landfill stabilization, and provide a new approach for the practical landfill to achieve fast stabilization.

New insight into the role of FDOM in heavy metal leaching behavior from MSWI bottom ash during accelerated weathering using fluorescence EEM-PARAFAC

Fluorescence excitation-emission matrix (EEM) spectroscopy is a powerful tool to characterize DOM that interacts with heavy metals in MSWI bottom ash (IBA). Here, two fresh IBA samples collected from large MSWI plants were subjected to 33 days of accelerated weathering. Carbon content and fluorescence characterization of DOM and leaching behavior of heavy metals (Cu, Ba, Cr, Ni, and oxyanions) were monitored during the weathering. The mineralogical and chemical properties of IBA during the weathering process were also characterized. EEM combined with parallel factor analysis showed that fluorescent DOM could be decomposed into humic-like (C1, C2) and tryptophan-like substances (C3), while the accelerated weathering process can be further divided into three phases. Fitted cubic polynomials described well the changes in the specific intensity of fluorescence components. Humification and freshness indexes and SUVA results suggested the leached DOM contained a higher proportion of condensed aromatic structures and/or conjugation of aliphatic chains post-weathering. The results also revealed that adsorption of humic-like substances onto neo-formed reactive surfaces occurred quickly in the early stage of accelerated weathering; thereafter, biodegradation of lower molecular mass-hydrophilic organic carbon fraction plays a vital role in further reduction of Cu and Cr leaching in subsequent weathering. Oxyanions (Mo and Sb) became more mobile after 3 days of accelerated weathering, but their leaching was effectively reduced after the weathering process. A novel method for an IBA weathering treatment combined with enhanced microbial degradation is proposed. These findings provide new and inspiration for improving accelerated weathering technology.

Compositional modelling and crushing behaviour of MSWI bottom ash material classes

At present, in Europe, 18 million tonnes of MSWI Bottom Ash (BA) is annually stockpiled or used in low-grade applications (e.g. in road bases). Therefore, alternative applications, such as aggregate or as a cement component in concrete, are stimulated. Physical and chemical characteristics remaining after treatment, however, prevent its extensive application in building materials. Hence, knowledge is needed on the distinct properties of the material classes making up a heterogeneous BA, enabling the assessment of its characteristics and the resulting applicability. Furthermore, a user-friendly composition assessment procedure is necessary to evaluate the output of physical treatment processes. Crushing is a commonly applied treatment and its effect on the material classes comprising BA is still unknown. In this paper, the latter are identified and classified into slag, magnetic slag, glass, refractory, metals, and unburned material classes. The individual characteristics of each material class are identified and a suitable tracer for tracking these classes in heterogeneous samples is defined. Furthermore, a fast method to quantify the distribution of material classes based just on the oxide composition is developed and applied to approximate the changes in the configuration of BA through crushing. It is concluded that, although the jaw crushing of BA results in a more homogeneous distribution, beneficiation of material classes occurs and selective crushing is possible in order to improve the quality of the BA and therefore its subsequent application.

Review of leaching behavior of municipal solid waste incineration (MSWI) ash

Incineration is widely adopted in modern waste management because it provides an effective way to minimize municipal solid waste that needs to be disposed of in landfills. The ash residue is often disposed by landfilling. Alternatively, the incineration ash may be recycled and reused for various applications. The crucial issues, however, are the leaching of harmful elements during the use and the end-of-life phases. This review summarizes extensive studies on leaching behavior of municipal solid waste incineration ash. Specifically, pollutants generated through leaching, factors governing leaching, methodologies to study leaching, leaching mechanisms, and treatments to reduce leaching. Many types of pollutants are generated through leaching from municipal solid waste incineration ash, in which heavy metals and organic contaminants are the most toxic and concerned. Ash properties, pH and liquid to solid ratio are the main factors governing municipal solid waste incineration ash leaching. Leaching behavior of municipal solid waste incineration ash is complicated and existing methods to evaluate leaching may not be able to represent the field conditions. Solubility and sorption are the two major leaching mechanisms. Many treatment methods have been proposed. However, not all methods are effective and some approaches are associated with high energy and high cost, which makes them less economically feasible and attractive.

Characteristics of geotextile clogging in MSW landfills co-disposed with MSWI bottom ash

As a main byproduct of municipal solid waste incineration (MSWI), bottom ash (BA) has become a big challenge in operating MSWI plants. The most common method for BA treatment is co-disposal with MSW in landfills, which may cause clogging in the leachate collection system (LCS). This research investigated the characteristics of geotextile clogging in landfills with BA co-disposal. The co-disposal of BA changed the characteristics of leachate, especially increasing the concentration of Ca²⁺. During the experiment, 0.14 g CaCO₃ was precipitated in the MSW geotextile, while it increased to 0.52 g CaCO₃ in the BA co-disposed geotextile. Based on mass balance of calcium and thermogravimetric (TG) analysis, the formation of biofilm was the main contributor to the mass increment, accounting for about 82% and 57% mass increment in the MSW and BA co-disposed geotextile, respectively. Moreover, CO₂ in landfill gas played an important role in the clogging process, including CaCO₃ precipitation and biofilm formation. The results suggested that the co-disposal of BA with MSW can increase the risk of geotextile clogging in landfills.

Recycling ground MSWI bottom ash in cement composites: Long-term environmental impacts

In the present study, the long-term leaching behaviors of heavy metals in the cement composites prepared by the municipal solid wastes incineration (MSWI) bottom ash are evaluated based on the modified NEN 7375 protocol. The leaching test of the compact and ground cement composites were performed in both the deionized water and saline water for 180 days. The results showed that the heavy metals investigated could be classified into three categories according to their leaching behaviors. In the first category, the concentrations of Cu, Cd, Pb, As, V and Ba in the leachate increased with the leaching time. Zn and Sn can be included into the second category, because a decline in their leaching concentrations was observed after the initial increase. In the third category, the concentration of Ni in the leachate decreased initially, but increased afterward. The results revealed that the concentrations of most heavy metals were within the corresponding regulation, except for As in saline water. The kinetic study revealed that, for most heavy metals, the leaching kinetic is controlled by diffusion in the deionized water, while by the surface wash-off in the saline water. Finally, the mechanical tests confirmed that the cement composites prepared by MSWI bottom ash were durable in the saline water. The overall results demonstrate that the MSWI bottom ash can be a promising alternative as the cementitious component applied in cements or concretes for civil engineering.

Separation and characterization of magnetic fractions from waste-to-energy bottom ash with an emphasis on the leachability of heavy metals

Magnetic fractions were extracted from pulverized waste-to-energy (WTE) bottom ashes using a combined wet-dry extraction method. The resulting magnetic and non-magnetic fractions were subjected to compositional, mineralogical, and redox state analyses by X-ray diffraction (XRD), X-ray fluorescence, and X-ray photoelectron spectroscopy (XPS), respectively. The distribution and leaching toxicity of heavy metals were assessed to evaluate potential effects on the environment. Compositional analyses revealed that Fe accounted for 35% of the magnetic fraction of pulverized ashes, which was approximately seven times that of the raw ash. In addition to Fe, elemental Ni, Mn, and Cr were also significantly enriched in the magnetic fractions. The mineralogical analysis determined that Fe was primarily present as hematite and magnetite, and metallic iron was also identified in the magnetic fraction samples. The XPS analysis further proved the existence of zero-valence Fe. However, a significant amount of Fe remained in the non-magnetic fractions, which could partially be ascribed to the intergrowth structure of the various minerals. The elevated concentrations of toxicity characteristic leaching procedure (TCLP)-extracted Mn, Ni, Cr, Cu, Pb, and Zn were primarily ascribed to the lower buffering capability of the magnetic fractions, with the enrichment of Mn, Ni, and Cr in the magnetic fractions also contributing to this elevation.

Inhibitory effect of high calcium concentration on municipal solid waste leachate treatment by the activated sludge process

This research focused on the inhibitory effects of Ca on the aerobic biological treatment of landfill leachate containing extremely high Ca concentrations. When the Ca concentration in leachate to be treated was more than 4500 mg l^-1, the total organic carbon removal rate was significantly reduced and the processing time to achieve the same removal efficiency was 1.4 times that in the control treatment without added Ca. In contrast, the total nitrogen and ammonia nitrogen (NH₄⁺-N) removal efficiencies were positively related to the Ca concentration, increasing from 65.2% to 81.2% and from 69.2% to 83.7%, respectively, when the dosage of added Ca increased from zero to 8000 mg l^-1. During aerobic treatment, the reductions of solution Ca concentration were in the range of 1003-2274 mg l^-1 and were matched with increases in the Ca content in the residual sludge. The inhibition threshold of Ca in the leachate treated by the activated sludge process appeared to be 4500 mg l^-1, which could be realized by controlling the influent Ca concentration and using an appropriate sludge return ratio in the activated sludge process.

Migration of nitrate, nitrite, and ammonia through the municipal solid waste incinerator bottom ash layer in the simulated landfill

Simulated landfill was operated for 508 days to investigate the effect of municipal solid waste incinerator (MSWI) bottom ash layer on the migration of nitrate, nitrite, and ammonia when it was used as the intermediate layer in the landfill. The result suggested that the MSWI bottom ash layer could capture the nitrate, nitrite, and ammonia from the leachate. The adsorption of the nitrate, nitrite, and ammonia on the MSWI bottom ash layer was saturated at the days 396, 34, and 97, respectively. Afterwards, the nitrogen species were desorbed from the MSWI bottom ash layer. Finally, the adsorption and desorption could reach the equilibrium. The amounts of adsorbed nitrate and nitrite on the MSWI bottom ash layer were 1685.09 and 7.48 mg, respectively, and the amount of the adsorbed and transformed ammonia was 13,773.19 mg, which was much higher than the desorbed. The water leaching test and synthetic precipitation leaching procedure (SPLP) results showed that the leachable nitrate, nitrite, and ammonia in the MSWI bottom ash were greatly increased after the landfill operation, suggesting that the adsorbed nitrogen could be finally leached out. Besides, the results also showed that MSWI bottom ash layer could affect the release of nitrate and ammonia at the initial stage of the landfill. However, it had little effect on the release of nitrite.

Retention and leaching of nitrite by municipal solid waste incinerator bottom ash under the landfill circumstance

The retention and leaching of nitrite by municipal solid waste incinerator (MSWI) bottom ash could affect its migration in the landfill. In this study, the effect of the dosage of MSWI bottom ash as well as the variation of the landfill environmental parameters including pH, anions and organic matter on the nitrite retention and leaching behavior was investigated by batch experiments. The highest removal percentage (73.0%) of nitrite was observed when the dosage of MSWI bottom ash was 10 g L(-1) in 2 mg L(-1) nitrite solution. Further increase of the dosage would retard the retention, as the nitrite leaching from MSWI bottom ash was enhanced. The optimum retention of nitrite was observed when the pH was 5.0, while the leaching of nitrite showed a consistent reduction with the increase of pH. Besides, the presence of Cl(-), SO4(2)(-) and acetic acid could enhance the leaching of nitrite and mitigate the retention process. However, the retention of nitrite was enhanced by PO4(3)(-), which was probably due to the formation of the apatite, an active material for the adsorption of the nitrite. These results suggested that MSWI bottom ash could affect the migration of nitrite in the landfill, which was related to the variation of the landfill circumstance.