Unsaturated Phase Environmental Processes in MSWI Bottom Ash
Abstract: Popular Abstract in English Every year, Sweden produces 1.5 million tons of incineration residues from waste to energy plants. Among these residues bottom ash(BA) due to its large volume and good geotechnical quality can be reused as an alternative construction material. However there are some negative environmental impacts of this practice such as leaching of salts and metals to soils and groundwater. Environmental processes occuring in the unsaturated phase can determine BA’s environmental impacts as well as provide interesting insights into improving its reuse. However, due to dominant conceptualisation of BA’s environmental impacts as a saturated phase phenomenon, very little work has been done on understanding the usaturated phase environmental processes. There is virtaully nothing known about BA’s dielectric properties which can be an impediment in the development of innovative tools e.g. microwave based monitoring and metal recovery systems. Similarly, release of hydrogen gas in anaerobic phase is considered a problem and its recovery for beneficial use has not been considered yet. Finally, residual organic matter is known to degrade and support microbial respiratory processes in BA. However, little work has been done on exploring its impact on leaching of metals and eco-toxicity of ash leachates.Therefore, the objective of this thesis was to explorethese unsaturated enviromental processes and highlight their role in envrionmental monitoring, reuse and resource recovery.To achieve this objective, the thesis was divided into three subdomains; 1) physical-dielectrics and non invasive monitoring , 2) chemicalanaerobic corrosion and hydrogen production, and 3) biological-microbial respiration and its impacts on the ash quality. During this work, methods such as frequency domain relectometry, respiration tests, hydrogen evolution experiments, batch leaching and ecotoxicity tests were employed. Methods such as gas chromatocgraphy and scanning electron microscopy(SEM) were also used when needed. For data analysis, statistical techniques such as polynomial regression, hierarchical clustering and principal components analysis (PCA) were used. Moreover, during the work on dielectrics, physical models were also used to establish the relationship between dielectric permittivity and volumetric water content. The results of dielectrics showed that in dry state BA acts a non conductor with strong dispersion tendencies especially at high frequencies.From the measured dielectric spectra, it was possible to estimate the moisture content as well as the effective electrical conductivity of BA.The dielectric properties of BA indicate the potential use of microwaves in noninvasive monitoring of moisture,weathering, residual metal contentand wet metal recovery.The results of hydrogen gas formation showed that the gas production from fresh BA was possible at mild conditions of temperature and pressure, and it showeda promise for commercial development. Comparison of hydrogen production with metal recovery showed that as compared to metal recovery,the reaction of metallic Al with alkaline solutions, to generate hydrogen , was more efficient. From the results on respiration tests, it was found that the respiration in fresh ash played a positive role by lowering the pH, reducing the leaching of critical metals and modifying the ash leachate eco-toxicity. However in case of the weathered ash, the microbial respiration negatively affected the ash quality by increasing the leaching heavy metals. The leaching of metals(Cu, Cr, Mo, Ni, Pb, & Zn) and total organic carbon was further enhanced by the addition of external organic matter. Further work on a bench scale hydrogen recovery system, the use of substrate induced respiration in fresh BA for carbonation and the use of microwaves in environmental monitoring and wet metal recovery is suggested.
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