title: Factors influencing the quality of bottom ash from municipal solid waste incineration in Switzerland
Creators: name: Glauser, Andreas
Publisher: name: Universität Bern
Year: 2021
Subjects: subject: 550 Earth sciences &amp; geology

description: Sustainable waste management has always been important in Switzerland and especially waste incineration has a long tradition. Municipal solid waste (MSW) that cannot be recycled has to be thermally treated in one of the 30 MSW incineration (MSWI) plants, where mass and volume are reduced, energy is recovered and organic compounds are destroyed. About 20 wt.-% of the waste input, presently ~800’000 t/y, remain as bottom ash. Due to the high particulate metal content, metal recovery is mandatory prior to deposition. In recent years, new processes for enhanced metal recovery have been developed and implemented in Swiss MSWI plants and on landfills. However, despite these improvements ~90 wt.-% of the bottom ash remains after metal recovery, in different residual bottom ash fractions, which are deposited on Type D landfills in Switzerland.

One aspect of this thesis contributes to a better understanding of the factors influencing the composition and quality of bottom ash. Based on an existing dataset of a 10 years monitoring of raw bottom ashes from the Canton of Zürich, the influence of optimised incineration on total organic carbon (TOC) is evaluated. Thereby, lowering the concentrations of TOC in bottom ash, leads to significant quality improvements with respect to DOC emissions. On landfills, DOC emissions are crucial for the release of heavy metals, especially Cu (aq), as the mobilisation is enhanced by complexation with organic ligand. Thus, based on a combined approach of column experiments and modelling, temporal aspects of DOC emission on bottom ash landfills are studied. By including data on DOC emissions of existing bottom ash landfills, forecasts concerning the aftercare duration indicate DOC emissions <20 mg/l within the legal aftercare duration of 50 years.

Another focus of this thesis is the characterisation of residual bottom ash fractions from different Swiss MSWI plants after the metal recovery processes. Thereby, the effect of bottom ash treatment on the remaining fractions is studied and their recycling potential is investigated based on Swiss and international legal requirements. Especially enhanced treatment processes operate with a strong fractionation of the bottom ash into different grainsize categories for the specific recovery of metals. In some of these processes, further washing with process water is involved, additionally influencing the composition of bottom ash. Thus, the result are carbonate- and sulphate-rich fine fractions, usually removed before metal recovery, that feature elevated heavy metal, Cl and TOC contents. In contrast, coarse bottom ash fractions, remaining after enhanced metal recovery, show a chemistry related to transit minerals and an improved quality due to depleted heavy metal, Cl and TOC contents. However, to qualify for recycling according to the Swiss Waste Legislation, further treatment of these fractions is necessary. By testing different chemical and mechanical treatment methods, this thesis provides a basis for progressing studies towards recycling of suitable bottom ash fractions as secondary raw material in Switzerland.