Ten-years monitoring of MSWI bottom ashes with focus on TOC development and leaching behaviour

Abstract

In Switzerland MSWI bottom ash has to comply with the legal threshold value for TOC of < 2 wt-% in order to be landfilled. However, TOC contents of this magnitude lead to elevated DOC emissions and associated emissions of ammonium and Cu (aq). Since 2008 the Canton of Zürich therefore pursues a strategy to lower TOC contents in bottom ash by 2020 to 0.5 wt-%. To observe the development of TOC and other constituents, bottom ash has been monitored from 2008 to 2018. Monitoring results indicate that TOC contents < 0.5 wt-% in bottom ash lead to DOC eluate concentrations < 20 mg/l. DOC concentrations of this magnitude are close to Swiss legal criteria for discharge of landfill leachate into surface waters (10 mg/l). The emission results have been obtained by batch eluate tests according to Swiss Waste Legislation. Such laboratory tests only partially simulate real conditions occurring on landfills. To approximate landfill conditions, column tests with recent bottom ashes combined with tests on simple emission forecasting complete the study. The comparison of results from batch and column tests shows similar cumulative concentrations, indicating that batch tests are suitable to evaluate bottom ash quality. The tested modelling approach, based on constant conditions and exponential decrease in concentration, proved adequate to simulate column progressions. The modelled emission forecasts for DOC lies within 33% of column test results. Further, the model demonstrates the differences in flow regime between eluate tests and landfills and promotes better understanding of temporal aspects and the influence of landfill relevant parameters on pollutant mobilisation.

Keywords: Bottom ash; Emission forecast; Leaching tests; Monitoring; Municipal solid waste incineration; TOC.