Epsilon Archive for Student Projects

Abstract

Determining the stability of sewage sludge biochar is crucial to ensure its contribution to carbon sequestration. As a primary objective, this thesis work aimed to evaluate the stability of sewage sludge biochars in agricultural soil using established methodologies: chemical oxidation and incubation experiments (92 days) coupled with modelling. Employing these methods yielded an inconclusive assessment of biochar stability. The chemical oxidation using potassium permanganate results indicated that the sewage sludge biochar used in this study is stable, with about 86 % stable biochar C that could persist in soil for around 100 years. However, findings from the 92 days incubation experiment, combined with modelling, suggested that sewage sludge biochar is not that stable (only 7% carbon remaining after 100 years). The inconsistency between both approaches likely owes to the shortness of incubation time and the use of a single first-order model, which is likely to lead to an underestimation of stability. Additionally, the study investigated the potential contribution of carbonates to carbon dioxide emissions and the aim was to differentiate between carbon dioxide emissions originating from organic carbon in biochar and those potentially contributed by carbonates. To accomplish this, the biochar was treated with acid to eliminate carbonates, before incubating both treated and untreated samples. By measuring and comparing the cumulative carbon dioxide emissions from both sets, the carbonate contribution could be isolated. This was done by subtracting the emissions from the treated (carbonate-free) samples from those of the untreated samples, allowing for a more precise evaluation of organic carbon-derived emissions from biochar. Statistical analysis revealed a significant impact of carbonates on carbon dioxide emissions (p=0.0005). In future research, the incubation experiment needs to be extended for a longer period (e.g., 1 year) to obtain more reliable estimates of the carbon stability of sewage sludge biochars.