Proper management of organic waste resources is important to reduce negative impacts on the environment and human health. Organic waste resources, such as sewage sludge, contain nutrients beneficial for plant growth but can cause problems like eutrophication if mismanaged. Sewage sludge may also contain substances that are potentially toxic, such as heavy metals and PFAS.
In the research project RenCARBio, three alternatives for managing sewage sludge have been explored: biogas and biofertiliser production through anaerobic digestion, biochar production through pyrolysis, and a combined process involving both anaerobic digestion and pyrolysis. This report presents results from Work Package 6, which focused on environmental impacts and costs. Environmental impacts were assessed using life cycle assessment (LCA). The analyses are based on data from IVAR’s two biogas plants (SNJ and Grødaland), pilot test results and measurements carried out within the project, supplemented with data from the literature. This report presents only the climate change impact category, while a scientific article including also ecotoxicity and human toxicity impacts has been written. The cost assessment was conducted by mapping potential cost components, revenues, and avoided costs for the three alternatives from the perspective of a treatment facility.
The aim of the study presented in this report is to identify which aspects have the greatest influence on environmental impact and economic performance when comparing the three treatment alternatives.
The economic assessment showed that costs and potential revenues from the treatment alternatives are site-specific, making it difficult to generalize. Both capital expenditures (CAPEX) and operational expenditures (OPEX) depend on the chosen technology, plant size, and location. A combination of technologies will likely result in total costs equal to the sum of the costs for each individual technology. Scenarios involving pyrolysis are expected to have high energy costs due to the need for drying the feedstock. Alternatives involving anaerobic digestion (biogas production) may generate revenue from biogas sales, while pyrolysis alternatives may potentially reduce costs related to the storage and distribution of digestate. Biochar production may also provide revenue through both biochar sales and carbon dioxide removal credits.
The choice of energy carrier that biogas is assumed to replace in the anaerobic digestion scenarios has the greatest influence on the resulting climate footprint. Another important factor is the assumptions related to the long-term carbon storage effect of biochar and digestate. Additionally, assumptions about the extent to which mineral fertiliser can be replaced by biochar and digestate play a significant role. The use of chemicals and fossil energy carriers in the treatment process (anaerobic digestion) also contributes notably to the overall climate impact.
While anaerobic digestion is well suited for feedstocks with high water content, pyrolysis requires drier feedstocks with specific material properties and homogeneity. However, this study does not address the
practical factors that may affect the feasibility of these two technologies. Furthermore, the application of biochar and digestate as fertilisers depends on their fulfilment of regulatory requirements