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Onsite wastewater treatment systems (OWTS) are vital in rural and peri-urban areas, but conventional designs such as septic tanks with soil infiltration often provide insufficient treatment. This contributes to eutrophication, groundwater contamination, and the spread of emerging micropollutants. To address these challenges, this thesis investigates a biochar-based multi-module filter system (MmBF) and examines the institutional factors influencing the adoption of innovative OWTS technologies in Sweden.

Each of the triplicate MmBFs configurations comprised six sequential modules with aerobic and anoxic conditions to promote organic matter oxidation, nitrification, denitrification, and faecal bacteria attenuation. The MmBFs achieved consistently high removal, with chemical oxygen demand reduced by about 95%, inorganic nitrogen by 71%, and Escherichia coli by 1.8–2.3 log10. Pharmaceutical active compounds were also effectively treated, with overall concentrations reduced by more than 99%, most within the first aerobic stage. Phosphate removal remained limited, indicating a need for design improvements. To improve nutrient removal, sawdust and Polonite® were amended at 10% and 30% in two of the MmBFs, each applied to separate modules. The 30% sawdust amendment enhanced nitrate reduction by up to 83% for a short term, while 30% Polonite® achieved short-term phosphate retention of 31 ± 19%. Both effects declined over time, suggesting the need for periodic media replacement.

A qualitative study complemented the technical evaluation by examining the socio-institutional context of OWTS transitions. Interviews with municipal officers, regulators, developers, and knowledge intermediaries, analysed through a Multi-Level Perspective framework, showed that regulatory ambiguity, fragmented responsibilities, and risk-averse permitting hinder adoption. At the same time, enabling factors were identified, including reference demonstration units, stronger municipal engagement, and more accessible knowledge-sharing platforms. While developers and researchers drive technical innovation, municipalities and national authorities retain decisive roles in approval and diffusion.

The thesis concludes that advancing decentralised wastewater treatment in Sweden requires both technical and institutional progress. For biochar-based systems, further work should address media replacement, installation design, and potential investigations for nutrient recovery, to support market-ready products. At the institutional level, municipalities and national authorities must collaborate with innovators to create transparent approval pathways and strengthen legitimacy for novel systems.