Study of cast seaweed harvesting technologies used in the bay of Køge and their implications for effective biogas production: Applying a circular bio-economy approach

This paper focuses on the technical methods used to maximize the collection of cast seaweed from the Bay of Køge within Solrød Municipality in Denmark. The study assesses the efficiency of various seaweed harvesting technologies, as well as the pre-treatment of the seaweed (i.e. removal of sand) at the Solrød Biogas plant. As the digestate is utilized as fertilizer (N, P, K) on surrounding agricultural farmland, it is also crucial to minimize the presence of heavy metals in the digestate resulting from the use of seaweed as feedstock. The paper discusses the various ways in which sand-laden seaweed used as feedstock can contribute to energy production. It highlights the circularity and advantages of collecting cast seaweed and how this can produce nitrogen-rich fertilizer, thereby reducing CO₂ emissions. The theoretical framework employed in this study is based on the Circular Bio-Economy school of thought, which supports the cascading and reuse of biomass resources to boost and prolong their value and usefulness. Interviews with pertinent parties involved in the collection of cast seaweed and the operation of the biogas technology within Solrød Municipality serve as the basis for the research data. Moreover, data from Solrød evaluation reports and reviews of seaweed digestion data are utilized, as well as pertinent scientific literature. Among other outcomes, it is concluded that the seaweed harvesting technology now in use is useful in marine environments near to the shoreline, but less effective for collecting seaweed from further up the beach. However, it will be necessary to develop the technology, or utilize complementary harvesting technologies, in the future to improve collection efficiency and reduce the amount of sand collected with the seaweed. Problematic factors include the likelihood of sand accumulating in the biogas reactor tank, as well as damage to the cyclone stirrer and the macerator. To support the Circular Bio-Economy in terms of the collection and use of cast seaweed within Solrød Municipality, new technological methods should hence be promoted.