Bringing nutrients from sea to land – mussels as fertiliser from a life cycle perspective

Abstract

Abstract Mussels can be used as fertiliser in organic farming, particularly on stockless farms, which need external nutrients but have limited access to fertiliser products. Mussels are filter feeders and accumulate nutrients bound in e.g. phytoplankton. When harvested and used onshore, their accumulated nutrients are removed from the water, thus decreasing eutrophication. This LCA study assessed the environmental impact of mussel farming on the Swedish coast of the Baltic Sea for subsequent use as agricultural fertiliser. The functional unit was related to the agronomic value of the mussels, including plant-available nitrogen, phosphorus and liming effect. Flow of cadmium to soil was also assessed. In one scenario the mussels were composted and in another they were inertly stored to avoid nitrogen losses and other emissions from storage. Use of mussels on agricultural land fulfils three functions: fertilisation, liming and a reduction in eutrophication. The results clearly showed the importance of integrated assessment of these three functions. Due to significant nitrogen losses from the composting process, the compost scenario resulted in a higher impact on acidification, global warming potential and energy use than the inert storage scenario. Alternative techniques to composting for handling mussels should thus be developed, for example direct use of the mussels as fertiliser without prior storage, or inert storage. Energy use was strongly related to production of materials for mussel cultivation, so choice and life length of materials is important when introducing mussel farming. The cadmium content of mussels is relatively high, but they often supply less cadmium to the field than the fertiliser and liming products they replace. Reducing eutrophication by cultivating mussels for fertiliser proved more energy-efficient than reducing eutrophication in wastewater treatment plants.