Life cycle assessment of recirculating aquaculture systems with innovative biochar filter for enhanced nutrient recirculation

Artikel i vetenskaplig tidskrift, 2025

Reducing pollution caused by losses of carbon (C), nitrogen (N), and phosphorus (P) from the technosphere and turning these flows into resources is a focus of current research towards a circular economy. Producing biochar from biomass pyrolysis and using it in agriculture is a promising way to mitigate climate change and improve soil quality. In this study, we explore the environmental performance of using biochar for nutrient recovery in recirculating aquaculture systems (RAS) and generating a nutrient-enriched biochar for agricultural use. Through prospective life cycle assessment (LCA), the study looks into two different RAS configurations, one with a conventional biofilter and one with the innovative biochar filter. The latter is also explored using two different system perspectives: in the first, the biochar production is considered an activity that happens solely for the purpose of the fish farming but with the added function that it captures and transports nutrients along with stable C to agricultural soil; in the second, biochar is considered already produced and destined for agriculture but it takes a detour to a fish farm to collect some nutrients en route. The main environmental hotspots for the conventional system are related to fish feed production and electricity usage. When the biofilter is replaced by a biochar filter and biochar is generated for the main purpose of being a filter, additional large impacts from forestry biomass production and construction of a pyrolysis plant are associated with the RAS. This is only partially counteracted by recovered heat and nutrients, but for climate impact, the gains related to C sequestration are considerable. A sensitivity analysis revealed considerable variability in the performance of the first RAS biochar configuration due to variations in NH4+ adsorption capacity. When biochar is considered a “sunk cost” - a resource generated for other purposes - the weight of impacts shifts back to fish feed production and to biochar container construction. With regard to overall performance, RAS with biochar shows promising results compared to conventional RAS, but there are variations between impact categories. The innovative technology is promising also when compared with benchmarks in the literature. The technology still needs proof of concept, both concerning the action as a filter and the behaviour as a fertiliser product, but performing an LCA at early stages provided useful insights into further development. It is clear that fish feed is underexplored in LCA contexts. Further work could also look into how the fish sludge could be valorised and what the best system integration is for the innovative technology.