Exploratory environmental assessment of large-scale cultivation of seaweed used to reduce enteric methane emissions

Abstract

Animal husbandry contributes to roughly 15% of total anthropogenic GHG emissions, of which the majority originates from methane emissions associated with ruminant enteric fermentation. Recent studies have shown that methane emissions from ruminants can be heavily reduced (up to 90% reduction) by introducing the seaweed Asparagopsis taxiformis in the feed composition. The largest hurdle in implementing a wider use of the seaweed feed supplement is to scale up the production. Today most of the production is performed in extensive ocean farming systems. In the small coastal city Lysekil, located on the Swedish west coast, an initiative for a land-based seaweed production system is currently in progress. In this study, Life Cycle Assessment (LCA) methodology was applied to assess the environmental impact of this production system. The assessment included all important stages in the seaweed production system, from cradle to farmgate. The results showed that a large part of the assessed impacts were attributed to the salt (NaCl) sourcing needed to increase the salinity to achieve optimal growing conditions. In the default scenario, the salt input contributed to 48% of the total GHG emissions. The scenario analysis showed that lower emissions could be obtained by changing the salt source from rock salt, sourced in Denmark, to sea salt sourced by using evaporation ponds in France. Moreover, increasing the water recycling rate could reduce salt demand and consequently the environmental impact. The results also displayed a large thermal energy demand used to increase the temperature in the water to more fitting conditions. Despite this, the environmental impact from thermal energy was relatively low, causing 16% of the total GHG emissions. The reason for this was explained by the use of low-impact district heating, mostly reliant on the excess heat from the local oil refinery. The scenario analysis showed that choices regarding thermal energy source and allocation method significantly impacted the overall results of the assessment. Furthermore, we argue that although the environmental impact from the excess heat from the oil refinery displayed low impact, the seaweed farm should consider using other thermal energy sources, since it is difficult to foresee the impact of the excess heat from the petrochemical industry as its main products are being phased out in decarbonization and climate neutrality schemes. An alternative thermal energy source with promising results was electric heat pumps powered by wind. We concluded that LCA methodology could be an important tool to optimize production systems before they are brought into operation.