Life cycle assessment of macroalgal biorefinery for the production of ethanol, proteins and fertilizers e A step towards a regenerative bioeconomy

Seaweed is a key biomass for the development of a biobased economy because it contains valuable components such as proteins, sugars, nitrogen and phosphorus. This paper analyses innovative offshore seaweed cultivation for the production of biorefinery feedstock. The biomass is converted into three products: bioethanol, liquid fertilizer and protein-rich ingredient for fish feed. We performed comparative life cycle assessment of a base case and six alternative production scenarios in order to maximize the benefits and minimize the trade-offs in environmental performance of future macroalgal biorefineries (MABs). The results show that the base case provides a net reduction in climate change factors, i.e. −0.1·10² kg CO2 eq. per ha of sea cultivated despite a cumulative net energy demand of 3.9·10⁴ MJ/ha, 13% of which originates from fossil sources. Regarding the environmental performance of the system, we obtained a reduction in marine eutrophication of −16.3 kg N eq./ha, thanks to bioextraction of nitrogen. For the base case the net impact on human toxicity (carcinogenic effects) was 2.1·10⁻⁴ comparative toxic units per ha of cultivation. The increase in human toxicity is seven times greater than the system can deal with, however reduction of materials for the cultivation lines, i.e. iron ballast, reduces human toxicity to 0.2·10⁻⁵ comparative toxic units. Externalities from the use of biofertilizer affect the non-carcinogenic effects of the system, resulting in 20.3·10⁻⁴ comparative toxic units per ha. Hotspots in the value chain show that biomass productivity is the main constraint against being competitive with other energy and protein producing technologies. Minor changes in plant design, i.e. use of stones instead of iron as ballast to weight the seeded lines, dramatically reduces human toxicity (cancer). Including engineered ecosystem services in the LCA significantly improves the results. As such, an increase in soil carbon stock represents 15% of the climate change mitigation provided by the MAB system. The study shows that MABs can contribute to a regenerative circular economy through environmental restoration and climate mitigation.