• Energy Research

Abstract Offshore cultivation of seaweed provides an innovative feedstock for biobased products supporting blue growth in northern Europe. This paper analyzes two alternative exploitation pathways: energy and protein production. The first pathway is based on anaerobic digestion of seaweed which is converted into biogas, for production of electricity and heat, and digestate, used as fertilizer; the second pathway uses seaweed hydrolysate as a substrate for cultivation of heterotrophic microalgae. As a result the seaweed sugars are consumed while new proteins are produced enhancing the total output. We performed a comparative Life Cycle Assessment of five scenarios identifying the critical features affecting resource efficiency and environmental performance of the systems with the aim of providing decision support for the design of future industrial scale production processes. The results show that all scenarios provide environmental benefits in terms of mitigation of climate change, with biogas production from dried Laminaria digitata being the most favorable scenario, quantified as −18.7*10 2 kg CO 2 eq./ha. This scenario presents also the lowest consumption of total cumulative energy demand, 1.7*10 4 MJ/ha, and even resulting in a net reduction of the fossil energy fraction, −1.9*10 4 MJ/ha compared to a situation without seaweed cultivation. All scenarios provide mitigation of marine eutrophication thanks to bioextraction of nitrogen and phosphorus during seaweed growth. The material consumption for seeded lines has 2–20 times higher impact on human toxicity (cancer) than the reduction achieved by energy and protein substitution. However, minor changes in cultivation design, i.e. use of stones instead of iron as ballast to weight the seeded lines, dramatically reduces human toxicity (cancer). Externalities from the use of digestate as fertilizer affect human toxicity (non-cancer) due to transfer of arsenic from aquatic environment to agricultural soil. However concentration of heavy metals in digestate does not exceed the limit established by Danish regulation. The assessment identifies seaweed productivity as the key parameter to further improve the performance of the production systems which are a promising service provider of environmental restoration and climate change mitigation.

This study presents a comparative analysis of the environmental and economic performances of four integrated waste and wastewater management scenarios in the city of Aarhus in Denmark. The purpose of this analysis is to deliver decision support regarding whether (i) the installation of food waste disposers in private homes (AS1) or (ii) separate collection and transport of organic waste to biogas plants is a more viable environmental and economic solution (AS2). Higher environmental benefits, e.g., mitigation of human health impacts and climate change, are obtained by transforming the existing waste combustion system into scenario (ii). Trade-offs in terms of increased marine eutrophication and terrestrial ecotoxicity result from moving up the waste hierarchy; i.e., from waste incineration to biogas production at wastewater treatment plants with anaerobic sludge digestion. Scenario (i) performs with lower energy efficiency compared to scenario (ii). Furthermore, when considering the uncertainty in the extra damage cost to the sewer system that may be associated to the installation of food waste disposers, scenario (ii) is the most flexible, robust, and less risky economic solution. From an economic, environmental, and resource efficiency point of view, separate collection and transport of biowaste to biogas plants is the most sustainable solution.