• 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.

Abstract Offshore cultivated seaweed (or macroalgae) used as feedstock for biobased products is a rapidly developing research field for an innovative new industry. A model system including seaweed cultivation, biorefining and usage phases of the products is assessed on the basis of real experimental studies. The aim is to provide a dynamic model of the biogenic carbon cycle with a view to carbon neutrality of future macroalgae-based biorefinery systems. The model takes a holistic view of the system, including all processes directly and indirectly connected to the biorefinery in a cradle to cradle perspective. In the biorefinery, the biomass is converted to ethanol and the solid protein residue is isolated and used as an ingredient for fish feed. The aqueous extract enriched in minerals and organic nutrients is used as liquid fertilizer. Annual cultivation and processing of 1 ton of seaweed (dry weight) evaluated over a time horizon of 100 years results in a net reduction of 9.3 tons of atmospheric carbon (34 ton CO2). From one cultivation cycle, i.e. 1 ton of seaweed (dry weight), a net reduction of 0.035 tons of atmospheric carbon (0.13 tons of CO2), assessed 100 years later, is achieved. The main processes providing climate mitigation are carbon assimilation by growing seaweed and carbon retention in soil. The model can be used to more accurately quantify climate services provided by green industries, thus strengthening Life Cycle Assessment as a decision-support tool for sustainable management of offshore cultivated seaweed. The model is flexible since it can be adapted to different international case studies by entering local parameter values.

Abstract Eutrophication is an environmental problem in a majority of shallow water basins all over the world. The undesired macroalgae has been proposed as a biomass resource for bioethanol production and we have analysed the environmental sustainability of two case studies: Orbetello Lagoon (OL), Italy, and Koge Bay (KB), Denmark. Today, macroalgae are collected and stored in landfills to provide a solution for the excess production. An emergy assessment revealed that the main environmental support for macroalgae growth relates to water in both case studies. In OL, rain represents 51% of the emergy use, and in KB runoff from agricultural land constitutes 86%. The environmental support needed for producing one Joule of bioethanol is somewhat more than for a number of other bioethanol feedstocks being 2.12 × 106 solar equivalent Joules (seJ) for OL and 2.56 × 106 seJ for KB. However, a high percentage of the environmental support comes from local renewable flows being 40% for OL and 88% for KB. The difference between the two case studies is partly due to the contribution of energy from waves, which plays an important role in carrying macroalgae towards the coast in Koge Bay. Energy-wise, one J of fossil energy is required directly or indirectly to produce 0.09 J of bioethanol for OL or 0.44 J of bioethanol for KB, i.e. the energy return on (energy) invested (EROI) is less than 1. An alternative scenario was developed in order to investigate improvements of system efficiency. This was analysed with the full-requirement approach as well as with a marginal-requirement approach accounting only what the bioethanol production requires of additional processes, i.e. mainly transportation and conversion of the macroalgae in a biorefinery facility which is assumed to be situated close to an existing industry producing waste heat. Both emergy and EROI analyses showed that only a relatively small amount of resources has to be added to the existing system to produce the bioethanol, e.g. the EROI increased to above 1 in both systems. With the marginal approach, macroalgae may be appreciated as a resource for bioethanol production instead of considered as an environmental problem.

Abstract 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.