Potential environmental impacts from fish sludge biogas production in Norwegian salmon farming
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Salmon is generally considered an environmentally friendly food to consume, however salmon escape, sea lice and nutrient leaching into the ocean are serious environmental problems of the present day open net pen farming systems. Adopting closed containment systems would alleviate those problems as well as produce substrate for biogas production. In this master thesis, the potential environmental benefit of fish sludge biogas production was estimated using the methodology of life cycle assessment (LCA). The environmental potential was estimated for salmon producers in the surroundings of a newly stablished biogas plant Biokraft in Skogn, Sør-Trondelag, and well as on the national level for the Norway of Norway. The BioValueChain model programmed in the software SimaPro was used to estimate the environmental impacts of each life cycle stage focusing on global warming and eutrophication potential. For the modelling, I considered that biofuel would replace diesel and the digestate would replace mineral fertilizers. The results showed a negative global warming potential of -206.7 kg CO2 eq, primarily due to the avoided emissions from the substitution of diesel which were - 258 kg CO2 eq. The negative global warming values are interpreted as a positive environmental impact. The largest positive emissions came from the transportation phase and fish sludge transport distance was more important than the transport of the digestate to the fields as fertilizer. If obtaining a positive environmental impact, fish sludge transportation distance should be kept short and less than 283 km and digestate fertilizer should be transported no more than 541 km. Sensitivity analyses were also done on the parameters that were most uncertain and could influence the model results. The substrate dry matter content can change the outcome of the biogas production because of the transportation stage as well as the potential biogas production is an important parameter. The environmental impact of eutrophication was 1.1.98 kg PO4-3 for the reference scenario, however, it was difficult to determent if this is an environmental benefit because there is not real control assessing the LCA whithout biogas production. The environmental potential of biogas production from fish sludge from the major salmon producers in the surrounding of Biokraft (Marine Harvest, Lerøy Midnor and Nova Sea) was estimated to be 3 600 ton CO2 eq per year, while the estimate for all of Norway assuming sludge is transported 250 km was 9 323 ton CO2 eq per year. In conclusion there is a significant potential of biogas production from fish sludge to reduce the environmental impact associated with salmon fish farming in Norway.