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The world is currently heavily dependent on fossil sources and the use of fossil fuels leads to increased global warming. One of the solutions to this environmental problems is biogas. Biogas is produced through anaerobic digestion of organic material and can be used for electricity or heat production,upgraded and used as vehicle fuel or induced in the industry.Lillerudsgymnasiet is a school that has farm animals, food waste and crop residues. The school is interested in building a small-scale biogas plant for biogas production where they can digest different substrates to produce electricity or heat for their own use.The purpose of this study is to discover the biogas potential during co-digestion of different substrates that are available in agriculture. The goal is to investigate different scenarios to be able to give a recommendation on how Lillerudsgymnasiet should co-digest its substrate in order to optimize its biogas yield.Two different digestion experiments were carried out in an AMPTSⅡ-reactor, where three different cases could be digested per experiment. The substrates were digested in different combinations and the substrates investigated were horse manure with softwood shavings, horse manure with straw, cow manure, pig manure, food waste, silage and crop residues. Samples were also taken for total solids, volatile solids, pH, ammonium, volatile fatty acids and total organic carbon before and after digestion. The amount of methane gas per kilogram substrate and per year was investigated and calculated for the various substrate mixtures.The result shows that the highest methane yield per gram VS (organic material) is obtained when the substrates are mixed and digested based on an optimal C/N value, the yield is then 218 Nml/g VS. The second highest yield is obtained when the substrates are mixed and digested in the same ratio as they are available at Lillerudsgymnasiet and horse manure with softwood shavings has been excluded, the yield is then 216 Nml/g VS. The lowest methane yield per gram VS is obtained when the substrates are mixed and digested in the same ratio as is available at the school but with a maximum amount of horse manure with straw. The methane yield is then 156 Nml/g VS. The result also shows that the highest biogas yield per kilogram of substrate, at Lillerudsgymnasiet, is obtained when the substrates are mixed to obtain an optimal C/N value, the methane gas yield is then 48 NL/kg substrate. The highest amount of methane gas per year, at the school, is obtained when the substrate is digested in the same ratio as is available at the school. The amount of methane gas is then approximately 75,200 Nm3/year.The amount of metals (chromium, copper, zinc, nickel, mercury, cadmium and lead) in the digestate is far below the maximum permitted content for the digestate to be considered suitable as biofertilizer, according to SPCR 120 certification.The result shows that approximately 10% of all manure from horses, pigs and cows in Sweden is digested into biogas, which means that there is a great potential to increase biogas production from agriculture in Sweden. In China, 19% of the biogas production potential is used in rural areas, which shows that there is an opportunity to increase biogas production there as well. In conclusion, Lillerudsgymnasiet is recommended to digest its substrate in the mixture obtained when the C/N content is optimized, if the school can have access to a larger amount of food waste. Otherwise, the substrate should be digested in the ratio it is available at Lillerudsgymnasiet. Finally, the school is recommended to exclude the horse manure with softwood shavings from the decomposition |
