Analysis of the distribution of plant nutrients in a biogas production system : a look at VH Biogas, co-digesting manure from several farms

Sammanfattning: This study examined nutrient flows in a system where 20 farms deliver manure to a biogas plant for digestion, and then receive digestate back. Some other substrates were also anaerobically digested at the plant e.g. slaughter residues. The farms had different types of production: Organic and conventional dairy and conventional pig. The aim was to analyse the re-distribution of plant nutrients this creates. Biogas pro-duction is of interest since it has the potential to reduce the use of fossil fuels and potentially also greenhouse gas emission. The use of digestate as fertilizer has the potential for a more even distribution of plant nutrients across the landscape. Data was taken from the biogas production plant and then analysed and organized to be able to see what the different types of farms contributed with in terms of plant nutrients. The results showed that there is a redistribution of plant nutrients between the farms and one type of farm can lose or gain nutrients compared to the other types of farms. The “winner” of the different types of farms seemed to be the organic dairy farms as they gained mineral nitrogen, whereas the others lost mineral N. The organic dairy farms lost phosphorous and potassium however, so it is dependent on each in-dividual farm what plant nutrient is valued the most. Conventional pig farms gained K whereas the dairy farms lost K. All types of farms lost P which was probably due to the solid fraction of the digestate being separated and not included in the digestate going back to the farms. The study also showed that the substrates other than manure had an influence on the plant nutrient content of the digestate. Slaughter residues stood for a lot of the total ingoing organic nitrogen to the biogas plant.