Nya metoder för återcirkulation av växtnäringsämnen från avfall

The aim of this seminar was to high-lighten methods being developed to recover plant nutrients from organic wastes and to present methods applied in society to improve recirculation of plant nutnents. Historically, recirculation of plant nutrients has been limited with the exception of certain Asian cultures. Even today, organic wastes are only partially recycled. In future, dissolved plant nutrients present in wastes can be recycled as inorganic fertilizers. Pros and cons of different recovery methods for P from wastewater are discussed.A new method to extract dissolved anions from liquid wastes through ion exchange was presented. To dissolve P in ash, alkaline or acid solutions can be used depending on whether the ash originated from combustion at high or low temperature. Using an alkaline solution to ash from high combustion temperature resulted in less dissolution of heavy metals. Dissolved phosphoms could be precipitated as sodium phosphate. Biogas digestion of slaughter house wastes and of residues from ethanolproduction mixed with cereals was tested at a plant in Linköping. Nitrogen contents in the dewatered biogas residues were higher than in animal slurnes while phosphorus and nitrogen contents were lower. Silage of grass and clover mixed with source separated organic household wastes will be digested at a new biogas plant in Västerås. The biogas residues will be separated into two fractions with ca 30 and 2-3% dry matter content and returned to cropland in the vicinity of Västerås. The fertilizer effect of biogas digested food wastes was tested in a field trial over 5 years. After digestion, the material was dewatered and two fractions were used as fertilizers. The fraction with a higher dry matter content resulted in lower yields Cereal yields amounted to 72-105% as compared to yields obtained with inorganic fertilizer. Different strategies for mixed digestion of animal slurries together with other organic wastes from the society were discussed. The state-of-the-art concerning composting of source separated householdand park wastes in Sweden was presented.