En förstudie om näring- och energiåtervinning från källsorterande avloppssystem

Sammanfattning: The world's demand for nitrogen and phosphorus is increasing at the same time as high concentrations of the nutrients flow out from water treatment plants. The presence of nitrogen and phosphorus in nature is a contributing factor to eutrophication and thus damages ecosystems. Wastewater from households contains a large part of these nutrients and can advantageously be recycled instead of separated to nature. Stockholm Vatten och Avfall plans to build a source-sorting sewage system in Stockholm Royal Seaport to build a sustainable urban area in Stockholm's inner city. A source-sorting sewage system contains three separate sewage fractions: black water, food waste and greywater. The different sewage fractions have different character and can advantageously be treated separately to efficiently recycle nutrients and energy. The study presents a solution for the nutrient factory for Stockholm Royal Seaport. The solution has been developed through discussions, study visits and literature studies with priority given to recycling nitrogen, phosphorus and biogas. An energy, chemical and space requirement for the system solution is also presented. An operating cost analysis for the purchase of chemicals and sales of manufactured products is also presented. A calculation model was made in Excel where the solution was evaluated together with a sensitivity analysis that discusses the credibility of the presented solution. The nutrient factory consisted of separate digestion of food waste and blackwater in anaerobic membrane bioreactors. The aqueous phases are further treated by struvite precipitation and ammonia stripping. The digestate is dewatered and recycled to agriculture. The digestate from blackwater was treated by hydrothermal carbonization and wet oxidation where the aqueous phase is further treated by ammonia stripping and the solid phase was biochar, which was recycled in agriculture. Greywater was only used for heat exchange. The nutrient factory produced struvite crystals, ammonium sulphate, biochar, digestate and an aqueous phase in need of further treatment. The system recovered 84% phosphorus, 69% nitrogen and produced 20 cubic meters of methane gas per person per year. Ammonia stripping accounted for the greatest energy and chemical needs. Ammonia stripping was not profitable when calculating the purchase of chemicals and the sale of ammonium sulphate. Struvit crystal and biochar production were profitable if seen from the same premises. Further studies to find other nitrogen recovery processes are of interest if energy and chemical demands wish to decrease. Higher biogas production is believed to take place in the co-digestion of food waste and blackwater. On the other hand, co-digestion would impair the quality of the sludge. Treatment of greywater needs to be studied further for potential higher recovery of nutrients and energy.