Life cycle assessment of grain production using source-separated human urine and mineral fertiliser

摘要

Source-separation of human urine is one promising technique for closing the nutrient cycle, reducing nutrient discharge and increasing energy efficiency. Separated urine can be used as a valuable fertiliser in agriculture, replacing mineral fertiliser. However, a proper handling of the urine at farm level is crucial for the environmental performance of the whole system. This study started from an agricultural point of view, demonstrating how grain production systems using human urine might be designed. The main objective was to evaluate the consequences on environmental impact and resource management when human urine replaced mineral fertiliser in arable farming. Production of winter wheat and spring barley when only mineral fertilisers were used was compared to a scenario where a combination of human urine and mineral fertilisers was used. The method for assessing the two different scenarios was Life Cycle Assessment (LCA), and the functional unit was 1 kg of grain. In the systems analysis, a change-orientated perspective was used whereby all major changes in the agricultural system using urine (the urine-separating scenario) were taken into account, compared to the conventional scenario. When urine is separated from the remaining wastewater, the production of drinking water as well as the wastewater handling is affected. These changes were taken into account through subtraction of the burdens avoided when separating urine. Production of capital goods, e.g. storage tanks, was also included in the urine-separating scenario in those cases where differences between the scenarios appeared. The results obtained were quite similar as regards the two grain production systems. Differences appeared instead when comparing the conventional scenario to the urineseparating scenario. For both scenarios, most of the energy required was fossil fuel. The use of fossil fuel was slightly higher in the scenario using human urine as fertiliser, but electricity consumption was higher in the conventional scenario. Whether a urine-separating scenario will decrease the energy usage depends on many factors, and is not self-evident. The construction phase might make a considerable contribution and the sense in which the existing water and wastewater system is affected will also be important. With the assumptions made in this study, the urine can be transported more than 40 km one way without exceeding the total energy used in the conventional scenario. However, minimising transports is just one of several key issues from an energy point of view. The contribution of greenhouse gases, expressed as GWP, from the two scenarios was of the same magnitude, although slightly less from the urine-separating scenario. For both scenarios, nitrous oxide originating from soil emissions gave the highest contribution. The difference in contribution to eutrophication was considerable between the two scenarios, due to the avoided emissions of eutrophying substances in the urine-separating scenario. Which scenario contributed most to acidification depended on in what sense nitrogen compounds contribute to acidification. A considerable part of the phosphorus required as mineral fertiliser can be replaced by phosphorus in human urine. When half of the nitrogen required in winter wheat was applied as human urine, approximately 40% of the phosphorus required came from the urine. Guaranteed quality is of major importance when discussing the use of human urine on arable land. The composition as regards heavy metals, organic pollutants, pathogens and plant nutrients must therefore be guaranteed. The level of heavy metals in human urine is very low. The contribution of e.g. cadmium is even lower than in some "cadmium-free fertilisers". The hygienic risks can be almost eliminated with adequate storage. However, the risks related to pharmaceuticals in urine must be further investigated.