Improving life cycle inventories for nitrogen and phosphorus in sewage sludge used in agriculture
Examensarbete för masterexamen
Industrial ecology (MPTSE), MSc
When treating wastewater, large amounts of sewage sludge are generated. Sewage sludge contains substances that may be of use, e.g. nutrients such as nitrogen and phosphorus, but also substances that are harmful such as pathogens and heavy metals. In order to reuse the nutrients in sewage sludge it can be used as a fertilizer on agricultural fields in Sweden but there are regulations that specify how the sludge must be hygienized before use. One way of hygienizing sewage sludge in Sweden is by storing it. However, this method will probably not will be allowed as an hygienization method in the future, since the Swedish Environmental Protection Agency has not listed it as one of the hygienization methods they recommended to be acceptable in future legislation on the use of sludge in agriculture. A way of assessing the environmental impact when sewage sludge is stored and applied to agricultural fields as a fertilizer is by using life cycle assessment (LCA). Previous reviews of LCAs performed in this field have shown a variety when it comes to which flows that are included in the life cycle inventories, and there is a need to improve the data quality when the flows are quantified. Therefore, this study was made to investigate if the life cycle inventory for nitrogen and phosphorus compounds could be improved and if there are differences in the characterization of nitrogen and phosphorus compounds in different methods, and what LCA practitioners needs to be aware of when assessing this kind of system. The three impact categories chosen were global warming potential, acidification potential and eutrophication potential. The study was performed by literature searches and by personal contact with experts on nutrients and the practice when using sewage sludge in agriculture in Sweden. Also, an LCA case study was performed to illustrate how choices of inventory data and LCIA methods affect the environmental impact results. The study concluded that sewage sludge storing and application to agricultural fields have a significant environmental impact regarding the three impact categories and the used characterization methods. The literature review combined with the personal contact and LCA case study indicated that there are many factors that affect the emissions of nitrogen and phosphorus from sewage sludge, e.g. soil type, soil chemistry, pH, how the sewage sludge has been treated, when the sewage sludge is incorporated into the soil and weather conditions. Therefore, it was shown that it is important to do a life cycle inventory for the specific system and not to use generic numbers of emissions since it can vary a great deal.
Sewage sludge , Life cycle assessment , Nitrogen , Phosphorus , Mineral fertilizers , Agriculture , Storage