Using LCA and LCC in Planning Industrial Symbiosis : A study of the handling of sewage sludge in Malmö, Sweden

Abstract: Sewage sludge is currently being disposed by spreading it out on fields, an action that recycles important nutrients such as phosphorus, but also leads to heavy metal contamination. With impeding regulation changes, possibly making it harder or impossible to keep current practice, waste water treatment plants are reviewing their options. One solution could be mono-incineration with phosphorus recovery. However, to make the sludge have a heating value high enough to avoid support fuel it needs to be thermally dried, which requires large amounts of heat. Moreover, large investments would have to be made, creating a more complex system than the current one. Industrial symbiosis could be the solution for making it both more economically and environmentally sustainable and possible, as it is possible to utilise waste heat for the drying, and collaborating with a waste incineration company to incinerate the sludge. Setting up an industrial symbiosis exchange is not always simple; knowing who benefits from what, and who should pay for what investment can be complicated. Moreover, it is often assumed that industrial symbiosis exchanges are environmentally sustainable, but it is not always the case. To better understand how costs should be allocated, and how exchanges should look to be both economically and environmentally sustainable, the methods life cycle analysis (LCA) and life cycle cost analysis (LCC) are suitable to use, as they allow a full view of the system, which can be broken down into different processes. The aim of this study is to see how LCA and LCC can be used on a planned symbiosis project to assess environmental and economical impacts. The results that were found was that using waste heat instead of primary produced heat was not necessarily better, both economically and environmentally in the categories acidification, eutrophication, and global warming potential. If the drying could take place solely during warmer months, through use of storage, then the heat could be produced through waste incineration, creating electricity to sell and replace marginal electricity. There was no clear cut answer to which scenario was better of the thirteen looked at in this study, as different scenarios were better in different categories, which proved the necessity of doing an LCA and a LCC, or similar methods. Moreover, the larger investments were not always the most profitable, even in the best economical scenario, showing the risk of unequal cost distribution. Similarly, the best scenario to avoid global warming potential involved using storage of dried sludge, increasing emissions for the one responsible for the storage, whilst decreasing emissions for incineration substantially. In summary, performing a LCA and a LCC on a planned symbiosis exchange can both show how different choices affect different categories, and help mitigate risks of uneven distribution of both costs and emissions.