The purpose of this study was to compare the energy use and climate impact of extracting phosphorus (P) from sewage sludge, compared with mineral phosphorus from mining. The two processes that are studied are struvite precipitation and extraction of phosphorus from sludge ash (AshDec). We also looked at the potential recycling rate of plant nutrients in the different systems.

In the study, we have calculated the environmental impact of introducing the processes for P-recovery, related to a conventional sewage treatment plant with chemical precipitation. It is therefore only the differences that arise in comparison with the reference plant that are included in the calculations. You can think like this: there is an existing sewage treatment plant, where you introduce P recycling. All changes in environmental impact that arise in connection with the investment in this new process are burdens the new P-product.

The results show that the AshDec scenario has the best recovery rate of phosphorus and energy balance, while the struvite scenario shows the best return rate of nitrogen and the lowest climate impact. However, the sensitivity analyzes show that the results are strongly influenced by different assumptions. For the AshDec scenario, assumption about nitrous oxide emissions from incineration of sludge as well as the handling of energy surplus is crucial. For the struvite scenario, assumptions around the reconstruction of the sewage treatment plant to biological phosphor cleaning is of great significance, especially if the biogas production is affected.