Background Protecting the Baltic Sea from eutrophication problems remains a high policy priority for the EU. Nitrogen concentrations in rivers flowing into the Baltic Sea have increased in recent decades. This problem has been linked to wastewater and agriculture. Wastewater treatment plants can remove >95 % of phosphorus, but usually Baltic Sea environments are already under pressure from eutrophication so an agreement is urgently needed to clarify which treatment process would have the best overall desired effect in terms of ecological and economic benefits. Such agreement should be informed by reliable evidence and work is required to confirm the most cost efficient ways to protect the long-term environmental integrity of lakes and rivers from the Baltic Sea area. Objectives The LIFE+ 2012 N-SINK project aims to demonstrate cost efficient wastewater treatment processes for nitrogen removal in order to reduce eutrophication of the Baltic Sea. In particular, it will demonstrate an innovative sediment filtration process for reducing the nitrogen load when wastewater nitrogen is released as nitrate. This will use the natural ecosystem service provided by the sediment. The basis for this innovation is that micro-organisms living in the sediment have an enormous capacity to reduce nitrate to nitrogen gas through denitrification. In this demonstration, wastewater released from sewage plants as a point source will be directed to a wider area near the sediment where denitrification takes place. With this new sediment filtering system the nitrogen load can be reduced in an economically and environmentally sustainable way. Outcomes expect to highlight how the efficiency of nitrogen removal could be increased, especially in small-medium sized WWTPs. A phased project approach will occur. The first phase involves planning and preparing the main demonstration plant. Two project sites will be selected for the demonstration and two sites will act as as control sites where the year-to-year variation in the denitrification activity will be measured. In the second phase, the project will demonstrate the changes and effectiveness of nitrogen removal in recipient waters by collecting long-term data from several WWTPs and from the downstream rivers and lakes. The target areas include Lake Vanajavesi and its drainage basin as well as River Porvoonjoki and its drainage basin. Finally, the economic and environmental costs of various actions to decrease nitrogen loading into lakes and rivers will be mapped and calculated using a state-of-the-art spatial optimisation framework model. The optimisation model is to be used for comparing the costs and effects of a number of nutrient abatement measures that can be carried out at different parts of the watershed and for determining the best combination of spatial and cost parameters. Such analysis will also allow the new sediment filtration measure to be compared (economically) against existing treatment measures. Expected results: The project will produce a series of reports covering the following: Water quality changes in recipient waters caused by the application of water purification techniques; The new alternative method for nitrogen removal that exploits a natural ecosystem service; The environmental effectiveness of nitrogen removal using the newest methods in measuring denitrification and N2O emissions; Scenarios of nitrogen fluxes in watersheds and the role of nitrogen removal at landscape level; and The socio-economic effects of nitrogen removal at landscape level.