PROJECT DESCRIPTION
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 (WWTPs) can remove >95% of phosphorus, but commonly just 70% of nitrogen. Hundreds of WWTPs are currently waiting for a decision on whether they are required to invest in new nitrogen-removal processes, and many of these WWTPs are located in remote inland areas. Reducing phosphorous had previously been the main aim of the WWTPs and a shift in their emphasis towards nitrates would require significant changes to the overall approach (as efficient nitrogen treatment systems may require using additional phosphorus as well as substrate). Furthermore, nitrate removal in wastewater treatments may increase the risk of enhanced nitrous oxide emissions and generate new environmental problems.
Baltic Sea environments are already under pressure from eutrophication so an agreement is urgently needed to clarify which treatment process perform best ecologically and economically. Such agreement should be informed by reliable evidence and research must be carried out into 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 aimed to demonstrate cost-efficient wastewater treatment processes for nitrogen removal in order to reduce eutrophication of the Baltic Sea. In particular, it planned to demonstrate an innovative sediment filtration process for reducing the nitrogen load when wastewater nitrogen is released as nitrate. This process would 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 the planned demonstration, wastewater released from sewage plants as a point source would be directed to a wider area near the sediment where denitrification takes place. With this new sediment filtering system, the nitrogen load would be reduced in an economically and environmentally sustainable way. Outcomes were expected to highlight how the efficiency of nitrogen removal could be increased, especially in small-medium sized WWTPs.
A phased project approach was planned. The first phase involved planning and preparing the main demonstration plant. Two project sites would be selected for the demonstration, and two sites would act as control sites where the year-to-year variation in the denitrification activity would be measured. In the second phase, the project aimed to 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 included 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 would be mapped and calculated using a state-of-the-art spatial optimisation framework model. The optimisation model was to be used for comparing the costs and effects of a number of nutrient-abatement measures that could be carried out at different parts of the watershed and for determining the best combination of spatial and cost parameters. Such analysis would also allow the new sediment filtration measure to be compared (economically) with existing treatment measures.
RESULTS
The LIFE+ 2012 N-SINK project showed that wastewater discharge from the WWTP increases the denitrification rate in the sediments of the recipient water bodies, and that this natural ecosystem service can be utilised in nitrogren removal and increased by the spatial optimisation (diffusor pipe) of the wastewater discharge. The project moreover demonstrated that the tested method does not interfere with the working of the WWTP, nor cause harmful effects to the receiving ecosystems. Monitoring at the project demonstration sites highlight that the method is particularly suitable for smaller WWTPs.
Measurement samples were found to vary greatly according to the time of year, as well as the functioning of the WWTP. Problems with nitrogen removal at the WWTP caused a heightened activity of the sediment microbes. The beneficiary believes the process could be further enhanced by installing the diffusor pipe in a way that better follows the forms of the lake bottom. The diffusor pipe system was nevertheless shown to be a cost-effective solution for nitrogen removal at the WWTPs. (The total cost of reducing 100 tonnes of nitrogen load to Lake Vanajavesi is €380 000 per year. The costs increase substantially if the target is increased – e.g. cost of reaching 200 tonnes of nitrogen rises to €1.2 million).
Significant increases can be added by introducing agricultural measures too. The beneficiaries thus concluded that water protection policies aimed at reducing significant amounts of nitrogen load should adopt a multi-sectoral approach, i.e. policies concerning both agriculture and wastewater treatment. The modelling includes some uncertainties, and the project team emphasise that phosphorous fractions should be included in future modelling given that some nitrogen reduction measures can lead to an increase in phosphorous loading.
Overall, the project measures have a great potential to decrease nutrient loading, as well as reduce energy and resources in wastewater treatment and agriculture. They therefore have a role to play in informing national and EU environmental and agricultural policy, while helping achieve the aims of the Water Framework Directive. Several representatives from public authorities, including the ministry of environment and governmental agencies, were invited to join the initiative’s steering and advisory board, ensuring awareness of the project conclusions. A close relationship with WWTPs in Finland was also established during the project. The demonstration method installed on the Petäjävesi site will remain in use by the WWTP.
Further information on the project can be found in the project's layman report (see "Read more" section).
