N-SINK - Reduction of waste water nitrogen load: demonstrations and modelling

 Director Lauri Arvola, demonstration work package leader Marja Tiirola, EU Life+ Environment project 1.8.2013-31.7.2017

In this project we demonstrate the cost-benefit value of the current wastewater treatment and nitrogen removal processes, for the first time by integrating the hydrological and chemical data and mass-balance calculations with the measured values of denitrification - the missing nitrogen sink. The most advanced stable isotope techniques will be used in the measurement allowing us to get accurate values of how much we can depend on the ecosystem services in the nitrogen removal in the current systems. Monitoring and modelling results will be used, for the first time, to evaluate the denitrification in natural system and enhanced by the wastewater nitrate.

 Sediment has a remarkable capacity to reduce nitrate load to nitrogen gas through denitrification. Denitrification is most importantly related to nitrogen concentration above the sediment and water residence time in the lake, as previous studies have consistently shown. Therefore we claim that spatial optimization of the wastewater discharge would be an efficient way to reduce nitrate-based nutrient load in the environment now, when most new environmental permits for new or reconstructed treatment plants have a nitrification sanction in Finland and many Baltic countries which care for the sensitive Baltic sea ecosystem. Currently the discharge of purified waste water has mostly been implemented using a one-point outlet system, either through a drain or a pipe, and many times the water is further mixed to the productive water layers of the lake. A new sediment filtration approach is suggested in which the nitrified water will be in contact with the reducing microbes of the sediment for longer period, which will result in the efficient denitrification of a portion of the nitrate load. Compared to the point outlet method/practice, nitrate will be spread to the sediment-water interphase, instead of mixing with the productive water layers. Nitrate is also expected to temporarily increase the redox-potential of the sediment layers, thus improving the quality of near-bottom layers and preventing phosphorus release. N2O emissions are predicted to be minor compared to the forced denitrification in wastewater treatment plants, where the N2O gas can be easily discharged to the atmosphere in the high water circulation speed and mixing.

 In the implementation action we demonstrate this new technical innovation for nitrogen removal –N-SINK sediment filtration. This technique is based on the observed field results and small-scale laboratory experiments, where we have shown the relationship between nitrogen concentration and denitrification in the above-sediment layers. The set of demonstration actions will be the first full-scale trial to test the sediment filtration technique, and it is unique both in its scale (will be tested in wastewater treatment utilities treating up to 20000 m3/day) and top-scientific monitoring techniques. This method would be a promising and minimum-cost way for the reduction of the nitrogen load by the ecosystem services itself. More information on project pages  http://www.helsinki.fi/lammi/NSINK/