Kväverening vid bergarbeten i Forsmark : Utvärdering av kvävereningssystemets funktion under utbyggnadsfasen av slutförvaret för kortlivat radioaktivt avfall

Sammanfattning

During the construction and operation of the final repository for short-lived radioactive waste (SFR) at Forsmark, nitrogen compounds are generated in drainage and leachate water, primarilyoriginating from undetonated explosives. If discharged, nitrogen compounds, mainly nitrate andammonium, may affect sensitive aquatic environments through eutrophication and toxic effects.To comply with the environmental permit requirement of 70% nitrogen removal, a nitrogentreatment system including a nitrifying sprayfilter and three denitrifying bioreactors wereinstalled. This study aims to evaluate the effectiveness and performance of the nitrogen treatmentsystem, focusing on the nitrification and denitrification processes, as well as to assess if theestablished nitrogen removal target is technically and environmentally relevant. The evaluation isbased on operational conditions and monitoring data collected between 2025-03-20 and 2025-10-22, supplemented with a tracer test to compare theoretical and measured hydraulic retentiontime. Results show that the average nitrogen removal rates in the bioreactors were 3.9-6.8 g NO2-+NO3-d-1 m-3. The sprayfilter exhibited a declining ammonium removal efficiency, with a maximum of 27%, likely limited by phosphate deficiency. The combined nitrogen removal across the sprayfilterand bioreactors, expressed as total nitrogen, was 81%, thereby meeting the preliminary targetduring the period. Correlation analyses revealed bioreactor-specific correlations betweenremoval efficiency (%) and DOC and PO43--P. For the isolated bioreactors, the removal efficiencyadditionally correlated with temperature and water flow. Temperature, along with nitrate and totalnitrogen, showed strong positive correlations with nitrogen removal rates in all bioreactors. The tracer test indicated that the actual hydraulic retention time in BR2 was substantially shorterthan the theoretical value, suggesting the presence of preferential flow paths. Overall, the studydemonstrates that the nitrogen treatment system has significant potential but requires continuedadjustment and monitoring to further evaluate and optimize treatment performance, while alsoproviding important insights into hydraulic constraints and biological processes relevant forsystem optimization.