Nitrous oxide formation in membrane bioreactor (MBR) systems : Occurrence and Mitigation of N₂O in the MBR Line: A Comparison with the Activated Sludge Line at Henriksdal Wastewater Treatment Plant, Stockholm

Sammanfattning

Nitrous oxide (N₂O) emissions from wastewater treatment plants represent a significant contribution to greenhouse gas emissions and are strongly influenced by biological nitrogen removal processes and operational conditions. This thesis investigates the occurrence and drivers of N₂O emissions at Henriksdal wastewater treatment plant in Stockholm, Sweden, through a comparison between a membrane bioreactor (MBR) line and a conventional activated sludge (CAS) line. Two monitoring scenarios were evaluated: Scenario A representing the baseline sensor placement used at the plant, and Scenario B involving relocation of sensors to assess the influence of monitoring location and potential emission hotspots. Continuous water-phase N₂O measurements were conducted in both treatment lines. N₂O emissions were quantified using gas–liquid mass transfer modelling based on the Unisense methodology and expressed as emission factors relative to influent nitrogen load to enable direct comparison between the treatment lines. The monitoring was complemented by grab sampling of key nitrogen species, including ammonium, nitrite, nitrate, phosphate, and volatile fatty acids. In addition, historical data from the CAS line were analysed, including both gas-phase and water-phase N₂O measurements together with corresponding operational parameters, to evaluate their impact on emission dynamics. Under Scenario A, the emission factor for the CAS line was higher than for the MBR line, and the CAS line also exhibited greater temporal variability and higher emission peaks. Following sensor relocation in Scenario B, substantially lower emission factors were obtained for both treatment lines, demonstrating the strong influence of sensor placement on calculated emissions. Variations in ammonium load, dissolved oxygen availability, and aeration conditions were identified as key drivers of N₂O formation. Finally, an energy assessment showed higher specific energy consumption for the MBR line compared with the CAS line, indicating differences in energy demand between the two treatment technologies.