Measurement of nitrous oxide emissions at the Käppala Association

Käppala Wastewater Treatment Plant (WWTP), located on Lidingö in Stockholm, is Sweden's third largest wastewater treatment plant and receives municipal wastewater from approximately 554,000 population equivalents (2024). The plant is located underground, which means that all process air can be collected and directed to a chimney.

Nitrous oxide emissions via the chimney have been measured since 2018 and the data is used for e.g. environmental reporting. The measurements are made using an GMS 810 from Sick that measures both N2O and CH4. The N2O emissions correspond to an average of 1% of the total amount of nitrogen that reaches the plant via the wastewater, but the emissions vary over the day and over the year. The average concentration measured in the off-gas air from the whole plant is around 3 ppm. Nitrous oxide emissions contribute on average to 35% of Käppala’s total climate emissions. Methane emissions are also significant, accounting for 45% of total climate emissions.

To increase understanding of the emissions, new sensors were installed in 2024 to measure nitrous oxide concentrations in the water phase at three points in one of a total of eleven biological treatment lines. They were placed in the pre-denitrification, nitrification and post-denitrification basins. The sensors are from Unisense and are of the type N2O wastewater system.

The Käppala Association is participating in the DEEPNOC project, which is partly funded by Svenskt Vatten Development Fund, and hopes to find control strategies that can reduce emissions at the Käppala WWTP.

Several new activities were launched in 2025. A CFD-N2O modelling of the investigated biological treatment line was carried out by the Belgian company AM-Team. Among several things, the aim was to investigate the best placement of nitrous oxide sensors in the water phase and investigate the N2O pathways in the basin. A new gas analyser for N2O will be installed in the ventilation duct that collects all exhaust air from the already investigated biological treatment line. Together with the doctoral student Christoffer Wärff, working at RISE Research Institutes of Sweden, the Käppala Association has launched a multi-year project to build a digital twin to investigate whether it can be used to reduce nitrous oxide emissions from the Käppala WWTP. The project is called DT-N2O and are partly financed by Smart Built Environment.

Belongs to project

Reduce the emissions of nitrous oxide from wastewater treatment