On line sensors are widely used to monitor wastewater quality in plants, and their use to monitor water quality in sewer systems has also been proposed. Since Autumn 2016, DTU Environment has established a monitoring site at Ålebækken (Virum). On-line sensors have been installed at a Combined Sewer Overflow structure, which discharges to the Mølle Å creek when the sewer capacity is exceeded. The project aims at collecting high-resolution data, which can then be used to assess the impact of CSO discharge and to develop new effecting methods for sewer water quality monitoring. This project involves maintenance of the existing sensors, collection of water quality sample, and analysis of the collected data. Depending on the students, modelling work can also be included in the project.