The wastewater system in the Netherlands comprises approximately 100,000 km of sewer pipes and more than 350 wastewater treatment plants (wwtp). The system collects and treats the majority of domestic and industrial wastewater. A small amount of untreated wastewater, however, is occasionally discharged to rivers and lakes, often resulting in a deterioration of surface water quality. Responsible authorities (water boards and municipalities) attempt to reduce these discharges to abide by new European regulations. In the search for effective measures knowledge is required on the principle in-sewer processes. Knowledge on hydrodynamics is readily available; knowledge on water quality processes is as yet insufficient. This dissertation considers two novel monitoring techniques that can be applied in sewer systems and that measure water quality parameters. The first technique (UV/VIS spectroscopy) has been applied to study the wastewater arriving at the wwtp in Eindhoven. Results show a large variation in the amount of pollutants originating from the sewer system and arriving at the treatment plant. Especially during large storm events after a long dry period (the occurrence of which might increase according to climate change scenarios) an extreme amount of pollutants can cause a disruption of wwtp operation. The second technique (fiber-optic DTS) measures wastewater temperatures along a sewer section. It has been successfully applied to locate illicit connections in a stormwater system. The application in combined sewer systems allows a detailed study of any process that influences in-sewer temperatures such as discharges of wastewater from house-connections and the inflow of stormwater run-off.