Illicit connections to storm sewers have a negative impact on surface water quality in Berlin. To improve water quality in the city, the Berliner Wasserbetriebe intends to locate and eliminate these illicit connections. The current approach using visual and CCTV inspection to pinpoint the exact locations of illicit discharges has proven insufficient: only a limited number of wrong connections have been found this way over the last years. To support utilities in obtaining a cost-effective approach for the search of illicit connections, a stepwise approach has been developed and tested in a storm sewer in Berlin as part of the DWC research project (digital-water.city). This paper presents the approach and the obtained results. • The Berlin Fennsee is suffering from surface water quality issues probably related to illicit connections to storm sewers in the area. • In a stepwise approach, a network of electrical conductivity (EC) sensors was first deployed to identify hotspot areas that likely contain illicit connections. • In a second step, Distributed Temperature Sensing (DTS) was used in one hotspot area to identify the exact location of an illicit connection with multiple discharges per day.