The combined sewer overflow issue in the city of Berlin is becoming an increasing threat to the water quality of the surface water bodies, as the number and volumes of combined sewer overflow (CSO) events occurring per year may be on the rise due to climate variations among other aspects. For this reason, a case study was formulated to investigate the implementation of storage tanks in one of Berlin’s sub catchments, Wilmersdorf, in order to reduce the Occurrence of CSO to a once per year event on average. The investigation was made using InfoWorks Collection system (CS), one dimensional urban planning software used widely for sewer system modelling. The network of the Wilmersdorf catchment (majorly consisting of combined sewers, with small portions of separate rain and separate foul sewers) was modelled with the aid of InfoWorks. The implementation of tanks in the network was divided into two main parts: centralized and decentralized tanks. The centralized tanks addressed the issue of CSO, in order to reduce the CSO occurrence to once per year, firstly by using a short design storms representing a one year return period, to implement initial storage volumes, then this network was validated using rain series records for one year (1990) and for thirty years (1980-2010). The decentralized tanks were implemented at much smaller storage volumes compared to the centralized tanks, in localized locations to solve small surface floods in the separate rain sewer system, or to reduce the pollutant load Biological oxygen demand (BOD) of the CSO, by storing water from the separate foul system in the catchment. The results obtained for the centralized storage tanks show major reductions in CSO, with four centralized tanks implemented in central parts of the catchment. The target of once per year CSO event was achieved for the one year rain series (1990), but not for the thirty years rain series (1980-2010). Results for the decentralized storage tanks show reduction of surface flooding for the studied local areas in the catchment, with sometimes a reduction of surface floods also downstream of the targeted areas. On the other hand, the pollutant load (BOD) was reduced by negligible amounts with decentralized tanks at the studied separate foul system locations, with results showing that the overall BOD load reduction in the overflow volume is also accompanied with CSO overall volume reduction.