There have been rapid land use change both in the center and in the outskirts of Addis Ababa.rnHowever, the existing conventional wastewater drainage system has been in use in therncentral part for around 40 years while sewer system has not yet developed in the expandingrnresidential areas. In view of that, this research was conducted to identify how ineffectivernexisting sewer system and onsite wastewater management facilities are contributing for therndischarge of untreated wastewater into rivers in addition to assessing inflow and infiltrationrnquantity in the existing sewer. For these purposes, Senga Tera and Tulu Dimtu study areasrnwere selected to represent urban densification and urban expansion respectively. Thernexisting land use plans of both areas, and as-built data and sewer master plan for the existingrnsewer system were collected and reviewed. Flow rate were manually measured at selectedrnmanholes on field for model calibration in addition to analyzing six months of ArcGIS waterrnuse dataâ€™s. Moreover, the number of stories and floor areas of high-rise buildings which arernnow under construction were surveyed in Senga Tera. Similarly, field research was conductedrnto identify the existing onsite wastewater management facilities inTulu Dimtu. Then, steadyrnstate analysis and modeling for the existing sewer system and for the proposed nonconventionalrnrnsewer system in Tulu Dimtu were conducted using Sewer GEMS V8i.rnAccordingly, it was revealed that 87% and 53% of the existing manholes in in Senga Terarncould be surcharged and overflowing due to rapid land use change during wet weatherrnseason respectively. It was also found that around 15.59 l/s of wastewater was crossrnconnected into the nearby storm water drainage system. As a result, overflows and leakagernthrough the walls of the existing cesspools in Tulu Dimtu and Sanitary Sewer Overflowsrn(SSOs) from manholes and sewer cross connections in Senga Tera area have been found tornbe factors for the discharge of untreated wastewater first into the nearby storm water drainagernsystem and then outfall into nearby rivers. Moreover, the quantity of average inflow andrninfiltration which was estimated in Senga Tera was around 71%. This in turn could contributernto induce SSOs on the existing sewer system in addition to become a factor for the hydraulicrnoverloading of the existing Kality Wastewater Treatment Plant. Therefore, it is recommendedrnto enhance the existing sewer system through master plan review and to use non-rnconventional sewer system instead of cesspools in the expansion areas.