Lifespan and environmental efficiency of marine outfall systems commonly used for disposal of domestic wastewater in many Mediterranean countries depend on the effective hydraulic performance. Many of these outfall systems show a low hydraulic performance, especially those with thick-walled diffuser pipes and sharp-edged ports in pipe wall (SEWWT). System conversion to bell-mouthed ports increases their hydraulic performance and service lifespan. In this study old-style outfalls with sharp-edged ports were newly designed according to various flow rates and taking into account the characteristics of the Black Sea. Then, these systems were converted to bell-mouthed ones and the differences in flow rates of both systems were studied under the aspect of increasing the system performance.