Sensitivity study of various design parameters on separation efficiency of pipe combined sewer overflow using CFD

Abstract

Combined sewer overflows (CSOs) represent a significant challenge in urban drainage systems, particularly during heavy rainfall events when their hydraulic capacity is exceeded. Effective separation of suspended solids in CSO structures is essential to reduce the pollutant load discharged into receiving waters. This study investigates the sensitivity of separation efficiency to various design parameters of a tube CSO using computational fluid dynamics (CFD). The chamber, designed as a part of a multifunctional hydraulic object of a real-world sewer network project, was analysed using numerical modelling, employing the Volume of Fluid (VOF) model and Lagrangian particle approach. The influence of two key design aspects, the total chamber length and overflow slot geometry configurations, was examined under different flow rate conditions. The results show a strong and consistent correlation between increased chamber length and improved separation efficiency, confirming the importance of sufficient residence time for directing the solids to the pumps. In contrast, modifications to slot geometry resulted in only minor and inconsistent changes in performance, suggesting that its impact may be dependent on specific flow dynamics and chamber configuration.