1D unsteady air ventilation model for sanitary sewer systems

Understanding transient sewer airflow is important for odour management in urban drainage systems. Variations in sewage flow can induce unsteady air motion in pipes and ventilation through manholes. Previous three-dimensional (3D) CFD study has demonstrated the significance of transient air ventilation in sewers, but its computational cost limits the applications in sewer systems. To address this gap, a one-dimensional (1D) unsteady two-phase model is developed for sewer systems using a decoupled strategy, in which the air and water phases are solved separately to accommodate their different timescales. The model accounts for pressure differentials, interfacial drag, and wall friction, allowing efficient simulation of transient airflow and manhole ventilation over a wide range of timescales. Key parameters were calibrated using prior field-validated 3D CFD simulation of pump-induced airflow. An idealised sewer network consisting of a main trunk and two laterals (10 pipes and 10 manholes) was designed as a demonstration case. The model predicted pressure variations and manhole ventilation driven by an upstream pumping event (144 L/s discharge, 3 min pump operation), yielding total vented and induced air volumes of 1.8 and 22.8 m3, respectively. Finally, the validity of a steady-state approximation was evaluated under periodic water inflow conditions, and a practical criterion was proposed for the configurations examined in this study, indicating that airflow can be reasonably treated as steady when the Strouhal number is below 0.05. The proposed model provides an efficient tool for ventilation assessment and subsequent pollutant transport simulations in sewer systems.