Unsteady Air Flow Induced by Pumping Events in Sewer Systems

Pumping events can cause air pressure fluctuations in sanitary sewers and lead to air ventilation through openings. In this study, field measurement of air flow was conducted in a sewer system downstream of a pump station, and the measured air pressure was used to validate a 3D computational fluid dynamics model to study the physical process of the air flow. The pump start-up generated sharp positive spikes on air pressure and caused air emission through the manhole openings. The value of the peak air pressure decreased at downstream manholes. The air pressure inside the manholes quickly became negative with stable air suction as the air was moved downstream by the water surface drag. When the pump was shut off, a negative air pressure spike appeared in all manholes. The effect of orifice size, pump discharge, and pipe length on air pressure and air ventilation were numerically investigated. Analytical solutions were also developed to estimate the pressure spikes in manholes.