Transport of non-flushable wipes in sewers and its application in sewer management

Flushing wipes into sewers has caused severe blockage issues in urban sewer systems, especially during the COVID-19 pandemic; however, the transport mechanism of wipes in sewers has not been reported. To address this knowledge gap, the transport of non-flushable wipes with different densities was systematically studied in a circular pipe. The critical shear stress and flow velocity for the incipient motion of wipes were found to increase with increasing wipe density and with decreasing relative wipe size, starting from 0.02 Pa and 0.05 m/s, respectively. Non-dimensional equations were developed for characterizing the incipient motion using two parameters: the critical Shield number and particle Froude number. The mean wipe velocity, mean ambient flow velocity, and wipe density could be well described by a power relationship. The ratio of wipe velocity to local flow velocity increased with the increase of wipe vertical position in the pipe. Different movement modes of wipes were observed and classified by using the ambient cross-sectional average velocity Ua: the sliding mode started first, followed by rolling/saltation at Ua = 0.18 m/s and suspension at Ua = 0.41 m/s. The threshold values for each mode were found to increase with increasing wipe density. Finally, the research results were applied in sewer management.