Transport of large solids in unsteady flow in sewers

Abstract

At the upstream end of wastewater collection systems, the depth of flow is small and large solids have not disintegrated to any extent. Traditional tractive stress analysis for motion of solids in sewers may not be applicable for large solids which have a larger vertical dimension than the depth of the flow. This paper presents criteria to determine the conditions under which large solids move in sewers which have unsteady flow pulses based on over 1200 experimental runs. The important parameters are the density of the solid, the slope of the sewer and the flow (or in the case of short pulses, the volume of fluid in the pulse). The solids do not move as fast as the fluid. If the duration of the pulse is not sufficiently long, the solid may be picked up and deposited further downstream as the fluid ‘outruns’ the solid.

Keywords:

• sewers
• solid transport
• wastewater collection systems
• tractive stress