Erosion characteristics of raw sewage: investigations for a pumping station in northern Germany under energy efficient pump control

Flow controlled sewage pumping stations offer high potential for energy savings. But along with a reduced flow velocity, flow-controlled pumping increases the risk of deposits formation. This work presents an experimental procedure to assess the erosion behaviour of municipal wastewater as a basis for solid transport characterization considering an energy efficient pump control. Raw sewage, sampled at the inflow channel to a pumping station in the city of Rostock (northern Germany), has been investigated under dry weather inflow conditions by means of a self-constructed laboratory-scale erosion measurement. Received data have been processed into critical bed shear stress points (for incipient erosion and total resuspension) and into erosion rates. Both bed shear stress points increase with the settling duration, from initially 0.016 N/m (incipient erosion) and 0.2 N/m (total resuspension) after 20 minutes settling, to respectively 0.14 N/m and 1 N/m after 3 days settling. With a reduced flow rate within the energy efficient control, the pump pauses decrease, from 64 min (regular control with higher flow rate) down to 20 min. Thus, both respective bed shear stress points are below the bed shear stress level of the energy saving control (0.2 N/m ), and a resuspension of the settled particles is guaranteed.