Impact of surface condition and roughness on sediment formation: an experimental sewer system operated with real wastewater.

Regular sewer cleaning in North Rhine-Westphalia (Germany) generates annual costs of around 50 million Euros. This leads to the question of whether and to what extent sewer cleaning is necessary. To determine the effect of roughness, sewer surface condition and discharge, experiments with real wastewater were performed, using a sewer test track with acrylic glass tubes (DN 300) prepared with abrasive paper and nature stone tiles at the wastewater treatment plant (WWTP) Bochum-Ölbachtal (Ruhrverband, Germany). A logarithmic relationship between deposit height and time was found to lead to maximum deposit heights of 5 to 60 mm. Surface structure analysis by texture measuring indicated that deposits within the first 28 days after cleaning are highly influenced by the surface condition of the sewer and not necessarily by roughness. Furthermore, under dry weather conditions deposit heights are nearly stable after this time, indicating the limiting effect of sewer cleaning. Deposit formation amounted to 1.75-1.80 mm/d at a roughness of ks = 0.10 mm (fine but catchy microstructure) and 0-0.1 mm/d at ks 1.25 mm (wavy microstructure) at steady state and transient discharge within the first 28 days after sewer cleaning.