Sediments in combined sewers may negatively affect the performance of sewer systems and significantly promote the formation of hydrogen sulfide. To prevent this, German sewer systems are cleaned regularly but at high costs. In order to optimize the cleaning intervals, this study investigates how deposits build up and change over a defined period under constant conditions. The experiments were performed using real wastewater at a pilot plant consisting of three 10 m long acrylic glass pipes with a diameter of 300 mm, a slope of I = 1‰, a discharge of Q = 2 L/s and a roughness of kS = 0.4 mm. Within the first 30 d, a slow increase of the deposit height (averaged over the pipe length) of 0.75 mm/d was observed. The deposits did not build up continuously, but in between times also decreased due to erosion. The daily changes in deposit amounts occurred partly as a function of the rainfall conditions and could go beyond the average growth many times. Within a day, the deposit height (averaged over pipe length) changed by up to 20 mm, at single measuring points even up to 100 mm. After about 50 d under constant test conditions, the deposit height came to a state of equilibrium between sedimentation and erosion.

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