Intermediate sedimentation is the fourth and last step of mechanical cleaning. The flow rate inside these large tanks is reduced to 2 centimetres per second, so that flaky sediments and smaller particles can settle down to the tank bottom. A bottom scraper with blades pushes the primary sludge to the end of the tank, from where it is removed. Every day between 80 and 120 tonnes of solids (primary sludge) are removed from the primary clarifiers and transported to the sludge thickening tanks. Floating sludge that rises to the water surface as scum is mostly comprised of cigarette butts or pollen. Cigarette butts should not end up in the sewer system, and need to be extracted from the tanks to avoid operational breakdowns in the downstream technical installations.
Primary sedimentation completes the mechanical cleaning stage, and its effect can be seen in the primary clarifier outflow. Solids can no longer be identified with the naked eye, and around 30% of the impurities have now been removed from the wastewater. The remaining impurities are available in dissolved form and will be removed in the two following biological cleaning stages.