In the intermediate clarifiers, the activated sludge is separated from the wastewater. Due to the large size of the tanks, the wastewater flow is slowed down to only one centimetre per second. This gives the activated sludge and its microorganisms time to settle out to the bottom of the tank. Flight scrapers push the activated sludge into a channel, from where it is withdrawn. The bulk of this sludge is pumped back to the aeration tanks as return sludge. The microorganisms get in contact with fresh wastewater and are reactivated for the cleaning process. The build-up of activated sludge, also referred to as excess sludge, is withdrawn and pumped into the sludge thickening tanks. Intermediate sedimentation marks the last step of the first biological cleaning stage.