n 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. A bottom scraper pushes the activated sludge into a channel, from where it is withdrawn.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The build-up of activated sludge, also referred to as excess sludge, is withdrawn and pumped into the sludge thickening tanks. As the wastewater leaves the intermediate clarifiers, it flows over dentated sills into the next stage. Intermediate sedimentation marks the last step of the first biological cleaning stage. The wastewater is transported into the intermediate pumping station and from here it is pumped into the second biological cleaning stage.