The main tunnel has an area of 21 sq m. It is about 4.5 metres wide and 5 metres high. The tunnel will start to be constructed at level minus 27 metres (i.e. 27 metres below sea level) in Bromma and then follow a falling line towards Sickla, where the level is at minus 43 metres. The tunnel will slope at an angle of 1 ‰, which means a drop of 14 metres over the overall 14 km route of the tunnel. The gradient of the tunnel will allow wastewater to flow all the way to Sickla due to gravity.
At its lowest level, the tunnel will travel beneath the bed of Lake Mälaren, 90 metres below sea level. The wastewater in this section will pass through a number of pipes in the tunnel in order to prevent sedimentation at the lowest level. This will allow the pipes to be flushed clean at regular intervals to prevent deposits and sludge.