Secondary sedimentation

The mixture of water, oxygen and active sludge from the biological treatment unit is transferred to the two secondary sedimentation tanks (32 tanks in all), which are superimposed in order to build the installation as compact as possible with regard to the limited space at disposal for the whole wastewater treatment plant. The upper tanks are not covered, because the water is considered odourless. Most settled sludge is returned to the aeration tanks. A small part of the sludge corresponding with the sludge growth is withdrawn from the process. This so called surplus sludge is pumped after thickening with centrifuges to the sludge digester.