The Secondary Settling Tank sedimentation tanks or settling basins or clarifiers allow the micro-organisms and other solids to settle after secondary or biological treatment. This sludge is pumped back into the inlet end of the primary sedimentation tanks and settles with the raw sludge. At this stage the BOD may be reduced to approximately 75 - 80% of the inffluent level and suspended solids are removed.
In the activated sludge process, the biological sludge mass has to be separated from the treated water to produce clear final effluent. This solid-liquid separation process is usually achieved by gravity sedimentation in traditional secondary settling tanks (SSTs, often referred as secondary settlers, final clarifier or secondary thickeners). From the biological reactor the mixed enters the secondary clarifier where it should be sufficiently clarified in order to produce an effluent of acceptable quality.
The sludge should also be adequately thickened so that the desired solids level in the bisectors can be maintained through sludge re-circulation. Furthermore, secondary settlers should function as storage tanks to store sludge under high solids loading rate and high surface overflow rate typically under peak wet weather conditions. Should any of these functions fail, suspended solids (SS) will be carried over the effluent weirs and escape with effluent. Besides the resulting poor effluent quality, excessive loss of SS may result in the decrease of mixed suspended solids and hence the sludge age, what affects the whole biological process.