Final settlement tanks (FSTs) are similar in design to primary settlement tanks and are an integral part of the activated sludge process. They are used to settle out the biological material flowing from the secondary treatment plant producing supernatant that has low levels of organic material and suspended matter that is suitable for additional tertiary treatment or discharge into a water course. They also enable the site to collect the microorganisms produced during treatment so that they can be re-used to seed the incoming settled sewage at the front end of the process. The flow returned to the start of the aeration process is called ‘Returned Activated Sludge’ (RAS).

Humus tanks perform a similar function in conjunction with the more traditional percolating filter type plants that are still in common use. On these plants the settled solids are normally removed for treatment in a sludge processing plant.

The terms Humus Tanks and Final Settlement Tank (FST) are often interchanged and can be taken to perform either of the function above.

Measurement of the position of the sludge blanket location is an important part of automating and optimising the control of the plant and of preventing pollution incidents, further details are provided by following the links below.