In a sewage (or industrial wastewater) treatment plant, the activated sludge process is a biological process that can be used for one or several of the following purposes: oxidizing carbonaceous biological matter, oxidizing nitrogenous matter: mainly ammonium and nitrogen in biological matter, removing nutrients (nitrogen and phosphorus

The general arrangement of an activated sludge process for removing carbonaceous pollution includes the following items:

• Aeration tank where air (or oxygen) is injected in the mixed liquor.
• Settling tank (usually referred to as “final clarifier” or “secondary settling tank”) to allow the biological flocs (the sludge blanket) to settle, thus separating the biological sludge from the clear treated water.

Treatment of nitrogenous matter or phosphate involves additional steps where the mixed liquor is left in anoxic condition (meaning that there is no residual dissolved oxygen).