Primary purpose of an anoxic tank in wastewater treatment is to promote denitrification, which is the biological conversion of nitrate (NO3-) to nitrogen gas (N2) in the absence of oxygen. Denitrification is a crucial process for the removal of nitrogen compounds from wastewater, particularly in cases where strict effluent standards for nitrogen are required.

Nitrate Addition: Wastewater is direct into the anoxic tank, usually following preliminary treatment processes to remove large solids. In some cases, nitrate-rich return active sludge (RAS) from the aerobic tank is also added to the anoxic tank to provide a source of nitrate.
Absence of Oxygen: Anoxic tank is maintain under conditions where oxygen is limit or absent. This can be achieve by reducing aeration, using mixers or diffusers that prevent oxygen intrusion, or by the presence of other electron acceptors that outcompete oxygen, such as sulfate or ferric iron.
Denitrification: Inside the anoxic tank, specific groups of microorganisms, known as denitrifiers, utilize the available nitrate as an electron acceptor to oxidize organic matter. These microorganisms convert nitrate to nitrogen gas (N2), which is release to the atmosphere, thereby removing nitrogen from the wastewater.
Mixing: Mixing adequate is maintain within the anoxic tank to ensure contact between the denitrifying microorganisms, organic matter, and nitrate. This is achieve using mechanical mixers or other mixing devices.