General biological denitrification refers to nitrification and denitrification. Nitrification refers to the transformation of ammonium salt into nitrite and then into nitrate. Denitrification is to convert nitrate into nitrogen, that is, to achieve denitrification. Among them, nitrification means that autotrophic bacteria use CO2 as carbon source, and denitrification means that heterotrophic bacteria need to consume organic matter in water and can only do it in an anoxic environment (there are more nitrates) (there are nitrates, so they are anoxic).

Therefore, the anoxic tank is placed in front of the aerobic tank to provide denitrifying bacteria with enough organic matter as the carbon source for denitrification, and some aerobic tanks return to the anoxic tank for denitrification.

If it is placed behind the aerobic tank, other microorganisms in the aerobic environment will consume the organic matter in the water, and it is difficult to achieve full denitrification after entering the anoxic tank.

In addition, maintaining anaerobic environment is for phosphorus removal, that is, "anaerobic phosphorus release and aerobic phosphorus absorption". Anaerobic must be strictly anaerobic.

(anaerobic and nitrate), nitrate exists in large quantities because of lack of oxygen. Anaerobic+anoxic+aerobic is the dual function of nitrogen and phosphorus removal.