How to remove nitrogen by AO process?

A/o process, also known as pre-denitrification, is to design an anoxic tank first and then an aerobic tank. Sewage will convert ammonia nitrogen into nitrate or nitrite in aerobic tank, and return to anoxic tank for denitrification through circulation in mixed solution, so that nitrate will be converted into nitrogen and discharged. The advantage of pre-denitrification is that it does not need additional carbon source, so microorganisms can survive with organic matter in sewage as carbon source, and sludge bulking can also be controlled before denitrification.