/O process was better. Denitrification of the sludge in the sedimentation unit was directly related to denitrification in the anoxic unit. Therefore, to ensure that denitrification in sedimentation unit does not seriously affect the separation of sludge and water, appropriate control of the aerobic operation and the maintenance of denitrification in the sedimentation unit will contribute more to the denitrification efficiency of the system rather than simply controlling the level of anoxia.