Abstract: The present invention relates to a process for treating wastewater that includes the steps of: —a contact step, wherein wastewater contacts bacteria retained on a support surface and the dissolved oxygen concentration of the wastewater is maintained at 2.0 mg/l or less; an aeration step, wherein gas is passed through wastewater that has passed through the contact step and the dissolved oxygen concentration of the wastewater is reduced as the wastewater passes through the aeration step; a sedimentation step, wherein wastewater that has passed through the aeration step is substantially separated into treated water and sludge; and a sludge recycling step, wherein sludge from the sedimentation step is passed to the contact step. The invention also relates to a processing unit on which the aforementioned process may be operated.