A common final clarifier is provided downstream of a two-stage or three-stage activated sludge (AS) system that includes: (A) one or more flow equalization basin (FEB) reactors and a nitritation reactor as the first AS stage, and an anammox reactor as the second AS stage, or (B) a carbonaceous biological oxygen demand (BOD) removal reactor and one or more FEBs as the first AS stage, a nitritation reactor as the second AS stage, and an anammox reactor as the third AS stage. A first return activated sludge (RAS) flow is conducted from the final clarifier to the first AS stage and a second segregated RAS flow is conducted to the second AS stage. Alternatively, a third segregated RAS flow is conducted to the third AS stage.
