Abstract: The present invention provides a method and system for the treatment of wastewater. One embodiment includes an anaerobic reactor and aerobic reactor. A terminal settling tank includes an upflow anaerobic reactor comprising an inlet mounted at the external lowest region of the anaerobic reactor. An introducing mechanism is directly connected to the inlet and located at the lower region in the anaerobic reactor and has a perforated drain pipe therein. A mixer is provided at the main shaft. A discharge is provided for allowing reaction water, which contains some portion of sludge generate by the mixer, to flow into the aerobic reactor. A sludge-accumulator is provided for collecting the sludge, which is settled by gravity during the reaction and produced by the mixer, to the center of the anaerobic reactor.